HVAC Design Overview Of Variable Air Volume Systems

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PDHonline Course M252 (4 PDH)HVAC Design Overview ofVariable Air Volume SystemsInstructor: A. Bhatia, B.E.2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPDH Course M252www.PDHonline.orgHVAC Design Overview of Variable Air Volume SystemsA. Bhatia, B.E.VARIABLE AIR VOLUME SYSTEMSIn central air conditioning systems there are two basic methods for delivering air to theconditioned space 1) the constant air volume (CAV) systems and 2) the variable air volume(VAV) systems. As the name implies, constant volume systems deliver a constant airvolume to the conditioned space irrespective of the load with the air conditioner cycling onand off as the load varies. The fan may or may not continue to run during the off cycle. VAVsystems, on the other hand, are designed to simultaneously meet a variety of cooling andheating loads in a relatively efficient manner. The system achieves this by varying thedistribution of air depending on the cooling or heating loads of each area. The air flowvariation allows for adjusting the temperature in a single zone without changing thetemperature of air in the whole system, minimizing any instances of overcooling oroverheating. This flexibility has made this one of the most popular HVAC systems for largebuildings with varying conditioning needs such as office buildings, schools, or apartments.How a VAV system works?What distinguishes a variable air volume system from other types of air delivery systems isthe use of a variable air volume box in the ductwork. The most basic VAV box consists of anenclosure with a small air valve (damper) that regulates the air flow in response to theroom's thermostat.In a VAV system, an air handling unit (AHU) cools or heats air to accommodate the zonewith the most extreme requirements, supplying the air through ducts to various zones. Atthe individual zone or space, the amount of air to be provided is regulated by air valveswithin a VAV box or terminal. As the load decreases in a particular zone, the VAV air valvethrottles the airflow matching the space requirements. Also when there is a sudden increasein load, the temperature sensor located in the zone will detect a rise in temperature andrequest the VAV box to open the air valve and increase the amount of cool air flow.When a VAV box air valve closes, the static pressure in the adjacent runout and trunk ductwill increase, resulting in increased airflow through the adjacent ductwork. This change inairflow will affect the space temperature in the new area supplied by that ductwork becausea higher volume of air (usually cooled) is now flowing into the area. The space thermostat inthis area will eventually sense this change in temperature, and re-position its air valve for 2007 A. BhatiaPage 2 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgreduced flow. This reduced flow will cause a further increase in static pressure in adjacentductwork and increase flow even further to other air terminals. This effect continuesunabated and will eventually cause unbalance in the entire air distribution ductwork.It is apparent that space thermostats alone can never stabilize a space temperature.Therefore, an additional static pressure control is added in the system, which maintains theduct static pressure within a set range. A static pressure sensor senses the increase in ductstatic pressure and provides signal to fan controller to reduce the speed. A reduction in fanspeed will reduce the airflow in direct proportion to the speed. At this point, it is importantto revisit “fan laws”:1. Volume varies directly with speed ratioCFM1 /CFM2 RPM1 /RPM22. Pressure varies with square of speed ratioSP1 /SP2 (RPM1 /RPM2)23. Power varies with cube of speed ratioW1 /W2 (RPM1 /RPM2)3ExampleA fan installed in a fixed system is operating at: CFM 10,000 SP 1.50" BHP 5.00 RPM 1,000How will the SP, BHP and RPM vary, if the fan is required to move 25% less air (7500 CFM)through this system?By rearranging the CFM fan law:RPM2 [CFM2 /CFM1] x RPM1RPM2 [7500/10000] x 1000 750 RPMThe corresponding static pressure is:SP2 [RPM2 /RPM1]SP2 [750 /1000] 2007 A. Bhatia22x SP1x 1.5 0.843”Page 3 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgThe resulting BHP is:BHP2 [RPM2 /RPM1]BHP2 [750 /1000]33x BHP1x 5 2.11In this example the static pressure reduces with the fan speed. But note that in VAVsystem, the static pressure sensor keeps the duct pressure constant within the set range. Itis only when the VAV box air valve closes; the duct pressure increases beyond the set pointand the sensor simply signals the fan to reduce RPM to the point where the duct staticpressure reaches within the set-point.VAV SystemIn the schematic above, the VAV system brings outside air and return air to the Air Handling Unitwhere both are mixed. The mixed air is drawn through a cooling coil, which drops the temperature toa fixed supply air temperature. The temperature in the individual rooms (#1 & #2) is measured