ASHRAE HVAC APPLICATIONS SI CH 52-2015 Evaporative Cooling.pdf

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1、52.1CHAPTER 52 EVAPORATIVE COOLINGGeneral Applications . 52.1Indirect Evaporative Cooling Systems for Comfort Cooling. 52.2Booster Refrigeration. 52.8Residential or Commercial Cooling 52.10Exhaust Required . 52.10Two-Stage Cooling. 52.11Industrial Applications. 52.11Other Applications . 52.14Control

2、 Strategy to Optimize Energy Recovery 52.15Air Cleaning and Sound Attenuation . 52.15Economic Factors. 52.17Psychrometrics . 52.17Entering Air Considerations 52.18VAPORATIVE cooling is energy-efficient, environmentallyE friendly, and cost-effective in many applications and all cli-mates. Application

3、s range from comfort cooling in residential,agricultural, commercial, and institutional buildings, to industrialapplications for spot cooling in mills, foundries, power plants, andother hot environments. Several types of apparatus cool by evaporat-ing water directly in the airstream, including (1) d

4、irect evaporativecoolers, (2) spray-filled and wetted-surface air washers, (3) sprayed-coil units, and (4) humidifiers. Indirect evaporative coolingequipment combines the evaporative cooling effect in a secondaryairstream with a heat exchanger to produce cooling without addingmoisture to the primary

5、 airstream.Direct evaporative cooling reduces the dry-bulb temperatureand increases the relative humidity of the air. It is most commonlyapplied to dry climates or to applications requiring high air ex-change rates. Innovative schemes combining evaporative coolingwith other equipment have resulted i

6、n energy-efficient designs.When temperature and/or humidity must be controlled withinnarrow limits, heat and mechanical refrigeration can be combinedwith evaporative cooling in stages. Evaporative cooling equip-ment, including unitary equipment and air washers, is covered inChapter 41 of the 2012 AS

7、HRAE HandbookHVAC Systems andEquipment.1. GENERAL APPLICATIONSCoolingEvaporative cooling is used in almost all climates. The wet-bulbtemperature of the entering airstream limits direct evaporative cool-ing. The wet-bulb temperature of the secondary airstream limitsindirect evaporative cooling.Design

8、 wet-bulb temperatures are rarely higher than 25C, mak-ing direct evaporative cooling economical for spot cooling, kitchens,laundries, agricultural, and industrial applications. In regions withlower wet-bulb temperatures, evaporative cooling can be effectivelyused for comfort cooling, although some

9、climates may requiremechanical refrigeration for part of the year.Indirect applications lower the air wet-bulb temperature and canproduce leaving dry-bulb temperatures that approach the wet-bulbtemperature of the secondary airstream. Using room exhaust as sec-ondary air or incorporating precooled ai

10、r in the secondary airstreamcould lower the wet-bulb temperature of the secondary air and furtherenhances the cooling capability of the indirect evaporative cooler.Direct evaporative cooling is an adiabatic exchange of energy. Heatmust be added to evaporate water. The air into which water is evapo-r

11、ated supplies the heat; thus, the dry-bulb temperature is lowered andthe moisture content increases. The amount of heat removed from theair equals the amount of heat absorbed by the water evaporated as heatof vaporization. If water is recirculated in the direct evaporative cool-ing apparatus, the wa

12、ter temperature in the reservoir approaches thewet-bulb temperature of the air entering the process. By definition, noheat is added to, or extracted from, an adiabatic process; the initial andfinal conditions fall on a line of constant wet-bulb temperature, whichnearly coincides with a line of const

13、ant enthalpy.The maximum reduction in dry-bulb temperature is the differencebetween the entering air dry- and wet-bulb temperatures. If air iscooled to the wet-bulb temperature, it becomes saturated and theprocess would be 100% effective. Effectiveness is the depression ofthe dry-bulb temperature of

14、 the air leaving the apparatus divided bythe difference between the dry- and wet-bulb temperatures of theentering air. Theoretically, adiabatic direct evaporative cooling isless than 100% effective, although evaporative coolers are 85 to 95%(or more) effective.When a direct evaporative cooling unit

15、alone cannot provide de-sired conditions, several alternatives can satisfy application require-ments and still be energy-effective and economical to operate. Therecirculating water supplying the direct evaporative cooling unit canbe increased in volume and chilled by mechanical refrigeration toprovi

16、de lower leaving wet- and dry-bulb temperatures and lower hu-midity. Compared to the cost of using mechanical refrigeration only,this arrangement reduces operating costs by as much as 25 to 40%.Indirect evaporative cooling applied as a first stage, upstream from asecond, direct evaporative stage, re

17、duces both the entering dry- andwet-bulb temperatures before the air enters the direct evaporativecooler. Indirect evaporative cooling may save as much as 60 to 75%or more of the total cost of operating mechanical refrigeration to pro-duce the same cooling effect. Systems may combine indirect evapo-

