25369182 HVAC Handbook Applications Engineering Manual for Desiccant Systems

COMMERCIAL DESICCANT SYSTEM APPLICATION GUIDE

Chapter 1 - Introduction 5

CHAPTER 1INTRODUCTION

Desiccant SystemsDesiccant systems and components are a useful addition to the engineer'stool kit. They allow independent control of humidity, as well as potentialenergy savings and increased comfort levels.

REALIZING THE BENEFITSThe growing popularity of desiccant systems is analogousto the addition of cooling systems to fans systems in an-other important respect. Namely, the system must be de-signed differently to take advantage of benefits and to mini-mize installed cost.

One hundred years ago, engineers could cool a building us-ing only fans, but the air flows used were very large, and airflow velocities had to be kept high. When mechanical cool-ing arrived in the engineer's tool kit, air flows and air veloci-ties could be greatly reduced because of the greater cool-ing capacity of cold air. But if air flows were not reduced, amechanical cooling system could be cost-prohibitive. Thetransition to desiccant systems is similar. A building's airhandling system must be designed to take advantage of thetechnical differences between the desiccant system and theolder technology it replaces or supplements.

That's the reason for this application guide: to provide en-gineers with an "efficient thought process" about the ad-vantages and limitations of desiccants. With this knowledge,desiccants can be applied as quickly as other air condition-ing technologies, and possibly more cost-effectively.

The guide will describe how desiccant systems work, whatcomponents they contain and how to install and maintainthem. It also describes general application principles for suchsystems, and gives examples of how those principles areapplied in practice to specific buildings.

THE FUTUREInformation contained in this guide pertains primarily tocommercial buildings, and as this edition goes to press, des-iccant system applications are expanding very rapidly. Wesincerely hope that the reader will find this information use-ful. And as his or her experience with desiccants expands,we look forward to hearing the reader's suggestions to im-prove and expand the guide in future editions.

BENEFITS FOR THE SECOND CENTURY OFAIR CONDITIONINGDesiccant technology now stands where mechanical cool-ing stood in the 1930's. Desiccant systems have been usedby industrial engineers for years to achieve productivity andenergy benefits which far outweigh their installed cost. Now,with lower cost desiccant components, commercial build-ings are using desiccant systems because they provide ben-efits beyond those of simple air cooling.

In many ways, the rise of desiccant systems is parallel to the80-year-old transition from fan-only cooling to mechanicalcooling. Mechanical cooling did not reduce the need for fansand blowers. Likewise, desiccant technology will not reducethe need for mechanical cooling; it will simply shift part ofthe cooling load to thermal energy sources. Also, just asmechanical cooling add cost to a fan-only system, desic-cant equipment can sometimes cost more than mechanicalcooling. But just as cooling coils add functionality to a ven-tilation system, desiccant systems provide benefits whichare beyond the reach of mechanical cooling systems. Spe-cifically, desiccant systems can provide:

• Precise control of humidity, independent oftemperature.

• Dew points below the practical limits of coolingtechnology.

• Comfortable conditions during cool, muggyweather

• Lower operating cost

• Lower peak electrical demand.

• Ability to use low-cost thermal energy to controlboth humidity and temperature.

• Dry duct systems in accordance with ASHRAEStandard 62, avoiding microbial and fungal growthassociated with sick building syndrome.


Chapter 2 - Fundamentals 7

CHAPTER 2FUNDAMENTALS

Operating PrinciplesDesiccant components are mechanically quite simple, durable and reliable.

Figure 1. Desiccant wheel operating principles

DESICCANTSDesiccants are materials which can attract and hold mois-ture. Nearly any material is a desiccant—even glass can col-lect a small amount of moisture. But desiccants used in com-mercial equipment are selected for their ability to hold largeamounts of moisture. For example, the silica gel packetsoften sealed into vitamin bottles can hold moisture equalto about 20% of their dry weight. Liquid desiccant materialscan hold even more moisture. But all desiccants used in com-mercial systems work the same way.

HOW DESICCANTS WORKDesiccants remove water vapor by chemical attractioncaused by differences in vapor pressure. When air is humid,it has a high water vapor pressure. In contrast, there arevery few water molecules on a dry desiccant surface, so thewater vapor pressure at the desiccant surface is very low.Water molecules move from the humid air to the dry desic-cant in order to equalize this pressure differential.

With desiccants, moisture removal occurs in the vaporphase. There is no liquid condensate. Consequently, desic-cant dehumidification can continue even when the dewpoint of the air is below freezing. This is different from cool-ing-based dehumidification, in which the moisture freezesand halts the process if part of the coil surface is below 32°F.

Desiccants can be either liquids or solids, and there are manydifferent materials of both types. The principles describedhere apply to both liquid and solid systems. However, thegreat majority of systems built for commercial buildings usesolid desiccants.

DESICCANT WHEELSFigure 1 shows the basic desiccant component—the wheel.The desiccant material, usually a silica gel or some type ofzeolite, is impregnated into a support structure. This lookslike an honeycomb which is open on both ends. Air passes

through the honeycomb passages, giving up moisture tothe desiccant contained in the walls of the honeycomb cells.The desiccant structure is formed into the shape of a wheel.The wheel constantly rotates through two separate airstreams. The first air stream, called the process air, is driedby the desiccant. The second air stream, called reactivationor regeneration air, is heated. It dries the desiccant.

Moist "process" air is passed through a rotating wheel,which looks like a ceramic honeycomb. The desiccantin the wheel absorbs moisture. Then the wheel slowlyrotates into a second, heated air stream.

The hot "reactivation" air removes moisture from thedesiccant so it can absorb more humidity when thewheel rotates back into the process air stream.

ReactivationHeater

HoneycombMatrix

Desiccant Wheel

Dry Process Air tothe HVAC SystemHumid Process Air

MoistReactivation Air

Desiccant Wheel Operation


Chapter 2 - Fundamentals8

A desiccant wheel rotates slowly, and contains moredesiccant than an enthalpy wheel. By heating thereactivation air, it can remove much more water vaporthan an enthalpy wheel.

An enthalpy wheel has a small amount of desiccant, soit can move moisture from the supply air to the exhaust.But without heat for reactivation, its dehumidificationcapacity depends on the dryness of the exhaust air.

75°70 gr/lb

85°120 gr/lb

20 rpm

76°77 gr/lb

Outdoor Air

Exhaust Air

85°120 gr/lb

250°120 gr/lb

85°120 gr/lb

0.2 rpm

121°42 gr/lb

Outdoor Air

Outdoor Air

Reactivation Heater

Moisture Exchange(Enthalpy Wheel)

Moisture Removal(Desiccant Wheel)

ENERGY RECOVERY VS. DESICCANT WHEELSDesiccant wheels are often confused with energy recoverywheels. The confusion is understandable. Both devices looknearly the same, because energy recovery wheels and des-iccant wheels are constructed with honeycomb media. Also,total heat or "enthalpy" wheels contain desiccant. Also, sen-sible-only heat wheels are sometimes used as post-coolersin desiccant systems. But there are important functional dif-ferences between these devices which appear so similar.

Heat wheels are optimized to transfer sensible heat betweentwo air streams, while desiccant wheels are optimized to re-move moisture. These different purposes lead to differencesin materials and in wheel rotation speed. An energy recov-ery wheel rotates at a comparatively high speed (20 rpm),to maximize the heat transfer between air streams. A desic-cant wheel rotates 60 times more slowly (10 to 20 rph). Theslow rotation speed allows the desiccant to adsorb moremoisture, and it minimizes the amount of heat carried overfrom the hot reactivation air into the cooler process air.

If the exhaust air is dry, an energy recovery wheel can trans-fer some moisture out of the incoming air. But energy re-covery wheels contain less desiccant than desiccant wheels.Also, the honeycomb material, air seals and support struc-ture of an enthalpy wheel cannot endure the temperatureand moisture differences typical of desiccant wheel opera-tion. Consequently, the wheels perform quite differently.

As seen in figure 2, thermal energy used for reactivationallows desiccant wheels to remove much more moisture thanenergy recovery wheels. Desiccant wheels can deliver air be-low the moisture condition of the exhaust air. That level ofdryness cannot be reached with energy recovery wheels.

DESICCANTS CHANGE VAPOR TO HEATOne aspect of desiccant wheel behavior can be confusingto the first-time user of the technology; air leaves a desic-cant wheel dry, but warmer than when it entered the wheel.For example, if air enters a desiccant wheel at 70°F and 50%rh, it will leave the wheel at about 100°F and 4% rh.

This non-intuitive behavior becomes easier to understandas the reverse of evaporative cooling. When water is sprayedinto air, it evaporates by using part of the sensible heat inthe air—so the dry bulb temperature falls as water vapor isadded to the air. That process is intuitive to children run-ning through sprinklers in summertime.

Desiccants produce the opposite phenomenon. As water va-por is removed from air, the dry bulb temperature of the airrises. The amount of temperature rise depends on theamount of water removed. More water removal produces agreater temperature rise. The initial user naturally asks: howcan desiccant systems save cooling energy if dehumidifica-tion adds sensible heat to the air? Part of the answer is thatsome heat is moved to reactivation by a heat exchanger.(Chapter 3, figure 4) The rest of the answer depends on theapplication.

For example, if air is dry, it may not be necessary to cool itif the space is already overcooled—as in a supermarket,where display cases cool the aisles as well as the product.Alternatively, dry air can be cooled using low-cost indirectevaporative cooling such as cooling towers, or with highlyefficient vapor compression systems operating at highevaporator temperatures. In such cases, desiccants can saveenergy and energy cost.

Figure 2. Desiccant wheels compared to energy recovery wheels


Chapter 3 - Types Of Desiccant Systems 9

COMMERCIAL DESICCANT SYSTEMSFigure 1 shows a typical example of a desiccant system de-signed for a commercial building. It includes a desiccantwheel for humidity control, and a conventional vapor com-pression cooling system for temperature control.

Such designs combine the best of both technologies, andpoint to one of the principal advantages of desiccant-as-sisted HVAC systems, namely that they can control humid-ity independently of temperature. The desiccant subsystemis controlled by a humidistat and the cooling coil is con-trolled by a thermostat. This allows humidity control regard-less of what the space may or may not need for heat re-moval.

But fundamentally, there are two different types of desic-cant systems: those which use only desiccants for all cool-ing and humidity control, and those like the system in fig-ure 3, which combine desiccants with conventional compo-nents.

All-Desiccant & Hybrid SystemsDesiccant systems combine a desiccant wheel with additional cooling and heatingcomponents. These may be conventional gas-driven or vapor-compression coolers,or they may be evaporative coolers combined with heat exchangers.

ALL-DESICCANT SYSTEMSFigure 2 shows how a desiccant wheel can be combined witha rotary heat exchanger to form a complete air condition-ing system. Air is dried by the desiccant wheel, and thencooled by the heat exchanger.

This configuration has useful advantages when largeamounts of fresh air are needed, and when the exhaust aircan be evaporatively cooled and used for post-cooling theair leaving the desiccant wheel. Under those circumstances,an all-desiccant system is the same physical size as conven-tional alternatives because the ventilation air required forthe building defines the overall system's air flow.

The system also uses very little electrical power, so it hasadvantages when electrical demand charges are high. Whenthese two circumstances combine, such as when largeamounts of ventilation air must be added to an existingbuilding in an area with high peak demand charges, the all-desiccant system will reduce both energy and first cost com-

pared to other ways ofadding the increasedfresh air.

The disadvantage of theal l-desiccant system isthat, at peak design tem-peratures, it delivers sup-ply air at temperaturesabove 70°F.

CHAPTER 3TYPES OF DESICCANT SYSTEMS

Figure 1. Hybriddesiccant system usingboth desiccant and vapor-compression cooling.



The only exceptions are in far-north and high-altitude cli-mates, where the ambient moisture is so low that evapora-tive cooling can provide lower air temperatures.

So in most climates, if the building does not need a largepercentage of ventilation air, and when the exhaust air can-not be collected and brought back to the unit for post-cool-ing, the all-desiccant system has a disadvantage comparedto a hybrid system. Since it cannot cool air below 70°F on a"design day", the all-desiccant system must use largeamounts of air to remove a given heat load. Such systemsare physically much larger than an equivalent conventionalcooling system . The conventional system would supply 55°air, and therefore remove the same internal sensible heatload using less air.

For example, consider a small office building maintained at75°F with an internal sensible heat load of 180,000 Btu/h(15 tons). If the supply air can be cooled to 55°F, the systemwill have to supply 8,333 cfm:

Btu/h = cfm x 1.08 x ∆t

cfm = (180,000) ÷ (1.08 x (75 - 55))

cfm required @55°F = 8,333

But if the supply air can only be cooled to 70°F, the tem-perature difference between supply and return is only 5°F ,so the air flow needed to remove the load is much greater:

Btu/h = cfm x 1.08 x ∆t

cfm required @ 70°F = (180,000) ÷ (1.08 x (75 - 70))

cfm = 33,333

However, if that office building needs a great deal of out-side air, the all-desiccant system could handle the ventila-tion load, and a separate system arranged to handle the in-ternal heat load. In that circumstance, the all-desiccant sys-tem has advantages over a conventional system.

The desiccant system's 70°F delivered air removes some in-ternal load since the space is being maintained at 75°F. Andthe heat exchanger in the desiccant system can operateduring cooler months, to recover waste heat from the build-ing exhaust. Since the system size is governed by the re-quired outside air quantity and not by the internal load, theall-desiccant system is the same size as a conventional alter-native. So installed cost is close to the same, and the desic-cant system costs much less to operate because it uses solittle electrical power.

In summary, an all-desiccant system is generally attractivewhen:

• Large amounts of air must be exhausted from thebuilding.

• The exhaust air can be brought back to where themake-up air enters the building.

• Electrical demand charges are high.

• Supplying outside air at 70°F is adequate for theapplication.

In other circumstances, the engineer may wish to considera hybrid desiccant system.

Figure 2 All-Desiccant system, including an indirect evaporative cooler.

All-Desiccant Air Conditioning SystemDesiccant Wheel

Reactivation Heater

Heat Wheel

Evaporative Pad

Exhaust Air

Supply Air

75° 100° 125°

72°48gr/lb

Chicago 2.5% Design91°, 92gr/lb



1 Air enters the process side of the desiccant wheelfrom outside the building. It is hot and humid.

2 Air leaves the process side of the desiccant wheelhotter, and much drier than when it entered thesystem. In most cases, this air is too hot to senddirectly to the building. It must be cooled.

3 Dry air leaves the first stage of post-cooling at alower temperature. The sensible heat has beenremoved from the process air and transferred tothe reactivation air by a heat exchanger. Theschematic here shows a rotary heat wheel, but heatpipes and plate-type heat exchangers are used bymany system suppliers instead of heat wheels.Regardless of the type of heat exchanger, itprovides a double benefit: the process air is cooledusing only the energy needed to push the airthrough the exchanger. So the operating cost ofthe cooling is very low. Secondly, the heat fromprocess is used to pre-heat incoming reactivationair, which saves slightly on the cost of thermalenergy.

4 Point 4 represents the additional cooling whichcan be accomplished by the heat exchanger if theair on the other side of the heat exchanger is

HYBRID DESICCANT SYSTEMSFigure 5 shows the wide variety of components which canform hybrid desiccant systems, i.e: systems which include adesiccant component along with gas cooling or conventionalcoils.

Figure 6 shows the psychrometric behavior of different sys-tem alternatives. Note especially the dry bulb temperatureleaving the system. To a great extent, the leaving air tem-perature determines which applications are economicallypractical for each system alternative.

Some applications, such as hotel corridors and ice rinks, arenot sensitive to a leaving air temperature of 78 to 85°F on adesign day during the summer. So an indirect evaporativepost-cooler is the best cooling option because it is quiteeconomical to install and operate. Other applications likehospital operating rooms must have air at relative humidi-ties below 50% rh and temperatures below 65°F. In thosecases, gas cooling or conventional cooling coils will be re-quired downstream of the desiccant wheel.

To understand each equipment alternative, we will track theprocess air as it moves through the system. The diagrams infigure 6 assume the system is arranged to handle 100% out-door air on the process side, and 100% outdoor air for reac-tivation.

Figure 5. Hybrid desiccant system, including a variety of heating and cooling options.

Hybrid Desiccant SystemHardware Options

Heat WheelPost-cooling

Direct-Fired GasReactivation Heater

Desiccant Wheel(Reactivation Sector)

Reactivation Fan

Heat ExchangerReactivation Pre-Heat

OptionalIndirect Evaporative

Post-Cooler

Filter

Desiccant WheelProcess Air Sector

Process Air Fan

OptionalCooling Coil

OptionalDirect Evaporative

Post-Cooler

Reactivation Air

To Conditioned Space Process Air

Exhaust

Heating The Air

Cooling The Air

2 13

456

A B C D E



evaporatively cooled. In this option, the incomingreactivation air is cooled by an evaporative padbefore it enters the heat exchanger. Since the airon the reactivation side of the exchanger is cooler,more heat can be removed from the process side.This diagram shows roughly what happens on adesign day, so the evaporative cooling effect is notvery large. But when outside air temperature andmoisture is lower—99% of the time during theyear—the cooling effect of the evaporative pad willbe substantial. This reduces the need for anysubsequent post cooling.

5 Point 5 shows the temperature and moistureleaving the system when a gas cool ing orconventional cooling system follows the heatexchanger. Air is sent to the building at a very cooland very dry condition. This configuration is popularbecause it keeps air distribution ducts and filtersdry and free of microbial growth. Low temperature,dry supply air allows the system to do a great dealof cooling and dehumidification with less air thanconventional cooling systems.

6 An alternative to conventional cooling coils is asecond evaporative cooling pad, this time on theprocess air side. Direct evaporative cooling seldomchills air as deeply as a conventional coil. Also, thesupply air is saturated at a comparatively hightemperature (73 to 78°F on a design day). So suchsystems cannot be used to control humidity unlessa relatively warm, highly humid environment isneeded, as in a greenhouse.

The evaporative cooling option (point 6) is less expensiveto install, and uses very little energy compared to conven-tional post-cooling alternatives. So this option has advan-tages when electrical power cost reduction is the principalgoal of a project rather than humidity control.

To date, hybrid systems have been popular, perhaps becausethey combine the best characteristics of each technology:desiccants for moisture removal and conventional coolingfor sensible heat removal. Hybrid systems are nearly always

Figure 6. State points within a desiccant system.

5

Cooling Coil OptionPost-cooling with vapor

compression orabsorption systems

provides greater coolingcapacity and a dry

system at an increasedoperating cost

Direct EvaporativeCooling Option

Can save operatingcosts, but may increase

unit installed costbecause more air will

be needed for the samesensible cooling load,

and humidity in ductwork is high.

Indirect EvaporativeCooling Option

Increases the postcooling effect of the

heat wheel, reducingoperating expense for

the cost of providingwater for the cooling

pads

Heat Exchanger Post-CoolingSaves considerable operatingcost by moving heat from theprocess air to the reactivation air.Both cooling and heating energyrequirements are reduced by theheat exchanger.

Desiccant DehumidificationRemoves moisture from thesupply air, but adds heat inproportion to the amount of waterremoved. Low-cost post-coolingis essential to achieving overallcost savings.

Outside Air Entering The System

OptionalWaste Heat RecoverySaves a modest amountof heating energy inreturn for an increase ininstalled cost.

Indirect Post-CoolingOptionSaves substantial coolingenergy on the processside at the expense ofslightly increased gascost in reactivation.

Direct-Fired HeaterReactivates the desiccantat very low cost.

Psychrometrics of Each Desiccant System Alternative

75° 100° 125°

75° 100° 125° 250°

ProcessAir

Path

ReactivationAir

Path

1

234

6

D

E

AB C



smaller than all-desiccant systems, because they can pro-vide air at low levels of both temperature and humidity. Sosmaller hybrid systems can do the same work as larger all-desiccant or all-cooling units.

LIQUID DESICCANT SYSTEMSOver the last 15 years, manufacturers of desiccant systemsfor commercial buildings have concentrated primarily ondesiccant wheel type units, which use solid desiccants. Butin industrial markets, liquid desiccant systems have beenused very effectively since the 1920's. In recent years, manu-facturers of liquid desiccant equipment have been expand-ing their activity in commercial markets.

The unique characteristics of liquid desiccant systems areeffective in commercial applications, especially in largerbuildings, where the advantages of liquid desiccants pro-vide cost-effective competition to both solid desiccants andto conventional cooling systems.

How Liquid Desiccants WorkLiquid desiccants, such as lithium chloride, can absorb up to1200 times their dry weight in water. The concentration ofsalt in the liquid solution determines the absorption char-acteristics of the liquid, which is sprayed into the processair. If the solution is concentrated, it can absorb moisturefrom drier air streams, and if the solution is dilute, it ab-sorbs moisture from more humid air streams.

So by controlling the concentration of the solution, one cancontrol the humidity of the air that passes through the liq-uid spray. In order to control the temperature of the pro-cess air, one simply adjusts the temperature of the liquiddesiccant being sprayed into that air.

Liquid systems are very simple in concept, as describedabove. In hardware, they are somewhat more complex, be-cause liquid desiccant solution can be corrosive, and becausethe components of the system can be located in differentparts of a building with interconnecting piping. In the past,this flexibility of component arrangement has meant thatin smaller sizes, liquid desiccant systems were more expen-sive to install than dry desiccant systems.

Unique advantages of liquid systems In larger sizes, liquid and solid desiccant systems are closerin cost, and the advantages of liquid systems can be signifi-cant. Specifically, liquid systems:

• Kill bacteria and viruses, clearing the air of biologicalcontamination

• Can operate effectively with very low-temperaturereactivation energy (as low as 130°F)

• Can connect many process air conditioner sectionswith a single regenerator section, saving costs forlarge installations where many air inlets may bescattered widely through a building.

• Can use low-cost cooling tower water for removingsensible heat from the desiccant dehumidificationprocess, eliminating any need for mechanicalcooling equipment in many cases.

In summary, although a full discussion of liquid desiccantsystems is beyond the scope of this application guide, thetechnology is well-proven. As manufacturers continue to re-duce costs and simplify installation, liquid systems will beapplied in low-rise construction as well as in the larger com-mercial and institutional and industrial buildings where liq-uids have enjoyed success in the past.

Liquid Desiccant System and Flow Diagram

Figure 7. Liquid desiccant system


Chapter 4 - Configurations & Consequences 15

CHAPTER 4CONFIGURATIONS + CONSEQUENCES

100% Outside Air SystemsDesiccant systems are especially useful for providing dry and coolfresh air. A desiccant system can remove heat and moisture fromventilation air, freeing conventional systems to remove the buildingsinternal heat loads.

