Working parameters of domestic refrigerators filled with propane-butane mixture

Working parameters of domestic refrigerators filled with propane-butane mixture E. Bodio, M. Chorowski and M . W i l c z e k

Institute of Power Engineering and Fluid Mechanics, Technical University of Wroclaw, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Received 26 June 1991; revised 20 July 1992

S o m e h y d r o c a r b o n s like p r o p a n e - b u t a n e m i x t u r e are p r o s p e c t i v e alternatives to C F C refrigerants. T h e r m o d y n a m i c analysis has p r o v e d t h a t p r o p a n e - b u t a n e m i x t u r e is c h a r a c t e r i z e d by p r o p e r t i e s similar to t h o s e o f the R I 2 refrigerant. D o m e s t i c refrigerators c h a r g e d with p r o p a n e - b u t a n e m i x t u r e are investigated here. T h e tests p r o v e that R12 c a n be r e p l a c e d by the p r o p o s e d mixture.

(Keywords: refrigerant; CFC; substitute; R12; propane; butane; mixture; household refrigerator)

Conditions de fonctionnement des r6frig6rateurs domestiques charg6s avec le m61ange propane-butane Certains hydrocarbures comme le mblange propane-butane constituent des substituts potentiels des CFC. Une analyse thermodynamique a montr~ que le mblange propane-butane prbsentait des propriktbs semblables d celles du R12. On examine des rbfrigbrateurs domestiques chargbs avee le mklange propane-butane. Les essais ont prouvb que le R12 pouvait ~tre remplacb par ce mblange.

(Mots cl6s: frigorig6ne; CFC; substitut; R12; propane; butane; m61ange; r6frig6rateur domestique)

It has been proved that fully halogenated refrigerants (CFCs) such as R12 and R I 1 are ozone depleting compounds. They are also partly responsible for the so-called 'greenhouse effect'. To prevent the degeneration of the natural environment, some governments have decided on a drastic reduction of the use of R l l and R12 (The Montreal Protocol, 1987). For instance, RI 1 and R12 are supposed to be totally abandoned in Sweden by 1995. The chemical industry has developed new refrigerants: R123 to substitute for R11 and R134a for R12. The physical and chemical properties of R134a have not been fully recognized. The lubricants formerly used for R12 systems are incompatible with R134a. A new refrigerant is chemically aggressive in contact with some construction materials, In this situation, other chemicals should also be taken into account as substitutes for CFCs in domestic vapour compression refrigerators. Usually, the following are required in refrigerants: • • • • • •

chemical stability over a range of temperatures environmental acceptability (including stratosphere) low cost of production chemical neutrality of lubricants and refrigerators' construction materials non-toxicity non-flammability.

If all the above requirements are to be met, then the only presently known substitute for R 12 is R 134a. If flammability is allowed, then the possibilities of finding alternative refrigerants are much greater. The demand of inflammability is reasonable in the case of large systems, but if a small domestic refrigerator is considered, then in case of leakage no more than 100 g of refrigerant is involved 1.2.

Table l

Chosen thermodynamic properties of hydrocarbons and refrigerants R12 and R134a t-2,4, to: critical temperature; th: boiling temperature; p: density; r: heat of vapourization. Tableau 1 Propriktbs thermodynamiques choisies des hydrocarbures et des jkigorig~nes R12 et R134a ~2.4

Name

t~(°C)

tb(°C)

p ( k g m 3)

r ( k J k g ')

