HVAC Systems II Transcript

8/11/2019 HVAC Systems II Transcript

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HVAC Systems II: All-Air Systems and Temperature Control

Slide 1: HVAC Systems IIWelcome to the course HVAC Systems II: All-Air Systems and Temperature Control.

Slide 2: ASHRAEThis course was produced with the support of the American Society of Heating, Refrigerating, and

Air-Conditioning Engineers, or ASHRAE.

Slide 3: How to Use This CourseFor the best viewing results, we recommend you maximize your browser window now. Thescreen controls allow you to navigate through the e-learning experience. Using your browsercontrols may disrupt the play of the normal course.

Click each button to learn more about the course navigation, or if you are already familiar with thisinterface, you may click the Next button to advance to the next screen.

Slide 4: ObjectivesThis course is the second in a series of three courses: HVAC Systems I-III. In the first course, youlearned the processes included in HVAC and examined a simple type of all-air system.

At the completion of this course, you will be able to explain the impact of constant volume,variable air volume (VAV), and reheat on the energy efficiency of all-air systems; describemethods for implementing zones in single-duct systems; explain the purpose of independentperimeter systems; describe an economizer cycle and its benefits; describe a simple dual-ductall-air system and its impact on energy efficiency; and describe a simple multizone all-air systemand its impact on energy efficiency.

In the following courses, you will look at some other configurations for all-air systems and look atalternatives for incorporating water as the thermal medium.

Slide 5: IntroductionHeating, ventilation, and air conditioning systems are essential to maintain a comfortable andproductive environment. One of the primary determinants of comfort in an environment is thetemperature. Therefore, effective energy-efficient temperature control is a high priority in HVACsystems.

Slide 6: Temperature ControlIn the previous course, you began looking at all-air systems. In all-air systems, temperaturecontrol is provided by varying either the quantityor the temperatureof the supplied air through the

use of constant flow volume, variable air volume, reheat, face and bypass dampers, or acombination of these.

Click each tab to learn more about these temperature control methods.

Constant Flow Volume:In the constant flow volume method, the same volume of air is continually supplied, but at higheror lower temperatures. Flow control of the heating or cooling source medium is used to vary theair temperature, for example by supplying more or less refrigerant, or chilled or hot water.


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VAV:In the VAV method, the temperature of the supplied air is kept constant, but the amount of air is

increased or decreased in order to manage the temperature in the zone.

Reheat:In the reheat method, the air is cooled to remove humidity, and then reheated according to thesensible heat load required in the building. Using this method is restricted by some building codesbecause of its poor energy efficiency.

Face and Bypass Dampers:In the face and bypass dampers method, part of the airflow is cooled to remove humidity, andthen blended with bypassed air to match sensible load.

Slide 7: Constant VolumeTake a look at the constant volume approach. In the single-duct, single-zone system you looked

at in the first course in this series, temperature control could be achieved by changing the supplyair temperature. In this case, the supply fan will run constantly, always supplying the samevolume of air to the space.

When the space needs more cooling, more cold water will be sent to the cooling coil. Becausemore water is moving through the coil, it will pick up heat from the airstream more quickly and theair will be colder. Notice that more cold water is being sent to remove the heat, not colder water,which still results in colder air.

When the space needs more heating, more hot water, or steam will be sent to the heating coil.

Constant volume systems tend to have poor energy efficiency because fans run continuously.


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Slide 8: Single-Duct, Zoned Reheat, Constant VolumeThe approach you saw in HVAC Systems I for a single-duct, single zone system is not very

flexible.

The reheat system is a modification that permits the space to be divided into different zones.Zone control is achieved by reheating the cool airflow to the temperature required for a particularzone.

Take a look at the schematic diagram for a typical primary air reheat system. As implied by theword reheat, heat is applied as a secondary process to centrally cooled air. The heating mediummay be hot water, steam, or electricity.

Slide 9: Terminal UnitsTerminal units are added to the distribution system to provide localized control for each zone.

Terminal units in reheat systems are located either under a window or in the overhead ductworksystem.

These units are designed to permit heating of primary air from the supply duct or secondary airthat is induced from the conditioned space. The conditioned air is supplied from a central unit at afixed cold air temperature designed to offset the maximum cooling load in any zone supplied bythe air handling unit.

A zone thermostat controls the amount of reheat required for the zone.

