Desiccant Wheel Dehumidification 2

DESICCANT WHEEL DEHUMIDIFICATION

Desiccant Wheel Dehumidification

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Contents

1 Introduction.................................................................................................................41.1 Dehumidification - using desiccant.....................................................................51.2 Desiccant Life.....................................................................................................6

2 Analyzing the desiccant wheel dehumidifiers............................................................72.1 Performance........................................................................................................72.2 Wheel Designing.................................................................................................82.3 Selection of desiccant wheels...........................................................................122.4 Hybrid Cycles...................................................................................................12

3 Energy Savings Mechanism......................................................................................134 Purge Sections/Carryover.........................................................................................175 References.................................................................................................................19

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1 Introduction

The term “dehumidification” applies to all types of processes in which water vapor is removed from a gas.Sorbents are materials which attract and hold certain vapor or liquid substances. The process is referred to absorption if a chemical change takes place and as adsorption if no chemical change occurs. Desiccants, in both liquid and solid forms, are a subset of sorbents that have a high affinity to water molecules.Liquid desiccants absorb water molecules,chemical process, while solid desiccants adsorb water molecules, physical process, and hold them on their vast surfaces (specific surface areas are typically hundreds of square meters per gram).

Dry desiccant systems operate in a manner similar to liquid desiccant systems, but use a desiccant coating on a rotary enthalpy heat exchanger. Dry desiccant systems do not require energy for desiccant regeneration.

Desiccant Cooling:The use of desiccant dehumidification systems for latent heat (humidity) removal in air conditioning systems inconjunction with sensible heat exchange and/or evaporative cooling equipment to perform cooling. This process may use vapor compression equipment to provide the final stage of cooling. Desiccant cooling equipment is used to treat outdoor air by reducing both the sensible and latent (temperature and humidity) loads during cooling seasons. Some systems also include winter heating and humidification components for all season service.Desiccant DehumidificationStatic:The removal of moisture from humid air by exposing the air to a desiccant without the use of an air-moving device. Once the desiccant achieves moisture equilibrium with the ambient air, the dehumidification process stops and the desiccant must be replaced with regenerated desiccantActiveThe removal of moisture from the air by exposing the humid air to a desiccant using an air-moving device and a concurrent reactivation (regeneration) process such as by applying thermal heat. The reactivation process is concurrent and continuous for rotary dehumidifiers.Passive Dehumidification:The removal of moisture from the air by exposing the humid air to a desiccant with the use of an air-moving device and a temperature/humidity sink (i.e., difference in the partial vapor pressure of moisture between the ventilation (outdoor) air and exhaust air streams). For example, enthalpy wheels that are used to reduce the impact of ambient temperature and humidity of ventilation air on cooling and heating systems.

Desiccant air-conditioning is a promising emerging technology to supplement electrically driven vapor compression systems that rely almost exclusively on

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refrigerants that causes depletion of the ozone layer. Desiccant-based air-conditioning can be used as stand-alone systems or in combination with conventional systems.Desiccant systems can provide both temperature and humidity control and in some applications use less energy than conventional vapor compression systems.The wet part of wheel rotates through a regeneration zone where heat releases the water vapor into the second air flow, an exhaust flow.

The following benefits have emerged by using desiccant wheel dehumidifiers: Ability to control temperature and humidity independently Improved indoor air quality

Reduced amount of airborne bacteria and fungus. Desiccant-based systems can improve indoor air quality because of precise humidity control. Where conventional systems are used in humid climates, there is potential for microbial growth in the ducts and condensate drain pans because of inadequate moisture removal. This is not a problem for a desiccant-based system because there is typically very little water on a post-desiccant cooling coil or, subsequently, in the drain pan and the air distribution ducts.

Lowers utility bills, due to a reduction in the moisture portion of the cooling load, leading to less use of electricity for cooling, and increases in profits. Desiccant systems often permit reductions in the size of the conventional system (vapor compression unit), because part of the cooling load (dehumidification load) is shifted to the desiccant system. Size reduction not only saves energy, but it also decreases electrical demand and may reduce initial capital investment.

