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 3 K belowand preferably 8 K below the lowest temperature to which it will beexposed. When subjected to the lowest temperature in the system,coolant viscosity should be low enough to allow sati
5、sfactory heattransfer 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 chi
7、ller. 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 density, sp
8、e-cific heat, viscosity, and thermal conductivity are based on the aver-age bulk temperature of coolant in the heat exchanger, noting thatfilm temperature corrections are based on the average film temper-ature. Trial solutions of the secondary coolant-side coefficient com-pared to the overall coeffi
9、cient and total log mean temperaturedifference (LMTD) determine the average film temperature. Wherethe secondary coolant is cooled, the more viscous film reduces theheat transfer rate and raises the pressure drop compared to what canbe expected at the bulk temperature. Where the secondary coolant is
10、heated, the less viscous film approaches the heat transfer rate andpressure 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 device resu
11、lts 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 heat transfer
12、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 function of th
13、e 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 electricmotor).
14、 Small centrifugal pumps, operating in the range of approx-imately 3 L/s at 240 kPa to 9 L/s at 210 kPa, for 60 Hz applications,typically have 45 to 65% efficiency, respectively. Larger pumps,operating in the range of 30 L/s at 240 kPa to 95 L/s at 210 kPa, for60 Hz applications, typically have 75 t
15、o 85% efficiency, respec-tively.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 motor is used as the driver, motor ineffi-cie
16、ncy 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 and brine line insulation heat gains,as w
17、ell as the delivered beneficial cooling, tabulating typical second-ary coolant performance values helps in coolant selection. A 27 mmID smooth steel tube evaluated for pressure drop and internal heattransfer coefficient at the average bulk temperature of 6.7C and atemperature range of 5.6 K for 2.1
18、m/s tube-side velocity providescomparative data (Table 1) for some typical coolants. Table 2 ranksthe same coolants comparatively, using data from Table 1.For a given evaporator configuration, load, and temperaturerange, select a secondary coolant that gives satisfactory velocities,heat transfer, an
19、d pressure drop. At the 6.7C level, hydrocarbonand halocarbon secondary coolants must be pumped at a rate of 2.3to 3.0 times the rate of water-based secondary coolants for the sametemperature range.Higher pumping rates require larger coolant lines to keep thepumps pressure and power requirement with
20、in reasonable limits.Table 3 lists approximate ratios of pump power for secondary cool-ants. Heat transferred by a given secondary coolant affects the costand perhaps the configuration and pressure drop of a chiller andother heat exchangers in the system; therefore, Tables 2 and 3 areonly guides of
21、the relative merits of each coolant.The preparation of this chapter is assigned to TC 10.1, Custom EngineeredRefrigeration Systems.13.2 2010 ASHRAE HandbookRefrigeration (SI)Other ConsiderationsCorrosion must be considered when selecting coolant, inhibitor,and system components. The effect of second
22、ary 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 stability for anticipated mois-ture, air, and con
23、taminants 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 considered for the application.DESIGN CONSIDERATI
24、ONSSecondary 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 level ofsecondary coolant (e.g., in the expansion tan
25、k 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 lowest secondary coolant temper-ature dictate the des
26、ign 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 in case of heatradiation from a fire in the area
27、, 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 found in ASHRAEStandard 15. Design standards for p
28、ressure 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 methods in Chapters 3 and 22 of the 2009 ASHRAEHan
29、dbookFundamentals. 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 pressure dropthrough the system for proper response an
30、d 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 tanks can shave peak loads for brief periods, li