Reheat systems, when permitted, are used in a variety of locations, including hospital laboratories,office buildings, spaces with wide load variations, spaces with high latent loads, and process orcomfort applications where close thermal control of space conditions is required.


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Slide 10: Advantages and Disadvantages of Reheat SystemsReheat systems can provide excellent control over space humidity. However, using them shouldgenerally be avoided, due to their high energy consumption and operating costs.

Lets take a closer look of the advantages and disadvantages of these systems.

The advantages of reheat systems are:

They permit simple zone control for areas with unequal loads. They provide simultaneous heating or cooling of perimeter areas with different

exposures. They maintain very close control over space humidity. Generally, they can accomplish dehumidification independently of sensible cooling,

which other systems cannot. They offer the designer infinite zoning capabilities during design. And, they allow zones to be easily revised during construction or occupancy. Changes

require only the addition of a heating coil or terminal unit, and a thermostat.

The disadvantages of reheat systems are:

They have high energy consumption if not properly designed and controlled. They are generally considered the least efficient approach and many energy codes

severely restrict their use. They function by adding heat to cooled air to provide zone temperature control. This

basically wastes energy and is therefore uneconomical.

They require heating coils with piping or electrical supply for every zone.

Slide 11: Improving Reheat Energy EfficiencyThe economic problems of reheat systems can be reduced by certain methods.

Click each tab to learn more.

Vary Supply Air Volume:Varying supply air volume using a VAV approach is something you will look at in more detailshortly. A constant volume system results in maximum reheat energy usage. Allowing the airflowvolume to decrease as the space load decreases will eliminate reheat until a minimum airflow isreached. The minimum airflow is generally limited by the number of occupants, who will require acertain amount of fresh air to maintain indoor air quality, or by humidity control. This approach

results in a VAV reheat system.

Reset Supply Air Temperature:Resetting the supply air temperature is another approach. If it is cooler outside, or not as sunny,the system could provide cool air but at a slightly warmer supply temperature. Then reheat is lesslikely to be needed in most zones. Discriminating controls can be used to accomplish thisadjustment automatically.

Heat Recovery:


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A third approach is using heat recovery; for example, using heat rejected from the refrigerationcycle of another internal heat source. However, note that the air handler in a reheat system

initially cools air more than necessary to meet building loads. This results in a waste ofrefrigeration that is not mitigated by the use of heat recovery.

Slide 12: Variable Air Volume (VAV)Variable air volume can be used to achieve full heating or cooling flexibility in a much moreenergy-efficient way than comparable constant-volume systems. This air distribution method isgenerally considered to be the most efficient. It can be combined with reheat, and applied in dual-duct systems as well.

In these systems, the supply air temperature is kept constant and the airflow is adjusted. Duringcooling season, the space is provided with cool air to counteract the heat loads of the space.Spaces with greater heat loads are provided with more air.

Slide 13: Single-Duct , Single-Zone VAVA single duct, single zone VAV all-air system may be a local system where the system is withinthe zone and does not require much ductwork. Or it may be a central system where majorcomponents are at a distance from the zone and the air is distributed by a network of ducts.These systems are often used in:

Small department stores. Small individual stores in a shopping center. Individual classrooms in a school. Computer rooms. Hospital operating rooms. And, large open areas such as gymnasiums, atriums, and warehouses.


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Slide 14: Methods of Varying Supply Air VolumeA simple VAV system is used in cooling-only applications where simultaneous heating and

cooling in different zones is not required. An example is the interior of an office building withvarying cooling loads and no heat loss.

There are several methods of varying the system supply air volume in response to loads. Thesemethods include:

Speed control with variable drive. Dumping excess air. And, using a fan bypass.

Speed control is preferred because Variable Frequency Drives (VFDs) are reliable, relativelyinexpensive, and the most energy-efficient VAV option. Less energy is used when fan operationis slowed to match reduced airflow needs. This assumes that there are varying loads that would

result in reduced airflow needs from time to time. This is discussed in more detail in the courseson fans.

Dumping excess supply air is a method that keeps the fan speed and volume or air supplyconstant, but diverts excess air into a return air ceiling plenum or directly into the return air ducts.

A drawback is overcooling under low load conditions due to air leakage through the ceiling andradiant cooling from the ceiling surface. Another drawback is if supply air is used for systembalancing, there may not be enough air. Lastly, this is not an energy-efficient approach.

