1、13.1CHAPTER 13SECONDARY COOLANTS IN REFRIGERATION SYSTEMSCoolant Selection 13.1Design Considerations 13.2Applications. 13.5ECONDARY coolants are liquids used as heat transfer fluidsS that change temperature as they gain or lose heat energy with-out changing into another phase. For lower refrigeratio
2、n tempera-tures, this requires a coolant with a freezing point below that ofwater. This chapter discusses design considerations for compo-nents, system performance requirements, and applications for sec-ondary coolants. Related information can be found in Chapters 3, 4,22, 30, and 31 of the 2009 ASH
3、RAE HandbookFundamentals.COOLANT SELECTIONA secondary coolant must be compatible with other materials inthe system at the pressures and temperatures encountered for max-imum component reliability and operating life. The coolant shouldalso be compatible with the environment and the applicable safetyr
4、egulations, and should be economical to use and replace.The coolant should have a minimum freezing point of 5F belowand preferably 15F below the lowest temperature to which it willbe exposed. When subjected to the lowest temperature in the sys-tem, coolant viscosity should be low enough to allow sat
5、isfactoryheat transfer and reasonable pressure drop.Coolant vapor pressure should not exceed that allowed at themaximum temperature encountered. To avoid a vacuum in a low-vapor-pressure secondary coolant system, the coolant can bepressurized with pressure-regulated dry nitrogen in the expansiontank
6、. However, some special secondary coolants such as thoseused for computer circuit cooling have a high solubility for nitro-gen and must therefore be isolated from the nitrogen with a suit-able diaphragm.Load Versus Flow RateThe secondary coolant pump is usually in the return line up-stream of the ch
7、iller. Therefore, the pumping rate is based on thedensity at the return temperature. The mass flow rate for a givenheat load is based on the desired temperature range and requiredcoefficient of heat transfer at the average bulk temperature.To determine heat transfer and pressure drop, the specific g
8、rav-ity, specific heat, viscosity, and thermal conductivity are based onthe average bulk temperature of coolant in the heat exchanger, not-ing that film temperature corrections are based on the average filmtemperature. Trial solutions of the secondary coolant-side coeffi-cient compared to the overal
9、l coefficient and total log mean temper-ature difference (LMTD) determine the average film temperature.Where the secondary coolant is cooled, the more viscous filmreduces the heat transfer rate and raises the pressure drop comparedto what can be expected at the bulk temperature. Where the second-ary
10、 coolant is heated, the less viscous film approaches the heattransfer rate and pressure drop expected at the bulk temperature.The more turbulence and mixing of the bulk and film, the betterthe heat transfer and higher the pressure drop. Where secondarycoolant velocity in the tubes of a heat transfer
11、 device results in lam-inar flow, heat transfer can be improved by inserting spiral tapes orspring turbulators that promote mixing the bulk and film. This usu-ally increases pressure drop. The inside surface can also be spirallygrooved or augmented by other devices. Because the state of the artof he
12、at transfer is constantly improving, use the most cost-effectiveheat exchanger to provide optimum heat transfer and pressure drop.Energy costs for pumping secondary coolant must be consideredwhen selecting the fluid to be used and the heat exchangers to beinstalled.Pumping CostPumping costs are a fu
13、nction of the secondary coolant selected,load and temperature range where energy is transferred, pump pres-sure required by the system pressure drop (including that of thechiller), mechanical efficiencies of the pump and driver, and elec-trical efficiency and power factor (where the driver is an ele
14、ctricmotor). Small centrifugal pumps, operating in the range of approx-imately 50 gpm at 80 ft of head to 150 gpm at 70 ft of head, for60 Hz applications, typically have 45 to 65% efficiency, respec-tively. Larger pumps, operating in the range of 500 gpm at 80 ft ofhead to 1500 gpm at 70 ft of head,
15、 for 60 Hz applications, typicallyhave 75 to 85% efficiency, respectively.A pump should operate near its peak operating efficiency for theflow rate and pressure that usually exist. Secondary coolant temper-ature increases slightly from energy expended at the pump shaft. Ifa semihermetic electric mot
16、or is used as the driver, motor ineffi-ciency is added as heat to the secondary coolant, and the total kilo-watt input to the motor must be considered in establishing load andtemperatures.Performance ComparisonsAssuming that the total refrigeration load at the evaporatorincludes the pump motor input
17、 and brine line insulation heat gains,as well as the delivered beneficial cooling, tabulating typical second-ary coolant performance values helps in coolant selection. A 1.06 in.ID smooth steel tube evaluated for pressure drop and internal heattransfer coefficient at the average bulk temperature of
