1、48.1CHAPTER 48ULTRALOW-TEMPERATURE REFRIGERATIONAutocascade Systems . 48.1Custom-Designed and Field-Erected Systems . 48.2Single-Refrigerant Systems 48.2Cascade Systems 48.3Low-Temperature Materials. 48.6Insulation . 48.9Heat Transfer . 48.9Secondary Coolants . 48.10LTRALOW-TEMPERATURE refrigeration
2、 is defined hereUas refrigeration in the temperature range of 58 to 148F.What is considered low temperature for an application depends on thetemperature range for that specific application. Low temperaturesfor air conditioning are around 32F; for industrial refrigeration,31 to 58F; and for cryogenic
3、s, approaching 0R. Applicationssuch as freeze-drying, as well as the pharmaceutical, chemical, andpetroleum industries, use refrigeration in the low temperature rangeas designated in this chapter.The 58 to 148F temperature range is treated separatelybecause design and construction considerations for
4、 systems thatoperate in this range differ from those encountered in industrialrefrigeration and cryogenics, which bracket it. Designers and build-ers of cryogenic facilities are rarely active in the low-temperaturerefrigeration field. One major type of low-temperature system is thepackaged type, whi
5、ch often serves applications such as environ-ment chambers. The other major category is custom-designed andfield-erected systems. Industrial refrigeration practitioners are thegroup most likely to be responsible for these systems, but they maydeal with low-temperature systems only occasionally; the
6、experienceof a single organization does not accumulate rapidly. The objectiveof this chapter is to bring together available experience for thosewhose work does not require daily contact with low-temperature sys-tems.The refrigeration cycles presented in this chapter may be usedin both standard packa
7、ged and custom-designed systems. Cascadesystems are emphasized, both autocascade (typical of packagedunits) and two-refrigerant cascade (found in custom-engineeredlow-temperature systems).AUTOCASCADE SYSTEMSAn autocascade refrigeration system is a complete, self-containedrefrigeration system in whic
8、h multiple stages of cascade cooling ef-fect occur simultaneously by means of vapor/liquid separation andadiabatic expansion of various refrigerants. Physical and thermody-namic features, along with a series of counterflow heat exchangersand an appropriate mixture of refrigerants, allow the system t
9、o reachlow temperature.Autocascade refrigeration systems offer many benefits, such as alow compression ratio and relatively high volumetric efficiency.However, system chemistry and heat exchangers are complex, re-frigerant compositions are sensitive, and compressor displacementis large.Operational C
10、haracteristicsComponents of an autocascade refrigeration system typicallyinclude a vapor compressor, an external air- or water-cooled con-denser, a mixture of refrigerants with descending boiling points, anda series of insulated heat exchangers. Figure 1 is a schematic of asimple system illustrating
11、 a single stage of autocascade.In this system, two refrigerants with significantly different boilingpoints are compressed and circulated by one vapor compressor. As-sume that one refrigerant is R-23 (normal boiling point, 115.6F)and the second refrigerant is R-404a (normal boiling point, 52F).Assume
12、 that ambient temperature is 77F and that the condenser is100% efficient.With properly sized components, this system should be able toachieve 76F in the absorber while the compression ratio is main-tained at 5.1 to 1. As the refrigerant mixture is pumped through themain condenser and cooled to 77F a
13、t the exit, compressor dis-charge pressure is maintained at 221 psig. At this condition, virtu-ally all R-404a is condensed at 95F and then further chilled tosubcooled liquid. Although R-23 molecules are present in both liq-uid and vapor phases, the R-23 is primarily vapor because of thelarge differ
14、ence in the boiling points of the two refrigerants. A phaseseparator at the outlet of the condenser collects the liquid by gravi-tational effect, and the R-23-rich vapor is removed from the outlet ofthe phase separator to the heat exchanger.At the bottom of the phase separator, an expansion device a
15、dia-batically expands the collected R-404a-rich liquid such that the out-let of the device produces a low temperature of 2.2F at 32 psig(Weng 1995). This cold stream is immediately sent back to the heatexchanger in a counterflow pattern to condense the R-23-rich vaporto liquid at 1.4F and 221 psig.
