Why Does the Compressor Continuously Trigger Overheat Protection?
Overheat protection for compressors is designed to prevent burnout of the compressor motor. When the motor temperature or compressor body temperature exceeds a specified threshold, built-in or external protection devices cut off the power supply to the compressor, so as to protect the unit.
A refrigeration compressor draws low-temperature and low-pressure gaseous refrigerant from the system directly into the compressor shell. The inhaled refrigerant first cools the motor before being compressed. Therefore, the suction superheat of refrigerant vapor is a critical factor contributing to compressor overheating.
The primary causes of excessive discharge temperature include high suction temperature, excessive motor heat generation, high compression ratio, high condensing pressure, and improper refrigerant selection.
1. High Suction Temperature
Suction temperature is measured relative to the evaporation temperature. To prevent liquid refrigerant from returning to the compressor, the suction line is generally required to maintain 20°C suction superheat. Poor thermal insulation of the suction line will push the superheat far above 20°C. The higher the suction temperature, the higher the cylinder suction temperature and discharge temperature. For every 1°C rise in suction temperature, the discharge temperature increases by 1 ~ 1.3°C.
2. Motor Heating
For suction gas-cooled compressors, refrigerant vapor absorbs heat from the motor as it flows through the motor chamber, further raising the cylinder suction temperature. Motor heat output is affected by power consumption and efficiency, while power consumption is closely related to displacement, volumetric efficiency, operating conditions and friction resistance.
For suction gas-cooled semi-hermetic compressors, the temperature rise of refrigerant inside the motor chamber is roughly between 15°C and 45°C. For air-cooled compressors, refrigerant does not pass through motor windings, so motor heating does not occur.
3. Excessively High Compression Ratio
Discharge temperature is strongly affected by compression ratio; the higher the compression ratio, the higher the discharge temperature. Reducing the compression ratio can effectively lower discharge temperature, mainly by raising suction pressure and lowering discharge pressure.
Suction pressure is determined by evaporation pressure and suction line resistance. Increasing evaporation temperature can effectively raise suction pressure, rapidly reduce the compression ratio and thereby lower discharge temperature.
Some users mistakenly believe that lower evaporation temperature leads to faster cooling. This view has obvious drawbacks. Although reducing evaporation temperature increases the cooling temperature difference, the cooling capacity of the compressor decreases, so the cooling speed may not improve. Furthermore, lower evaporation temperature reduces the coefficient of performance, increases load, extends running time and raises power consumption.
Reducing suction line resistance can also raise suction pressure. Practical measures include timely replacing clogged suction filters and minimizing the length of evaporator and suction pipelines.
In addition, insufficient refrigerant charge is another cause of low suction pressure. Refrigerant shall be replenished promptly after leakage. Practice proves that raising suction pressure is simpler and more effective than other methods to reduce discharge temperature.
Excessively high discharge pressure is mainly caused by elevated condensing pressure. Insufficient condenser heat exchange area, fouling deposits, inadequate cooling air or water flow, and excessively high cooling water or ambient air temperature will all lead to high condensing pressure. Selecting an appropriate condenser area and maintaining sufficient cooling medium flow are essential.
Compressors designed for air conditioning and high-temperature applications feature low operating compression ratios. When used for low-temperature freezing, the compression ratio multiplies, resulting in extremely high discharge temperature and insufficient cooling capacity, which triggers overheating. Avoid operating compressors beyond their applicable scope, and keep them running at the minimum feasible compression ratio. In many low-temperature systems, overheating stands as the primary cause of compressor failure.
4. Re-expansion and Gas Mixing
Upon commencement of the suction stroke, high-pressure gas trapped in the cylinder clearance volume undergoes re-expansion. After re-expansion, the gas pressure falls back to suction pressure. The energy consumed to compress this trapped gas is lost during re-expansion. A smaller clearance volume reduces power loss induced by re-expansion and increases suction volume, significantly improving the compressor’s coefficient of performance.
During re-expansion, gas absorbs heat by contacting hot surfaces including the valve plate, piston top and cylinder head. Hence, the gas temperature cannot drop to the suction temperature when re-expansion completes.
The actual suction stroke only starts after re-expansion. When new gas enters the cylinder, it mixes with the re-expanded gas and heats up; meanwhile, the mixed gas absorbs heat from cylinder walls and rises further in temperature.
Accordingly, the gas temperature at the start of compression is higher than the suction temperature. However, the re-expansion and suction processes are extremely brief, so the actual temperature rise is limited, generally less than 5°C.
Re-expansion originates from cylinder clearance volume and constitutes an unavoidable disadvantage of traditional reciprocating compressors. Residual gas that cannot be discharged from discharge holes of the valve plate will trigger re-expansion.
5. Compression Temperature Rise and Refrigerant Type
Different refrigerants possess distinct thermophysical properties, leading to different discharge temperature rises after undergoing identical compression processes. Therefore, different refrigerants should be selected for different refrigeration temperature requirements.
Conclusions and Recommendations
Under normal operating ranges, properly operated compressors should not suffer overheating such as excessive motor temperature or overhigh discharge temperature. Compressor overheating serves as an important fault signal, indicating severe problems within the refrigeration system or improper operation and maintenance of the compressor.
