ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 40-2012 COOLING TOWERS.pdf

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1、40.1CHAPTER 40COOLING TOWERSPrinciple of Operation . 40.1Design Conditions 40.2Types of Cooling Towers 40.2Materials of Construction 40.9Selection Considerations . 40.10Application. 40.10Performance Curves. 40.17Cooling Tower Thermal Performance . 40.18Cooling Tower Theory 40.18Tower Coefficients 40

2、.21Additional Information. 40.23OST air-conditioning systems and industrial processes gen-Merate heat that must be removed and dissipated. Water iscommonly used as a heat transfer medium to remove heat from re-frigerant condensers or industrial process heat exchangers. In thepast, this was accomplis

3、hed by drawing a continuous stream ofwater from a utility water supply or a natural body of water, heatingit as it passed through the process, and then discharging the waterdirectly to a sewer or returning it to the body of water. Water pur-chased from utilities for this purpose has become prohibiti

4、vely ex-pensive because of increased water supply and disposal costs.Similarly, cooling water drawn from natural sources is relativelyunavailable because the ecological disturbance caused by the in-creased temperature of discharge water has become unacceptable.Air-cooled heat exchangers cool water b

5、y rejecting heat directlyto the atmosphere, but the first cost and fan energy consumption ofthese devices are high and the plan area required is relatively large.They can economically cool water to within approximately 20F ofthe ambient dry-bulb temperature: too high for the cooling waterrequirement

6、s of most refrigeration systems and many industrialprocesses.Cooling towers overcome most of these problems and thereforeare commonly used to dissipate heat from refrigeration, air-conditioning, and industrial process systems. The water consump-tion rate of a cooling tower system is only about 5% of

7、 that of aonce-through system, making it the least expensive system tooperate with purchased water supplies. Additionally, the amountof heated water discharged (blowdown) is very small, so the eco-logical effect is greatly reduced. Lastly, cooling towers can coolwater to within 4 to 5F of the ambien

8、t wet-bulb temperature,which is always lower than the ambient dry-bulb, or approxi-mately 35F lower than can air-cooled systems of reasonable size(in the 250 to 500 ton range). This lower temperature improves theefficiency of the overall system, thereby reducing energy use sig-nificantly and increas

9、ing process output.PRINCIPLE OF OPERATIONA cooling tower cools water by a combination of heat and masstransfer. Water to be cooled is distributed in the tower by spray noz-zles, splash bars, or film-type fill, which exposes a very large watersurface area to atmospheric air. Atmospheric air is circul

10、ated by(1) fans, (2) convective currents, (3) natural wind currents, or(4) induction effect from sprays. A portion of the water absorbs heatto change from a liquid to a vapor at constant pressure. This heat ofvaporization at atmospheric pressure is transferred from the waterremaining in the liquid s

11、tate into the airstream.Figure 1 shows the temperature relationship between water andair as they pass through a counterflow cooling tower. The curvesindicate the drop in water temperature (A to B) and the rise in theair wet-bulb temperature (C to D) in their respective passagesthrough the tower. The

12、 temperature difference between the waterentering and leaving the cooling tower (A minus B) is the range.For a steady-state system, the range is the same as the water tem-perature rise through the load heat exchanger, provided the flowrate through the cooling tower and heat exchanger are the same.Ac

13、cordingly, the range is determined by the heat load and waterflow rate, not by the size or thermal capability of the cooling tower.The difference between the leaving water temperature and enter-ing air wet-bulb temperature (B minus C) in Figure 1 is the approachto the wet-bulb or simply the approach

14、 of the cooling tower. Theapproach is a function of cooling tower capability. A larger coolingtower produces a closer approach (colder leaving water) for a givenheat load, flow rate, and entering air condition. Therefore, theamount of heat transferred to the atmosphere by the cooling tower isalways

15、equal to the heat load imposed on the tower, whereas thetemperature level at which the heat is transferred is determined bythe thermal capability of the cooling tower and the entering air wet-bulb temperature.Thermal performance of a cooling tower depends mainly on theentering air wet-bulb temperatu

16、re. The entering air dry-bulb tem-perature and relative humidity, taken independently, have an insig-nificant effect on thermal performance of mechanical-draft coolingtowers, but do affect the rate of water evaporation in the coolingtower. A psychrometric analysis of the air passing through a cool-i

17、ng tower illustrates this effect (Figure 2). Air enters at the ambientcondition point A, absorbs heat and mass (moisture) from theThe preparation of this chapter is assigned to TC 8.6, Cooling Towers andEvaporative Condensers.Fig. 1 Temperature Relationship Between Water and Air in Counterflow Cooli

