Working fluids for mechanical refrigeration — Invited paper presented at the 19th International Congress of Refrigeration, The Hague, August 1995

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~il ELSEVIER

Int. J. Refrig. Vol. 19, No. 8, pp. 485-496, 1996 Copyright © 1996 Published by Elsevier Science Ltd and IIR Printed in Great Britain. All rights reserved PII: S0140-7007(96)00008-4 0140-7007/96/$15.00

REVIEW PAPER Working fluids for mechanical refrigeration - Invited paper presented at the 19th International Congress of Refrigeration, The Hague, August 1995 A l b e r t o Cavallini I s t i t u t o di Fisica T e c n i c a , via Venezia 1, Universit~t, I 35131 P a d o v a , I t a l y Received f o r publication 9 February 1996 The phasing out of fully halogenated halocarbons becomes effective at the end of 1995 by international agreement. Under the same ozone depletion issue, the companion fluids HCFCs are suffering a similar fate, as they are considered controlled substances with a virtual phase-out by 2020, and more drastic reductions may be proposed in the near future. Some international action might also be agreed upon on refrigerants with regard to the threatened environmental emergency of anthropogenic global warming. Therefore, in choosing replacement fluids primary concern must be given to minimising the total warming impact, which, for most applications, calls'for improved energy efficiency. During recent years, industry has scrutinised and proposed a number of new synthesised products as immediate drop-in or long-term replacements for fluids harmful to the environment. Together with some single-component new generation refrigerants, quite a few two-component, three-component or even four-component mixtures, both with zeotropic and azeotropic behaviour, are being considered. The main issues associated with the use of the new generation refrigerants are discussed, such as behaviour with oil; flammability; efficient use of temperature glides, fractionation and heat-transfer degradation with zeotropic mixtures. The full environmentally friendly option of resorting to natural fluids is also considered by examining some recent innovative applications as refrigerants of some hydrocarbons, ammonia, carbon dioxide, water and air. Copyright © 1996 Published by Elsevier Science Ltd and IIR

(Keywords:mechanicalrefrigeration;workingfluids;refrigerants)

Fluides de travail pour le froid m6canique - Rapport pr6sent6 au 19e Congr6s International du Froid, La Haye aofit 1995 L'~limination progressive des halocarbures totalement halog~n~s est devenue effective it la fin de 1995 sur accord international. L 'appauvrissement de la stratosphere en ozone, fait subir le m~me sort it tous les fluides de type HCFC, dont on pr~voit l'~limination pratiquement complbte pour 2020 et qui risquent des limitations encore plus radicales. Une action relative aux frigorig~nes pourrait dtre conclue au plan international, eu ~gard it la menace grandissante que fait peser sur l'environment le r~chauffement de la planbte dfi it l'activit~ humaine. Par consequent, le souci de minimiser l'impact total sur le r~chauffement doit presider au choix des fluides de remplacement, ce qui, dans la plupart des applications, n~cessite d'am~liorer les performances ~nerg~tiques. Au cours des dernibres ann~es, les industriels ont ~tudi~ et proposk un certain nombre de nouveaux produits de synth~se comme produits de remplacement imm~diat ou it long terrne des fluides nuisibles it l'environnement. Sont actuellement it l'~tude, en m~me temps que les frigorig~nes purs, d'assez nombreux mdlanges it deux, trois ou quatre composants, azdotropes aussi bien que non-az~otropes. Les principales questions associ~es it l'utilisation de nouveaux frigorigbnes sont it l'dtude, comme le comportement avec l'huile, l'inflammabilit~, ainsi que l'utilisation efficace des glissements de temperature, la s~paration et la ddgradation du transfert de chaleur avec les mdlanges z~otropes. Le recours it desfluides naturels, les plus recherch~s au titre de l'environnement, est ~galement examin~ it travers les rdcentes innovations en matibre d'applications de frigorigbnes tels que certains hydrocarbures, l'ammoniac, le dioxyde de carbone, l'eau et l'air.

The end of 1995 marked a turning-point for the refrigerant substitution process; it involved the phasing-out of fully halogenated halocarbons, known as CFCs, by international agreement between all developed countries, while developing countries benefited from a more relaxed phase-out schedule. The partly halogenated companion products H C F C s , regarded within the Montreal Protocol scenario as

necessary transition fluids, are nevertheless destined to a similar fate under the same ozone depletion issue. The Copenhagen and Vienna amendments to the Montreal Protocol established H C F C s as controlled substances with a phase-out in new equipment by the year 2020, and at the 9th meeting in Montreal in autumn 1997, the parties adhering to the Protocol might approve a more accelerated phase-out schedule.

485

486

A. Cavallini

household refrigeration retail refrig.(current losses)

nn ---n

retail refrig.(reduced losses) commercial chillers unitary air conditioners autom.a.c.(current losses) autom.a.c.(reduced losses) 0

10

20

30

40

50

60

70

f 80

90

100

%

Figure l

Relative proportion of direct/indirect effects to TEWI for selected refrigeration applications of HCFC and H F C alternatives (ITH = 100

years) Figure 1 Proportion des effets directs/indirects sur le T E W I de produits de remplacement de H C F C et H F C (horizon - 100 ans) pour les applications suivantes

Since December 1993, EU Ministers for environment protection have agreed upon earlier HCFC phase-out dates, with a total ban by the end of 2014. Individual countries have imposed much tighter restrictions; for example in Sweden the phasing-out of HCFCs in new equipment has been set for the beginning of 1998, while in Germany the phase-out of HCFC22 in new equipment will be in force from the beginning of the year 2000. Austria too set a tighter HCFC phase-out schedule than that of the EU (HCFC phase-out date as refrigerant in new equipment will be in force from the beginning of 2002). In Switzerland, all HCFCs will be banned by 2005 and in New Zealand by 2015, while, in the USA, selected HCFCs are scheduled to undergo an early phase-out; HCFC22 elimination from new equipment has been set for 2010, and that of HCFC123 for 2020, while HCFC141b production and consumption will be banned as from 2003. For export-oriented companies it is most disturbing that regulations on HCFC phase-out are so different from country to country, and subject to frequent and unpredictable updates. As seen, the ozone issue has resulted in wellestablished international action, with the complete ban of all ozone-harmful compounds within a few years. This same action (especially the CFC ban), can also greatly benefit the other, perhaps even more threatening, environmental emergency of our times: that of anthropogenic global warming, for which a specific, international agreement has not so far been established. Nevertheless, in the preparatory meetings for the assembly of the parties involved in the United Nations Framework Convention on Climate Change (Rio Convention) some pressure has been placed on the setting of restrictions in the use of trace greenhouse gases, which of course include non-chlorinated halocarbons HFCs, now considered the most promising candidates for long-term replacement of CFCs and HCFCs on the basis of their harmlessness to the stratospheric ozone layer. It will no doubt be a difficult task for policymakers to make general decisions on refrigerant regulations in

