What are the main reasons for compressor exhaust overheating?

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 condensation pressure, and improper refrigerant selection.

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 air pipe 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.

Motor heating

For the return-air cooling compressor, the refrigerant vapor is heated by the motor as it flows through the motor cavity, and the cylinder suction temperature is once again increased. The calorific value of the motor is affected by power and efficiency, and the power consumption is closely related to displacement, volumetric efficiency, working conditions, friction resistance, etc.

In the return air cooling type semi-hermetic compressor, the temperature rise of the refrigerant in the motor cavity is roughly between 15 and 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.

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 pipe. Increasing the evaporation temperature can effectively increase the suction pressure and rapidly reduce the compression ratio, thereby reducing the exhaust temperature.

Some users are partial to believe 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 refrigeration 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 prolonged, 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 the dirty return air filter, and minimizing the length of the evaporation pipe and the return air line. In addition, insufficient refrigerant is also a factor of low suction pressure. The refrigerant must be replenished in time after it is lost. 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 exhaust 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 has a low operating compression ratio. After being used for refrigeration, 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.

Anti-expansion and gas mixing

After the start of the suction stroke, the high-pressure gas trapped in the cylinder clearance will undergo an anti-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 anti-expansion on the one hand, and the larger the air intake on the other hand, which greatly increases the energy efficiency ratio of the compressor.

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 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.

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

Compression temperature rise and refrigerant types

Different refrigerants have different thermal and physical properties, and the exhaust 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 the high temperature of the motor and the excessively high exhaust steam temperature in 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.