Analysis of the performance of mixed refrigerant chillers based on the impact of exergy glide temperature matching coefficient

A glide temperature matching model based on the exergy matching coefficient was established for the phase change heat transfer process of non-azeotropic mixed refrigerants. The exergy loss rate of the temperature difference and the exergy loss rate of the glide temperature in the condenser and evaporator were analyzed, and the temperature matching coefficient of mixed refrigerants parameter πglide was derived to indicate mixed refrigerants with large refrigeration capacity. The model was experimentally verified by the performance of three types of mixed refrigerants in chillers. The results show that the higher the glide temperature, the bigger the condenser πglide of the mixed refrigerants and the worse the temperature-matching effect. The smaller the πglide of the evaporator, the better the temperature-matching effect, and at the same time, the greater the cooling capacity. This model can be used for predicting the different component mixtures of working fluids.


Introduction
A dual-temperature chiller that utilizes the glide temperature of non-azeotropic mixed refrigerants to simultaneously produce two temperature chilled water at 7℃ and 18℃ [1] has broad application prospects in THIC air conditioning systems [2][3] .In the actual phase change heat transfer process of mixed refrigerants, how to utilize the glide temperature of the refrigerant [4][5] to achieve a near Lorentz cycle and improve the refrigeration capacity of the unit has always been a hot research topic for scholars at home and abroad [5][6] .However, due to the particularity of the dual temperature chiller system and the different thermal performances of diverse component mixed refrigerants, it is of great practical significance to choose the appropriate mixed refrigerants to decrease irreversible exergy loss and to form a good temperature matching between the mixed refrigerants and the heat exchange fluid.That is, to indicate the working fluid with low exergy heat loss and high cooling capacity during heat exchange processes.This article mainly analyzes the inevitable exergy loss caused by the temperature difference heat transfer of ideal fluids and the exergy loss caused by the glide temperature of actual fluids.A parameter model based on the exergy loss rate (temperature matching coefficient of mixed working fluids) is established to study the heat transfer mechanism of condensers and evaporators, which is used to describe mixed working fluids with large refrigeration capacity and provide a reference for the design of dual temperature chillers and their heat exchangers.

Glide temperature
The temperature difference between the condenser and the evaporator is the driving force for heat exchange, but it also causes irreversible loss in the refrigeration cycle.The effective method to decrease irreversible heat transfer losses is to form an equal temperature difference between the mixed refrigerants and the heat exchange fluid.However, in reality, due to the temperature glide characteristic of the mixed refrigerants, the heat exchanger may experience heat transfer pinch points and maximum temperature difference.As shown in Figure 1, the glide temperatures of R32/R600, R32/R236fa, and R1270/R600 mixtures display a trend of increasing at first and decreasing at last as the mass ratio of low boiling point mixtures increases under their respective suitable pressures.Among them, the glide temperatures of R32/R236fa and R1270/R600 are between 0-20℃, R32/R600 is between 0-50℃, and the maximum glide temperature is the highest among the three mixtures Figure 1 Glide Temperature of Mixed Working Fluids with Low Boiling Point [7].

Heat exchange loss rate model
The matching of heat exchange temperature has a significant impact on the phase change heat transfer in the condenser and the evaporator.As long as there is a temperature difference, the exergy loss generated will inevitably exist.As shown in Figure 2, the exergy loss generated includes the ideal heat transfer part and the part generated by the glide temperature difference.Therefore, it is necessary to distinguish between the inevitable heat transfer exergy loss caused by temperature difference and the actual heat transfer exergy loss caused by glide temperature.For the heat transfer of mixed working fluids, the exergy loss analysis method is used.The exergy loss of refrigerant and heat exchange fluid during phase change heat transfer can be expressed as: In the condenser, the exergy loss is caused by the appearance of pinch points: In the evaporator, the exergy heat caused by the occurrence of the minimum heat transfer temperature difference: E is the exergy loss, Q is heat exchange, T0 is the temperature at the selected reference state point, ΔTpinch is the pinch point or minimum temperature difference, Ef is the exergy loss due to temperature difference, Ep is the actual heat transfer exergy lose of fluid, mainly due to the heat transfer lose of glide temperature, df is temperature difference exergy lose rate, dp is glide temperature exergy lose rate, and πglide is temperature matching coefficient of the mixed refrigerants, with the ideal πglide value of 1, indicating the best matching between the mixed refrigerants and the heat source.The smaller the πglide, the better the temperature-matching effect and the greater heat transfer.

