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 50 to 100C.What is considered low temperature for an application depends on thetemperature range for that specific application. Low temperaturesfor air conditioning are around 0C; for industrial refrigeration,35 to 50C; and for cryogenics
3、, approaching 0 K. Applicationssuch as freeze-drying, as well as the pharmaceutical, chemical, andpetroleum industries, use refrigeration in the low temperature rangeas designated in this chapter.The 50 to 100C 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, 82C) andthe second refrigerant is R-404a (normal boiling point, 46.7C).Assume
12、that ambient temperature is 25C and that the condenser is100% efficient.With properly sized components, this system should be able toachieve 60C 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 25C at
13、 the exit, compressor dis-charge pressure is maintained at 1524 kPa (gage). At this condition,virtually all R-404a is condensed at 35C 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 di
14、fference 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 devi
15、ce adia-batically expands the collected R-404a-rich liquid such that the out-let of the device produces a low temperature of 19C at 220 kPa(gage) (Weng 1995). This cold stream is immediately sent back tothe heat exchanger in a counterflow pattern to condense the R-23-rich vapor to liquid at 18.5C an
16、d 1524 kPa (gage). The R-23-richliquid is then adiabatically expanded by a second expansion deviceto 60C. As it absorbs an appropriate amount of heat in the ab-sorber, the R-23 mixes with the expanded R-404a and evaporates inthe heat exchanger, providing a cold source for condensing R-23 onthe high-
17、pressure side of the heat exchanger. Leaving the heat ex-changer at superheated conditions, the vapor mixture then returns tothe suction of the compressor for the next cycle.The preparation of this chapter is assigned to TC 10.4, Ultralow-TemperatureSystems and Cryogenics.Fig. 1 Simple Autocascade R
18、efrigeration SystemFig. 1 Simple Autocascade Refrigeration System48.2 2010 ASHRAE HandbookRefrigeration (SI)As 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 b
19、oilingpoint refrigerant.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 dow
20、nstream. This process 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 conden
21、sing of refrigerants. 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
22、overallheat transfer coefficients 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.Expans
23、ion Devices. Each expansion 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 maintai
24、n a space at a low temperature 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 refrigerationc
25、apacity exceeds that of 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, whic
26、h may be a multistageplant 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 eit
27、her selected or custom-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 finalpr
28、oduct to the evaporator. 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 addres
29、sed.SINGLE-REFRIGERANT 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 refrigerants
30、have been R-22 and ammonia (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 about20C, two-stage
31、 or compound systems are widely used. These sys-tems are explained in Chapter 2 of this volume and in Chapter 2 ofthe 2009 ASHRAE HandbookFundamentals. Advantages of two-stage compound systems that become particularly prominent whenthe evaporator operates at low temperature includeImproved energy ef
32、ficiency because of removal 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 pressure
33、ratios that are lower than 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 ev
34、apo-rator because the liquid 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 app
35、lications. However, atvery 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 t
36、he low-stage compressor per 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 par
37、tic-ularly in the petroleum 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 77.8C, so its use must berestricted to temperatures
38、safely above that temperature.The pressure ratios the compressors must operate against intwo-stage systems are also important. A condensing temperature of35C is assumed, with the intermediate pressure being the geometricmean of the condensing and evaporating pressures. Many low-temperature systems m
39、ay be small enough that a reciprocatingcompressor would be favorable, but the limiting pressure ratiowith reciprocating compressors is usually about 8, a value chosento limit the discharge temperature. An evaporating temperature of70C is about the lowest permissible for systems using reciprocat-ing
40、compressors. For evaporating temperatures lower than 70C,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
41、 pressure ratios than thereciprocating 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 the90C evaporating temperat
42、ure.Some conclusions from 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,
43、 has little appeal for low-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 conventiona
44、l compressor refrigeration cycleswith the lowest stage ranging from 60 to 100C. 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
45、) confronts some of the problems 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, which isthe condenser of the low-temperature circuit and the evaporato
46、r ofthe high-temperature circuit. 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
47、R-13 and R-23, was a popularchoice, 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-Temperatur
48、e Circuit.The cascade system 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 therm
49、al disadvantage of needing 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 the low-temperature circuit when the system shuts down. At roo
