ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 42-2012 LIQUID COOLERS.pdf

1、42.1CHAPTER 42LIQUID COOLERSTypes of Liquid Coolers 42.1Heat Transfer 42.3Pressure Drop . 42.4Vessel Design 42.4Application Considerations. 42.5LIQUID cooler (hereafter called a cooler) is a heat exchangerA in which refrigerant is evaporated, thereby cooling a fluid(usually water or brine) circulati

2、ng through the cooler. This chapteraddresses the performance, design, and application of coolers.TYPES OF LIQUID COOLERSVarious types of liquid coolers and their characteristics are listedin Table 1 and described in the following sections.Direct-ExpansionRefrigerant evaporates inside the tubes of a

3、direct-expansioncooler. These coolers are usually used with positive-displacementcompressors, such as reciprocating, rotary, or rotary screw compres-sors, to cool various fluids, such as water, water/glycol mixtures,and brine. Common configurations include shell-and-tube, tube-in-tube, and brazed-pl

4、ate.Figure 1 shows a typical shell-and-tube cooler. A series of baf-fles channels the fluid throughout the shell side. The baffles createcross flow through the tube bundle and increase the velocity of thefluid, thereby increasing its heat transfer coefficient. The velocityof the fluid flowing perpen

5、dicular to the tubes should be at least0.6 m/s to clean the tubes and less than the velocity limit of the tubeand baffle materials, to prevent erosion.Refrigerant distribution is critical in direct-expansion coolers.If some tubes are fed more refrigerant than others, refrigerant maynot fully evapora

6、te in the overfed tubes, and liquid refrigerant mayescape into the suction line. Because most direct-expansion cool-ers are controlled by an expansion valve that regulates suctionsuperheat, the remaining tubes must produce a higher superheat toevaporate the liquid escaping into the suction line. Thi

7、s unbalancecauses poor overall heat transfer. Uniform distribution is usuallyachieved by adding a distributor, which creates sufficient turbu-lence to promote a homogeneous mixture so that each tube gets thesame mixture of liquid and vapor.The number of refrigerant passes is another important item i

8、ndirect-expansion cooler performance. A single-pass cooler mustcompletely evaporate the refrigerant before it reaches the end of thefirst pass; this requires relatively long tubes. A multiple-pass cooleris significantly shorter than a single-pass cooler, but must be properlydesigned to ensure proper

9、 refrigerant distribution after the first pass.Internally and externally enhanced tubes can also be used to reducecooler size. Typical tube diameters are in the range of 8 to 16 mm.A tube-in-tube cooler is similar to a shell-and-tube design,except that it consists of one or more pairs of coaxial tub

10、es. Thefluid usually flows inside the inner tube while the refrigerant flowsin the annular space between the tubes. In this way, the fluid side canbe mechanically cleaned if access to the header is provided.Brazed- or semiwelded-plate coolers are constructed of platesbrazed or laser-welded together

11、to make an assembly of separatechannels. Semiwelded designs have the refrigerant side welded andthe fluid side gasketed and allow contact of the refrigerant with thefluid-side gaskets. These designs can be disassembled for inspec-tion and mechanical cleaning of the fluid side. Brazed types do nothav

12、e gaskets, cannot be disassembled, and are cleaned chemically.Internal leaks in brazed plates typically cannot be repaired. Thistype of evaporator is designed to work in a vertical orientation. Uni-form distribution in direct-expansion operation is typically achievedby using a special plate design o

13、r distributor insert; flooded andpumped overfeed operations do not require distribution devices.Plate coolers are very compact and require minimal space.Most tubular direct-expansion coolers are designed for horizon-tal mounting. If they are mounted vertically, performance may varyconsiderably from

14、that predicted because two-phase flow heat trans-fer is a direction-sensitive phenomenon and dryout begins earlier invertical upflow.FloodedIn a flooded shell-and-tube cooler, refrigerant vaporizes on theoutside of the tubes, which are submerged in liquid refrigerant in aclosed shell. Fluid flows th

15、rough the tubes as shown in Figure 2.Flooded coolers are usually used with rotary screw or centrifugalcompressors to cool water, water/glycol mixtures, or brine.The preparation of this chapter is assigned to TC 8.5, Liquid-to-RefrigerantHeat Exchangers.Fig. 1 Direct-Expansion Shell-and-Tube CoolerFi

16、g. 2 Flooded Shell-and-Tube Cooler42.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)A refrigerant liquid/vapor mixture usually feeds into the bottomof the shell through a distributor that distributes the mixture equallyunder the tubes. The relatively warm fluid in the tubes heats therefrigeran

17、t liquid surrounding the tubes, causing it to boil. As bub-bles rise through the space between tubes, the liquid surrounding thetubes becomes increasingly bubbly (or foamy, if much oil is present).The refrigerant vapor must be separated from the mist generatedby the boiling refrigerant to prevent li