If the root cause of compressor overheating lies within the refrigeration system, the problem can only be solved by optimizing system design and maintenance. Simply replacing the compressor cannot fundamentally eliminate overheating faults.
A refrigeration compressor draws low-temperature and low-pressure gaseous refrigerant from the system directly into the compressor shell. The inhaled refrigerant first cools the motor before being compressed. Therefore, the suction superheat of refrigerant vapor is a critical factor contributing to compressor overheating.
The primary causes of excessive discharge temperature include high suction temperature, excessive motor heat generation, high compression ratio, high condensing pressure, and improper refrigerant selection.
1. High Suction Temperature
Suction temperature is measured relative to the evaporation temperature. To prevent liquid refrigerant from returning to the compressor, the suction line is generally required to maintain 20°C suction superheat. Poor thermal insulation of the suction line will push the superheat far above 20°C. The higher the suction temperature, the higher the cylinder suction temperature and discharge temperature. For every 1°C rise in suction temperature, the discharge temperature increases by 1 ~ 1.3°C.
2. Motor Heating
For suction gas-cooled compressors, refrigerant vapor absorbs heat from the motor as it flows through the motor chamber, further raising the cylinder suction temperature. Motor heat output is affected by power consumption and efficiency, while power consumption is closely related to displacement, volumetric efficiency, operating conditions and friction resistance.
For suction gas-cooled semi-hermetic compressors, the temperature rise of refrigerant inside the motor chamber is roughly between 15°C and 45°C. For air-cooled compressors, refrigerant does not pass through motor windings, so motor heating does not occur.
3. Excessively High Compression Ratio
Discharge temperature is strongly affected by compression ratio; the higher the compression ratio, the higher the discharge temperature. Reducing the compression ratio can effectively lower discharge temperature, mainly by raising suction pressure and lowering discharge pressure.
Suction pressure is determined by evaporation pressure and suction line resistance. Increasing evaporation temperature can effectively raise suction pressure, rapidly reduce the compression ratio and thereby lower discharge temperature.
Some users mistakenly believe that lower evaporation temperature leads to faster cooling. This view has obvious drawbacks. Although reducing evaporation temperature increases the cooling temperature difference, the cooling capacity of the compressor decreases, so the cooling speed may not improve. Furthermore, lower evaporation temperature reduces the coefficient of performance, increases load, extends running time and raises power consumption.
Reducing suction line resistance can also raise suction pressure. Practical measures include timely replacing clogged suction filters and minimizing the length of evaporator and suction pipelines.
In addition, insufficient refrigerant charge is another cause of low suction pressure. Refrigerant shall be replenished promptly after leakage. Practice proves that raising suction pressure is simpler and more effective than other methods to reduce discharge temperature.
Excessively high discharge pressure is mainly caused by elevated condensing pressure. Insufficient condenser heat exchange area, fouling deposits, inadequate cooling air or water flow, and excessively high cooling water or ambient air temperature will all lead to high condensing pressure. Selecting an appropriate condenser area and maintaining sufficient cooling medium flow are essential.
Compressors designed for air conditioning and high-temperature applications feature low operating compression ratios. When used for low-temperature freezing, the compression ratio multiplies, resulting in extremely high discharge temperature and insufficient cooling capacity, which triggers overheating. Avoid operating compressors beyond their applicable scope, and keep them running at the minimum feasible compression ratio. In many low-temperature systems, overheating stands as the primary cause of compressor failure.
4. Re-expansion and Gas Mixing
Upon commencement of the suction stroke, high-pressure gas trapped in the cylinder clearance volume undergoes re-expansion. After re-expansion, the gas pressure falls back to suction pressure. The energy consumed to compress this trapped gas is lost during re-expansion. A smaller clearance volume reduces power loss induced by re-expansion and increases suction volume, significantly improving the compressor’s coefficient of performance.
During re-expansion, gas absorbs heat by contacting hot surfaces including the valve plate, piston top and cylinder head. Hence, the gas temperature cannot drop to the suction temperature when re-expansion completes.
The actual suction stroke only starts after re-expansion. When new gas enters the cylinder, it mixes with the re-expanded gas and heats up; meanwhile, the mixed gas absorbs heat from cylinder walls and rises further in temperature.
Accordingly, the gas temperature at the start of compression is higher than the suction temperature. However, the re-expansion and suction processes are extremely brief, so the actual temperature rise is limited, generally less than 5°C.
Re-expansion originates from cylinder clearance volume and constitutes an unavoidable disadvantage of traditional reciprocating compressors. Residual gas that cannot be discharged from discharge holes of the valve plate will trigger re-expansion.
5. Compression Temperature Rise and Refrigerant Type
Different refrigerants possess distinct thermophysical properties, leading to different discharge temperature rises after undergoing identical compression processes. Therefore, different refrigerants should be selected for different refrigeration temperature requirements.
Conclusions and Recommendations
Under normal operating ranges, properly operated compressors should not suffer overheating such as excessive motor temperature or overhigh discharge temperature. Compressor overheating serves as an important fault signal, indicating severe problems within the refrigeration system or improper operation and maintenance of the compressor.
If the root cause of compressor overheating lies within the refrigeration system, the problem can only be solved by optimizing system design and maintenance. Simply replacing the compressor cannot fundamentally eliminate overheating faults.
