Performance of Eco-friendly Refrigerants 410A and R407C

Heat Transfer Performance Comparison of R407C and R410A
R410A has excellent heat transfer performance. The evaporation heat transfer coefficient and condensation heat transfer coefficient of R410A are higher than those of R407C. In many application scenarios, the heat transfer performance of R410A is also superior to R22. Evaporation experimental studies have found that the heat transfer coefficient of R410A in a smooth horizontal tube is about 50% higher than that of R407C. Compared with the evaporation test results of R22, the heat transfer coefficient of R410A is 10% - 50% higher than that of R22. Using a horizontal tube with micro-fins, the heat transfer coefficient of R410A is 80% - 150% higher than that of a smooth tube. The evaporation test of the plate heat exchanger also confirmed the superiority of the heat transfer performance of R410A. Under the same conditions, the heat transfer coefficient of R410A is 0 - 15% higher than that of R22. Condensation tests show that the condensation heat transfer coefficient of R410A in a smooth tube is 20% higher than that of R407C. Outside the smooth tube, the condensation heat transfer of R410A is 35% - 50% higher than that of R407C and about 11% - 17% higher than that of R22. However, the heat transfer coefficient of R407C is 24% - 37% lower than that of R22. Outside the tube with micro-fins, the condensation heat transfer coefficient of R410A is 35% - 55% higher than that of R407C and 3% - 7% higher than that of R22. On the contrary, the heat transfer coefficient of R407C is 33% - 52% lower than that of R22. The fact that the heat transfer performance of R407C is poor can also be illustrated by the refrigerant replacement test results of existing equipment. In a test of a 100kW refrigeration capacity screw water chiller unit, it was found that the heat transfer coefficient of R407C in the shell-and-tube condenser was 25% - 51% smaller than that of R22.
The low heat transfer coefficient of R407C is related to its non-azeotropic property: First, there is a large phase change temperature gradient during isobaric evaporation or condensation. Second, there is a significant concentration difference between the vapor and liquid phases. When R407C evaporates or condenses, it not only has to overcome the thermal resistance of the condensate layer but also has to overcome the negative effects of the phase change temperature gradient and the vapor-liquid concentration difference on heat transfer. The phase change temperature gradient refers to the temperature difference when a mixture changes from saturated vapor to saturated liquid at a certain pressure. The phase change temperature gradient of R407C at atmospheric pressure is approximately 7K. The existence of the phase change temperature gradient directly reduces the heat transfer performance of R407C. During isobaric condensation, as the condensation process progresses, the condensation temperature required for the vapor-liquid equilibrium of R407C becomes lower and lower. For constant wall temperature condensation, the effective temperature pressure used to promote steam condensation will become smaller and smaller, and the heat transfer efficiency will decrease. Similarly, the phase change temperature gradient also has the effect of reducing heat transfer efficiency during the evaporation process.
The vapor-liquid concentration difference of the three components of R407C is caused by the different relative volatilities among the components. The high boiling point component R134a is not easy to volatilize, while the low boiling point components R32 and R125 are more volatile than R134a. When the vapor and liquid phases coexist, the concentration of R134a with a high boiling point in the liquid phase is higher than its vapor phase concentration, while the concentrations of R125 and R32 with low boiling points in the vapor phase are higher than those in the liquid phase. Imagine there is a thin layer of condensate on the tube wall, and there is a mixed vapor diffusion layer between the condensate and the mainstream vapor. Due to the higher boiling point of R134a, it is more easily condensed than the other two components. The concentration of R134a vapor near the liquid-vapor interface is lower than that of the mainstream vapor. Therefore, in the diffusion layer vapor flow, there is a diffusion process of component R134a from the mainstream vapor to the liquid-vapor interface. On the contrary, the boiling points of components R32 and R125 are lower and they are more volatile and not easy to condense. In the vapor flow near the interface, the concentrations of R32 and R125 are higher than their respective concentrations in the mainstream vapor, thus forming a concentration gradient from the liquid-vapor interface to the mainstream vapor in the diffusion layer. The diffusion layer constitutes an additional thermal resistance during the condensation of non-azeotropic refrigerants. The saturated condensation temperature of the vapor at the interface further decreases with the increase in local concentration, also increasing the resistance of the condensation process.
