ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 41-2012 EVAPORATIVE AIR-COOLING EQUIPMENT.pdf

1、41.1CHAPTER 41EVAPORATIVE AIR-COOLING EQUIPMENTDirect Evaporative Air Coolers 41.1Indirect Evaporative Air Coolers 41.3Indirect/Direct Combinations . 41.5Air Washers. 41.6Humidification/Dehumidification . 41.7Sound Attenuation . 41.9Maintenance and Water Treatment. 41.9HIS chapter addresses direct a

2、nd indirect evaporative equip-Tment, air washers, and their associated equipment used for aircooling, humidification, dehumidification, and air cleaning. Resi-dential and industrial humidification equipment are covered inChapter 22.Principal advantages of evaporative air conditioning include Substan

3、tial energy and cost savingsReduced peak power demandImproved indoor air qualityLife-cycle cost effectivenessEasily integrated into built-up systemsWide variety of packages availableProvides humidification and dehumidification when neededEasy to use with direct digital control (DDC)Reduced pollution

4、 emissionsNo chlorofluorocarbon (CFC) useFor same amount of cooling, less water is evaporated than withconventional air conditioningSound attenuationPackaged direct evaporative air coolers, air washers, indirectevaporative air coolers, evaporative condensers, vacuum coolingapparatus, and cooling tow

5、ers exchange sensible heat for latent heat.This equipment falls into two general categories: those for (1) aircooling and (2) heat rejection. This chapter addresses equipmentused for air cooling.Adiabatic evaporation of water provides the cooling effect ofevaporative air conditioning. In direct evap

6、orative cooling, waterevaporates directly into the airstream, reducing the airs dry-bulbtemperature and raising its humidity level. Direct evaporative equip-ment cools air by direct contact with the water, either by an extendedwetted-surface material (e.g., packaged air coolers) or with a seriesof s

7、prays (e.g., an air washer).In indirect evaporative cooling, secondary air removes heatfrom primary air using a heat exchanger. In one indirect method,water is evaporatively cooled by a cooling tower and circulatesthrough one side of a heat exchanger. Supply air to the space passesover the other sid

8、e of the heat exchanger. In another commonmethod, one side of an air-to-air heat exchanger is wetted andremoves heat from the conditioned supply airstream on the dry side.Even in regions with high wet-bulb temperatures, indirect evapora-tive cooling can be economically feasible. This is especially t

9、rue ifbuilding return air from an air-conditioned building is used on thewet side of an air-to-air heat exchanger. The return airs lower wet-bulb temperature, which derives from mechanical refrigeration,may be used to extend indirect evaporative cooling performance inmore humid climates.It is often

10、desirable to combine the effects of direct and indirectevaporative processes (indirect/direct). The first stage (indirect)sensibly cools the air, thereby lowering its wet-bulb temperature,and passes it through the second stage (direct) where it is evapora-tively cooled further. Combination systems u

11、se both direct and indi-rect evaporative principles as well as secondary heat exchangers andcooling coils. Secondary heat exchangers enhance both cooling andheat recovery (in winter), and the coils provide additional cooling/dehumidification as needed. Used in both dual-duct and unitary sys-tems, se

12、condary heat exchangers save energy by eliminating theneed for terminal reheat in some applications (in such systems, airmay exit below the initial wet-bulb temperature).Direct evaporative coolers for residences in low-wet-bulb re-gions typically require 70% less energy than direct-expansion aircond

13、itioners. For instance, in El Paso, Texas, the typical evaporativecooler consumes 609 kWh per cooling season, compared to3901 kWh per season for a typical vapor-compression air condi-tioner with a seasonal energy-efficiency ratio (SEER) of 10. Thisequates to an average demand of 0.51 kW based on 120

14、0 operatinghours, compared to an average of 3.25 kW for a vapor-compressionair conditioner. Depending on climatic conditions, many buildings can use indi-rect/direct evaporative air conditioning to provide comfort cooling.Indirect/direct systems achieve a 40 to 50% energy savings in mod-erate humidi

15、ty zones (Foster and Dijkstra 1996).DIRECT EVAPORATIVE AIR COOLERSIn direct evaporative air cooling, air is drawn through porouswetted pads or a spray and its sensible heat energy evaporates somewater. Heat and mass transfer between the air and water lowers theair dry-bulb temperature and increases

16、the humidity at a constantenthalpy (wet-bulb temperature remains nearly constant). The dry-bulb temperature of the nearly saturated air approaches the ambientairs wet-bulb temperature.Saturation effectiveness is a key factor in determining evaporativecooler performance. The extent to which the leavi

17、ng air temperaturefrom a direct evaporative cooler approaches the thermodynamic wet-bulb temperature of the entering air defines the direct saturationefficiency e,expressed ase= 100 (1)wheree= direct evaporative cooling saturation efficiency, %t1= dry-bulb temperature of entering air, Ct2= dry-bulb

