The main reasons for high exhaust temperature of refrigeration compressors are high return air temperature, large motor heating, high compression ratio, high condensing pressure and improper refrigerant selection. Among them, high return air temperature is a key factor, which directly affects the cylinder suction temperature and exhaust temperature. In order to prevent liquid return, the return air pipeline usually needs to maintain a return air superheat of no more than 20°C, but if the insulation measures are not good, the superheat may rise sharply. For every 1°C increase in return air temperature, the exhaust temperature will increase by 1 to 3°C accordingly.
In addition, motor heating is also a reason for the increase in exhaust temperature. In a return air cooling refrigeration compressor, the refrigerant vapor will flow through the motor cavity and be heated by it, thereby increasing the cylinder suction temperature. The heat generated by the motor is closely related to power and efficiency, while the power consumption is affected by multiple factors such as displacement, volumetric efficiency, working conditions and friction resistance.
In addition, too high a compression ratio is also an important reason for the increase in exhaust temperature. The larger the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can effectively reduce the exhaust temperature. Specific measures include increasing the suction pressure and reducing the exhaust pressure. The suction pressure is affected by the evaporation pressure and the resistance of the suction line. Therefore, increasing the evaporation temperature can effectively increase the suction pressure, thereby reducing the compression ratio and the exhaust temperature.
It is worth noting that some users may mistakenly believe that the lower the evaporation temperature, the faster the cooling speed, but in fact this is not always the best choice. Although lowering the evaporation temperature can increase the freezing temperature difference, it will also reduce the refrigeration capacity of the compressor, so the freezing speed may not be significantly improved. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient will be, increasing the operating time and power consumption.
In addition, reducing the resistance of the return air line can also increase the return air pressure, thereby reducing the exhaust temperature. Specific measures include timely replacement of clogged return air filters, shortening the length of the evaporator pipe and the return air line, etc. At the same time, it is also very important to ensure that the refrigerant is sufficient, because the refrigerant needs to be replenished in time after leakage to maintain the normal operation of the system.
Practice has proved that the method of reducing the exhaust temperature by increasing the suction pressure is simpler and more efficient than other methods. The core reason for the high exhaust pressure is the increase in condensing pressure. This may be caused by factors such as insufficient condenser heat dissipation area, fouling, insufficient cooling air volume or water, and excessive cooling water or air temperature. Therefore, it is crucial to select an appropriate condensing area and ensure sufficient cooling medium flow.
In a high temperature environment, the design operation compression ratio of the air conditioning compressor is low, and the compression ratio after freezing will increase exponentially, resulting in excessive exhaust temperature and insufficient cooling. In order to avoid overloading the compressor, it should be ensured to operate at the minimum pressure ratio. Especially in some low-temperature systems, overheating problems are often the primary cause of compressor failure.
# Clearance expansion and gas mixing
When the suction stroke starts, the high-pressure gas in the cylinder clearance will undergo a clearance expansion process. During this process, the gas pressure gradually returns to the suction pressure, and the energy consumed during this period is inevitably lost. The reduction in clearance volume not only reduces the energy loss caused by clearance expansion, but also helps to increase the suction volume, thereby significantly improving the energy efficiency ratio of the compressor.
During the gap expansion phase, the gas comes into contact with and absorbs heat from the hot surfaces of the valve plate, piston top, and cylinder top. Therefore, the gas temperature does not drop below the suction temperature at the end of the gap expansion. The actual suction process begins after the gap expansion is complete, when the gas entering the cylinder mixes with the previously expanded gas, causing a slight temperature increase. In addition, the mixed gas absorbs heat from the cylinder wall, further increasing the temperature. Therefore, the gas temperature at the beginning of the compression process is usually higher than the suction temperature. However, please note that due to the relatively short duration of the gap expansion and suction process, the actual temperature increase is quite limited, usually not exceeding 5°C.
Gap expansion is a phenomenon caused by the cylinder gap and is an inevitable defect of traditional piston compressors. Gap expansion occurs when the gas in the valve plate exhaust hole cannot be completely discharged.
# Compression temperature rise and refrigerant type
Different types of refrigerants have different thermophysical properties, and the amount of exhaust temperature increase after the same compression process will also be different. Therefore, when selecting a refrigerant, full consideration should be given to different refrigeration needs.
# Conclusion and Suggestions
The refrigeration compressor should not have overheating phenomena such as high motor temperature or high exhaust temperature within the normal working range. Once the compressor overheats, it is often an important fault signal, indicating that there may be serious problems with the refrigeration system or improper use and maintenance of the compressor. If the overheating problem comes from the refrigeration system itself, simply replacing a new compressor may not completely solve the problem, and it is necessary to start with improving the design and maintenance of the refrigeration system.