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Alternative heat pump working fluids to CFCs
Heat Recovery S)wtems &CHP Vol. 15, No. 3, pp. 273-279, 1995
~ ) Pergamon
Elsevier ScienceLtd Pdnted in Great Britain 0890-4332/95$9.50+ .00
0890-4332(94)E0021-B
ALTERNATIVE
HEAT
PUMP
WORKING
FLUIDS
TO
CFCs* SUKUMAR DEVOTTA National Chemical Laboratory, Pune 411 008, India (Received with revisions 18 February 1994)
Abstract---CFC-I 1, CFC-12, CFC-114 and HCFC-22 are used as mechanical vapour compression heat pump working fluids for low and high temperature applications. Because of the Montreal Protocol, alternative heat pump working fluids to CFCs have to be identified and developed. This paper discusses the various potential alternatives screened from various families of compounds. HFC-134a is the most popular choice as an alternative to CFC-12, although HFC-152a appears to be a better alternative. Among the HFEs, HFE-134 is a potential long-term alternative. HFC-143 and HFE-134 appear to be attractive as substitutes for CFC-I14 for high temperature applications. For centrifugal compressor applications, HCFC-123 is currently the most popular choice as an interim alternative to CFC-I 1. HFC-245ca and HFC-143 are also potential long-term substitutes for CFC-11. The various implications with respect to these alternatives are discussed.
NOMENCLATURE HGWP ODP Pc Tc
Global warming potential relative to CFC-I 1 Ozone depletion potential Critical pressure (MPa) Critical temperature (°C)
1. I N T R O D U C T I O N M a n y chlorofluorocarbons (CFCs) and hydrochiorofluorocarbons ( H C F C s ) have been used as heat p u m p working fluids in the past two decades or so. They possess most o f the desirable characteristics, such as thermal and chemical stability, t h e r m o d y n a m i c efficiency, non-toxicity, non-flammability, low cost, etc., but C F C s are being blamed for the stratospheric ozone layer depletion. The Montreal Protocol mandates that C F C s and H C F C s should be phased out on a global scale. Heat p u m p s are generally used for space heating and for process heat recovery and recycling in industrial sectors. U N E P Technology Options R e p o r t [1] presents a comprehensive review o f the current status o f heat p u m p installation a r o u n d the world. Most o f the smaller heat p u m p s use hermetic reciprocating compressors, while the bigger sizes use both semi-hermetic and open types o f reciprocating, scroll and screw compressors. Turbo-compressors are used for very large sizes. Commercial heat p u m p s have been developed using CFC-11, H C F C - 2 2 , CFC-12 and CFC-114, for condensing temperatures o f up to 50°C, 60°C, 70°C and 120°C respectively. C F C - 1 2 is the most widely used fluid, followed very closely by H C F C - 2 2 . CFC-500, CFC-502 and C F C - I I (used exclusively with centrifugal compressors) are also used to some extent. Although CFC-114 accounts only for a b o u t 1% o f the C F C s used in heat pumps, it is exclusively used when the condensing temperature is in excess o f 80°C. The designs o f heat pumps using these fluids are well established. It was estimated [1] that the annual global consumption o f C F C s was about 1700 tonnes. F o r temperatures above 150°C, R718 (water) is very attractive. However, the use o f water mandates the use o f unlubricated (dry) compressors, which are fairly expensive when c o m p a r e d to refrigeration compressors. Heat pumps are still viewed as expensive for energy recovery; additional *NCL Communication No: 5918. 273
274
SUKUMAR DEVOTTA Table 1. Comparative data for some HFC alternatives to CFC12
Molecular formula Mol. wt, kgkmol ' Tt , C Pc, MPa Ts , C ODP HGWP Flammability
CFCI2
HFCI43a
HFCI34a
HFC227
HFCI52a
HFCI34
CCI2F _, 120.9 112.0 4. I 13 -29.79 1.0 2.08 NF
CHCIF 2 84.04 73.1 3.826 -47.6 0.0 0.828 F
CF3CHFCF, 102.0 101 7 4.1)55 26.2 0.0 0.343 NF
CH3CF~ 170.0 101.9 2.952 17.3 0.0 NA NF
CHF2CHF 2 66.0 I I3.5 4.49 -24.7 0.0 0.04 F
CHF2CHF 2 102.03 115.6 3.77 -19.7 0.0 NA NA
F--Flammable, N A - - N o t available, NF
Non-flammable, UC
Uncertain
initial fixed capital costs have made the return on investment less attractive. Therefore, the steam heat pumps have yet to realise their full potential and are mostly limited to industrial sectors. Water has no environmental controversies. If the potential for high temperature heat pumps has to be safeguarded, alternatives have to be identified soon. This will be the major factor determining the future of mechanical vapour compression heat pumps. 2. ALTERNATIVES TO CFCs Any alternative has to have a low, or preferably zero, "Ozone Depleting Potential (ODP)". Ideally, one should also screen for alternatives which satisfy other environmental concerns. The substitutes should have relatively low "Global Warming Potentials (GWP)" and also should not be "volatile organic compounds (VOCs)" having high smog potential. These constraints limit the domain within which alternatives could be found. It is now acknowledged that, in order to satisfy the environmental regulations, one has to compromise on other factors, such as flammability, energy efficiency, manufacturing feasibility (and hence cost), import, re-design of the system etc. Midgley [2] with his molecular screening approach, using the periodic table to identify potential refrigerants, concluded that they should be made with some combination of carbon, hydrogen, nitrogen, oxygen, sulphur, fluorine, chlorine and bromine. With toxicity and stability, complications associated with sulphur and nitrogen compounds, these were eliminated. Although Midgley identified a few more families of compounds, ultimately the choice was for CFCs. McLinden and Didion [3], using various thermodynamic constraints and, eliminating chlorine and bromine due to their active participation in ozone layer depletion, concluded that the potential refrigerants should consist of carbon, fluorine, hydrogen and oxygen. From the combinations of H, F and C leading to fluorocarbons (FCs) and hydrofluorocarbons (HFCs), FCs were discarded because of their longer atmospheric life and hence undesirably high GWP. Some thermodynamic screening for refrigeration [4-11] and heat pumps [12-18] from many classes of compounds has been reported. Some theoretical and experimental evaluations of HCC-160 [12] and HCC-280a [16] as medium temperature and high temperature working fluids, respectively, have been reported. Devotta and Gopichand [13] found that there are attractive flammable refrigerants, including HFC-152a. Vamling et al. [18] found that most of the flammable fluids, including HFC-152a, give better (COP) values than CFC-114 for heat pumps. Holland e t al. [19] have theoretically assessed HC-290 (propane), HC-600 (n-butane) and HC-600a (isobutane) and found them to be attractive working fluids for both medium and high temperature ranges. The quest for alternatives does not appear to be over with HFCs. Some more alternatives with zero ODP have been found from combinations of H, F, C and O, e.g. partially fluorinated ethers Table 2. Comparative data for some hydrocarbon alternatives to CFC-12 Refrigerant