18、temperature of leaving air, C= thermodynamic wet-bulb temperature of entering air, CThe preparation of this chapter is assigned to TC 5.7, Evaporative Cooling.t1t2t1ts-ts41.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)An efficient wetted pad (with high saturation efficiency) can re-duce the

19、air dry-bulb temperature by as much as 95% of thewet-bulb depression (ambient dry-bulb temperature less wet-bulbtemperature), although an inefficient and poorly designed pad mayonly reduce this by 50% or less.Although direct evaporative cooling is simple and inexpensive,its cooling effect is insuffi

20、cient for indoor comfort when the ambientwet-bulb temperature is higher than about 21C; however, cooling isstill sufficient for relief cooling applications (e.g., greenhouses,industrial cooling). Direct evaporative coolers should not recircu-late indoor air; exhaust should equal incoming conditioned

21、 air.Random-Media Air CoolersThese coolers contain evaporative pads, usually of aspen wood orabsorbent plastic fiber/foam (Figure 1). A water-recirculating pumplifts sump water to a distributing system, and it flows down throughthe pads back to the sump.A fan in the cooler draws air through the evap

22、orative pads anddelivers it for space cooling. The fan discharges either through theside of the cooler cabinet or through the sump bottom. Random-media packaged air coolers are made as small tabletop coolers (0.02to 0.09 m3/s), window units (0.05 to 2.1 m3/s), and standard duct-connected coolers (2.

23、4 to 8.5 m3/s). Cooler selection is based on acapacity rating from an independent agency.When clean and well maintained, commercial random-media aircoolers operate at approximately 80% effectiveness and reduce10 m and larger particles in the air. In some units, supplementaryfilters are added to redu

24、ce the particle count of delivered air whenthe unit is operating with or without water circulation. Evaporativepads may be chemically treated to increase wettability. An additivemay be included in the fibers to help them resist attack by bacteria,fungi, and other microorganisms.Random-media cooler d

25、esigns with face velocity of 0.5 to 1.3 m/swith a pressure drop of 25 Pa are the norm. Aspen fibers packed toapproximately 1.5 to 2 kg/m2of face area, based on a 50 mm thick pad,are standard. Pads mount in removable louvered frames, which areusually made of painted galvanized steel or molded plastic

26、. Troughsdistribute water to the pads. A centrifugal pump with a submerged inletdelivers water through tubes that provide an equal flow of water toeach trough. It is important to thermally protect the pump motor. Thesump or water tank has a water makeup connection, float valve, over-flow pipe, and d

27、rain. It is important to provide bleed water or a timeddump of the sump (or both) to prevent build-up of minerals, dirt, andmicrobial growth.The fan is usually a forward-curved, centrifugal fan, complete withmotor and drive. The V-belt drive may include an adjustable-pitchmotor sheave to allow fan s

28、peed to increase to use the full motorcapacity at higher airflow resistance. The motor enclosure may bedrip-proof, totally enclosed, or a semi-open type specificallydesigned for evaporative coolers.Rigid-Media Air CoolersBlocks of corrugated material make up the wetted surface ofrigid-media direct e

29、vaporative air coolers (Figure 2). Materialsinclude cellulose, plastic, and fiberglass, treated to absorb water andresist weathering effects. The medium is cross corrugated to maxi-mize mixing of air and water. In the direction of airflow, the depthof medium is commonly 300 mm, but it may be between

30、 100 and600 mm, depending on the desired thermal performance. Themedium has the desirable characteristics of low resistance to air-flow, high saturation effectiveness, and self-cleaning abilities. Thestandard design face velocity of a rigid medium is 2 to 3 m/s. Staticpressure loss for a 300 mm medi

31、a pad varies from 34.8 to 74.6 Pa atsea level.Direct evaporative air coolers using this material can handle asmuch as 280 m3/s and may include an integral fan. Saturation effec-tiveness varies from 70 to over 95%, depending on media depth andair velocity. Air flows horizontally while the recirculati

32、ng waterflows vertically over the medium surfaces by gravity feed from aflooding header and water distribution chamber. The header may beconnected directly to a pressurized water supply for once-throughoperation (e.g., gas turbines, cleanrooms, data centers), or a pumpmay recirculate the water from

33、a lower reservoir constructed ofheavy-gage corrosion-resistant material. The reservoir is also fittedwith overflow and positive-flowing drain connections. The uppermedia enclosure is of reinforced galvanized steel or other corrosion-resistant sheet metal, or of plastic.Flanges at the entering and le

34、aving faces allow connection ofductwork. In recirculating water systems, a float valve maintainsproper water level in the reservoir, makes up water that has evapo-rated, and supplies fresh water for dilution to prevent an overcon-centration of solids and minerals. Because the water recirculationrate