Increases comfort Expands facility's life span

The prime applications for desiccants are: Supermarkets, where the latent heat loads are very high Ice Arenas Archival storage facilities, that require very dry air Pharmaceutical facilities and hospitals, where maximum clean air is required

1.1 Dehumidification - using desiccant.In most modern, atmospheric-pressure industrial dehumidifiers, the desiccant is held in a lightweight, wheel-shaped matrix that rotates between two separate airstreams (Fig.). The desiccant is contained in the walls of thin air channels that extend through the depth of the wheel. Channel diameters vary, but typically are about 2 mm. Wheel diameter depends on how much air must pass through it. Large airflow volumes require a larger diameter wheel.Process air passes through a portion of the rotating desiccant wheel, giving off its water vapor to the desiccant in the air passage walls. Dry air leaves the wheel and is carried to the point of use by fans or blowers.Simultaneously, the rotating wheel passes through a second, smaller airstream carrying heated reactivation air. The hot air drives the water vapor from the desiccant. As the wheel rotates from the reactivation air into the process airstream, it can once again remove moisture.As the air dries, its temperature rises in proportion to the amount of water removed. Drier air means warmer air. This process is the reverse of evaporative cooling. When water is

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evaporated into air, the heat needed for evaporation comes from that same air and the sensible temperature falls. Conversely, when air is dehumidified, the heat needed to evaporate the water originally is liberated and the temperature of the airstream rises.Because a desiccant dehumidifier removes water vapor rather than condensed liquid from the air, there is no risk of freezing. This type of equipment is most often used for applications requiring dewpoints below 50 F.

Energy transfer The mass of the molecular sieve heat wheel is a coated desiccant matrix. The wheel rotates slowly, typically at about 20 rpm, between the building exhaust and supply air streams. The desiccant medium transfers heat with an efficiency of 75 percent to 90 percent by adsorbing and transferring vapor from one air stream to the other. Because moisture is transferred in vapor form, there are no wet surfaces to support microbial growth or chemical byproducts associated with boiler steam humidification.

1.2 Desiccant Life

The desired desiccant properties for optimum dehumidification performance are a suitable isotherm shape and a large moisture sorption capacity. The isotherms of silica gel are almost linear. The moisture sorption capacity is high; the desiccant is reactivated at relatively low temperatures and is suitable for moderate dehumidification. Molecular sieves have very steep isotherms at low relative humidity. The desiccant is reactivated at relatively high temperatures and is used for deep dehumidification.

Usefulness of the desiccant material depends largely on the quantity and type of contamination in the air streams. In a commercial air-conditioning environment, desiccants last between 10,000 hours and 100,000 hours before they need replacement (ASHRAE 1993, Chapter 19). Adsorbents (solid desiccants used in TWDS) tend to be less reactive chemically and more sensitive to clogging, a function of the type and particulate material in the air stream. They may also be sensitive to hydrothermal stress,

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which results from thermal expansion and contraction of the desiccant material due to rapid changes in desiccant moisture content (ASHRAE 1993). Because the application of TWDS in commercial air-conditioning is new, the long-term performance (over 10 years) of the desiccant wheel is not clear. According to the manufacturers, a well-maintained desiccant wheel will last for approximately 100,000 hours of operation (10 to 15 years).

2 Analyzing the desiccant wheel dehumidifiers

2.1 Performance

(Reference [3])For comparing desiccant wheel performance there are two parameters, Moisture Removal Capacity (MRC) or performance, and Regeneration Specific Heat Input (RSHI) or energy efficiency.MRC is the mass of moisture removed per hour, (kg/hr), and RSHI as hourly regeneration energy supplied to the device, normalized by MRC, (kJ/kg).

(1)

(2)

where:

MRC = moisture removal capacity, (kg/hr = standard density of air

Q = process air volume flow rateΔGPP = absolute humidity depression of the processRSHI = regeneration specific heat input

= thermal energy input rate

Moisture Mass Balance, defined as:

Moisture Balance Mass = MRC / MRR (3)

where MRR, Moisture Removal Regeneration, is analogous to MRC

Q = regeneration air volume flow rate

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2.2 Wheel Designing

Residence time and the basic mass transfer parameters are critical. Residence time combines the effects of face velocity, open area, and wheel depth.Overall mass transfer is governed by driving potential, airside transfer coefficient, diffusion within the desiccant, and surface area.The driving potential is the difference in partial pressure of water vapor between the air and the surface of the desiccant.Water vapor pressures in terrestrial dehumidificationapplications are on the order of two to five kilopascals (kPa) at the wheel inlet. Vapor pressures of a few hundred pascals exist locally at the wheel outlet. Vapor pressure at the desiccant surface varies with desiccant type and temperature and is on the order of hundreds of Pascal’s.