Using a fan bypass to reduce the supply air volume is a third approach; but again, this is notenergy efficient.


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Slide 15: Advantages and Disadvantages of a Single-Zone VAV SystemThe advantages of single-zone VAV systems include:

They can closely and effectively maintain temperature and humidity. They can be shut down when required without affecting other areas. They are fairly energy efficient. They are easy to control. And, they can be adapted to economizer cycles using more or less outside air, according

to what is best for energy efficiency.

The disadvantages of single-zone VAV systems include:

They only provide one zone. They respond to only one set of space conditions, and as a result, they are only suitable

when: Variations are approximately uniform throughout the building or area served. The load is stable, such as an interior room. Costs are a constraint, such as in a single-family residence.

Slide 16: Single-Duct Zoned VAVSingle-duct zoned VAV systems seek to overcome some of the flexibility disadvantages of asingle-zone system. Zones are served by VAV boxes, which change the volume flow of air intothe zone under the control of a zone temperature sensor. The VAV box is generally moresophisticated than a simple damper. Air from the main duct is at a relatively high and variablepressure compared with the outlet to the room. The pressure drop creates noise, especially at lowsupply volume. The box has a liner to attenuate the sound. The pressure variations also make the

flow varyindependent of the thermostat demandsif a simple damper is used. Instead, apressure independent box is required, containing an airflow sensor that maintains a constantairflow through the box until it is reset to a new value by the temperature requirements.

Slide 17: Throttling and ReheatFor a single-duct zoned VAV to be efficient, the distribution of airflow is throttled down until thehighest load zone has just enough air at the maximum opening. The other zones throttle downthe airflow as needed to their minimum set flow rate. If this cooling rate is still too much, thesupply air can be reheated by a terminal unit at the point of entering the space, or blended withunconditioned room return air.

VAV reheat or VAV dual-duct can be applied to both interior and perimeter spaces.

A VAV system with reheat allows for control of both temperature and humidity within a space.

Slide 18: Humidity ControlIn a single-duct system:

Without reheat, temperature and humidity control offers sensible cooling flexibility, butcannot provide humidity control independent of temperature.

With reheat, there is flexibility for both temperature and humidity control by using acooling coil to cool supply air to the desired humidity, and a reheat coil to raise the dry-


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bulb temperature to the desired value. A VAV system with reheat allows for control ofboth temperature and humidity within a space.

Slide 19: Pressure DistributionAir distribution is very important in VAV systems because supply air volumes are continuallychanging. In many countries, one basic design goal is to size supply and return fans along withthe distribution system to maintain a slight positive pressure in the building spaces. Alternatively,such as in Nordic countries, there may be a goal to maintain a slight negative pressure.

A problem is that the positions of zone control dampers in VAV systems are constantly changing.Consequently, a single combination of supply fan, return fan, and air distribution systemparameters cannot meet the pressure requirements at all operating conditions.

The best solution is to control the fan pressure via a variable frequency drive. This has the addedadvantage of energy conservation. For example, here is an illustration of a typical pressuredistribution. Reducing the capacity will cause a drop in pressure at points A and B. The resultingpressure can be monitored and the VFD can change the speed to ensure that the deviation fromthe desired zone pressure is minimal.

Slide 20: Independent Perimeter SystemsSome VAV systems incorporate an independent perimeter system.

A VAV system serves the interior spaces and handles the building envelope solar gains. Thisprovides only cooling throughout the year.

A second independent system serves the perimeter non-solar loads. This could be a water-basedheating system that functions only in winter or an electric baseboard radiation system. It couldalso be a separate constant-volume air system, using an outdoor or indoor temperature scheduleand constant-volume air supply to offset gains or losses from heat flowing by conduction orconvection at the envelope.


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Slide 21: Independent Perimeter SystemsSince the VAV system takes care of the internal zone load variations and all outdoor airventilation requirements, the perimeter system does not require zone control (except to make itmore economical) or an outdoor air component.

The electric or water-based heating-only systems would function only in the winter, and are thepreferred envelope isolation system for cold climates.

Coordination between the two systems will be important to ensure overall energy efficiency.

Also, note that envelope isolation systems are not generally required in buildings with energy-efficient envelope assemblies, especially high-performance glazing.

Slide 22: Fan-Powered Terminals

Fan-powered terminals provide a minimum or constant airflow to a particular zone, while thevolume of conditioned primary air that is supplied is varied. The airflow is kept constant byrecirculating return air and keeping the sum of the primary air that meets the load and therecirculated air constant.