Propylene Propane l-Butylene Isobutylene lsobutane n-Butane RI2 R134a

+ 91.4 + 96.8 + 146.6 + 144.7 + 135.0 + 152.0 + 112.0 + 100.5

- 47.8 - 42.1 - 6.3 - 7.0 - 11.7 - 0.5 - 29.7 - 26.5

1.955 2.019 2.550 2.500 2.668 2.703 6.240 5.080

440.16 425.92 391.58 397.02 366.03 387.81 166.00 208.8

Propane-butane mixture as a refrigerant

If refrigerant flammability is allowed, then the search for alternatives to CFCs can be focused on compounds that exist naturally, e.g. hydrocarbons. It is well known that hydrocarbons mixed with air form explosive mixtures. However, hermetic, high-reliability domestic refrigerators can be charged with hydrocarbons, either singly or mixed. Table I shows the chosen thermodynamic properties of selected hydrocarbons; the values for R12 and R134a are given for comparison. It follows from Table 1 that pure hydrocarbons cannot be exact alternatives for R12. Propane can be considered as an alternative for R22. But if some hydrocarbons are mixed, then the mixture can have properties similar to those of R 12. Propane-butane mixture is used as a fuel in many countries all over the world, but the mixture is not applied as a refrigerant. It is possible to obtain a propane-butane mixture with thermodynamical properties

0140-7007/93/050353~)4

~" 1993Butterworth-Heinemannand IIR

Rev. Int. Froid 1993 Vol 16 No 5

353

Working parameters of domestic refrigerators." E, Bodio et al,

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Figure I Saturated vapour pressure versus temperature; 1 propane, 2 mixture (70% propane, 30% butane), 3 -- R12, 4 mixture (50% propane, 50% butane), 5 - butane Figure 1 Pression de vapeur satur~e par rapport ?*la temperature; 1) propane," 2) mblange (70% de propane et 30% de butane)," 3) R12." 4) mblange (50% de propane et 50% de butane)," 5) butane

very close to those of RI2. Figure 1 shows the relation saturated vapour pressure versus temperature for propane, butane, refrigerant R12, and two different concentration p r o p a n e - b u t a n e mixtures. It follows from Figure 1 that:

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neither single propane nor butane can exactly replace refrigerant R12 saturated vapour pressure for propane butane mixture of propane concentration equal to 50% is very close to the refrigerant R I2 saturated vapour pressure.

This conclusion can be confirmed with experimental data 5. Figure 2 gives vapour-liquid equilibria for a prop a n e - b u t a n e mixture. The mixture consisting of 45% of propane is characterized by a boiling temperature close to - 20°C at a pressure of 0.1 MPa. Typical pressures for domestic refrigerators charged with R12 refrigerant are 0.1 M P a for the evaporator and

354

Int. J. Refrig. 1993 Vo116 No 5

0.8 MPa for the condenser. The pressures should be the same for a refrigerator charged with a p r o p a n e - b u t a n e mixture. Dewar and boiling lines for a p r o p a n e b u t a n e mixture for the pressures 0.1 and 0.8 MPa were calculated with the use of a Peng Robinson equation of state 3. The calculation results are shown in Figure 3. The calculated boiling and condensing temperatures of the p r o p a n e butane (45% propane} mixture are similar to the values obtained for R 12. The lowest boiling temperature in the evaporator is equal to - 27.3°C. and the highest temperature in the condenser is equal 52. I °C. The mixture's latent heat of evaporation calculated for 0.1 MPa pressure is equal to 401.2 kJ kg -~ and a mixture density in normal conditions is 2.395 kg m 3 I f latent heats of evaporation are given m kJ m- 3 then one obtains 960 kJ m -:~ for the mixture and 1035.8 kJ m 3 for RI2. If the same cooling powers are to be achieved, it is necessary to increase the volume flow of the mixture by 8% above that for R12. It follows from the above that the working parameters of a domestic refrigerator filled with a p r o p a n e - b u t a n e mixture (45% propane) should be comparable to those obtained using R12. All the above considerations enabled us to state that a proper p r o p a n e - b u t a n e mixture is a prospective refrigerant for domestic refrigerators. The following questions should be then answered: •

• •

Is a refrigerator, or a part of it (e.g. compressor), construction to be changed if the propane mixture is used? Which lubricants can be used'? W h a t hazards can be caused by a leak'?