18、quid carryover to the compres-sor. The simplest separation method is provided by a dropout areabetween the top row of tubes and the suction connections. If thisdropout area is insufficient, a coalescing filter may be required be-tween the tubes and connections. Perry and Green (2007) give addi-tiona

19、l information on mist elimination. Another approach is to addanother vessel, or “surge drum,” above the suction connections. Thediameter of this vessel is selected so that the velocity of the liquiddroplets slows to the point where they fall back to the bottom of thesurge drum. This liquid is then d

20、rained back into the flooded cooler.The size of tubes, number of tubes, and number of passes shouldbe determined to maintain fluid velocity typically between 1 and2.4 m/s for copper alloy tubing. Velocities beyond these limits maybe used if the fluid is free of suspended abrasives and fouling sub-st

21、ances (Ayub and Jones 1987; Sturley 1975) or if the tubing is man-ufactured from special alloys, such as titanium and stainless steel,that have better resistance to erosion. In some cases, the minimumvelocity may be determined by a lower Reynolds number limit tominimize precipitation fouling and cor

22、rosion issues.One variation of this cooler is the spray shell-and-tube cooler.In large-diameter coolers where the refrigerants heat transfer coef-ficient is adversely affected by the refrigerant pressure, liquid can besprayed to cover the tubes rather than flooding them. A mechanicalpump circulates

23、liquid from the bottom of the cooler to the sprayheads.Flooded shell-and-tube coolers are generally unsuitable for otherthan horizontal orientation.In a flooded plate cooler (Figure 3), refrigerant vaporizes in ver-tical channels between corrugated plates with the liquid inlet at thebottom and the v

24、apor outlet at the top (i.e., vertical upflow). Thewarm fluid flow may be either counter or parallel to the refrigerantflow. Both thermosiphon (gravity feed) and pumped overfeed oper-ation are used. Surge drums are required for pumped overfeed oper-ation but usually not for thermosiphon operation be

25、cause thecorrugated plates demist flow under most conditions.BaudelotBaudelot coolers (Figure 4) are used to cool a fluid to near itsfreezing point in industrial, food, and dairy applications. In thiscooler, fluid circulates over the outside of vertical plates, which areeasy to clean. The inside sur

26、face of the plates is cooled by evaporat-ing the refrigerant. The fluid to be cooled is distributed uniformlyalong the top of the heat exchanger and then flows by gravity to acollection pan below. The cooler may be enclosed by insulated wallsto avoid unnecessary loss of refrigeration.Table 1 Types o

27、f CoolersType of Cooler Subtype Usual Refrigerant Feed Device Usual Capacity Range, kW Commonly Used RefrigerantsDirect-expansion Shell-and-tube Thermal expansion valveElectronic modulation valve7 to 18007 to 180012, 22, 134a, 404A, 407C, 410A, 500, 502, 507A, 717Tube-in-tube Thermal expansion valve

28、 18 to 90 12, 22, 134a, 717Brazed-plate Thermal expansion valve 2 to 700 12, 22, 134a, 404A 407C, 410A, 500, 502, 507A, 508B, 717, 744Semiwelded plate Thermal expansion valve 175 to 7000 12, 22, 134a, 500, 502, 507A, 717, 744Flooded Shell-and-tube Low-pressure float 90 to 7000 11, 12, 22, 113, 114Hi

29、gh-pressure float 90 to 21 100 123, 134a, 500, 502, 507A, 717Fixed orifice(s)Weir90 to 21 10090 to 21 100Spray shell-and-tube Low-pressure float 180 to 35 000 11, 12, 13B1, 22High-pressure float 180 to 35 000 113, 114, 123, 134aBrazed-plate Low-pressure float 2 to 700 12, 22, 134a, 500, 502, 507A, 7

30、17, 744Semiwelded plate Low-pressure float 175 to 7000 12, 22, 134a, 500, 502, 507A, 717, 744Baudelot Flooded Low-pressure float 35 to 350 22, 717Direct-expansion Thermal expansion valve 18 to 90 12, 22, 134a, 717Shell-and-coil Thermal expansion valve 7 to 35 12, 22, 134a, 717Fig. 3 Flooded Plate Co

31、olerFig. 4 Baudelot CoolerLiquid Coolers 42.3R-717 (ammonia) is commonly used with flooded Baudelotcoolers using conventional gravity feed with a surge drum. A low-pressure float valve maintains a suitable refrigerant liquid level inthe surge drum. Baudelot coolers using other common refrigerantsare

32、 generally direct-expansion, with thermostatic expansion valves.Shell-and-CoilA shell-and-coil cooler is a tank containing the fluid to be cooledwith a simple coiled tube used to cool the fluid. This type of coolerhas the advantage of cold fluid storage to offset peak loads. In somemodels, the tank

