The reasons for the overheating of the exhaust temperature of a semi sealed piston machine

The main reasons for overheating of exhaust temperature are as follows: high return air temperature, large motor heating capacity, high compression ratio, high condensing pressure, and improper refrigerant selection.

(1) High return air temperature

The temperature of the return air is relative to the evaporation temperature. In order to prevent liquid return, the general return air pipeline requires a return air superheat of 20 ° C. If the insulation of the return air pipeline is not good, the superheat will far exceed 20 ° C.

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

(2) Motor heating

For a return air cooled compressor, the refrigerant vapor is heated by the motor as it flows through the motor chamber, and the cylinder suction temperature is once again raised. The heat generation of a motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, operating conditions, friction resistance, etc.

The temperature rise range of the refrigerant in the motor chamber of the return air cooling semi sealed compressor is approximately between 15 and 45 ° C. In air-cooled (air-cooled) compressors, the refrigeration system does not go through the winding, so there is no problem with motor heating.

(3) Compression ratio 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 significantly reduce the exhaust temperature, including increasing the suction pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance of the suction pipeline. Raising the evaporation temperature can effectively increase the suction pressure, quickly reduce the compression ratio, and thus lower the exhaust temperature.

Some users have a biased belief that the lower the evaporation temperature, the faster the cooling rate, but this idea actually has many problems. Although reducing the evaporation temperature can increase the freezing temperature difference, the refrigeration capacity of the compressor decreases, so the freezing speed may not be fast. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient, while the load increases, the operating time prolongs, and the power consumption increases.

Reducing the resistance of the return air pipeline can also increase the return air pressure. Specific methods include timely replacement of dirty and clogged return air filters, and minimizing the length of the evaporator and return air pipelines as much as possible. In addition, insufficient refrigerant is also a factor contributing to low suction pressure. Refrigerant leakage should be promptly replenished. Practice has shown that reducing exhaust temperature by increasing suction pressure is simpler and more effective than other methods.

The main reason for high exhaust pressure is that the condensation pressure is too high. Insufficient heat dissipation area of the condenser, scaling, insufficient cooling air volume or water volume, and high cooling water or air temperature can all lead to excessive condensation pressure. Choosing the appropriate condensing area and maintaining sufficient cooling medium flow rate is very important.

The high temperature and air conditioning compressor design has a relatively low operating compression ratio, which increases exponentially when used for freezing. The exhaust temperature is very high, but the cooling cannot keep up, causing overheating. It is necessary to avoid using the compressor beyond the range and keep the compressor operating at the possible minimum pressure ratio. In some low-temperature systems, overheating is the primary cause of compressor failure.

(4) Anti expansion and gas mixing

After the suction stroke begins, the high-pressure gas trapped in the cylinder clearance will undergo a counter expansion process. After counter expansion, the gas pressure is restored to the suction pressure, and the energy consumed for compressing this part of the gas is lost during counter expansion. The smaller the clearance, on the one hand, the lower the power consumption caused by anti expansion, and on the other hand, the larger the suction volume, which greatly increases the energy efficiency ratio of the compressor.

During the process of counter expansion, the gas contacts and absorbs heat from the high-temperature surfaces of the valve plate, piston top, and cylinder top, so the gas temperature will not decrease to the suction temperature at the end of counter expansion.

After the anti inflation is completed, the actual inhalation process begins. After the gas enters the cylinder, it mixes with the counter expansion gas and the temperature rises; On the other hand, the mixed gas absorbs heat and heats up from the wall. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, due to the short duration of the anti expansion and suction processes, the actual temperature rise is very limited, usually less than 5 ° C.

Reverse expansion is caused by cylinder clearance, which is a disadvantage that traditional piston compressors cannot avoid. If the gas in the exhaust hole of the valve plate cannot be discharged, there will be counter expansion.

(5) Compression temperature rise and refrigerant type

Different refrigerants have different thermal and physical properties, and the amount of increase in exhaust temperature after undergoing the same compression process varies. Therefore, for different refrigeration temperatures, different refrigerants should be selected.

Conclusion and recommendations

The compressor should not have overheating phenomena such as high motor temperature and high exhaust temperature during normal operation within the range of use. Compressor overheating is an important fault signal, indicating serious problems in the refrigeration system or improper use and maintenance of the compressor.

If the root cause of compressor overheating lies in 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 overheating issues.