16、The R-23-rich liquid is then adia-batically expanded by a second expansion device to 76F. As it ab-sorbs an appropriate amount of heat in the absorber, the R-23 mixeswith the expanded R-404a and evaporates in the heat exchanger,providing a cold source for condensing R-23 on the high-pressureside of
17、the heat exchanger. Leaving the heat exchanger at super-heated conditions, the vapor mixture then returns to the suction ofthe compressor for the next cycle.The preparation of this chapter is assigned to TC 10.4, Ultralow-TemperatureSystems and Cryogenics.Fig. 1 Simple Autocascade Refrigeration Syst
18、emFig. 1 Simple Autocascade Refrigeration System48.2 2010 ASHRAE HandbookRefrigerationAs can be seen from this simple example, the autocascade effectderives from a short cycle of the refrigerant circuit within the sys-tem that performs only internal work to condense the lower boilingpoint refrigeran
19、t.The concept of the single-stage cycle can be extended to multiplestages. Figure 2 shows the flow diagram of a four-stage system. Thecondensation and subsequent expansion of one refrigerant providesthe cooling necessary to condense the next refrigerant in the heatexchanger downstream. This process
20、continues until the last refrig-erant with the lowest boiling point is expanded to achieve extremelylow temperature.Design ConsiderationsCompressor Capacity. As can be seen from Figures 1 and 2, asignificant amount of compressor work is used for internal evapo-rating and condensing of refrigerants.
21、The final gain of the system istherefore relatively small. Compressor capacity must be enough toproduce an appropriate amount of final refrigerating effect.Heat Exchanger Sizing. Because there is a significant amountof refrigerant vapor in each stage of the heat exchanger, the overallheat transfer c
22、oefficients on both the evaporating and condensingsides are rather small compared to those of pure components atphase-changing conditions. Therefore, generous heat-transfer areashould be provided for energy exchange between refrigerants on thehigh- and low-pressure sides.Expansion Devices. Each expa
23、nsion device is sized to providesufficient refrigerating effect for the adjacent downstream heatexchanger.Compressor Lubrication. General guidelines for lubrication ofrefrigeration systems should be adopted.CUSTOM-DESIGNED AND FIELD-ERECTED SYSTEMSIf refrigeration is to maintain a space at a low tem
24、perature to storea modest quantity of product in a chest or cabinet, the packaged low-temperature system is probably the best choice. Prefabricated walk-in environmental chambers are also practical solutions when theycan accommodate space needs. When the required refrigerationcapacity exceeds that o
25、f packaged systems, or when a fluid must bechilled, a custom-engineered system should be considered.The refrigeration requirement may be to chill a certain flow rateof a given fluid from one temperature to another. Part of the designprocess is to choose the type of system, which may be a multistagep
26、lant using a single refrigerant or a two-circuit cascade system usinga high-pressure refrigerant for the low-temperature circuit. Thecompressor(s) and condenser(s) must be selected, and the evapora-tor and interstage heat exchanger (in the case of the cascade system)must be either selected or custom
27、-designed.The design process includes selection of (1) metal for piping andvessels and (2) insulating material and method of application. Theproduct to be refrigerated may actually pass through the evaporator,but in many cases a secondary coolant transfers heat from the finalproduct to the evaporato
28、r. Brines and antifreezes that perform sat-isfactorily at higher temperatures may not be suitable at low tem-peratures. Compressors are subjected to unusual demands whenoperating at low temperatures, and, because they must be lubricated,oil selection and handling must be addressed.SINGLE-REFRIGERANT
29、 SYSTEMSSingle-refrigerant systems are contrasted with the cascade system,which consists of two separate but thermally connected refrigerantcircuits, each with a different refrigerant (Stoecker and Jones 1982).In the industrial refrigeration sector, the traditional refrigerantshave been R-22 and amm
30、onia (R-717). Because R-22 will ulti-mately be phased out, various hydrofluorocarbon (HFC) refriger-ants and blends are proposed as replacements. Two that might beconsidered are R-507 and R-404a.Two-Stage SystemsIn systems where the evaporator operates below about 4F,two-stage or compound systems ar
31、e widely used. These systems areexplained in Chapter 2 of this volume and in Chapter 2 of the 2009ASHRAE HandbookFundamentals. Advantages of two-stagecompound systems that become particularly prominent when theevaporator operates at low temperature includeImproved energy efficiency because of remova
32、l of flash gas at theintermediate pressure and desuperheating of discharge gas fromthe low-stage compressor before it enters the high-stage com-pressor.Improved energy efficiency because two-stage compressors aremore efficient operating against discharge-to-suction pressureratios that are lower than