compliance with the global warming issue. It is a wellknown fact that to determine the overall contribution of refrigerants to global warming, a systematic approach has to be employed taking into account both the direct contribution of the working fluid as a greenhouse gas and the indirect contribution of carbon dioxide emission resulting from the production of the energy required to operate the equipment throughout its normal life. For this purpose, GWP (Global Warming Potential) indices referring to CO2 have been determined to provide a simplified means for describing the relative influence of each greenhouse gas emission on future radiative forcing and thereby direct effect on the global climate. These indices are calculated from a cumulative radiative forcing over a suitable Integration Time Horizon, ITH. In the evaluation of the indirect contribution to the Total Equivalent Warming Impact (TEWI) of electrically powered equipment, due consideration has to be given to the global warming impact of the local power-generation plant that depends on the resource energy mix used and on electric-power generation, transmission and distribution efficiency. Emissions of CO2 due to electricity generation can vary considerably from one country to another. Regional average values are 0.51 kg CO2kWh~- l (delivered to end users) for western Europe, 0.67 kg kWh~ 1 for North America, and 0.58 kg kWhe' for Japan. On the basis of the above averaged data and current technology, Figure 1 shows the relative proportion of direct/indirect effects on TEWI for selected refrigeration applications of HCFC and HFC alternatives, based on an 1TH = 100 years 1. Evidently, in some applications there may still be plenty of room for improvement in global warming impact by moving further from the H C F C - H F C alternative to non-GWP compounds, or through drastic modifications of system features. Practical means to reduce refrigerant emissions into the atmosphere, such as improving design, maintenance and service practices to minimise leakages, or reducing system charge and implementing refrigerant reclaiming procedures can be pursued. This is especially true, for

Working fluids for mechanical refrigeration

487

mobile air,

industrial

domestic commercial

air 0

5

10

20

15

25

30

35

% Figure 2 1986 consumption of all halogenated fluids in refrigeration and air conditioning Figure 2 Consommation de fluides halogtn6s dans le froid et le conditionnement d'air en 1986

example, in retail refrigeration and automotive air conditioning, both non-hermetic systems which feature refrigerant loss rates typically of the order of 20-35% of the total charge per year. On the other hand, very little can be done to reduce TEWI by focusing merely on the refrigerant in other refrigeration applications, where the major contribution to TEWI comes from the CO2 produced for energy consumption; in these cases, efficiency improvement offers a far greater opportunity to reduce TEWI. To evaluate the relative impact on anthropogenic global warming of a refrigeration application, data in Figure 1 is to be considered in conjunction with the distribution of refrigerant consumption; this data is reported in Figure 2, relative to the Montreal Protocol reference year 1986. TEWI index is not therefore a refrigerant property, unlike ODP or GWP, but depends to a large extent on the particular application and location considered. How this concept can effectively be used in setting refrigerant regulations remains an open question.

Alternative synthesised refrigerants Table 1 indicates some alternative synthesised chlorinated refrigerants now available on the market2'3; most of them are intended to serve as short-term, drop-in replacements for CFCs with minor system modifications. All these alternatives are based on HCFCs; the presence of chlorine thus ensures adequate lubricity and solubility with traditional mineral oils and alkylbenzene lubricants. Furthermore, all the chlorinated refrigerants offered as drop-in replacements for CFC 12 and R502 are in the form of zeotropic or near-azeotropic mixtures. They are tailored to give approximately the same performance (primarily capacity with positive displacement compressors and energy consumption) within the same systems as the CFCs they are replacing (look-alike alternatives). A factor of prime importance to this goal is a close matching of the saturation pressure-to-temperature correlation at evaporator conditions. Of course, even material compatibility with motor insulation (in hermetic systems) and elastomers

should not be overlooked when retrofitting existing equipment. Table 2 lists some chlorine-free alternative refrigerants, mostly based on HFCs; some of them have already been introduced onto the market, while others are still undergoing close experimentation. Several mixtures are considered, especially as alternatives for R502 and HCFC22, for which no non-flammable single-component alternatives with similar pressure-temperature property profiles have yet been found. Although some of these mixtures have been formulated to display approximately the same volume-refrigerating capacity in common applications as the old-generation refrigerants they are to substitute, strictly speaking, in no event can they be considered as true drop-in alternatives in systems with positive displacement compressors, owing to the fact that they require at least substitution of the traditional lubricants. It is a well-known fact that, due to the absence of chlorine, both mineral oils and alkylbenzene lubricants do not have adequate lubricity, and are not miscible with HFC refrigerants. An intensive research effort that lasted for several years came to the conclusion that polyol ester oils (with proper additives) are the best choice for use as lubricants with HFC refrigerants in almost all applications. A major concern regarding the use of POE lubricants is their high hygroscopicity; with moisture levels in the system exceeding limits of the order of 100ppm, oil breakdown can occur, possibly bringing about copper plating and rusting, which can seriously reduce compressor life4. In centrifugal compressors the lubricant does not come into contact with the circulating refrigerant, and mineral oil can be the best choice also with HFC working fluids. Another point that should be carefully considered with POE lubricants is their high tendency to dissolve contaminants and manufacturing chemicals, like wire and shell drawing lubricants, residual in the system. The implementation of controlled procedures in processing the equipment during manufacturing, assembly and installation, and the use of proper filter-dryers can avoid the possible occurrence of system failures caused by the peculiar characteristics of POEs s.

488

A. Cavallini

Table 1 Alternative refrigerants containing HCFCs Tableau 1 FrigorigOnes des remplacement contenant des HCFC ASHRAE no. (rating)

Composition (% mass f.)