CFC 12
HC290
HCC270
HC600a
HC600
Molecular formula
CCI2F~ 120.9 112.0 4.113 -29.79 2.941 2.08 --
-CH2 CH2H 2 42.08 124.7 5.49 32.85 0.0 NA 2.4/10.4
CH(CHs) 3
Mol. wt, kgkmol '
CHsCH _, CH~ 44.1 96.8 4.25 -42.1 0.0 0.0008 2.2/9.5
CH3CH 2 CH2CH~ 58.13 152.0 3.8 -0.5 0.0 0.0008 1.9/8.5
Tc, C Pc, MPa
TH, C ODP HGWP LEL/UEL vol % N A - - N o t Available
58.13 135.0 3.65 -11.7 0.0 0.0008 1.9/8.5
A l t e r n a t i v e heat p u m p w o r k i n g fluids
275
Table 3. Comparative data for some HFE and FE alternatives to CFC-12
Molecular formula Mol. wt, kg kniol ~ Tc, C Pc, MPa Ta, 'C ODP HGWP Flammability
CFCI2
HFEI34
HFE227
FE218
FE216
CCI2F 2 120.9 112.0 4.113 -29.79 1.0 2.08 NF
C:H2F40 118.03 147.1 3.951" 6.2 0.0 NA NF
C3HF70 186.03 114.6 2.64 -4.2 0.0 NA NF
C3F802 220.02 99.2 2.33 - 10.0 0.0 NA NF
C3F60 166.02 88.7 3.09 -29.9 0.0 NA NF
*Estimated, N A - - N o t available, NF--Non-flammable
(HFEs) as well as some cyclic fluorinated compounds [4, 5, 8-11, 15]. The fluorinated ethers, notably high molecular weight ones, are commercially manufactured, although the volume of production of low molecular weight ethers may not be significant. The synthesis of these compounds is being established [4, 20]. From the structural features of these compounds, it is expected that they are likely to be adequately stable but will still have lower atmospheric lives and hence lower GWPs. Although fluorinated ethers were identified as potential refrigerants and patented as early as the 1930s [10], they were not commercially exploited for as long as CFCs, which are simpler fluids compared with the other compounds, could serve the purpose. The current environmental concerns have forced us to re-examine these fluids. 2.1. Alternatives to H C F C - 2 2 HCFCs have been included as controlled substances under the Montreal Protocol in the 1992 Copenhagen convention. Therefore, considerable initiatives have already been taken by industry to look for alternatives. Most of the search is jointly considered along with R502 (an azeotropic mixture HCFC-22/CFC-115). The Air-Conditioning and Refrigeration Institute [21] in the U.S.A. has already launched an initiative as the "R-22 Alternative Refrigerants Evaluation Program (R-22 AREP)". HC-290, HFC-134a, R-717 (ammonia) and some blends of HFC-32, HFC-125, HFC134a, HFC-143a and HC-290 have been identified as potential alternatives to HCFC-22. Although the emphasis will be for air-conditioning and refrigeration, these results would also be extremely useful for domestic heat pumps. In general, there is great enthusiasm to use propane or propane/butane/isobutane mixtures as the alternative, particularly in the European sector. Recently, two domestic heat pumps running on R-502 and HCFC-22 in Germany were effectively converted to run on a propane/butane mixture with some improvements of COP. A study undertaken by the lEA Heat Pump Centre in the Netherlands [22] had concluded that the risks arising out of using hydrocarbons were grossly exaggerated. 2.2. Alternatives to CFC-12 Some of the potential alternatives to CFC-12 from HFCs, hydrocarbons and HFEs are listed in Tables 1, 2 and 3 respectively. HFC-134a is considered to be the most potential substitute for CFC-12 for both refrigeration and heat pump applications [6, 8, 13]. HFC-134a is significantly costlier than CFC-12. The various tests under PAFT which is commercially available through international vendors and AFEAS have not indicated any adverse effects of HFC-134a. A few experimental assessments of HFC-134a have been reported for heat pump applications. The performance of HFC-134a under retrofit and for identical conditions has been found to be either similar or even better than that of CFC-12, in applications where water is either the source or the sink. This improvement is generally attributed to the better heat transfer characteristics of HFC-134a [23]. A large scale 25 MW district heat pump, originally designed for CFC-500 with a two stage turbo compressor, has been successfully converted to HFC-134a and operated for more than a year [24]. The converted heat pump has been reported to be operating without much difference in the original performance, with most of the components unaffected. HFC-152a is a better choice because of its higher COP, zero ODP and relatively low Global Warming Potential (GWP) [6-8]. However, problems associated with its flammability aspects have
CFC-114
H FC-236cb
HFC-254cb
HFE-134
H F C - 143
HFC-236ca
F - - F l a m m a b l e , N A - - N o t available, N F - - N o n - f l a m m a b l e , U C - - U n c e r t a i n
CF3CH2CF 3 CF2HCF2CH 3 CHF2OCHF z CH2FCHF 2 CHF2CF2CHF 2 152.01 116.03 118.0 84.02 152.01 130.6 146.1 153.4 157.8 138.9 3.177 3.753 3.751 4.233 3.299 - 1.11 -0.78 4.67 5.00 5.00 0.0 0.0 0.0 0.0 0.0 NA NA NA NA NA NF UC UC F NF
HFC-236fa
Table 4. C o m p a r a t i v e data for some H F C and H F E alternatives to CFC-114
Molecular formula C C I F ~ C C I F 2 CF3CF.~CH2F Mol. wt, k g k m o l ~ 170.9 152.0 T~, C 145.7 130.1 Pc, M P a 3.263 3.118 T a. C 3.78 - 1.22 ODP 0.8 0.0 HGWP 1.97 NA Flammability NF NF
Refrigerant
HFC-236ea CF~CHFCHF~ 152.01 141.1 3.533 6.50 0.0 NA NF
©
o~
277
A l t e r n a t i v e heat p u m p w o r k i n g fluids Table 5. Comparative data for some HCFC alternatives to CFC-II
Molecular formula Mol. wt, kgkmol i Tc , C Pc, MPa T a, C ODP HGWP Flammability
CFC-I1
HCFC-226ea
HCFC-226da
HCFC-235ca
HCFC-123
HCFC-123a
HCFC-141b
CCI~F 137.4 198.4 4.41 23.8 1.0 1.0 NF
CF3CHFCF2CI 186.45 158.3 2.985 17.6 0.0 NA NA
CF3CHCICF 3 186.45 158.5 3.063 14.1 0.0 NA NA
CF3CF2CH2CI 168.45 170.3 3.083 28.1 -NA NF
CHCI2CF 3 152.91 183.8 3.676 27.9 0.02 0.024 NF
CHCIFCCIF, 152.9 189.62 3.889 29.9 -NA NF
CCI2FCH 3 116.9 208.0 4.339 32.1 0. I 0.126 F
N A - - N o t available, NF--Non-flammable