33、 for a single-stage compressor.Avoidance of high discharge temperatures typical of single-stagecompression. This is important in reciprocating compressors butof less concern with oil-injected screw compressors.Possibility of a lower flow rate of liquid refrigerant to the evapo-rator because the liqu
34、id is at the saturation temperature of theintermediate pressure rather than the condensing pressure, as istrue of single-stage operation.Refrigerant and Compressor SelectionThe compound, two-stage (or even three-stage) system is anobvious possibility for low-temperature applications. However, atvery
35、 low temperatures, limitations of the refrigerant itself appear:freezing point, pressure ratios required of the compressors, andFig. 2 Four-Stage Autocascade SystemFig. 2 Four-Stage Autocascade SystemUltralow-Temperature Refrigeration 48.3volumetric flow at the suction of the low-stage compressor pe
36、r unitrefrigeration capacity. Table 1 shows some key values for four can-didate refrigerants, illustrating some of the concerns that arise whenconsidering refrigerants that are widely applied in industrial refrig-eration systems. Hydrocarbons (HCs), which are candidates partic-ularly in the petroleu
37、m and petrochemical industry, where the entireplant is geared toward working with flammable gases, are notincluded in Table 1.The freezing point is not a limitation for the halocarbon refrig-erants, but ammonia freezes at 108F, so its use must be restrictedto temperatures safely above that temperatu
38、re.The pressure ratios the compressors must operate against intwo-stage systems are also important. A condensing temperature of95F is assumed, with the intermediate pressure being the geometricmean of the condensing and evaporating pressures. Many low-temperature systems may be small enough that a r
39、eciprocatingcompressor would be favorable, but the limiting pressure ratiowith reciprocating compressors is usually about 8, a value chosento limit the discharge temperature. An evaporating temperature of94F is about the lowest permissible for systems using reciprocat-ing compressors. For evaporatin
40、g temperatures lower than 94F,consider using a three-stage system. An alternative to the reciprocat-ing compressor is the screw compressor, which operates with lowerdischarge temperatures because it is oil flooded. The screw com-pressor can therefore operate against larger pressure ratios than there
41、ciprocating compressor, and is favored in larger systems.The required volumetric pumping capacity of the compressoris measured at the compressor suction. This value is an indicator ofthe physical size of the compressor; the values become huge at the130F evaporating temperature.Some conclusions from
42、Table 1 areA single-refrigerant, two-stage system can adequately serve aplant in the higher-temperature portion of the range consideredhere, but it becomes impractical in the lower-temperature portion.Ammonia, which has many favorable properties for industrialrefrigeration, has little appeal for low
43、-temperature refrigerationbecause of its relatively high freezing point and pressure ratios.Special Multistage SystemsSpecial high-efficiency operations to recover volatile com-pounds such as hydrocarbons use the reverse Brayton cycle. Thisconsists of one or two conventional compressor refrigeration
44、 cycleswith the lowest stage ranging from 76 to 148F. This final stageis achieved by using a turbo compressor/expander and enables thecollection of liquefied hydrocarbons (Emh 1997; Enneking andPriebe 1993; Jain and Enneking 1995).CASCADE SYSTEMSThe cascade system (Figure 3) confronts some of the pr
45、oblems ofsingle-refrigerant systems. It consists of two separate circuits, eachusing a refrigerant appropriate for its temperature range. The twocircuits are thermally connected by the cascade condenser, whichis the condenser of the low-temperature circuit and the evaporatorof the high-temperature c
46、ircuit. Typical refrigerants for the high-temperature circuit include R-22, ammonia, R-507, and R-404a. Forthe low-temperature circuit, a high-pressure refrigerant with a highvapor density (even at low temperatures) is chosen. For many years,R-503, an azeotropic mixture of R-13 and R-23, was a popul
47、archoice, but R-503 is no longer available because R-13 is an ozone-depleting chlorofluorocarbon (CFC). R-23 could be and has beenused alone, but R-508b, an azeotrope of R-23 and R-116, has supe-rior properties, as discussed in the section on Refrigerants for Low-Temperature Circuit.The cascade syst
48、em has some of the thermal advantages of two-stage, single-refrigerant systems: it approximates flash gas removaland allows each compressor to take a share of the total pressure ratiobetween the low-temperature evaporator and the condenser. The cas-cade system has the thermal disadvantage of needing
49、 to provide anadditional temperature lift in the cascade condenser because the con-densing temperature of the low-temperature refrigerant is higherthan the evaporating temperature of the high-temperature refrigerant.There is an optimum operating temperature of the cascade condenserfor minimum total power requirement, just as there is an optimumintermediate pressure in two-stage, single-refrigerant systems.Figure 3 shows a fade-out vessel, which limits pressure in thelow-temperature circuit when the system shuts down. At room tem-perature, the pressu
