The cause of overheated exhaust temperature of semi-sealed piston machine

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

(1) The temperature is high

The return temperature is relative to the evaporation temperature. In order to prevent liquid return, the general return pipe requires a return air superheat of 20°C. If the return air pipe is not well insulated, the superheat is far more than 20°C.

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

(2) Motor heating

For reair-cooled compressors, the refrigerant vapor is heated by the motor as it flows through the motor chamber, and the suction temperature of the cylinder is again raised. The heat output of motor is affected 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 the refrigerant in the motor chamber is roughly between 15 and 45°C. The refrigeration system in the air-cooled (air-cooled) compressor does not pass through the winding, so there is no motor heating problem.

(3) Compression ratio is 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 suction line resistance. Increasing the evaporation temperature can effectively increase the suction pressure, rapidly reduce the compression ratio, and thus reduce the exhaust temperature.

Some users believe that the lower the evaporation temperature, the faster the cooling speed, this idea actually has many problems. Although reducing 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 has increased, the operation time is extended, and the power consumption will increase.

Reducing the resistance of the return pipe can also increase the return air pressure, including timely replacement of dirty and blocked return air filter, and reducing the length of the evaporation pipe and return pipe as much as possible. In addition, insufficient refrigerant is also a factor in low suction pressure. After refrigerant leakage, replenish it in time. 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 high exhaust pressure is the high condensing pressure. Insufficient cooling area of the condenser, scaling, insufficient cooling air or water, cooling water or air temperature is too high can lead to high condensing pressure. It is very important to select the appropriate condensing area and maintain sufficient cooling medium flow.

The high temperature and air conditioning compressor design has low operating compression, and the compression ratio is doubled after freezing, and the exhaust temperature is very high, and the cooling cannot keep up, resulting in overheating. It is necessary to avoid using the compressor out of range and to 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 suction 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 used to compress this part of the gas is lost in the reverse expansion. The smaller the clearance, on the one hand, the smaller the power consumption caused by anti-expansion, on the other hand, the larger the suction volume, the compressor energy efficiency ratio is greatly increased.

In the process of reverse expansion, the gas absorbs heat in contact with the high temperature surface of the valve plate, the top of the piston and the top of the cylinder, so the gas temperature does not decrease to the suction temperature at the end of the reverse expansion.

After the reverse expansion is over, the true inspiratory 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 and heats up. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, because the antiexpansion process and the inspiratory process are very short, the actual temperature rise is very limited, generally less than 5°C.

Antiexpansion is caused by the 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 reverse expansion.

(5) compression temperature rise and refrigerant type

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

Conclusion and suggestion

Compressor in the normal operation of the range of use should not have the motor high temperature and exhaust steam temperature too high overheating phenomenon. Compressor overheating is an important fault signal, indicating a more serious problem in the refrigeration system, or improper use and maintenance of the compressor.

If the root cause of compressor overheating is the refrigeration system, it can only be solved by improving the design and maintenance of the refrigeration system. Replacing a new compressor can not fundamentally eliminate the overheating problem