to be mitigated. HFC-152a is likely to be as stable a fluid as HFC-134a, but, with more H atoms, it will pose miscibility problems similar to HFC-134a. It is already commercially available and it is cheaper than HFC-134a. The compressor discharge temperature is also likely to be relatively high, this will require some metallurgical considerations for the discharge ports and better insulating materials for the motor windings for applications with hermetic compressors. Initially, there was a significant enthusiasm about the use of HFC-152a, but this has considerably subsided because of the undue concerns over its flammability. From Table 2 it is clear that no hydrocarbon has its boiling point close to CFC-12. Only cyclopropane (HCC-270) has its characteristics close to CFC-12. It is felt that the stability of cyclopropane, owing to its acute bond angle, will not make it suitable for heat pump applications. Therefore, it may be only prudent to consider mixtures of HC-290/HC-600/HC-600a to match the design features of the CFC-12 compressor as is being undertaken in domestic refrigerators. Some of the potential fluorinated and partially fluorinated ethers are listed in Table 3. The partially fluorinated ethers are preferred, as they have a much lower GWP than fluorinated ethers. From a comparative assessment of various fluorinated ethers, Devotta et al. [8] have identified HFE-134 as a potential candidate for refrigeration applications, although the compressor size may be suitable for centrifugal applications. Therefore, for large capacity heat pumps, like district heating, HFE-134 could be considered. 2.3. Alternatives to C F C - I14 From about 30 HFCs, HFEs and FEs, the following fluids were selected by Devotta and Rao Pendyala [15] as potential substitutes for CFC-II4: HFC-143, HFC-236ca, HFC-236cb, HFC236ea, HFC-236fa, HFC-254cb, HFE-134. Table 4 presents some basic data for these fluids. It is generally known that simple fluids are preferred both from performance and other aspects. The process development aspect is more difficult with bigger molecules like propane derivatives. Simpler molecules offer better efficiency, e.g. HFC-143 and HFE-134 offering better energy efficiency than the propane derivatives. HFC-143 was found to be attractive from the energy point of view [15]. The operating pressures are very close to CFC-114, although (PR) values are relatively high. The compressor size will be smaller than that for CFC-114. The use of HFC-143 warrants more stringent safety designs to mitigate the risks of flammability. For industrial applications, where high temperature heat pumps are generally used, this may not be a major hurdle. There is no report on the process development efforts for HFC-143. The use of HFE-134 also appears to be appealing from an energy efficiency point of view [15]. HFE-134 was found to be attractive as an alternative to CFC-12 for refrigeration [8]. There are already some initiatives for the use of HFE-134 as an alternative to CFC-I 1, CFC-12 and CFC-114 Table 6. Comparative data for some HFC alternatives to CFC-II CFC-I I Molecular formula Mol. wt, kg kmol t Tc. C Pc, MPa T B. C ODP HGWP Flammability
CCI3F 137.4 198.4 4.41 23.8 1.0 1.0 NF
HFC-347ccd
HFC-338eea
CF3CF.~CF2CH ~ CF3CFHCFHCF 184.02 202.02 144.2 148.6 2.572 2.747 15.1 25.4 0.0 0.0 NA NA NA NA
N A - - N o t available, NF--Non-flammable. UC--Uncertain
HFC-245 fa
HFC-245ca
CF3 CH_,CF_~H 134.0 157.5 3.644 15.3 0.0 0.089 UC
CF,HCF2CH_~ F 134.0 178.4 3.885 25.0 0.0 NA UC
278
SUKUMAR DEVOTTA Table 7. Comparative data for the HFE alternatives to CFC-II
Molecular formula Mol. wt, kgkmol ~ Tt . C Pc, MPa T~, C ODP HGWP Flammability
CFC-I 1
HFE-245fa
CCI3F 137.4 198.4 4.41 23.8 1.0 1.0 NF
CFsCH2OCF2H 150.05 171.0 3.376 29.2 0.0 NA --
HFE-245cb
HFE-143
CFsOCF2CH s CH2FOCHF ., 150.05 100.0 185.17 186.83 3.42 4.141 34.06 30.06 0.0 0.0 NA NA UF F
HFE-254cb CHF2OCF2CH ~ 132.0 189.44 3.56 36.44 0.0 NA F
N A - - N o t available, NF--Non-flammable, UF--Uncertain flammability, F--Flammable
in refrigeration [21] but the plans for its process development and commercial production are not yet clear. It is worth investigating these fluids for a whole range of applications of heat pumps. 2.4. Alternatives to CFC- I1 As in the case of refrigeration, the use of CFC-11 is limited to large size space heating applications with water cooled condensers using the conventional centrifugal compressors. The condensing temperatures of heat pumps using CFC-11, in spite of its higher critical temperature, are limited to those of HCFC-22 because CFC-11 has a very poor thermal and chemical stability. The HCFC-123 boiling point is reasonably close to CFC-II and its molecular weight is marginally higher. Therefore, it is being projected as an "aerodynamically and thermodynamically acceptable drop-in" substitute for CFC-11 in centrifugal chillers. HCFC-123 could be considered as an interim working fluid for centrifugal heat pumps [13, 25]. Most of the centrifugal chillers manufactured now are supposed to be compatible with both CFC-11 and HCFC-123. HCFC-123 is available in the international market. With the preliminary toxicity test results, the allowable exposure limit (AEL) for HCFC-123 has recently been fixed at 30 ppm, compared to 1000 ppm, for CFC-11. Although HCFC-123 is considered to possess the best potential as a substitute for CFC-11, it is still a transition substitute. It will be a long time before one can confidently use HCFC-123 as an alternative to CFC-11 in centrifugal heat pumps. HFC-254ca and HFE-143 are also potential long-term candidates for this application [9, 26]. 3. C O N C L U S I O N HFC-134a is the most popular choice as an alternative to CFC-12. Except for its flammability, H FC-! 52a offers better efficiency. HFE-134 also appears to be a long-term alternative. HFC-143 and HFE-134 are the potential short- and long-term candidates for CFC-114 replacement in high temperature heat pumps. For CFC-11, HFC-245ca and HFE-143 are the potential long-term substitutes. The substitution of CFCs by any alternative would involve substantial changes, including various components, such as insulation, lubricants, heat exchangers, motors, etc. Tests have to be carried out to optimise the system performance and to ensure the reliability and safety of the system. The chemical stability of HFCs [27] and fluorinated ethers may not be a limiting condition in high temperature pump applications. Performance data of heat pump systems with alternatives must be generated first, so that eventually it will be possible to substitute CFCs by more environmentally friendly compounds. Among many synthetic lubricants specially developed for use with HFCs, particularly for HFC-134a, ester lubricants are the recommended ones for refrigeration applications. These lubricants are also likely to exhibit similar characteristics to other HFEs. The acceptance of heat pumps has always been debated on the grounds of economic feasibility, with respect to other conventional heating processes. Recycling of energy also leads to a reduction in CO2 emission and, consequently, a reduction in the "greenhouse effect", this is likely to give an additional advantage to heat pumps and the economic calculations should be modified accordingly. REFERENCES 1. United Nations Environment Programme, Montreal Protocol 1991 Assessment--Report of the Refrigeration, Air-conditioning and Heat Pumps Technical Options Committee, UNEP, Nairobi (1991).
