1. The oil temperature of the screw refrigeration unit is too high
First of all, it is necessary to clarify the normal oil temperature range of the screw refrigeration unit. According to industry specifications and equipment operation standards, when the screw refrigeration unit is running, the normal operating temperature of the lubricating oil is usually controlled between 40℃~65℃. If the oil temperature is high during operation, the unit will trigger the protection mechanism.
Lubricating oil plays three core roles in screw refrigeration units:
Lubrication (reduce friction in screw rotors, bearings and other moving parts);
cooling (taking away the heat generated during compression);
Sealing (filling the rotor gap to prevent refrigerant leakage);
2. The core hazards of excessive oil temperature
Lubricating oil performance declines sharply
If the oil temperature continues to exceed 65°C, the lubricating oil indicators will deteriorate rapidly: the viscosity will drop significantly, and a stable oil film cannot be formed, resulting in dry friction and wear. Research shows that if the lubricating oil is operated above 70°C for a long time, the service life will be shortened by more than 50%; if the oil temperature continues to exceed 80°C, it will quickly carbonize and lose its function, becoming a "fuse" for unit failure.
Oil temperature that is too high will increase wear
Screw rotor: small gap, isolated by oil film. If the oil temperature is too high, the oil film will fail, causing dry friction on the rotor, causing wear and scratches. In severe cases, it may become stuck, causing the unit to be shut down for overhaul;
Bearing: bears the rotor force and relies on lubricating oil for lubrication and cooling. Excessive oil temperature leads to insufficient lubrication, and the bearings overheat, wear, and lock. This caused the inner ring of the angular contact bearing of a cold storage unit to break and the main shaft to jam;
Shaft seal: seals refrigerant and lubricating oil. If the oil temperature is too high, it will accelerate its aging and deformation, leading to seal failure and exacerbating the increase in oil temperature and unit failure.
Efficiency dropped significantly and energy consumption increased significantly
The essence of refrigeration of screw refrigeration units is the cycle of "compressing refrigerant → cooling refrigerant → evaporating and absorbing heat". The cooling effect of lubricating oil affects the compression efficiency. When the oil temperature is too high, on the one hand, the cooling capacity of the lubricating oil decreases and the exhaust temperature increases. If it exceeds a certain value, the cooling capacity will be reduced; on the other hand, the failure of the oil film will increase the friction resistance, increase the motor load, and increase power consumption.
3. Common causes of excessive oil temperature
The reasons for the high oil temperature of screw refrigeration units are complex and not caused by a single factor. Based on the practical experience of front-line engineers and equipment failure cases, the explanation is as follows:
Oil cooler failure
The oil cooler is responsible for cooling the high-temperature lubricating oil separated by the oil separator and then circulating it back to the unit. Common faults include pipeline scaling, blockage, and reduced heat exchange efficiency.
Cooling water (or wind) system abnormality
If the cooling water temperature is too high (exceeding 32℃) and the flow rate is insufficient, the cooling effect of the oil cooler will be reduced. Excessive cooling water temperature may be caused by poor cooling tower heat dissipation or fan failure; insufficient flow may be caused by insufficient pressure in the water supply system, Y-type filter blockage, or the cooling water valve is not fully opened. This will cause the oil temperature to rise simultaneously with the cooling water temperature, resulting in the phenomenon of "high water temperature, high oil temperature" phenomenon.
Insufficient lubricating oil
Insufficient addition of lubricating oil to the unit, leakage, or oil separator failure leading to lubricating oil loss will result in insufficient lubricating oil circulation, unable to fully cover moving parts, and unable to effectively take away heat, resulting in increased local friction and rapid rise in oil temperature.
Deterioration of lubricating oil quality
After long-term use, lubricating oil will deteriorate due to oxidation and contamination (mixed with refrigerant, moisture, impurities). The viscosity of deteriorated lubricating oil decreases, the heat dissipation ability becomes poor, and sludge and carbon deposits are generated, which clog pipelines and oil filters, further aggravating the increase in oil temperature.
Unit operating load problem
If the screw refrigeration unit is operating at full load or overload for a long time, or the refrigeration demand suddenly increases, the compressor will continue to operate at high speed, the compression ratio will increase, and the heat generated will increase significantly, exceeding the heat dissipation capacity of the lubricating oil and cooling system, causing the oil temperature to rise.
Insufficient or excessive refrigerant
Insufficient refrigerant will cause abnormal high and low pressure differences, which will affect the evaporator liquid level and oil tank oil level, reduce the oil supply, and cause difficulty in oil return. The oil film strength will decrease, the rotor dry friction will increase, and a large amount of heat will be generated, causing the oil temperature to rise. If the refrigerant is excessive, the compressor exhaust pressure will increase, and the heat generated during the compression process will increase. At the same time, the excess refrigerant will be mixed into the lubricating oil, reducing the lubrication and cooling performance of the lubricating oil, and indirectly causing the oil temperature to rise.
Inspiratory superheat is too high
The suction superheat refers to the difference between the actual suction temperature and the saturation temperature of the refrigerant. If the suction superheat exceeds 15°C, the temperature of the gas sucked by the compressor will be too high, and the heat generated during the compression process will further increase, thereby driving the oil temperature to rise. This situation is mostly caused by frost on the evaporator, insufficient heat exchange area, or improper adjustment of the throttling mechanism.
Bearing wear is serious
After the bearings are worn, the clearance will increase, and the rotor will shake and rub during operation, generating a large amount of extra heat. At the same time, the bearings are poorly lubricated, which will further increase the oil temperature. In severe cases, the bearings will lock up, causing the unit to shut down.
Operating environment issues
If the screw refrigeration unit is installed in a closed machine room with poor ventilation (ventilation volume <3 times/hour), or other heat sources in the machine room, the ambient temperature around the unit will be too high (exceeding 40℃), the heat dissipation efficiency will decrease, and the oil temperature will rise. In addition, the air inlet of the unit is blocked and the air circulation is blocked, which will also affect the heat dissipation inside the equipment and cause the oil temperature to be too high.
Summary: The oil temperature of the screw refrigeration unit is too high, which is essentially an imbalance phenomenon in which "heat generation is greater than heat dissipation." Common causes include cooling systems, lubricating oil systems, operating conditions, mechanical components, and environmental operations 5. Among them, oil cooler failure, insufficient or deteriorated lubricating oil, and excessive unit load are the most important causes.
(Source: Refrigeration Encyclopedia)