ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 4-2012 AIR HANDLING AND DISTRIBUTION.pdf

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1、4.1CHAPTER 4AIR HANDLING AND DISTRIBUTIONAir-Handling Units 4.3Air-Handling Unit Psychrometric Processes . 4.4Air-Handling Unit Components. 4.6Air Distribution 4.10AIR-HANDLING SYSTEMS. 4.10Single-Duct Systems. 4.10Dual-Duct Systems. 4.12Multizone Systems 4.13Special Systems 4.13Terminal Units. 4.15

2、Air Distribution System Controls . 4.17Automatic Controls and Building Management Systems . 4.18Maintenance Management System. 4.18Building System Commissioning. 4.18ERY early in the design of a new or retrofit building project,Vthe HVAC design engineer must analyze and ultimately selectthe basic sy

3、stems, as discussed in Chapter 1, and whether produc-tion of primary heating and cooling should be decentralized (seeChapter 2) or central (see Chapter 3). This chapter covers theoptions, processes, available equipment, and challenges of all-airsystems; for all-water, air-and-water, and local termin

4、al systems,see Chapter 5. For additional system selection tools, refer to theHVAC System Analysis and Selection Matrix in ASHRAE Hand-book Online+ (https:/handbook.ashrae.org).Building air systems can be designed to provide complete sensi-ble and latent cooling, preheating, dehumidification, and hum

5、idifi-cation capacity in air supplied by the system. No additional coolingor humidification is then required at the zone, except for certainindustrial systems. Heating may be accomplished by the same air-stream, either in the central system or at a particular zone. In someapplications, heating is ac

6、complished by a separate heat source. Theterm zone implies the provision of, or the need for, separate thermo-static control, whereas the term room implies a partitioned area thatmay or may not require separate control.The basic all-air system concept is to supply air to the room atconditions such t

7、hat the sensible and latent heat gains in the space,when absorbed by supply air flowing through the space, bring the airto the desired room conditions. Because heat gains in the space varywith time, a mechanism to vary the energy removed from the spaceby the supply air is necessary. There are two su

8、ch basic mecha-nisms: (1) vary the amount of supply air delivered to the space byvarying the flow rate or supplying air intermittently; or (2) vary thetemperature of air delivered to the space, either by modulating thetemperature or conditioning the air intermittently.All-air systems may be adapted

9、to many applications for comfortor process work. They are used in buildings of all sizes that requireindividual control of multiple zones, such as office buildings,schools and universities, laboratories, hospitals, stores, hotels, andeven ships. All-air systems are also used virtually exclusively in

10、special applications for close control of temperature, humidity,space pressure, and/or air quality classification (e.g., ISO 14644-1Class 3 space), including cleanrooms, computer rooms, hospitaloperating rooms, research and development facilities, and manyindustrial/manufacturing facilities.Advantag

11、es Operation and maintenance of major equipment can be per-formed in an unoccupied area (e.g., a central mechanical room).It also maximizes choices of filtration equipment, vibration andnoise control, humidification and dehumidification options, andselection of high-quality and durable equipment, in

12、cludingenhanced filtration.Piping, electrical equipment, wiring, filters, and vibration- andnoise-producing equipment are away from the conditioned area,minimizing (1) disruption for service needs and (2) potentialharm to occupants, furnishings, and processes.These systems offer the greatest potenti

13、al for using outdoor air foreconomizer cooling instead of mechanical refrigeration.Seasonal changeover is simple and adapts readily to automaticcontrol.A wide choice of zoning, flexibility, and humidity control underall operating conditions is possible. Simultaneous heating of onezone and cooling of

14、 another zone during off-season periods isavailable.Air-to-air and other heat recovery may be readily incorporated.Designs are flexible for optimum air distribution, draft control,and adaptability to varying local requirements.The systems are well-suited to applications requiring unusualexhaust or m

15、akeup air quantities (negative or positive pressuriza-tion, etc.).All-air systems adapt well to winter humidification and dehumid-ification for high latent loads.All-air systems take advantage of load diversity. In other words, acentral air-handling unit serving multiple zones needs to be sizedonly

16、for the peak coincident load, not the sum of the peak loads ofeach individual zone. In buildings with significant fenestrationloads, diversity can be significant, because the sun cannot shineon all sides of a building simultaneously.By increasing the air change rate and using high-quality controls,t