Alternative heat pump working fluids
279
2. T. Midgley, From the periodic table to production, Ind. Engng Chem. 29, 241-244 (1937). 3. M. O. McLinden and D. A. Didion, The search for alternative refrigerants--a molecular approach. Proc. I.I.F.-I.LR.-Commissions BI, B2, El, E2, Purdue, U.S.A., Vol. 2, pp. 91-99 (1988). 4. J. L. Adcock, S. B. Mathur, W. A. Van Hook, H. Q. Huang, M. Narkhade and B.-H. Wang, Fluorinated ethers: a new series of CFC substitutes, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 386-395 (1991). 5. A. L. Beyerlein, D. D. DesMarteau, S. H. Hwang, D. N. Smith and J. Powell, Physical property data on fluorinated propanes and butanes as CFC and HCFC alternatives, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 396-405 (1991). 6. S. Devotta and S. Gopichand, Comparative assessment of HFC34a and some refrigerants as alternatives to CFC12, Int. J. Refrig. IS(l), 112-118 (1992). 7. S. Devotta and S. Gopichand, Comparative assessment of some flammable refrigerants as alternatives to CFCI2, Proc. 1992 Int. Conf - - E n e r g y E~iciency and New Refrigerants, Purdue University, U.S.A., pp. 249-257 (1992). 8. S. Devotta, S. Gopichand and V. R. Pendyala, Assessment of HFCs, fluorinated ethers and amines as alternatives to CFC12. Int. J. Refrig. 16(2), 84-90 (1993). 9. S. Devotta, S. Gopichand and V. R. Pendyala, Comparative assessment of some HCFCs, HFCs and fluorinated ethers as alternatives to CFC-I l, Int. J. Refrig. 17(1), 32-39 (1994). 10. W. L. Kopko, Beyond CFCs: Extending the search for new refrigerants, Int. J. Refrig. 13, 79-85 (1990). l 1. J. R., Sand, S. K. Fischer and P. A. Joyner, Modeled performance of non-chlorinated substitutes for CFC-I 1 and CFC-12 in centrifugal chillers, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 406-415 (1991). 12. K. Baernthaler, J. Fresner, J. Schnitzer and F. Moser, Test results with ethyl chloride (Rl60) as a new heat pump medium, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 466-475 (1990). 13. S. Devotta and S. Gopichand, Theoretical assessment of R134a and R123 for heat pumps, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 185-197 (1990). 14. S. Devotta and S. Gopichand, Theoretical assessment of HFC134a and alternatives to CFCI2 as working fluids for heat pumps, Appl. Energy 41, 285-299 (1992). 15. S. Devotta and V. Rao Pendyala, Thermodynamic screening of some HFCs and fluorinated ethers for high temperature heat pumps as alternatives to CFC-114, Int. J. Refrig. (accepted) (1994). 16. J. Fresner, J. Schnitzer and F. Moser, 1-Propyl chloride (R280a)---a potential heat pump medium for high temperature applications, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 218-229 (1990). 17. D. E. Preisegger, HFC 134a and HFC 227--future fluids for heat pumps? Proc. Workshop on High Temperature Heat Pumps, University of Hannover, pp. 59-66 (1989). 18. L. Vamling, M. Hogberg and T. Berntsson, CFC alternatives for high temperature heat pump applications, Proc. 4th Int. Conf (BHR) on Applications and Efficiency of Heat Pump Systems, Munich, Germany, pp. 71-82. STI/Springer-Verlag, Oxford, U.K. (1990). 19. F. A. Holland, F. A. Watson and S. Devotta, Thermodynamic Design Data for Heat Pump Systems. Pergamon Press, Oxford, U.K. (1982). 20. G. Siegemund, A. Feiring, B. Smart, F. Behr and B. McKusick, Fluorine compounds, Organic, In Ullmann's Encyclopaedia o f Industrial Chemistry, Vol. A I I, 5th Edn, VCH Verlagsgesellschaft mbH, Germany (1988). 21. Air-conditioning and Refrigeration Institute, ARI research plan, ARI, Virginia, U.S.A., March (1992). 22. Anon., Annex 20 completes study on working fluid safety, lEA Heat Pump Centre Newsletter 11(3), 4 (1993). 23. S. Devotta, M. G. Parande and V. R. Patwardhan, Experimental assessment of performance and heat transfer characteristics of HFC-134a in a water chiller (to be published). 24. Anon., Annex 20 completes study on working fluid safety, IEA Heat Pump Centre Newsletter 11(3), 7 (1993). 25. S. Devotta and S. Gopichand, Derived thermodynamic design data for refrigeration and heat pump systems operating on HCFC-123, Heat Recovery Systems & CHP 13(3), 213-218 (1993). 26. N. D. Smith, K. Ratanaphruks, M. W. Tufts and A. S. Ng, R-243ca: a potential far-term alternative for R-I 1, ASHRAE J., 19-23, February (1993). 27. H. O. Spauschus, HFC 134a as a substitute refrigerant for CFCI2, Proc. LLF--l.l.R.--Commissions BI, B2, El, E2, Purdue, U.S.A., pp. 397-400 (1988).
