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Refrigeration compressor overheat failure analysis

Refrigeration compressor overheat failure analysis The main reasons for the exhaust temperature overheating are as follows: high return air temperature, large motor heating capacity, high compression ratio, high condensation pressure, and improper refrigerant selection. (1) High return air temperature 　　Return gas temperature is relative to the evaporation temperature. In order to prevent liquid return, the return air pipeline generally requires a return air superheat of 20°C. If the return air line is not well-insulated, the superheat will far exceed 20°C. The higher the return air temperature, the higher the cylinder intake temperature and exhaust temperature. Each time the return air temperature increases by 1°C, the exhaust gas temperature will increase by 1 to 1.3°C. (2) Motor heating 　　 For the return air-cooled compressor, the refrigerant vapor is heated by the motor as it flows through the motor cavity, and the cylinder suction temperature is raised again. The calorific value of the motor is affected by the power and efficiency, and the power consumption is closely related to the displacement, volumetric efficiency, working conditions, friction resistance, etc. 　　Return air cooled semi-sealed compressor, the temperature rise of refrigerant in the motor cavity is roughly between 15~45°C. In the air-cooled (air-cooled) compressor, the cooling system does not pass through the winding, so there is no problem of motor heating. (3) The compression ratio is too high 　　 The exhaust temperature is greatly affected by the compression ratio. The larger the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can significantly reduce the exhaust temperature. Specific methods include increasing the suction pressure and reducing the exhaust pressure. 　　 The suction pressure is determined by the evaporation pressure and the resistance of the suction line. Increasing the evaporation temperature can effectively increase the suction pressure, quickly reduce the compression ratio, and thus reduce the exhaust temperature. 　　 Some users think that the lower the evaporating temperature is, the faster the cooling rate is. This idea actually has many problems. Although lowering the evaporating temperature can increase the freezing temperature difference, the cooling capacity of the compressor is reduced, so the freezing speed is not necessarily fast. Moreover, the lower the evaporating temperature, the lower the cooling coefficient, while the load is increased, the operating time is extended, and the power consumption will increase. 　　Reducing the resistance of the return air line can also increase the return air pressure. The specific methods include timely replacement of dirty and clogged return air filters, and reducing the length of the evaporator and return air lines as much as possible. In addition, insufficient refrigerant is also a factor for low suction pressure. After the refrigerant is lost, it should be replenished in time. Practice has shown that reducing the exhaust temperature by increasing the suction pressure is simpler and more effective than other methods. The main reason why the exhaust pressure is too high is that the condensing pressure is too high. Insufficient heat dissipation area of ​​the condenser, fouling, insufficient cooling air volume or water volume, too high cooling water or air temperature, etc. can cause the condensation pressure to be too high. It is very important to choose a suitable condensation area and maintain sufficient cooling medium flow. 　　 High-temperature and air-conditioning compressors are designed to have a relatively low operating compression, which is used to double the compression ratio after freezing, the exhaust temperature is high, and the cooling cannot keep up, causing overheating. It is necessary to avoid using the compressor beyond the range and make the compressor work at the smallest possible pressure ratio. In some low-temperature systems, overheating is the primary cause of compressor failure. (4) Anti-expansion and gas mixing 　　 After the intake stroke starts, the high-pressure gas trapped in the cylinder clearance will undergo a reverse expansion process. After de-expansion, the gas pressure returns to the suction pressure, and the energy consumed for compressing this part of the gas is lost during de-expansion. The smaller the clearance, the smaller the power consumption caused by anti-expansion on the one hand, and the larger the suction volume on the other hand, the compressor energy efficiency ratio is therefore greatly increased. 　　 During the anti-expansion process, the gas contacts the high-temperature surfaces of the valve plate, the top of the piston, and the top of the cylinder to absorb heat, so the gas temperature will not drop to the suction temperature at the end of the anti-expansion. 　　After the anti-expansion ends, the inspiratory process starts. After the gas enters the cylinder, on the one hand, it mixes with the anti-expansion gas and the temperature rises; on the other hand, the mixed gas absorbs heat from the wall surface and heats up. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, because the anti-expansion process and the suction process are very short, the actual temperature rise is very limited, generally less than 5°C. 　　 Anti-expansion is caused by the cylinder clearance, which is a shortcoming that traditional piston compressors cannot avoid. If the gas in the exhaust hole of the valve plate is not discharged, there will be anti-expansion. (5) Compression temperature rise and refrigerant type 　　 Different refrigerants have different thermophysical properties, and the exhaust gas temperature rises differently after undergoing the same compression process. Therefore, for different refrigeration temperatures, different refrigerants should be used. conclusion and suggestion 　　Compressor should not have overheating phenomena such as high temperature of the motor and excessively high exhaust temperature in normal operation of the compressor in the use range. Compressor overheating is an important fault signal, indicating that there are serious problems in the refrigeration system, or improper use and maintenance of the compressor. 　　If the root cause of compressor overheating is the refrigeration system, the problem can only be solved by improving the design and maintenance of the refrigeration system. Replacing a new compressor cannot fundamentally eliminate the problem of overheating.

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