The main reasons for the overheating of exhaust temperature are as follows: high return air temperature, large heating capacity of motor, high compression ratio, high condensation pressure and improper selection of refrigerant.
(1) The return air temperature is high
The return air temperature is relative to the evaporation temperature. In order to prevent the return of liquid, the general return gas pipeline requires a return gas superheat of 20 C. If the air return 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. For every 1 C increase in return air temperature, the exhaust temperature will increase by 1 ~ 1.3 C.
(2) Motor heating
For the return air cooling compressor, the refrigerant vapor is heated by the motor when it flows through the motor cavity, and the suction temperature of the cylinder is increased again. The calorific value of motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, working condition and friction resistance.
The temperature rise range of refrigerant in the motor cavity of the semi-sealed compressor with return air cooling is about 15 ~ 45 C. In 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 temperature is greatly influenced by the compression ratio. The greater the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can obviously reduce the exhaust temperature, and the specific methods include increasing the suction pressure and reducing the exhaust pressure.
The suction pressure is determined by evaporation pressure and suction pipeline resistance. Increasing the evaporation temperature can effectively increase the suction pressure and rapidly reduce the compression ratio, thus reducing the exhaust temperature.
Some users think that the lower the evaporation temperature, the faster the cooling speed, which 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. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient, while the load increases, the longer the operation time, and the higher the power consumption.
Reducing the resistance of the return air pipeline can also improve the return air pressure. The specific methods include replacing the dirty and blocked return air filter in time and reducing the length of evaporation pipe and return air pipeline as much as possible. In addition, insufficient refrigerant is also a factor of low suction pressure. Refrigerant should be replenished in time after leakage. Practice shows that it is simpler and more effective than other methods to reduce the exhaust temperature by increasing the suction pressure.
The main reason for the high exhaust pressure is that the condensation pressure is too high. Insufficient heat dissipation area of condenser, fouling, insufficient cooling air volume or water volume, and too high temperature of cooling water or air can all lead to too high condensation pressure. It is very important to choose a suitable condensation area and maintain sufficient cooling medium flow.
High temperature and air conditioning compressors are designed with low operating compression ratio, which is doubled after freezing, and the exhaust temperature is high, but the cooling can't keep up, resulting in overheating. Therefore, it is necessary to avoid using the compressor beyond the scope and make the compressor work at the lowest possible pressure ratio. In some cryogenic systems, overheating is the primary cause of compressor failure.
(4) Reverse expansion and gas mixing
After the intake stroke begins, the high-pressure gas trapped 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 gas is lost in the reverse expansion. The smaller the clearance, on the one hand, the smaller the power consumption caused by reverse expansion, on the other hand, the larger the suction volume, so the energy efficiency ratio of the compressor is greatly increased.
In the process of reverse expansion, the gas absorbs heat by contacting with the high-temperature surfaces of the valve plate, the piston top and the cylinder top, so the gas temperature will not drop to the suction temperature at the end of reverse expansion.
After the anti-inflation is over, the real inhalation process begins. After the gas enters the cylinder, on the one hand, it is mixed with the anti-expansion gas, and the temperature rises; On the other hand, the mixed gas absorbs heat from the wall and heats up. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, the actual temperature rise is very limited, generally less than 5 C, because the reverse expansion process and inspiratory process are very short.
Reverse expansion is caused by cylinder clearance, which is an unavoidable shortcoming of traditional piston compressor. 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 types
Different refrigerants have different thermophysical properties, and the exhaust temperature rises differently after the same compression process. Therefore, different refrigerants should be selected for differenrefrigeration temperatures.t
Conclusions and suggestions
The normal operation of the compressor within the scope of use should not have overheating phenomena such as high motor temperature and too high exhaust temperature. Overheating of the compressor is an important fault signal, indicating that there is a serious problem in the refrigeration system, or that the compressor is improperly used and maintained. If the source of compressor overheating lies in the refrigeration system, we can only solve the problem by improving the design and maintenance of the refrigeration system. Replacing a new compressor can't fundamentally eliminate the overheating problem.
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