Airside transfer coefficient is governed by fluid dynamic phenomena and is typically correlated for both heat and mass transfer to Reynolds number, Prandtl number, and geometry, including number of transfer units (NTU). NTU relates rotor surface area exposed to the thermal loads embodied in the airstreams.

Prandtl number is a function of air thermophysical properties.Reynolds number is a ratio of momentum to viscous forces. Air velocity enters the correlations in the momentum term. The correlations change to reflect the flow regime present in the flutes. Flute velocity determines the regime, which may be roughly categorized as laminar or turbulent. In laminar flow,viscous forces dominate, so that nearly all air motion is in the direction of the bulk flow, along the axis of the flute. In turbulent flow, momentum is strong enough to produce substantial eddies within the bulk flow that continuously mix the air as it passes through the flute. This mixing generally means turbulent flow produces higher heat/mass transfer, but in doing so, also generates higher-pressure drops. The pressure drops incurred by turbulent airflow put an unacceptable load on the face and circumferential seals and drastically increase seal wear and fan power requirements. Laminar flow keeps pressure drops within HVAC application ranges and has the added benefit of keeping the internal surfaces of the matrix relatively clean because airflow moving parallel to the flute walls tends not to deposit dirt there.

NTU is a parameter commonly applied to heat exchangers that can also be applied to rotary mass exchangers. It is typically defined for the thermal component as the ratio of convective heat transfer at a given matrix-to-air temperature potential to the thermal capacity of the air over that potential:

(4)

where:NTU = number of transfer unitsΔT = temperaturej = hot or cold side of the wheelm = mass flow rate of air

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h = convective heat transfer coefficientcp = specific heat of airA = convective transfer surface area.This calculation must be performed on the hot and cold sides of the wheel (j) separately.The resulting values can be combined with the use of the parameter C*:

(5)

where C* is the ratio of minimum to maximum air heat capacity rates:

(6)

Heat exchange effectiveness for a direct counterflow heat exchanger is then calculatedwith total NTU:

(7)

where:εcf = heat exchange effectiveness.

Heat exchange effectiveness (and thereby outlet temperatures) for a rotary exchanger is then correlated using a parameter tailored to rotary devices that represents the thermal capacitance of the matrix:

(8)

where:M = the mass of the matrixΦ = its rotational frequency.

These correlations are valid for values of Cr over 0.4 (high wheel speed; temperature does not vary with rotational angle but with distance through the wheel only). This is the case for enthalpy exchangers; dehumidifiers might have a heat capacitance one-tenth this value.

NTU for mass transfer is similarly defined:

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(9)

where:hm = the mass transfer analog to thermal convection coefficient h.

Mass transfer parameters are modeled analytically by heat transfer analogy or computed numerically.Diffusion and surface area are closely related in wheel dynamics. Given the restrictions of residence time, the limitations of the former require a lot of the latter to achieve acceptable grain depression. Air is in contact with the desiccant only for a few hundredths of a second, making mass transfer for a given flute primarily a surface phenomenon. When performance depends on a single pass, surface area is critical in inherently slower processes like mass diffusion in solids.Diffusion comes into play as the desiccant/matrix slowly rotates within the same airflow; mass diffusion within the desiccant must keep the surface as dry as possible (on the adsorption side) until it can be regenerated and vice-versa during desorption.

Maximizing surface area means packing a lot of matrix into as small an area as possible, which leads to flutes with small cross sections. This is convenient because laminar flow is best achieved in small flow channels. This also means matrix walls should be as thin as possible to maximize open area and keep flute velocities as low and residence time as long as possible. This too is convenient because thin walls are less likely to waste unexposed desiccant by relying on slower solid-side diffusion to utilize drying potential. Surface area as a function of matrix design is complimented by the effect chemistry can produce with desiccant pore structure. Silica gels typically have on the order of 100 million square feet of surface area within their pores for each cubic foot of material.Activated carbon has several times that volumetric surface area but has lower water vapor uptake because its pore void space is too small to hold much water.

Residence time is the result of a number of important parameters.