18、20F and atemperature range of 10F for 7 fps tube-side velocity provides com-parative data (Table 1) for some typical coolants. Table 2 ranks thesame coolants comparatively, using data from Table 1.For a given evaporator configuration, load, and temperaturerange, select a secondary coolant that gives
19、 satisfactory velocities,heat transfer, and pressure drop. At the 20F level, hydrocarbon andhalocarbon secondary coolants must be pumped at a rate of 2.3 to3.0 times the rate of water-based secondary coolants for the sametemperature range.Higher pumping rates require larger coolant lines to keep the
20、pumps pressure and brake horsepower requirement within reason-able limits. Table 3 lists approximate ratios of pump power for sec-ondary coolants. Heat transferred by a given secondary coolantaffects the cost and perhaps the configuration and pressure drop ofa chiller and other heat exchangers in th
21、e system; therefore, Tables2 and 3 are only guides of the relative merits of each coolant.The preparation of this chapter is assigned to TC 10.1, Custom EngineeredRefrigeration Systems.13.2 2010 ASHRAE HandbookRefrigerationOther ConsiderationsCorrosion must be considered when selecting coolant, inhi
22、bitor,and system components. The effect of secondary coolant and inhib-itor toxicity on the health and safety of plant personnel or consumersof food and beverages must be considered. The flash point andexplosive limits of secondary coolant vapors must also be evaluated.Examine the secondary coolant
23、stability for anticipated mois-ture, air, and contaminants at the temperature limits of materialsused in the system. Skin temperatures of the hottest elements deter-mine secondary coolant stability.If defoaming additives are necessary, their effect on thermal sta-bility and coolant toxicity must be
24、considered for the application.DESIGN CONSIDERATIONSSecondary coolant vapor pressure at the lowest operating tem-perature determines whether a vacuum could exist in the secondarycoolant system. To keep air and moisture out of the system,pressure-controlled dry nitrogen can be applied to the top leve
25、l ofsecondary coolant (e.g., in the expansion tank or a storage tank).Gas pressure over the coolant plus the pressure created at the lowestpoint in the system by the maximum vertical height of coolantdetermine the minimum internal pressure for design purposes. Thecoincident highest pressure and lowe
26、st secondary coolant temper-ature dictate the design working pressure (DWP) and materialspecifications for the components.To select proper relief valve(s) with settings based on the systemDWP, consider the highest temperatures to which the secondarycoolant could be subjected. This temperature occurs
27、 in case of heatradiation from a fire in the area, or normal warming of the valved-off sections. Normally, a valved-off section is relieved to an uncon-strained portion of the system and the secondary coolant can expandfreely without loss to the environment.Safety considerations for the system are f
28、ound in ASHRAEStandard 15. Design standards for pressure piping can be found inASME Standard B31.5, and design standards for pressure vessels inSection VIII of the ASME Boiler and Pressure Vessel Code.Piping and Control ValvesPiping should be sized for reasonable pressure drop using thecalculation m
29、ethods in Chapters 3 and 22 of the 2009 ASHRAEHandbookFundamentals. Balancing valves or orifices in each ofthe multiple feed lines help distribute the secondary coolant. Areverse-return piping arrangement balances flow. Control valves thatvary flow are sized for 20 to 80% of the total friction press
30、ure dropthrough the system for proper response and stable operation. Valvessized for pressure drops smaller than 20% may respond too slowly toa control signal for a flow change. Valves sized for pressure dropsover 80% can be too sensitive, causing control cycling and instability.Storage TanksStorage
31、 tanks can shave peak loads for brief periods, limit the sizeof refrigeration equipment, and reduce energy costs. In off-peakhours, a relatively small refrigeration plant cools a secondary coolantstored for later use. A separate circulating pump sized for the maxi-mum flow needed by the peak load is
32、 started to satisfy peak load.Energy cost savings are enhanced if the refrigeration equipment isused to cool secondary coolant at night, when the cooling mediumfor heat rejection is generally at the lowest temperature.The load profile over 24 h and the temperature range of the sec-ondary coolant det
33、ermine the minimum net capacity required for therefrigeration plant, pump sizes, and minimum amount of secondaryTable 1 Secondary Coolant Performance ComparisonsSecondary CoolantConcentration(by Weight), %Freeze Point,F gpm/tonaPressure Drop,bpsiHeat Transfer Coefficientchi, Btu/hft2FPropylene glyco
