1、3.1CHAPTER 3CARBON DIOXIDE REFRIGERATION SYSTEMSApplications . 3.2System Design 3.3System Safety 3.5Piping. 3.6Heat Exchangers and Vessels. 3.8Compressors for CO2Refrigeration Systems. 3.8Lubricants 3.9Evaporators 3.10Defrost 3.10Installation, Start-up, and Commissioning 3.11ARBON dioxide (R-744) is
2、 one of the naturally occurringCcompounds collectively known as “natural refrigerants.” It isnonflammable and nontoxic, with no known carcinogenic, muta-genic, or other toxic effects, and no dangerous products of combus-tion. Using carbon dioxide in refrigerating systems can beconsidered a form of c
3、arbon capture, with a potential beneficialeffect on climate change. It has no adverse local environmentaleffects. Carbon dioxide exists in a gaseous state at normal tempera-tures and pressures within the Earths atmosphere. Currently, theglobal average concentration of CO2is approximately 390 ppm byv
4、olume.Carbon dioxide has a long history as a refrigerant. Since the1860s, the properties of this natural refrigerant have been studiedand tested in refrigeration systems. In the early days of mechanicalrefrigeration, few suitable chemical compounds were available asrefrigerants, and equipment availa
5、ble for refrigeration use was lim-ited. Widespread availability made CO2an attractive refrigerant.The use of CO2refrigeration systems became established in the1890s and CO2became the refrigerant of choice for freezing andtransporting perishable food products around the world. Meat andother food prod
6、ucts from Argentina, New Zealand and Australiawere shipped via refrigerated vessels to Europe for distribution andconsumption. Despite having traveled a several-week voyage span-ning half the globe, the receiving consumer considered the condi-tion of the frozen meat to be comparable to the fresh pro
7、duct. By1900, over 300 refrigerated ships were delivering meat productsfrom many distant shores. In the same year, Great Britain imported360,000 tons of refrigerated beef and lamb from Argentina, NewZealand, and Australia. The following year, refrigerated bananaships arrived from Jamaica, and tropic
8、al fruit became a lucrativecargo for vessel owners. CO2gained dominance as a refrigerant inmarine applications ranging from coolers and freezers for crew pro-visions to systems designed to preserve an entire cargo of frozenproducts.Safety was the fundamental reason for CO2s development andgrowth. Ma
9、rine CO2-refrigerated shipping rapidly gained popular-ity for its reliability in the distribution of a wide variety of fresh foodproducts to many countries around the world. The CO2marinerefrigeration industry saw phenomenal growth, and by 1910 some1800 systems were in operation on ships transportin
10、g refrigeratedfood products. By 1935, food producers shipped millions of tons offood products including meats, dairy products, and fruits to GreatBritain annually. North America also was served by CO2marinerefrigeration in both exporting and receiving food products.The popularity of CO2refrigeration
11、 systems reduced once suit-able synthetic refrigerants became available. The development ofchlorodifluoromethane (R-22) in the 1940s started a move awayfrom CO2, and by the early 1960s it had been almost entirelyreplaced in all marine and land-based systems.By 1950, the chlorofluorocarbons (CFCs) do
12、minated the major-ity of land-based refrigeration systems. This included a wide varietyof domestic and commercial CFC uses. The development of the her-metic and semihermetic compressors accelerated the developmentof systems containing CFCs. For the next 35 years, a number ofCFC refrigerants gained p
13、opularity, replacing practically all otherrefrigerants except ammonia, which maintained its dominant posi-tion in industrial refrigeration systems.In the 1970s, the atmospheric effects of CFC emissions werehighlighted. This lead to a concerted effort from governments, sci-entists, and industrialists
14、 to limit these effects. Initially, this took theform of quotas on production, but soon moved to a total phaseout,first of CFCs and then of hydrochlorofluorocarbons (HCFCs).The ozone depleting potential (ODP) rating of CFCs and HCFCsprompted the development of hydrofluorocarbon (HFC) refriger-ants.