(10)

where:ΔGPP = absolute humidity depression per second of residence time, grains/lbair/sMRC = moisture removal capacity, lb/hrVfl = flute velocity, ft/minQ = process air volume flow rate, ft3/mind = wheel depth, ft.

RSHI is an indicator of energy consumed by the regeneration heater. RSHI does not include the effect of a heat exchanger that can be employed at the process air outlet to recover heat of adsorption and preheat regeneration air. This configuration is commonly found in ventilation air conditioning applications. To include the effect of heat recovery, we use the term RSHIHX:

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(11)

where:εHX = the heat exchanger effectiveness.

This formulation assumes the PI and the heater receive air from the same source, and the heat exchanger is operated with balanced airflows, as is the case with many ventilation air pre-conditioners. Regeneration specific heat drop (RSHD) is an indicator of the energy consumed by thewheel.

(12)

RSHD focuses on the energy performance of the matrix itself by focusing on sensible energy drop in the regeneration air as it passes through the wheel rather than the energy supplied to the regeneration air. It is very nearly independent of face velocity for many wheel configurations, although there are exceptions. RSHD is also much less sensitive to mass-flow ratio than RSHI, again for many wheels but not all, and trends in the opposite direction as RSHI in some instances. Unlike high RSHI, high RSHD does not necessarily indicate reduced efficiency. High RSHD may indicate poor grain depression, as might RSHI, or it may show that the wheel is able to utilize lower temperature air for regeneration, or that the matrix is picking up a lot of heat. RSHI does not register these and other phenomena on it own. RSHD is a distinct parameter that adds to the understanding of a wheel’s energy consumption characteristics.

Heat dump-back is another feature of dehumidifier wheels that becomes important when process outlet temperature is a design requirement. Some processes benefit from the sensible energy evolved from the desiccation process; for others, this represents a load that must be removed. In quantifying heat dump-back, we calculate adsorption heat ratio:

(13)

where:AHR = adsorption heat ratioTPO = temperature achieved upon reaching measured grain depression.TPO,adiabatic = is the temperature achieved upon reaching the measured grain depression with no change in enthalpy-essentially evaporative cooling in reverse. If AHR = 1.0, theprocess is adiabatic. Fractional AHR indicates the degree of heat dump-back.

2.3 Selection of desiccant wheels

Some considerations for selection of desiccant wheels are:

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Appropriate desiccant materials Large desiccant content Wheel depth and flute size (for large contact surface area and low pressure drop) Size and cost

The actual performance depends on several additional factors. These include:

Inlet process air temperature and humidity Desired exit process air humidity Inlet reactivating air temperature and hurnidity Face velocity of the two air streams Size of reactivation segment

Higher inlet process air humidity results in higher exit humidity and temperature (more heat of sorption is released).

Lower face velocity of the process stream results in lower exit humidity and highher temperature.

Higher regeneration temperatures result in deeper drying, hence lower exit process air humidity and higher temperature.

When lower exit air temperature is required, the exit process air should be cooled by a heat exchanger. The following is a range of typical parameters for rotary desiccant wheels:

Rotation speed: 4 to 20 rpmDesiccant fraction: 70 to 80%Flute size: I to 2 mmReactivation segment: 25 to 30% of wheelFace velocity: 300 to 700 fpmReactivating temperature: 100 to 300gradF

2.4 Hybrid Cycles

A limited number of hybrid systems consisting of desiccant dehumidifiers and electrically driven vapor compression air-conditioners are presently in use in supermarkets. This application is uniquely suited for this purpose since the latent heat loads are high due to tile large number of people and frequent traffic through doors. Also, low relative humidity air is advantageous for open-case displays.

Vapor compression systems are inefficient below a dew point of 45 to 50gradF. When used in supermarkets, they require high airflow rates, the air must be reheated for comfort, and the evaporator coils must be defrosted frequently. Hybrid systems offer improved performance and lower energy cost in these cases.

The figure shows a typical hybrid air-conditioning system for supermarkets. A mixture of outdoor and recirculated air is first passed through the desiccant and sensible heat exchanger wheels, where it is dehumidified and precooled. It then enters the conventional chiller before it is introduced to the interior of the supermarket. The

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sensible heat exchanger wheel is cooled by outdoor air and the desiccant wheel is regenerated by air heated with natural gas. Energy cost can be further reduced by preheating the reactivating air stream with waste heat rejected from the condenser of the refrigeration and/or air-conditioning systems.