bythermostats, which directly control the dampers in the VAV units. The supply air fan isspeed controlled by a variable speed drive, which controls the air volume by keeping theduct static pressure constant. The pressure is measured by the sensor (P) locatedapproximately 2/3rd of the way down the main duct starting air handling unit. As the zonedampers throttle back, the duct pressure rises, and the fan is controlled to reduce the ductpressure.When VAV systems are equipped with a return fan, some form of fan tracking control isrequired to keep the CFM leaving the conditioned building in a controlled relationship to the 2007 A. BhatiaPage 4 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgCFM being supplied to the building. There are several ways of doing this. One method is tomaintain a constant CFM differential between supply and return air flows. Another method isto vary the return fan CFM to maintain a pre-determined space pressure. You mayencounter either method in the field. We will study this, later in this course.VAV Systems – BenefitsHere are a few of the advantages of VAV systems:1. Efficiency –VAV systems are low on energy consumption. During lean periods, the air handling unitfan can run at a slower speed (through the use of a variable speed drive) and theamount heating and cooling mediums running through the coil can be reduced (through2-way modulating valves or solenoid control valves). A VAV system allows the airhandling unit, chillers, and boilers to run a part load majority of time.2. Individual Temperature Control –VAV system provides optimum control of temperature especially in the areas with wideload variations. Since each VAV box is linked to a thermostat, the ability to controltemperature in a space is independent of other spaces. Rooms with similar loadingpatterns are often placed on the same VAV box, and through the use of a VAV system,areas with very different loading patterns can be placed on the same air handling unit.3. Humidity Control –Humidity refers to moisture content in the air. It must be controlled in conjunction withair temperature to provide a comfortable environment. VAV systems do an excellent jobof controlling building humidity since the air handler delivers constant-temperature, lowdew-point air at all load conditions.4. Cost –Variable-air-volume (VAV) systems are sized for a diversified "block" load (peakinstantaneous demand) rather than a sum of zone "peak" loads. As a result theequipment sizes (air handling units/refrigeration plant/ducts etc) are lowered thusreducing the first initial cost.5. Flexibility –A VAV system is capable of serving multiple zones with varying load demands; thisallows a single air-handling unit to handle varying loads in an economical way. 2007 A. BhatiaPage 5 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgVAV systems are popular because they can easily accommodate added control zones.VAV Systems are virtually self balancing since boxes are set for maximum and minimumCFM. This feature almost completely eliminates costly and time consuming air quantitysetting procedures.DISADVANTAGES1. Each zone requires a terminal unit. This may cause spacing problems in smaller zones.2. Terminal units must be in an easily accessible location for its particular zone, which maycomplicate the system design.3. Terminal units will require a power source to operate, resulting in extra electrical wiring.4. Terminal units will require periodic maintenance in addition to usual attention paid to thecentral unit.5. VAV systems may require a secondary mechanical system to meet peak demands. Also,a secondary system may be necessary to provide heating to zones when the majority ofthe building requires cooling, or vice versa.6. Circulation may become a problem when zone needs are met using minimal air flow.DESIGN CHALLENGES1. Inadequate ventilation - Providing proper ventilation with a simple VAV systempresents a challenge. ASHRAE Standard 62-1989, "Ventilation for Acceptable Indoor AirQuality," mandates minimum outdoor air requirements of 20 CFM/person for ventilation.Since with VAV systems, the amount of air delivered varies with load, restricting the airflow can lead to inadequate outside air flow. The result is stuffiness and discomfort.2. Poor Air Distribution - As a VAV system reaches its design set-point, the volume of airdelivered to a room is decreased. This affects the air distribution. A standard diffusermay work well for constant volume applications, but not so well at part load airvelocities. When airflow is reduced, the distribution pattern is changed and can causestratification or drafts to occur.3. Control problems - (1) How to sense small changes in static pressure, (2) How tobalance return air systems with variations in supply air, and (3) How to maintain aconstant flow of outside air with variations in supply and return airflows.4. Space Requirements - The installation of a VAV box requires considerable space, bothin the vertical and horizontal directions. As a rule of thumb, the linear duct