34、l 39 5.1 2.56 2.91 205Ethylene glycol 38 6.9 2.76 2.38 406Methanol 26 5.3 2.61 2.05 473Sodium chloride 23 5.1 2.56 2.30 558Calcium chloride 22 7.8 2.79 2.42 566Aqua ammonia 14 7.0 2.48 2.44 541Trichloroethylene 100 123 7.44 2.11 432d-Limonene 100 142 6.47 1.48 321Methylene chloride 100 142 6.39 1.86
35、 585R-11 100 168 7.61 2.08 428aBased on inlet secondary coolant temperature at pump of 25F.bBased on one length of 16 ft tube with 1.06 in. ID and use of Moody Chart (1944) for anaverage velocity of 7 fps. Input/output losses equal one Vel. HD(V2/2g) for 7 fps veloc-ity. Evaluations are at a bulk te
36、mperature of 20F and a temperature range of 10F.cBased on curve fit equation for Kerns (1950) adaptation of Sieder and Tates (1936)heat transfer equation using 16 ft tube for L/D = 181 and film temperature of 5Flower than average bulk temperature with 7 fps velocity.Table 2 Comparative Ranking of He
37、at Transfer Factors at 7 fps*Secondary Coolant Heat Transfer FactorPropylene glycol 1.000d-Limonene 1.566Ethylene glycol 1.981R-11 2.088Trichloroethylene 2.107Methanol 2.307Aqua ammonia 2.639Sodium chloride 2.722Calcium chloride 2.761Methylene chloride 2.854*Based on Table 1 values using 1.06 in. ID
38、 tube 16 ft long. Actual ID and length varyaccording to specific loading and refrigerant applied with each secondary coolant, tubematerial, and surface augmentation.Table 3 Relative Pumping Energy Required*Secondary Coolant Energy FactorAqua ammonia 1.000Methanol 1.078Propylene glycol 1.142Ethylene
39、glycol 1.250Sodium chloride 1.295Calcium chloride 1.447d-Limonene 2.406Methylene chloride 3.735Trichloroethylene 4.787R-11 5.022*Based on same pump pressure, refrigeration load, 20F average temperature, 10Frange, and freezing point (for water-based secondary coolants) 20 to 23F below low-est seconda
40、ry coolant temperature.Secondary Coolants in Refrigeration Systems 13.3coolant to be stored. For maximum use of the storage tank volumeat the expected temperatures, choose inlet velocities and locate con-nections and tank for maximum stratification. Note, however, thatmaximum use will probably never
41、 exceed 90% and, in some cases,may equal only 75% of the tank volume.Example 1. Figure 1 depicts the load profile and Figure 2 shows thearrangement of a refrigeration plant with storage of a 23% (byweight) sodium chloride secondary coolant at a nominal 20F. Duringthe peak load of 50 tons, a range of
42、 8F is required. At an averagetemperature of 24F, with a range of 8F, the coolants specific heatcpis 0.791 Btu/lbF. At 28F, the weight per unit volume of coolantLat the pump = 1.183(62.4 lb/ft3)/(7.48 gal/ft3); at 20F, L=1.185(62.4 lb/ft3)/(7.48 gal/ft3).Determine the minimum size storage tank for 9
43、0% use, minimumcapacity required for the chiller, and sizes of the two pumps. The chillerand chiller pump run continuously. The secondary coolant storage pumpruns only during the peak load. A control valve to the load source divertsall coolant to the storage tank during a zero-load condition, so tha
44、t the ini-tial temperature of 20F is restored in the tank. During low load, only therequired flow rate for a range of 8F at the load source is used; the bal-ance returns to the tank and restores the temperature to 20F.Solution: If x is the minimum capacity of the chiller, determine theenergy balance
45、 in each segment by subtracting the load in each segmentfrom x. Then multiply the result by the time length of the respectivesegments, and add as follows:6(x 0) + 4(x 50) + 14(x 9) = 06x + 4x 200 + 14x 126 = 024x = 326x = 13.58 tonsCalculate the secondary coolant flow rate W at peak load:W = (50 200
46、)/(0.791 8) = 1580.3 lb/minFor the chiller at 15 tons, the secondary coolant flow rate isW = (15 200)/(0.791 8) = 474.1 lb/minTherefore, the coolant flow rate to the storage tank pump is1580.3 474.1 = 1106.2 lb/min. Chiller pump size is determined by474.1/(1.183 62.4)/7.48 = 48 gpmCalculate the stor
47、age tank pump size as follows:1106.2/(1.185 62.4)/7.48 = 112 gpmUsing the concept of stratification in the storage tank, the interfacebetween warm return and cold stored secondary coolant falls at the ratepumped from the tank. Because the time segments fix the total amountpumped and the storage tank
48、 pump operates only in segment 2 (seeFigure 1), the minimum tank volume V at 90% use is determined asfollows:Total mass = (1106.2 lb/min)(60 min/h)(4 h)/0.90 = 295,000 lbandV = 295,000/(1.185 62.4)/7.48 = 29,840 galA larger tank (e.g., 50,000 gal) provides flexibility for longer seg-ments at peak lo
49、ad and accommodates potential mixing. It may be desir-able to insulate and limit heat gains to 8000 Btu/h for the tank and lines.Energy use for pumping can be limited by designing for 46 ft head. Withthe smaller pump operating at 51% efficiency and the larger pump at52.5% efficiency, pump heat added to the secondary coolant is 3300 and7478 Btu/h, respectively.For cases with various time segments and their respective loads, themaximum load for segment 1 or 3 with the smaller pump operating can-not exceed the