The advantages of these hybrid systems are Air-conditioning requirement is reduced by up to 20%. The vapor compression system operates at a higher coefficient of performance

(COP) since the evaporator coils are at a higher temperature. Airflow requirements are reduced; electric fan energy is saved and duct sizes are

reduced. The refrigeration cases run more efficiently since the frequency of defrost cycles

is greatly reduced.

3 Energy Savings MechanismRef [5]

The energy saving mechanism is presented using a Two Wheel Desiccant System.

As shown in the figure , a TWDS consists of a desiccant wheel, a rotary heat exchanger (sometimes referred to as a sensible heat wheel), a supply fan, an exhaust fan, and a heat source for regenerating the desiccant. The desiccant wheel is made of finely divided desiccant material, usually silica gel, titanium silicates, or some type of zeolite (a mineral containing hydrous silicates). The desiccant material is impregnated into a fibrous support structure, which looks like corrugated cardboard that has been rolled into the shape of a wheel or into a wheel-shaped rotor with a lightweight structural honeycomb core of man-made, fire-retardant material.The rotary heat exchanger, which exchanges (recovers) heat rather than moisture, resembles the desiccant wheel in appearance and design.

Any form of thermal energy stream can be used to dry and regenerate the desiccant, including electric - resistance heaters, solar hot water coils, heat reclaim coils, hot water or steam from boilers, or natural gas burners. Most commercial applications use either direct- or indirect-fired natural gas burners.

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Fig. Schematic of the Two-Wheel Desiccant System

The TWDS can control or lower humidity, but has a low ability to lower sensible heat. Therefore, in most commercial applications the TWDS is supplemented with either a vapor compression or an evaporative cooling system.

The energy-saving mechanism of a TWDS is explained by comparing the dehumidification and cooling process of the conventional and the desiccant-based systems. Both systems can be operated in various modes (recirculation, pure ventilation, and mixed). It is assumed that both systems take in 100% outdoor air. The following steps describe the psychometric process for a hybrid desiccant dehumidification and supplemental cooling system.

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Fig.(a) Comparison of a Hybrid Desiccant-Based Dehumidification and Supplemental Cooling Process with a Conventional Dehumidification and Cooling Process(b) Desiccant Reactivation Process

DehumidificationA: Intake--hot and humid outdoor air enters the desiccant wheel at point A on the psychrometric chart (Figure (a)). A-B: Dehumidification--as the moisture from the outdoor air is removed by sorption, the heat generated when the water is sorbed remains in the air stream, increasing the air stream's sensible load. There is a slight increase in the enthalpy (i.e., the energy content of the air stream increases), when latent heat is being converted into sensible heat. At state B, the air is hot and dry and cannot be directly used to cool the conditioned area.CoolingB-C: Heat loss or post-cooling--the dehumidified outdoor air enters the rotary heat wheel, where it exchanges heat with the exhaust (return) air stream from the conditioned space. In this process, the hot and dry outdoor air cools down, and the cold exhaust air is pre-heated for reactivating the desiccant wheel. C-D: Supplemental cooling--the air leaving the rotary heat wheel is colder than the air leaving the desiccant wheel, but further cooling is often required before it can enter the conditioned space. This can be achieved by using a conventional direct-expansion vapor compression cooling system. D-E: Space cooling load--the exhaust air leaving the conditioned space is at state E. RegenerationE-H: Heat recovery--the exhaust air stream enters the rotary heat wheel where it exchanges heat with the hot and dry air leaving the desiccant wheel. Part of the heat lost in step B-C is recovered by this process (Figure (b)). H-I: Heat addition--the hot exhaust air is further heated to increase the vapor pressure at the desiccant. I-J: Reactivation--the hot exhaust air stream dries and reactivates the saturated desiccant. For comparing the above process with that of a conventional system, the following steps for cooling and dehumidification with a conventional vapor compression system are shown on the psychrometric chart (Figure (a)): Sensible coolingA: Intake--hot and humid outdoor air enters the evaporator coil of a conventional vapor compression system at point A on the psychrometric chart (Figure (a)). A-F: Sensible cooling--the hot and humid outdoor air stream is cooled until it reaches saturation. At this point, the air is cold enough to be used in the conditioned space, but cannot be circulated because it is saturated with moisture. To remove moisture, the air must be cooled to below its dew-point temperature. Latent cooling (dehumidification) and reheatF-G: Dehumidification--the evaporator continues to cool the saturated air stream and condenses the moisture, further reducing the dry-bulb and the humidity. If the humidity requirement is low (less than 40 grains/lb of dry air), the air must be cooled to less than 43°F in order to condense enough moisture. In this state, it is too cold to be circulated to conditioned space.G-D: Reheat--the cold, dry air stream is mixed with hot air or reheated to the desired circulation temperature (state D).