NBP* (°C) (bubble/glide)

ODP (RI1 = 1)

GWP* (CO2 = 1)

Replacing

R22 (A1) R 123 (B1)

-40.9

0.055

1700

27.9

0.020

93

R 124

- 13.2

0.022

480

R141b

32.2

0.110

630

CFC 114 mixture component CFC11

R142b (A2)

-9.1

0.065

2000

mixture component

Brand name

R502 mixture component CFC 11

R401A (A 1/A 1)

R22/152a/124 (53/13/34)

(-33.0/6.3)

0.037

1100

CFC12

Suva TM MP 39

R401B (A 1/A 1)

R22/152a/124 (61 / 11/28)

( - 34.6/5.9)

0.040

1200

C FC 12 R500

Suva TM MP 66

R401C (A 1/A 1) R402A (A I/A 1) R402B (A 1/A 1) R403A (A 1/A 1) R403B (A 1/A 1)

R22/152a/124 (33/15/52) R125/290/22 (60/2/38) R 125/290/22 (38/2/60) R290/22/218 (5/75/20) R290/22/218 (5/56/39)

(-28.3/4.7)

0.030

850

CFC 12

(-48.9/2.0)

0.021

2600

R502

(-47.1/2.3)

0.033

2200

R502

( - 50.0/2.5)

0.041

R502

(-49.5/0.9)

0.030

2700 t > 8000 § 3700 t > 14000§

Suva TM MP 52 Suva TM HP 80 Suva TM HP 81 Isceon TM 69-S lsceon TM 69-L

R405A (A 1/A 1) R406A (A1/A2) R408A (A1/AI)

R22/152a/142b/C318 (45/7/5.5/42.5) R22/600a/142b (55/4/41) R 125/143a/22 (7/46/47) R 125/143a/290/22 (42/6/2/50) R22/124/142b (60/25/15) R22/124/142b (65/25/10) R22/124/600 (50/47/3) R22/218 (44/56) R1270/22/152a (1.5/87.5/11) R1270/22/152a (3/94/3) R22/218/142b (70/5/25)

(-35.5/5.5)

0.028

(-36.0/9.9)

0.057

(-44.4/0.7)

R409A (A1/A1)

R509 (A I) R411A (AI/A2) R41 IB (A1/A2) R412A (A1/A1)

R502 CFC12

G2015

1800

CFC12

G H G 12

0.026

3000

R502

(-45.6/1.0)

0.027

2500

R502

( - 34.3/8.5)

0.048

1400

CFC 12

( - 35.5/7.7)

0.048

1400

CFC 12

(-32.1/6.1)

0.038

1100

CFC12

HCFC22

Forane TM FX 10 Meforex TM DI 44 Forane TM FX 56 Forane TM FX 57 Meforex TM DI 36 Arcton TM TP5R2 G2018A

-47.5 (azeo)

0.024 0.048

(-40.1/8.1)

4700 $ > 19000§ 1500

0.052

1600

R502

G2018B

0.055

2000 ~ >3300 §

R500

Arcton TM TP5R

* Boiling point or (bubble point/temperature glide) at p = 1 atm. Temperature glide: (Tae w t l T H = 100 years ~;According to ref. 28, where GWPx00 = 7000 is attributed to FC218 §According to ref. 29, where GWPI00 > 34000 is attributed to FC218 (Some ASHRAE designations or ratings are still under public review)

Another point worth mentioning is that the use of POE lubricants with HFC refrigerants in positive displacement compressors, as compared to the use of previous refrigerants and mineral oil, may result in a higher susceptibility to slugging damage under flooded start conditions. Moreover, the noise power level emitted by a positive displacement compressor working with POE lubricants is somewhat higher than with mineral oils; this seems due to a different sound absorption action of the foamed and dispersed oil interface in the compressor crankcase.

R502

-

Tbubble )

The use of synthesised refrigerants causes some concern regarding possible future restrictions on the basis of their global warming impact. This fact apart, efforts to identify and develop suitable look-alike longterm alternatives for CFC12 and R502 seem so far to come up to expectation; the major question is still the long-term reliability of equipment using HFC134a and HFC NEARMs (NEar Azeotropic Refrigerant Mixtures) with the new lubricants. The situation is somewhat different with HCFC22, most widely used in almost all air-conditioning applica-

Working fluids for mechanical refrigeration HFC-32

489 HFC-32

410A~ "-'I°BF R-

/ \

HFC-125

Figure 3

4.5/21,5/74

HFC-134a

Flammabilityboundariesfor mixtures of HFC-32/125/134a Figure 3 Limites d'inflamrnabilit~ des m~langes HFC-32-125/134a

Figure HFC-23

HFC-134a

4 Vapourleakage path for a mixture of HFC-23/32/134a Figure 4 Evolution en cas defuites de vapeurpour un m~lange de HFC23/32/134a

tions, for which all the non-flammable look-alike alternatives fall in the category of NARMs (Non Azeotropic Refrigerant Mixtures) ~. With regard to this point, the preliminary conclusions of the international co-operative AREP program on identification and testing of possible substitutes for HCFC22 (based on compressor calorimeter, system drop-in and softoptimisation test results) can be summed up as follows: (a)

no single refrigerant which clearly outperforms all the other possible alternatives in all types of systems tested has emerged; (b) there may be many viable new-generation alternatives which could yield performance ratings similar to and, after extensive systems redesign, possibly even better than HCFC22 as far as capacity and efficiency are concerned; (c) there are many issues still to be fully addressed before HCFC22 can be completely abandoned, such as fractionation of NARMs, flammability, long-term reliability of equipment using new refrigerants and lubricants, lubrication and heat transfer. Among the various non-flammable alternatives to HCFC22 listed in Table 2, three directions seem to be gaining most favourable support depending on application and system design: the use of a look-alike zeotropic mixture such as R407C; the use of higher pressure, nearly-azeotropic mixtures R410A or R410B; and the use of the lower pressure refrigerant HFC134a 7. By examining these alternatives, all the major issues related to CFC and HCFC substitution emerge and will be briefly discussed.

Look-alike zeotropic alternatives

The special features of zeotropic mixtures present both drawbacks and possible advantages when used in refrigeration technology in place of a single-component refrigerant. Specially tailored zeotropic blends used in heat pumps with distillation, partial condensation devices and rectifiers may offer an effective means of obtaining capacity modulation, by suitably changing

the circulation-mixed refrigerant composition. The development of these techniques is still at an early stage of research and will not be treated in any further detail here 8. However, some major concerns in the use of zeotropic mixtures are just consequences of fractionation, which derives from the difference in composition between liquid- and vapour-phase at thermodynamic equilibrium. Fractionation may cause a change in overall composition of the residual mixture as a consequence of leakage from a component of the refrigerating circuit where the refrigerant is present in both phases. It is evidently desirable that the equipment be topped up after a leakage with the as-formulated composition refrigerant, without suffering noticeable adverse changes in performance, and without the composition shifting to the flammable region if, as is the case with R407C, the non-flammable mixed refrigerant contains a flammable component. This occurs in most situations with the proposed mixtures, as experimenting and simulation results indicate. Only in extreme cases would servicing a system after a fluid loss require the removal of the residual refrigerant and its replacement with a new charge. The possibility of formulating mixtures with one flammable component and still obtaining a non-flammable refrigerant is being widely exploited in creating chlorinefree mixed alternatives to R502 and HCFC22, as is shown in Table 2. Either HFC32 or HFCI43a are present in all the halogenated mixtures listed in Table 2; both these fluids are moderately flammable (ASHRAE rating A2), but with desirable thermodynamic properties from which blends may benefit9. The change in composition during fractionation of zeotropic mixtures has to be duly taken into account in the formulation of a safe blend in terms of flammability, and in fact ASHRAE Standard 34-1992 on number designation and safety classification of refrigerants requires zeotropic blends (containing a flammable and/ or a toxic component) to be assigned a dual safety group classification, the first for the as-formulated composition of the blend, and the second for the worst case of fractionation. Figure 3 shows the vapour flammability boundaries (at ambient pressure, and temperatures 25 and 100°C) of

490 Table 2

A. Cavallini Chlorine-free alternative refrigerants

Tableau 2 L'rigorig~ne de remplacement sans chlore ASHRAE no. (rating)

Composition (% mass f.)