18、rative cooling, direct evaporative cooling, heaters, and mechanicalrefrigeration, in any combination.The psychrometric chart in Figure 1 illustrates what happenswhen air is passed through a direct evaporative cooler. In the exam-ple shown, assume an entering condition of 35C db and 24C wb.The initia

19、l difference is 35 24 = 11 K. If the effectiveness is 80%, thedepression is 0.80 11 = 8.8 K db. The dry-bulb temperature leavingthe direct evaporative cooler is 35 8.8 = 26.2C. In the adiabaticevaporative cooler, only part of the water recirculated is assumed toevaporate and the water supply is reci

20、rculated. The recirculatedwater reaches an equilibrium temperature approximately the same asthe wet-bulb temperature of the entering air.The performance of an indirect evaporative cooler can also beshown on a psychrometric chart (Figure 1). Many manufacturersdefine effectiveness similarly for both d

21、irect and indirect evapora-tive cooling equipment. With indirect evaporative cooling, the cool-ing process in the primary airstream follows a line of constantmoisture content (constant dew point). Indirect evaporative coolingeffectiveness is the dry-bulb depression in the primary airstreamdivided by

22、 the difference between the entering dry-bulb temperatureof the primary airstream and the entering wet-bulb temperature ofthe secondary air. Depending on heat exchanger design and relativeThe preparation of this chapter is assigned to TC 5.7, Evaporative Cooling.52.2 2015 ASHRAE HandbookHVAC Applica

23、tions (SI)quantities of primary and secondary air, effectiveness ratings maybe as high as 85%.Assuming 60% effectiveness, and assuming both primary andsecondary air enter the apparatus at the outdoor condition of 35C dband 24C wb, the dry-bulb depression is 0.60(35 24) = 6.6 K. Thedry-bulb temperatu

24、re leaving the indirect evaporative cooling processis 35 6.6 = 28.4C. Because the process cools without adding mois-ture, the wet-bulb temperature is also reduced. Plotting on the psy-chrometric chart shows that the final wet-bulb temperature is 22.1C.Because both wet- and dry-bulb temperatures in t

25、he indirect evapo-rative cooling process are reduced, indirect evaporative cooling cansubstitute for part of the refrigeration load.HumidificationAir can be humidified with a direct evaporative cooler by threemethods: (1) using recirculated water without prior treatment of theair, (2) preheating the

26、 air and treating it with recirculated water, or(3) heating recirculated water. Air leaving an evaporative coolerused as either a humidifier or a dehumidifier is substantially sat-urated when in operation. Usually, the spread between leavingdry- and wet-bulb temperatures is less than 1 K. The temper

27、aturedifference between leaving air and leaving water depends on thedifference between entering dry- and wet-bulb temperatures and oncertain physical features, such as the length and height of a spraychamber, cross-sectional area and depth of the wetted media used,quantity and velocity of air, quant

28、ity of water, and spray pattern. Inany direct evaporative humidifier installation, air should not enterwith a dry-bulb temperature of less than 4C; otherwise, the watermay freeze.Recirculated Water. Except for the small amount of energyadded by shaft work from the recirculating pump and the smallamo

29、unt of heat leakage through the unit enclosure, evaporativehumidification is strictly adiabatic. As the recirculated liquid evap-orates, its temperature approaches the thermodynamic wet-bulbtemperature of the entering air.The airstream cannot be brought to complete saturation, but itsstate point cha

30、nges adiabatically along a line of constant wet-bulbtemperature. Typical saturation or humidifying effectiveness ofvarious air washer spray arrangements is between 50 and 98%. Thedegree of saturation depends on the extent of contact between airand water. Other conditions being equal, low-velocity ai

31、rflow isconducive to higher humidifying effectiveness.Preheated Air. Preheating air increases both the dry- and wet-bulb temperatures and lowers the relative humidity; it does not, how-ever, alter the humidity ratio (i.e., mass ratio of water vapor to dry air)or dew-point temperature of the air. At

32、a higher wet-bulb tempera-ture, but with the same humidity ratio, more water can be absorbedper unit mass of dry air in passing through the direct evaporativehumidifier. Analysis of the process that occurs in the direct evapora-tive humidifier is the same as that for recirculated water. The desiredc

33、onditions are achieved by heating to the desired wet-bulb tempera-ture and evaporatively cooling at constant wet-bulb temperature tothe desired dry-bulb temperature and relative humidity. Relativehumidity of the leaving air may be controlled by (1) bypassing airaround the direct evaporative humidifi

34、er or (2) reducing the numberof operating spray nozzles or the area of media wetted.Heated Recirculated Water. Heating humidifier water in-creases direct evaporative humidifier effectiveness. When heat isadded to the recirculated water, mixing in the direct evaporativehumidifier may still be modeled

