Causes of high compressor exhaust temperature and solutions

It’s likely the exhaust valve is leaking; check the exhaust valve

The main reasons for excessive exhaust temperature in compressors are as follows: high return air temperature, high motor heating, high compression ratio, high condensing pressure, and improper selection of refrigerant. 1. High return air temperature: The level of the return air temperature is determined in relation to the evaporation temperature. To prevent backflow, the return gas line generally requires a return gas superheat of 20°C. If the return air pipeline is not properly insulated, the superheat will far exceed 20°C. The higher the return air temperature, the higher the intake and exhaust temperatures in the cylinder. For every 1°C increase in the return air temperature, the exhaust air temperature will rise by 1–1.3°C. 2. Motor heating: In recirculating-cooling compressors, the refrigerant vapor is heated by the motor as it passes through the motor chamber, which raises the suction temperature in the cylinder once again. The heat generation of a motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, operating conditions, frictional losses, and so on. For the return-air cooling type semi-hermetic compressor, the temperature rise of the refrigerant in the motor compartment is generally between 15 and 45°C. In air-cooled compressors, the refrigerant does not pass through the windings, so there is no issue of motor heating. 3. Excessively high compression ratio: The exhaust temperature is greatly influenced by the compression ratio; the higher the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can significantly lower the exhaust temperature; methods to achieve this include increasing the intake pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance in the suction line. Increasing the evaporation temperature can effectively raise the suction pressure, rapidly reduce the compression ratio, and thereby lower the exhaust temperature. Some users mistakenly believe that the lower the evaporation temperature, the faster the cooling rate; this idea actually has many problems. Although lowering the evaporation temperature can increase the freezing temperature difference, the cooling capacity of the compressor decreases, so the freezing speed is not necessarily faster. Moreover, the lower the evaporation temperature, the lower the coefficient of performance, while the load increases, the operating time lengthens, and power consumption rises. Reducing the resistance in the return air circuit can also increase the return air pressure; methods to achieve this include replacing the dirty or clogged return air filter in a timely manner, and minimizing the length of the evaporation tubes and return air circuits as much as possible. Furthermore, a lack of refrigerant is also a factor contributing to low suction pressure. The refrigerant should be replenished promptly in case of leakage. Practice has shown that reducing the exhaust temperature by increasing the intake pressure is simpler and more effective than other methods. The main reason for excessive exhaust pressure is too high condensation pressure. Insufficient cooling surface area of the condenser, fouling, inadequate volume of cooling air or water, and excessively high temperatures of the cooling water or air can all lead to excessive condensation pressure. It is very important to select an appropriate condensation area and maintain a sufficient flow rate of the cooling medium. At high temperatures, the operating compression ratio of air-conditioning compressors is relatively low; after cooling, this compression ratio increases significantly, resulting in very high exhaust temperatures. The cooling system is not able to keep up, leading to overheating. It is necessary to avoid overusing the compressor and to operate it at the lowest possible pressure ratio. In some low-temperature systems, superheating is the primary cause of compressor failures. 4. Anti-expansion and gas mixing: Once the intake stroke begins, the high-pressure gas remaining in the cylinder clearance undergoes an anti-expansion process. After anti-expansion, the gas pressure returns to the intake pressure, and the energy consumed to compress this portion of gas is lost during the anti-expansion process. The smaller the clearance, the lower the power consumption resulting from back expansion on one hand, and the greater the amount of air drawn in, which in turn **increases** the compressor’s efficiency ratio. During the reverse expansion process, the gas absorbs heat by coming into contact with the high-temperature surfaces of the valve plate, the top of the piston, and the top of the cylinder; as a result, the gas temperature does not drop to the intake temperature at the end of the reverse expansion. Only after the anti-expansion is complete does the actual inhalation process begin. Once the gas enters the cylinder, it mixes with the counter-expanding gas, causing the temperature to rise ； On the other hand, the mixed gas absorbs heat from the wall surface and heats up. Therefore, the gas temperature at the start of the compression process is higher than the intake temperature. However, since the anti-expansion process and the suction process are very brief, the actual temperature rise is extremely limited, generally less than 5°C. Anti-expansion is caused by the clearance in the cylinder, and it is a drawback that traditional piston compressors cannot avoid. If the gas in the valve plate’s exhaust holes cannot be discharged, reverse expansion will occur. The patented disc-shaped valve plate of Tanaka Company’s exhaust valves is highly special; it eliminates gaps in the exhaust ports and gas retention, thereby effectively controlling back expansion. Since its invention, disc valve compressors have maintained the record for highest efficiency. 5. Compression temperature rise and type of refrigerant: Different refrigerants have different thermophysical properties, resulting in varying increases in exhaust temperature after undergoing the same compression process. Therefore, different refrigerants should be selected for different cooling temperatures. Figures 1-3 show the temperature rise caused by adiabatic compression of different refrigerants at a condensation temperature of 50°C and a return gas superheat of 20°C. Considering a return air superheat of 20°C and motor heating at 30°C, the theoretical exhaust temperature will exceed 150°C, requiring additional cooling. For systems with an evaporation temperature above 0°C (such as air conditioners), the exhaust temperature should not exceed 110°C, so there is no issue of overheating. Conclusion and recommendation: When the compressor operates within its specified range, there should be no phenomena of overheating such as high motor temperatures or excessively high exhaust temperatures. Compressor overheating is an important fault indicator, suggesting serious problems with the refrigeration system or improper use and maintenance of the compressor. If the root cause of compressor overheating lies in the refrigeration system, the problem can only be resolved by improving the design and maintenance of the refrigeration system. Replacing it with a new compressor cannot fundamentally eliminate the overheating problem.

1. The intake and exhaust valves are leaking; repair the valves. 2. The clearance at the piston dead center is too large; adjust the clearance. 3. If the clearance at the piston ring openings is too large or the rings are severely worn, replace them. 4. The cooling effect is poor; check the cooling system.

An excessively high inlet air temperature of the machine can also lead to an excessively high exhaust temperature. The intake air temperature should be controlled at ≤40℃

Reasons for excessively high compressor discharge temperature: 1. Leakage in the inlet and exhaust valves. 2. Leakage in the next stage’s exhaust valve results in a high compression ratio for this stage. 3. There are many cases of excessive clearance. 4. Gas leakage due to damage to the piston rings at this level and the next lower level.

Is there a problem with the cooler, resulting in low cooling efficiency?

Leaking intake and exhaust valves, excessive clearance, severe wear of piston rings, poor cooling effect of the cylinder liner, and high compression ratio.

It’s mainly the exhaust valve that is leaking; the other issues aren’t that serious. Just repair the valve. With gas flowing back and forth during compression, how can it not get hot? Once the valve is fixed, there will be no problem.

It could also be that the conversion rate of the previous system was low, resulting in an excessive amount of material undergoing flashing

Is the compressor in the original post a piston-type or a centrifugal type?