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ALTERNATIVE
HEAT
PUMP
WORKING
FLUIDS
TO
CFCs* SUKUMAR DEVOTTA National Chemical Laboratory, Pune 411 008, India (Received with revisions 18 February 1994)
Abstract---CFC-I 1, CFC-12, CFC-114 and HCFC-22 are used as mechanical vapour compression heat pump working fluids for low and high temperature applications. Because of the Montreal Protocol, alternative heat pump working fluids to CFCs have to be identified and developed. This paper discusses the various potential alternatives screened from various families of compounds. HFC-134a is the most popular choice as an alternative to CFC-12, although HFC-152a appears to be a better alternative. Among the HFEs, HFE-134 is a potential long-term alternative. HFC-143 and HFE-134 appear to be attractive as substitutes for CFC-I14 for high temperature applications. For centrifugal compressor applications, HCFC-123 is currently the most popular choice as an interim alternative to CFC-I 1. HFC-245ca and HFC-143 are also potential long-term substitutes for CFC-11. The various implications with respect to these alternatives are discussed.
NOMENCLATURE HGWP ODP Pc Tc
Global warming potential relative to CFC-I 1 Ozone depletion potential Critical pressure (MPa) Critical temperature (°C)
1. I N T R O D U C T I O N M a n y chlorofluorocarbons (CFCs) and hydrochiorofluorocarbons ( H C F C s ) have been used as heat p u m p working fluids in the past two decades or so. They possess most o f the desirable characteristics, such as thermal and chemical stability, t h e r m o d y n a m i c efficiency, non-toxicity, non-flammability, low cost, etc., but C F C s are being blamed for the stratospheric ozone layer depletion. The Montreal Protocol mandates that C F C s and H C F C s should be phased out on a global scale. Heat p u m p s are generally used for space heating and for process heat recovery and recycling in industrial sectors. U N E P Technology Options R e p o r t [1] presents a comprehensive review o f the current status o f heat p u m p installation a r o u n d the world. Most o f the smaller heat p u m p s use hermetic reciprocating compressors, while the bigger sizes use both semi-hermetic and open types o f reciprocating, scroll and screw compressors. Turbo-compressors are used for very large sizes. Commercial heat p u m p s have been developed using CFC-11, H C F C - 2 2 , CFC-12 and CFC-114, for condensing temperatures o f up to 50°C, 60°C, 70°C and 120°C respectively. C F C - 1 2 is the most widely used fluid, followed very closely by H C F C - 2 2 . CFC-500, CFC-502 and C F C - I I (used exclusively with centrifugal compressors) are also used to some extent. Although CFC-114 accounts only for a b o u t 1% o f the C F C s used in heat pumps, it is exclusively used when the condensing temperature is in excess o f 80°C. The designs o f heat pumps using these fluids are well established. It was estimated [1] that the annual global consumption o f C F C s was about 1700 tonnes. F o r temperatures above 150°C, R718 (water) is very attractive. However, the use o f water mandates the use o f unlubricated (dry) compressors, which are fairly expensive when c o m p a r e d to refrigeration compressors. Heat pumps are still viewed as expensive for energy recovery; additional *NCL Communication No: 5918. 273
274
SUKUMAR DEVOTTA Table 1. Comparative data for some HFC alternatives to CFC12
Molecular formula Mol. wt, kgkmol ' Tt , C Pc, MPa Ts , C ODP HGWP Flammability
CFCI2
HFCI43a
HFCI34a
HFC227
HFCI52a
HFCI34
CCI2F _, 120.9 112.0 4. I 13 -29.79 1.0 2.08 NF
CHCIF 2 84.04 73.1 3.826 -47.6 0.0 0.828 F
CF3CHFCF, 102.0 101 7 4.1)55 26.2 0.0 0.343 NF
CH3CF~ 170.0 101.9 2.952 17.3 0.0 NA NF
CHF2CHF 2 66.0 I I3.5 4.49 -24.7 0.0 0.04 F
CHF2CHF 2 102.03 115.6 3.77 -19.7 0.0 NA NA
F--Flammable, N A - - N o t available, NF
Non-flammable, UC
Uncertain
initial fixed capital costs have made the return on investment less attractive. Therefore, the steam heat pumps have yet to realise their full potential and are mostly limited to industrial sectors. Water has no environmental controversies. If the potential for high temperature heat pumps has to be safeguarded, alternatives have to be identified soon. This will be the major factor determining the future of mechanical vapour compression heat pumps. 2. ALTERNATIVES TO CFCs Any alternative has to have a low, or preferably zero, "Ozone Depleting Potential (ODP)". Ideally, one should also screen for alternatives which satisfy other environmental concerns. The substitutes should have relatively low "Global Warming Potentials (GWP)" and also should not be "volatile organic compounds (VOCs)" having high smog potential. These constraints limit the domain within which alternatives could be found. It is now acknowledged that, in order to satisfy the environmental regulations, one has to compromise on other factors, such as flammability, energy efficiency, manufacturing feasibility (and hence cost), import, re-design of the system etc. Midgley [2] with his molecular screening approach, using the periodic table to identify potential refrigerants, concluded that they should be made with some combination of carbon, hydrogen, nitrogen, oxygen, sulphur, fluorine, chlorine and bromine. With toxicity and stability, complications associated with sulphur and nitrogen compounds, these were eliminated. Although Midgley identified a few more families of compounds, ultimately the choice was for CFCs. McLinden and Didion [3], using various thermodynamic constraints and, eliminating chlorine and bromine due to their active participation in ozone layer depletion, concluded that the potential refrigerants should consist of carbon, fluorine, hydrogen and oxygen. From the combinations of H, F and C leading to fluorocarbons (FCs) and hydrofluorocarbons (HFCs), FCs were discarded because of their longer atmospheric life and hence undesirably high GWP. Some thermodynamic screening for refrigeration [4-11] and heat pumps [12-18] from many classes of compounds has been reported. Some theoretical and experimental evaluations of HCC-160 [12] and HCC-280a [16] as medium temperature and high temperature working fluids, respectively, have been reported. Devotta and Gopichand [13] found that there are attractive flammable refrigerants, including HFC-152a. Vamling et al. [18] found that most of the flammable fluids, including HFC-152a, give better (COP) values than CFC-114 for heat pumps. Holland e t al. [19] have theoretically assessed HC-290 (propane), HC-600 (n-butane) and HC-600a (isobutane) and found them to be attractive working fluids for both medium and high temperature ranges. The quest for alternatives does not appear to be over with HFCs. Some more alternatives with zero ODP have been found from combinations of H, F, C and O, e.g. partially fluorinated ethers Table 2. Comparative data for some hydrocarbon alternatives to CFC-12 Refrigerant