The outside air ventilation requirements of ASHRAE Standard 62 can present a considerable challenge to HVACsystem designers. Conventional equipment does not

deal effectively with large amounts of outside air. Thermalcomfort and humidity control can suffer unless special mea-sures are taken to remove heat and moisture from fresh air.

Desiccant systems are ideal for preconditioning largeamounts of fresh air. This section of the application guidewill survey alternatives for 100% outside air desiccant sys-tems. Each system configuration has its own advantages andlimitations, which will become apparent as the systems arecompared to each other.

ISSUES FOR 100% OUTSIDE AIR SYSTEMSIn designing desiccant systems, removing the sensible heatproduced by desiccant dehumidification is an important is-sue. In a 100% outside air system, the issue is especially im-portant because fresh air carries a great deal of moisture,so the desiccant process will produce a great deal of sen-sible heat.

As described in the previous sections, heat can be removedfrom the supply air by using a heat exchanger to move theheat to the reactivation air. The temperature of the incom-ing reactivation air is critical. The colder the air, the colderthe process air can be made, so the overall system can domore work.

There are two places to get that reactivation air: from thebuilding exhaust or from the outside environment. If thedesigner can use building exhaust air, the system can coolthe process air more deeply. But if only outside air can beused, there will be more heat for other systems to removeafter the supply air leaves the desiccant system.

Regardless of the source of the reactivation air, it can bemade still cooler through evaporation. If the owner will con-

sider that alternative, the dry process air on the other sideof the heat exchanger can be cooled more deeply.

The most practical way to evaluate the advantages and limi-tations of each approach is to calculate the specific supplyair temperatures and moisture levels which can be achievedwith each design alternative, and consider their effect onthe rest of the HVAC system.

ASSUMPTIONSTo compare the alternatives on equal terms, one must makesome basic design assumptions. For the alternatives in thischapter, the key assumptions include:

The building :

• needs 10,000 cfm of outside air

• has additional cooling systems to remove internalsensible heat loads

• needs to be maintained at 75°F and 50% rh. (Thecenter of the ASHRAE summer comfort zone)

The location:

• is Detroit, Michigan

• has a 1% peak design enthalpy which occurs at 83°F,123 gr/lb, based on ASHRAE data. (RP 754)

The owner:

• is willing to consider the use of evaporative coolingto reduce cost of operation

• would, if economically feasible, prefer to havebetter-than-typical humidity control in duct workand in the building itself.


Chapter 4 - Configurations & Consequences16

System 1 - 100% outside air, with exhaust air used for post-cooling.

System 2 - 100% outside air, with outside air used for post-cooling.

EXHAUST RECOVERYSystem 1 and 2 differ in only one respect: system 1 usesbuilding exhaust air to cool the process air after the desic-cant wheel, and system 2 uses outside air for post-cooling.

In all other respects, the systems are the same. They pro-cess 10,000 cfm of fresh air and deliver it dry, for subse-quent cooling by other systems downstream. Both systemscan remove 437 lbs of moisture per hour from the fresh air.Consequently, that air is so dry that it can remove 64 lbs perhour from the building, when the desired control level is75°, 50% rh. So both systems have ample moisture removalcapacity, and it is very unlikely that any cooling coil down-stream of the desiccant system will have to remove any mois-ture at all.

Because the cooling air comes from different places, thetwo systems do different amounts of work. System 1 doesmore work, delivering air at 89°F. System 2 delivers air at95°. This is because, on the cooling side of the heat ex-changer, system 1 uses 75° air from the building, where

system 2 uses air from the outside at 83°. In almost all cases,the lower temperature is more desirable because it reducescooling requirements in the rest of the HVAC system.

However, in some buildings, it may not be practical to bringthe exhaust air back to the same location as the fresh airinlet duct work. For example, in a light industrial buildingwith many internal fire walls and a dozen different processexhaust points, the return duct work may be more costlythan the small additional cost to add capacity to the otherrooftop air conditioning units. Or in cases where a very smallamount of fresh air is needed, rather than 10,000 cfm, theadditional sensible cooling capacity may already be availablein other parts of the system at no additional cost, comparedwith a high cost for return duct work.

But in most cases, and in particular those cases where asmuch as 10,000 cfm of fresh air is needed, the use of returnair for post-cooling quickly pays off any small cost of a re-turn duct system to bring the exhaust air back to the unitbefore it leaves the building.

Make-up Air With Exhaust Recovery

Temperature ( °F ) 83 144 89 75Moisture (gr/ lb) 123 55 55 65Air Flow (scfm) 10,000 10,000 10,000 BLDG

A B C D

MBtu/h Tons Lbs/hrSensible (151) (12.6) --------

Latent 68 5.7 64

Sensible (65) (5.4) --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Post-cooling by heat exchanger alone

DW HW

A B C

D 32 150

150

A

BC

89°, 55gr

D

Supply Air

Make-up Air - No Exhaust Recovery

Temperature ( °F ) 83 144 95 75Moisture (gr/ lb) 123 55 55 65Air Flow (scfm) 10,000 10,000 10,000 BLDG

A B C D


Latent 68 5.7 64

Sensible (129) (10.8) --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Post-cooling by heat exchanger alone

32 150

150

A

BC

95°, 55gr

D

Supply Air

DW HW

A B C

D



System 4 - 100% outside air, with outside air indirect evaporative post-cooling.

System 3 - 100% outside air, with exhaust air energy recovery and indirect evaporative post-cooling.

INDIRECT EVAPORATIVE COOLINGSystems 3 and 4 are similar to systems 1 and 2, but 3 and 4use indirect evaporative post cooling.

This feature adds slightly to the purchase cost of the equip-ment, but saves on downstream cooling capacity in the restof the HVAC system.

For example, note that the supply air temperature for sys-tem 3 is 8 degrees lower than what system 1 can provide(81° compared to 89°). On 10,000 cfm, that allows the sys-tem 3 configuration to save 7 tons of cooling capacity. Aswith the previous systems, system 3 outperforms system 4,because the exhaust air can cool the supply air more deeplythan can the outside air.

The major advantage to indirect evaporative cooling is itsvery low operational cost, and simple maintenance comparedwith conventional cooling systems. The only cost to coolthe air evaporatively is the cost of the water, and the mod-est cost to overcome the additional air flow resistance ofthe evaporative pad (less than 0.15"WC). That is usually lessthan 1/10th the cost of running an equivalent vapor com-pression cooling system.

Of course, these benefits do not come without some cost.For example, while simpler than a vapor compression cool-ing system, the evaporative cooling system will require someadditional maintenance beyond the maintenance of the des-iccant wheel and the heat exchanger. Also, saving 7 tons on10,000 cfm may not justify the increased purchase cost andmaintenance cost if there are 7 extra tons of cooling capac-ity downstream of the desiccant system.

These facts suggest that indirect evaporative post-coolingis likely to yield the best cost-benefit ratio when:

• The system is large enough so the net coolingsavings and peak electrical demand reduction islarge in absolute terms.

• The building is large enough to have a maintenancestaff which will already be familiar with servicerequirements of simple evaporative coolers orcooling towers.

• The exhaust can be returned to the same place asthe supply, so the extra cooling effect of the dryexhaust air can maximize the cooling savings.



Latent 68 5.7 64

Sensible 22 1.8 --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Post-cooling by indirect evaporation

DW HW

A B C

D

EC

E

Temperature ( °F ) 83 144 81 75 65Moisture (gr/ lb) 123 55 55 65 82Air Flow (scfm) 10,000 10,000 10,000 BLDG 10,000

A B C D E

32 150

150

A

BC

81°, 55gr

D

Supply Air

E



Latent 68 5.7 64

Sensible (76) (6.3) --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Post-cooling by indirect evaporation

Temperature ( °F ) 83 144 90 75 77Moisture (gr/ lb) 123 55 55 65 133Air Flow (scfm) 10,000 10,000 10,000 BLDG 10,000

A B C D E

A B C

D

E

DW HW

EC

32 150

150

A

BC

90°, 55gr

D

Supply AirE



System 5 - 100% outside air, with exhaust air heat recovery, and assist from conventional cooling.

System 6 - 100% outside air, with assist from conventional cooling, but no heat recovery.

HYBRID SYSTEMS WITH AND WITHOUTEXHAUST RECOVERYSystems 5 and 6 are hybrid desiccant systems. In otherwords, they use conventional or gas cooling coils after theheat exchanger so the system can deliver air to the buildingat the 55° temperature which is typical of AC systems.

That conventional assist allows these system to remove 64lbs of water vapor and 216,000 Btu/h from inside the build-ing, in addition to removing all the temperature and mois-ture loads from the incoming fresh air.

To do that, they use 30 and 36 tons respectively of conven-tional equipment, which is mounted after the heat ex-changer. System 5 uses less conventional cooling, becauseit makes use of recovered cooling by using building exhaustair on the cooling side of the heat exchanger. System 6 hasno energy recovery, so it must use an additional 6 tons ofconventional cooling to achieve the same 55° supply air tem-perature. Each of these alternatives has its own advantagescompared to the other, and both have significant differencesfrom systems 1 through 4.

Systems 5 & 6 vs. Systems 1-4• 5&6 remove 18 tons of sensible load from the

building, 1-4 add sensible heat load to the building.• 5&6 use more electrical power• 5&6 cost more to purchase

System 5 advantages over 6• Uses less electrical power for the same cooling work• Reduces winter heating costs• Reduces annual operating costs

With these advantages, system 5 is especially useful for build-ings which have return air duct work, and for mid-conti-nent and northern climates where the cost of heating upoutside air in the winter is reduced by exhaust recovery.

System 6 advantages over 5• Lower installed cost by avoiding return air duct work• Allows multiple, independent exhaust points

System 6 is advantageous where first cost is more of a con-cern than operating cost, and where there are a reducedbenefit to winter heat recovery; such as in hot and humidclimates. Eliminating a central, combined exhaust makes thissystem useful in applications where air must be exhaustedfrom a building at many different points.


MBtu/h Tons Lbs/hrSensible 216 18.0 --------

Latent 68 5.7 64

Sensible 302 25.2 --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

DW HW

A B C

E

D

CC

Temperature ( °F ) 83 144 89 55 75Moisture (gr/ lb) 123 55 55 55 65Air Flow (scfm) 10,000 10,000 10,000 10,000 BLDG

A B C D E

Post-cooling by heat exchanger alongwith gas or conventional cooling coil

32 150

150

A

BD

55°, 55gr

F

Supply Air

C



Latent 68 5.7 64

Sensible 302 25.2 --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Temperature ( °F ) 83 144 95 55 75Moisture (gr/ lb) 123 55 55 55 65Air Flow (scfm) 10,000 10,000 10,000 10,000 BLDG

A B C D E


DW HW

A B C

E

D

CC 32 150

150

A

BD

55°, 55gr

F

Supply Air

C



HYBRID AND ALL-DESICCANT SYSTEMSWITH EVAPORATIVE COOLINGSystem 7 includes all the components and same flow sche-matic as system 6, but also includes an evaporative pad toboost the cooling effect of the heat exchanger.

This allows system 7 to cool the air leaving the desiccantwheel to 90°, which in turn reduces the amount of conven-tional cooling capacity needed to lower the supply air tem-perature to 55°. System 7 needs 32 tons of conventionalcooling, compared to 36 tons in system 6. That 4 tons ofcapacity is not really a significant saving at installation time,but it saves a considerable amount of money over a year'soperation. As the temperature and moisture outside re-duces, the evaporative cooling effect increases. Then theconventional equipment can be shut off entirely for thou-sands of hours of the year, when the temperature and hu-midity outside is reduced. For example, during spring, andfall, and during evenings and mornings in the summer.

System 7 - 100% outside air, with indirect evaporative post-cooling and conventional assist.

System 8 - 100% outside air, with both indirect and direct evaporative cooling, assisted by dehumidification inside the building

System 8 is the classic, all-desiccant makeup air configura-tion, which provides sensible cooling with no assistance fromvapor compression coils. The system cools the incoming airfrom 82°F down to 68°F, and reduces the moisture from123 gr/lb to 81 gr/lb.

However, in contrast to all the other systems described here,system 8 does not dehumidify the building. In fact, it adds102 lbs of water per hour to the internal moisture load. Thatmoisture must be removed by other systems. If other sys-tems do not remove that moisture, system 8 cannot coolthe air as shown here. Rising moisture in the return air wouldreduce effectiveness of the indirect post-cooler, and thesupply air temperature and moisture would rise.

System 8 is best applied where other desiccant or conven-tional dehumidification systems can remove the moistureload remaining in the supply air. Large buildings with mul-tiple systems can use this system to add fresh air withoutoverloading existing electrical capacity. System 8 provides10,000 cfm of cooled, fresh air, but only needs power fortwo fans and two fractional hp drive motors.



Latent 68 5.7 64

Sensible 302 25.2 --------Latent 463 38.6 437

System Can Remove

Inte

rnal

Tota

l

Post-cooling by indirect evaporationalong with gas or conventional coolingcoil

Temperature ( °F ) 83 144 90 55 75 77Moisture (gr/ lb) 123 55 55 55 65 133Air Flow (scfm) 10,000 10,000 10,000 10,000 BLDG 5,300

A B C D E F

A B C

E

F

DW HW

EC

CC

D

32 150

150

A

BC

55°, 55gr

E

Supply AirF

D

All-Desiccant With Exhaust Recovery

Post-cooling uses both indirect anddirect evaporative cooling pads

32 150

150

A

BC

68°, 81gr

E

Supply Air

D

A B C

E

F

HW

EC

D

ECDW

GH

Temperature ( °F ) 83 144 82 68 75 67 129 250Moisture (gr/ lb) 123 55 58 81 65 78 78 78Air Flow (scfm) 10,000 10,000 10,000 10,000 BLDG 10,000 10,000 3,000

A B C D E F G H


Latent -109 -9.0 -102

Sensible 162 13.5 --------Latent 286 23.8 270

System Can Remove

Inte

rnal

Tota

l



COMPARING OUTSIDE AIR ALTERNATIVESFigure 7 compares all of the makeup air systems accordingto four characteristics:

• Loads they remove from the building (or add to it)

• Loads they remove or add to the incoming fresh air

• Thermal energy they need to operate at the designcondition

• Supplemental gas or conventional cooling neededto bring the supply air to a building-neutraltemperature of 75° (or to 55° in systems 5-7).

The figure divides the systems into three groups. Systems 1through 4 are considered all-desiccant systems, becausethey do not contain gas or conventional cooling compo-nents. Systems 5 through 7 are hybrid systems, because theycombine desiccants with conventional cooling. System 8 isan all-desiccant system, but it uses direct evaporative cool-ing, so it does not remove moisture like the other systems.

COMMON CHARACTERISTICS OF 1-7Each approach has advantages and limitations, but the first7 systems share some common characteristics:

Dry & Fungus-Free Duct WorkEvery alternative shown here delivers dry air to the build-ing. The warning in ASHRAE Standard 62 against saturatedair in duct systems can be satisfied by any of these alterna-tives. The low humidity also allows all internal cooling coilsto run dry, reducing the hazard of microbial growth in drainpans and insulation.

Internal Latent Loads Removed By Makeup AirAll of these alternatives remove so much moisture from themakeup air, that any internal cooling coils can be designedto operate at a higher evaporator temperature, which canreduce their power consumption.

Rock-Solid Humidity ControlThe moisture removal capacity of all these systems allowsvery stable humidity levels inside the building throughoutthe whole range of weather conditions. This differs fromconventional cooling systems, where humidity levels shiftconstantly, because dehumidification is an arbitrary resultof whatever cooling happens to be needed.

Improved Temperature ControlWithout the moisture load to remove, the internal coolingsystem can control temperature much more evenly, becausethere is no need to over-cool and re-heat the air as incom-ing ventilation air changes in temperature and moisturecontent.

Reduced Peak Power DemandAll these systems remove moisture through thermal energyrather than by using electric power. Part of the sensible loadcreated by dehumidification is removed by a heat exchanger,so the net peak power demand is reduced.

CHARACTERISTICS OF SYSTEMS 1 - 4These are all-desiccant system, in that they contain nosupplemental conventional cooling. In addition, they sharethese characteristics:

Lowest first costThe all-desiccant systems are less costly than the hybrid sys-tems, because they contain fewer components.

Remove moisture, but add heatThe post-cooling heat exchanger, even when assisted by theevaporative cooler, does not have enough capacity to re-move all the sensible heat produced by dehumidification,so these systems remove the latent load from both the in-coming air and from the building itself, but they deliver airwhich must be cooled by other systems inside the building.

Exhaust recovery improves winter economicsIn the summer months, exhaust recovery reduces post-cool-ing expense, but not by much. For example, system 1 usesexhaust recovery and system 2 does not. System 1 has onlysaved 5.3 tons of post-cooling. But during winter months,the value of waste heat recovery can be very great, perhapsreducing makeup air heating costs by 60% to 80%.

Cost advantage at high internal sensible heat loadsIf extra sensible capacity already exists inside the buildingfor other reasons, the small additional sensible load fromthe all-desiccant makeup air system may be inconsequen-tial. This would keep costs down by avoiding the need for asupplemental cooling system on the makeup air.

CHARACTERISTICS OF SYSTEMS 5-7These systems all include extra cooling capacity to deliverair at 55°, so they can all remove not only moisture, but alsoremove some of the heat from inside the building. In addi-tion, they share these characteristics:

They do more work, so they cost moreSystems 5-7 all include supplemental cooling coils, so theycost more than systems 1-4. But the hybrid systems also domuch more work than the all-desiccant systems, deliveringair at 55° to the building instead of 78 or 90°.

Cost advantage for buildings with low sensible loadsIf the makeup air represents not only 80% of the moistureload on the building, but also a high percentage of the to-tal sensible load, then all the internal loads may be removedby cooling the required makeup air. That way, there may bevery little need for internal cooling systems, which wouldlower the overall installation costs for the building.

SYSTEM 8 - ALL-DESICCANT ALTERNATIVESystem 8 cools, but does not dehumidify the building. Whenother systems in the building can remove moisture, system8 provides 10,000 cfm of nearly-conditioned air for almostno power consumption (other than for fans). When existingbuildings with limited electrical service need more fresh airto meet ASHRAE Std. 62, system 8 may be ideal.



Bldg.

Bldg.

Bldg.

Bldg.

Bldg.

Bldg.

Bldg.

Bldg.

0.07.2

18.1

0.07.5

5.40.0

7.6

13.5

0.07.6

-12.6

-13.5

DW HW1 0

5.7

-12.6

38.6

-5.4

DW HW 025.7

-18.1

38.6

-10.9

DW HW

EC

035.7

-5.4

38.6

1.8

DW HW

EC

045.7

38.6

-6.7

Non-Desiccant Tons

Additional Equipment Used

In System After System

Gas Consumption

Therms/hr @ Design

Thermal Energy

From The Building

Sensible Latent

Loads Removed (Tons)

From The Fresh Air

Sensible Latent


30.6

0.07.7

DW HW CC05

5.718.0

38.6

25.2

36.0

0.09.8

DW HW CC06

5.718.0

38.6

25.2

31.5

0.07.6

08

DW HW

EC

CC

075.7

18.0

38.6

25.2

HW

EC

ECDW

23.813.5

-9.0

6.3 7.6 9.00.0

Figure 1 - Capacity comparison of 100% OSA systems



In the previous section, we discussed 100% outside airsystems. In those cases, desiccant systems haveimpressive advantages because there is so much moisture

to be removed from incoming air during the summer. Thatsame summer moisture load can also adversely affectconventional systems with as little as 15% outside air. Whenthe proportion of outside air rises to 30%, conventionalcooling-only systems have real difficulty controll ingtemperature, because so much of their capacity is beingused to remove moisture,

So when a building or a system needs more than 15% outsideair, engineers often use a single desiccant system to replaceone of the several rooftop units on the building. Thedesiccant system easily handles 30% outside air, so the otherrooftops can either use much less outside air, or perhapsnone at all, allowing them to cool air more efficiently.

ISSUES FOR 30% OUTSIDE AIR SYSTEMSAs in the all-outside air systems, the fundamental issue re-mains: how to remove the sensible heat produced when thedesiccant removes moisture from the air? A heat exchangerfollows the desiccant wheel, but on the other side of theheat exchanger, the engineer can chose to use either out-side air or return air from the building. And that air can beused "as-is", or it can be cooled with an evaporative pad.

Following the heat exchanger on the supply air side, theengineer can remove heat with a conventional cooling coil.Or the warm air can be sent directly to the building, whereother systems may have extra sensible capacity since theyno longer need to remove moisture.

In this section, we will examine pro's and cons of each ap-proach. These example use a system with 3,000 cfm of out-side air, and 7,000 cfm of return air, rather than one whichdraws all 10,000 cfm from the outside.

30% Outside Air SystemsConventional rooftop packaged equipment has diff iculty removingmoisture loads when the proportion of fresh air increases above 15%.These system layouts show how desiccant systems can be configured toair condition a building which needs a high proportion of outside air.

ALL-DESICCANT SYSTEMS FOR 30% OSASystem diagrams 9, 10 and 11 show all-desiccant systemsfor 30% outside air. But each diagram shows different alter-natives for removing the sensible heat created by dehumidi-fication.

System 9 uses exhaust air energy recovery. Also, the exhaustair is pre-cooled by evaporation before being used in theheat exchanger. By using both exhaust recovery and indi-rect evaporation, system 9 delivers air to the building at 73°F,even at peak design conditions.

System 10, by comparison, delivers air to the building at atemperature of 83°. The supply temperature is higher, be-cause system 10 uses warmer, more humid outside air forpost-cooling rather than the relatively cool and compara-tively dry exhaust air from the building.

System 11 delivers the supply air at an even higher tem-perature—88°F. This is because there is neither evaporativecooling nor exhaust energy recovery for the post-coolingheat exchanger.

Note that all three systems deliver the supply air at a verydry condition of 49 gr./lb. Consequently all of these systemsremove the entire moisture load from the ventilation air,and they can also remove 103 lbs of water vapor per hourfrom the building itself.

Note also that systems 9,10 and 11 do not contain any con-ventional cooling. So they provide cooling and dehumidifi-cation using very little electrical power.



System 9 - 30% outside air, with exhaust air and indirect evaporation used for post-cooling.

System 11 - 30% outside air, with outside air used for post-cooling without evaporative cooling.

System 10 - 30% outside air, with outside air and indirect evaporation used for post-cooling.