Heating elements can be included in the terminal.

They are commonly used in zones with large variations in internal load, such as conferencerooms or occupied spaces during periods of reduced cooling load, in order to ensure good aircirculation.

Slide 23: Air-Side Economizer CycleAn air-side economizer cycle is used when the total heat content, including both sensible heat

and latent heat from the humidity of the outdoor air, is lower than the return air. Or you might saysimply when it is colder and/or drier outside than inside.

Conventional air conditioning has to bring in some outdoor air in order to ventilate the space andreduce contaminants. Since the outdoor air may be hot and require energy to be cooled, thesystem brings in as little outdoor air as possible. It recirculates indoor air which has already beencooled once. However, if the outside air is actually cooler or drier than the inside air, thisapproach wastes energy. When outside air is cooler or drier, we wantto bring it into the buildingfor free cooling.

The economizer reduces the energy used for cooling by taking in more cool outdoor air than isrequired for ventilation, and releasing the excess warm return air to the outdoors. This can save alot of energy. When it is hot outside, the minimum amount of warm outdoor air needed forventilation is brought in to keep the maximum of conditioned air inside the space and minimizethe amount of conditioning that has to be done.

The economizer requires a large outdoor air intake and relief of exhaust components, which arenot justified if favorable conditions rarely occur.


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Slide 24: Advantages and Disadvantages of Zoned VAV SystemsIn summary, the advantages of zoned VAV systems include:

They are easy to control and allow good zone control. They can be highly energy efficient.


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They can adapt to an economizer cycle. They have good flexibility for zone changes. And, they are suitable for offices, classrooms, and other applications.

All of these advantages make them the system of choice for most commercial and institutionalbuildings.

However, there are disadvantages of zoned VAV systems as well, such as:

Humidity control under widely varying latent load can be difficult. There is a possibility of poor ventilation, which may result in unacceptable indoor air

quality, particularly under low zone loads. And, control coordination with heating systems can be complex.

Slide 25: Dual-Duct All-Air Systems

Let's move on from the single-duct systems and examine another approach.

Dual-duct systems distribute air from a central air handler to the conditioned spaces through twoparallel ducts. Zone control is achieved by mixing air from the two air streams in a terminal mixingbox.

In multizone systems, which you will look at next, the requirements of the different zones of abuilding are achieved by mixing cold air and warm air and using dampers at the central air-handling unit in response to zone thermostats.

Slide 26: Advantages and Disadvantages of Dual-Duct All-Air SystemsThe advantages of dual-duct all-air systems include:

They do a good job of controlling temperature and humidity. They can accommodate a variety of zone loads. They facilitate adding zones or subdividing existing zones.


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And, they can be adapted to constant or variable volume.

The disadvantages of dual-duct all-air systems include:

Energy consumption is relatively high. Substantial space is required to accommodate two sets of ducts running throughout the

building. Ductwork not only takes up space, but is extensive and expensive. A large number of terminal mixing boxes are required, which are expensive and may

require maintenance. And, these systems hinder the implementation of economizer cycles.

Slide 27: App licationDual-duct all-air systems were extensively used in office buildings in the 1950s and 1960s.

They can produce good results in moderately humid climates where the outdoor designconditions do not exceed 25C or 78F wet bulb and 35C or 95F dry bulb, and the minimumoutdoor air is not more than 35 to 40 percent of total air.

These systems are less popular because of their high energy consumption and high initial cost.The energy consumption can be reduced by applying VAV control to reduce total supply airduring periods of reduced cooling and heating loads.

Slide 28: Using VAV to Reduce Energy ConsumptionThis illustration shows that VAV controls can be employed in the summer cooling mode. It also

shows reheat in the terminal unit. In a dual-duct system, this would be replaced by hot duct air.Understand that there are many variations of the dual-duct system, some with two supply fans,and some with a single supply fan and terminal reheat applied from heat recovery, instead ofheating at the point of entry.

Click the glowing markers to learn more about VAV controls.

In Summer

In summer, cold air supply should be kept just low enough to meet the space cooling anddehumidification (dew point) requirements. This is usually around 10C to 13C or 50F to 55Fwith the air nearly saturated. Warm air temperature is governed by the return air from theconditioned spaces. The hot duct temperature will always be higher than the average return air

temperature, even if no heat is added by the heating coil. The higher temperature is due to theheat contributed by outdoor air, fan energy, and recessed lighting fixtures.

