Analysis of the system of R32 refrigerant

1.1 basic characteristics

O32, ODP, GWP and boiling point, molar mass, critical temperature, critical pressure and critical density of R32 and several commonly used refrigerants. R32 is closest to R410A in boiling point. In fact, R32 is the main component of R410A. The GWP of R32 is moderate (675), which is much lower than the promotion and application of R410A and R22.R32 refrigerants in the air source heat pump water heater industry, and will also promote the implementation of China's low carbon economy policy. At the same time, R32 has a lower GWP value, which is in line with the alternative direction of low GWP refrigerant advocated by European countries. The molar mass of R32 is only 0.6 times that of R22, which is 71.7% of R410A. Since the charge is generally proportional to the molar mass, the R32 charge is only 0.6 times that of R22, which is 71.7% of R410A.

LFL is the lowest combustible limit, the minimum concentration at which a flammable refrigerant can sustain flame propagation in its homogeneous mixture with air under specified test conditions. Typically, the LFL is expressed as the volume fraction of refrigerant in the air. The smaller the LFL value, the stronger the latitude; the LFL means no such that the refrigerant is not flammable in the air. The heat of combustion HOC represents the amount of heat released by a flammable refrigerant of 1 kmol at 25 and 101.325 kPa, when it is completely burned and the combustion products are in a gas phase state. TLVTWA is a chronic toxicity indicator, the larger the value, the lower the toxicity.

R32 is a flammable and R22 equivalent, weakly flammable refrigerant.

1.2 Basic thermal physical quantity comparison

1.2.1 Condensing pressure and evaporation pressure

At the same condensation temperature, the condensing pressure of R32 is higher than that of R410A and R22; as the condensing temperature increases, the condensing pressure also increases. For example, at a condensation temperature of 60, the condensing pressures of R22, R410A and R32 are 2.3 MPa, 3.82 MPa and 3.93 MPa, respectively, that is, R32 is 1.62 times R22 and 1.029 times R410A. Larger values ​​of condensing pressure, on the one hand, can use smaller diameter heat exchange tubes to overcome large resistance losses; on the other hand, higher requirements are placed on the pressure and reliability of the system. However, because the pressure is only about 3% higher than that of the R410A, the heat exchanger used in the existing R410A system can basically withstand the pressure requirements of the R32 system; however, in specific applications, special attention must be paid to the pressure requirements of the system. Make appropriate adjustments when designing the system. In order to meet the pressure requirements of the system, the condensation temperature must be lowered, that is, the outlet temperature of the hot water is lowered. For example, reducing the condensing temperature from 65 ° C to 60 ° C, the condensing pressure of the R32 system will be reduced from 4.38 MPa to 3.93 MPa. That is, for the R32 system, the author believes that the maximum water temperature can only reach 55 ° C, can not reach 60 °C. At the same evaporation temperature, the evaporation pressure of R32 is slightly higher than that of R410A, but larger than that of R22 and R407C. For example, at an evaporation temperature of -15 ° C, the evaporation pressure of R32 is 64.8% higher than that of R22. Higher evaporation pressure means that the air source heat pump water heater using R32 refrigerant may be used at a lower ambient temperature.

1.2.2 Saturated liquid density and saturated gas specific volume

At the same temperature, the saturated liquid density of R32 is the lowest. At 60°, it is only 94% of R410A and 75% of R22. The density of liquid refrigerant directly affects the charge of the refrigerant. At the same volume, the density is small. The refrigerant charge is small.

At the same temperature, the specific capacity of the R32 saturated gas is slightly lower than that of R22, but higher than that of R410A. The large specific volume of the saturated refrigerant means that when the displacement of the compressor is the same, the mass flow rate of the refrigerant having a larger specific volume is lowered, and the amount of cooling (heat) is lowered. However, this does not indicate that the refrigerant has poor physical properties, because efficiency is the ratio of its cooling (heat) capacity to its power consumption, so the amount of power consumption must also be considered.

1.2.3 saturated liquid specific heat capacity

At the same temperature, the larger the specific heat capacity of the saturated liquid, the greater the heat released by the same subcooling, and the larger the heat exchanger area required. As can be seen from Figure 5, the specific heat capacity of the R32 saturated liquid is higher than that of R410A, R407C and R22. For heat pump water heaters, the advantage of using R32 is that the same quality of refrigerant can release more heat to the water and provide better heating.