These questions have been answered experimentally.

Experimental investigations To confirm the theoretical considerations, four typical domestic refrigerators filled with a p r o p a n e - b u t a n e mix-

Working parameters of domestic refrigerators: E. Bodio et al. 280

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Figure 3 Vapour liquid equilibrium, propane butane mixture, Peng Robinson equation of state. (a) 0.1 MPa, (b) 0.8 MPa, 1 Dewar line, 2 boiling line Figure 3 l~'quilibre vapeu~liquide, mdlange propan~butane, dquation

d'~;tat Pen~Robinson. (a) 0,1 MPa; (b) 0,8 MPa; 1) conduite Dewar; 2) eonduite d'dbullition

ture have (chamber 176dm3). following

been investigated: TSi 35/0, TS135/1, TS135/2 volume 135dm3), TS176 (chamber volume The refrigerators were charged with the hydrocarbon mixture: methane - 0. 1%, ethane

- 4.5%, p r o p a n e - 34%, butane 61.3%, hydrocarbons C 5+ - 0.1%. The mixture composition was obtained chromatographically. The following parameters were measured: evaporator temperature, chamber temperature, condenser temperature, compressor temperature, environment temperature, low and high pressures, electrical energy consumption, net cooling power. The refrigerator monitoring took place for over eight months. The refrigerators were placed in a room of nonstabilized temperature (the ambient temperature varied from 18°C to 25°C). The thermostats were set to provide an evaporator temperature of - 12°C. The average results based on eight months of investigations are given in Table 2. Refrigerators TSl35/l and TS176 were then charged with R12 refrigerant. Table 3 gives comparison R12 data. The temperature changes of compressors, evaporators and condensers of the investigated refrigerators are shown in Figure 4. Condenser temperature analysis shows that the process of condensing is completed approximately in the centre of the condenser tube, meaning that the condenser heat exchange surface is sufficient, and no changes in condenser construction are necessary. In spite of relatively long compressor working times, their exit temperatures did not exceed 63°C. This temperature is only slightly higher than that for R12. The following lubricants were used during the experiments: freol 16 in TS 135/0 and TS 176; freol 27 in TS135/2; and karylobenzene in TS135/1. No other negative effects like noise, vibration or compressor overheating were observed during the experiments. The net cooling power was measured, and was found to be equal to 25 W for the TS 135 refrigerators, and 35 W for the TS 176 refrigerator. The energy consumption of the p r o p a n ~ b u t a n e mixture-charged refrigerators was comparable with the R 12 charged refrigerators (see Table 3). If the thermostat screws were set close to their maximal values, the refrigerators charged with the mixture worked in a continuous way. Despite long-term compressor work without any turn off, its temperature was stable, and did not exceed the value measured during an on/off working mode, and no noise, vibration, overheating, etc., were observed. After three months of constant work a sample of the mixture (gaseous phase) was extracted from refrigerator TS 135/0 and analysed. The following composition of the

Table 2 Working parameters of domestic refrigerators charged with propane butane mixture (ev: evaporation; con: condensation; temp: temperature) Tableau 2 Conditions de fonctionnement des r~J~ig~rateurs domestiques charges avec le mblange propane butane Refrigerator

TS135/0 TS135/1 TS135/2 TS176

Pressure ev./con. (MPa)

Energy consumed (kWh day ~)

Time on/off (min/min)

0.1/0.7 0,1/0.8 0.1/0.8 0.1/0.8

1.0 1.0 1.2 1.1

10/14 31/11 32/16 35/20

Compressor temp (°C) +37to +22to +37 to +37to

+51 +47 +63 +53

Evaporator temp (°C)

Chamber temp. (°C)

14to - 1 1 -20to 12 - 16 to - 6 -16to -7

- 1 to 0 0 t o +1 + I to +2 +1 to +3

Table 3 Working parameters of domestic refrigerators charged with RI2 refrigerant (ev: evaporation; con: condensation; temp: temperature) Tableau 3 Conditions deJbnctionnement des rkfrig~rateurs domestiques charges avee le frigorig~ne RI2 Refrigerator TS135/1 TS176

