(1) The high return air temperature is related to the evaporation temperature. In order to prevent the liquid from flowing back, the superheat of the return air is generally required to be 20℃. If the heat preservation of the return air pipe is not good, the superheat will far exceed 20℃. The higher the return air temperature, the higher the suction temperature and exhaust temperature of the cylinder. When the return air temperature increases by1C, the exhaust air temperature will increase by1~1.3 C. ..
(2) Motor heating For the return air cooling compressor, the refrigerant vapor is heated by the motor when it flows through the motor cavity, and the suction temperature of the cylinder rises again. The calorific value of motor is influenced by power and efficiency, while power consumption is closely related to displacement, volumetric efficiency, working condition and friction resistance. The temperature rise range of the refrigerant in the motor cavity of the return air-cooled semi-sealed compressor is about 15 ~ 45℃. In the air-cooled (air-cooled) compressor, the refrigeration system does not pass through the winding, so there is no motor heating problem.
(3) If the compression ratio is too high, the exhaust temperature is greatly affected by the compression ratio, and the higher the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can obviously reduce the exhaust temperature, and the specific methods include increasing the suction pressure and reducing the exhaust pressure. The suction pressure is determined by evaporation pressure and suction pipe resistance.
Increasing the evaporation temperature can effectively increase the suction pressure and rapidly reduce the compression ratio, thus reducing the exhaust temperature. Some users think that the lower the evaporation temperature, the faster the refrigeration speed, which actually has many problems. Although lowering the evaporation temperature can increase the freezing temperature difference, the refrigeration capacity of the compressor decreases, so the freezing speed is not necessarily fast. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient, while the load increases and the longer the operation time, the higher the power consumption.
Reducing the resistance of the return air duct can also increase the return air pressure. The specific methods include replacing the dirty and blocked return air filter in time and shortening the length of evaporation pipe and return air pipeline as much as possible. In addition, insufficient refrigerant is also a factor of low suction pressure. Refrigerant leakage should be replenished in time. Practice shows that it is simpler and more effective to reduce the exhaust temperature by increasing the suction pressure than other methods. The main reason of high exhaust pressure is too high condensation pressure.
Insufficient heat dissipation area of condenser, scaling, insufficient cooling air or water volume, and too high temperature of cooling water or air will all lead to too high condensation pressure. It is very important to choose an appropriate condensation area and maintain sufficient cooling medium flow. The compressor designed for high-temperature air conditioning has a low compression ratio, which doubles after freezing, and the exhaust temperature is high, but the refrigeration can't keep up, resulting in overheating.
Therefore, it is necessary to avoid using the compressor beyond the scope and make the compressor work at the lowest pressure ratio possible. In some cryogenic systems, overheating is the main cause of compressor failure.
(4) After the suction stroke of reverse expansion and gas mixing begins, the high-pressure gas trapped in the cylinder gap will have a process of reverse expansion. After the reverse expansion, the gas pressure returns to the suction pressure, and the energy consumed by compressing this part of gas is lost in the reverse expansion. The smaller the gap, the smaller the power consumption caused by reverse expansion, and the larger the suction volume, so the energy efficiency ratio of the compressor is greatly improved.
In the process of reverse expansion, the gas absorbs heat by contacting with the high-temperature surfaces of the valve plate, piston top and cylinder top, so the gas temperature will not drop to the suction temperature at the end of reverse expansion. After the anti-inflation is over, the real inhalation process begins. After the gas enters the cylinder, on the one hand, it is mixed with anti-expansion gas, and the temperature rises; On the other hand, the mixed gas absorbs heat from the wall and heats up. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. But the actual temperature rise is very limited, generally less than 5℃, because the reverse expansion process and suction process are very short.
Reverse expansion is caused by cylinder clearance, which is an inevitable shortcoming of traditional piston compressor. If the gas in the valve plate exhaust hole cannot be exhausted, reverse expansion will occur. The disc valve plate and exhaust valve plate patented by Gulen Company are very special, which can eliminate the gap and gas retention in the exhaust hole and fundamentally control the reverse expansion. Since its invention, the disc valve compressor has maintained the highest efficiency record.
(5) Different types of refrigerants have different thermophysical properties, and the exhaust temperature rises differently after the same compression process. Therefore, different refrigerants should be selected for different refrigeration temperatures. Figure 1-3 shows the temperature rise caused by adiabatic compression of different refrigerants when the condensation temperature is 50 C and the return air superheat is 20°C. Considering that the return air superheat is 20°C and the motor fever is 30°C, the theoretical exhaust temperature will exceed150 C, which requires additional cooling. For evaporation temperature above 0 C (such as air conditioning), the exhaust temperature should not exceed110 C, so there is no overheating problem.
Conclusion and suggestion: Within the scope of application, the compressor should not be overheated due to high motor temperature and high exhaust temperature during normal operation. Overheating of the compressor is an important fault signal, indicating that there is a serious problem in the refrigeration system or improper use and maintenance of the compressor. If the source of compressor overheating lies in the refrigeration system, we can only solve the problem by improving the design and maintenance of the refrigeration system. Replacing a new compressor cannot fundamentally eliminate the overheating problem.