1.2.4 Saturated liquid viscosity and saturated gas viscosity

At the same temperature, the viscosity of R32 is lower than that of the other three refrigerants. At 60 o'clock, the viscosity of the R32 saturated liquid is 94.6% for R410A and 67.1% for R22. The viscosity of the saturated gas of R32 is higher than that of R22 and lower than that of R407C and R410A. At 10 o'clock, the viscosity of the R32 saturated gas is 1.5% higher than that of the #82# refrigeration and air conditioning volume 10 R 22 and 6% lower than that of the R410A. Due to the small viscosity of the saturated liquid and the saturated gas refrigerant, the fluid can be reduced on the one hand. Friction loss with the pipe wall and the inside of the fluid; on the other hand, the thickness of the boundary layer during heat transfer can be reduced, thereby increasing the heat transfer coefficient. In addition, a refrigerant having a small viscosity can reduce the resistance loss of the refrigerant flow process. Therefore, compared with the other three types of refrigerants, the R32 liquid refrigerant has a small flow resistance and a good heat transfer effect.

1.2.5 Thermal conductivity of saturated liquid and thermal conductivity of saturated gas

The thermal conductivity of R32 saturated liquid is much higher than that of the other three refrigerants; the thermal conductivity of saturated gas is basically equivalent to that of R410A, but higher than the other two refrigerants. Due to the higher thermal conductivity of the refrigerant, the greater the heat transfer coefficient in the evaporator and condenser, which is beneficial to improve the heat transfer performance of the system: therefore, R32 refrigerant can be considered to have better heat transfer performance for air. The source heat pump water heater is beneficial to improve energy efficiency.

1.2.6 latent heat of vaporization

It can be seen from the change law of latent heat of vaporization with temperature that the latent heat of vaporization of R32 is higher than that of the other three kinds of refrigerants at the same temperature. At 60, the latent heat of vaporization of R32 is 60.7% higher than that of R410A and 25.4% higher than that of R22. At 10, the latent heat of vaporization of R32 is 43% higher than that of R410A, and 52% higher than that of R22. The higher the amount of heat absorbed or released per unit mass of refrigerant. Therefore, R32 is used in heat pump water heaters, which can reduce the power consumption of refrigerant delivered by the compressor and improve COP.

Theoretical cycle analysis of 2R32

The calculation selects the condensation temperature 60, the evaporation temperature varies with the ambient temperature, the subcooling degree 5, the superheat degree 5, and the compressor compression process is isentropic compression, and the calculation uses Solkane6.0 software.

2.1 compression ratio

At the same evaporation temperature, the compression ratio of R407C is the largest; R32 is substantially close to R410A, but slightly lower than R22. The data in the figure shows from another side that at the evaporation temperature of -20, the system should reach the condensation temperature of 60. The four refrigerant systems used in the calculation will exceed the reliable pressure ratio operating range of the compressor and be in an unsafe area. In order to meet the heating demand at low temperatures, the pressure ratio of the compressor can be ensured only by lowering the condensation temperature. That is to say, in the low temperature environment, in order to ensure the reliability of the compressor, the hot water outlet temperature of the heat pump water heater needs to be lowered.

The compression ratio of the R32 refrigerant system cycle is lower than that of R22 and R407C over the entire evaporation temperature range, which is basically equivalent to that of R410A. Since the compressor of the air source heat pump water heater often works in the high condensing temperature range, the third phase of Rao Rongshui: the characteristics of the refrigerant R32 and its theoretical cycle analysis for the air source heat pump water heater #83# easily cause a high compression ratio. Therefore, from the perspective of compression ratio, R32 is a refrigerant that is more suitable for heat pump water heaters.

2.2 COP

At the same evaporation temperature, the R410A cycle has the lowest COP, the R22 cycle is the highest, and the R32 cycle is slightly lower than R407C. For example, at an evaporation temperature of 10, the COP of the R32 cycle is 92.1% of R22, which is 106.8% of the R410A cycle; at the evaporation temperature of -12, the COP of the R32 cycle is 92.6% of R22, which is 108.7% of the R410A cycle. Compared with traditional electric water heaters and gas water heaters, an important advantage of air source heat pump water heaters is that they have good energy saving effects. The best evaluation index reflecting energy saving is the energy efficiency ratio COP of heat pump water heater. Therefore, from the perspective of improving system theory COP, R32 is more suitable for heat pump water heater than R410A, and it can reflect the environmental protection characteristics of energy saving and carbon emission reduction.

