Comparison and Analysis of the Performance of Refrigerants R407C and R410A

　　一. Comparison of heat transfer performance between R407C and R410A

　　 R410A has good heat transfer performance. The evaporation heat transfer coefficient and condensation heat transfer coefficient of R410A are higher than R407C. In many applications, R410A The heat transfer performance is also excellent R22. The evaporation test study found that the heat transfer coefficient of R410A in a smooth horizontal tube is about 50% higher than that of R407C; compared with the results of the R22 evaporation test, the heat transfer coefficient of R410A is 10% to 50% higher than that of R22. Using horizontal tubes with micro-fins, the heat transfer coefficient of R410A is 80% to 150% higher than that of smooth tubes. The evaporation test of the plate heat exchanger also confirmed the superior heat transfer performance of R410A. Under the same conditions, the heat transfer coefficient of R410A is 0-15% higher than that of R22.

　　The condensation test shows that the condensation heat transfer coefficient of R410A in the smooth tube is 20% higher than that of R407C. Outside the smooth tube, the condensation heat transfer of R410A is 35% to 50% higher than that of R407C, and about 11% to 17% higher than that of R22; however, the heat transfer coefficient of R407C is 24% to 37% lower than that of R22. Outside the tube with micro-fins, the condensation heat transfer coefficient of R410A is 35% to 55% higher than that of R407C and 3% to 7% higher than that of R22. On the contrary, the heat transfer coefficient of R407C is 33% to 52% lower than that of R22. The fact that the heat transfer performance of R407C is poor can also be explained by the results of the refrigerant replacement test of the existing equipment. In the test of a 100kW refrigeration screw water refrigeration unit, it is found that the heat transfer coefficient ratio of R407C in the shell-and-tube condenser R22 is 25%～51% smaller.

The low heat transfer coefficient of 　　R407C is related to its non-azeotropy: one is that there is a large phase transition temperature gradient during equal pressure evaporation or condensation, and the other is the vapor-liquid two-phase There is a significant concentration difference between. When R407C is evaporating or condensing, it not only has to overcome the thermal resistance of the condensate layer, but also overcome the negative effects of phase transition temperature gradient and vapor-liquid concentration difference on heat transfer. The phase transition temperature gradient refers to the temperature difference of the mixture from saturated steam to saturated liquid under certain pressure. The phase transition temperature gradient of R407C at atmospheric pressure is about 7K. The existence of phase transition temperature gradient directly reduces the heat transfer performance of R407C. During isobaric condensation, as the condensation process progresses, the condensation temperature required by R407C vapor-liquid balance is getting lower and lower. For constant wall temperature condensation, the effective temperature and pressure used to promote steam condensation will become smaller and lower, and the heat transfer efficiency will decrease. . In the same way, the phase change temperature gradient also has the effect of reducing the heat transfer efficiency for the evaporation process.

The difference in vapor-liquid concentration of the three components of 　　R407C is caused by the relative volatility of the components. The high boiling point component R134a is less volatile, and the low boiling point components R32 and R125 are more volatile than R134a. When the vapor and liquid coexist, the concentration of R134a with high boiling point in the liquid phase is higher than its vapor phase concentration, while the concentration of R125 and R32 with low boiling points in the vapor phase is higher than the concentration of liquid phase.

　　 Figure 1 qualitatively shows the concentration changes of the three components of R407C during condensation.

　　 In Figure 1, it is assumed that there is a thin layer of condensate on the pipe wall, and there is a mixed vapor diffusion layer between the condensate and the mainstream steam. Due to the higher boiling point of R134a, it is easier to be condensed than the other two components. The concentration of R134a vapor near the liquid-vapor interface is lower than that of mainstream vapor. Therefore, in the diffusion layer vapor flow, the component R134a occurs The diffusion process from the mainstream steam to the liquid-vapor interface. On the contrary, the components R32 and R125 have low boiling points, strong volatility, and are not easy to be condensed. In the steam flow close to the interface, the concentration of R32 and R125 is higher than their respective concentrations in the mainstream steam, thus forming a diffusion layer The concentration gradient from the liquid-vapor interface to the mainstream steam. The diffusion layer constitutes an additional thermal resistance when the non-azeotropic refrigerant condenses. The saturated condensation temperature of the steam at the interface further decreases as the local concentration increases, which also increases the resistance of the condensation process.

An important reason for the higher heat transfer coefficient of 　　R410A is its quasi-azeotropic property. Although R410A consists of a mixture of two components (R32 and R125), there is no significant difference in volatility between the two components. During the evaporation or condensation process, the concentration of the vapor and liquid components of R410A Very similar, the phase change temperature gradient is less than 0.2K. Reflected in the thermodynamic engineering drawing of R410A, the isotherms of the vapor-liquid two-phase zone are almost parallel to the isobars. Therefore, the thermodynamic and physical properties of R410A are very close to azeotropic refrigerants or pure refrigerants. As a quasi-azeotropic mixture, the heat transfer mechanism of R410A during evaporation and condensation is similar to that of pure refrigerant, there is no obvious component diffusion phenomenon, and the phase change temperature gradient has minimal effect on the heat transfer efficiency, which makes the heat transfer of R410A The coefficient is higher than that of non-azeotropic refrigerant R407C. The main reason why the heat transfer coefficient of R410A is higher than R22 is that it has more favorable heat transfer control physical quantities, such as higher thermal conductivity and lower viscosity coefficient.

　　二. Comparison of coefficient of performance between R410A and R407C

The excellent heat transfer performance of 　　 R410A is conducive to improving the coefficient of performance of air conditioning and refrigeration systems. R410A also has two other favorable conditions for improving the coefficient of performance: lower flow force and higher compression efficiency.

　　The experiment found that the flow pressure drop of R410A is smaller than that of R407C and R22, while the pressure drop of R407C is close to the value of R2. For example, when evaporating and flowing in a smooth tube, the pressure drop of R410A is 30% smaller than that of R22. When flowing in a plate evaporator, the pressure loss of R410A is 15% to 35% lower than that of R22. When condensing in a smooth tube, the pressure drop of R410A is 30% lower than that of R22. The drop is 35%-50% less than R22. The smaller the pressure drop required for refrigerant flow, the smaller the useless work consumed by the compressor on the pressure drop, and the better it is to improve the coefficient of performance.

　　
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