30% OSA With Exhaust Recovery


Latent 109 9.1 103

Sensible 43 3.6 --------Latent 232 19.3 219

System Can Remove

Inte

rnal

Tota

l

Post-cooling by indirect evaporationinto exhaust air

Temperature ( °F ) 83 75 77 106 73 75 65Moisture (gr/ lb) 123 65 83 49 49 65 82Air Flow (scfm) 3,000 7,000 10,000 10,000 10,000 BLDG. 3,000

A B C D E

150

DW HW

A

B

C

F

EC

G

D E

F G

32

150

A

D

C

73°, 49gr

G

Supply Air

EB

30% OSA - No Exhaust Recovery


Latent 109 9.15 103

Sensible (65) (5.4) --------Latent 232 19.3 219

System Can Remove

Inte

rnal

Tota

l

Post-cooling by indirect evaporationinto outside air

Temperature ( °F ) 83 75 77 106 83 82 77Moisture (gr/ lb) 123 65 83 49 49 123 133Air Flow (scfm) 3,000 7,000 10,000 10,000 10,000 3,000 3,000

A B C D E

150

F G

32

150

A

D

C

83°, 49gr

G

Supply Air

EB

DW HW

A

B

C

F

EC

G

D E



Latent 109 9.1 103

Sensible (118) (9.9) --------Latent 232 19.3 219

System Can Remove

Inte

rnal

Tota

l

Post-cooling by Heat Exchanger Only

Temperature ( °F ) 83 75 77 106 88 82Moisture (gr/ lb) 123 65 83 49 49 123Air Flow (scfm) 3,000 7,000 10,000 10,000 10,000 3,000

A B C D E

150

F

32

150

A

D

C

88°, 49grSupply Air

EB

DW HW

A

B

C

F

D E



System 12 - 30% outside air, with conventional assist for post-cooling.

System 13 - 30% outside air, with desiccant on the outside air only, and using conventional assist for post-cooling

HYBRID SYSTEMS FOR 30% OSADiagrams 12 and 13 show hybrid systems which use con-ventional cooling coils and either compressor-based or gascooling to deliver the supply air to the building at a tradi-tional temperature of 55°F.

System 12 uses a desiccant wheel to dry the mixed returnair and outside air. In contrast, system 13 uses a much smallerdesiccant wheel to dry only the outside air. Consequently,system 13 is less expensive to install, and less expensive tooperate. On the other hand, system 12 has three times themoisture removal capacity because it dries the entire sup-ply air stream. Both systems can remove the same amountof sensible heat from the building—18 tons.

Unlike systems 9, 10 and 11, these systems can provide allthe needed temperature and humidity control for a smallcommercial building. So they are especially useful for smallerbuildings or areas within buildings where a single system isthe most economical and practical alternative.

COMPARING 30% OSA DESICCANT SYSTEMSLooking at the first column of figure 8, one can quickly seethe difference between the desiccant-only and the hybridsystems. The all-desiccant systems remove very little, if any,sensible heat from the building. Only system 9 removes avery small 1.8 tons of sensible heat load, and systems 10and 11 actually add sensible heat load to the building.

But looking at the latent load capacity in the first column,one can also see that every system removes all the mois-ture from the 30% outside air, and can still remove addi-tional moisture loads from inside the building.

All of these systems have enough moisture removal capac-ity to allow the building owner to reset the humidity to alower-than-traditional level, and to ensure that the level willbe maintained even in extremely humid conditions. Thatcapacity would be useful, for instance, in laboratory researchbuildings, where constant humidity is important for consis-tent research results, and where large amounts of fresh airmust be brought into the building to replace air exhaustedthrough chemical fume hoods.



Latent 103 9.1 103

Sensible 237 20.0 --------Latent 232 19.3 219

System Can Remove

Inte

rnal

Tota

l

Temperature ( °F ) 83 75 77 106 88 55 82Moisture (gr/ lb) 123 65 83 49 49 49 123Air Flow (scfm) 3,000 7,000 10,000 10,000 10,000 10,000 3,000

A B C D E

150

F

32

150

A

D

C


FB

DW HW

A

B

C

G

D E

CC

F

G

E


30% OSA - Dry The OSA Only


Latent 34 2.8 32

Sensible 242 20.2 --------Latent 149 12.4 141

System Can Remove

Inte

rnal

Tota

l

Temperature ( °F ) 83 154 97 82 75 55Moisture (gr/ lb) 123 47 47 60 65 60Air Flow (scfm) 3,000 3,000 3,000 10,000 7,000 10,000

A B C D E

150

F

32

150

A

B

E


F DC


AB C D

ECC

FHWDW



Another difference between desiccant-only and hybrid sys-tems is their respective installed costs. The desiccant-onlysystems cost less, because they have fewer components.So if a building has sufficient additional conventional sys-tems to remove the building's sensible heat loads, the low-est-cost solution for additional outside air would be to in-stall a desiccant-only system. However, if the building hasno supplemental sensible heat removal equipment, a hy-brid system is probably the best choice for adding moreventilation air.

In summary, the owner and the design engineer would bewell-advised to investigate the use of desiccant systemswhenever one or more of the following circumstances ap-plies to the project:

• Need for more than 15% outside air. (e.g.: to meetcodes based on ASHRAE Standard 62-89)

• Higher than normal inside moisture loads. (e.g.:food processing buildings)

• Need for lower-than-conventional humidity controllevel. (e.g.: research, biomedical buildings andelectronic assembly)

• Humidity cannot be allowed to swing more than2% rh. (e.g.: museums and research labs)

• Electrical power service to a building is limited. (e.g.:retrofits to add ventilation air)

• Duct work must be kept dry to prevent fungalgrowth. (e.g.: hospitals and medical office buildings)

• Warm temperatures with low humidity is preferredto cold, saturated environments. (e.g.: nursinghomes, movie theatres and conference rooms)

Figure 2 - Capacity comparison of 30% OSA systems

9

bk

bl

Non-Desiccant Tons

Additional Equipment Used

In System After System

Gas Consumption

Therms/hr @ Design

Thermal Energy

From The Building

Sensible Latent


From The Supply Air

Sensible Latent


DW HW

0.0

4.0

16.5

0

9.11.8

19.3

3.6

0.0

25.1

0

9.1

-7.2 -5.4

0.0

29.3

0

9.1

-11.7

19.3

-9.9

4.0

4.0

19.3

bmDW HW

G

CC 0.0

29.3

0

9.118.0 19.320.0

4.0

bnCC

HWDW

0.0

24.0

02.8

12.42.2

18.0 20.2

DW HW

EC

DW HW

EC

Building

Building

Building

Building

Building


Chapter 5 - Components Of Desiccant Systems 27

Desiccant System HardwareMechanically, desiccant systems are rather simple. By examining eachcomponent in turn, the reader can understand what must be done indesign and maintenance to ensure minimum cost and maximum reliability.

From the outside, commercial desiccant systems looklike conventional rooftop packaged equipment. That'sbecause the system enclosures are essentially identical

to conventional equipment. Additionally, most commercialdesiccant systems are hybrids combining desiccantcomponents with conventional or gas cooling equipment.So there are may components of desiccant systems whichwill be familiar to owners and designers of commercialbuildings.

Therefore, we will discuss the components which may beless familiar because they are more specific to desiccantsystems, including:

• Desiccant wheel assemblies

• Reactivation heaters

• Heat exchangers for post-cooling

• Indirect evaporative post-coolers

• Filtration requirements

• Control components

DESICCANT WHEEL ASSEMBLIESDesiccant wheel assemblies consist of "core material", whichcontains the desiccant, a wheel support structure, a drivesystem and a set of air seals on both faces of the wheel toprevent air from leaking from the moist reactivation to thedry process air stream.

Desiccant core materialThe desiccant material is coated, impregnated or formed inplace on a support matrix that looks like the corrugatedboard used to make cardboard boxes. The material is not,of course, cardboard, since it must withstand soaking andheating to temperatures between 130° and 250°F six timesevery hour. Typically, the basic material which forms thesupport matrix is a mix of different fibres including glass,

Desiccant wheels are made as single wheels, or as wheelsections supported by a spoke-and rim assembly

The geometry of core material in desiccant wheels variesaccording to manufacturer. Both sinusoidal and hexagonal airpassages are used in commercial equipment.

ReactivationHeater

Dry Process Air tothe HVAC SystemHumid Process Air

MoistReactivation Air

CHAPTER 5COMPONENTS OF DESICCANT SYSTEMS



ceramic binders and high-temperature plastics. And in onecase, the corrugated material is an aluminum alloy. Each des-iccant wheel manufacturer has a somewhat different pro-cess for making core material, and each technique has itsown advantages and disadvantages.

But in all cases, the core material is very durable. It resiststemperature and moisture extremes and is quite fault-tol-erant. Manufacturers warranties vary widely, but apart fromwhat the manufacturers will actually guarantee, field expe-rience shows that desiccant core material lasts for well over5 years, and cases of over 20 years of continuous operationare not uncommon. For all practical purposes the ownercan think of the desiccant wheel as being equal to, or bet-ter than a compressor in terms of longevity and reliability.

Many manufacturers offer a choice of desiccants for load-ing into the core material. And in recent years, much re-search has been invested in developing advanced desiccantmaterials which have sorption characteristics which are bet-ter for different applications.

In commercial applications, the desiccant materials are usu-ally adsorbents like silica gel, activated alumina and molecu-lar sieves rather than absorbents, such as lithium chloride.An AD-sorbent is like a ceramic sponge. It has a limited ca-pacity for water vapor, usually 18 to 22% of its own weight.Adsorbents are very durable and can be designed to attractmoisture at specific relative humidities, so they can be"tuned" for particular applications. AB-sorbents, by contrast,hold far more water vapor, but are more sensitive to over-saturation, so they are usually used for industrial applica-tions where their great capacity is an advantage and theirreduced fault-tolerance is less of a disadvantage.

Lithium chloride is an example of an AB-sorbent desiccant,and it was used in early commercial desiccant systems withmixed results. The material performed well when properlyreactivated, but when reactivation heat was lost, the saltmigrated out of the wheel as a liquid, reducing capacity andsoftening the wheel itself. Large commercial systems nolonger use lithium chloride, although one manufacturer ofsmall systems uses low-cost, disposable lithium chloride/paper-based wheels rather than the semi-ceramic materialsused by most competitors. In that smaller equipment, themanufacturer suggests periodic replacement of the wheel.

The more common AD-sorbent materials used in desiccantwheels are like fine powders. Such small particles present agreat deal of surface area to air flowing through the corematerial. The desiccant is coated onto the core material orformed in-place through chemical reactions so that it isbound tightly to its support structure. The desiccant lastsabout as long as the core material—between 5 and 20 years.Each manufacturer can provide performance data for equip-ment over the life of a desiccant wheel. Generally there issome loss of desiccant capacity over time—on the order of15% over 5 years. Most manufacturers rate the performanceof the desiccant equipment at the midpoint of the life of

the desiccant wheel. Usually, the rating is conservative. Theonly things that commonly shorten the life of the core ma-terial and the desiccant are sticky particulates such as to-bacco smoke.

Like a cooling coil, a desiccant wheel can become cloggedby fibrous dust, bird feathers, insects, etc. Consequently,the inlets of the process and reactivation air streams arealways equipped with filters. Tobacco smoke, which is muchtoo small to be trapped by coarse filters, will clog some ofthe pores in the desiccant material, reducing its moistureadsorption capacity. However, tests conducted in govern-ment laboratories have shown that even at smoke concen-trations significantly greater than dedicated smoke lounges,the reduction in desiccant capacity is less than 15% overfive years.

Desiccant wheel assemblies vary mechanically betweenmanufacturers and between different wheel diameters. Insmaller wheels, typically below 5 ft. in diameter, the desic-cant wheel is a monolithic structure—the core material is

Smaller wheels are made in one piece

Larger wheels are made in sections for strength



formed in the shape of a wheel, and a round casing at itsperimeter protects its edges. Above 5 ft. in diameter, thecore material may need more support internally, so the corematerial is cut into sections and inserted into a spoke-and-rim assembly, supported in a frame by a hub and bearingassembly at its center.

The advantage of a monolithic wheel is it's convenience forreplacement; as a single unit, the wheel can be easily rolledinto and out of the unit . In contrast, the advantage of thespoke-and-rim design is its great strength, which is usefulfor large wheels where the weight of the water in the desic-cant places considerable load on the core material.

In commercial systems, the wheel is rotated at speeds whichvary by manufacturer, but are generally between 6 rph and20 rph. The drive system consists of a motor which turns adrive belt or chain at the perimeter of the wheel. By drivingthe wheel from the rim rather than at the center hub, me-chanical advantage is increased substantially, so very little

power is needed to drive even the largest wheels. For ex-ample, a 12 ft. diameter wheel can easily be turned by a 1/2hp motor.

Each manufacturer has a different arrangement fortensioning the drive belt and providing sufficient frictionbetween belt and wheel to rotate the wheel with a full loadof water. Over 10 years, these drive belts and chains can breakor slip. So most manufacturers provide rotation fault de-tectors to signal the operator if the desiccant wheel is notrotating properly—a fact that may not be evident by casualobservation since normal wheel rotation is so slow (one revo-lution every 6 minutes).

The air seals at the edge of the desiccant wheel and whichseparate the process from the reactivation air can affectdesiccant equipment performance. If air leaks around thewheel, or if humid reactivation air leaks into the dry processair, then the equipment capacity is reduced. There are twostrategies used by equipment manufacturers for seals.

The first uses flexible strips of rubber or other elastomer to"wipe" across the surface of the wheel lightly. Air pressurefrom fans keeps these strips tight against the wheel face.The advantage of such seals is low friction and low cost.These rarely need replacement, and they do not stress thewheel drive system with a high friction load. The disadvan-tage is that they allow some leakage, and if fans and sealsupports are not located carefully, such seals can leak sig-nificant amounts of air.

The alternate seal strategy uses compressible bulb sealswhich press against the face of the wheel. Their advantageis air tightness—most can resist leaks at pressure differen-tials across the seals of 6" WC. But they cost more thanwiper seals, they create a larger friction load on the drivesystem, and wear out more quickly than wiper seals. Essen-tially, compressible seals gain a significant performance ad-vantage in return for a somewhat greater maintenance cost.Wiper seals have the

advantage of lowwear and long life.

Compressible seals arevirtually air-tight, butwill need replacementabout every five years.

Desiccant wheels are driven from their periphery, so they canbe rotated by small drive motors.

Flexible rubber sheetingor extrusion which rideslightly on the wheel face



The rotor casing, called the "cassette" by some manufactur-ers, is an important part of the desiccant wheel assemblyfor two reasons. First, it provides the support structure forthe seals. The volume of air leakage can depend on casingstiffness. Secondly, the cassette provides structural sup-port for the wheel itself. If the casing is weak or crooked,the wheel may not rotate properly, or may leak air duringrotation. Consequently, desiccant equipment manufactur-ers tend to invest more design and materials in the framethan what one might otherwise assume is necessary to sup-port a lightweight, slowly-rotating wheel. The net result ofthose investments is assured equipment capacity, longer lifeand very low maintenance expense compared to the vaporcompression components of conventional systems.

Reactivation heatersAs seen in previous chapters, desiccant system recover heatfrom the process air stream to use in reactivation. But thatheat is usually less than 20% of the total energy needed forreactivation. Other heat sources are always needed. Thesecan include electric resistance heaters, solar hot water coils,

heat reclaim coils and hot water or steam coils fed fromboilers. Industrial desiccant system use all of these sources.

But in general, natural gas heaters allow an owner to savebetween 50 and 75% of the cost of reactivating the desic-cant compared to other means. Consequently, as a matterof practical economics, gas heaters are the heaters of choicefor nearly all commercial desiccant system installations.There are two basic types of natural gas heaters used in com-mercial desiccant systems: direct-fired and indirect-fired gasburners.

Direct-fired heaters burn natural gas directly into the reac-tivation air stream. This allows 90 to 92% heating efficiency,so it is the equipment of choice for many desiccant sys-tems, particularly for larger units. Not all systems areequipped with direct-fired burners, however, because thereare different advantages to indirect-fired burners.

Indirect-fired burners burn the natural gas outside the re-activation air stream, and the combustion heat is transferredto the reactivation air through a heat exchanger. Since a heatexchanger comes between the flame and the reactivationair stream heating efficiency is reduced to 50-70% comparedto the 92% efficiency of direct fired burners. So operationalcosts of indirect-fired burners are slightly higher. However,using a heat exchanger reduces the maximum temperatureof the reactivation air. A direct-fired burner produces tem-peratures of 1300°F. If the air is not mixed well with the com-bustion gases, the desiccant wheel and/or the air seals canbe scorched. With an indirect-fired burner, those potentialproblems are eliminated.

Also, some desiccant units use small gas-fired boilers whichcirculate water through heating coils for both reactivationand for winter heating. That strategy has advantages in firstcost. In winter, there is seldom a need for desiccant reacti-vation. One boiler can serve both heating and reactivationneeds through coils rather than with separate gas heaters.

Direct-fired natural gas reactivation heaters are over 90%efficient, so they are often used on larger units

Gas-fired boilers are less efficient than direct-fired burners, butcan provide heating for both reactivation and supply air,minimizing first costs.

A temperature controller can vary reactivation energy as themoisture load reduces, to avoid waste.

ReactivationT

Process



Reactivation heater modulationThe heating capacity of the reactivation burners is usuallycontrolled, so that no more energy is used for reactivationthat is necessary to remove the moisture from the desic-cant. That energy requirement varies in direct proportionto the changes in moisture load. As the moisture load onthe desiccant wheel increases, more heat must be added tothe reactivation air to remove the moisture.

One method of reactivation energy modulation maintains aconstant "reactivation-leaving" air temperature. A tempera-ture controller is mounted in the reactivation air leaving thedesiccant wheel. If the temperature of that air falls, moremoisture is being pulled off the wheel, so more heat shouldbe added to the reactivation air before it enters the desic-cant wheel. Conversely, if the reactivation-leaving air tem-perature rises, it means there is less moisture being pulledfrom the wheel, so less heat is needed in reactivation.

POST-COOLING HEAT EXCHANGERSMost commercial desiccant systems include a heat exchangermounted after the desiccant wheel to transfer sensible heatfrom the process air to the reactivation air stream. Thereare three common types of heat exchangers used in thislocation: plate-type, heat wheels and heat pipes. Each hasadvantages and disadvantages.

Plate-type heat exchangersPlate-type heat exchangers, often called "air-to-air" exchang-ers, are the lowest in cost, and the lowest in heat exchangeefficiency. (On the order of 40 to 65%) These are made ofthin sheets of metal or plastic, arranged so that one air

Sensible heat recovery wheels are sometimes used as post-coolers. They are controllable and highly efficient.

Heat pipes also serve as post-coolers when low maintenance ispreferred to high heat exchange efficiency

stream flows across one side of the plates, and the other airstream flows across the other side of the plates. The hotterair stream heats the cooler one through the plates. In addi-tion to having the virtue of low cost, for all practical pur-poses, plate-type exchangers separate the two air streams,so leakage is unlikely. On the other hand, their efficiency is afunction of their surface area, so even minimally-efficientplate exchangers tend to be rather large, adding size to theoverall desiccant system.

Heat wheelsHeat wheels are the highest in cost, but also have the high-est heat exchange efficiency. (Between 80 and 95%) Heatwheels look very much like desiccant wheels. They are madeof fluted media. And the media is held in a cassette assem-bly similar to desiccant wheels. But where desiccant wheelsare optimized for moving moisture, heat wheels are opti-mized for moving heat. Heat exchange efficiency is a func-tion of wheel rotation speed. Specifically, where desiccantwheels rotate at 6 to 10 revolutions per HOUR, heat wheelsrotate at 10 to 20 revolutions per MINUTE.

Heat wheels are also much more compact than plate ex-changers, occupying a length of perhaps 11" in the direc-tion of air flow compared to several feet for plate exchang-ers. Also, the high heat exchange efficiency translates di-rectly into reduced cost for cooling downstream. More heatis moved to reactivation, so less heat must be removed fromprocess, reducing the size, cost and complexity of any re-maining conventional cooling equipment. Additionally, thedesigner can modulate the cooling effect of the heat wheelby changing its speed.

HotProcess Air

Cool, IncomingReactivation Air



However, in spite of these significant advantages, not all des-iccant systems use heat wheels. Such equipment costs morethan other types of heat exchangers, it requires some main-tenance attention, and it does not entirely separate the hu-mid reactivation air from the dry process air. None of theselimitations have prevented heat wheels from being a popu-lar form of post-cooling heat exchanger for desiccants.

Heat pipesHeat pipes are smaller and less expensive than heat wheels,and somewhat less efficient in terms of heat exchange (55to 75% efficient). Heat pipe exchangers look like conven-tional cooling coils, with one end of the coil in the hot airstream and the other end in the cool air stream. A dividerplate in the coil separates the air streams.

Heat pipes have the useful advantages of compactness anddesign flexibility. They occupy between 4 and 8 inches inthe direction of air flow, depending on how many rows ofpipes are included in the assembly. Also, the heat pipes neednot cover the entire face of the process air outlet. This isbecause the temperature of the air leaving the process sec-tor of the wheel varies according to how far the wheel hasrotated away from the reactivation sector. That is to say, asthe wheel leaves the reactivation sector, it is quite hot, andit looses heat as it rotates through the process sector. Soheat pipes can be arranged to capture the hottest part ofthe process air, and allow the cooler air to bypass aroundthe heat pipe array. This saves cost without making majorsacrifices in heat exchange efficiency. Also, the heat piperequires no maintenance other than occasional cleaning.

These advantages are balanced by the increased conventionalcooling capacity needed to remove remaining heat from the

process air stream. And there is no practical way to controlthe heat exchange capacity of a heat pipe other than a rela-tively costly and complex air bypass system.

Heat pipes and heat wheels are the two most popular post-cooling heat exchangers for desiccant systems. Customerpreference determines which type is provided by the manu-facturer on larger systems. In smaller, more standardizedsystems, either one or the other is built into the system.

However, not all desiccant systems include heat exchang-ers. In some applications such as supermarkets and ice rinks,the extra supply air heat is an advantage. And in other caseswhen conventional cooling is provided as part of the sys-tem, it may be more cost-effective to add a bit of capacityto the conventional cooling equipment rather than pay forthe cost of an additional heat exchanger.

EVAPORATIVE POST-COOLERSIn addition to heat exchangers, desiccant systems often in-clude evaporative coolers to remove heat produced by de-humidification. In these devices, air is forced through a wet-ted contact media. As the air picks up moisture from themedia, its temperature drops because its sensible heat isused to evaporate the moisture. The air is then more hu-mid, but cooler.

Evaporative cooling is adiabatic, that is to say, the air is cooledwithout the addition of external energy. So the only electri-cal power required for cooling is used by the water pump(less than 1/8 hp) and by the fans which push the air throughthe evaporative pad. Consequently, the energy and first-costadvantages of evaporative cooling are significant.

