Analysis of Working Principle of Oil Circuit System of Screw Air Compressors
The oil circuit system of a screw air compressor consists of an oil tank, oil cooler, oil filter, oil stop valve, thermostatic valve and other components.
The lower volume of the air-oil separator serves as the oil tank, equipped with an oil filling port, oil drain plug and oil level gauge.
Conventional screw air compressors are not fitted with an oil pump. Lubricating oil circulation is realized by the pressure difference between the upstream side of the filter element and the oil injection port of the air end. When the compressor operates, pressure is built up inside the air-oil separator under the action of the minimum pressure valve. This pressure forces lubricating oil to flow through the oil cooler, then the oil filter and the oil stop valve, and feed oil to the upper and lower oil injection holes of the air end. The oil removes heat generated during air compression, lubricates and seals the working chamber of the air end, and reduces internal leakage.
After the atomized oil injected into the compressor mixes with air and undergoes compression, the mixture flows back into the air-oil separator via the exhaust check valve.
1 Oil Cooler
Oil coolers adopt the same cooling modes as air coolers: air cooling and water cooling. In poor ambient conditions, the fins of air-cooled coolers are easily covered by dust, impairing cooling performance. In severe cases, excessive air-oil temperature triggers automatic shutdown. Therefore, accumulated dust on fin surfaces shall be blown off regularly with low-pressure air. If blowing fails to remove contaminants, solvent cleaning is required to keep the heat dissipation surfaces of the cooler clean.
If the tubes of a water-cooled cooler become clogged, they must be soaked in solvent, and internal scale deposits shall be mechanically removed to achieve thorough cleaning.
2 Oil Filter
The oil filter fitted with a differential pressure transmitter removes impurities from oil to maintain lubricating oil cleanliness and protect the operation of the compressor air end. A clogged filter leads to insufficient oil supply to the air end and rising air-oil temperature, shortening the service life of all moving parts inside the air end.
When the oil filter is blocked, the differential pressure transmitter sends a signal and the indicator light turns on. The unit shall be shut down promptly for inspection or replacement. The decision to replace the filter element depends on actual operating conditions.
3 Oil Stop Valve
The oil stop valve is mainly composed of a valve body, valve spool, floating plug, spring and other parts. It is one of the critical components of the compressor. Working principle: Immediately after startup, the high-pressure chamber of the air end supplies air to the end of the oil stop valve. The piston overcomes spring force to push the floating plug and open the valve spool, enabling oil supply.
The oil stop valve remains open during unit operation. After unit shutdown, the valve shall close in a timely manner to prevent oil from flooding into the air end.
4 Thermostatic Valve
Water-cooled compressors are all equipped with a thermostatic valve at the oil inlet of the cooler. It controls the bypass flow of lubricating oil passing through the cooler and ensures that the air-oil temperature of the loaded compressor is higher than the pressure dew point. An excessively low oil injection temperature will reduce the air-oil temperature inside the air end, causing condensed water to separate out inside the air-oil separator and cooler. Such condensed water cannot be easily carried away by the gas circuit system, which degrades lubricating oil quality and shortens its service life.
Working principle of the thermostatic valve:
At the initial operation stage, lubricating oil temperature is low. The oil flows through the bypass passage, directly passing through the oil filter and oil stop valve into the air end.
When the air-oil temperature rises to 71°C, the temperature sensing element inside the thermostatic valve extends and pushes the spool to move inside the valve body. The bypass passage gradually throttles down, and the passage leading to the oil cooler opens progressively. The flow area ratio of the two passages is determined by the air-oil temperature.
When the air-oil temperature reaches 75°C, the bypass port is fully closed, and all lubricating oil flows to the oil cooler for cooling circulation.
The lower volume of the air-oil separator serves as the oil tank, equipped with an oil filling port, oil drain plug and oil level gauge.
Conventional screw air compressors are not fitted with an oil pump. Lubricating oil circulation is realized by the pressure difference between the upstream side of the filter element and the oil injection port of the air end. When the compressor operates, pressure is built up inside the air-oil separator under the action of the minimum pressure valve. This pressure forces lubricating oil to flow through the oil cooler, then the oil filter and the oil stop valve, and feed oil to the upper and lower oil injection holes of the air end. The oil removes heat generated during air compression, lubricates and seals the working chamber of the air end, and reduces internal leakage.
After the atomized oil injected into the compressor mixes with air and undergoes compression, the mixture flows back into the air-oil separator via the exhaust check valve.
1 Oil Cooler
Oil coolers adopt the same cooling modes as air coolers: air cooling and water cooling. In poor ambient conditions, the fins of air-cooled coolers are easily covered by dust, impairing cooling performance. In severe cases, excessive air-oil temperature triggers automatic shutdown. Therefore, accumulated dust on fin surfaces shall be blown off regularly with low-pressure air. If blowing fails to remove contaminants, solvent cleaning is required to keep the heat dissipation surfaces of the cooler clean.
If the tubes of a water-cooled cooler become clogged, they must be soaked in solvent, and internal scale deposits shall be mechanically removed to achieve thorough cleaning.
2 Oil Filter
The oil filter fitted with a differential pressure transmitter removes impurities from oil to maintain lubricating oil cleanliness and protect the operation of the compressor air end. A clogged filter leads to insufficient oil supply to the air end and rising air-oil temperature, shortening the service life of all moving parts inside the air end.
When the oil filter is blocked, the differential pressure transmitter sends a signal and the indicator light turns on. The unit shall be shut down promptly for inspection or replacement. The decision to replace the filter element depends on actual operating conditions.
3 Oil Stop Valve
The oil stop valve is mainly composed of a valve body, valve spool, floating plug, spring and other parts. It is one of the critical components of the compressor. Working principle: Immediately after startup, the high-pressure chamber of the air end supplies air to the end of the oil stop valve. The piston overcomes spring force to push the floating plug and open the valve spool, enabling oil supply.
The oil stop valve remains open during unit operation. After unit shutdown, the valve shall close in a timely manner to prevent oil from flooding into the air end.
4 Thermostatic Valve
Water-cooled compressors are all equipped with a thermostatic valve at the oil inlet of the cooler. It controls the bypass flow of lubricating oil passing through the cooler and ensures that the air-oil temperature of the loaded compressor is higher than the pressure dew point. An excessively low oil injection temperature will reduce the air-oil temperature inside the air end, causing condensed water to separate out inside the air-oil separator and cooler. Such condensed water cannot be easily carried away by the gas circuit system, which degrades lubricating oil quality and shortens its service life.
Working principle of the thermostatic valve:
At the initial operation stage, lubricating oil temperature is low. The oil flows through the bypass passage, directly passing through the oil filter and oil stop valve into the air end.
When the air-oil temperature rises to 71°C, the temperature sensing element inside the thermostatic valve extends and pushes the spool to move inside the valve body. The bypass passage gradually throttles down, and the passage leading to the oil cooler opens progressively. The flow area ratio of the two passages is determined by the air-oil temperature.
When the air-oil temperature reaches 75°C, the bypass port is fully closed, and all lubricating oil flows to the oil cooler for cooling circulation.