An important reason for the high heat transfer coefficient of R410A lies in its quasi-azeotropic property. Although R410A is a mixture composed of two components (R32 and R125), there is no significant difference in volatility between these two components. During the evaporation or condensation process, the vapor phase component concentration and liquid phase component concentration of R410A are very similar, and the phase change temperature gradient is less than 0.2K. Reflected on the thermodynamic engineering diagram of R410A, the isotherms and isobars in the vapor-liquid two-phase region are almost parallel. Therefore, the thermodynamic and physical properties of R410A are very close to those of 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 refrigerants, and there is no obvious component diffusion phenomenon. The influence of the phase change temperature gradient on the heat transfer efficiency is minimal, which makes the heat transfer coefficient of R410A higher than that of the non-azeotropic refrigerant R407C. The main reason for the higher heat transfer coefficient of R410A than R22 lies in its more favorable heat transfer control physical quantities, such as having a higher thermal conductivity and a lower viscosity coefficient.

Performance Coefficient Comparison of R410A and R407C
The excellent heat transfer performance of R410A is conducive to improving the performance coefficient of air conditioning and refrigeration systems. R410A also has two other favorable conditions for improving the performance coefficient: lower flow resistance and higher compression efficiency. Experiments have 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% - 35% lower than that of R22. When condensing in a smooth tube, the pressure drop of R410A is 35% - 50% smaller than that of R22. The smaller the pressure drop required for refrigerant flow, the less useless work the compressor consumes on the pressure drop, which is more conducive to improving the performance coefficient. In terms of refrigerant compression efficiency, the compression efficiency of R410A is higher than that of R22 and R407C. The isentropic compression efficiency and gas delivery efficiency of R410A measured on the reciprocating compressor test bench are respectively about 5% higher than those of R22. The isentropic compression efficiency and gas delivery efficiency measured on the scroll compressor test bench are respectively 2% - 15% and 3% - 10% higher than those of R22. In contrast to R410A, the compression efficiency of R407C measured on the reciprocating compressor test bench is similar to that of R22. However, the compressor volumetric efficiency and isentropic efficiency measured on a small air conditioning system are respectively 3% - 7% and 6% - 14% lower than those of R22. In contrast, the compression efficiency of R410A is 5% - 20% higher than that of R407C. Research shows that among the two alternative refrigerants of R22, using R410A can achieve a higher system performance coefficient than R407C. The data obtained on the compressor test bench show that the performance coefficient of R410A exceeds that of R407C by more than 10%. The conclusion that the performance coefficient of R410A is high in the optimized designed air conditioning and refrigeration system is further confirmed. The performance coefficients of R410A are respectively 10% and 5% higher than those of R407C and R22.
The performance coefficient advantage of R410A has also been confirmed in the substitution tests of the operating R22 air conditioning and refrigeration systems. For three different operating R22 systems, using the methods of replacing the refrigerant and the scroll compressor, using the same evaporator and condenser, and conducting field operation tests under the same working conditions, the results show that the performance coefficient obtained using R410A is higher than the values of R22 and R407C: In the substitution test of a 9.2kW refrigeration capacity household air conditioner, the performance coefficient produced by R410A is 5% higher than that of R22. The comparative test of the roof-mounted 27kW air conditioner found that the performance coefficient of R410A is 3% and 11% higher than those of R22 and R407C respectively. The substitution test of the 35kW water chiller unit found that the performance coefficient of R410A is 5% and 6% higher than those of I122 and R407C respectively. Currently, more and more energy-saving R410A air conditioners have come out of the laboratory and entered the market for sale, and the performance coefficient of some air conditioning products exceeds 6.