CFC 12
HC290
HCC270
HC600a
HC600
Molecular formula
CCI2F~ 120.9 112.0 4.113 -29.79 2.941 2.08 --
-CH2 CH2H 2 42.08 124.7 5.49 32.85 0.0 NA 2.4/10.4
CH(CHs) 3
Mol. wt, kgkmol '
CHsCH _, CH~ 44.1 96.8 4.25 -42.1 0.0 0.0008 2.2/9.5
CH3CH 2 CH2CH~ 58.13 152.0 3.8 -0.5 0.0 0.0008 1.9/8.5
Tc, C Pc, MPa
TH, C ODP HGWP LEL/UEL vol % N A - - N o t Available
58.13 135.0 3.65 -11.7 0.0 0.0008 1.9/8.5
A l t e r n a t i v e heat p u m p w o r k i n g fluids
275
Table 3. Comparative data for some HFE and FE alternatives to CFC-12
Molecular formula Mol. wt, kg kniol ~ Tc, C Pc, MPa Ta, 'C ODP HGWP Flammability
CFCI2
HFEI34
HFE227
FE218
FE216
CCI2F 2 120.9 112.0 4.113 -29.79 1.0 2.08 NF
C:H2F40 118.03 147.1 3.951" 6.2 0.0 NA NF
C3HF70 186.03 114.6 2.64 -4.2 0.0 NA NF
C3F802 220.02 99.2 2.33 - 10.0 0.0 NA NF
C3F60 166.02 88.7 3.09 -29.9 0.0 NA NF
*Estimated, N A - - N o t available, NF--Non-flammable
(HFEs) as well as some cyclic fluorinated compounds [4, 5, 8-11, 15]. The fluorinated ethers, notably high molecular weight ones, are commercially manufactured, although the volume of production of low molecular weight ethers may not be significant. The synthesis of these compounds is being established [4, 20]. From the structural features of these compounds, it is expected that they are likely to be adequately stable but will still have lower atmospheric lives and hence lower GWPs. Although fluorinated ethers were identified as potential refrigerants and patented as early as the 1930s [10], they were not commercially exploited for as long as CFCs, which are simpler fluids compared with the other compounds, could serve the purpose. The current environmental concerns have forced us to re-examine these fluids. 2.1. Alternatives to H C F C - 2 2 HCFCs have been included as controlled substances under the Montreal Protocol in the 1992 Copenhagen convention. Therefore, considerable initiatives have already been taken by industry to look for alternatives. Most of the search is jointly considered along with R502 (an azeotropic mixture HCFC-22/CFC-115). The Air-Conditioning and Refrigeration Institute [21] in the U.S.A. has already launched an initiative as the "R-22 Alternative Refrigerants Evaluation Program (R-22 AREP)". HC-290, HFC-134a, R-717 (ammonia) and some blends of HFC-32, HFC-125, HFC134a, HFC-143a and HC-290 have been identified as potential alternatives to HCFC-22. Although the emphasis will be for air-conditioning and refrigeration, these results would also be extremely useful for domestic heat pumps. In general, there is great enthusiasm to use propane or propane/butane/isobutane mixtures as the alternative, particularly in the European sector. Recently, two domestic heat pumps running on R-502 and HCFC-22 in Germany were effectively converted to run on a propane/butane mixture with some improvements of COP. A study undertaken by the lEA Heat Pump Centre in the Netherlands [22] had concluded that the risks arising out of using hydrocarbons were grossly exaggerated. 2.2. Alternatives to CFC-12 Some of the potential alternatives to CFC-12 from HFCs, hydrocarbons and HFEs are listed in Tables 1, 2 and 3 respectively. HFC-134a is considered to be the most potential substitute for CFC-12 for both refrigeration and heat pump applications [6, 8, 13]. HFC-134a is significantly costlier than CFC-12. The various tests under PAFT which is commercially available through international vendors and AFEAS have not indicated any adverse effects of HFC-134a. A few experimental assessments of HFC-134a have been reported for heat pump applications. The performance of HFC-134a under retrofit and for identical conditions has been found to be either similar or even better than that of CFC-12, in applications where water is either the source or the sink. This improvement is generally attributed to the better heat transfer characteristics of HFC-134a [23]. A large scale 25 MW district heat pump, originally designed for CFC-500 with a two stage turbo compressor, has been successfully converted to HFC-134a and operated for more than a year [24]. The converted heat pump has been reported to be operating without much difference in the original performance, with most of the components unaffected. HFC-152a is a better choice because of its higher COP, zero ODP and relatively low Global Warming Potential (GWP) [6-8]. However, problems associated with its flammability aspects have
CFC-114
H FC-236cb
HFC-254cb
HFE-134
H F C - 143
HFC-236ca
F - - F l a m m a b l e , N A - - N o t available, N F - - N o n - f l a m m a b l e , U C - - U n c e r t a i n
CF3CH2CF 3 CF2HCF2CH 3 CHF2OCHF z CH2FCHF 2 CHF2CF2CHF 2 152.01 116.03 118.0 84.02 152.01 130.6 146.1 153.4 157.8 138.9 3.177 3.753 3.751 4.233 3.299 - 1.11 -0.78 4.67 5.00 5.00 0.0 0.0 0.0 0.0 0.0 NA NA NA NA NA NF UC UC F NF
HFC-236fa
Table 4. C o m p a r a t i v e data for some H F C and H F E alternatives to CFC-114
Molecular formula C C I F ~ C C I F 2 CF3CF.~CH2F Mol. wt, k g k m o l ~ 170.9 152.0 T~, C 145.7 130.1 Pc, M P a 3.263 3.118 T a. C 3.78 - 1.22 ODP 0.8 0.0 HGWP 1.97 NA Flammability NF NF
Refrigerant
HFC-236ea CF~CHFCHF~ 152.01 141.1 3.533 6.50 0.0 NA NF
©
o~
277
A l t e r n a t i v e heat p u m p w o r k i n g fluids Table 5. Comparative data for some HCFC alternatives to CFC-II
Molecular formula Mol. wt, kgkmol i Tc , C Pc, MPa T a, C ODP HGWP Flammability
CFC-I1
HCFC-226ea
HCFC-226da
HCFC-235ca
HCFC-123
HCFC-123a
HCFC-141b
CCI~F 137.4 198.4 4.41 23.8 1.0 1.0 NF
CF3CHFCF2CI 186.45 158.3 2.985 17.6 0.0 NA NA
CF3CHCICF 3 186.45 158.5 3.063 14.1 0.0 NA NA
CF3CF2CH2CI 168.45 170.3 3.083 28.1 -NA NF
CHCI2CF 3 152.91 183.8 3.676 27.9 0.02 0.024 NF
CHCIFCCIF, 152.9 189.62 3.889 29.9 -NA NF
CCI2FCH 3 116.9 208.0 4.339 32.1 0. I 0.126 F
N A - - N o t available, NF--Non-flammable