NBP* (°C) (bubble/glide)

GWP* (CO2 = 1)

Replacing

R23

-82.1

12100

R32 (A2)

-51.7

580

* C F C I 3/BFC/13B1 *R503/mix. comp. mixture component

R 125 (Al)

-48.6

3200

mixture component

R134a (A 1) R 143a (A2)

-26.1

1300

-47.4

4400

*CFCI2/HCFC22 mixture component mixture component

-24.7

150

mixture component

R 152a (A2)

R404A (A1/A1)

R218/134a/600a (9/88/3) R 125/143a/134a (44/52/4)

R407A (A 1/A 1) R407B (A1/AI) R407C (A1/A1)

R410A (A1/A1) R410B (A1/A1)

R507

(Al) (R508) Ir (A1) II

( - 35,0/5.2)

Brand name

(-46.5/0.8)

1800~t >4200 § 3700

* C F C 12

IsceonTM 49 (RX2)

* R502/HCFC22

R32/125/134a (20/40/40) R32/125/134a (10/70/20) R32/125/134a (23/25/52)

(-45.5/6.6)

1900

* R502

Forane TM FX70 Meforex TM M55 ReclinTM 404A SuvaTM HP 62 Klea TM 407A

(-47.3/4.4)

2600

CFC 12/*R502

Klea TM 407B

(-44.0/7.2)

1600

*HCFC22

R32/125/143a (10/45/45) R32/125 (50/50) R32/125 (45/55) R32/125/143a/134a (10/33/36/21) R23/32/134a (4.5/21.5/74) R125/143a (50/50) R125/290/218 R23/116 (39/61) R23/116 (46/54) R32/134a (25/75) R290/600

(-49.7/0.9)

3500

* R502

Genetron TM 407C Klea TM 407C MeforexTM M95 ReclinTM HX3 SuvaTM 9000 Forane TM FX 40

(-52.7/<0.1)

1900

HCFC22

(-51.8/<0.1)

2000

HCFC22

Genetron TM AZ 20 Solkane TM 410A SuvaTM 9100

(-49.4/4.1)

3000

HCFC22/*R502

ReclinTM HX4

(-43.0/10.2)

1600

*HCFC22

Forane TM FX 220

-46.7 (azeo)

3800

R502

Genetron TM AZ 50 Meforex TM M57

-54.6 (nearm) -85.7 (azeo)

12300

*BFC13BI * C F C I 3/*R503

IsceonTM 89 (RX4) Klea TM 508

- 8 8 (nearm)

12300

*CFC13/*R503

Suva TM 95

(-40.4/7.2) (-31.6/12.3)

1100 3

*HCFC22 *CFC12

OZI2

(50/50) R717

-33.3

neglig.

HCFC22/R502

(B2) * Boiling point or (bubble point/temperature glide) at p = 1 atm. Temperature glide: (Tdcw - Tbubb~e) t l T H = 100 years ~;According to ref. 28, where GWPI00 = 7000 is attributed to FC218 §According to ref. 29, where GWP10o > 34000 is attributed to FC218 IIAssignment pending * Look-alike alternative (Some ASHRAE designations or ratings are still under public review)

ternary mixtures of HFC32, HFC125 and HFC134a. The nominal compositions of several blends considered as alternatives to R502 and HCFC22 are indicated (symbol *, marked L). Further, the diagram shows the composition of the charge in a system mistakenly

vapour-charged from a large canister, where the liquid is at the nominal composition L (points marked V/L). Finally, a third point V is reported for any blend, corresponding to the first vapour composition which could leak from the system, where the charge had been

Working fluids for mechanical refrigeration

T

b

geq

T c = Ta2

Tc Ta~ J

Tot. T, t

m

_1

nop

Figure 5 Comparison between idealized Carnot and Lorenz reverse cycles Figure 5

Comparaison entre les cycles id~aux inverses de Carnot et de

Lorenz

liquefied; data in Figure 3 are calculated with reference to equilibrium compositions at +10°C. Checking that this two-step fractionation path keeps within the non-flammable region is a rather conservative criterion to rate a blend as to flammability; it is not part of an official test procedure, though it has been used by some refrigerant manufacturers 2. Figure 3 evidences the substantial improvement in safety displayed by the present formulation of R407C blend, as compared to the previous composition (30/10/ 60% mass fraction) that two major refrigerant manufacturers were at first considering. Figure 4 shows the composition path referring to a low temperature (-33°C), slow vapour leak for the zeotropic blend HFC23/32/134a (4.5/21.5/74) proposed as a lookalike substitute for HCFC22 in air-conditioning applications. The situation depicted has been identified as corresponding to the worst case of fractionationl°. Details of requirements for fractionation analyses in relation to flammability of refrigerant blends are specified in ASHRAE Standard 34, 1992 (section 8, currently under public review), and in UL Standard 2182, 1995. Fractionation of zeotropic refrigerant mixtures strictly requires that system charging and refilling be made with liquid-phase refrigerant from the bottom of a charging cylinder, to make sure that the composition of the fluid entering the system is practically the nominal composition of the blend. Modelling and computer simulations of leakage and charging processes with zeotropic refrigerant mixtures are very useful tools for rapid analysis of fractionation scenarios; an excellent methodology for leakage analyses is reported in ref. 11. Fractionation may cause problems not only in connection with leakages or charging of a circuit. Early tests of zeotropic mixtures in packaged multi-split equipment, with a few indoor units fed by a single outdoor unit showed erratic system operation due to composition shift when one of the indoor evaporators was not working. Furthermore, as a result of fractionation, the composition of the circulating refrigerant may