35、 adiabatically. The state point of themixture should move toward the specific enthalpy of the heatedwater. By raising the water temperature, the air temperature (bothdry- and wet-bulb) may be raised above the dry-bulb temperature ofentering air. The relative humidity of leaving air may be controlled

36、by methods similar to those used with preheated air.Dehumidification and CoolingDirect evaporative coolers may also be used to cool and dehumid-ify air. If the entering water temperature is cooled below the enteringwet-bulb temperature, both the dry- and wet-bulb temperatures of theleaving air are l

37、owered. Dehumidification results if the leaving watertemperature is maintained below the entering air dew point. More-over, the final water temperature is determined by the sensible andlatent heat absorbed from the air and the amount of circulated water,and it is 0.5 to 1 K below the final required

38、dew-point temperature.The air leaving a direct evaporative cooler being used as a dehu-midifier is substantially saturated. Usually, the spread betweendry- and wet-bulb temperatures is less than 0.5 K. The temperaturedifference between leaving air and leaving water depends on the dif-ference between

39、 entering dry- and wet-bulb temperatures and oncertain design features, such as the cross-sectional area and depth ofthe media or spray chamber, quantity and velocity of air, quantity ofwater, and the water distribution.Air CleaningDirect evaporative coolers of all types perform some air clean-ing.

40、See the section on Air Cleaning and Sound Attenuation fordetailed information.2. INDIRECT EVAPORATIVE COOLING SYSTEMS FOR COMFORT COOLINGFive types of indirect evaporative cooling systems are most com-monly used for commercial, institutional, and industrial coolingapplications. Figures 2 to 6 show s

41、chematics of these dry evapora-tive cooling systems.Indirect evaporative cooling efficiency is measured by theapproach of the outdoor air dry-bulb condition to either the roomreturn air or scavenger outdoor air wet-bulb condition on the wetside of the air-to-air heat exchanger. The wet-bulb depressi

42、on effi-ciency (WBDE) is expressed as follows:Fig. 1 Psychrometrics of Evaporative CoolingEvaporative Cooling 52.3WBDE = 100wheret1= supply air inlet dry-bulb temperature, Ct2= supply air outlet dry-bulb temperature, Ct3= wet-side air inlet wet-bulb temperature, CThe heat pipe air-to-air heat exchan

43、ger in Figure 2 uses a directwater spray from a recirculation sump on the wet side of the heat pipetubes (Scofield and Taylor 1986). When either room return or scav-enger outdoor air passes over the wet surface, outdoor air enteringthe building is dry-cooled and produces an approach to the wet-sidew

44、et-bulb temperature in the range of 60 to 80% WBDE for equalmass flow rates on both sides of the heat exchanger. The WBDE is afunction of heat exchanger surface area, face velocity, and complete-ness of wetting achieved for the wet-side heat exchanger surface.Face velocities on the wet side are usua

45、lly selected in the range of 2to 2.3 m/s.Figure 3 illustrates an indirect evaporative cooling (IEC) heatexchanger. This cross-flow, polymer tube air-to-air heat exchangeruses a sump pump to circulate water to wet the outside of the hori-zontal heat exchanger tubes. A secondary air fan draws either b

46、uild-ing return or outdoor air vertically upward over the outside of thewetted tubes, causing evaporative cooling to occur. Outdoor airentering the building passes horizontally through the inside of thepolymer tube bundle and is sensibly (dry) cooled. Latent and sensi-ble cooling may occur in the ou

47、tdoor makeup air stream if the sec-ondary airs wet-bulb temperature is lower than the outdoor airsdew-point temperature.The heat wheel (Figure 4) and the run-around coil (Figure 5)both use a direct evaporative cooling component on the cold sideto enhance the dry-cooling effect on the makeup air side

48、. The heatwheel (sensible transfer), when sized for 2.5 m/s face velocitywith equal mass flows on both sides, has a WBDE around 60 to70%. The run-around coil system at the same conditions producesa WBDE of 35 to 50%. The adiabatic cooling component is usu-ally selected for an effectiveness of 85 to

49、95%. Water coil freezeprotection is required in cold climates for the run-around coilloop.Air-to-air heat exchangers that are directly wetted produce acloser approach to the cold-side wet-bulb temperature, all thingsbeing equal. First cost, physical size, and parasitic losses are alsot1t2t1t3-Fig. 2 Heat Pipe Air-to-Air Heat Exchanger with Sump BaseFig. 3 Cross-Flow Plate Air-to-Air Indirect Evaporative Cooling Heat Exchanger(Munters)Fig. 4 Rotary Heat Exchanger with Direct Evaporative CoolingFig. 5 Coil Energy Recovery Loop with Direct Evaporative