2.3 exhaust temperature

The exhaust gas temperature of different refrigerants varies with evaporation temperature. At the same evaporation temperature, R32 has the highest exhaust temperature, R407C is the lowest, and R410A is close to R22.

From the theoretical cycle calculation, under the condition of 60 condensation temperature, after the evaporation temperature is lower than 5, the exhaust temperature of the compressor will exceed 110, which will deteriorate the working environment of the compressor lubricating oil. Therefore, from the perspective of exhaust temperature, R32 is not an ideal refrigerant for heat pump water heaters; however, the exhaust temperature of the compressor can be adjusted by means of refrigerant spray to ensure that it is within a reliable operating range.

2.4 volumetric heat

The volumetric heat of R32 is the highest, and the minimum of R407C. When the evaporation temperature is 10, the volumetric heat of R32 is 58% higher than that of R22, 67% higher than R407C and 14% higher than R410A. This shows that in order to achieve the same The heating effect of the R407C refrigerant compressor is larger than that of the R32. The higher volume of heat can reduce the displacement of the compressor, reduce the size of the compressor cylinder design, reduce the size of the compressor, and reduce the cost of the compressor. From the perspective of the heat of the capacitive mechanism, R32 is a good refrigerant for heat pump water heaters; when the exhaust volume is the same, the best heating effect can be achieved by using R32 refrigerant.

2.5 return air volume

The variation of specific gas volume with evaporation temperature is shown in Fig. 16. It can be seen from the figure that under the same evaporation temperature and condensation temperature, the specific gas to volume ratio of R32 is higher than that of R410A, slightly lower than that of R22 and R407C. . This difference is more pronounced in areas of low evaporation temperatures. This means that at the same evaporation temperature, if the displacement of the compressor is the same, the mass of the compressor suction R32 is slightly higher than that of R22#84# refrigeration and air conditioning, but lower than R410A, higher than R407C. From the perspective of return air specific volume, R32 is more suitable for heat pump water heater than R22 and R407C, but slightly worse than R410A.

3 conclusions

1) Comparing the basic characteristics of four refrigerants R32, R22, R407C and R410A, R32 has lower viscosity, higher thermal conductivity, higher vaporization latent heat, condensation pressure and evaporation from the perspective of heat transfer and flow. pressure. Due to the high condensing pressure of the R32 system, higher requirements are placed on the pressure bearing performance of the system. In the system design, appropriate methods must be adopted to control the highest pressure to ensure the safety of the system. Considering the reliability of the system, the maximum effluent temperature of the R32 system is recommended to be 55.

2) Through theoretical cycle calculation and analysis, R32 system has lower compression ratio, higher COP and good volumetric heat capacity, suitable for air source heat pump water heater; however, R32 system has higher exhaust temperature and needs to pass Refrigerant spray and other methods reduce the exhaust temperature to ensure the reliability of the compressor.

3) R32 refrigerant can be used for air source heat pump water heater after reasonable system matching, and get higher COP. In the current environment of energy saving and emission reduction, we can consider promoting R32. At the same time, considering air source heat pump water heater The national standard for energy efficiency limits is being developed. With the implementation of this standard, the application of R32 refrigerant, which is conducive to improving the COP of the system, to air source heat pump water heaters may be promoted.

4) R32 refrigerant with lower GWP value, promoted and applied in China's air source heat pump water heater industry, will help to achieve a low carbon economy development strategy.

Domestically, there are many alternatives for refrigerants for air source heat pump water heaters, such as R417A, R404A, R407C, R134a and CO2. The effects achieved by various refrigerants in practical applications are not only related to the characteristics of the refrigerant itself, but also to the type of heat pump water heater and the conditions of use. Therefore, a refrigerant with excellent performance can only maximize its energy saving and environmental protection performance by combining it with a heat pump water heater product with excellent system design, and provide maximum economic benefits for users and manufacturers.