491

be considerably richer in the more volatile, highercapacity component when flooded evaporators are employed, such as in large water chillers with screw or centrifugal compressors; a zeotropic refrigerant blend with a significant glide like R407C is not therefore considered suitable for use in these systems, although some applications have been attempted. It is worthwhile noting that a composition shift in the circulating refrigerant can also be caused (in both zeotropic and azeotropic blends) by differential solubility of the mixture components in the lubricant. The existence of temperature glides during constant pressure condensation and evaporation processes is a feature connected with fractionation of zeotropic mixtures which, depending on the circumstances, can benefit or penalise system operation. In air-source heat pumps, because of the temperature glide exhibited by R407C, frost is formed on the outside heat exchanger at higher outdoor temperature than with R22. On the other hand, in direct expansion air conditioners, the existence of a temperature glide improves dehumidification of processed air. It may be difficult to deal with temperature glide in a reversible heat pump. In water-source heat pumps, the water exchanger is usually piped in parallel flow during the heating mode, and freeze-up problems may arise as a result of the low temperature of the refrigerant entering the exchanger ~2. To overcome this problem, it may sometimes be necessary to select a heat exchanger with significant refrigerant-side pressure drop in order to decrease the temperature glide. In fact, a pressure drop affects glide: it reduces the glide in a direct expansion evaporator, whereas it increases the temperature glide in flow condensers. When considering zeotropic mixtures, this unique characteristic of variable temperature phase change under constant pressure offers a potential improvement of the refrigeration efficiency by closely matching the refrigerant temperature profile with the sink and/or source fluid's temperature profile. As an ideal reference for the vapour compression refrigeration process with a zeotropic mixture a Lorenz cycle can be assumed in place of a reverse Carnot cycle 13'14. The ideal Lorenz cycle considers two constant-heat capacity processes during which heat transfer takes place, and two constantentropy adiabatic processes involving compression and expansion. To gain a first-approximation assessment of the potential of this temperature-matching issue, one can Lrst refer to the fully idealized situation depicted in the diagram (T is absolute temperature, S is entropy rate) in Figure 5, where the situation examined considers constant specific heat for external fluids through predetermined temperature transitions: (T01 -~ T02) for the fluid cooled in the evaporator; (T~1 ~ Ta2 ) for the condenser cooling fluid. With single-component refrigerants (or azeotropic mixtures), the reverse Carnot cycle is the ideal reference for the refrigeration process; the Carnot cycle (bcdeb) in Figure 5 operating between temperatures Tc = Ta2 at the condenser and Te = T02 at the evaporator represents the limit for compatibility under the temperature constraints of the external fluids. The corresponding evolutions of the external fluids are f ~ q at the hightemperature exchanger (condenser), and h ~ c at the low-

492

A. Cavallini 0.7

dml

to the Lorenze cycle rather than to the Carnot cycle, one must consider that the ideal COP of the Lorenz cycle can be computed by the usual Carnot expression:

ds

C O P = _ Te

0

Tc-~o

0.1 O.I

C

O.q

0 0.2 0 . 4 0 . 6 0 . 8

1 1.2 1.4 1.61.8 2

d Figure 6 Diagram to compute the temperature gain values At* in the reference temperatures of refrigerant in a reverse Lorenz cycle, as compared to those in a Carnot reverse cycle. Atml is the countercurrent logarithmic mean temperature difference; the other symbols are defined in the diagram Figure 6 Diagramme pour l'estimation des valeurs du gain de temperature At* pour les temperatures de r~fdrence du frigorigkne clans un cycle de Lorenz inverse, par comparaison avec celles du cycle de Carnot inversd. A t m l est la diffdrence des tempdratures en moyenne logarithmique g~contre-courant; les autres symboles sont ddfinis clans la diagramme

temperature exchanger (evaporator), with area(fqpmf) = area(ebmoe) and area(hcmnh) = area(dcmod). Although the two heat exchangers have zero effective mean temperature differences (and would therefore require heat transmittance-~ oc), they are sources of unavoidable thermodynamic irreversibility due to the temperature mismatch between the refrigerant and the external fluid during heat transfer, which takes place over finite temperature differences. Area(stons) and area(otupo) in Figure 5 represent the exergy loss rates at the evaporator and the condenser respectively, with Tal being the natural choice for the reference ambient temperature in this circumstance. Working with a zeotropic mixed refrigerant makes it conceptually possible to eliminate any thermodynamic irreversibility by perfectly matching the refrigerant temperature profiles in counter-flow heat exchangers through execution of the ideal reverse Lorenz cycle (hgfch), performing the same cooling duty as the previously considered reverse Carnot cycle. In this case, the evolutions of the external fluids are h ~ c and f ~ g, respectively, for the evaporator and the condenser. The area enclosed by a (reversible) cycle in the diagram in Figure 5 represents the required power input. Thus, the difference in area between the two cycles yielding the same refrigeration duty emphasises the superior energy efficiency of the Lorenz cycle as compared to the Carnot cycle. To calculate the increase in COP obtained by referring

where irc and Te are the mean thermodynamic temperatures of the refrigerant in the evaporator and the condenser, respectively (ratio of the enthalpy variation over the entropy variation). Moreover, in typical conditions of normal refrigeration and heat pump operations, no appreciable error is made by approximating the mean thermodynamic temperatures with the mean arithmetic temperatures of the refrigerant heattransfer processes. One can then compute the improvement in COP by considering the Carnot expression in relation to the temperature gains At* both in the reference condensation temperature (At~ = T ~ - irc) and in the reference evaporation temperature (At~ = ire -T~), obtainable by referring to the Lorenz cycle instead of the Carnot cycle. As shown in Figure 5, the temperature gains At* are equal to half the external fluid temperature transitions for the fully idealized situation examined above: At~hd = (Ta2 - Tal)/2,

Ate]id = (Tin - T02)/2

The ideal limit assumptions of zero temperature approach and perfectly matched flow heat capacities of the streams in the heat exchangers can be relaxed by considering finite temperature approaches Am in the exchangers with change of refrigerant phase at constant temperature, and flow heat capacity mismatch parameters R for the exchangers with change of refrigerant phase at gliding temperature: R = Ar/Ae

where Ar = temperature range of the refrigerant Ae = temperature range of the external fluid. The temperature gain values At* both at the evaporator and the condenser can then be calculated, under the constraint of equal values for the counter-current logmean temperature differences in the corresponding exchangers of the Carnot and Lorenz ideal refrigeration cycles. Figure 6 shows the results of this computation, expressed in terms of non-dimensional parameters. The temperature profiles shown in Figure 6 refer to the hightemperature exchanger, but the diagram can be applied to the evaporator as well, with reference to appropriate equivalent parameters. Once the temperature gains Ate and Ate, and thus also the reference temperatures T~ and ir~ have been computed, the relative COP improvement can be calculated by applying the relationship:

A(COP)_ cop

1 rc - L

At;+ Tc .'~ Ate)

which clearly shows the role of the glide-to-lift ratios in determining the effectiveness of the Lorenz concept.