15、Subsequent environmental research shifted the focus fromozone depletion to climate change, producing a second ratingknown as the global warming potential (GWP). Table 1 presentsGWPs for several common refrigerants. Table 2 compares perfor-mance of current refrigerants used in refrigeration systems.I
16、n recent years, CO2has once again become a refrigerant of greatinterest. However, high-pressure CO2systems (e.g., 490.8 psia at asaturation temperature of 30F, or 969.6 psia at 80F) present somechallenges for containment and safety.Advances in materials science since the 1950s enable the designof co
17、st-effective and efficient high-pressure carbon dioxide systems.The attraction of using CO2in modern systems is based on itsThe preparation of this chapter is assigned to TC 10.3, Refrigerant Piping.Table 1 Refrigerant DataRefrigerant Number Refrigerant Group Chemical FormulaTemperature at14.7 psia,
18、 F Safety Group GWP at 100 YearsR-22 HCFC CHClF241.4 A1 1700R-134a HFC CF3CH2F 15 A1 1300R-410A HFC blend HFC-32 (50%) 62.1 A1/A1 2000HFC-125 (50%)R-507A HFC blend HFC-125 (50%) 52.8 A1 3900HFC-143a (50%)R-717 Ammonia NH327.9 B2 0R-744 Carbon dioxide CO2109.1 A1 1Source: ANSI/ASHRAE Standard 34.3.2
19、2010 ASHRAE HandbookRefrigerationattractive thermophysical properties: low viscosity, high thermalconductivity, and high vapor density. These result in good heattransfer in evaporators, condensers, and gas coolers, allowingselection of smaller equipment compared to CFCs and HFCs. Car-bon dioxide is
20、unique as a refrigerant because it is being consideredfor applications spanning the HVAC this is con-sidered to be low compared to all commonly used refrigerants.APPLICATIONSTranscritical CO2RefrigerationIn a transcritical refrigeration cycle, CO2is the sole refrigerant.Typical operating pressures a
21、re much higher than traditional HFCand ammonia operating pressures. As the name suggests, the heatsource and heat sink temperatures straddle the critical temperature.Development on modern transcritical systems started in the early1990s with a focus on mobile air-conditioning systems. However,early m
22、arine systems clearly were capable of transcritical operationin warm weather, according to their operating manuals. For exam-ple, marine engineers sailing through the Suez Canal in the 1920sreported that they had to throttle the “liquid” outlet from the con-denser to achieve better efficiency if the
23、 sea water was too warm.They did not call this transcritical operation and could not explainwhy it was necessary, but their observation was correct.The technology suggested for mobile air conditioning was alsoadopted in the late 1990s for heat pumps, particularly air-sourceheat pumps for domestic wa
24、ter heating. In Japan, researchers andmanufacturers have designed a full line of water-heating-systemequipment, from small residential units to large industrial applica-tions, all incorporating transcritical CO2heat pump technology. Awide variety of such units was produced, with many different com-p
25、ressor types, including reciprocating, rotary piston, and scroll.Current commercial production of pure transcritical systems isprimarily in small-scale or retail applications such as soft drink vend-ing machines, mobile air conditioning, heat pumps, domestic appli-ances, and supermarket display free
26、zers. Commercial and industrialsystems at this time tend to use CO2as secondary refrigerant in aTable 2 Comparative Refrigerant Performance per Ton of RefrigerationRefrig-erant NumberEvapora-torPressure, psiaCon-denser Pressure, psiaNet Refrig-erating Effect,Btu/lbRefrigerant Circulated, lb/minSpeci
27、fic Volume of Suction Gas, ft3/lbR-22 42.8 172.2 69.9 0.81 1.248R-134a 23.6 111.2 63.6 0.89 1.945R-410A 69.3 271.5 72.2 0.77 0.873R-507A 55.0 211.6 47.4 1.20 0.814R-717 34.1 168.5 474.3 0.12 8.197R-744 326.9 1041.4 57.3 0.51 0.269Source: Adapted from Table 9 in Chapter 29 of the 2009 ASHRAE Handbook
28、Funda-mentals. Conditions are 5F and 86F.Fig. 1 CO2Expansion-Phase ChangesFig. 1 CO2Expansion-Phase Changes(Adapted from Vestergaard and Robinson 2003)Fig. 2 CO2Phase DiagramFig. 2 CO2Phase Diagram(Adapted from Vestergaard and Robinson 2003)Carbon Dioxide Refrigeration Systems 3.3two-phase cascade s
29、ystem in conjunction with more traditional pri-mary refrigerants such as ammonia or an HFC.In a transcritical cycle, the compressor raises the operating pres-sure above the critical pressure and heat is rejected to atmosphere bycooling the discharge gas without condensation. When the cooledgas passe
30、s through an expansion device, it turns to a mixture of liq-uid and gas. If the compressor discharge pressure is raised, theenthalpy achieved at a given cold gas temperature is reduced, sothere is an optimum operating point balancing the additional energyinput required to deliver the higher discharg
31、e pressure against theadditional cooling effect achieved through reduced enthalpy. Sev-eral optimizing algorithms have been developed to maximize effi-ciency by measuring saturated suction pressure and gas cooleroutlet temperature and regulating the refrigerant flow to maintain anoptimum discharge p
32、ressure. Achieving as low a temperature at thegas cooler outlet as possible is key to good efficiency, suggestingthat there is a need for evaporatively cooled gas coolers, althoughnone are currently on the market. Other devices, such as expanders,have been developed to achieve the same effect by red
