Analysis of Alternative Work for Faced Refrigerant

- Aug 21, 2018-

The refrigerant R22 widely used in refrigeration enterprises has been counted down, and how domestic air-conditioning enterprises lay out the battle for refrigerant replacement has become a hot topic.

The new version of the "Special Requirements for Safe Heat Pumps, Air Conditioners and Dehumidifiers for Household and Similar Electrical Appliances" will be officially implemented on May 1, 2013, explicitly requiring non-environmental refrigerant models to be disapproved.

The ideal substitute standard for R22:

The replacement of refrigerants is a worldwide problem, and the main difficulty lies in the choice of alternative refrigerants. 

The ideal alternative should be non-toxic, non-flammable, thermally stable under working and storage conditions, compatible with system materials, similar or superior to R22 thermal performance, with high energy efficiency, zero ODP and low GWP. To date, no ideal alternatives have been found that fully meet the requirements. Replacing refrigerant R22, which has a destructive effect on the ozone layer, two internationally recognized refrigerants are R410A and R407C. The characteristics of the two collected in terms of heat transfer performance, coefficient of performance and steam pressure are introduced to help colleagues. 

Heat transfer performance: R407C heat transfer performance is poor

R410A has good heat transfer performance, R410A has higher evaporation heat transfer coefficient and condensation heat transfer coefficient than R407C. The evaporation test found that the heat transfer coefficient of R410A is 10% to 50% higher than that of R22 compared with the R22 evaporation test results. 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 performance is superior, and the heat transfer coefficient of R410A is 0-15% higher than that of R22 under the same conditions. 

The condensation test shows that the R410A has a condensation heat transfer coefficient of 35% to 55% higher than R407C and 3% to 7% higher than R22. In contrast, the heat transfer coefficient of R407C is 33% lower than that of R22. ~52%.

R407C has a low heat transfer coefficient and is related to its non-azeotropic behavior: first, there is a large phase transition temperature gradient during isostatic evaporation or condensation, and second, there is a significant concentration between vapor and liquid phases. difference. When R407C evaporates or condenses, it not only overcomes the thermal resistance of the condensate layer, but also overcomes the negative impact of the phase change temperature gradient and the 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 at a certain pressure, and the phase transition temperature gradient of R407C at atmospheric pressure is about 7K. The presence of a phase change temperature gradient directly reduces the heat transfer performance of R407C. During isostatic condensation, as the condensation process advances, the condensation temperature required for R407C vapor-liquid equilibrium becomes lower and lower. For constant wall temperature condensation, the effective temperature and pressure for propelling 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 for the evaporation process. The vapor-liquid concentration difference of the three components of R407C is caused by the difference in relative volatility between the components. The high-boiling component R134a is not volatile, and the low-boiling components R32 and R125 are more volatile than R134a. When the vapor-liquid two phases coexist, the concentration of R134a with a high boiling point in the liquid phase is higher than that of the vapor phase, and the concentration of R125 and R32 with a lower boiling point in the vapor phase is higher than the concentration of the liquid phase. 

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

Performance coefficient: R410A has better heat transfer performance 

The excellent heat transfer performance of R410A is beneficial to improve the performance coefficient of air conditioning refrigeration system. The R410A also has two other advantages for increasing the coefficient of performance: lower flow force and higher compression efficiency.

It is found that the flow pressure drop of R410A is smaller than that of R407C and R22, and the pressure drop of R407C is close to the value of R22. For example, when evaporating 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 of R410A The drop is 35% to 50% smaller than R22. The smaller the pressure drop required for the refrigerant flow, the less the useless work of the compressor on the pressure drop, and the better the coefficient of performance.

In terms of refrigerant compression efficiency, R410A has a higher compression efficiency than R22 and R407C. The isentropic compression efficiency and gas transmission efficiency of R410A measured on the reciprocating compressor test bench are about 5% higher than that of R22 respectively; the isentropic compression efficiency and gas transmission efficiency measured on the scroll compressor test bench are respectively It is 2% to 15% and 3% to 10% higher than R22. Compared with R410A, the compression efficiency measured by R407C on the reciprocating compressor test bench is similar to that of R22. The compression efficiency of R410A is 5%-20% higher than that of R407C.

Studies have shown that R410A can achieve a higher system performance coefficient than R407C in the two alternative refrigerants of R22. The data obtained on the compressor test bench shows that the coefficient of performance of R410A exceeds 10% of R407C. The conclusion that R410A has a high coefficient of performance in the optimized design of air-conditioning refrigeration system is further confirmed. The coefficient of performance of R410A is higher than that of R407C and R22 respectively. 10% and 5%.

The performance factor advantage of the R410A is also confirmed in the alternative test of the R22 air conditioning refrigeration system already in operation. For the three different R22 systems in operation, the method of replacing the refrigerant and the scroll compressor, using the same evaporator and condenser, and performing the field operation test under the same working conditions, the results show the performance obtained by R410A. The coefficient is higher than the values of R22 and R407C.

Steam pressure: R407C steam pressure is lower than R410A.

R410A has a high steam pressure and is subject to some restrictions in engineering applications. It is necessary to develop an application technology suitable for high-pressure systems, and the vapor pressure of R407C is suitable for all applications of R22.

The vapor pressure of R410A is 60-70% greater than the vapor pressure of R22. Therefore, the refrigeration system must have better sealing performance and use appropriate interface technology to prevent leakage. All refrigeration components, including compressors, heat exchangers, piping, etc., must meet the special operating pressure requirements of R410A.

If the design pressure of the R410A refrigeration system is 2500 kPa, then the condensation temperature must be controlled at about 40 °C, and the water-cooled application can meet the upper pressure limit. If the air cooling design is adopted, the condensing pressure of the R410A system will exceed the upper pressure limit of the current air conditioning system. At the condensing temperature of 50 °C, the system condensing pressure will reach 3000 kPa. The current R22 refrigeration system can not withstand such a large pressure. Must be designed specifically.

Comprehensive analysis: For the replacement of environmentally friendly refrigerants, the sales of stellar products should use different alternative refrigerants according to their different applications.

For low condensation temperatures, such as water cooling, evaporative cooling, R410A technology can reduce the size of the equipment and improve the coefficient of performance.

For air-cooled air conditioners with large loads, R407C is superior to R410A in terms of investment saving, because the technical requirements of the R407C system are very close to those of the R22 system. And the Stellar R&D department has completed the replacement output work for the R407C.

For the current R22 equipment retrofit project, the use of R407C is more reliable than R410A, not only low cost, but also simple and easy.