Working fluids for mechanical refrigeration The analytical correlation among the parameters defined in Figure 5 is:

as = 5 - -

(1 - R)

+ 1)

d

for R :fi 1

-1

and

I

ds=-~-

In

1

1)

dI for R = 1

As an example, in heat exchangers with a temperature range Ae = 10°C and a pinch point Am = 5°C (representative design parameters of an air-cooled condenser), the maximum (R = 1) temperature gain is At* = ds Ae = 0.09 x 10 = 0.9°C. For a reversible cycle taking place between 0 and 40°C, this temperature gain brings about a COP improvement of 2.25% (condenser) and 2.58% (evaporator), giving a total improvement of 4.83%. This improvement lessens as the temperature lift of the cycle increases. Observing that in most applications fractionation may limit the maximum acceptable temperature glide of a zeotropic refrigerant to 7-10°C, the diagram in Figure 5, although referring to theoretical processes, indicates that in common situations one should not expect much in terms of energy efficiency improvement if advantage is taken of the temperature glides with a zeotropic refrigerant such as R407C in the heat exchangers. Furthermore, it must be noted that, while in some systems with water- (or brine)-to-refrigerant heat exchangers it may be relatively straightforward to resort to counter-flow arrangements, with air-torefrigerant heat exchangers only 'quasi-counterflow' arrangements with special design are practicable. It can be concluded that only with large temperature ranges of external fluids (i.e. 20°C or more), reduced cycle temperature lifts and close approach temperature in the heat exchangers can substantial improvements in energy efficiency be pursued by matching the gliding temperature profiles during change of phase of suitably tailored zeotropic mixtures. In principle other potential performance benefits are possible by exploiting zeotropic glides, for example by using two-phase liquid line/suction line heat exchangers or combined subcooler/evaporator unit in the refrigerating circuit15; these possibilities, which require substantial hardware design changes, are not dealt with here. When working with zeotropic mixtures, generally a further penalty has to be paid in terms of heat-transfer degradation, during both the condensation and evaporation processes 16. In condensation heat transfer, the added mass-transfer resistance in the vapour phase caused by accumulation of the more volatile compound at the vapour-liquid interface is responsible for the lower coefficients measured as compared to pure fluids, with shell-side condensation being particularly penalised by this occurrence. In boiling heat transfer with zeotropic mixtures, loss of wall superheat and mass-transfer resistance are responsible for a degradation in heat-transfer rates, as

493

compared to pure components under equivalent working conditions. Moreover, as refrigerants are mixed, liquid thermal conductivity decreases and liquid viscosity increases more than predicted by a simple linear interpolation of the component properties, and this penalises heat transfer, both in film condensation and in flow evaporation. Generally speaking, it can be assumed that the value of the change-of-phase temperature glide of a zeotropic mixture gives an indication of the potential heat-transfer degradation in phase-change processes in comparison with the mass-fraction linearly interpolated values of pure components. The above discussion, of course, does not necessarily imply that the heat-transfer coefficient of a zeotropic mixture serving as a replacement refrigerant is always smaller than that of the original fluid. Enhanced heat transfer with change of phase of zeotropic mixtures is still wide open to basic and applied research; some preliminary experimentation is being carried out in research laboratories and some first results are starting to appear in open literature.

High- and low-pressure alternatives In the preceding paragraph, a few concerns about the use of zeotropic mixtures as substitute refrigerants, and namely of R407C as an alternative to HCFC22, have been discussed in some detail. Only by gaining complete field experience will some presently open points, which cannot be overlooked, receive clear answers. Whenever possible, choosing a look-alike alternative as substitute refrigerant is certainly the most attractive option, as it calls for minor modifications and adaptations to currently operating equipment, therefore making R407C the most likely replacement for HCFC22 in the interim in most applications, except when fractionation may be the cause of concern. Alongside R407C, another two alternatives are being considered for replacing HCFC22, both of them free from concerns .about zeotropic behaviour: the higher pressure nearly azeotropic blends of the R410 family, and the lower pressure fluid HFC134a. However, both these options require complete system redesign for optimum operation. R410A has about 50% higher saturation pressure vs temperature as compared to HCFC22, and this deeply affects the characteristics of system components. Compressors, especially for air-cooled systems, may need complete redesign with stronger pressure-containing structures to pass standard tests, higher bearing loads and larger motors (compressors of the same displaced volume are compared). The lower vapour volume flowrate for a given cooling capacity requires optimisation of heat exchangers, with smaller diameter tubing and possibly different circuiting. It is believed that a fully optimised system for use of R410 could display a better energy efficiency as compared to its HCFC22 counterpart, as a result of the more favourable pressure level, which lends itself to improved compressor efficiency and reduced energy losses in other system components, even contrary to simple thermodynamic cycle analyses that indicate a slightly better efficiency for HCFC22. This efficiency

494

A. Cavallini

advantage could balance the cost of redesign and retooling, and could permit compliance to new higher efficiency standards that may be implemented for unitary systems in the future. R410 blends are gaining favourable support as the most likely HCFC22 long-term alternative for general air-conditioning applications, except with centrifugal chillers, for which HFC134a appears to be a more attractive choice. HFC134a, which is a look-alike alternative for CFC12, requires a volumetric compressor displacement about 50% greater than that of a HCFC22 compressor of the same cooling capacity, due to lower pressure vs temperature correlation at saturation. For the same reason, on the basis of the same cooling capacity, a HFC134a system needs larger tubing than its HCFC22 counterpart to keep energy losses within appropriate limits. As a consequence, the system would need to be physically larger and more expensive to manufacture. HFC 134a is therefore considered a possible long-term substitute for HCFC22 only in centrifugal or large screw compressor chillers, for which shell-side evaporation and condensation make the zeotropic R407C blend unsuitable, and where the higher speed (for centrifugal compressors) and strengthened shell structures needed for R410 set this alternative aside. An additional alternative for those same systems could be the R404A blend, a very low-glide mixture with properties close to those of HCFC22, and therefore requiring minor redesign. This mixture is now being investigated as a possible replacement for R502. However, one possible drawback is its high GWP index value, but, of course, TEWI value should be given higher priority. As with all mixtures containing HFC125, a loss of efficiency may be experienced at high condensation temperatures, due to the low critical temperature of this mixture component. Natural fluids

A fully environment-friendly option, at least with regard to the direct effect of the refrigerant, can be achieved only by resorting to natural fluids, that is, substances that are part of the global ecological system, and therefore have no yet unknown harmful side effects. Among the various possible natural fluids, certain hydrocarbons (HC), ammonia, carbon dioxide, water and air are applicable as refrigerants. Besides their friendliness to the global environment, natural fluids possess other positive properties, such as their low cost, abundant availability not subject to monopoly, no recycling is required except for local requirements (e.g. flammable fluids), favourable thermodynamic and transfer properties (most HCs considered as replacement refrigerants, ammonia, and partly CO2) and lower molecular mass than that of synthesised refrigerants, which leaves room for improved compressor design. Unfavourable aspects of natural fluids mainly concern local and environmental safety, such as in the use of the highly flammable HCs and the moderately flammable and toxic NH3; the high process pressure and low critical temperature of CO2; the extremely low process pressure with water; and the low energy efficiency of air cycles of the present generation.

