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Compressor Exhaust Overheating Reason Oct 10, 2018

Compressor exhaust overheating reason


The main reasons for the overheating of the exhaust gas are as follows:


The return air temperature is high, the motor is heated, the compression ratio is high, the condensing pressure is high, and the refrigerant is improperly selected.


Compressor exhaust overheating is not so complicated, nothing more than these reasons!


(1) high return air temperature


The return air temperature is relative to the evaporation temperature. In order to prevent backflow, the return air line generally requires a return air superheat of 20 °C. If the return line is not well insulated, the degree of superheat will far exceed 20 °C.


The higher the return air temperature, the higher the cylinder suction and exhaust temperatures. For every 1 °C increase in the return air temperature, the exhaust gas temperature will increase by 1 to 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 cavity, and the cylinder suction temperature is again increased. The heat generated by the motor is affected by power and efficiency, and the power consumption is closely related to displacement, volumetric efficiency, working conditions, and frictional resistance.


In the return air-cooled half-sealed compressor, the temperature rise of the refrigerant in the motor cavity is approximately between 15 and 45 °C. In the air-cooled (air-cooled) type compressor, the cooling 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, and the larger the compression ratio, the higher the exhaust gas temperature. Reducing the compression ratio can significantly reduce the exhaust gas temperature by 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 and rapidly reduce the compression ratio, thereby reducing the exhaust gas temperature.


Some users believe that the lower the evaporation temperature, the faster the cooling rate. This idea 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, the higher the load, the longer the operation time, and the higher the power consumption.


Reducing the resistance of the return 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 tube and the return line. In addition, insufficient refrigerant is also a factor of low suction pressure. After the refrigerant is lost, it should be replenished in time. Practice has shown that reducing the exhaust gas temperature by increasing the suction pressure is simpler and more efficient than other methods.


The main reason for the excessive 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, and too high cooling water or air temperature can cause excessive condensation pressure. It is important to choose the right condensing area and maintain a sufficient flow of cooling medium.


The high-temperature and air-conditioning compressors are designed to have a low compression ratio, which is used to double the compression ratio after freezing, and the exhaust temperature is high, and the cooling cannot keep up, causing overheating. This should avoid using the compressor over the range and operate the compressor at the lowest possible pressure ratio. In some cryogenic systems, overheating is the primary 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 an inverse expansion process. After the anti-expansion, the gas pressure is restored 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 reverse expansion on the one hand, and the larger the inhalation volume on the other hand, the greater the energy efficiency ratio of the compressor.


During the reverse 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 that the gas temperature does not decrease to the suction temperature at the end of the reverse expansion.


After the end of the anti-expansion, 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 heat up. Therefore, the gas temperature at the beginning of the compression process is higher than the suction temperature. However, since the anti-expansion process and the inhalation process are very short, the actual temperature rise is very limited, generally less than 5 °C.


The reverse expansion is caused by the cylinder clearance and is a disadvantage that the conventional piston compressor cannot avoid. If the gas in the vent hole of the valve plate is not discharged, there will be reverse expansion.


(5) Compressed temperature rise and refrigerant type


Different refrigerants have different thermophysical properties, and the exhaust gas temperature rises differently after undergoing the same compression process. Therefore, different refrigerants should be used for different cooling temperatures.


conclusion and suggestion


If the compressor is operating normally within the range of use, there should be no overheating such as high motor temperature and high exhaust steam temperature. Overheating of the compressor is an important fault signal indicating a serious problem with the refrigeration system or improper use and maintenance of the compressor.


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