17、hese systems can maintain the closest operating condition of0.25F dry bulb and 0.5% rh. Some systems can maintainessentially constant space conditions.Removal and disposal of cold condensate from cooling coils, andcapture and return of steam condensate from heating coils, is gen-erally simpler and m

18、ore practical in an all-air system.Operation and maintenance costs of central air-handling equip-ment are less than for many other terminal systems.DisadvantagesDucts installed in ceiling plenums require additional duct clear-ance, sometimes reducing ceiling height and/or increasing build-ing height

19、. In retrofits, these clearances may not be available.Larger floor plans may be necessary to allow adequate space forvertical shafts (if required for air distribution). In a retrofit appli-cation, shafts may be impractical.Transport energy used by the fans to distribute air and overcomeduct and equi

20、pment static resistance is a larger part of the totalbuildings HVAC energy use than in other systems.In commercial buildings, air-handling equipment rooms representnonrentable or non-revenue-generating spaces.The preparation of this chapter is assigned to TC 9.1, Large Building Air-Conditioning Syst

21、ems.4.2 2012 ASHRAE HandbookHVAC Systems and Equipment Accessibility to terminal devices, duct-balancing dampers, etc.,requires close cooperation between architectural, mechanical,and structural designers. Accessible ceilings are recommended.Air balancing, particularly on large systems, can be cumbe

22、rsome.Permanent heating is not always available sufficiently early toprovide temporary heat during construction.Mechanical failure of a central air-handling component, such as afan or a cooling-coil control valve, affects all zones served by thatunit.Heating and Cooling CalculationsBasic calculation

23、s for airflow, temperatures, relative humidity,loads, and psychrometrics are covered in Chapters 1 and 17 of the2009 ASHRAE HandbookFundamentals. System selection shouldbe related to the need, as indicated by the load characteristics. Thedesigner should understand the operation of system components,

24、their relationship to the psychrometric chart, and their interactionunder various operating conditions and system configurations. Thedesign engineer must properly determine an air-handling systemsrequired supply air temperature and volume; outdoor air require-ments; desired space pressures; heating

25、and cooling coil capacities;humidification and dehumidification capacities; return, relief, andexhaust air volume requirements; and required pressure capabilitiesof the fan(s).The HVAC designer should work closely with the architect tooptimize the building envelope design. Close cooperation of all p

26、ar-ties during design can result in reduced building loads, which allowsthe use of smaller mechanical systems.ZoningExterior zones are affected by weather conditions (e.g., wind,temperature, sun) and, depending on the geographic area and sea-son, may require both heating and cooling at different tim

27、es. Thesystem must respond to these variations. The need for separateperimeter zone heating is determined by the following:Severity of heating load (i.e., geographic location)Nature and orientation of building envelopeEffects of downdraft at windows and radiant effect of cold glasssurfaces (i.e., ty

28、pe of glass, area, height, U-factor)Type of occupancy (i.e., sedentary versus transient).Operating costs (i.e., in buildings such as offices and schools thatare unoccupied for considerable periods, fan operating cost can bereduced by heating with perimeter radiation during unoccupiedperiods rather t

29、han operating the main or local unit supply fans.)Separate perimeter heating can operate with any all-air system.However, its greatest application has been in conjunction with VAVsystems for cooling-only service. Careful design must minimizesimultaneous heating and cooling. See the section on Variab

30、le AirVolume for further details.Interior spaces have relatively constant conditions because theyare isolated from external influences. Cooling loads in interior zonesmay vary with changes in the operation of equipment and appliancesin the space and changes in occupancy, but usually interior spacesr

31、equire cooling throughout the year. A VAV system has limitedenergy advantages for interior spaces, but it does provide simple tem-perature control. Interior spaces with a roof exposure, however, mayrequire treatment similar to perimeter spaces that require heat.Space HeatingAlthough steam is an acce

32、ptable medium for central system pre-heat or reheat coils, low-temperature hot water provides a simpleand more uniform means of perimeter and general space heating.Individual automatic control of each terminal provides the idealspace comfort. A control system that varies water temperatureinversely w

33、ith the change in outdoor temperature provides watertemperatures that produce acceptable results in most applications.For best results, the most satisfactory ratio can be set after installa-tion is completed and actual operating conditions are ascertained.Multiple perimeter spaces on one exposure se

