In order to make the heat pump operate efficiently and safely in a low temperature environment, many technologies have been developed and improved at home and abroad. The main research focuses on the use of oil burners as auxiliary heaters in low temperature environments to improve the low temperature performance of heat pumps and the use of frequency conversion systems. Under low temperature conditions, the compressor can work at high speed to increase the circulation of the refrigerant, and simultaneously spray the fluid into the compression chamber to prevent it from overheating.

The research on the heat pump technology used in cold regions has found that the use of the scroll compressor economizer system can significantly improve the heat generation of the heat pump, and effectively solve the problem that the temperature cannot be started in the ultra-low temperature environment and the exhaust temperature of the compressor is too high. The unit runs smoothly and has good performance.

Cyclic performance analysis The DDD compression process analysis model Compared with the conventional system, the main feature of the scroll compressor refrigeration cycle with economizer is the intermediate air supply, so the whole compression process is divided into three stages: (1) before the air supply Quasi-low pressure stage compression process. At this point the system relies solely on the reduction in the volume of the element so that the compressor compresses the superheated vapor from state 1 drawn from the evaporator to state 2, which is a pure compression process. The correction can be considered in accordance with the isentropic process.

The qi process analysis model takes a throttling cycle as an example to analyze the influence of the qi process.

(1) Air supply volume For the scroll compressor, the working chamber and the air supply port are extremely short from communication to disengagement time. In the analysis, the air supply process of the scroll compressor can be modeled as an adiabatic supercharged and equal volume mixing process. Assuming that the refrigerant flow rate through the condenser is 1, and the refrigerant flow rate through the economizer is X, the amount of heat exchange through the economizer.

q=x(h7-h6)=(1-x)(h5-h8)(2) The state of the final state of the qi process is assumed to be an isentropic compression process, then the state 2 is h: h2=h1+ Wiq is h3=xh7+(1-x)h2(3) The main performance parameters of the unit: heat: qk=h4-h5 unit working fluid compression process: w0=(1-x)(h2-h1)+( H4-h3) Heating coefficient of performance: COPh=qk/w04 Performance analysis A performance analysis of a heat pump unit with a working volume of 64 cm3 for a scroll compressor is assumed. Evaporation temperature: -20 ° C, -15 ° C, -10 °C (air-side heat exchanger considers the heat transfer temperature difference of 10 °C); condensation temperature: 48 ° C; suction superheat: 8 ° C; condenser outlet subcooling: 6 ° C.

In order to comprehensively compare the performance of the qi system (economizer system), under the same low temperature conditions, the frequency conversion system and the single-stage system are respectively adopted, and the operating performance of each system unit is as shown.

It can be seen that in the working environment with the evaporation temperature of -15 ° C and the condensation temperature of 48 ° C, the heat capacity of the heat pump unit operating according to the economizer cycle can be increased by about 25% compared with the heat pump unit operating according to the single stage cycle. The heating coefficient of performance can be increased by about 20.3%. Compared with the heat pump unit working with the inverter system, the heat pump unit operating according to the economizer cycle can increase the heating capacity by 7%, and the heating coefficient can be increased by 0.7%. However, the frequency conversion system of the inverter system is in the overclocking area and is not allowed for long-term. Running. Therefore, the use of an economizer system can significantly improve the low temperature performance of air source heat pumps.

Results Analysis and discussion Under the condition that other conditions are unchanged, the change of other performance parameters is measured by changing the intermediate air pressure, and the appropriate intermediate pressure value is found. For the convenience of research, the following intermediate pressure values ​​take the relative value, that is, the ratio of the actual air supply pressure to the geometric mean value of the compressor suction and discharge pressures.

The variation of each main performance parameter with the intermediate air pressure under specific working conditions (1) The change of heating capacity with the relative qi pressure is as shown, the heating capacity changes almost linearly with the relative qi pressure, the qi pressure increases, and the heating capacity Also increase accordingly. This is because when the other conditions are constant, the supplemental gas pressure becomes larger, that is, the amount of supplemental gas increases. This not only increases the flow of refrigerant in the condenser, but also increases the power consumption of the compressor. Both of these can increase the condenser load. The corresponding unit heat is also increased. It can be seen that the heating capacity decreases with the decrease of the evaporation temperature, but the reduction speed is different. When the evaporation temperature is lowered from -10 ° C to -15 ° C, the heat production is reduced by about 7%. When the evaporation temperature is lowered from -15 ° C to -20 ° C, the heating capacity is reduced by about 20%.

This shows that the lower the evaporation temperature, the more obvious the effect of qi. The change of heating capacity with the relative qi pressure at different evaporation temperatures (2) The input power increases with the relative qi pressure. The input power of the compressor increases with the increase of the relative qi pressure, but the increase is very slow. Within the range of calculation conditions, the maximum and minimum values ​​of compression work differ by only about 5%. This is because on the one hand, the increase in the air supply pressure increases the gas volume of the compressor, resulting in an increase in the compression work. On the other hand, the increase of the qi pressure increases the amount of qi, and it can improve the cooling effect of the second half of the compression process, which reduces the compressor enthalpy. This in turn reduces the compression work (parameter). The former plays a leading role, but after the combination of the two factors, the input power of the compressor does not change much.

(3) The coefficient of heating coefficient can be seen from the change of relative qi pressure. The heating COP is not obvious with the change of relative qi pressure. When the relative qi pressure is 0.7~1.2, the change is relatively fast; After 1.2, the change was very slow.

Conclusion (1) Compared with the single-stage compression system and the variable frequency system, the scroll compressor economizer system can significantly improve the unit heat and performance coefficient, greatly increasing its low temperature adaptability. (2) The intermediate air supply can increase the heating capacity and power consumption of the unit, but the speed of heating increase is greater than the speed of power consumption, so the heating coefficient of the system can be improved by supplementing the air.

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