18、ng Tower40.2 2012 ASHRAE HandbookHVAC Systems and Equipment water, and exits at point B in a saturated condition (at very lightloads, the discharge air may not be fully saturated). The amount ofheat transferred from the water to the air is proportional to the dif-ference in enthalpy of the air betwe

19、en the entering and leaving con-ditions (hB hA). Because lines of constant enthalpy correspondalmost exactly to lines of constant wet-bulb temperature, thechange in enthalpy of the air may be determined by the change inwet-bulb temperature of the air.Air heating (vector AB in Figure 2) may be separa

20、ted into com-ponent AC, which represents the sensible portion of the heatabsorbed by the air as the water is cooled, and component CB,which represents the latent portion. If the entering air condition ischanged to point D at the same wet-bulb temperature but at a higherdry-bulb temperature, the tota

21、l heat transfer (vector DB) remains thesame, but the sensible and latent components change dramatically.DE represents sensible cooling of air, whereas EB represents latentheating as water gives up heat and mass to the air. Thus, for the samewater-cooling load, the ratio of latent to sensible heat tr

22、ansfer canvary significantly.The ratio of latent to sensible heat is important in analyzing waterusage of a cooling tower. Mass transfer (evaporation) occurs only inthe latent portion of heat transfer and is proportional to the changein specific humidity. Because the entering air dry-bulb temperatur

23、eor relative humidity affects the latent to sensible heat transfer ratio,it also affects the rate of evaporation. In Figure 2, the rate of evapo-ration in case AB (WB WA) is less than in case DB (WB WD)because the latent heat transfer (mass transfer) represents a smallerportion of the total.The evap

24、oration rate at typical design conditions is approximately1% of the water flow rate for each 12.5F of water temperature range;however, the average evaporation rate over the operating season isless than the design rate because the sensible component of total heattransfer increases as entering air tem

25、perature decreases. The evapo-ration rate is also directly proportional to the load; this must be takeninto account when estimating annual water usage.In addition to water loss from evaporation, losses also occurbecause of liquid carryover into the discharge airstream and blow-down to maintain accep

26、table water quality. Both of these factors areaddressed later in this chapter.DESIGN CONDITIONSThe thermal capability of any cooling tower may be defined bythe following parameters:Entering and leaving water temperaturesEntering air wet-bulb (and sometimes dry-bulb) temperaturesWater flow rateThe en

27、tering air dry-bulb temperature affects the amount ofwater evaporated from any evaporative cooling tower. It also affectsairflow through hyperbolic towers and directly establishes thermalcapability in any indirect contact cooling tower component operat-ing in a dry mode. Variations in tower performa

28、nce associated withchanges in the remaining parameters are covered in the section onPerformance Curves.The thermal capability of a cooling tower used for air condition-ing is often expressed in nominal cooling tower tons. A nominalcooling tower ton is defined as cooling 3 gpm of water from 95F to85F

29、 at a 78F entering air wet-bulb temperature. At these condi-tions, the cooling tower rejects 15,000 Btu/h per nominal coolingtower ton. The historical derivation of this 15,000 Btu/h coolingtower ton, as compared to the 12,000 Btu/h evaporator ton, is basedon the assumption that at typical air-condi

30、tioning conditions, forevery 12,000 Btu/h of heat picked up in the evaporator, the coolingtower must dissipate an additional 3000 Btu/h of compressor heat.Newer, high-efficiency compressor systems have significantly re-duced the amount of compressor heat generated. For specific appli-cations, nomina

31、l capacity ratings are not used, and the thermalperformance capability of the cooling tower is usually expressed asa water flow rate at specific operating temperature conditions (en-tering water temperature, leaving water temperature, entering airwet-bulb temperature).TYPES OF COOLING TOWERSTwo basi

32、c types of evaporative cooling devices are used. Thedirect-contact or open cooling tower (Figure 3), exposes water di-rectly to the cooling atmosphere, thereby transferring the source heatload directly to the air. A closed-circuit cooling tower, involvesindirect contact between heated fluid and atmo

33、sphere (Figure 4),essentially combining a heat exchanger and cooling tower into onerelatively compact device.Of the direct-contact devices, the most rudimentary is a spray-filled cooling tower that exposes water to the air without any heattransfer medium or fill. In this device, the amount of water