In spite of the obvious advantages of evaporative coolers,they are not always used in desiccant systems. They require

Adding an evaporative pad to the cool side of the heatexchanger allows cooling without adding water vapor to thesupply air

Filtration requirementsfor desiccant systems are

simple, but easy accessand regular replacementare essential for proper

performance.



attention to water quality, and need controls or maintenanceto ensure that water pipes do not freeze in winter climates.Also, as with heat exchangers, it may be more cost-effec-tive to add conventional cooling capacity rather than invest-ing in the cost of installing another system component.

In large, built-up desiccant systems such as those used onhigh-rise buildings or fresh air for laboratories and hospi-tals, cooling tower water is often used to post-cool the pro-cess air. Where a cooling tower is already an assumption inthe overall building design, such a strategy can save sub-stantial conventional cooling capacity at nearly negligibleinstalled cost and no incremental maintenance cost. But if acooling tower is not part of the system assumptions, as inthe case of low-rise commercial construction, it is not com-mon to add one simply for desiccant post-cooling. Coolingtower maintenance issues usually make such a design lessdesirable from a small-building owner's perspective.

FILTRATIONFilters are an important component of desiccant systems,as they are in conventional systems. Like conventional sys-tems, the filtration need not be extreme—it must simply bethere.

That is to say, the process and reactivation air must be fil-tered to prevent the desiccant wheel and heat exchangersfrom clogging with gross particulates like feathers, insectsand grass clippings. But unless the building needs extra fil-tration, as in the case of hospitals and industrial processes,a conventional 30% disposable pleated filter is quite ad-equate to protect system components.

CONTROLSAs in conventional systems, controls for desiccant systemsare a mixture of components provided by the manufacturerand the control subcontractor. We will divide the discussioninto two sections—controls needed for internal system op-eration, and external controls needed to instruct the equip-ment to provide a particular supply air temperature andhumidity.

Internal controlsInside the system, the equipment manufacturer generallyprovides controls to ensure, as a minimum, that:

• The reactivation burner does not ignite if there isno reactivation air flow.

• The reactivation fan continues to operate for a shortperiod after the reactivation burner has turned offto allow the burner to cool down.

• The reactivation burner is shut down if the airtemperature entering or leaving the desiccantwheel is excessive, and a fault indicator trippedaccordingly.

• An alarm or fault indicator is tripped if the desiccantwheel is not rotating.

Also, manufacturers often provide controls inside the sys-tem to modulate reactivation energy in response to changesin moisture load, as previously described in the reactivationheater section.

Manufacturers generally believe It would be unwise and un-economical for the design engineer to perform these func-tions in an external control system or in a central buildingautomation system.

Dehumidification capacity controlsControlling dehumidification capacity is often a shared re-sponsibility between equipment manufacturer and the con-trol subcontractor. There are three common methods ofcontrolling dehumidification capacity:

• Variable air bypass (close-tolerances)

• Reactivat ion energy modulat ion (moderatetolerances)

• On-off reactivation control (loose tolerances)

Each of these methods is effective, depending on the de-gree of precision needed for the humidity control level inthe building.

Variable process air bypassThe most precise method of controlling humidity requires abypass air duct and variable-position dampers for the faceof the desiccant wheel and for the bypass duct. In this ar-rangement, when the humidity in the building rises abovethe setpoint, the bypass damper closes and the face damperopens, allowing more air to flow through the desiccantwheel. Then when the humidity control level is satisfied, thebypass damper opens to allow more air to flow past thedesiccant wheel so the building is not over-dried.

ReactivationAir

Process Air

Bypass

Supply Air

H

A process air bypass system provides the most precise control,at the expense of some mechanical complexity



This method is preferred for industrial process applications,where control within ±1 or 2% rh is essential. Bypass controlis used less frequently in commercial buildings, partly be-cause it costs more and partly because the dampers andlinkages require some degree of maintenance attention. Fi-nally, many commercial buildings do not need precision con-trol. When control within ±5 or 7%rh is sufficient to meetthe needs of the building and it's internal operations, othermethods of control are quite adequate.

Reactivation energy modulationIn this scheme, a humidistat varies the amount of heat pro-duced by the reactivation heaters. When the building risesabove setpoint, the controller adds heat to the reactivationair stream, which provides more drying power for the des-iccant. When the humidity in the building falls belowsetpoint, the reactivation heat is reduced, so the desiccantdoes not remove as much moisture.

This scheme has the advantage of low cost and mechanicalsimplicity. On the other hand, the control system must havelag-time built in so that the heater does not over-respondto a rise in humidity. For example, if the humidistat in thebuilding calls for more dehumidification, but does not im-mediately see a fall in humidity, it may continue to call formore dehumidification long after the heater input has beenincreased. Then the dehumidifier may "overshoot" the de-sired control level because it is still being fed with extra re-activation heat.

These problems can be avoided by locating the humidistatnear the supply air outlet of the system, so that changes donot take long to be sensed by the humidistat. Also, a step-function can be built into the control, so that the humidis-tat calls for a small increment of extra heat, then waits forone wheel rotation before calling for additional heat.

Winter operations add one additional consideration to reac-tivation modulation as a means of control. When supply airflows through the wheel without reactivation, moisture willbuild up in the desiccant wheel. Excess moisture supportsfungal growth within the core material. In springtime, asthe humidistat calls for dehumidification, the warm, humidwheel can give off odors reminiscent of dirty laundry untilthe heat kills microbial growth accumulated through thewinter. But this problem can be easily avoided by periodi-cally rotating and reactivating the desiccant wheel throughany season when humidity is too low for the humidistat tocall for dehumidification. If this wheel rotation and reacti-vation is performed once every eight hours for ten min-utes, the problem will not occur in the spring. Manufactur-ers may or may not include this feature in their internal con-trols. So the owner and design engineer should be aware ofthe issue when dehumidifier capacity control is accomplishedby a central energy management or by a building automa-tion system.

On-Off reactivation controlThis method is a variation on the modulating reactivationheat scheme. Instead of modulating the reactivation heataccording to the moisture load, the heaters are simplyturned on or off according to control signals from a humi-distat mounted in the building.

The on-off method is even lower in cost than the modu-lated reactivation method, and gives good results in manysituations. For example, when the desiccant system ismounted on the incoming ventilation air, the large mass ofreturn air buffers the effect of changes in outside humidity.

The results can be nearly as precise as face-and-bypassdampers, for much less cost and complexity. However, thesame cautions as used in the reactivation modulationmethod apply: the wheel should be rotated and reactivatedseveral times a day even in periods of low humidity to avoidmicrobial growth during inactive seasons.

ReactivationAir

H

ProcessAir

Modulating reactivation energy provides simpler capacitycontrol, in return for some reduced precision.

ReactivationAir

H

ProcessAir

On-off reactivation energy control is the lowest cost alternativeand is the least precise, but often adequate for commercialbuildings where lag-time is not an issue.



Temperature controlsMany desiccant systems cool the air with conventional va-por compression cooling systems, or with gas cooling sys-tems. Such systems are well-understood, as are the meth-ods through which these are controlled. For purposes ofthis guide, we will discuss methods of controlling directevaporative and indirect evaporative coolers, which may beless familiar to building owners and design engineers.

As described earlier, air leaves a desiccant wheel warm anddry. In most cases, the air is too warm to send directly tothe building, so some sensible heat must be removed. Then,if sensible heat is to be removed from the building as well,the dry air must be cooled below the control condition inthe occupied space. There are three stages of cooling avail-able with evaporative cooling and heat exchangers.

Stage one - heat exchanger alonePart of this cooling can be accomplished with the heat ex-changer which, in many cases, follows the desiccant wheel.In most desiccant systems, this heat exchanger is not con-trolled. That is to say, it cools the air as much as possible,regardless of the thermostat setting in the building, becausethere is always a need to remove heat. However, if the des-iccant system is mounted on the ventilation air, there maybe a need for winter heat, and with that need, there will bea need to control the heating effect produced by the desic-cant wheel.

In those cases, the warm air from the desiccant wheel canbypass the post-cooling heat exchanger when the spaceneeds heat. Or, if the heat exchanger is a heat wheel, thewheel speed can be modulated in proportion to the heat-ing requirement. If the wheel rotates slowly, most of theheat remains in the supply air. When the wheel rotates morequickly, the supply air is cooled.

Stage two - indirect evaporative coolingWhen the heat exchanger by itself does not cool the supplyair sufficiently, the next stage of cooling is an evaporativecooling pad located on the cold side (i.e.: the reactivationside) of the heat exchanger. As the temperature in the build-ing rises above set point, the control system turns on thewater feed to the evaporative cooling pad. The cooling air isreduced in temperature, so more heat can be removed fromthe supply air.

Stage three - direct evaporative coolingWhen the building does not require humidity control, orwhen the building must kept cool and humid, as in the caseof greenhouses, livestock barns or vegetable storage, addi-tional sensible heat can be removed by direct evaporativecooling after the heat exchanger. The warm air is passedthrough an evaporative cooling pad, where it picks up mois-ture and therefore becomes cooler.

This can bring the temperature of the supply air low enoughto remove sensible heat from the building, rather than sim-ply bring the air to the same temperature as the building.Direct evaporative cooling has the advantages of low firstcost and low operating cost. In return for those advantages,the water supply system and the evaporative pad and drainpan must be maintained regularly, and the supply air is es-sentially saturated. Consequently the system is no longer adehumidification system, although such systems can be veryuseful for maintaining low temperatures and high humidi-ties as described above.

When the supply air must remain dry, the third stage of cool-ing can be accomplished by either conventional vapor-com-pression or gas cooling systems.

Final cooling - Vapor compression or gas coolingDesigning conventional cooling systems to follow desiccantwheels is quite straightforward, and in many cases, a desic-cant system will include such systems on-board, so the pack-age is a complete comfort-conditioning system.

The third stage of cooling will operate for relatively few hoursduring the year, and will be operating at only partial capac-ity for much of the rest of the year. So, if the post-cooler isnot equipped with capacity modulation, the building couldsuffer from poor temperature control and spikes in electri-cal power demand as the post-cooler turned on and off rap-idly when outdoor temperatures and moisture levels aremoderate.

When post-cooling is provided by the desiccant systemmanufacturer, one can usually assume that the post-cool-ing will include capacity control. But the point is worth check-ing, and definitely requires attention when the controls con-tractor supplies temperature and humidity controls ratherthan the manufacturer of the desiccant system.


Chapter 6 - Installation Design Tips 37

Designing Desiccant System InstallationsThis information serves as a useful review for engineers, and as a guide toreviewing construction drawings and specifications for end users.

CHAPTER 6INSTALLATION DESIGN TIPS

DUCT WORKDuct work must be air tight. Although this instruction seemsobvious, the point needs somewhat more emphasis in des-iccant systems than in conventional systems.

Return air duct work must be air tight, to avoid losing de-humidification capacity. Capacity is wasted when the fandraws in un-treated air through cracks on the negative-pres-sure side of the system. An engineer or owner is well-ad-vised to specify that return air duct work for any desiccantsystem be sealed and tested for gross air leaks. As the re-quired humidity control level goes lower, it becomes evenmore important to avoid air leaks in the return ducts.

When an all-desiccant system uses evaporative cooling padsfor final cooling of the supply air, the air is likely to be satu-rated for many hours during the cooling season. If the sup-ply air duct work immediately following the pad has an in-side lining of insulation, the system could eventually developthe same fungal growth that is so common in conventionalsystems downstream of cooling coils. To avoid that prob-lem, the duct work immediately downstream of the coolingsystem should be lined with a washable, non-porous sur-face. Also, access doors should be located in the duct workto allow for cleaning of that lining.

These guidelines would not be out of place for conventionalsystems as well as desiccant systems. Manufacturers of alltypes of rooftop equipment often profess astonishment thatdesign engineers and owners who would never tolerate pip-ing leaks sometimes accept that air duct work will leak as amatter of course. Avoiding air leaks is a very low-cost way to

eliminate problems. Tight duct work also improves comfortand save hundreds of thousands of dollars in annual oper-ating costs. Such great benefits are gained at the minorcost of a few rolls of duct sealer and a few hours of applica-tion labor at time of construction.

CONTROLS + SENSORSPerhaps the most important point about controls and sen-sors is to be clear about who supplies which items, and todefine who is responsible for installation.

In general, the desiccant system manufacturer can providethe internal system controls more economically than thecontrols contractor. Likewise, the external sensors are usu-ally less costly when purchased through the controls con-tractor. In any case, the engineer is advised to rely on thedesiccant system manufacturer for advice on controls andsensors which will work properly with the equipment. Con-trols contractors are not always familiar with desiccant sys-tems, and when they are, they may make unwarranted as-sumptions based on industrial desiccant system practices.

Sensor location can affect the success of any system instal-lation, and desiccant systems are no different. For example,if a humidistat is located near the supply air discharge, thesystem may not control humidity properly if there is a largemoisture load in the occupied space. This is because thehumidistat would never see the effect of the internal load,so the desiccant would cease drying prematurely. So, as withany system, locate the sensors wherever one needs to con-trol the temperature and moisture conditions most closely.



COOLING COIL DRAIN PANS + TRAPSIn many hybrid desiccant systems, the conventional coolingcoils are designed to run dry. That is to say, the desiccantwheel will remove the moisture, so it does not condenseout on a cooling coil. However, there are still times when acooling coil will condense moisture temporarily. For example,on system start-up, before the systems comes to full equi-librium with the loads, the coil may condense water. Or ifthe desiccant wheel does not have full reactivation due toheater malfunction or other infrequent event, the coil maycondense moisture from the air.

Consequently, experienced engineers and owners specifythat cooling coils should have sloped drain pans with con-densate piping and full-sized P-traps to carry water out ofthe system and onto the roof or into a condensate drain.The P-trap must be long enough so it contains a column ofwater larger than the pressure rating of the system fan. Oth-erwise, water will not stay in the trap, and air will leak intoor out of the system through the condensate drains.

UTILITIESMost modern desiccant systems are designed for single-point connection of each utility. But for each service, thereare some items that the engineer should consider when de-signing the installation.

Electrical powerThe main disconnect for the desiccant system cuts all powerto the unit, which is an important safety feature when theunit is being serviced. However, the service technician usu-ally needs some source of power for work lights, drills, powerwrenches, etc. So if the manufacturer does not provide aseparate service power circuit for lights and power tools,the service technicians will be grateful for such a separateconnection on the roof, but outside the unit. Thetechnician's alternative is to connect a long extension corddown off the roof, through a roof access door and into thenearest building power socket—a major time-waster.

Natural gasThe supply air and reactivation air heaters in desiccant sys-tems are designed to work within a specific range of gaspressures. If the pressure is above or below the specifiedrange, the system will shut down on various types of faults.

Often the desiccant system uses enough natural gas to jus-tify a separate supply line to the roof at elevated pressure(2 to 5 psi). Other times, different appliances or heaters willneed more gas, and the desiccant unit will be supplied withlower-pressure gas from a secondary line. In both cases, theengineer should be sure that the appropriate gas pressureregulators are installed to provide the specified pressure atthe unit—not 300 ft. away from the unit.

In other words, if the desiccant system is supplied with gasfrom the main, it will need a pressure regulator installednear the unit to be sure the gas pressure is not too high.And if a smaller desiccant unit is supplied with gas from in-

side a large building's internal gas distribution system, theengineer must be sure that the pressure will be high enoughwhen the gas reaches the desiccant unit.

WaterSome desiccant systems use evaporative coolers instead ofconventional cooling coils. Part of the normal maintenancefor evaporative coolers is flushing the pads and the watersump every two months during the cooling season. Also,the water supply system must be drained before winter. Themaintenance technician will be grateful to the engineer whomakes these tasks easy to accomplish through proper fix-tures on the water supply piping.

Figure 1 shows the needed fixtures. Starting from the topof the diagram, the shut-off valve stops water flow to thesystem when the technician needs to flush the pads andthe sump. Then the hose bib, mounted just below the shut-off valve, provides the technician with the flow needed toflush the pads. Below the roof line, the hose bib allows thetechnician to drain the water supply piping before winter.

Modem connectionsLarger desiccant systems, particularly those installed on su-permarkets, are often equipped with modems, so that theiroperation can be monitored remotely. When telecom wiresare brought to the unit, some cautions apply.

Figure 1. Water supply piping for units which use evaporativepads for cooling

To Evaporative Pads

Water Supply

Roof

Shut-off Valve

Hose BibFor Flushing PadsDuring Summer

Shut-off Valve

Hose BibFor Draining The SystemBefore Winter



Telecom signals can be distorted by high power electricalcables. It is tempting to run all wires to the unit through acommon electrical conduit, in order to minimize roof pen-etrations. When that is done, the telecom wire must beshielded cable, to avoid line distortion from the power wire.

Alternatively, modem wire can be brought to the unitthrough the supply or return air ducts. In that case, localfire codes must be consulted to determine what type ofcable insulation in acceptable for use in duct work. For ex-ample, PVC insulation may not be acceptable because itcould generate chlorine gas if it should catch fire.

CLEARANCE AROUND THE UNITMost commercial desiccant systems are mounted on roof-tops, so clearances do not present the same challenges asin a mechanical room. However, it is useful to review theitems which may have to be removed from the system atsome point, in order to ensure that they are not blocked byother mechanical equipment, or by piping. The engineershould check the manufacturer's equipment arrangementdrawing to be certain of how much space must be allowednext to the unit to provide access to key components.

FiltersFilters must be changed every three months, or sooner industy environments. Check the width of the recommendedfilters to be certain they can be removed and replaced with-out being blocked by a nearby structure.

Desiccant wheel (or heat wheel)If the wheel is designed as a single piece, it will have to berolled out of the unit if it ever needs replacement. Checkthe diameter of the wheel, and allow 1.5 to 2 times this dis-tance beside the unit to roll it out if needed.

Cooling coilsIf a cooling coil needs replacement, it generally is pulled fromthe side of the unit. Check the width of the longest coil,and allow 1.5 to 2 times that length for replacement clear-ance.

Evaporative padsIf the desiccant system uses evaporative pads rather thancooling coils, the technician must be able to remove thepads for flushing every two months. These are usually re-moved through the side of the unit. Check the width of thewidest piece of pad material, and allow 1.5 to 2 times thatdistance beside the unit for removal clearance.

Figure 2.Clearancesfor removingkey components

Cooling Coil

Reactivation Air

Process Air

Plan View

Once in 10 years

Every 2 months

Access Frequency

EvaporativePad

Filter EvaporativePad

Filter

Desiccant WheelHeat Wheel


Chapter 7 - Maintenance 41

Maintaining Desiccant SystemsCommercial desiccant systems are very much like air handlers. Themaintenance they require is quite modest compared to conventionalvapor compression cooling systems.

ESSENTIAL MAINTENANCELike any other mechanical equipment, desiccant systemsmust be maintained. But the good news is that these sys-tems are very simple, and easy to service.

Owners also report that the easiest way to optimize opera-tion and to avoid problems is for the service technician toreally understand the system. With that understanding,maintenance practices will be obvious, and therefore thereis a better chance that they will be accomplished regularly.

FILTERSDesiccant systems require only low-cost, 30% pleated filters.However, if the three keys to success in Real Estate are: "lo-cation, locations, location", then the three keys to reliabledesiccant systems are: "Filters, filters, filters."

If filters are not changed, air flow will be reduced. Thenoccupants will complain about being too hot or too cold.According to desiccant system manufacturers, owners andindependent service companies, 90% of "problems" reportedby occupants of desiccant-equipped buildings can be tracedto clogged filters.

Clogged filters on the supply, or process air side cause thethermal discomfort described above, because with less airflowing through the building, the system removes less heat.

Clogged filters on the reactivation side cause two problems.First, there is not enough air to remove the moisture fromthe desiccant wheel, so system performance is reduced.Then as filters load still further, there is not enough air to

absorb the heat from the burners, so the unit shuts downbecause the temperature in reactivation is too high as theair enters the wheel and too low as the air leaves the wheel.

The bottom line is simple: replace all inlet air filters at leastevery three months...and more often if the outside air isexceptionally dusty. This simple, low-cost measure will elimi-nate more than 90% of problems with desiccant systems.

One point which is not obvious to most technicians is that adesiccant system has TWO air inlets, not one. A desiccantsystem has incoming air on the supply side, but also a sec-ond air stream for reactivation. Both sets of inlet filters needquarterly replacement.

DESICCANT WHEEL MAINTENANCEAssuming the unit is equipped with an ADsorbent wheel(Silica gel, Titanium silicate, Activated alumina, etc.), there isno regular maintenance needed for the wheel material.

The only exception would be if the filters had not been well-maintained, so that dust built up on the wheel face. In thatevent, the wheel face should be vacuumed with a soft brushto remove excess particulate.

Desiccant wheel drive assemblyThe drive belt around the desiccant wheel needs to be tightenough to turn the wheel, but not so tight as to put anexcessive load on the drive motor shaft bearings. Most des-iccant units are equipped with automatic tensioning devices,but belt tension should be checked at least twice a year, tobe certain the belt is neither too slack, nor too tight.

CHAPTER 7MAINTENANCE



Manufacturers have specific belt tension checking proce-dures, but "too tight" usually means the belt cannot be de-flected more than 2" by hand pressure. "Too loose" meansthat the belt slips as the wheel turns.

On larger wheels, there is often a speed reducer betweenthe drive motor and belt drive pulley. The oil in these speedreducers needs to be changed once a year. If this is notdone, the speed reducer gears eventually wear out so thatthe speed reducer must be replaced in two or three yearsinstead of 10 to 15 years. Changing the oil takes between10 minutes and 25 minutes in most desiccant units.

Desiccant wheel support bearingsBecause desiccant wheels rotate so slowly, the bearingswhich support the wheel need only be greased once a year.Smaller single-piece wheels are sometimes supported by roll-ers under the wheel. Each of these rollers has a bearing lu-brication point.

Other wheels, particularly larger wheels, are supported by acentral hub, with either internal or outboard bearings, whichneed periodic lubrication. Lubrication once a year is usuallysufficient. But since the bearings in the system fans needquarterly lubrication, most owners check and lubricate thedesiccant wheel bearings when fan bearings are greased.

FAN MAINTENANCE

Fan beltsIf the desiccant system is large enough to use belt-drivenfans, the fan belts must be checked once every three monthsto make sure they are not too tight or too loose. Too tight

means the fan bearings are running hot. Too loose meansthe belts are flapping as the fan turns at full speed. The beltsshould also be inspected for excessive wear. Excessive wearwould include cracks in the rubber, or frayed spots on thebelt. Belts usually need replacement every three to five yearsunless they are overtightened, in which case they wear morequickly.

Fan bearingsOn belt-driven fans, bearings should be lubricated everythree months.

MAINTAINING YOUR SERVICE TECHNICIANThe most important maintenance element is the servicetechnician. If that individual understands the system, it willbe maintained. If the service tech does not understand thesystem, it will be ignored and will cause difficulties.