31、mit 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 started to satisfy peak load.Energy cost savings
32、 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 determine the minimum net capacity required for ther
33、efrigeration plant, pump sizes, and minimum amount of secondaryTable 1 Secondary Coolant Performance ComparisonsSecondary CoolantConcentration(by Mass), %Freeze Point,C L/(s kW)aPressure Drop,bkPaHeat Transfer Coefficientchi, W/(m2KPropylene glycol 39 20.6 0.0459 20.064 1164Ethylene glycol 38 21.6 0
34、.0495 16.410 2305Methanol 26 20.7 0.0468 14.134 2686Sodium chloride 23 20.6 0.0459 15.858 3169Calcium chloride 22 22.1 0.0500 16.685 3214Aqua ammonia 14 21.7 0.0445 16.823 3072Trichloroethylene 100 86.1 0.1334 14.548 2453d-Limonene 100 96.7 0.1160 10.204 1823Methylene chloride 100 96.7 0.1146 12.824
35、 3322R-11 100 111.1 0.1364 14.341 2430aBased on inlet secondary coolant temperature at pump of 3.9C.bBased on one length of 4.9 m tube with 26.8 mm ID and use of Moody Chart (1944) foran average velocity of 2.13 m/s. Input/output losses equal V2/2 for 2.13 m/s velocity.Evaluations are at a bulk temp
36、erature of 6.7C and a temperature range of 5.6 K.cBased on curve fit equation for Kerns (1950) adaptation of Sieder and Tates (1936)heat transfer equation using 4.9 m tube for L/D = 181 and film temperature of 2.8Clower than average bulk temperature with 2.134 m/s velocity.Table 2 Comparative Rankin
37、g of Heat Transfer Factors at 2 m/s*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 27 m
38、m ID tube 4.9 m long. Actual ID and length varyaccording to specific loading and refrigerant applied with each secondary coolant,tube material, and surface augmentation.Table 3 Relative Pumping Energy Required*Secondary Coolant Energy FactorAqua ammonia 1.000Methanol 1.078Propylene glycol 1.142Ethyl
39、ene 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, 6.7C average temperature, 6 Krange, and freezing point (for water-based secondary coolants) 11 to 13 K below low-est s
40、econdary 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
41、 never 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% (by mass)sodium chloride secondary coolant at a nominal 6.7C. During thepeak load of 176 kW, a rang
42、e of 4.4 K is required. At an average tem-perature of 4.4C, with a range of 4.4 K, the coolants specific heatcpis 3.314 kJ/(kgK). At 2.2C, the density of coolant Lat the pump= 1183 kg/m3; at 6.7C, L= 1185 kg/m3.Determine the minimum size storage tank for 90% use, minimumcapacity required for the chi
43、ller, 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 that the ini-tial temperature of 6.7C is restor
44、ed in the tank. During low load, onlythe required flow rate for a range of 4.4 K at the load source is used; thebalance returns to the tank and restores the temperature to 6.7C.Solution: If x is the minimum capacity of the chiller, determine theenergy balance in each segment by subtracting the load
45、in each segmentfrom x. Then multiply the result by the time length of the respectivesegments, and add as follows:6(x 0) + 4(x 176) + 14(x 31.7) = 06x + 4x 704 + 14x 443.8 = 024x = 1146.8x = 47.8 kWCalculate the secondary coolant flow rate W at peak load:W = 176/(3.314 4.4) = 12.07 kg/sFor the chille
46、r at 52.8 kW, the secondary coolant flow rate isW = 52.8/(3.314 4.4) = 3.62 kg/sTherefore, the coolant flow rate to the storage tank pump is12.07 3.62 = 8.48 kg/s. Chiller pump size is determined by1000 3.62/1183 = 3.06 L/sCalculate the storage tank pump size as follows:1000 8.48/1185 = 7.16 L/sUsin
47、g 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 pump operates only in segment 2 (seeFigure 1), the minimum tank vol
48、ume V at 90% use is determined asfollows:Total mass = 8.48 kg/s 4 h 3600 s/h/0.9 = 135 700 kgandV = 135 700/1185 = 114.5 m3A larger tank (e.g., 190 m3) provides flexibility for longer segmentsat peak load and accommodates potential mixing. It may be desirable toinsulate and limit heat gains to 2.3 k
49、W for the tank and lines. Energy usefor pumping can be limited by designing for 160 kPa. With the smallerpump operating at 51% efficiency and the larger pump at 52.5% effi-ciency, pump heat added to the secondary coolant is 970 and 2190 W,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 net capacity of the chiller minus insulation and pumpheat gain to the secondary coolant. For various combinations of seg-men