34、surfaceexposed to the air depends on the spray efficiency, and the time ofcontact depends on the elevation and pressure of the water distribu-tion system.Fig. 2 Psychrometric Analysis of Air Passing Through Cooling Tower Fig. 3 Direct-Contact or Open Evaporative Cooling TowerCooling Towers 40.3To in

35、crease contact surfaces as well as time of exposure, a heattransfer medium, or fill, is installed below the water distributionsystem, in the path of the air. The two types of fill in use are splash-type and film-type (Figure 5A). Splash-type fill maximizes contactarea and time by forcing the water t

36、o cascade through successiveelevations of splash bars arranged in staggered rows. Film-type fillachieves the same effect by causing the water to flow in a thin layerover closely spaced sheets, principally polyvinyl chloride (PVC),that are arranged vertically.Either type of fill can be used in counte

37、rflow and cross-flowcooling towers. For thermal performance levels typically encoun-tered in air conditioning and refrigeration, a tower with film-type fillFig. 4 Indirect-Contact or Closed-Circuit Evaporative Cooling TowersFig. 5 Types of (A) Fill and (B) Coils40.4 2012 ASHRAE HandbookHVAC Systems

38、and Equipment is usually more compact. However, splash-type fill is less sensitiveto initial air and water distribution and, along with specially config-ured, more widely spaced film-type fills, is preferred for applica-tions that may be subjected to blockage by scale, silt, or biologicalfouling.Ind

39、irect-contact (closed-circuit) cooling towers contain twoseparate fluid circuits: (1) an external circuit, in which water isexposed to the atmosphere as it cascades over the tubes of a coilbundle, and (2) an internal circuit, in which the fluid to be cooledcirculates inside the tubes of the coil bun

40、dle. In operation, heatflows from the internal fluid circuit, through the tube walls of thecoil, to the external water circuit and then, by heat and mass trans-fer, to atmospheric air. Because the internal fluid circuit nevercontacts the atmosphere, this unit can be used to cool fluids otherthan wat

41、er and/or to prevent contamination of the primary coolingcircuit with airborne dirt and impurities. Some closed-circuitcooling tower designs include cooling tower fill to augment heatexchange in the coil (Figure 6).Coil Shed Cooling Towers (Mechanical Draft). Coil shed cool-ing towers usually consis

42、t of isolated coil sections (nonventilated)located beneath a conventional cooling tower (Figure 7). Counter-flow and cross-flow types are available with either forced- or induceddraft fan arrangements. Redistribution water pans at the towers basemay be used to feed cooled water by gravity flow to th

43、e tubular heatexchange bundles (coils). These units are similar in function toclosed-circuit fluid coolers, except that supplemental fill is alwaysrequired, and the airstream is directed only through the fill regions ofthe cooling tower. Often, designs allow water from the fill section toimpinge dir

44、ectly on the coil(s). These units are arranged as field-erected, multifan cell towers and are used primarily in industrial pro-cess cooling. Modular factory-assembled versions are also available.Direct-Contact Cooling TowersNon-Mechanical-Draft Cooling Towers. Aspirated by spraysor a differential in

45、 air density, these towers do not contain fill and donot use a mechanical air-moving device. The aspirating effect of thewater spray, either vertical (Figure 8) or horizontal (Figure 9),induces airflow through the cooling tower in a parallel flow pattern.Because air velocities for the vertical spray

46、 tower (both enteringand leaving) are relatively low, such cooling towers are susceptibleto adverse wind effects and, therefore, are normally used to satisfya low-cost requirement when operating temperatures are not criticalto the system. Some horizontal spray cooling towers (Figure 9) usehigh-press

47、ure sprays to induce large air quantities and improve air/water contact. Multispeed or staged pumping systems are normallyrecommended to reduce energy use in periods of reduced load andambient conditions.Chimney (hyperbolic) towers have been used primarily for largepower installations, but may be of

48、 generic interest (Figure 10). Theheat transfer mode may be counterflow, cross-flow, or parallel flow.Air is induced through the cooling tower by the air density differen-tials that exist between the lighter, heat-humidified chimney air andthe outdoor atmosphere. Fill can be splash or film type.Prim

49、ary justification of these high first-cost products comesthrough reduction in auxiliary power requirements (elimination offan energy), reduced property area, and elimination of recirculationand/or vapor plume interference. Materials used in chimney con-struction have been primarily steel-reinforced concrete; early tim-ber structures had size limitations. Mechanical-Draft Cooling Towers. Figure 11 shows five dif-ferent designs for mechanical-draft (conventional) cooling towers.Fans may be on the inlet air side (forced-draft) or the exit air side(induced-draft). The type of fan sel