In Winter

In winter, cold air temperature is sometimes reset 3C to 6C or 5F to 10F higher for economyof operation. If internal loads are small, outdoor air may be used for the cold duct supply,permitting shutdown of the chiller. Maintaining the cold air during the heating season at 13C to16C or 55F to 60F and raising the warm air temperature as the outdoor temperature decreases


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permits better humidity control and better air balancing between the hot and cold ducts. However,this increases the energy consumption. The warm air temperature can be adjusted based on the

outdoor temperature.

When the cold air temperature is lower than it needs to be in order to satisfy the coolingrequirement of the warmest zone, an energy penalty results. Conversely, when the warm airtemperature is higher than it needs to be in order to satisfy the heating requirement of thecoldest zone, this also imposes an energy penalty.

Slide 29: Multizone SystemsA multizonesystem is a specific type of HVAC system. It doesnt mean any system arrangementwith more than one zone. These systems have been successfully applied to small- and medium-size commercial buildings, and to larger buildings with relatively few zones.

You should understand how the requirements of the different zones of a building are met in a

multizone system. Navigate through this guided diagram with the arrows in the upper right.

Mixing DampersCold air and warm air produced at the air handling unit are mixed to provide appropriate supplyair temperatures, using dampers at the air handling unit.

ThermostatsThe dampers are controlled by zone thermostats.

DuctsThe mixed conditioned air is distributed to specific zones throughout a building by a system ofmultiple single-zone ducts. The distribution system downstream of the air handler is a collectionof zone-dedicated ducts. There will be as many ducts as there are zones, to a typical maximum of

about 12 zones.

Return AirReturn air is usually handled in a conventional manner by combining the return air from all zones.

Slide 30: Advantages/Disadvantages of Multizone SystemsExamine the advantages and disadvantages of multizone systems.

Click each tab to learn more.

Advantages:

The advantages of multizone systems include the following:

They provide good control over temperature. Only ducts and diffusers, plus thermostats, are located outside the mechanical room. They provide easy system control.

Disadvantages:

The disadvantages of multizone systems include the following:

They require many individual ducts, which limits the number of zones.


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They can have high energy consumption. Temperature control of the hot deck is a concern. Implementing an economizer cycle is difficult.

Slide 31: VariationsTwo-fan arrangements for VAV control are not common with multizone systems due to higher firstcosts, lack of packaged equipment with such variations, and the ability of the basic system toprovide reasonable humidity control. When VAV approaches are used, they are seldom applied toan entire distribution system, except for television studios and other noise-critical applications.

Slide 32: Similarity to Dual-DuctThe multizone system is conceptually similar to the dual-duct system in many respects. Multizonesystems using a variety of packaged equipment can provide a small building with some of theadvantages of dual-duct systems at a lower first cost. Most packaged air handling units, however,

lack the sophisticated control for comfort and operating economy that can be built into dual-ductsystems.

Slide 33: SummaryThat concludes the course HVAC Systems II: All-Air Systems and Temperature Control. Take amoment to review what you have learned.

Temperature control may be achieved by constant volume with variable temperature, variable airvolume, reheat, face and bypass dampers, or a combination of methods.

Constant volume systems tend to have poor energy efficiency because of the continual fanoperation.


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Reheat functions by adding heat to cooled air to provide zone temperature control. This is

basically wasteful of energy and therefore considered to be the least efficient and mostuneconomical approach.

Variable air volume is more energy efficient, especially when variable speed is used to controlairflow.

Systems may support a single zone or be divided into multiple zones. Single-duct and dual-ductsystems can support multiple zones through terminal units or VAV boxes.

An independent perimeter system may be used to take care of heat loads at the buildingenvelope.

An economizer cycle can be used in a single-duct system to take advantage of relatively cool and

dry outside air for cooling the building.

Dual-duct systems tend to be inefficient because separate airflows are heated and cooled, andthen mixed at the point of entry to the space.

Multizone systems can also have high energy consumption because they produce both cold andwarm air at the air handling unit, and then mix the air and distribute it to multiple single-zoneducts.

And finally, compared to constant volume, reheat, dual duct, and multizone systems, variable airvolume is the most efficient approach.

Slide 34: Thank You!Thank you for participating in this course.

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HVAC Systems II Transcript