length before 2007 A. BhatiaPage 6 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orga VAV box should be three times the diameter of the inlet. This length is required for theair profile in the duct to even out before entering the box. In the vertical direction, theVAV box can require up to 18", which can be a problem if above ceiling heights arerelatively small.5. VAV Box Location - Since VAV boxes must be located in the branch ductwork, they areoften placed above the ceiling. In situations where an acoustical ceiling is used, there iseasy access to the box. However, in situations where a plaster ceiling is present, anaccess panel must be provided to allow access to the box.It is important to note that the shortcomings of the applied VAV technology are not intrinsicand these shall have limited impact if the design, installation and operation of the systemare properly addressed.GENERAL APPLICATIONSVariable air volume systems are very effective in medium to large scale buildings thatcontain many smaller HVAC zones such as office buildings, schools, apartment buildings, orhospitals. These applications require a mechanical system capable of handling large airvolumes while paying attention to the specific needs of each separated area.Inappropriate Uses for a VAV SystemNot acceptable for some critical areas such as hospitals because, at low load conditions, lessair is discharged from supply outlets. This may not meet strict ventilation, humidity controland pressure regulation requirements to prevent infiltration of outside air in a cleanenvironment.Laboratories/Hospitals and Clean room applications- Traditional VAV systems would not be agood choice to use in a situation where pressurizing spaces is critical. For instance, ahospital hallway serving isolation rooms must be positively pressured so that contagiousgerms do not seep out of the isolation room. Since a traditional VAV system cannot assure aconstant volume of delivered air, it works poorly in positively pressured situations.VAV systems may not be as desirable for smaller scale structures or structures that requireuniform heating or cooling in a large area, because the added control mechanisms wouldadd to expenses while not adding any desired benefits. Structures such as single familyhomes or warehouses may find an alternative HVAC system to be much more economical.TYPES OF VAV SYSTEMSThere are many different types of VAV units: 2007 A. BhatiaPage 7 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.org1. Single Duct2. Dual Duct3. Reheat4. Fan Powered5. Series FanSingle Duct VAV systemA single duct VAV (or cooling only VAV) is one of the most basic air terminal unit consistingof a damper, actuator, flow sensor and selected controls. It is simply an enclosure withsingle inlet and single outlet with an air valve in between.Common Applications - It is used only in spaces where the variation in load is relativelysmall. It is most often used in an interior zone of the building for cooling only. Generally,these boxes should be used only where minimum ventilation is not a concern and,therefore, can be set at zero.Dual Duct VAVA dual duct VAV consists of two primary air inlet connections, two primary air dampers, inletvelocity sensors, and a zone controller mounted on the side of the box. The dual duct VAVterminal is supplied with both hot and cold air. When the space is calling for full cooling, thecold deck damper opens to its max design CFM while the hot deck damper goes closed. Theopposite happens when the space is calling for full heating. What makes this box so uniqueis that when the space temperature is satisfied the two dampers will mix its hot and cold airto produce the constant design flow without over heating or cooling the space.Common Applications – Exterior zones in buildings where overhead heating and cooling is desired but use ofauxiliary heat coil is not feasible. 2007 A. BhatiaPage 8 of 43

www.PDHcenter.com PDH Course M252www.PDHonline.orgAlso where zero to low minimum flow is acceptable during changeover.The following is a list of pros and con's for this type box: Keeps a constant airflow thru out the space. Will not allow air to get stagnant or stale. Allows more airflow to be filtered through main air handling unit. Brings in a more constant amount of fresh air (outside air). The Air Handling Unit will need to produce hot and cold air all year. A boiler and chiller will have to be run all year.Two main ducts from the Air Handler will have to be run through out the building to supplyboth hot and cold air, needing more above ceiling space.Reheat VAVA reheat VAV box is very similar to a single duct VAV box. The only difference here is thatthe electric heat strips or a hot water coil is added to the VAV box for heating. With adecrease in cooling load, the room thermostat resets the regulator to the minimum volumesetting. With a further decrease in load, the thermostat opens the reheat valve/coil.Common Applications –Exterior zones (adjacent to outside walls) where convective and radiated