The introduction of natural refrigerants in new sectors of refrigeration requires thorough research in the development of improved technology in processes and components suitably adapted to their positive and negative properties, and the formulation of safety codes of practice and standards for system and component design, installation and operation 17. In particular, for ammonia and hydrocarbon applications there is a great need for uniform international regulations and standards based on accepted quantitative risk assessment studies; this, however, seems a distant possibility although this effort is currently under way within the EU 18'19. Equipment working with natural refrigerants will no doubt be favoured in the future by an expected market awareness towards environmental protection. The International Institute of Refrigeration is dedicating particular attention to innovative applications for natural refrigerants. An international conference on New Applications of Natural Working Fluids in Refrigeration and Air Conditioning took place in Hannover, Germany in May 1994. A meeting of the I.I.R. Sections B and E with almost the same title will be held in Aarhus, Denmark, in September 1996, and another meeting on a related theme will probably take place the following year in Norway. A brief outline of the present situation on this subject is reported in the following2°-z7.

Hydrocarbons Due to their high flammability, the use of hydrocarbons in new sectors is confined to simple, hermetically sealed systems with a limited charge operating at non subatmospheric pressures. The use of hydrocarbons in domestic applicances, both as refrigerants (R600a or R600a/R290 mixture) and as blowing agent for insulating foam (cyclopentane) is gaining wide acceptance in Germany and other European countries. In these applications, the charge of hydrocarbon is limited to approximately 50 g. In single-temperature and three-star refrigerators the energy consumption compares favourably with appliances using HFC 134a. In some northern European countries some leading manufacturers of residential heat pumps are offering small- to medium-sized units with propane or iso-butane used as working fluids. It is believed that safety implications are fully covered by simple technical measures. In the commercial sector, some systems are being converted to indirect ones which enables use of hydrocarbons (or ammonia) in the remote machine-room under appropriate safety conditions. Another possibility currently under investigation involves use of indirect ammonia systems with normal brines directly supplied to the medium-temperature cabinets, while the low-temperature cabinets work with individual propane systems whose condensers are cooled by the brine circuit. In certain countries such as the USA, the attitude towards flammable refrigerants is exactly the opposite, in fact they are not pursued due to potential liability exposure.

Ammonia The extension of the practices for safe use of ammonia to

Working fluids for mechanical refrigeration new sectors in refrigeration may benefit from the considerable practical experience gained in using this fluid since mechanical refrigeration was introduced, in compliance with existing codes and design standards. Moreover, recent important steps have been made towards an extended use of ammonia: •

• •

The introduction of synthetic oils (PAG) that are soluble in ammonia, making this fluid compatible with dry expansion operation in small units, and favouring introduction of simple solutions to the problem of automatic oil return to the compressor. Due to the very low liquid/gas volume ratio for ammonia, problems may be experienced for correct distribution and wetting of the heat transfer surface in dry evaporators; The availability of canned motor-type compressors which through extensive use of aluminium makes it possible to achieve fully hermetic circuits; The construction of ammonia-compatible, soldered plate heat exchangers which helps reduce the amount of charged refrigerant considerably, thus satisfying a very important safety requirement with all flammable refrigerants.

Further developments are necessary on semi-hermetic compatible compressors, direct-expansion plate-type evaporators and non-welded steel and aluminium pipe connections. Recent new applications of ammonia involve small direct-expansion systems for supermarket refrigeration, water chillers and heat pumps for air conditioning of residential buildings; all of them are, of course, closed indirect systems. Safety precautions in the machine-room involve closing each machine or groups of them in suitably ventilated compartments; discharging ammonia safely outside or through a special absorber system in the event of leakages. Ammonia is also used extensively as a working fluid in large heat pump plants for district heating in the Nordic countries of Europe. Central cooling, thermal storage and precooling of combustion air of gas-turbine cogeneration plants are other sectors where ammonia is being more and more extensively used. Carbon dioxide

This non-toxic and incombustible environmentfriendly refrigerant is fully compatible with normal lubricants and common construction materials. It has a low critical temperature (31°C), while the critical pressure is a little below 74 bar. In normal refrigeration applications therefore, the high-pressure heat-release process in most cases does not involve condensation of the refrigerant, but wide glide temperature cooling at trans-criticat pressure, which has to be taken into account (for example, as discussed previously with relation to zeotropic mixtures) for energy-efficient operations. The coordinates of the critical point also indicate that, when working with CO2, the pressure levels are far higher than in conventional systems. This requires the development of suitable components. While the highpressure level is usually regarded as a drawback, it does have some positive features, such as small component

49,5

dimensions and low compression ratios which improve compressor efficiency2°'21• Carbon dioxide has been successfully used in a prototype automotive air conditioner (an application with high relative direct global warming impact when using HFC134a), and excellent prospects are also predicted in commercial refrigerating units with associated tap-water heating, and in high-temperature-range heat pumps. Another interesting application is the use of CO2 as a secondary refrigerant with change of phase (for example, in low-temperature commercial applications), remotely condensed in a refrigerating system using ammonia or HC290 under appropriate safety conditions. Water

Water has certain unfavourable features when used directly as a refrigerant, that is: •

• •

pressure levels are very low, and conversely the specific volume of the vapour phase at low temperature is very high (180m3kg -~ at 2°C saturation temperature), which produces a volumetric refrigerating capacity of approximately 0.5-1% of that of conventional refrigerants; pressure ratios at normal temperature lifts are rather high (i.e. from 5 to 7); when used with mechanical positive displacement compressors, enormous swept volumes are necessary: only turbo or specially designed rotary machines (such as cycloid type compressors) are suitable.