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D-E: Cooling load--the exhaust air leaves the conditioned space (state E).

The amount of energy saved depends primarily on the ability of the hybrid system to shift part of the cooling load (dehumidification load) to a low-grade thermal source and to eliminate reheat (step G-D). The fan power is slightly increased because of increased air pressure drop through the desiccant and sensible wheels. The amount of energy saved and the reduction in electric demand depend on several factors.

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4 Purge Sections/Carryover

Figure 2. Purge section forces a PI to RI leak to prevent RI to PO carryover contamination inherent in wheel rotation.

(Ref [3])As the matrix rotates out of the regeneration airflow, it carries with it both regeneration air trapped in the flutes and heat, contained in the air and in the matrix itself. This amounts to a small, constant rotation leak or carryover from RI to PO, which is acceptable in most instances. Purging purposely misaligns one of the seals on the RI/PO face of the wheel to eliminate this leak by forcing a purge leak from PI to RI. Figure 2 diagrams the purge concept. Purge sections are commonly used in industrial applications when very low PO dew points are required.Purges can also be necessary in applications that demand minimal carryover of regeneration air into the supply air. In the case of carryover, the purge prevents regeneration air trapped in the flutes from carrying contaminants into the supply air. This could be a concern if the unit is directfired and if there are combustion products in the regeneration air, or if the regeneration air comes from an indoor or outdoor source that may have high levels of volatile organic compounds (VOCs) or other pollutants.Seal leakage and rotation carryover combined are not large enough to be a concern.

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Another potential concern is co-sorption. Co-sorption is the potential for desiccants to adsorb other chemicals with the water vapor. If the desiccant were able to pick up considerable amounts of undesirable chemicals from an exhaust flow and dump them back into the supply air, this would create a much more powerful carryover effect than wheel rotation could produce, and essentially concentrate the pollutants in the required space.This is very unlikely for several reasons.First of all, in actively regenerated systems, the pollutants would have to be picked up by the desiccant at elevated temperatures and released at low temperature, the opposite sense in which sorbents work.In passive systems, this reasoning does not apply because the regeneration air is not heated.There are two lines of reasoning for these enthalpy exchange systems.One is size exclusion. Pollutant molecules larger than the desiccant pores are physically excluded from adsorption, making carryover impossible. Three angstroms is sometimes cited as a practical pore size in which water vapor fits, but many pollutants cannot.The other is that co-sorption does not happen in the presence of water vapor. Sorption on the molecular level is a very electrically influenced phenomenon. Water vapor is a highly polar molecule; that is, it has strongly positive and negative ends. Analyses predict that desiccants will always adsorb the most polar molecules first. Experience shows this to be true. Even ammonia, which is moderately polar, is not picked up in appreciable quantities when water vapor is present. Carryover does not currently appear to be an issue for rotary desiccant equipment.

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5 References

[1] Shan K. Wang, Zalman Lavan, Paul Norton, 2000,“Air Conditioning and Refrigeration Engineering”, CRC Press

[2] W. P. Jones, 1994,“Air Conditioning Engineering”, Edward Arnold

[3] S.J. Slayzak and J.P. Ryan, 2000,“Desiccant Dehumidification Wheel Test Guide”NREL, U.S. Department of Energy Laboratory

[4] Yunus A. Cengel, Michael A. Boles, 1998,„Thermodynamics-An Engineering Approach“Mc Graw-Hill Higher Education

[5] Thomas H. Kuehn, James W. Ramsey, James L. Threlked, 1998,“Thermal Environmental Engineering”Prentice Hall Inc.

[6] http://www.pnl.gov/fta/8_tdd.htm - The U.S. Department of Energy: FederalTechnology Alerts

[7] http://www.advancedbuildings.org/_frames/fr_t_heat_desiccant_cooling.htm

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Desiccant Wheel Dehumidification 2