to be mitigated. HFC-152a is likely to be as stable a fluid as HFC-134a, but, with more H atoms, it will pose miscibility problems similar to HFC-134a. It is already commercially available and it is cheaper than HFC-134a. The compressor discharge temperature is also likely to be relatively high, this will require some metallurgical considerations for the discharge ports and better insulating materials for the motor windings for applications with hermetic compressors. Initially, there was a significant enthusiasm about the use of HFC-152a, but this has considerably subsided because of the undue concerns over its flammability. From Table 2 it is clear that no hydrocarbon has its boiling point close to CFC-12. Only cyclopropane (HCC-270) has its characteristics close to CFC-12. It is felt that the stability of cyclopropane, owing to its acute bond angle, will not make it suitable for heat pump applications. Therefore, it may be only prudent to consider mixtures of HC-290/HC-600/HC-600a to match the design features of the CFC-12 compressor as is being undertaken in domestic refrigerators. Some of the potential fluorinated and partially fluorinated ethers are listed in Table 3. The partially fluorinated ethers are preferred, as they have a much lower GWP than fluorinated ethers. From a comparative assessment of various fluorinated ethers, Devotta et al. [8] have identified HFE-134 as a potential candidate for refrigeration applications, although the compressor size may be suitable for centrifugal applications. Therefore, for large capacity heat pumps, like district heating, HFE-134 could be considered. 2.3. Alternatives to C F C - I14 From about 30 HFCs, HFEs and FEs, the following fluids were selected by Devotta and Rao Pendyala [15] as potential substitutes for CFC-II4: HFC-143, HFC-236ca, HFC-236cb, HFC236ea, HFC-236fa, HFC-254cb, HFE-134. Table 4 presents some basic data for these fluids. It is generally known that simple fluids are preferred both from performance and other aspects. The process development aspect is more difficult with bigger molecules like propane derivatives. Simpler molecules offer better efficiency, e.g. HFC-143 and HFE-134 offering better energy efficiency than the propane derivatives. HFC-143 was found to be attractive from the energy point of view [15]. The operating pressures are very close to CFC-114, although (PR) values are relatively high. The compressor size will be smaller than that for CFC-114. The use of HFC-143 warrants more stringent safety designs to mitigate the risks of flammability. For industrial applications, where high temperature heat pumps are generally used, this may not be a major hurdle. There is no report on the process development efforts for HFC-143. The use of HFE-134 also appears to be appealing from an energy efficiency point of view [15]. HFE-134 was found to be attractive as an alternative to CFC-12 for refrigeration [8]. There are already some initiatives for the use of HFE-134 as an alternative to CFC-I 1, CFC-12 and CFC-114 Table 6. Comparative data for some HFC alternatives to CFC-II CFC-I I Molecular formula Mol. wt, kg kmol t Tc. C Pc, MPa T B. C ODP HGWP Flammability
CCI3F 137.4 198.4 4.41 23.8 1.0 1.0 NF
HFC-347ccd
HFC-338eea
CF3CF.~CF2CH ~ CF3CFHCFHCF 184.02 202.02 144.2 148.6 2.572 2.747 15.1 25.4 0.0 0.0 NA NA NA NA
N A - - N o t available, NF--Non-flammable. UC--Uncertain
HFC-245 fa
HFC-245ca
CF3 CH_,CF_~H 134.0 157.5 3.644 15.3 0.0 0.089 UC
CF,HCF2CH_~ F 134.0 178.4 3.885 25.0 0.0 NA UC
278
SUKUMAR DEVOTTA Table 7. Comparative data for the HFE alternatives to CFC-II
Molecular formula Mol. wt, kgkmol ~ Tt . C Pc, MPa T~, C ODP HGWP Flammability
CFC-I 1
HFE-245fa
CCI3F 137.4 198.4 4.41 23.8 1.0 1.0 NF
CFsCH2OCF2H 150.05 171.0 3.376 29.2 0.0 NA --
HFE-245cb
HFE-143
CFsOCF2CH s CH2FOCHF ., 150.05 100.0 185.17 186.83 3.42 4.141 34.06 30.06 0.0 0.0 NA NA UF F
HFE-254cb CHF2OCF2CH ~ 132.0 189.44 3.56 36.44 0.0 NA F
N A - - N o t available, NF--Non-flammable, UF--Uncertain flammability, F--Flammable
in refrigeration [21] but the plans for its process development and commercial production are not yet clear. It is worth investigating these fluids for a whole range of applications of heat pumps. 2.4. Alternatives to CFC- I1 As in the case of refrigeration, the use of CFC-11 is limited to large size space heating applications with water cooled condensers using the conventional centrifugal compressors. The condensing temperatures of heat pumps using CFC-11, in spite of its higher critical temperature, are limited to those of HCFC-22 because CFC-11 has a very poor thermal and chemical stability. The HCFC-123 boiling point is reasonably close to CFC-II and its molecular weight is marginally higher. Therefore, it is being projected as an "aerodynamically and thermodynamically acceptable drop-in" substitute for CFC-11 in centrifugal chillers. HCFC-123 could be considered as an interim working fluid for centrifugal heat pumps [13, 25]. Most of the centrifugal chillers manufactured now are supposed to be compatible with both CFC-11 and HCFC-123. HCFC-123 is available in the international market. With the preliminary toxicity test results, the allowable exposure limit (AEL) for HCFC-123 has recently been fixed at 30 ppm, compared to 1000 ppm, for CFC-11. Although HCFC-123 is considered to possess the best potential as a substitute for CFC-11, it is still a transition substitute. It will be a long time before one can confidently use HCFC-123 as an alternative to CFC-11 in centrifugal heat pumps. HFC-254ca and HFE-143 are also potential long-term candidates for this application [9, 26]. 3. C O N C L U S I O N HFC-134a is the most popular choice as an alternative to CFC-12. Except for its flammability, H FC-! 52a offers better efficiency. HFE-134 also appears to be a long-term alternative. HFC-143 and HFE-134 are the potential short- and long-term candidates for CFC-114 replacement in high temperature heat pumps. For CFC-11, HFC-245ca and HFE-143 are the potential long-term substitutes. The substitution of CFCs by any alternative would involve substantial changes, including various components, such as insulation, lubricants, heat exchangers, motors, etc. Tests have to be carried out to optimise the system performance and to ensure the reliability and safety of the system. The chemical stability of HFCs [27] and fluorinated ethers may not be a limiting condition in high temperature pump applications. Performance data of heat pump systems with alternatives must be generated first, so that eventually it will be possible to substitute CFCs by more environmentally friendly compounds. Among many synthetic lubricants specially developed for use with HFCs, particularly for HFC-134a, ester lubricants are the recommended ones for refrigeration applications. These lubricants are also likely to exhibit similar characteristics to other HFEs. The acceptance of heat pumps has always been debated on the grounds of economic feasibility, with respect to other conventional heating processes. Recycling of energy also leads to a reduction in CO2 emission and, consequently, a reduction in the "greenhouse effect", this is likely to give an additional advantage to heat pumps and the economic calculations should be modified accordingly. REFERENCES 1. United Nations Environment Programme, Montreal Protocol 1991 Assessment--Report of the Refrigeration, Air-conditioning and Heat Pumps Technical Options Committee, UNEP, Nairobi (1991).