34、rved by a centralsystem may be heated by supplying warm air from the central sys-tem. Areas with heat gain from lights and occupants and no heat lossrequire cooling in winter, as well as in summer. In some systems,very little heating of return and outdoor air is required when thespace is occupied. L

35、ocal codes dictate the amount of outdoor airrequired (see ASHRAE Standard 62.1 for recommended outdoorair ventilation). For example, with return air at 75F and outdoor airat 0F, the temperature of a 25% outdoor/75% return air mixturewould be 53.8F, which is close to the temperature of air supplied t

36、ocool such a space in summer. In this instance, a preheat coil installedin the minimum outdoor airstream to warm outdoor air can produceoverheating, unless it is sized so that it does not heat the air above 35to 40F. Assuming good mixing, a preheat coil in the mixed air-stream prevents this problem.

37、 The outdoor air damper should bekept closed until room temperatures are reached during warm-up.Low-leakage dampers should be specified. A return air thermostatcan terminate warm-up.When a central air-handling unit supplies both perimeter andinterior spaces, supply air must be cool to handle interio

38、r zones.Additional control is needed to heat perimeter spaces properly.Reheating the air is the simplest solution, but is often restrictedunder energy codes. An acceptable solution is to vary the volume ofair to the perimeter and to combine it with a terminal heating coilor a separate perimeter heat

39、ing system, either baseboard, overheadair heating, or a fan-powered terminal unit with supplementalheat. The perimeter heating should be individually controlled andintegrated with the cooling control. Lowering the supply watertemperature when less heat is required generally improves temper-ature con

40、trol. For further information, refer to Chapter 13 in thisvolume and Chapter 47 of the 2011 ASHRAE HandbookHVACApplications.Air Temperature Versus Air QuantityDesigners have considerable flexibility in selecting supply airtemperature and corresponding air quantity within the limitations ofthe proced

41、ures for determining heating and cooling loads. Thedifference between supply air temperature and desired room tem-perature is often referred to as the T of the all-air system. The rela-tionship between T and air volume is approximately linear andinverse: doubling the T results in halving of the air

42、volume.ASHRAE Standard 55 addresses the effect of these variables oncomfort.The traditional all-air system is typically designed to deliverapproximately 55F supply air, for a conventional building with adesired indoor temperature of approximately 75F. That supply airtemperature is commonplace becaus

43、e the air is low enough in abso-lute moisture to result in reasonable space relative humidity in con-ventional buildings with modest latent heat loads. However, lowersupply air temperatures may be required in spaces with high latentloads, such as gymnasiums or laundries, and higher supply air tem-pe

44、ratures can be applied selectively with caution. Obviously, not allbuildings are conventional or typical, and designers are expected notto rely on these conventions unquestioningly. Commercially avail-able load calculation software programs, when applied correctly,help the designer find the optimum

45、supply air temperature for eachapplication.In cold-air systems, the supply air temperature is designed sig-nificantly lower than 55F (perhaps as low as 44F) in an effort toreduce the size of ducts and fans. In establishing supply air temper-ature, the initial cost of lower airflow and low air temper

46、ature(smaller fan and duct systems) must be calculated against potentialproblems of distribution, condensation, air movement, and de-creased removal of odors and gaseous or particulate contaminants.Terminal devices that use low-temperature air can reduce the airAir Handling and Distribution 4.3distr

47、ibution cost. These devices mix room and primary air to main-tain reasonable air movement in the occupied space. Because theamount of outdoor air needed is the same for any system, the per-centage in low-temperature systems is high, requiring special carein design to avoid freezing preheat or coolin

48、g coils.Advantages of cold-air systems include lower humidity levels inthe building, because colder air has a lower maximum absolutemoisture content, and reduced fan energy consumption. However,these low-temperature air supply systems might actually increaseoverall building energy consumption, becau

49、se the cold-air processstrips more moisture from the air (i.e., greater latent heat removal)than is otherwise required in comfort applications. Again, commer-cially available software can help the designer evaluate the overallenergy effects of these decisions.Space PressureDesigners faced with the need to provide space pressure controlalong with temperature, humidity, and/or air filtration control willmost likely find that all-air systems are the only systems capable ofachieving this pressure control. Many special applications, such asisolation rooms, research labs, and