Owners say they train technicians to think about desiccantsystems as "air handlers, ....with a few extra components".With that basic paradigm, a technician has an easy way tothink abut the system, and has a mental structure for re-taining other information about the systems.

An ideal time to "maintain" the technician's knowledge isduring spring and fall semiannual maintenance. In thespring, the technician should review the system flow dia-gram to be reminded of what happens to the air at eachpoint during normal summer operation. Then again in thefall, the technician should review the flow diagram for win-ter operation, so that normal and abnormal operation willbe clear.

Lubricate BearingsCheck Drive Belt TensionEvery 2-3 months

ReplaceProcess inlet filtersevery 2-3 months

Check Belt TensionEvery 2-3 monthsChange Speed Reducer OilIn the springLubricate Support Bearingsin the spring

Lubricate BearingsCheck Drive Belt TensionEvery 2-3 months

ReplaceReactivation inlet filters

every 2-3 months

Process Air

Check Belt Tension + Bolt TightnessEvery 2-3 months

Change Speed Reducer OilIn the spring

Lubricate Support BearingsIn the spring and fall

Flush Pads + SumpAdd Water Treatment Pellets

Every 2-3 monthsConnect Water Supply

In the springDrain Water Piping

In the fall

Tighten Electrical ConnectionsEvery Spring

Reactivation Air



Such a simple review will be sufficient to refresh thetechnician's memory as to what the system does, and why.That knowledge, which takes less than ten minutes to "main-tain" every six months, saves hours or even days of confu-sion when problems arise.

COOLING COMPONENT MAINTENANCEIf the system is a hybrid, that is to say it includes conven-tional cooling or gas cooling coils, the technician will likelybe familiar with typical maintenance procedures. But somedesiccant systems include two components which may beless familiar. For that reason, we address them here.

Evaporative Cooling Pads & SumpsEvaporative cooling components are very simple devices, sothey need very little maintenance. Four items stand out asimportant:

• Flush the pads and sump every two months

• Replenish water treatment tablets (if used) everytwo months

• Drain the water supply piping in the fall

• Refill the water supply piping in the spring

Once again, if inlet air filters are replaced at least every threemonths, there is less maintenance required for the evapo-rative coolers. The pads and sump need to be flushed witha hose, because they collect particulate that escapes the fil-ters. So if filters are clean and functioning properly, therewill be little particulate to flush from the media and fromthe sump, reducing the time needed for normal mainte-nance.

Tablets containing algicide and water softeners can be usedextend the life of the pad material to more than 5 years bypreventing microbiological growth, and by precipitating min-erals so the evaporative media stays clear. New tablets aresimply dropped into the sump as needed, usually every twomonths during the cooling season.

In most North American locations, there is a risk of freezingduring winter months. Therefore, the piping that carrieswater to the system should be drained in the fall, and re-opened in the spring.

Engineers who design evaporative cooling systems in themountain states and in the southwest of the US are fullyaware of this simple semiannual practice. Those new to de-signing evaporative cooling systems should keep in mindthe piping recommendations outlined in chapter 6.

Eve

ry 2

-3M

on

ths

Replace inlet filters for bothsupply and reactivation air

Tighten all electricalconnections

Check tension and condition of alldrive belts

Lubricate bearings

Review system flow diagramand maintenance checklists

Flush evaporative pads andsump

Replenish water treatmentpellets

Open or close watersupply piping

Check heat wheel connectortightness and belt tension

Ge

ne

ral

Des

icca

nt W

heel

and

F

ans

Coo

ling

Com

pone

nts

Change oil in speed reducer

Change oil in speed reducer

Sp

rin

g

Fa

ll

Maintenance Tasks



Specifically, to avoid the future frustration of the mainte-nance technician, the engineer should equip the water sup-ply line with hose bibs mounted in tandem with shut-offvalves—both above and below the roof line. Above theroofline, the hose bib mounted between shut-off valves willallow for easy flushing and for draining the sediment fromthe sump. Below the roofline, a hose bib mounted above ashut-off valve allows easy draining of the water supply pip-ing before freezing weather.

Heat wheelsOn smaller systems, the heat wheel needs very little atten-tion if filters are changed every two months. Specifically,the wheel only needs a semiannual inspection to ensure theair seals remain in place and that the drive belt tension issufficient to turn the wheel, but not so tight as to stressthe motor bearings.

Larger systems often have hub-and-spoke assemblies. Inthose cases, the heat wheel should be examined every sixmonths to be certain that the bolts which hold the assem-bly together remain tight. Over a period of years, the rela-tively fast rotation of large wheels place a considerable loadon the hub-spoke-rim connectors. If they loosen, damagecan occur. So it is important to keep the wheel structuralassembly tight.

Also, as with desiccant wheels, the drive belt around theheat wheel needs to be tight enough to turn the wheel, butnot so tight as to put an excessive load on the drive motorshaft bearings. Also, belt tension should be checked at leastquarterly, to be certain the belt is neither too slack, nor tootight.

Manufacturers have specific belt tension checking proce-dures, but "too tight" usually means the belt cannot be de-flected more than 2" by hand pressure. "Too loose" meansthat the belt slips as the wheel turns.

Also on larger heat wheels, there is a speed reducer betweenthe drive motor and belt drive pulley. The oil in these speedreducers needs to be changed once a year.

ELECTRICAL CONNECTIONSExperienced maintenance technicians make a habit of tight-ening electrical connections at least once a year, just to becertain that normal vibration has not loosened an impor-tant wire or relay. Loose connections can create difficult-to-diagnose faults in any mechanical system, but such prob-lems can be easily avoided by annual tightening. Once again,investing 20 minutes may avoid wasted dollars and expen-sive hours by avoiding electrical faults at awkward times.


Chapter 8 - Evaluating Applications 45

CHAPTER 8EVALUATING APPLICATIONS

Six Application CriteriaPrudent owners and pragmatic engineers naturally ask: which buildings would benefit fromdesiccants, and which buildings would be better off with conventional technology?

Deciding whether desiccant systems apply to a givenbuilding requires an understanding of the differingneeds of each application, combined with the en-

ergy economics specific to the site. But there are six cir-cumstances when desiccant systems may have advantagesover competing technology. When two or more of thesecircumstances apply, the owner would be well-advised toinvestigate desiccants.

1. ECONOMIC BENEFIT TO LOW HUMIDITYDesiccants are exceptionally effective at maintaining lowhumidity. So when there is an economic benefit to suchhumidity control, desiccants are likely to be the best choice.

The classic, extreme example from industry is lithium bat-tery production. Above 6% rh, pure lithium produces enoughhydrogen gas to ignite, producing an explosion. Clearly, if amanufacturing company is making lithium batteries, thereis a strong economic incentive to maintaining low humidityto avoid explosions. A more common and less extreme ex-ample is the Supermarket, which avoids excess energy costby avoiding frost formation on refrigerated display cases andon frozen food. At typical store operating margins of 1.5%of sales, the $25,000 annual energy cost reduction producedby a desiccant system is the bottom-line equivalent of sell-ing an additional $1.7 million worth of groceries each year.

A more subtle example is the loading dock of a refrigeratedwarehouse. Worker`s compensation insurance costs are high,because of injuries from slips and falls. Keeping humiditylow avoids ice and condensation on floors. That is very ad-vantageous economically, as well as being the right thing todo from a purely human perspective.

Other examples of economic benefits to low humidity areas diverse as the last half-century of desiccant technology.For a quick rule-of-thumb, desiccants are worth evaluatingif the required humidity control level is below a 50°F dewpoint (below 55 gr/lb). Said another way: when a problemoccurs during the summer, but not during the winter, hu-midity may be a contributing factor, so desiccants may beeconomically advantageous.

2. HIGH MOISTURE LOADS WITH LOWSENSIBLE HEAT LOADSBecause desiccants convert a moisture load to a sensibleheat load, they do well where the moisture loads are highand the sensible heat loads are small.

Supermarkets provide a good example. A great deal of mois-ture must be removed from the store to avoid frosting onlow-temperature coils and frozen product. At the same time,so much excess cold air spills out of frozen food display casesso the store must be heated to maintain a comfortable shop-ping environment. In other words, the moisture load is highwhile the cooling load is low... or even non-existent.

Another example is a movie theatre. There are no lights,computers or machinery to generate sensible heat, the wallsare well-insulated and there are no windows. Also, and thepatrons are sedentary, keeping their heat production to anabsolute minimum. But each person breathes in and out,releasing large amounts of water vapor into the air, and thefresh air needed to ventilate the theatre carries very largeamounts of water vapor—particularly in the evening, whentheatres are most active. Once again, the moisture loads arehigh compared to the sensible cooling loads.



One easy way to compare the moisture load to the sensibleheat load is to calculate the sensible heat ratio (SHR) for theproposed system. The SHR is equal to the sensible heat loaddivided by the total heat load. For example, if the sensibleheat load totals 700 Btu/h, and the total load consists ofthat 700 Btu/h of sensible heat, plus 300 Btu/h or latentheat (moisture), the sensible heat ratio is:

In other words 70% of the total load is sensible heat and30% of the total load is latent heat (moisture). For a quickrule of thumb: when a system or a building has a sensibleheat ratio (SHR) of less than 0.8, the application may or maynot benefit from desiccants. But if the SHR is below 0.7,the application is almost certain to gain from desiccants.

3. NEED FOR MORE FRESH AIR ASHRAE Standard 62 now calls for 15 cfm of fresh air perperson in most buildings. That standard has been incorpo-rated into all three of the model building codes used in theUS, so it begins to have the force of law.

So many buildings need much more ventilation air than inthe past. Retail buildings are especially affected. Where a100,000 sq.ft. store would formerly require only 4,000 cfmof outside air, codes based on ASHRAE Standard 62 now de-mand that a store be provided with 0.3 cfm per square foot.So ventilation air flow jumps from 4,000 to 30,000 cfm tomeet new codes.

When such large amounts of fresh air are needed, conven-tional equipment is simply not designed to deal with theincreased load. For example, major manufacturers of roof-top packaged air conditioning equipment estimate thatwhen the proportion of outside air goes above 15% of thetotal supply air, conventional equipment has problems. Whenthe fresh air goes above 20% of the total, equipment willregularly fail to maintain the specified temperature levels ina traditional, low-rise commercial building.

For this reason, engineers sometimes combine a desiccant-based make-up air unit with conventional equipment on thesame building. The desiccant system handles the majorityof the total fresh air requirement, and the conventional unitscan then operate quite comfortably with less than 10% freshair without violating ventilation codes.

Similarly, engineers often use a desiccant system to add morefresh air to an existing building. The desiccant system satis-

fies the larger requirement for fresh air as needs of the oc-cupants change over time. Adding a desiccant system is of-ten much less costly and troublesome than re-working allexisting HVAC systems to provide additional fresh air.

As a rule of thumb, desiccant systems begin to have advan-tages over conventional systems when the proportion offresh air in a given system goes above 15%.

4. EXHAUST AIR AVAILABLE FOR POST-COOLING AND WINTER HEATINGThe cost of post-cooling the air as it leaves the wheel canfavor or limit the applicability of desiccant systems. Whenexhaust air can be brought back to the desiccant system atlittle or no incremental cost, the desiccant alternative gainsan advantage over conventional systems.

For example, a hospital HVAC system may use 100% outsideair to meet local ventilation codes. If the exhaust air can beevaporatively cooled and re-used through a heat exchangerto cool the warm process air, there may be no need for anyconventional post-cooling equipment. The overall installedcost of such a system might well be less expensive than theconventional alternative, and the operating cost would beconsiderably lower. In cooler climates, the advantage is evengreater, because the exhaust air can heat the incoming freshair during winter months, as well as cooling the fresh, dryair during humid months.

In many buildings, returning the air to the desiccant unit isnot practical, either because there is little exhaust air, orbecause the system operates for very few hours a day, orbecause air must be exhausted far away from the desiccantsystem. But with the increased amounts of ventilation airrequired by building codes which adopt ASHRAE Standard62-89, it is more likely that air can be exhausted throughthe desiccant system. That is because it is less likely that allthe air can be exhausted from any other point, and becausestandard commercial HVAC design practice usually includesreturn duct work to bring air back to recirculate throughthe unit. In such cases, there is no incremental cost to ex-hausting air at the desiccant system.

As a quick rule of thumb—if the system design already in-cludes return air duct work, the situation is neutral; neitherdesiccant nor conventional systems has an advantage. Butif recovering energy from the exhaust can reduce the in-stallation cost by down-sizing equipment, then desiccantsystems have an advantage. Conversely, if the desiccant sys-tem costs more than conventional, and if the payback timeis over 3 to 5 years, then conventional systems have a de-cided advantage.

In many ways, this criterion is linked to the need for freshair. If fresh air is a large proportion of the supply air, chancesare good that recovering energy from exhaust air will payoff quickly, and may even reduce the installed cost of adesiccant system to less than a conventional system.

700 Btu/h

700 Btu/h + 300Btu/h= 0.7 SHR

Sensible Heat Load

Sensible Heat Load

Latent Heat Load



Figure 1. Graphic evaluation template for desiccant applications

FAVORSDesiccant Systems V apor Compression

The Economic BenefitOf Dry Duct Work Need For Dry Duct Work

High CostsIf Duct Work Is Infected

HighHealth Risk

LowHealth RiskUnknown

Zero CostsIf Duct Work Is Infected

Exhaust Air Availability

Energy Recovery ReducesNet Installation Cost

ObviouslyYes No Extra Cost

ProbablyNot

Incremental Cost PaybackOver Three Years

The Installed Cost Of UsingExhaust Air For

Post-Cooling

Local Utility Costs

High Demand Charge Low Demand Charge

$20/kw $0/kw$8 per kw

Low Summer Gas Cost High Summer Gas Cost

30¢ 90¢60¢ per 100,000 Btu

Site-Specific Utility Costs

Outside Air As A Percent Of Supply Air

Supply AirIs 100% Outside Air

100% 0%15%

Supply Airis 100% Recirculated Air

A Need For More Fresh Air

Sensible Heat Ratio

High Moisture Load Low Moisture Load

0.6 1.00.8

A High Moisture LoadCompared To The

Sensible Heat Load

Required Dew Point

High Economic Benefit

35°F 65°F50°F

The Economic Benefit OfLow Humidity

Low Economic Benefit



5. LOW THERMAL ENERGY COST PLUS HIGHELECTRICAL DEMAND CHARGESGeography plays an important role for desiccant technol-ogy in unexpected ways. One might expect that desiccants,with their ability to remove moisture efficiently, would beespecially advantageous in humid Miami, and less so in dryDetroit. But that is not the case. In Miami, electrical costsare fairly low compared to Detroit, where electrical demandcharges are rather high. Also; sensible heat loads are highand continuous in Miami, where on an annual basis, sen-sible heat loads are lower in Detroit.

Clearly, "relatively low thermal energy cost " and "rather highelectrical demand charges" are nebulous. It is difficult toestablish better, more specific numerical guidelines for thiscriterion. For example, if a hospital pays $.90/therm for gas,one would not assume the thermal energy cost is particu-larly cheap. But if that hospital must operate boilers duringthe summer for sterilization, then the excess boiler capac-ity is essentially free when used for desiccant reactivation.

In another example, an existing building may have a verylow power cost. But if adding more outside air to meet IAQneeds requires more fans and refrigerant compressors, theincreased demand may lift the total power cost into anotherrate category. Also, adding power is not always easy or cheap,and may require changes in the basic service and electricaldistribution system. Such changes may turn a low-cost lo-cation into a high-cost location because of expenses otherthan power usage charges.

For a necessarily simplified rule of thumb , inexpensive ther-mal energy might be defined as a summertime cost of lessthan $0.60 per therm (100,000 Btu). An electrical demandcharge over $9.00/kw would usually be considered a "high"demand charge.

6. ECONOMIC BENEFIT TO DRY DUCT WORKFungus and bacteria are present throughout all indoor en-vironments, and are especially prevalent in air distributionduct work, where high humidity and accumulated dust arecommon. But in many cases, the rate of such microbialgrowth is very low.

In other cases where growth is more rapid and extensive, itmay not matter. For example, in many air conditioned struc-tures in humid climates, a distinctive musty odor is com-mon throughout the building. But until occupants complain,there is no economic incentive for owners to change thesystem.

In other buildings, the economic consequences of micro-bial growth are too important to ignore. Hospital operatingrooms, for example, place open wounds directly under airdischarge ducts. In such cases, complying with the recom-mendations of ASHRAE Standard 62 to maintain relative hu-midity in duct work below 70% is much more important thanit might be for a local pizza parlor or a post office. Conse-quently, for buildings like hospitals, nursing homes, medi-cal office buildings and schools, the economic consequences

Figure 2. Graphic evaluation of a Louisiana operating room

O p e r a t i n g R o o m - S h r e v e p o r t , L A

FavorsDesiccant Systems

FavorsVapor Compression

Exhaust Post-Cool

% Fresh Air

Dry Duct Work

Demand

Summer Gas

SHR

Dew Point

Criteria for an operating room stronglyfavor the use of desiccant systems.

This is especially true if the building has"free steam" during summer months whenboilers must operate at very low capacity



of fungal infection of duct work are focusing more atten-tion on using desiccant systems to keep duct work dry tominimize microbial growth in the HVAC system.

A rule of thumb is difficult to formulate in this area, be-cause there are so many unknowns and a wide variety ofopinions among experts. But one might suppose that if anowner or engineer believes that amount at risk in futurelegal costs is higher than the incremental cost of a desic-cant-assisted system, it might be prudent to evaluate thehybrid desiccant option in more detail.

EVALUATION EXAMPLESFigures 2,3 and 4 provide examples of how these criteriacan help decide whether desiccants deserve more study.Unfortunately, no such simple analysis can provide instant,comprehensive answers. But by looking briefly at these sixkey variables, an engineer can make a better prediction ofwhat answers are likely to result from a more rigorous analy-sis. Some examples are discussed below.

Hospital operating room, Shreveport, LouisianaFigure 2 shows an operating room. There is a need for a lowdew point, and failing to provide it may mean the hospitalloses revenue as surgeons take patients elsewhere for morecomplex procedures. Because the operating room often hasan all-outside air system, the latent loads are quite high.Also, the need for fresh air is extreme. Exhaust air is gener-ally not available for desiccant post-cooling in existing build-

ings, but in new construction, it would usually be accessibleat low incremental cost. The availability of cheap steam heatduring summer months improves economics, and the needto keep duct work dry is typical of a hospital environment.So the desiccant alternative is strongly favored.

Spec office building - Houston, TexasFigure 3 shows the results of a graphic assessment for alow-budget, spec office building to be built on Houston, TX.In this case, desiccants do not look promising.

There is no apparent economic benefit to the developerfrom maintaining a low humidity level. The building containsoffice equipment and is only two stories high, so the inter-nal heat generation and sensible heat loads through thebuilding envelope are quite high, and the moisture loadsare rather low by comparison. There is no need for a greatdeal of fresh air, and there is very little exhaust air availablefor desiccant post-cooling. Utility costs do not especiallyfavor desiccants, and there is normally no need for dry ductwork. Consequently, a desiccant system is not likely to beadvantageous in this case, unless the building was experi-encing problems with IAQ and needed to keep duct workdry, or to add fresh air.

Supermarkets in Detroit and MiamiFigure 4 compares supermarkets in two locations. In bothcases, desiccants make sense economically, but the benefitsare greater in Detroit, because of local utility cost differ-ences.

Figure 3. Graphic evaluation of a Spec office building

L o w - r i s e O f f i c e B u i l d i n g



Exhaust Post-Cool

% Fresh Air

Dry Duct Work

Demand

Summer Gas

SHR

Dew Point

Criteria for a low-budget commercial officebuilding are generally less favorable fordesiccants.

However, If there was a need to add morefresh air, or if the building had problemswith fungal infestation, these two criteriawould be more favorable for desiccantsystems. Then the owner should look atthe technical alternatives more closely.



Limitations of the graphic evaluationThis graphic technique has limited utility, because differentowners weight the importance of each variable differently.For example, if a hospital has had difficulty with contami-nated duct work, it may not matter what other factors fa-vor a conventional system... the desiccant system will bechosen regardless of other considerations.

Likewise, in the Supermarket industry, some owners havedecided that regardless of site utility cost, they will use des-iccant systems because of harder-to-quantify benefits suchas customer time-in-the-store, since modern desiccant sys-tems for Supermarkets are essentially the same cost as con-ventional equipment. In the opinion of some owners in thatindustry, the cost of analysis exceeds any cost saving thatmight result from a site-by-site decision process.

And in the case of an Ice Arena, the need for a low dewpoint makes all economics very clear. Regardless of whatother considerations might exist at a given site, if the rinkmust operate during summer months, then the desiccantalternative is an obvious choice.

Given those all-important weighting considerations, designengineers and building owners will probably find that suchgraphic evaluation is most useful as a "first cut", rather thanas the final decision-maker.

Figure 4. Graphic comparison of supermarkets in Miami and in Detroit

S u p e r m a r k e t - D e t r o i t



Exhaust Post-Cool

% Fresh Air

Dry Duct Work

Demand

Summer Gas

SHR

Dew Point

S u p e r m a r k e t - M i a m i



Exhaust Post-Cool

% Fresh Air

Dry Duct Work

Demand

Summer Gas

SHR

Dew Point

For a Supermarket in Detroit, nearly allthe criteria favor desiccant systems.

In Miami, local utility costs are lessfavorable for desiccant system.

This means that while desiccant systemsare still popular, the economics are not asfavorable as in Detroit.


Chapter 9 - Designing With Desiccant Systems 51

CHAPTER 9DESIGNING WITH DESICCANT SYSTEMS

Optimizing Systems With DesiccantsDesiccant systems have unique strengths and limitations. How can the engineertake advantage of the strengths of desiccant systems at each stage in the processof designing a system?

In abstract terms, designing HVAC systems with desiccantcomponents the same as designing with conventional com-ponents. The engineer determines the purpose of theproject, sets the control levels and design points, calculatesthe loads and lays out the equipment and controls whichwill accomplish the purpose of the project.

But as the engineer translates those abstract principles intoactual hardware, it will be helpful to understand how theunique strengths of desiccant systems can help the designerat each stage.

SYSTEM DESIGN STEPSSteps in designing any HVAC system include:

1. Define the purpose of the project

2. Establish the control levels and tolerances which,if met, will accomplish the purpose of the project

3. Determine the extreme weather conditions andcalculate the loads

4. Select, size and position components

5. Select and locate controls

Defining the purpose of the projectIn most cases, an engineer can assume that the primarypurpose of a project is to keep the occupants comfortable.But the key to minimizing the costs of a system is to under-stand the OTHER purposes as well.