Exergy Loss rate of phase change in condensers
The working conditions for phase change heat transfer of three mixed refrigerants in the heat exchanger are: cooling water 32℃-37℃, low and high temperature chilled water temperatures of the evaporator are 7℃-12℃ and 18℃-23℃ [8] .The heat exchange temperature and temperature difference of refrigerant in the condenser is displayed in Figure 3 and Figure 4.
As displayed in Figure 5-Figure 7, under the same working condition, the exergy loss rate of temperature difference in the condenser for three mixed refrigerants is lower than that of glide temperature.The main heat exergy loss in the condenser is caused by a mismatch in glide temperature.Among them, the R32/R600 temperature difference has the lowest exergy loss rate and the glide temperature has the highest exergy loss rate.The R32/R236fa has the highest temperature difference exergy loss rate and the lowest glide temperature exergy loss rate.Moreover, the temperature matching coefficient of the R32/R600 in the condenser is the highest, indicating that the higher the glide temperature, the greater the exergy loss of the mixed refrigerants in the condenser.These glide temperatures of R32/R236fa and R1270/R600 are close, while the temperature matching coefficient of R32/R236fa (0.6:0.4,mass ratio) in the condenser is the lowest.

Exergy loss rate of phase change in evaporators
The exergy loss of phase change in evaporators can be divided into the exergy losses of low and hightemperature evaporators.Its calculation model is the same as the condenser.The temperature and temperature difference of refrigerant in the low and high evaporator are displayed in Figure 8 and Figure 9.
As displayed in Figure 10-Figure 12, under the same operating conditions, the temperature difference heat transfer exergy loss rate in low and high-temperature evaporators is greater than that of glide temperature exergy loss rate in the evaporator phase transition of these three working fluids.That is, the main heat exergy loss in low and high-temperature evaporators is caused by the temperature difference, indicating the heat transfer effect of the evaporator is good.Moreover, the exergy losses in a low-temperature evaporator are lower than those in a hightemperature evaporator under the component, indicating that the temperature-matching effect of the mixed refrigerant in the low-temperature heat exchanger is better than that in the high-temperature.Meanwhile, among these three mixed working fluids, R32/R600 has the lowest low-temperature temperature matching coefficient and the highest low-temperature cooling capacity.R1270/R600 has the lowest temperature matching coefficient in high-temperature and the highest high-temperature cooling capacity.The value reflects the heat exchange capacity of the evaporator well.

Conclusion
(1) Through the model analysis and the comparison of experimental results, it was found that the πglide value of the mixed refrigerant in the condenser can be used to determine the optimal design of the condenser.The bigger the glide temperature of the mixed refrigerant, the bigger the condenser πglide value, and the worse the heat transfer temperature matching effect.Therefore, it is very important to increase this design of the condenser or increase the flow rate of cold water to enhance heat transfer.
(2) The πglide value of the mixed refrigerant in the evaporator can indirectly reflect the cooling capacity and can be used for optimizing the mixed refrigerant.The smaller the πglide value, the better the temperature-matching effect and the greater the cooling capacity.

Figure 2
Figure 2 Exergy Lose of Phase Change.

Figure 3
Figure 3 Temperature Distribution of Condensers.Figure 4 Temperature Difference of Condensers.

Figure 4
Figure 3 Temperature Distribution of Condensers.Figure 4 Temperature Difference of Condensers.

Figure 5
Figure 5 Temperature Difference Exergy Loss Rate. Figure 6 Glide Temperature Exergy Loss Rate.

Figure 7
Figure 7 Temperature Matching Coefficient of Mixed Refrigerant in Condensers.

Figure 8
Figure 8 Temperature Distribution of Evaporator. Figure 9 Temperature Difference of Evaporator.

Figure 10
Figure 10 Exergy Loss Rate in Figure 11 Exergy Loss Rate in Low-Temperature Evaporator.High-Temperature Evaporator.

Figure 12
Figure 12 Temperature Matching Coefficient in Low and High Evaporators.