The purpose of this investigation is to help improve (i) our understanding of refrigeration cycles and (ii) hence their efficiencies. The challenge is to use less energy, i.e. for the refrigerator to be more efficient. The requirement is to obtain quantitative information that will lead to a better understanding of the process irreversibilities and their distribution among the plant’s components. By determining the sources and magnitudes of the exergy losses, and minimising them, an optimal refrigeration cycle can emerge.

The scope of this paper encompasses two-stage vapour-compression with flash intercooling. Removal of the flash gas at an intermediate pressure P_i and recompressing it to the condenser pressure P_cond results in saving compressor power, i.e. flash intercooling reduces the power requirement for the compressors. Also, staging of compressors is necessary to prevent an excessive discharge temperature from the compressor.

A schematic diagram of the equipment, for the two-stage (designated I and II) system with an open-flash intercooler, is shown in Fig. 1 the corresponding temperature versus entropy diagram appears as Fig. 2.

The cold room considered was for storing frozen meat, requiring preservation at a temperature of 243 K, i.e. as recommended by the Intervention Board for Agricultural Products in the United Kingdom and Eire. The temperature difference between the cold room and the evaporator is assumed to be 5 K and the evaporator’s steady-state temperature is fixed, initially at 238 K, and then varied to become 236, 234, 232, 230 and 228 K successively.

The ambient temperature is assumed to be 293 K and the saturation temperature of the condenser is fixed at 298 K, and then altered by 2 K increments as follows:- to 300, 302, 304, 306 and eventually 308 K.

A sample hand-calculation for the 2-stage vapour-compression cycle is carried out below to indicate how the exergy method can be applied. Subsequently a computer program is used to predict values for the varied condenser’s and evaporator’s temperatures.

There is a direct casual relationship between the plant’s component irreversibilities and their effects on the plant’s rational efficiency . The fractions representing the proportion of the input lost through irreversibilities in the sub-regions are denoted (usually as percentages) by the appropriate efficiency defect ?_i.

The results presented in Fig. 3 and Fig. 4 are obtained by varying the condenser’s saturation-temperature from 298 to 308 K, assuming a constant ambient-temperature of 293 K. We see that for T_cond=298 K, the plant’s rational efficiency is 37% and the condenser’s efficiency defect ?_cond is 7%. If the condenser’s saturation-temperature is increased to 308 K, the plant’s rational-efficiency is reduced to 30% and the condenser’s efficiency-defect ?_cond to 12%.

The results seen in Fig. 5 are obtained by reducing the evaporator’s saturation-temperature from 238 to 228 K, assuming a constant cold-room temperature of 243 K.

For T_evap=238 K, the plant’s rational efficiency is 37% and the evaporator’s efficiency defect ?_evap is 5%. As the evaporator’s saturation-temperature is increased to 228 K, the plant’s rational-efficiency reduces to 25% and the evaporator’s efficiency defect ?_evap to 10%. The variations of the temperatures in the condenser and evaporator also affect other components of the plant. The components, which are affected most by the change of the saturation condenser temperature, are the high-pressure compressor (HPC), the condenser, and the high-pressure valve (HPV) see Table 5. Also, there is an increase in the plant’s total irreversibility rate of 12 kW. The components, which are affected most by the change of the saturation evaporator temperature, are the low-pressure compressor (LPC), the evaporator, and the low-pressure valve (LPV) see Table 6. Also, there is an increase in the total plant’s irreversibility rate of 15.6 kW. The temperature change affects not only the component’s irreversibility rates, but the irreversibility rate of the plant as a whole.

The results in Table 7 and Table 8 were used to plot the graphs of Fig. 6 and Fig. 7, with variables T_cond and T_evap respectively. The mean slopes of the curves give the coefficients of the structural bonds. For the condenser, the mean slope of the curve was found to be 2.40, and that for the evaporator 2.87. This means that, any reduction in the irreversibility rate of the condenser gives a 2.40 times greater reduction in the irreversibility rate of the plant. Also, any reduction in the irreversibility rate of the evaporator gives a 2.87 times greater reduction in the irreversibility rate of the plant.

Finally, Fig. 8 and Fig. 9 show the effects of varying the saturation condenser and evaporator temperatures on the plant’s rational efficiency. From these results, it is clear that ?T_cond and ?T_evap need to be optimised, because, the heat-transfer area A_s of both heat-exchangers is inversely proportional to the temperature difference ?T, i.e.

If ?T_cond or ?T_evap is reduced, the heat-transfer area of the condenser or the evaporator has to be increased. A thermo-economic optimisation needs to be carried out in order to find the optimal heat-transfer area of the two heat exchangers. The method of thermo-economic optimisation can be used, in order to determine the size of the two heat-exchangers, based on minimising the sum of the capital cost plus the net present values of the annual operating costs over the expected lifetime of the plant, for each considered plant output.

The analysis of the two-stage vapour-compression refrigeration plant’s performance by the exergy method demonstrates how powerful this method is for analysing behaviour. Employing the concepts of efficiency defect and rational efficiency have enabled the proportions of input lost through irreversibilities, in various plant sub-regions, to be evaluated easily.

Using the technique of the coefficient of structural bond has demonstrated that a change in any component variable in a plant component significantly influences the other plant components and the plant as a whole, and a reduction of irreversibility rate in a plant component gives a greater reduction in the irreversibility rate of the plant as a whole. The greater the value of ?T, the greater the irreversibility. Because ?T_cond and ?T_evap affect the plant’s rational efficiency, they need to be optimised for each particular heat-transfer area chosen for the two heat-exchangers.