On the other hand, water vapour compression plants may be constructed as open systems, with direct-contact heat transfer both at the evaporator and the condenser, thus eliminating the exergy losses due to the driving temperature differences at the surface exchangers of traditional installations; systems with very high energy efficiency are obtained, thus possibly making these installations cost-competitive with traditional ones, and for which the higher cost of the compressor can be compensated by the energy saved during the operative life cycle of the plant. Mechanical compressors for water vapour refrigerating systems are commercially available today for industrial applications in the capacity range from 500 to 5000 kW, with pressure ratios from 3 to 10 in singleand multi-stage arrangements. Several of these plants have recently been built and perform satisfactorily. Another application for water, which is gaining an increasing interest, is its use as a secondary refrigerant of high energy density, in the form of a pumpable suspension of microscopic ice-crystals (water-ice slurry); this can be obtained directly in the evaporator of a water-vapour refrigeration plant. Air

Energy improvements in standard air-cycle machines are necessary if they are to be made competitive with conventional systems in the normal refrigeration temperature range. These improvements can be achieved with high-performance components (compressors, expanders, heat exchangers) and modifications to the

496

A. Caval/ini

basic configurations which include open systems, regenerative heat exchangers, multi-stage compression and expansion and intelligent use of humidity in the processed air. Promising innovative applications are: systems for very-low temperature uses, refrigerated transport of foodstuffs, food chilling, plants with both heating and cooling loads, and perhaps automotive and railway-car air conditioning.

14

15 16

17

References 1

2

3 4 5 6 7 8

9 10 11 12 13

Fisher, S. K., Hughes, P.J., Fairchild, P. D., Kusik, C. L., Dieckmann, J. T., McMahon, E. M., Hobday, N. Energy and global warming impacts of CFC alternative technologies AFEAS and the U.S. DOE, December 1991, 1.13-1.15 Didion, D. A. Recent developments in the design of new refrigerant mixtures for existing and future systems. Proc Int Seminar New Technology in Refrigeration - Progress in the Application of New Refrigerants Padova (1994) 173-195 Umweltbericht der Bundesregierung 1994. Gritfith, R. W. Alternate refrigerants for the refrigeration industry, Proc ASHRAE/NIST Conf R-22/R-502 Alternatives Gaithersburg, MD (1993) 19-21 Muir, E. B. HFC replacements for R-22 Proc LLR.-AICARR Int. Conf. CFCs, The Day After Padova (1994) 249-257 Hiekman) K. E. Redesigning equipment for R-22 and R-502 alternatives A S H R A E J (1994) 36(1) 42-47 Godwin, D., Menzer, M. HCFC-22 phase out in North America - impact on future equipment production I.E.A. Heat Pump Center Newsletter (1995) 13(1) 29-31 Kruse, H., Chen, J. Cycle performance of alternative refrigerant mixtures Proe Int. Seminar on Heat Transfer, Thermophysical Properties and Cycle Performance of Alternative Refrigerants Kytakyushu (1993) 217-244 Cavallini, A. CFC and HCFC substitution - short- and longterm solutions I.I.R. Bulletin (1994) 94.6 2-15 Maeaudiere, S. New range of HCFC-22 alternatives Proc 1.1.R.AICARR Int Conf CFCs, The Day After Padova (1994) 283-290 Kim, M. S., Diction, D. A. Simulation of isothermal and adiabatic leak processes of zeotropic refrigerant mixtures HVA C&R Res (1991) 1 3-20 Glamm, P. Water-source heat pumps with HFC refrigerants LE.A. Heat Pump Center Newsletter (1995) 13(1) 25-28 Camporese, R., Cavallini, A., Fornasieri, E., Joppolo, C. M. Lo stato dell'arte e le prospettive del processo di sostituzione dei CFC e degli HCFC (in Italian) Proc FREE 94 - Fluidi refrigeranti, espandenti, estinguenti Padova (1994) 25 62

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24 25 26 27

28 29

Bobbo, S., Camporese, R., Cortella, G., Fornasieri, E. Theoretical evaluation of the performance of zeotropic mixtures in refrigeration cycles Proc 19th lnt Congress q] Refrigeration The Hague (1995) 72 79 Didion, D. A., Bivens, D. B. Role of refrigerant mixtures as alternatives to CFCs Int J Refrig (1990) 13(3) 163.-175 Kedzierski, M. A., Kim, J. H., Didion, D. A. Causes of the apparent heat transfer degradation for refrigerant mixtures A S M E HTD Vol 197 (Two-phase flow and heat transfer) (1992) 149158 Frivik, P. E. CFC replacement by natural working fluids - R&D and industrial challenges within the refrigeration, air conditioning and heat pump areas Proc Int Seminar on Heat Transfer. Thermophysical Properties and Cycle Performance of A lternative Refrigerants Kytakyushu (1993) 189-202 Anderson, K. A natural choice LE.A. Heat Pump Center Newsletter (1995) 13(1) 3 van Gerven, R. Safety aspects of natural working fluids I.E.A. Heat Pump Center Newsletter (1995) 13(1) 19-21 Lorentzen, G. Natural refrigerants, a complete solution Proc LLR. AICARR Int Conf CFCs, The Day After Padova(1994) 317-328 Lorentzen, G. The use of natural refrigerants: a complete solution to the CFC/HCFC predicament Int J Refrig (1995) 18(3) 190 197 Stuij, B. CFC and HCFC replacement - an international overview 1.E.A. Heat Pump Center Newsletter (1995) 13(1) 13 18 Kruse, H. Refrigeration and air conditioning systems working with natural fluids Proc Int Seminar New Technology in Refrigeration - Progress in the Application of New Refrigerants Padova (1994) 7-27 Paul, J. Water as natural refrigerant Proc L1.R. Int ConfNew Applications of Natural Working Fluids in Refrigeration and Air Conditioning Hannover (1994) 97-108 Reinhart,A. Ammonia in liquid chillers and heat pumps Proc 1,I.R. Int Conf New Applications of Natural Working Fluids in Refrigeration and Air Conditioning Hannover (1994) 65-70 Mosemarm, D. Air conditioning with NH 3 water chillers Proc LLR. Int Conf New Applications of Natural Working Fluids in Refrigeration and Air Conditioning Hannover (I 994) 71-96 Kruse, H., Siears, A. Technical potential for refrigeration, air conditioning and heat pump applications of air cycle systems Proc LLR. Int Conf New Applications of Natural Working Fluids in Refrigeration and Air Conditioning Hannover (1994) 179-193 prEN 378-3 (Draft) Refrigerating systems and heat pumps safety and environmental requirements part 3. Classification of Refrigerating Systems, Occupancies and Refrigerants UNEP 1994 Report of the Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee - 1995 Assessment 38-41