Energy 27 (2002) 127–134www.elsevier.com/locate/energy

Performance and dynamic flammability of R32/134amixtures in water-to-water heat pumps

Yang Zhaoa,*, Tian Guansana, Zhao Yi a, Zhang Guozhengb

a Thermal Energy Research Institute, Tianjin University, 92 Weijin Road, NanKai, Tianjin 300072,People’s Republic of China

b South China University of Technology, Guangzhou 510641, People’s Republic of China

Received 31 May 2000

Abstract

Thermodynamic properties and refrigerating performance for the alternative R32/134a mixtures in water-to-water heat pumps have been analyzed. Also, its dynamic model and flammability during the leakageprocess have been investigated. The results show that R32/134a (30/70) is a good alternative for water-to-water heat pumps. But the possibility of burning and explosive accidents during the leakage processcannot be excluded. 2002 Elsevier Science Ltd. All rights reserved

1. Introduction

A water-to-water heat pump plays an important role in developing and making use of wasteheat resources. It not only has the characteristics of high efficiency and energy savings, but alsocontributes to reducing heat and atmospheric pollution [1]. However, these advantages greatlyrely on the refrigerant used in the system. To help to protect the ozone layer and reduce thegreenhouse effect, the thermodynamic properties of an alternative to the traditional refrigerantsuch as R22 should be analyzed [2,3]. In addition, during the process of the filling, storage andtransportation of the alternative, leakage is nearly inevitable. Accordingly, it is vital to researchthe thermodynamic properties and dynamic flammability of the alternative caused by the leakage.

* Corresponding author. Fax:+86-27404741.E-mail address: zhaoyang@tju.edu.cn (Y. Zhao).

0360-5442/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0360-5442(01)00063-9

128 Y. Zhao et al. / Energy 27 (2002) 127–134

Nomenclature

Ms0 Initial quantity (kg)DMs0 leaking quantity (kg/s) per unit time at the initial time of leakageDMs Leakage quantity (kg/s) of the mixture per unit timeDMr The ratio of DMs to Ms0

Y(I) Mass fraction of the component I in the vapor-phase mixtureX(I) Mass fraction of the component I in the liquid-phase mixtureMs(t) Surplus quantity (kg) of the mixture in the container at the time tZ(I) Total mass fraction of the component I in the vapor and liquid phase mixture

2. Thermodynamic properties of the R32/134a mixtures

For routine water-to-water heat pump systems, R22 has typically been used as the refrigerantin the past. In this paper, the CSD (Carnhan–Starling–Desantis) equation of state is used to calcu-late the thermodynamic properties of various types of alternatives to R22. The results show thatR32/134a (30/70) can be considered as a replacement for R22 in water-to-water heat pumps.

Fig. 1 shows the predicted performance of the R32/134a mixtures, including the relative volu-metric capacity Qvr, energy efficiency ratio EERr, compressor–discharge temperature T2r, pressureratio Pr, and condensation pressure Pkr with respect to R22, where X is the R32 mass fraction inthe mixtures. It shows that all lines intersect at the point A, where X equals ca 30%, and allrelative parameters, such as Qvr, T2r and Pkr with respect to R22, equal about 1. So, the behaviorsof the alternative mixture are very similar to those of R22. Moreover, at the point A the energy

Fig. 1. Properties of R32/134a mixtures as a function of X.

129Y. Zhao et al. / Energy 27 (2002) 127–134

efficiency ratio EER of the alternative is slightly higher than that of R22. Fig. 2 shows temperaturechanges �T during phase change with X in the evaporator or condenser. There is a 5–7°C tempera-ture difference during phase change when X=0.3. But for water-to-water type heat pumps, it ispossible to make full use of these temperature differences.

The flammability of the mixture composed of non-flammable R134a and flammable R32 duringthe process of the storage and transportation should be considered. There must be a critical flam-mable concentration of the mixture. If the concentration of the flammable component R32 of themixture during the leakage process is lower than the critical flammable concentration, the mixturewill not ignite during ejection or diffusion even if it mixes with air at random percentage. Thetesting results show that the critical flammable concentration for this mixture is 33% and 38%R32 mass fraction at 100°C and 60°C, respectively, according to ASHRAE Standard 34, and is56% and 52% R32 mass fraction at 20°C and 80°C, respectively measured by Shiflett et al. [4].

3. Dynamic model during leakage process

The hypotheses are made as follows:

1. The volume of the system does not vary with the external conditions.2. The system always maintains a balance between vapor and liquid phases during the leakage pro-

cess.

Accordingly, the mathematical descriptions are as follows:

�DMs�dMs

dt. (1)

Fig. 2. Temperature changes with X (R32) for R32/134a mixtures.