35、 is low and because high-pressure nozzles are not needed to sat-urate the medium, pumping power is low compared to spray-filledair washers with equivalent evaporative cooling effectiveness.Remote Pad Evaporative Cooling EquipmentGreenhouses, poultry or hog buildings, and similar applicationsuse exha

36、ust fans installed in the wall or roof of the structure. AirFig. 1 Typical Random-Media Evaporative Cooler Fig. 2 Typical Rigid-Media Air CoolerEvaporative Air-Cooling Equipment 41.3evaporatively cools as it flows through pads located on the other endof the building. Water flowing down from a perfor

37、ated pipe wets thepads, with excess water collected for recirculation. In some cases,the pads are wetted with high-pressure fogging nozzles, which pro-vide additional cooling. Water for fogging nozzles must comedirectly from the fresh-water supply. The pad has an air velocity ofapproximately 0.8 m/s

38、 for random-media pads, 1.3 m/s for 100 mmrigid media, and 2.2 m/s for 150 mm rigid media.INDIRECT EVAPORATIVE AIR COOLERSPackaged Indirect Evaporative Air CoolersFigure 3 illustrates an indirect evaporative cooling (IEC) heatexchanger. This cross-flow, tube-type heat exchanger uses a sumppump to re

39、circulate water to wet the inside of the heat exchangertubes. A secondary-air fan causes either building return or outdoorair to flow through the inside of the tubes, causing evaporative cool-ing to occur. Outdoor air is sensibly cooled as it passes through theheat exchanger as it comes into contact

40、 with tubes that are cooled byevaporative cooling on the opposite side of the tube. Latent coolingmay also occur if the secondary air wet-bulb temperature is belowthe outdoor air dew point.These heat exchangers are capable of a 60 to 80% approach ofthe ambient dry-bulb temperature to the secondary a

41、irflow enteringwet-bulb temperature. The calculation is called wet-bulb depres-sion efficiency (WBDE) and defined asWBDE = 100 (2)whereWBDE = wet-bulb depression efficiency, %t1= dry-bulb temperature of entering primary air, Ct2= dry-bulb temperature of leaving primary air, Cts = wet-bulb temperatur

42、e of entering secondary air, CSupply-air-side static pressure losses for these heat exchangersrange from 60 to 185 Pa. Wet-side airflow pressure drop penaltiesrange from 100 to 225 Pa. Secondary airflow ratios are in the rangeof 1.5 to 1 down to a low of 0.5 L/s of outdoor air (OA) to 0.3 L/s ofseco

43、ndary airflow. The higher the ratios of wet-side air to dry air, thegreater the WBDE, with all other factors remaining constant. Cool-ing energy efficiency ratios (EER) for this type of heat exchangerrange from 40 to 80.With DX Refrigeration. Figure 4 illustrates a package unitdesign that combines t

44、he tube-type indirect evaporative cooling heatexchanger with a direct-expansion (DX) refrigeration final stage ofcooling. The geometry of the tube-type heat exchanger usually lim-its the size of this application to less than 19 m3/s of supply air.By placing the condenser coil in the wet-side air pat

45、h off the heatexchanger, the mechanical cooling components coefficient of per-formance (COP) significantly increases over that of an air-cooledcondenser system with the coil in the ambient air. When buildingreturn air is used as the secondary airflow, compressor energy inputsare often reduced from 0

46、.3 kW per kilowatt of cooling to 0.2 kW/kWor lower, because building return air from an air-conditioned build-ing has wet-bulb conditions in the range of 15.5 to 18C at a 24Croom dry-bulb temperature. Wet-side air leaving the heat exchangeris usually in the range of 21 to 24C db, but at 80 to 90% rh

47、, depend-ing on the heat exchangers wetting efficiency. Because refrigera-tion air-cooled condenser coils are unaffected by humidity, thiscooler airstream may be used to reduce the refrigeration condensingtemperature of the DX system, which increases compressor capac-ity and life by reducing vapor c

48、ompression temperature lift.Fig. 3 Indirect Evaporative Cooling (IEC) Heat Exchanger(Courtesy Munters/Des Champs)t1t2t1ts-Fig. 4 Indirect Evaporative Cooler Used as Precooler41.4 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)Figure 5 shows how a heat-pipe, indirect evaporative coolingheat excha

49、nger may be packaged with a DX-type refrigeration sys-tem, using building return air, to minimize cooling energy consump-tion for an all-outdoor-air design such as may be required for alaboratory or hospital application. The geometry of the heat pipelends itself to the treatment of larger airflow quantities. The dimen-sions shown in Figure 5 are for a nominal 23.6 m3/s supply air sys-tem with 775 kW of total load.In addition, the heat pipe heat exchanger has the distinct advan-tage over other air-to-air heat exchangers of being