Alternative heat pump working fluids
279
2. T. Midgley, From the periodic table to production, Ind. Engng Chem. 29, 241-244 (1937). 3. M. O. McLinden and D. A. Didion, The search for alternative refrigerants--a molecular approach. Proc. I.I.F.-I.LR.-Commissions BI, B2, El, E2, Purdue, U.S.A., Vol. 2, pp. 91-99 (1988). 4. J. L. Adcock, S. B. Mathur, W. A. Van Hook, H. Q. Huang, M. Narkhade and B.-H. Wang, Fluorinated ethers: a new series of CFC substitutes, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 386-395 (1991). 5. A. L. Beyerlein, D. D. DesMarteau, S. H. Hwang, D. N. Smith and J. Powell, Physical property data on fluorinated propanes and butanes as CFC and HCFC alternatives, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 396-405 (1991). 6. S. Devotta and S. Gopichand, Comparative assessment of HFC34a and some refrigerants as alternatives to CFC12, Int. J. Refrig. IS(l), 112-118 (1992). 7. S. Devotta and S. Gopichand, Comparative assessment of some flammable refrigerants as alternatives to CFCI2, Proc. 1992 Int. Conf - - E n e r g y E~iciency and New Refrigerants, Purdue University, U.S.A., pp. 249-257 (1992). 8. S. Devotta, S. Gopichand and V. R. Pendyala, Assessment of HFCs, fluorinated ethers and amines as alternatives to CFC12. Int. J. Refrig. 16(2), 84-90 (1993). 9. S. Devotta, S. Gopichand and V. R. Pendyala, Comparative assessment of some HCFCs, HFCs and fluorinated ethers as alternatives to CFC-I l, Int. J. Refrig. 17(1), 32-39 (1994). 10. W. L. Kopko, Beyond CFCs: Extending the search for new refrigerants, Int. J. Refrig. 13, 79-85 (1990). l 1. J. R., Sand, S. K. Fischer and P. A. Joyner, Modeled performance of non-chlorinated substitutes for CFC-I 1 and CFC-12 in centrifugal chillers, Proc. Int. CFC and Halon Conf., Baltimore, U.S.A. pp. 406-415 (1991). 12. K. Baernthaler, J. Fresner, J. Schnitzer and F. Moser, Test results with ethyl chloride (Rl60) as a new heat pump medium, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 466-475 (1990). 13. S. Devotta and S. Gopichand, Theoretical assessment of R134a and R123 for heat pumps, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 185-197 (1990). 14. S. Devotta and S. Gopichand, Theoretical assessment of HFC134a and alternatives to CFCI2 as working fluids for heat pumps, Appl. Energy 41, 285-299 (1992). 15. S. Devotta and V. Rao Pendyala, Thermodynamic screening of some HFCs and fluorinated ethers for high temperature heat pumps as alternatives to CFC-114, Int. J. Refrig. (accepted) (1994). 16. J. Fresner, J. Schnitzer and F. Moser, 1-Propyl chloride (R280a)---a potential heat pump medium for high temperature applications, Proc. 3rd Int. Workshop on Research Activities on Advanced Heat Pumps, University of Technology, Graz, Austria, pp. 218-229 (1990). 17. D. E. Preisegger, HFC 134a and HFC 227--future fluids for heat pumps? Proc. Workshop on High Temperature Heat Pumps, University of Hannover, pp. 59-66 (1989). 18. L. Vamling, M. Hogberg and T. Berntsson, CFC alternatives for high temperature heat pump applications, Proc. 4th Int. Conf (BHR) on Applications and Efficiency of Heat Pump Systems, Munich, Germany, pp. 71-82. STI/Springer-Verlag, Oxford, U.K. (1990). 19. F. A. Holland, F. A. Watson and S. Devotta, Thermodynamic Design Data for Heat Pump Systems. Pergamon Press, Oxford, U.K. (1982). 20. G. Siegemund, A. Feiring, B. Smart, F. Behr and B. McKusick, Fluorine compounds, Organic, In Ullmann's Encyclopaedia o f Industrial Chemistry, Vol. A I I, 5th Edn, VCH Verlagsgesellschaft mbH, Germany (1988). 21. Air-conditioning and Refrigeration Institute, ARI research plan, ARI, Virginia, U.S.A., March (1992). 22. Anon., Annex 20 completes study on working fluid safety, lEA Heat Pump Centre Newsletter 11(3), 4 (1993). 23. S. Devotta, M. G. Parande and V. R. Patwardhan, Experimental assessment of performance and heat transfer characteristics of HFC-134a in a water chiller (to be published). 24. Anon., Annex 20 completes study on working fluid safety, IEA Heat Pump Centre Newsletter 11(3), 7 (1993). 25. S. Devotta and S. Gopichand, Derived thermodynamic design data for refrigeration and heat pump systems operating on HCFC-123, Heat Recovery Systems & CHP 13(3), 213-218 (1993). 26. N. D. Smith, K. Ratanaphruks, M. W. Tufts and A. S. Ng, R-243ca: a potential far-term alternative for R-I 1, ASHRAE J., 19-23, February (1993). 27. H. O. Spauschus, HFC 134a as a substitute refrigerant for CFCI2, Proc. LLF--l.l.R.--Commissions BI, B2, El, E2, Purdue, U.S.A., pp. 397-400 (1988).
HRS 15/3~D