For example, in the case of supermarkets, the HVAC systemand the moisture level it produces in the store will dramati-cally affect the cost of operating the display case refrigera-tion systems. So the purpose of a supermarket system could

be better stated as: "Keep the customers comfortable whilekeeping the humidity as low as possible."

As another example, the purpose of a hospital operatingroom system is not really to maintain 72°F ±2°F. Its purposeis more likely to be: "to keep the surgical staff comfortablewhile keeping the patient safe from airborne infection."

In fact, the HVAC system has many purposes, and the engi-neer must attempt to satisfy all of them. As the engineerand owner discuss purposes, they might keep in mind thatdesiccant systems can be very cost-effective when needsof the project include either fresh air, or humidity control.

Desiccants are unique in their ability to control humidity atvirtually any level, so when a project has any humidity-re-lated aspect, the owner and engineer should be aware ofthat capability.

Desiccants are also exceptionally well-suited to providinglow-cost ventilation air, so when the purpose of the projectinvolves either indoor air quality or large amounts of ex-haust air, desiccants allow the engineer to achieve resultswhich are not economical with other technologies.

But beyond the specific capabilities of desiccants comparedto other technologies. as this stage of the project, the ownercan help the engineer and equipment supplier to met thereal need by defining the purpose of the project in termsof desired results.

For example, in the case of the operating room system, theowner's purposes could be stated most helpfully as: "Makethe surgical staff more comfortable working at our hospitalthan at others in the area. Do that while complying with allcode requirements, and without interrupting surgical op-erations."



Understanding that purpose helps the designer how to"weight' the dozens of technical trade-offs that must bemade during the project. In this case, the engineer will beless concerned with energy consumption, and more con-cerned with figuring out what conditions will keep surgeonscomfortable. And more concerned with planning a construc-tion schedule with minimal service interruption than figur-ing out how to save money in piping insulation. While en-ergy consumption and the cost of piping insulation may re-main significant, they are much less important to this projectthan construction speed and understanding the sources ofdiscomfort in cool environments.

Establish control levels and tolerances which achievethe purposes of the projectWith a clear understanding of the purposes of the project,the engineer can proceed to the next step, where he or shemust decide what environmental conditions will best achievethe project purpose. No loads or equipment size can be cal-culated until the control level is set.

In this stage, the designer should know that desiccants canachieve and maintain any level of humidity needed, andmaintain it within a range of 1% rh if that is necessary oruseful.

It is also useful to remember that conventional systems of-ten have trouble maintaining temperature and humiditycontrol at the same time. A desiccant system can assist aconventional system by removing the need for that con-ventional system to control humidity.

In other words, desiccants allow the engineer any controllevel and any tolerance which will help achieve the purposesof the project. Desiccant systems simply remove the con-trol limitations inherent in conventional systems.

But knowing that traditional limitations no longer apply isonly part of the job at this stage. The more difficult task isdeciding what the control levels and the tolerances shouldbe. The engineer and owner must come to common agree-ment that the control levels selected will achieve the projectpurpose, because in the final analysis, the engineer's sys-tem cannot, for example "make people comfortable". It canonly maintain temperature ranges, moisture levels, particu-late concentrations and gaseous contamination levels withincertain ranges. The owner must decide that those levels willachieve the project purposes.

To return to the example of the operating room, the sur-geons will be heavily protected by layers of fabric and plas-tic to prevent transmission of disease to or from the pa-tient on the operating table. So although the ASHRAE com-fort chart and hospital construction guidelines would sug-gest that temperatures between 72 and 75°F should be ad-equate for human comfort, those levels have proven ex-tremely uncomfortable for surgeons. Heavy gowning pre-vents normal body cooling through evaporation and con-vection. So to maintain comfort though 2 to 8 hours ofstrenuous physical labor, the temperature and humidity mustbe lower. Modern surgical procedures may need tempera-tures of 62 to 66°F with humidity at 50% rh.

ASHRAE Sensible Heat Design Data Underestimates The Moisture Load

Atlanta, Georgia ......... July 1990Weather Data Obtained From National Climatological Data Center

ASHRAE Research Project 754-RP has shown thatsensible heat load design data in the 1993 Handbookof Fundamentals does not reflect the true moisture load.

This graph of Atlanta dew points during July, 1990shows that the ASHRAE 1% sensible design condition(65°F dew point) is exceeded for more than 80% of thetime during the summer.

In practical terms, this means that unless engineers areaware of this discrepancy, they may greatly under-designthe dehumidification capacity of an HVAC system.

65

July 5th July 10th July 15th July 20th July 25th July 30th

55

75

Dew

Poi

nt (

°F)

Humidity Observations Which ExceedThe 1% ASHRAE Sensible Design

Figure 1. Actual weather data from Atlanta, GA shows thatmoisture levels are higher than assumed when using peaksensible design data



The point is that written technical references may conflict.And technical references may lag behind actual surgical prac-tice. So the discussion between owner and engineer needsto include the owner's experience as well as the engineer'sexperiences and understanding of code requirements.

Calculate heat and moisture loadsWith the control levels defined, the project can proceed toload calculation. But another variable must be defined first:the extreme weather conditions under which the systemmust maintain the specified conditions.

At this stage, it is very important for the engineer to under-stand the limitations of past ASHRAE references when de-signing systems which must control humidity. Stated briefly,ASHRAE design data for calculating sensible heat loads is notadequate for calculating peak moisture loads, because mois-ture load peaks come when sensible temperatures are lowerthan their peak values. Figure 1 shows how this is true.

This difference will be clear to readers of the 1997 editionof the ASHRAE Handbook Of Fundamentals. In that version,the weather extremes are calculated for both moisture andtemperature. In earlier editions, only peak temperature val-ues appear. Using older editions temperature values formoisture design would understate the peak moisture loadsby 15 to 40%.

Readers who may not yet have access to the 1997 Hand-book of Fundamentals can obtain proper moisture extremevalues for U.S and Canadian locations by requesting the re-

sults of ASHRAE research project 754. These results are con-tained in ASHRAE Transactions 1995, Volume 101, Part 2, in apaper authored by Donald Colliver, Ph.D., P.E., entitled: De-termination of the 1%, 2.5% and 5% occurrences of extremedew point temperatures and mean coincident dry bulb tem-peratures. Readers who need data for projects in countriesother that the U.S and Canada must consult the 1997 Hand-book, which contains information for 1600 international lo-cations.

Another point which may not be obvious to engineers newto desiccants is that calculating heat and moisture loadsseparately can be very helpful in designing any system, butmaintaining that separation is essential to proper design ofdesiccant-assisted systems.

One of the major advantages of desiccant systems is theirability to remove excess moisture, freeing the conventionalsystem from that task. But to size the desiccant compo-nent, one must separate the moisture load from the sen-sible heat load, sizing the desiccant subsystem for the mois-ture and the sizing the cooling system to remove the sen-sible heat loads.

This separation is essential even if the desiccant system willremove both temperature and moisture by itself, becausewithin the desiccant system, the required component sizesmay vary according to the balance between the two differ-ent loads.

Moisture Loads vs. Sensible Heat Loads In Outdoor Air

Outdoor Dry Bulb BIN ( °F)

97° 92° 87° 82° 77° 72° 67° 62°

-800

-600

-400

-200

200

400

600

800

1000

1200

TON

-HO

UR

S(T

ons

x H

ours

in e

ach

BIN

)

Latent Ton-Hours

Sensible Ton-Hours

1. The engineer sizes thesystem for the hottestoutdoor temperature,but there are only 29hours each year that thesystem must operate atthis condition. Eventhen, the moisture loadis greater than thesensible heat load forfresh air.SHR = 0.48

2. When the total load is highest, thetemperature is lower, but the moistureload is even higher than it was at the peaksensible design conditionSHR = 0.21

3. When the outdoor temperature is equalto the inside condition, no sensible coolingis needed to change the fresh airtemperature, but the moisture loadremains very high.SHR = 0

4. The fresh air can now remove sensibleheat from the space—provided that it'smoisture load is removed. Mostconventional systems stop cooling whenthe temperature is this low, which meansthat moisture builds up, creatinguncomfortably high humidity andincreasing the risk of fungal growth.

Assumptions:• Air Flow - 1,000 cfm• Control Level - 72°F, 50% rh• Location - New Jersey• Weather Data - USAF 88-29

Figure 2 On an annual basis, moisture removal accounts for more ton-hours than sensible heat loads when treating outside air



The benefits of separate calculations are very useful fromanother perspective: they highlight the sources of each typeof load, which has immediate implications for the layout ofthe system. For example, in a meat-packing plant, the bulkof the annual moisture load comes from the washdown cycleat the end of a shift, and during those few hours, there isno need for humidity control. So rather than sizing a desic-cant component to remove the washdown load, it wouldbe less expensive to simply purge the building with outsideair during the washdown cycle. Then the desiccant compo-nent can be sized for the more modest moisture loads gen-erated during actual meat-packing operations.

In an example from commercial buildings, the engineer willoften find, on calculating loads separately, that 80% of themoisture load in an office is brought into the building bythe make-up air. The immediate implication is that a desic-cant system could be used to remove that moisture load,while conventional systems may be better suited to remov-ing the internally-generated sensible heat loads.

Select, size and lay out componentsEach equipment manufacturer has a slightly different ap-proach to sizing systems which include desiccant compo-nents. These approaches vary according to the hardwarefeatures offered by the manufacturer.

But one principle is common to most experienced design-ers of desiccant systems. find the moisture load, and de-sign the desiccant system to remove it first. Then designthe other components which remove the sensible heat loads.

This principle results from the fact that the desiccant pro-cess converts moisture to sensible heat. So there is not muchpoint in sizing and placing the cooling components until itis clear what sort of sensible heat load may result from re-moving the moisture load.

This approach often leads to placing the desiccant compo-nent on the outside air system, because in most cases, thebulk of the moisture load comes from that source. Also, thepeak moisture loads often occur when the sensible heatloads are somewhat reduced from their peaks, as shown infigures 2 and 3.

When the desiccant wheel is located on the make-up air, itproduces the greatest sensible heat during periods of lowsensible heat loads from the building envelope. So othersystems in the building may well have excess sensible cool-ing capacity, which reduces the need for sensible coolingcapacity within the desiccant system.

Another useful principle familiar to desiccant system design-ers is that the hotter the reactivation air, the smaller thedesiccant wheel can be. Drier desiccant can remove moremoisture than desiccant which has not been as completelydried. Since drier desiccant removes more moisture, lessdesiccant is needed for a given moisture load... so the wheelcan be smaller.

On the other hand, it is quite common to reactivate desic-cants with moderate or low temperature air (below200°F).Low temperature air can be less costly than high tempera-ture air. For example, the engineer may choose to use low-pressure steam or even compressor-reject heat for reacti-vation. In some cases, this approach can save operating cost.But in most cases, the size of the desiccant equipment willbe larger than if high-temperature reactivation air were used.

A larger desiccant wheel may or may not result in higher-cost equipment, depending on where the load falls in thecapacity range of a given piece of equipment. But in gen-eral, low-temperature reactivation means lower operatingcost, but higher first cost. High-temperature reactivationusually means lower first cost.

COST-EFFECTIVE SYSTEM DESIGNSAt this stage of development of the market for commercialdesiccant equipment , there as many different approachesto system design as there are manufacturers. An optimaldesign for one manufacturers equipment may not be opti-mal for anothers'.

Consequently, the owner and engineer can best optimize asystem by speaking with several manufacturers at an earlystage, to gain a sense of which type of equipment will bebest-suited to the needs of a particular project.

Moisture Loads & Heat Loads PeakAt Different Times Of Day

70

80

90

100

110

Tem

pera

ture

(°F

) H

umid

ity R

atio

(gr

.lb.)

12:00 Noon Midnight 12:00 Noon

Moisture PeaksIn the Mornings

Temperature PeaksIn the Afternoon

°F

gr.lb.

Atlanta, Georgia ......... July 1st and 2nd, 1990Weather Data Obtained From National Climatological Data Center

Figure 3. Moisture loads usually peak at different times of theday than sensible heat loads

55


Chapter 10 - Case Histories

CHAPTER 10CASE HISTORIES

1. ICE ARENAA desiccant system removes moisture from the arena, al-lowing summertime operation by reducing the ice refrig-eration load, and eliminating fog. The system also saves en-ergy, but that benefit is minor compared to increased rev-enue from year-round operation.

2. REFRIGERATED WAREHOUSEThe loading docks of refrigerated warehouses are hazard-ous because of water and ice on the floors. Desiccant unitseliminate this hazard, saving the human cost of injuries, whilealso reducing the cost of workers compensation. Refrigera-tion costs are also reduced.

3. HOSPITAL OPERATING ROOMSSurgeons fill hospitals with patients, which generate revenue.If these key contributors are dissatisfied with their workingenvironment, they take their patients elsewhere. Desiccantsystems can solve the comfort problems created by newsurgical procedures and health safety precautions.

Using Desiccants Rather Than Conventional SystemsEach type of building has different functions, site characteristics and utility costs vary andeach owner has unique needs. So the reasons for using desiccant systems are different foreach application.

4. TARGETED SUPERMARKET SYSTEMHigh humidity in supermarkets causes frost on the coolingcoils of refrigerated display cases, and condensation on dis-play case doors. A desiccant system removes excess mois-ture, which saves operating costs by reducing power con-sumption of compressors and door heaters.

5. RESEARCH LAB MAKE-UP AIRLaboratories must exhaust large amounts of air, which cre-ates a need to condition an equally-large amount of make-up air. Desiccant make-up air systems allowed this buildingto improve temperature and humidity control, while savinghundreds of thousands of dollars in annual operating costs.

56



Many ice rinks do not operate during the spring, summer and fall. This limits their income potential, because the building is idle for so much of the year.

One reason for shutting down during these seasons andforgoing potential revenue is the cost of operating ice re-frigeration systems during warm, humid months. During thesummer, electrical demand charges are high at the sametime that heat loads are at their highest levels, so the re-frigeration system is especially costly to operate.

Another reason for halting operations is the fog which formsinside the arena during humid months. As the ice sheet coolsthe air in the building, the humid air reaches saturation, andfog forms above the ice. This leads to poor ice conditions(soft and slow), muggy air and rust-inducing condensationon structures.

In recent years, however, many rink operators have foundthat when the arena is dehumidified, it is practical to oper-ate year-round, which greatly increases revenues. The costto remove excess moisture varies according to utility ratesand system alternatives, but those costs are small comparedto the great economic benefit of year-round revenue.

UNDERSTANDING THE PROBLEMIn a skating rink, a large, cold ice surface is exposed to therelatively humid air inside the building. Moisture continuallycondenses from the air onto the ice. As it condenses, it trans-

fers heat to the ice, softening the surface and formingpuddles which must be frozen by the ice refrigeration sys-tem. That freezing must be accomplished through a thicklayer of existing ice, so the cooling process is not very effi-cient, particularly as ice builds up during humid months andoutside temperatures rise. In contrast, during winter monthsheat must be added to the space, because the rink surfaceover-cools the building.

This situation is similarto the situation in supermarkets. Thecooling system has excess capacity to remove sensible heat,but does not remove moisture efficiently.

In ice rinks, as in supermarkets, efficient dehumidificationcan increase building revenue—and save operational costat the same time.

THE PURPOSE OF THE SYSTEM &ESTABLISHING CONTROL LEVELSOne might expect that the ideal system would prevent allcondensation on the ice surface. But as a practical matter,this is neither necessary nor economical. Such a systemwould be very large, and cost a great deal to operate. A bet-ter goal for the system is to reduce the moisture levelenough so the rate of condensation has a negligible effecton the ice refrigeration system. Maintaining the space at a35°F dew point satisfies this goal. The ice sheet is typicallyheld at 25 to 28°F, so there will be very little condensation ifair is kept at a 35°F dew point.

Ice Arena

57



A theoretically "ideal" system would maintain the 35°F dewpoint at all times, including periods of peak moisture load-ing from spectators. But once again, such a system wouldbe prohibitively expensive. An arena filled to it's capacitywith many thousands of spectators has a tremendous mois-ture load due to respiration and the fresh air ventilation.

A more economical system will have the air handling capac-ity needed for the fully-loaded arena, but include only thedehumidification capacity needed to maintain a 35°F dewpoint during periods of "practice/recreational" loading.

THE BUILDINGThis example is adapted from an actual case history describedin the ASHRAE transactions (#3747, January 1994, V.100, Pt1).

The arena is a multipurpose municipal rink, located nearGrand Rapids. MI. The rink surface measures 85 ft. x 200 ft.The rink was not built originally for year-round operation,but it was equipped with two 100 hp refrigeration compres-sors to cool the brine which freezes the ice sheet. At a 15°Fsuction temperature, the brine system had a COP of 2.2.

DESICCANT SYSTEMThe desiccant system is described by the schematic in fig-ure 1. It delivers 8,500 cfm of air at a dew point of -1° F anda dry bulb temperature of 85°F on a design day. That airremoves 131 lbs of water vapor per hour from the arena.

This illustrates the fact that when the moisture load gov-erns the size of the supply air stream, the deep-drying ca-pacity of the desiccant-dried air allows the engineer to keep

the total supply air volume very low. Handling less air re-duces the installed cost of the desiccant alternative.

Also note the relatively high supply air temperature. In thisbuilding, as in supermarkets, there is excess sensible cool-ing capacity. The ice sheet helps to cool the building, andother cooling systems which borrow from the ice refrigera-tion system provide additional air cooling. So, the desiccantsystem does not need a post-cooling system, which lowersoverall installation costs.

In fact, extra heat is desirable in this application. The arenais kept at 60°F, so spectators quickly become uncomfort-able. The warm dry air from the desiccant system is distrib-uted across the spectator stands. This air flow pattern bothwarms the spectators, and avoids problems created whendehumidified air is blown directly at the ice sheet. High ve-locity dry air would remove ice through sublimation, leav-ing depressions in the ice surface.

FOUR YEARS LATER...The Rink Manager, Don Cooley, moved to another commu-nity, and was hired to build a new rink. He specified a desic-cant system for humidity control in the new building, andreports that in both facilities, the desiccant systems haveprovided the expected benefits.

Beyond fog elimination and energy reduction, Cooley re-ports that the ice surfaces are harder. Hard surfaces are pre-ferred by skaters. A hard surface also lasts longer, reducingthe number of interruptions required for ice resurfacingoperations. And in general, ice surface is also better becausethe "mushrooms" formed by condensate dripping from the

Figure 1. Simple desiccant system for an ice arena

Desiccant System for an Ice Rink

MBtu/h Tons Lbs/hrSensible (248) (20.6)

Latent 139 11.6 131

Sensible (211) (17.6)Latent 167 13.9 158

System Can Remove or (Add)

Inte

rnal

Tota

l

15032

150

A

D

C

87°, 5grSupply Air

B

E

Temperature ( °F ) 92 60 62 85 87 92Moisture (gr/ lb) 110 29 34 5 5 110Air Flow (scfm) 500 8,000 8,500 8,500 8,500 2,500

A B C D E F

DW

A

B

C

F

D E

58



roof has been eliminated. Equipment maintenance has con-sisted of quarterly filter changes and annual lubrication offan and drive bearings.

TIPS AND TRAPS FOR APPLYING DESICCANTSYSTEMS IN ICE RINKS

Engineers and owners can benefit from the experience ofdesiccant equipment manufacturers in this and in other icerink installations:

Use an enthalpy wheel to reduce the cost of higherfresh air requirementsIn many parts of the country, the ventilation air required bylocal codes will be much greater than in this example. This isparticularly true when ice surfacing machinery may gener-ate products of combustion inside the building. When largeamounts of fresh air are needed, it may be more economi-cal to design the desiccant system as an all-outside air sys-tem, and to use an enthalpy heat exchanger to pre-cooland pre-dry the air entering the desiccant wheel. The en-thalpy wheel eliminates the need for a vapor-compressionpre-cooling system and reduces the first cost of such sys-tems. And in wide open buildings like ice arenas, the costof return air duct work to bring the exhaust air back to theunit for use in the enthalpy wheel is minimal.

Avoid blowing jets of dry air towards the ice surfaceA focused jet of warm, dry air will erode the ice surface wherethe air hits the rink. It is good practice to have dry air abovethe rink surface, because that's where the dry air can dothe most good for the ice refrigeration system. But the airshould be designed to move at very low velocity if it is di-rected towards the ice rather than over the spectator stands.

Consider extra small desiccant units to meet peak loadsWhen there is a need for humidity control during infrequent,but critical peak periods—such as hockey finals in latespring—it may be more economical to install a second unitrather than increasing the size of the basic air handler tomeet the peak load. This avoids the inefficiency of operat-ing a large unit at part-load conditions for most of the year.

Consider the risk as well as the benefit of compressorheat recoveryBecause the ice refrigeration compressors operate almostcontinuously, there may be some benefits to using theirwaste heat to pre-heat the desiccant reactivation air. Thismay reduce the cost of desiccant operation. However, someusers have found that the increased piping cost and the cor-responding risk of refrigerant leaks reduces the benefits ofusing waste heat. Also, such low-grade heat is never enoughby itself to regenerate the desiccant, so gas burners are stillrequired. Refrigerant heat recovery always increases firstcost.

Consider renting portable desiccant units to establishcost savings and other benefitsGas-fired, portable desiccant dehumidifiers are availablethrough industrial painting equipment rental firms. This typeof equipment is available in air flows of up to 4500 cfm in asingle unit. Also, companies which provide building restora-tion services following floods, fires and disasters frequentlyrent smaller desiccant dehumidifiers.

Portable units can be used for temporary installations in ex-isting buildings, where owners may be unsure about the pos-sible benefits and costs of dehumidification. This avoids theexpense of purchasing equipment or making permanentchanges to the building in the experimental phase.

Photo of ice rink desiccant system

59



One of the largest cost items in the budget of awarehouse is the expense of workers compensation. Every year, thousands of warehouse

workers slip and fall on the ice and water which condenseson the cold floors of the loading docks in front of frozenfood storage areas.

Frozen product must be moved in large volume, and trucksmust be loaded and unloaded at lightening speed to avoidthermal damage which reduces the quality and value of theproduct. Consequently, fork lift trucks, and motorized pal-let jacks move across the confined space of the loading dockat speeds up to 15 m.p.h. When floors are wet and icy, acci-dents are inevitable at such high speeds.

UNDERSTANDING THE PROBLEMLoading docks are maintained at low temperatures. Thisavoids loading the warehouse refrigeration system withwarm moist air pulled through doors by the movement offork lifts and pallet jacks. For example, a typical ice creamwarehouse might be maintained at -20°F, and it's loadingdock maintained at 35°F. It follows that any time the weathermoisture level is above a 35°F dew point, air leaking into thedock area will carry enough moisture to condense on thefloor. Also, the floor near the -20° warehouse itself willfreeze that condensate, and ice will form on racks near thedoor inside the storage area.