Steam Pressure Comparison of R410A and R407C
Within the selected temperature range, according to the characteristics of steam pressure, the following aspects should be noted when using R410A:
(1) The steam pressure of R410A is 60% - 70% higher than that of R22. The refrigeration system of R410A must have better sealing performance and use appropriate interface technology to prevent leakage. All refrigeration components, including compressors, evaporators, condensers, pipes, etc., must meet the special working pressure requirements of R410A. Take necessary safety protection measures for high-pressure steam. Refrigeration system
(2) If the design pressure of the R410A refrigeration system is 2500kPa, the condensation temperature must be controlled at about 40°C. The water-cooled application scenario can meet the requirements of the upper pressure limit. If the air-cooled design is adopted for the R410A system, the condensation pressure will exceed the upper pressure limit of the current air conditioning system. For example, considering the summer air design temperature of 40°C, the condensation temperature of R410A will reach about 55°C, and the corresponding condensation pressure will reach and exceed 3400kPa. The current R22 system cannot withstand such a large pressure and must be specially designed.
(3) The critical temperature of R410A is relatively low, only 72.5°C. In the case of air cooling, if the ambient air temperature reaches above 55°C, R410A may undergo condensation in the critical temperature region, and the refrigeration load will decrease to about 65% of that at the 35°C ambient temperature, and the performance coefficient will also decrease significantly. As can be seen from Figure 2, the pressure curve of R407C is very close to that of R22, especially below 10°C, the saturation pressure curve of R407C almost coincides with that of R22. Only above 30°C, the saturation pressure of R407C is slightly higher than that of R22. Therefore, R407C has a very close evaporation pressure to R22, and the condensation pressure may be 100 - 270kPa higher than that of I122, but the exhaust temperature is 0 - 10°C lower than that of R22.

Different Characteristics and Application Prospects of R410A and R407C
The characteristics of R410A can also be analyzed from the thermodynamic engineering diagram. Within the evaporation temperature range of -40 - 10°C, the vapor density of R410A is 43% higher than that of R22, the latent heat of vaporization is 11% higher than that of R22, and the volumetric refrigeration effect of R410A is about 60% higher than that of R22. Considering the loss in compressor efficiency, the refrigeration capacity of the same hermetic compressor using R410A will be approximately 50% higher than that of R22. If an optimized R410A system is used to replace the R22 system, considering the reduction in compressor displacement and the reduction in heat exchangers, pipes, and control element sizes caused by the reduction in the working fluid circulation flow, for small-power air conditioners, the cost of R410A equipment may be cheaper than that of R22 equipment. However, for high-power air conditioners, the increase in equipment investment and equipment weight brought about by the safe operation of high-pressure steam may become a consideration. Because, as the load of the refrigeration system increases, the equipment investment and weight required for the safe operation of R410A will increase sharply.
In the technical development and use of environmental-friendly refrigerants R410A and R407C, different environmental-friendly refrigerant technologies should be adopted according to their different applications and different loads:
(1) For occasions with a lower condensation temperature, such as water cooling, the adoption of R410A technology can reduce the equipment volume and improve the performance coefficient.
(2) For small-load air-cooled air conditioning systems, such as when the rated load is below 4kW, the adoption of R410A technology can not only obtain a higher performance coefficient but also control equipment investment.
(3) For large-load air-cooled air conditioners, there are two development trends. From the perspectives of the difficulty of development and investment savings, R407C is superior to R410A because the technical requirements of the R407C system are very similar to those of the R22 system. From the perspectives of improving the performance coefficient and reducing the equipment size, the development of R410A technology becomes more attractive. As people's environmental awareness continues to strengthen and the international community's emphasis on energy conservation continues to increase, the share of R410A in large air conditioning systems will become larger and larger.
(4) For the renovation projects of R22 equipment, the use of R407C is more reliable than the adoption of R410A. It is not only lower in cost but also simple and feasible.