Causes of overheating of refrigeration compressor discharge

The main reasons for the overheating of the exhaust gas temperature are as follows: high return air temperature, large heating capacity of the motor, high compression ratio, high condensing 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 backflow, the general return gas pipeline requires a return gas superheat of 20°C. If the return line is not well insulated, the superheat will be well over 20°C.

The higher the return air temperature, the higher the cylinder intake and exhaust temperature. For every 1°C increase in the return air temperature, the exhaust temperature will increase by 1 to 1.3°C.

2. Motor heating

For return air cooled compressors, the refrigerant vapor is heated by the motor as it flows through the motor cavity, and the cylinder suction temperature is raised 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, operating conditions, and frictional resistance.

Return air cooling type semi-hermetic compressor, the temperature rise of 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 winding, so there is no motor heating problem.

3. the compression ratio is too high

The exhaust gas temperature is greatly affected by the compression ratio. The larger the compression ratio, the higher the exhaust gas temperature. Reducing the compression ratio can significantly reduce the exhaust temperature by increasing the suction pressure and reducing the exhaust pressure.

The suction pressure is determined by the evaporative pressure and the resistance of the suction line. Increasing the evaporating temperature can effectively increase the suction pressure and rapidly reduce the compression ratio, thereby reducing the exhaust temperature.

Some users unilaterally believe that the lower the evaporating temperature, the faster the cooling rate, but there are actually many problems with this idea. Although reducing 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. What's more, the lower the evaporating temperature, the lower the cooling coefficient, but the load increases, the operation time is prolonged, and the power consumption will increase.

Reducing the resistance of the return gas pipeline can also increase the return gas pressure. The specific methods include timely replacement of the dirty and blocked return gas filter, and reducing the length of the evaporation pipe and the return gas pipeline as much as possible. In addition, insufficient refrigerant is also a factor for low suction pressure. After the refrigerant leaks, 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 for the high exhaust pressure is that the condensing pressure is too high. Insufficient cooling area of the condenser, fouling, insufficient cooling air or water volume, too high cooling water or air temperature, etc., can lead to excessive condensing pressure. It is very important to choose a suitable condensation area and maintain sufficient cooling medium flow.

The operating compression ratio of high temperature and air-conditioning compressor design is low. After being used for freezing, the compression ratio is doubled, and the exhaust temperature is very high, but the cooling cannot keep up, resulting in overheating. For this reason, avoid overrunning the compressor and run the compressor at the smallest possible pressure ratio. In some cryogenic systems, overheating is the number one 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 have a reverse expansion process. After the reverse expansion, the gas pressure returns to the suction pressure, and the energy consumed for compressing 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, so the energy efficiency ratio of the compressor is greatly increased.

During the reverse expansion process, the gas absorbs heat in contact with the high temperature surfaces of the valve plate, the top of the piston and the top of the cylinder, so the gas temperature will not drop to the suction temperature at the end of the reverse expansion.

After the end of the anti-inflation, 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 surface to rise. The gas temperature at the beginning of the compression process is therefore higher than the suction temperature. However, because the reverse expansion process and the inhalation process are very short, the actual temperature rise is very limited, generally less than 5 °C.

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

5. Compression temperature rise and refrigerant type

The thermophysical properties of different refrigerants are different, and the temperature rise of the exhaust gas is different after the same compression process. Therefore, different refrigerants should be used for different refrigeration temperatures.

Conclusion and suggestion:

The compressor should not have overheating phenomena such as high motor temperature and high exhaust steam temperature during normal operation within the scope of use. Overheating of the compressor is an important fault signal, indicating that there is a serious problem with the refrigeration system, or that the compressor is not used and maintained properly.

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. Going to a new compressor won't fundamentally eliminate the overheating problem.