These problems are caused by moist air leaking into the load-ing dock through gaps between the trucks and the loadingdock doors. Although dock doors are equipped with cush-ion seals, there are always gaps and mismatches betweentruck and door dimensions.

Moisture also increases the load on the primary warehouserefrigeration system. Because these systems operate at verylow suction temperatures and high condensing tempera-tures, the compressors have to work very hard when mois-ture loads cause frosting on the cooling coils. Low-tempera-ture refrigeration systems are not efficient dehumidifiers.Once the frost has formed, it must be removed throughdefrost cycles, which consume still more energy. And de-frost cycles reduce cooling capacity in the warehouse, whichaffects product quality.

Removing moisture with desiccants reduces the load onthe primary refrigeration system. Also, in most cases, in-stalling a dedicated dehumidification system saves energy,reducing the cost of solving the safety problem.

THE PURPOSE OF THE SYSTEMThe desiccant system has two purposes: first to prevent con-densation in order to provide a safer working environmentand minimize workers compensation costs. The second pur-pose is to remove moisture load from the refrigeration sys-tem to minimize electrical power costs.

There are two ways to stop condensation: dehumidify theair in the dock area, or use air pressure to prevent humid airfrom infiltrating into the dock area.

Somewhat surprisingly, it is generally more energy efficientto dehumidify the dock than to prevent humid air infiltra-tion. To prevent infiltration, dry air must be brought intothe dock area, so that air pressure keeps humid air out ofthe dock area. However, that approach requires constantdehumidification of the incoming air, and for much of the

RefrigeratedWarehouse

60



time, that dehumidification load is greater than the load thatwould result from air infiltration. Either approach can pre-vent condensation problems, as long as the system hasenough capacity to keep the air dew point below the sur-face temperature of the floors and walls of the loading dock.

ESTABLISHING CONTROL LEVELSTo prevent condensation, in theory, one need only controlhumidity at the dew point to match the floor temperature.In practice, that would mean the air is constantly at or nearsaturation. Any small temperature fluctuation would causefog, condensation, and freezing, as well as encouraging mi-crobial growth and maximizing the load on the refrigera-tion system. To avoid these problems, the system shouldmaintain a control condition of 70% rh at the dock tem-perature of 35°F rather than maintaining near 100% rh.At 70% rh, the air has capacity to absorb moisture from anywet patches that might form in extreme weather conditions.Also, drier is generally better—because any reduction in dockhumidity reduces the cost of low-temperature refrigeration.

THE BUILDINGThis example is adapted from information provided byShaw's Supermarkets of Brockton, MA. That chain operatesa central frozen food warehouse in Wells, Maine.

The building was built in the late 80's. It has two refriger-ated sections in addition to a very large dry goods section.The refrigerated storage areas are served by separate load-ing docks. Each loading dock measures 300' long, x 45' wide

x 30' high. There are 13 truck doors on each dock, and twomuch larger, automated doors leading from the dock to thewarehouse. The loading docks are in line with each other,but separated by a fire stairway and office block, so it waslogical to use separate air handling systems for each dockto avoid long duct runs and to avoid penetrating fire walls.

To solve the condensation and frosting problems describedabove, the owner installed two 5,000 cfm desiccant systems.

DESICCANT SYSTEMEach desiccant system pulls 5,000 cfm from the loading dock.The duct work is arranged to draw from directly above thedock doors, where the air is warmest and most humid.

The air is dried to a -2°F dew point, and distributed alongthe back wall near the doors to the storage warehouse. Thereis no fresh air, other than what leaks into the dock throughthe truck doors.

Air leaving the desiccant system has been warmed to 52° bythe dehumidification process. The owner decided not to addpost-cooling to the desiccant system because there was ex-cess sensible cooling capacity in the dock coolers. In prac-tice, the owner has found that the air is seldom that warm,because the moisture load is usually under the maximum,and because there is quite a bit of mixing with the coolerdock air as the dry air leaves the distribution ducts. How-ever, the warmth of the dry air must be taken into accountwhen calculating energy costs, because its sensible heatmust be removed from the air by the dock coolers.

Unless humidity is controlled in theloading dock, ice forms above thedoorway inside a frozen foodwarehouse . Warm, humid air ispulled in at the top of the doorwayby the suction created when coldair flows out at the bottom of thedoor opening.

61



THREE YEARS LATERAccording to the Shaw's Energy Manager and the WarehouseManager, the results of the project were quite positive. Thedesiccant systems did indeed solve the safety problems andsave energy. The equipment manufacturer estimated annualoperating savings of $14,000 per year, but the actual sav-ings were larger. The owner measured a net energy cost re-duction of over $25,000 in the first year of operation.

In addition to the evidence provided by the reduced powerbills ,the facility manager could see the energy savings bythe fact that the dock coolers needed much fewer defrostcycles. Before desiccant systems, the dock coolers defrosted4 times each day during the summer. With desiccant sys-tems, defrost cycles reduced to once every three days.

TIPS & TRAPS FOR APPLYING DESICCANTSYSTEMS IN LOADING DOCKSEngineers and owners can benefit from the experience ofdesiccant system manufacturers and current system own-ers in this application. Suggestions include:

Distribute the dry air where it will do the most goodThe biggest condensation and ice problems occur near thedoors which connect the dock to the warehouse. So that iswhere the dry air should be distributed. Pulling air fromabove the truck doors sets up a circular flow which sweepsthe humid infiltrating air into the desiccant system beforeit can reach the colder surfaces near the warehouse doors.

Control system alternativesThis particular installation was originally designed to includecondensation controllers. These devices sense the cold sur-

face temperature, compare it to the air relative humidityand switch on the dehumidifiers if condensation is immi-nent. But the final control system depended instead on dewpoint sensors alone. The owner had difficulty finding a safeplace to install the surface temperature and surface rh sen-sors, because the surface of interest is the busy dock floor.

Dew point sensors are connected to the existing buildingautomation system. The dehumidifiers operate if the dewpoint is above 28°F.

Rooftop vs. below-ceiling installationLoading docks generally have ceilings high enough to ac-commodate desiccant units mounted inside under the roof.The advantage to this arrangement is that the units neednot be weather-tight, and there is no need to penetrate theroof for air duct and power connections. However, hangingunits high inside a building tends to reduce the maintenanceattention they get.

Filter maintenance is very important with desiccant units.So if the units are suspended, they should be arranged withcatwalks or other external access so belts can be adjusted,wheels replaced and filters changed.

Also, moisture will condense in the duct work carrying thewarm, humid reactivation air away from the unit unless it isheavily insulated. Regardless of the insulation, it should bemade of stainless steel or fiberglass, because condensationwould quickly erode conventional galvanized steel ducts onthe leaving side of the reactivation circuit. That duct shouldalso be sloped towards the outside or equipped with a low-point drain, so no water remains stagnant in the duct.

Figure 2. A desiccant system for a refrigerated warehouse loading dock

CB

Refrigerated Loading Dock System

MBtu/h Tons Lbs/hrSensible (91.8) (7.6)

Latent 58.3 4.8 55

Sensible (91.8) (7.6)Latent 58.3 4.8 55

System Can Remove or (Add)

Inte

rnal

Tota

l

150

150

A

52°, 5grSupply Air

Temperature ( °F ) 35 50 52 70Moisture (gr/ lb) 22 5 5 113Air Flow (scfm) 5,000 5,000 5,000 660

A B C D

DWA

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When units are mounted above the roof, their casings mustbe heavily insulated so the weather does not heat the dockair as it travels through the desiccant unit. Insulation alsohelps retard external condensation, which could corrode theunit casing as well as waste the capacity of the dock cool-ers. Finally, roof-mounted units must be very airtight. If theypull in untreated humid air through leaks in the casing, mois-ture could condense inside the unit and freeze or drip intothe dock through duct work.

Consider the risk as well as the benefit of compressorheat recoveryBecause the warehouse refrigeration compressors operatealmost continuously, there may be some benefits to usingtheir waste heat to pre-heat the desiccant reactivation air.This may reduce the cost of desiccant operation. However,some users have found that the piping cost and the risk ofrefrigerant leaks reduces the benefits of using waste heat.Also, such low-grade heat is never enough by itself to re-generate the desiccant, so gas burners are still required. Re-frigerant heat recovery always increases first cost.

Consider renting portable desiccant units to establishcost savings and other benefitsGas-fired, portable desiccant dehumidifiers are availablethrough industrial painting equipment rental firms. This typeof equipment is available in air flows of up to 4500 cfm in asingle unit. Also, companies which provide building restora-tion services following floods, fires and disasters frequentlyrent smaller desiccant units.

These units can be used for temporary installations in ex-isting buildings, where owners may be unsure about the pos-sible benefits and costs of dehumidification. This avoids theexpense of purchasing capital equipment or making changesto the building in the experimental phase.

Suggested air distributionfor the loading dockinstallation

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In recent years, medical cost containment efforts haveplaced a premium on keeping surgeons happy. Much ofthe care formerly given in hospitals can be done on an

outpatient basis, which leaves a hospital with empty beds.The overhead associated with those empty beds can causereal financial problems for a hospital. Since surgeons fill bedswith patients, they play a key role in the profitability of manyhospitals. Hospitals tend to be very responsive to their needs.

At the same time, surgical techniques have changed radi-cally over the last ten years. New electronic equipment addsheat to the operating room. And the risk of fluid-borne dis-eases such as AIDS has forced the surgical staff to add pro-tective layers of clothing, which makes them uncomfort-ably hot. Also, some newer procedures like hip replacementsand organ transplants are quite lengthy, so the discomfortand excess perspiration can last for hours. Consequently,operating rooms are cooled to much lower temperaturesthan in the past.

In the early 1980's, it was common to maintain the operat-ing room (OR) at 70 to 72°F. Now it is quite common forsurgeons to insist on temperatures of 65°F or below. At cooltemperatures, controlling humidity can be a problem. Hos-pital construction standards of the U.S. Public Health Ser-vice call for maintaining humidity between 30 and 60% in

operating rooms. Older systems have great difficulty achiev-ing that level when the room must be kept at 62 or 65°. Sohumidity rises above the specified level—and above the levelrecommended by most hospital licensing authorities.

UNDERSTANDING THE PROBLEMSThere are two problems: uncomfortable surgeons and thefact that different procedures call for different tempera-ture and humidity conditions.

Surgeon discomfortSurgeons are uncomfortable at high humidity because thebody cannot cool itself through normal evaporation. The sur-geon may perceive the problem as one of temperature, butin fact, humidity is causing the discomfort when the rh risesclose to saturation.

In conventional systems, the high relative humidity is causedby the falling temperature. Air leaving a cooling coil is nearlysaturated. It's relative humidity is close to 100%. In olderconventional systems, the air is reheated, which lowers itsrelative humidity before the air is delivered to the space.But when the space must be kept very cool, the air cannotbe reheated because it must be delivered at a temperaturelow enough to remove the heat in the space. When a cool-ing system is overextended, air must be delivered near satu-

HospitalOperating Rooms

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ration—perhaps as high as 75 or 85% rh—to be cold enoughto remove the sensible heat load in the operating room.

That high humidity causes the discomfort and noncompli-ance problems. In addition, it creates an environment whichfavors the growth of infectious bacteria and fungi in theduct work.

Different conditions for different proceduresThe hospital cannot afford to have idle operating rooms.They represent a considerable overhead cost, so they mustbe used continuously to avoid draining cash from the hos-pital budget. So surgeons and administrators are anxious toschedule operations close together to minimize idle time.

But conventional systems do not adapt well to fast set pointchanges to accommodate different procedures. And con-ventional systems have difficulty controlling several differ-ent rooms to different conditions at the same time.

Although a specialty cooling-reheating system can be cus-tom-designed to solve these problems, there are advan-tages to the desiccant approach. Most significantly, the lowdew point drying capacity of desiccants allows humidity con-trol by drying only the makeup air. This particularly attrac-tive in retrofit applications where a relatively simple add-oncan eliminate the need to replace or upgrade cooling equip-ment in the OR system.

THE PURPOSE OF THE SYSTEM &ESTABLISHING CONTROL LEVELSThe system is designed to keep the surgical staff comfort-able and to satisfy local codes.

The system should also use the least amount of energy pos-sible, but energy consumption is a secondary consideration.Energy cost differences will be measured in thousands ofdollars per year. But lost surgical revenues or costs associ-ated with certification problems or construction interrup-tions would be measured in the millions of dollars per year.From an economic point of view, comfort, compliance andavoiding construction interruptions are two orders of mag-nitude more important than any differences in energy costs.

The surgeon sets the temperature control level. In this case,surgeons and the engineering staff agreed on 68°F as thetemperature control level. For humidity, codes in Shreve-port, LA call for maintaining operating rooms between 45and 65% rh. The design engineer therefore specified thatthe new system must be capable of maintaining any or allof the OR's at 45% rh at 68°F.

THE BUILDINGThe Willis-Knighton Medical Center in Shreveport, LA, wasbuilt in the late 1970's. Given an existing building, the de-sign engineer was presented with many constraints whichwould not be typical of new construction.

Photo of the desiccantsystem, installed on themakeup air inlet for theoriginal operating roomHVAC system

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The first of these constraints was the original HVAC systemwhich served the eight operating rooms. The system used100% outside air, to comply with codes of the middle 1970's.Today's codes allow the use of recirculated air, but only ifthe total flow is increased from 15 air changes per hour to25 air changes per hour. Such a large increase in air volumewould demand more space for duct work, and such exten-sive reconstruction would halt surgical procedures for sev-eral months—a practical impossibility. Consequently, thedesign engineer decided to provide the specified conditionsby deep-drying the makeup air, which could be accomplishedwithout disrupting surgical operations.

Another constraint was the owner's need for chilled waterredundancy between the surgical section and the general-care sections of the hospital. In the past, the hospital staffwas able to re-balance the chilled water flow from point topoint in the medical complex as loads changed, and as dif-ferent pieces of refrigeration equipment needed service. Soalthough a low-temperature glycol chiller with re-heat wouldimprove humidity control in the OR's, such a design was sim-ply not acceptable from the perspective of the owner.

A second, low-temperature glycol system would prevent re-circuiting coolant flow for maintenance and eliminate thebenefits of partial redundancy and the ability to meet loadpeaks in different parts of the complex. Keeping the cool-ant type the same and keeping the all coolant mixtures andsupply temperatures constant was therefore imperative.

A third design factor was the presence of excess steam ca-pacity during summer months. In this community, and withthe boiler capacities and the steam system, hospital codesrequired both boilers to be operated year-round to provideinstant back-up for unexpected heating loads and for ster-ilization needs. During warm months of the year when hu-midity is high, this excess steam capacity could be put toproductive use reactivating a desiccant system. In this situ-ation, the desiccant system converts the wasted capacityof the steam system into useful capacity to control humid-ity—at no increase in overall operating cost.

Consequently, the hospital decided to install a hybrid desic-cant system at the inlet to the make-up air HVAC systemwhich serves the operating rooms. A photo of this systemis shown on the previous page, and it's flow schematic isshown above in figure 4.

THE DESICCANT SYSTEMA hybrid desiccant system delivers 7700 scfm to the eightoperating rooms at a constant year-round temperature of52°F and a moisture level of 40 grains per pound (a dewpoint of 42°F.) If necessary, that air is heated or humidifiedas it enters each operating room to bring the OR to the con-dition needed for each procedure. That design allows thecondition in each room to be completely independent ofthe conditions in all the other rooms.

Figure 4. Hybrid desiccant system controls temperature and moisture in eight operating rooms

Operating Room HVAC System

MBtu/h Tons Lbs/hrSensible 133 11 --------

Latent 208 2.6 30

Sensible 399 33.3 --------Latent 388 32.4 366

System Can Remove

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rnal

Tota

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Temperature ( °F ) 100 82 127 117 52 100Moisture (gr/ lb) 114 94 40 40 40 114Air Flow (scfm) 8,100 8,100 7,700 7,700 7,700 3090

A B C D E F

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ESC

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Note that in this system, chilled water is used to pre-cooland pre-dehumidify the incoming fresh air before it reachesthe desiccant wheel. This feature saves on the overall costof the system, because at the peak design condition, thedesiccant wheel would have to be much larger to removethe full moisture load. That larger wheel would have meantthe overall system would have been larger and thereforemuch more expensive to install. The design decision becamea simple matter of comparing the incremental cost of theadditional coil to the incremental cost of the larger desic-cant wheel. The chilled water pre-cooler is actually operatedfor less than 100 hours per year—only when the desiccantwheel cannot fully satisfy the humidistat which controls thesupply air.

A heat pipe post-cooler moves some of the heat of dehu-midification from the supply air to the reactivation air. Thebalance of the excess sensible heat is then removed by thechilled water post-cooling coil. Even after such post-cool-ing, the relative humidity in the supply duct work is no higherthan 70% rh, meeting ASHRAE Standard 62 guidelines foravoiding fungal infection of duct work.

TWO YEARS LATERJerry Ivey, the project engineer at the hospital, was con-tacted after one year's operation of the system. He con-firmed that the system eliminated complaints from the sur-gical staff about comfort in the OR, and also pointed outthe system responds very quickly to the needed changes inset points between different procedures scheduled for thesame OR. This avoids a considerable source of annoyance.Previously, it was not unusual for the surgical team to haveto simply wait 10 to 30 minutes while fully gowned andprepped for temperature and humidity conditions to stabi-lize before beginning procedures.

As a result of the success of this project, Mr. Ivey decided touse a desiccant system to control the OR's on a new hospi-tal being built by the Willis-Knighton Health Care System inBossier City, LA. In that case, he was able to gain somewhatmore space to return exhaust air to the unit, so its energywill be used to cool the dry air from the desiccant wheel.That investment in duct work saves 10 tons of sensible cool-ing capacity, and reduces operating expense for the greatmajority of the operating hours during the year.

TIPS & TRAPS FOR APPLYING DESICCANTSYSTEMS TO HOSPITAL OPERATING ROOMS Some suggestions for operating rooms include:

Consider exhaust air energy recovery for newconstructionIn the operating room application, a great deal of air—some-times 100% of the supply—is wasted through exhaust. Us-ing that costly, conditioned air to cool the incoming freshair is almost always an investment that pay back it's addi-tional equipment cost within months rather than years. Heatpipe exchangers offer the advantages of no maintenanceand complete separation of air streams, but heat wheelsoffer the advantages or controllability plus 20 to 40% greaterheat exchange efficiency compared to heat pipes. Both typesof exchangers are used extensively in hospital applications.

Locate the desiccant system at the fresh air inletThe least expensive and easiest way to apply a desiccant sys-tem to an operating room is to treat the makeup air alone.That arrangement minimizes construction interruptions forexisting buildings. And for new construction, a 100% out-side air system can satisfy codes by providing only 15 airchanges per hour. A recirculated air system would need 25air changes per hour, which would cost considerably moreto install. Also, the usual operating cost advantage of a re-circulated system usually disappears when heat recovery isused in an all-outside air system.

Consider "excess steam" for desiccant reactivationIn many hospitals, boilers must operate during summermonths at a fraction of their fully-loaded capacity, At thatpoint of operation, boilers are rather inefficient. Using suchexcess steam capacity for desiccant reactivation improvesefficiency and reduces the maintenance problems associ-ated with running boilers at extreme low-load conditions.

Raise chilled water supply temperatureSince the desiccant wheel will remove all or most of themoisture load, there is no need to run the main chillers atlow suction temperatures to provide dehumidification. Bysetting the chilled water supply temperature at 45 or 48°Finstead of 40 or 42°F, the chillers operate more efficiently,reducing peak power demand and increasing the system'ssensible cooling capacity. Raising the chilled water tempera-ture in desiccant-equipped systems saves both first cost andoperating costs, without compromising humidity control.

Consider indirect evaporative post-coolingMost hospitals have cooling towers to reject heat from thechillers. The water cooled by these towers can also be usedfor post-cooling supply air from a desiccant system, savingmore expensive vapor-compression capacity. Also, when theexhaust air can be used for post-cooling, the waste air canbe cooled with evaporative pads in a desiccant system, whichmay eliminate vapor-compression post cooling entirely. Thatstrategy would reduce first cost as well as operating cost,and the minor maintenance associated with evaporative padswould be familiar and easy for the hospital maintenance staffto accommodate in their normal routines.

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TargetedSupermarket System

Supermarkets operate on profit margins of less than2% of sales; and their annual energy costs are aboutthe same amount of money. So Supermarket owners

are naturally concerned about energy, and interested in des-iccant technology, which can reduce the cost of energy andtherefore put more money on the bottom line.

For example, in larger stores located in parts of the countrywith high electrical demand charges along with low-cost gas,a supermarket owner might expect to save between $15,000and $25,000 per year per store. From the owner's perspec-tive, that means that for every ten stores built with desic-cant systems, the annual savings are about equal to the an-nual profits of an entire store. So desiccants have an appealthat is similar to retail promotions: "For every ten storesbuilt with desiccants, you get one store free".

While that overly-casual economic assessment does not ap-ply in all cases, it helps explain why Supermarkets were thefirst commercial buildings to use desiccant systems as amatter of course. But in addition to energy savings, thereare other, more difficult-to-quantify reasons why desiccantsare popular with Supermarket owners.

Comfort has a higher-than-average value to Supermarketprofits. If customers are cold and uncomfortable in front offrozen food display cases, they will not linger to considerpurchasing those high-margin products. And if frost hasaccumulated on ice cream and frozen juice cans, the con-sumer will reject the product as outdated. Such frosted prod-

ucts represent a cost rather than a profit. Both problemsare caused by failures of conventional HVAC systems. Tradi-tional cooling systems do not solve the unique requirementsand problems posed by refrigerated display cases.

UNDERSTANDING THE PROBLEMIn most buildings, the sensible heat loads are high duringwarm months. But in Supermarkets, there is so much coolair spilling from the display cases that there is a heating loadall year long, even during the summer. The frozen food aislesmust be heated so that customers remain comfortable,

And in most buildings, high humidity does not cause majorproblems for the mechanical system. But in Supermarkets,the display cases must cool air to between -10°F and +25°F.When air is cooled to such low temperatures, many poundsof water are removed along with sensible heat. That con-densed water loads the case refrigeration system three ways.First, the water releases its heat of condensation. Secondly,the cooling coil must bring the water through a change ofstate to freeze it. Finally, the coil must be warmed for de-frosting and re-cooled after the frost is melted.

Clearly, minimizing the amount of water in the air will re-duce the load on the case refrigeration compressors. Mini-mizing air moisture will also reduce the frosting problemon the cold products inside the display cases. Desiccant sys-tems solve both problems by removing moisture and pro-vide heat at the same time.