Pressure ev./con. (MPa)

Energy consumed (kWh day-')

Time on/off (min/min)

Compressor temp (°C)

Evaporator temp (°C)

0.1/0.82 0.1/0.8

1.01 1.3

15/11 17/12

+43 to +48 + 4 6 t o +61

- 17 to - 9 - 2 2 t o - 11

Chamber temp. (°C) 1 to 0 1 to + I

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Hydrocarbons are chemically acuve gases. The ignition temperatures of propane and butane mixed with air are 490°C for propane and 5000C for butane. If mixed with air, both propane and butane form an explosive mixture if their volume concentrations are 1.8 8.5% for butane, and 2.1 9.5% for propane. It follows from our investigations that TS176 refrigerators should be charged with about 50 g of mixture and TS135 with 35 g. In case of TS176 refrigerator dehermetization, about 14 g of mixture (6.3 dm 3) will leave the system. This figure was obtained experimentally after the TS176 dehermetization had been carried out. If the room volume exceeds 342 dm 3 there is no danger of forming an explosive mixture. The volume of any existing room is much larger than this, hence it is practically impossible to reach a dangerous mixture concentration in a kitchen or any other similar room. The probability of mixture self-ignition is also very small. The compressor temperature does not exceed 63°C, and the ignition temperature of the hydrocarbons used is much higher. The hydrocarbons investigated are blood insoluble in atmospheric conditions, they do not react chemically with blood, and do not possess toxic properties. Conclusion

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mixture was recorded: methane 0.15%, ethane 12%, propane 48.51%, butane 38.86%, pentane 0.16%. It follows from the analysis that some amount of

Int.

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propane--butane mixture is a prospective alternative to CFC (especially RI 2) refrigerants one of the commonly used lubricants can also be used in the case of a propane-butane mixture domestic refrigerators can be charged with a propane-butane mixture with no changes being necessary in their construction electrical energy consumption is comparable with refrigerators charged with R12 refrigerant.

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Figure 4 Temperature changes in chosen points of the p r o p a n e butane filled refrigerators. (a) TS 135/1, (b) TS 135/2, (c) TS 176, 1 compressor head, 2 condenser pipe (middle), 3 condenser pipe (end), 4 environment, 5 -chamber, 6 evaporator Figure 4 Changements de temperature duns les points choisis des r~l?igOrateurs chargks en propane-butane. (a) TS 135/1, (b) TS 135/2. (c) TS 176.1) t~te du compresseur; 2) conduite du condenseur (milieu); 3) eonduite du eondenseur ( extrkmitk ) ; 4) ambiance; 5) ehambr e ; 6) {'vaporateur

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butane has been dissolved in the lubricant. Analysis of the lubricant led to the same conclusion. No chemical changes (e.g. polymerization, cracking) were observed either in the mixture or in the lubricant.

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Kruse, H., Hesse, U. Possible substitutes for fully halogenated chlorofluorocarbons using fluids already marketed lnt J Re~rig (July 1988) 11 276,283 Ktfiipers, L.M.J., De Wit, J.A., Jm~sen, M.J.P. Possibilities for the replacement of CFC 12 in domestic equipment Int J Refrig (July 1988) 11 284-291 Peng, D., Robinson, D.B. A new two-constant equation of state Ind Eng Chem Fundam (1976) 15 (l) 59-64 Rabeev, N.I, Krazev, B.G. Szizennyje Uglevodorodnkje Gazy Moskva, Nedra (1977) Skripka, V.G., Nikitlna, J.E, et al. Fazovye ravnovesia zidkostpar pri niskich temperaturach v dwoinych sistemach obrazowannych komponentami prirodnogo gaza Gazovaja Promyslennost (1970) 12