heat losses createan intermittent need for moderate heating as the terminal usually reheats at the minimumsetting. Interior zones where ventilation requirements preclude full shut-off of the terminalor minimum airflow require some added heat. 2007 A. BhatiaPage 9 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgThe following is a list of pro's and con's for this type of box. Has the ability to give the space cooling or heating without effecting temperatures inother areas. Gives the space a larger percentage of fresh air (outside air). Airflow is better filtered because all of the airflow is being supplied by the main airhandler. All airflow entering the space is being dehumidified by the main air handler. Airflow that has been cooled to 55 F then heated back up is very inefficient andexpensive to run. Boilers and hot water pumps would have to run all year round when using hot waterreheat.Bypass VAV BoxA bypass VAV box has one inlet and two outlets. One outlet is for supplying conditioned airto the space and other is open to the ceiling plenum for bypassing excess air. Thethermostat controls airflow to the space by varying the position of the volume regulatingdevice. If less air is required to the space, the regulating device closes down and divertssome of the air to the return ceiling plenum thru bypass opening. This type of VAV box hasa constant air flow input and therefore does not provide any energy saving benefit.Common Applications –Bypass terminals are used primarily with packaged roof-top air conditioning equipment witha direct expansion coil where zoning is desired, but relatively constant airflows across thesystem components (i.e. coils, fans) are required. This allows the coil to operate at 100%airflow at all times in order to avoid freeze-up. The system offers an economical VAV supplydesign with low first cost. It does not provide the energy saving advantages of variable fanvolume, but avoids the expense of a more sophisticated system. 2007 A. BhatiaPage 10 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgFan Powered VAVFan powered VAV terminal includes a fan in an enclosure. These units were originallydesigned and introduced for their ability to save energy. They take advantage of typical VAVsavings at the air handler and the chiller during the cooling periods, but the real savingskick in when heating is required. Fan powered terminals induce warm plenum air from theceiling and blend it with the primary air during the heating sequence when the VAV damperthrottles to the minimum set-point. This recaptures all the heat created in the zone andplenum by lights, occupants, solar loading and machinery or equipment such as computers,coffee machines, copiers etc. The fan powered unit returns this heat as free heating ratherthan wasting it back at the air handler. If additional heating is required, then supplementalheat is added to the sequence, but the unit still saves energy by warming blending air at75 F rather than reheating primary cooled air at 55 F.The addition of a fan to a VAV box also improves air movement at times when a space isnear its design temperature and supply air volumes are low. The addition of a fan, however,demands additional energy because the fan motor requires electric input for power.Common Applications1. Exterior zones where heating and cooling loads may vary considerably and occupancyvariations allow the central system to be shut-down or set-back during unoccupiedhours.2. Situations where central system economy is desired as central fans can be reduced insize because they only need to provide sufficient static to deliver air to the terminal.There are two different types of fan powered boxes; 1) parallel and 2) series.Parallel UnitsThe parallel fan-powered mixing box air terminal typically consists of a primary air inletconnection, a set of opposed blade primary air dampers, an inlet velocity sensor, a fan, abackdraft damper, a heater, and a damper (zone) controller mounted on the side of the box(refer figure below). 2007 A. BhatiaPage 11 of 43

www.PDHcenter.comPDH Course M252www.PDHonline.orgParallel units sometimes called variable volume units because the fan runs intermittently. Asthe room temperature decreases, the primary air damper closes towards its minimum flowsetting and the unit blower, situated in parallel outside the primary airstream is energized toprovide warm ceiling plenum air to the space. A further drop in space temperatureenergizes the supplementary heating coil. The resultant control provides variable volume airdiffusion to the space.The fan and VAV mixing box are aligned such that all the induced air enters the fan, but theconditioned air bypasses the fan and m

What distinguishes a variable air volume system from other types of air delivery systems is the use of a variable air volume box in the ductwork. The most basic VAV box consists of an enclosure with a small air valve (damper) that regulates the air flow in response to the room's thermostat.

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