130 Y. Zhao et al. / Energy 27 (2002) 127–134

For component I in the leaking vapor mixture:

�DMs�Y(I)�d(Ms×Z(I))

dt. (2)

For component I in the leaking liquid mixture:

�DMs�X(I)�d(Ms×Z(I))

dt. (3)

From the initial time to t=�t with first difference:

Ms(t0+�t)Ms0

�1�DMs0��tMs0

. (4)

and

Z(I)(t0��t)�1

Ms(t0+�t)/Ms0

�Z(I)s0�DMs0�t/Ms0

Ms(t0+�t)/Ms0

Y(I)s0. (5)

For any one time t=t0(J)+�t,

Ms

t0(J)+�tMs

�Ms

t0(J)Ms0

�DMs0

t0(J)Ms0

(6)

and

Zs((I)(t0(J)��t))�1

Ms(t0(J)+�t)/Ms(t0(J))�Zs((I)(t0(J))) (7)

�DMs(t0(J))�t/Ms0

Ms(t0(J)+�t)/Ms0

Ys((I)t0(J)).

Obviously, for every incremental time interval

Ms(t0(J))�Ms(t0(J�1)��t) (8)

and

DMs(t0(J))�DMs(t0(J�1)��t). (9)

4. Analysis of the calculation results

The solutions of the above equations can be obtained, which are arranged as follows.

131Y. Zhao et al. / Energy 27 (2002) 127–134

Fig. 3. Variation of flammable concentration in the leaking liquid over time.

4.1. Variation of the concentration of the flammable component during the leakage process

Figs. 3 and 4 show the relationship between the concentration of flammable component R32and time for liquid and vapor leakage, where X32 and Y32 are the mass fractions of the componentR32 in leaking liquid and vapor mixtures, respectively. Obviously, compared with liquid leakage,the variation of the Y32 is faster for vapor leakage and its initial flammable concentration, whichgradually decreases afterward, is the highest during the leakage process. Like the vapor phase or

Fig. 4. Variation of flammable concentration in the leaking vapor over time.

132 Y. Zhao et al. / Energy 27 (2002) 127–134

liquid phase leakage, the variation of the flammable concentration is faster at higher temperaturesthan that at lower temperatures. From Fig. 4 we can find that for vapor phase leakage, when theenvironmental temperature is �30°C, the initial leakage flammable concentration will exceed theflammability limit of the mixture at 60°C according to the ASHRAE Standard 34. So if themixture temperature rises to about 60°C caused by some reasons during the leakage process, thepossibility of burning and explosion cannot be excluded.

4.2. Dynamic flammability during leaking liquid phase

Fig. 5 is the variation of flammable concentration in the surplus vapor mixture over time forliquid phase leakage. Comparing it with Fig. 3, we find that the flammable concentration in thesurplus vapor mixture in the container is far higher than that in the leaking liquid mixture andeven exceeds the flammability limits of the mixture provided by ASHRAE Standard 34 when theenvironmental temperature is �30°C. Accordingly, in the leaking process, if the leakage movesto the vapor phase from the liquid phase in the container, the possibility of burning and explosioncannot be excluded.

4.3. Variation of the leakage rate over time

Figs. 6 and 7 show the variation of the leakage rate over time for liquid and vapor phaseleaking. Comparing these, we find that the leakage speed of the liquid phase is much faster thanthat of the vapor phase. The higher the environmental temperature is, the faster the leakage speedis. For liquid phase leakage, the leakage rate almost stays constant with time. However, for vaporphase leakage, the leakage rate decreases over time. The higher the environmental temperature

Fig. 5. Variation of flammable concentration in the surplus vapor phase over time.

133Y. Zhao et al. / Energy 27 (2002) 127–134

Fig. 6. Variation of leakage rate over time for liquid phase leakage.

Fig. 7. Variation of leakage rate over time for vapor phase leakage.

is, the more obvious this property becomes. Hence, the initial time is the most dangerous momentfor vapor phase leakage.

5. Conclusions

1. The R32/134a (30/70) used as an alternative to R22 in routine water-to-water heat pumpshas good thermodynamic properties and refrigerating performance.

134 Y. Zhao et al. / Energy 27 (2002) 127–134

2. When the leakage of this alternative mixture happens during storage and transportation, thevariation of the flammable component concentration in the leaking vapor mixture is faster thanliquid phase leakage. Moreover, the flammable concentration of the R32/134a (30/70) at the initialtime is the highest. It may exceed the flammability limit of the mixture at 60°C according to theASHRAE Standard 34 when the environmental temperature is �30°C. In this case, if the mixturetemperature rises to about 60°C caused by some reason during the leakage process, there is thepossibility of burning and explosion.

3. For liquid phase leakage, the leakage speed is much faster than that of the vapor phase. Thehigher the temperature, the faster the leaking speed is. In the leakage process, if the leakagemoves to vapor phase from liquid phase, the possibility of burning and explosive accidents cannot be excluded.

4. According to the flammability data from the ASHRAE standard 34 and the calculation resultsin this paper, the R32/134a (30/70) mixture may be flammable under certain conditions, whichis accordant with the result addressed by Shiflett et al. [4].

Acknowledgements

This project (No. 59876027) is supported by the NSFC, the Major State Basic Research andDevelopment Program (G20000263), the National Education Department for young teachers andTianjin SFC (993803211).

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