33、ucing theenthalpy during the expansion process and using the recovered workin the compressor to augment the electrical input.CO2Cascade SystemThe cascade system consists of two independent refrigerationsystems that share a common cascade heat exchanger. The CO2low-temperature refrigerant condenser s
34、erves as the high-temperaturerefrigerant evaporator; this thermally connects the two refrigerationcircuits. System size influences the design of the cascade heatexchanger: large industrial refrigeration system may use a shell-and-tube vessel, plate-and-frame heat exchanger, or plate-and-shelltype, w
35、hereas commercial systems are more likely to use brazed-plate, coaxial, and tube-in-tube cascade heat exchangers. In chillingsystems, the liquid CO2is pumped from the receiver vessel belowthe cascade heat exchanger to the heat load. In low-temperatureapplications, the high-pressure CO2liquid is expa
36、nded to a lowerpressure and a compressor is used to bring the suction gas back upto the condensing pressure.Using a cascade system allows a reduced high-temperaturerefrigerant charge. This can be important in industrial applicationsto minimize the amount of ammonia on site, or in commercial sys-tems
37、 to reduce HFC refrigerant losses.CO2cascade systems are configured for pumped liquid recircu-lation, direct expansion, volatile secondary and combinations ofthese that incorporate multiple liquid supply systems.Low-temperature cascade refrigeration application include coldstorage facilities, plate
38、freezers, ice machines, spiral and belt freez-ers, blast freezers, freeze drying, supermarkets, and many otherfood and industrial product freezing systems.Some theoretical studies e.g., Vermeeren et al. (2006) have sug-gested that cascade systems are inherently less efficient than two-stage ammonia
39、plants, but other system operators claim lowerenergy bills for their new CO2systems compared to traditionalammonia plants. The theoretical studies are plausible because intro-ducing an additional stage of heat transfer is bound to lower thehigh-stage compressor suction. However, additional factors s
40、uch asthe size of parasitic loads (e.g., oil pumps, hot gas leakage) on thelow-stage compressors, the effect of suction line losses, and theadverse effect of oil in low-temperature ammonia plants all tend tooffset the theoretical advantage of two-stage ammonia system, andin the aggregate the differe
41、nce in energy consumption one way orthe other is likely to be small. Other factors, such as reduced ammo-nia charge, simplified regulatory requirements, or reduced operatorstaff, are likely to be at least as significant in the decision whether toadopt CO2cascades for industrial systems.In commercial
42、 installations, the greatest benefit of a CO2cascadeis the reduction in HFC inventory, and consequent probable reduc-tion in HFC emission. Use of a cascade also enables the operator toretain existing HFC compressor and condenser equipment whenrefurbishing a facility by connecting it to a CO2pump set
43、 andreplacing the evaporators and low-side piping. End users in Europeand the United States suggest that CO2cascade systems are simplerand easier to maintain, with fewer controls requiring adjustment,than the HFC systems that they are replacing. This indicates thatthey are inherently more reliable a
44、nd probably cheaper to maintainthan conventional systems. If the efficiency is equivalent, then thecost of ownership will ultimately be cheaper. However, it is not clearif these benefits derive from the higher level of engineering inputrequired to introduce the new technology, or whether they can be
45、maintained in the long term.SYSTEM DESIGNTranscritical CO2SystemsRecent advances in system component design have made it pos-sible to operate in previously unattainable pressure ranges. Thedevelopment of hermetic and semihermetic multistage CO2com-pressors provided the economical ability to design a
46、ir-cooled tran-scritical systems that are efficient, reliable, and cost effective.Today, transcritical systems are commercially available in sizesfrom the smallest appliances to entire supermarket systems. Figures3 and 4 shows examples of simple transcritical systems. Heat rejec-tion to atmosphere i
47、s by cooling the supercritical CO2gas withoutphase change. For maximum efficiency, the gas cooler must be ableto operate as a condenser in colder weather, and the control systemmust be able to switch from gas cooler operation (where outflowfrom the air-cooled heat exchanger is restricted) to condens
48、er oper-ation (where the restriction is removed, as in a conventional sys-tem). Compared to a typical direct HFC system, energy usage can bereduced by 5% in colder climates such as northern Europe, but mayincrease by 5% in warmer climates such as southern Europe or theUnited States. In a heat pump o
49、r a refrigeration system with heatrecovery, this dual control is not necessary because the system oper-ates transcritically at all times.CO2/HFC Cascade SystemsCascade refrigeration systems in commercial applications gener-ally use HFCs, or occasionally HCs, as the primary refrigerant.Supermarkets have adopted cascade technology for operational andeconomic reasons (the primary refrigerant charge can be reduced byas much as 75%). Liquid CO2is pumped to low-temperature displayFig. 1 CO2Expansion-Phase ChangesFig. 3 Transcritical CO2Refrigeration Cycle in Appliances and Vendin
