Analysis of Overheating Failure of Refrigeration Compressor

Analysis of Overheating Failure of Refrigeration Compressor The main reasons for the overheating of exhaust temperature are as follows: high return air temperature, large heating capacity of the motor, high compression ratio, high condensation pressure, and improper refrigerant selection.



(1) High return air temperature

　　The return air 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 line is not well insulated, the superheat will far exceed 20°C.



　　The higher the return air temperature, the higher the cylinder suction temperature and exhaust temperature. Every time the return air temperature increases by 1°C, the exhaust temperature will increase by 1 to 1.3°C.



(2) Motor heating

　　 For the return air cooling type compressor, the refrigerant vapor is heated by the motor when it flows through the motor cavity, and the cylinder suction temperature is increased again. The amount of heat generated by the motor is affected by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, working conditions, and frictional resistance.



　　Return air cooling type semi-hermetic compressor, the temperature rise range of the refrigerant in the motor cavity is roughly between 15~45°C. In the air-cooled (air-cooled) compressor, the refrigeration system does not pass through the windings, so there is no motor heating problem.



(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 and rapidly reduce the compression ratio, thereby reducing the exhaust temperature.



　　Some users have a partial view that the lower the evaporation temperature, the faster the cooling rate. This idea actually has many problems. Although lowering the evaporation temperature can increase the freezing temperature difference, the cooling capacity of the compressor is reduced, so the freezing speed is not necessarily fast. What's more, the lower the evaporation temperature, the lower the refrigeration coefficient, but the load increases, 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. Specific methods include timely replacement of the dirty return air filter, and minimize the length of the evaporation pipe and the return air line. In addition, insufficient refrigerant is also a factor of low suction pressure. Refill in time after refrigerant leakage. Practice shows that reducing the exhaust temperature by increasing the suction pressure is simpler and more effective than other methods.



　　 The main reason for the excessively high discharge pressure 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 excessive condensing pressure. It is very important to choose a suitable condensing area and maintain sufficient cooling medium flow.



　　The high temperature and air-conditioning compressor design have a low operating compression ratio. After being used for freezing, the compression ratio is doubled, the exhaust temperature is very high, and the cooling cannot keep up, resulting in overheating. Therefore, it is necessary to avoid over-range use of the compressor and make the compressor work at the lowest possible pressure ratio. In some low temperature systems, overheating is the primary cause of compressor failure.



(4) Anti-expansion and gas mixing

　　 After the start of the suction stroke, the high-pressure gas remaining in the cylinder clearance will undergo a reverse expansion process. After the reverse expansion, the gas pressure returns to the suction pressure, and the energy used to compress this part of the gas is lost in the reverse expansion. The smaller the clearance, the smaller the power consumption caused by the anti-expansion on the one hand, and the larger the suction volume on the other hand, which greatly increases the compressor energy efficiency ratio.



During the anti-expansion process, the gas contacts the high temperature surface 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 is over, the real inhalation process begins. 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 to increase the temperature. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, since the reverse expansion process and the suction process are very short, the actual temperature rise is very limited, generally less than 5°C.



　　Reverse expansion is caused by cylinder clearance, which is an unavoidable shortcoming of traditional piston compressors. If the gas in the exhaust hole of the valve plate cannot be discharged, there will be anti-expansion.

(5) Compression temperature rise and refrigerant types

　　Different refrigerants have different thermophysical properties, and the exhaust gas temperature rises differently after the same compression process. Therefore, different refrigerants should be selected for different refrigeration temperatures.



conclusion and suggestion

　　 The compressor should not have overheating phenomena such as high temperature of the motor and excessive exhaust temperature during the normal operation of the compressor. Compressor overheating is an important fault signal, indicating that there is a serious problem in the refrigeration system, or the compressor is used and maintained improperly.



　　If the source of compressor overheating lies in the refrigeration system, the problem can only be solved by improving the design and maintenance of the refrigeration system. Changing to a new compressor cannot eliminate the overheating problem fundamentally.