33、can be opened for cleaning. Most applications areat low capacities (e.g., for bakeries, for photographic laboratories,and to cool drinking water).The coiled tube containing the refrigerant can be either inside thetank (Figure 5) or attached to the outside of the tank in a way thatallows heat transfe

34、r.HEAT TRANSFERHeat transfer for liquid coolers can be expressed by the followingsteady-state heat transfer equation:q = UAtm(1)whereq = total heat transfer rate, Wtm= mean temperature difference, KA = heat transfer surface area associated with U, m2U = overall heat transfer coefficient, W/(m2K)The

35、area A can be calculated if the geometry of the cooler is known.Chapter 4 of the 2009 ASHRAE HandbookFundamentals de-scribes the calculation of the mean temperature difference.This chapter discusses the components of U, but not in depth. Umay be calculated by one of the following equations.Based on

36、inside surface areaU = (2)Based on outside surface areaU = (3)wherehi= inside heat transfer coefficient based on inside surface area,W/(m2K)ho= outside heat transfer coefficient based on outside surface area,W/(m2K)Ao= outside heat transfer surface area, m2Ai= inside heat transfer surface area, m2Am

37、= mean heat transfer area of metal wall, m2k = thermal conductivity of heat transfer material, W/(mK)t = thickness of heat transfer surface (tube wall thickness), mrfi= fouling factor of fluid side based on inside surface area,(m2K)/Wrfo= fouling factor of fluid side based on outside surface area, (

38、m2K)/WNote: If fluid is on inside, multiply rfiby Ao/Aito find rfo.If fluid is on outside, multiply rfoby Ai/Aoto find rfi.These equations can be applied to incremental sections of theheat exchanger to include local effects on the value of U, and thenthe increments summed to obtain a more accurate d

39、esign.Heat Transfer CoefficientsThe refrigerant-side coefficient usually increases with (1) anincrease in cooler load, (2) a decrease in suction superheat, (3) adecrease in oil concentration, or (4) an increase in saturated suctiontemperature. The amount of increase or decrease depends on the typeof

40、 cooler. Schlager et al. (1989) and Zrcher et al. (1998) discuss theeffects of oil in direct-expansion coolers. Flooded coolers have a rela-tively small change in heat transfer coefficient as a result of a changein load, whereas a direct-expansion cooler shows a significant increasewith an increase

41、in load. A Wilson plot of test data (Briggs and Young1969; McAdams 1954) can show actual values for the refrigerant-sidecoefficient of a given cooler design. Collier and Thome (1994), Thome(1990, 2003), and Webb (1994) provide additional information on pre-dicting refrigerant-side heat transfer coef

42、ficients.The fluid-side coefficient is determined by cooler geometry,fluid flow rate, and fluid properties (viscosity, specific heat, thermalconductivity, and density) (Palen and Taborek 1969; WolverineTube 1984, 2004-2010). For a given fluid, the fluid-side coefficientincreases with fluid flow rate

43、 because of increased turbulence andwith fluid temperature because of improvement of fluid propertiesas temperature increases.The heat transfer coefficient in direct-expansion and floodedcoolers increases significantly with fluid flow. The effect of flow issmaller for Baudelot and shell-and-coil coo

44、lers.An enhanced heat transfer surface can increase the heat transfercoefficient of coolers in the following ways:It increases heat transfer area, thereby increasing the overall heattransfer rate and reducing the thermal resistance of fouling, evenif the refrigerant-side heat transfer coefficient is

45、 unchanged.Where flow of fluid or refrigerant is low, it increases turbulence atthe surface and mixes fluid at the surface with fluid away from thesurface. For stratified internal flows of refrigerants, it may convertthe flow to complete wetting of the tube perimeter.In flooded coolers, an enhanced

46、refrigerant-side surface may pro-vide more and better nucleation points to promote boiling ofrefrigerant.Pais and Webb (1991) and Thome (1990) describe many enhancedsurfaces used in flooded coolers. The enhanced surface geometriesprovide substantially higher boiling coefficients than integral finned

47、tubes. Nucleate pool boiling data are provided by Webb and Pais(1991). The boiling process in the tube bundle of a flooded coolermay be enhanced by forced-convection effects. This is basically anadditive effect, in which the local boiling coefficient is the sum of thenucleate boiling coefficient and

48、 the forced-convection effect. Webbet al. (1989) describe an empirical method to predict flooded coolerperformance, recommending row-by-row calculations.Based on this model, Webb and Apparao (1990) present theresults of calculations using a computer program. The results showsome performance differen

49、ces of various internal and external sur-face geometries. As an example, Figure 6 shows the contribution ofnucleate pool boiling to the overall refrigerant heat transfer coeffi-cient for an integral finned tube and an enhanced tube as a functionFig. 1Fig. 2Fig. 5 Shell-and-Coil Cooler11 hi AiAohotkAiAmrfi+ +-1AoAihi1 ho tkAoAmrfo+ +-42.4 2012 ASHRAE H