A common cause of premature reciprocating compressor failure is overheating. A reciprocating that is running too hot will surely fail before its time. A service tech should always look for this potential problem while servicing any refrigeration system.
The hottest location in a refrigeration system is at the discharge port of the valve plate inside the compressor head. It is difficult for a technician to measure the exact temperature at this location. It’s important to know the causes of overheating to repair the system.
Let’s learn the main causes of discharge port overheating: high return air temperature, large motor heating capacity, high compression ratio, high condensing pressure, and improper refrigerant selection.
(1) high return air temperature
The return air temperature is relative to the evaporation temperature. In order to prevent liquid back flow, the return air line generally requires a return air super heat of 20 °C. If the return line is not well insulated, the degree of super heat 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 exhaust, volumetric efficiency, working conditions, and frictional resistance etc.
About the return air-cooled semi hermetic 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 refrigerant 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 temperature, including increasing the suction pressure and reducing the discharge 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 discharge 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 running time, and the higher the power consumption.
(4) Refrigerant type
Different refrigerants have different thermos physical properties, and discharge gas temperature rises differently after undergoing the same compression process. Therefore, different refrigerants should be used for different cooling temperatures.
If the compressor is operating normally, there should be no overheating such as high motor temperature and high discharge temperature. Overheating of the compressor is an important failure signal indicating a serious problem with the refrigeration system or improper usage and maintenance of the compressor.