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THE PURPOSE OF THE SYSTEM &ESTABLISHING CONTROL LEVELSThe system must dry, heat and cool the store. In many cases,the lowest-cost solution involves one or more hybrid desic-cant systems to control the entire space. But in larger stores,or "Supercenter" general merchandise stores which have asmaller grocery section, conventional systems are used tocondition most of the store, but a desiccant system is usedfor the area containing refrigerated display cases. That isthe strategy preferred by the owner in the current example.

The exact control level is always a matter of compromise.Certainly, if the humidity were below 10% at 78°F, therewould be no frost on any of the coils. However, drying thestore to that level would be very costly and uncomfortable.In this case, the owner decided on a maximum control levelof 80°F and 40% rh (63 gr/lb, or a 53°F dew point). Lookingat figure 6, one can see why. At this level, the moisture loadon the medium-temperature cases has been reduced by al-most 50%. Yet the desiccant equipment can be installed forthe same cost as a conventional system, or perhaps less.

THE BUILDINGThe store is located in Albany, NY. It has a total of 43,840sq.ft. of retail area. The retail space is divided into three gen-eral areas. Each is served by a separate rooftop air condi-tioning and heating system:

• Produce & Dry Goods - 24,225 sq.ft., served by a36-ton gas-electric vapor compression cooling sys-tem delivering 14,400 cfm of supply air

• Checkout Area - 7,400 sq.ft., served by a 20-tongas-electric vapor compression cooling system, de-livering 8,000 cfm of supply air.

• Refrigerated Area - 12,214 sq.ft., served by an all-desiccant cooling system equipped with an auxil-iary vapor compression cooling system, delivering12,000 of supply air.

THE DESICCANT SYSTEMThe desiccant system flow diagram is outlined in figure 5. Itdelivers air to the refrigerated area at temperatures between84 and 72°F, depending on the heating requirement in theaisles. The moisture level is never greater than 36 gr/lb, whichis a dew point of 40°F. By delivering air that dry, the systemcan remove over 208 lbs of water vapor every hour fromthe building, and still keep the moisture level in the refrig-erated aisles below a 50°F dew point (40% rh at 80°F).

Note the temperature of the air leaving the desiccant wheel,and the cooling arrangement which follows. Air leaves thedesiccant wheel at only 122°F, even though the air has beendried all the way from 70 to 34 gr/lb. This relatively low tem-perature rise is made possible by reactivating the desiccantwheel with air heated to only 190°F rather than the morecommon 250°F. Since the wheel is not heated so high, thereis less waste heat carried over into the process air after re-activation is complete, so the supply air is cooler than inmany desiccant systems.

After the desiccant, a rotary heat wheel cools the processair, with the assistance of cool air produced by an evapora-tive cooler. A small amount of moisture is carried over intothe dry process air by the heat exchanger (15 lbs per hour).But the combination of the high-efficiency heat wheel andthe evaporative cooler does away with the need for 41 tonsof conventional post-cooling.

Figure 5. Targeted dry air desiccant system installed at a Price Chopper Supermarket in Albany, NY.

Supermarket "Targeted" System

32 150

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Latent 221 18.3 208

Sensible 143 11.9 --------Latent 253 21.1 239

Temperature ( °F ) 93 80 83 122 84 72 75 190Moisture (gr/ lb) 93 63 70 34 36 36 130 130Air Flow (scfm) 3,000 9,000 12,000 12,000 12,000 12,000 4,000 4,000

A B C D E F G H

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For more than 97% of the year, this low-cost indirect evapo-ration is the only cooling required by the system. One ofthe goals of the system is to keep the aisles warm in therefrigerated area, so air can be supplied at 84°, or some-times higher. But to be certain of meeting temperatureneeds even on days which go beyond extreme design, theowner elected to install a small supplementary vapor com-pression cooling system after the heat wheel. This 13-tonsubsystem operates for very few hours during the year—only during extremely hot and humid weather.

TWO YEARS LATERBenny Smith is Chief Engineer for the Price-Chopper Chain,which owns this building, and five others equipped with des-iccant units in addition to 52 stores equipped with eitherconventional or heat-pipe-assisted cooling systems. Mr.Smith reports that, as with the other 1,000 desiccant unitsinstalled in Supermarkets, this system delivers the expectedbenefits. In particular, the comfort in the aisles is greatlyenhanced compared to buildings with either conventionalcooling or heat-pipe-assisted conventional cooling.

Mr. Smith notes that even his service technicians remark onthe improved comfort levels and the improved product ap-pearance at the desiccant-equipped location. Energy con-sumption has been minimized as predicted.

Maintenance has been about the same as with the conven-tional cooling units. The slight increase in maintenanceneeded for the evaporative cooler is offset by the reducedfilter maintenance made possible by eliminating the below-case returns. (When air is brought back to the unit from thefloor, it carries more dust and dirt than air returned fromthe ceiling level)

The installed cost of the unit was the same or less than aconventional unit. While the desiccant hardware may costslightly more, the elimination of expensive below-case re-turn ducts offset the unit cost.

The targeted dry air concept has worked very well. The des-iccant unit is able to consistently maintain the dew point ofthe refrigerated area 5 to 8° lower than the dew point inthe balance of the store.

TIPS AND TRAPS FOR APPLYING DESICCANTSYSTEMS TO SUPERMARKETSEngineers and owners can benefit from the experience ofthis and other cases of desiccant systems in Supermarkets.

Reduce defrost cycles to reflect reduced humidityIn older supermarkets, the case defrost is set with timers. Innewer stores, defrost might be set through the central en-ergy management system. In either event, it is importantto change the defrost cycle frequency to match the reducedneed when humidity is controlled at low levels. Otherwise, asignificant part of the energy savings may not be realized.

Shut off anti-sweat heaters on cases and on doorsIn stores equipped with desiccant units, there is seldom aneed to operate the anti-sweat heaters. Since these heaterscan account for 20 to 35kw of electrical load, it is generallywise to leave them off unless humidity rises above the setpoint. Each store will respond somewhat differently to ris-ing humidity, so some experimentation is needed before theoptimal heater-on humidity set point can be established.

Consider targeting the dry air from aboveIn some cases, controlling humidity in the entire store maybe more economical. In other cases, overall costs can beless when only the refrigerated area is controlled at low

Figure 6. Moisture load on refrigerated cases reduces as the control dew point goes down

50.7lbs/hr

32.7lbs/hr26.3

lbs/hr19.9lbs/hr

80°F55% rh98 gr/lb66°F dpt

75°F55% rh70 gr/lb57°F dp t



lbs

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er 1

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humidity levels. When targeting the air rather than control-ling the entire store, balancing the air flows becomes evenmore important than usual. Each supply diffuser should beequipped with its own individual air flow damper. Also, thesupply air should be distributed through double-deflectionsupply grills so that the warm, dry air can be aimed directlyat the baseboard kick plates of the display cases. That way,warm air mixes with the cold layer of air at the floor, raisingthe aisle temperature to more comfortable levels. Also, aim-ing the air at the baseboards avoids having the warm airdrift into the cases.

Consider reducing air flow to meet the loadIn this example, the air flow was maintained at a fairly highlevel—1 cfm/sq.ft. This high circulation rate was necessarywith cooling-based equipment because air could not be drieddeeply—so more air had to be cooled to provide adequatemoisture removal.

Desiccants allow the possibility of using less air, but the pref-erences and experiences of owners vary considerably on thispoint. Some owners reduce circulation rates, because thedesiccant-dried air allows the moisture load to be removedwith much less air. Figures as low as 0.5 cfm/sq.ft. have beenused, and circulation rates between 0.6 and 0.7 cfm/sq.ft.are quite common for desiccant-equipped stores. By reduc-ing the air flow, the desiccant unit uses less fan horsepower,saving additional energy cost. But as in this case, not allowners agree that the reduced circulation rate can providegood air mixing and even temperature control. The savingspossible through lower circulation rates make this pointworth the owner's time to discuss with the design engineer.

Evaluate pluses and minuses of below-case air returnsReturning air from below the cases (rather than from theceiling) has the advantage of capturing cold air spilling fromthe cases, so the aisles are warmer and the cold air can beused to enhance the performance of the desiccant wheel.But such returns require duct work running behind or be-low the cases, which is relatively costly compared to ceilingreturns. Also, pulling air from the floor means that the airwill be dirtier than air pulled from the ceiling, so filters areneeded at the face of the return grill, and these filters mustbe changed frequently to avoid air flow problems.

Once again, owners are divided on this point. Many favorbelow-case returns because of their potential for superiorcomfort, while others prefer the lower-cost alternative ofceiling returns, and do not see comfort advantages for lowreturns.

Supply outside air via the desiccant systemASHRAE Standard 62 calls for large amounts of fresh air inretail stores. This outside air carries the bulk of the mois-ture load. A desiccant system removes moisture very effi-ciently compared to conventional systems. So when a storeis served by both desiccant and conventional systems, thedesigner should consider letting the desiccant unit(s) handlethe fresh air, and allowing the conventional units to coolrecirculated air only. Then the conventional units can be setup with higher-than-normal evaporator temperatures, sincethey do not need to dehumidify. Higher evaporator tem-peratures allow them to operate with less power consump-tion per ton of cooling, which saves energy and reduces thepeak electrical demand.

Photo of the targeted desiccant system installed at the Albany store

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Medical Research Building

Research laboratories contain the most sophisticatedplumbing and mechanical systems of all commercialor institutional buildings. An engineer must design

systems and select equipment to deal with hazardous chemi-cal and biological agents which enter and leave the buildingin highly complex mixtures. The mechanical systems mustprotect people, but must also protect the research results—which are the reason the building exists.

The ventilation and air conditioning system is especially chal-lenging. A large amount of air is exhausted from fume hoodsand from biological safety cabinets, which protect workersfrom hazardous agents used in the research. The exhaustair must be replaced by air from the outside. Conditioningthat air is very costly. Also, air balance is critical, because labequipment uses air pressure differences to control the airflows which protect workers.

Protecting workers is obviously critical, but protecting re-search results is equally critical. Research projects can lastfor years. If temperature and humidity varies widely, resultsfrom one time period may not match results from a differ-ent season of the year, raising questions about the validityof the research. So maintaining a stable temperature andhumidity becomes far more important in a lab than it mightbe in other buildings where comfort is the primary concern.The ventilation system must isolate the building fromweather changes, in spite of the fact that hundreds of thou-sands of cubic feet of weather air are introduced into thebuilding every minute.

Much of the load on such ventilation systems is moisture.For example, on a design day in Chicago, every 10,000 cfmof ventilation air carries more than 48 gallons of excess wa-ter vapor into the building every hour. That load must beremoved, along with excess sensible heat.

Desiccant systems are especially useful in lab ventilation sys-tems because they can remove moisture very efficiently, andbecause the engineer can use desiccant systems to de-couple the temperature control system from the humiditycontrol system. By using a desiccant system, the engineercan provide independent control of humidity, so that con-ditions remain stable in the building, regardless of changesin sensible heat loads.

THE BUILDINGThe Sanders Research and Education building at the MedicalCollege of Georgia in Augusta was built in 1970. It contains250,000 sq.ft. of classroom and laboratory space. The origi-nal air conditioning system was equipped with 1200 tons ofchiller capacity to maintain the building at 75°F during thesummer. Gas-fired steam boilers were in place to maintain70°F during the winter, and to provide the re-heat whichwas intended to control humidity. The temperature was al-lowed to fluctuate between a low of 70° and a high of 75°during the spring and fall.

To control dew point, the system sprayed water over thecoils which cool the make-up air. By controlling the flow of

72



water through the cooling coils, the make-up air systemcould provide air at different dew points. But to control rela-tive humidity, the air had to be over-cooled to the desireddew point, and then re-heated.

Over time, this system had more and more difficulty main-taining temperature and humidity within the range neededfor consistent research results. This was partly because thesensible heat loads in the building became greater as moreelectronic equipment was added to serve researchers. Also,over time, the sprayed coils became less effective coolers.In addition, ASHRAE research into IAQ problems suggestedthat highly saturated air in old duct work was not the bestway to avoid contamination in a research facility. Finally, thesystem was never designed for tight humidity tolerances,so it had difficulty responding to fast changes in weathertemperature and moisture. Rh above 65% was common in-side the research labs.

UNDERSTANDING THE PROBLEM + SETTINGCONTROL LEVELSProblems maintaining humidity prompted a new look at whatthe system was expected to accomplish. After careful needsanalysis, the design engineering firm, Nottingham, Brookand Pennington of Macon, GA, determined that the systemneeded somewhat more sensible capacity and a great dealmore moisture removal capacity. The existing system sim-ply did not have the capacity to control both temperatureand humidity since loads had grown and researcher's ex-pectations had risen. It was no longer acceptable for hu-midity to swing as high as 70% rh and as low as 45% as theweather changed.

For this project, the owner and engineer agreed that tem-perature should be controlled as before—between 70 and75°F, and that humidity should remain constant at 50% rh±5%rh

The engineers noted that if the make-up air system couldbe improved, the rest of the existing system could handlethe internal loads. Also, by re-working the make-up air sys-tem instead of the internal duct work, the building couldcontinue in operation without disrupting research and in-struction, saving money for the college.

While adding chillers was an option, costs would be high.There was no space in the existing mechanical rooms, andthe roof could not take such heavy, vibrating loads. So addi-tional chillers would have to be placed at ground level in anew mechanical room built on the side of the building, andthe chilled water would have to be pumped to the roofwhere the air inlets were located.

The problem could be stated clearly: how best to precondi-tion the make-up air so it is dry enough to remove any in-ternal moisture loads, and cool enough to avoid overload-ing the building's existing cooling system?

DESIGN ALTERNATIVESGiven the prohibitive cost of a chiller-based solution to trulymeet the loads, the engineer had a choice of using eitheran undersized conventional system or a desiccant-based sys-tem for the make-up air. Uncertain of the economics, theengineer decided to design both alternatives and send bothto bid. When the bids were received, the desiccant systemwas somewhat higher, but it would met all the loads, where

Figure 7. Construction bids for the desiccant and cooling-based alternatives.

Instal led Cost Comparison Based on Actual Bids

C o n v e n t i o n a l $ 1 , 7 1 0 , 0 0 0( W h e n e q u a l t o D e s i c c a n t )

Addit ional Chi l ler &Rework $750,000

Instal lat ion, r igging, etc.$358,000

Demol i t ion$60,000

Controls & Electr ical$52,000

Rooftop Systems Cost$490,000

Instal lat ion, r igging, etc.$707,420

Controls & Electr ical$62,000

Demol i t ion$60,000

Rooftop Systems Cost$855,580

$1,685,000 Desiccant

73



the undersized (but affordable) chillers would not meet theloads. The owner decided to spend somewhat more moneyfor the desiccant system, since it would perform the neededfunctions fully, and because it's cost of operations wouldbe much lower than the chiller alternative.

The cost of power was fairly low at $0.05 kwh, but the de-mand charges were rather high at $11.50 per peak Kw. Gascosts for desiccant reactivation were relatively low at $0.25per therm ($0.33 per therm after boiler losses). But in addi-tion, the boilers operated way below peak capacity duringsummer months. So the true incremental cost of summerboiler usage was less than the projection.

As shown in figure 8, the engineers expected that the des-iccant system would save approximately $200,000 annuallycompared to the cost of operating a conventional system.

THE DESICCANT SYSTEMTo remove the internal loads, the building needs 200,000cfm delivered to the space at 55°F and 60 gr./lb. This airflow is provided by four identical desiccant systems, Figure9 shows the components of each system. The tables in fig-

ure 9 indicate the sum of the capacities of all systems. Di-viding the load between four units ( figure 10) allowed foreasier rigging, and also provides a degree of redundancy, sothat if any one unit needs maintenance. The other threecan take the load and maintenance can be performed dur-ing off-peak conditions without interrupting research.

The desiccant systems use the building's boiler for reactiva-tion, drying most of the make-up air before it blends withthe additional makeup air. Note that in this case, the reacti-vation temperature is 250°F. By using such a high tempera-ture, the desiccant wheel dries air very deeply. That very dryair can then be blended with additional un-treated make-up air and the return air (point D), with the result that theblended air stream (point E) is still dry enough to removethe internal moisture loads and maintain 50% rh. The desic-cant system removes 5,800 lbs/hr (701 gallons) of water perhour from the incoming air.

The engineer could have chosen to dry all the make-up airrather than only 140,000 cfm, but then the desiccant wheelwould have to be larger, increasing the system cost. In thiscase, hotter reactivation (deeper drying) reduced the instal-lation cost

Figure 9. Desiccant-assisted air conditioning system, installed at the Medical College of Georgia

Figure 8. Estimate of annual operating costs for both alternatives

Annual Operat ing Cost Comparison

Desiccant$230,928

Convent ional$425,374

Demand Costs$244,400

Elect ical Usage$200,974

Electr ical Usage$86,865

Demand Costs$73,341

React ivat ion Costs$63,011

Addit ional Fan Energy$8,711

MBtu/h Tons Lbs/hrSensible 4,320 360 --------

Latent 544 45.5 514

Sensible 7,668 639 --------Latent 6,201 516 5,850

Research Building AC System

Temperature ( °F ) 95 150 95 95 95 55 80 190Moisture (gr/ lb) 115 50 50 115 75 60 142 142

Air f low (000 scfm) 140 140 140 50 200 200 70 70

A B C D E F G H

B

DW

A C E

D

F

EC

CCHW

GH

SC

C

32 150

150

A

B

55°, 60gr

E

Supply Air

F

D

System Can Remove

Inte

rnal

Tota

l

74



After the desiccant wheel, the dry make-up air is cooled bya heat wheel combined with an indirect evaporative cooler.The high efficiency of the heat wheel, combined with theevaporative cooling effect displaces 693 tons of sensiblecooling. That reduces the system's power consumption andgreatly reduces the building's peak electrical demand.

Following the heat wheel, the dry air is blended with addi-tional make-up air. The blended air is then cooled by con-ventional chilled water cooling coils before the air is deliv-ered to the building.

The desiccant wheels move the building's moisture load fromthe overloaded chillers to the under-utilized boilers. Thissaves money because during the summer, operating boilersis less expensive than operating more chillers.

TWO YEARS LATERDavid Smith, PE., is the Manager of Facility Engineering forthe Medical College of Georgia. He confirms that the desic-cant system operates as expected, saving money and con-trolling humidity. Data logs from the central Digital ControlSystem confirm that humidity is maintained between 45 and55% rh even during the cooler, damp conditions typical ofevenings and mornings, and during heavy summer rainstorms.

Further, Mr. Smith notes that about the time the systemwas installed, the manufacturer's desiccant wheel was testedindependently by the Georgia Tech Research Institute. Thattesting shows that the desiccant mixture used in these sys-

tems is capable of removing a high percentage of pollut-ants from the air in addition to water vapor. Although thedesiccant system was not selected for this reason, any air-cleaning benefits are welcome in a research environmentwhere lab exhausts can mix with air near the intakes in un-predictable ways.

TIPS & TRAPS FOR APPLYING DESICCANTSYSTEMS TO RESEARCH BUILDINGSEngineers and owners can benefit from the experience ofthis and other examples of desiccant systems in researchbuildings.

Consider using exhaust air to aid post-coolingIn this case, the exhaust air contained a high concentrationof complex chemical mixtures. Also, the air was exhaustedfrom numerous different points on the roof. So the ownerpreferred not to collect the air for use in the indirect evapo-rative cooler (heat wheel) which cools the hot air as it leavesthe desiccant. But in other circumstances, an owner canbenefit from using the relatively cool and dry exhaust airfrom the building for the indirect evaporative cooling pro-cess. If that had been possible in this case, an additional 142tons of conventional cooling could have been saved.

Consider using condenser heat for partial reactivationIn many larger buildings with a continuous sensible heat load,the cooling system is constantly rejecting heat to a coolingtower. That warm water leaving the condenser barrel on itsway back to the cooling tower can be used a first stage ofheat for desiccant reactivation. This would improve econom-

Figure 10. Rooftop layout of desiccant systems and air handlers at the Medical College.

Supply air to the building

Conventional cooling coilsAdditional outside air

Dry air from desiccant systems

Evaporative padHeat wheelReactivation steam coilsDesiccant wheel

Reactivation fan

Ventilation (outside) air

Roof Layout - 200,000 cfm Hybr id Desiccant System

75



ics in cases where the piping and pumping costs do not ex-ceed the savings. In the current case, such additional instal-lation cost would not have been justified.

Dry air allows higher chilled water temperaturesIn existing installations where chilled water capacity is oftenshort, a desiccant system allows the owner to re-set thechilled water temperature to a higher level, increasing thecapacity of the equipment. Since the chiller does not haveto cool air deeply enough to dehumidify it, there is no needto produce 42 or 48° chilled water. If the chiller can supplywater at 52 or 55°F, it can operate more efficiently, savingmoney and regaining capacity.

Avoid cross-contamination on the roof topAir leaving a desiccant system's reactivation circuit is hotand humid, because it is carrying away the building's mois-ture loads. But frequently, the inlets to the building are alsolocated on the roof. The engineer should take precautions

to avoid pulling the hot, humid exhaust into the fresh airsupply of the building. For example, the system dischargecan be directed upwards like a stack exhaust, and the sys-tem inlets arranged to pull air in horizontally at roof level.

Consider special desiccant alternatives for lab buildingsNo matter what precautions are taken on the roof top, thefact remains that any lab exhausts contaminated air. Any-thing that can be done to clean the air entering the build-ing reduces the chance that contaminants around the build-ing might end up inside. Some desiccant wheels can removegaseous pollutants, as in this case. The engineer may wishto investigate desiccant equipment with this in mind.

Figure 11. The desiccant wheel s installed on this building arecapable of removing some times of gaseous contaminats fromthe incominf fresh air

Some advanced desiccants removecommon IAQ pollutants, butcollection levels vary depending onthe material, pollutant types andconcentrations as well as on airtemperature and moisture. Theperformance of one material isillustrated here. In this test, highhumidity was 118 gr/lb, and lowhumidity was 80 gr/lb, which aretypical of outdoor and indoorenvironments, respectively.

Removing IAQ Pollutants With Desiccants

SO2

Ozone

Formaldehyde

Hexane (Low rh)

Toluene (High rh)

CO2

Hexane (High rh)

Inlet OutletProcess Air Contaminant & Concentration

53 ppb 4 ppb

1.72 ppm 0.18 ppm

1.72 ppm 1.1 ppm

60 ppm 12 ppm

1.2 ppm 0.3 ppm

425 ppm

5.75 ppm 2.4 ppm

1,325 ppm

Percent Removal

10 20 30 40 50 60 70 80 90

96

92

36

80

75

68

61

Source: Georgia Tech Research Institute

Photo of the four desiccant unitsinstalled on the roof of the medicalresearch building

Documents

25369182 HVAC Handbook Applications Engineering Manual for Desiccant Systems