The oil circuit system of a screw air compressor includes an oil reservoir (oil tank), oil cooler, oil filter, shut-off valve, and thermal control valve.
The lower volume of the oil-gas separator acts as the oil tank, and is equipped with an oil filler hole, an oil drain plug, and an oil level gauge.
Conventional screw air compressors are not equipped with a hydraulic pump. The circulation of lubricating oil is driven by the pressure difference between the upstream side of the filter element and the oil injection port of the main unit (air end). When the compressor is running, the gas inside the oil-gas separator first builds up pressure under the action of the minimum pressure valve. This pressure forces the lubricating oil to pass through the oil cooler, and then enter the shut-off valve via the oil filter, supplying oil to the upper and lower oil injection ports of the main unit. This takes away the heat generated during the air compression process, while lubricating and sealing the working chamber of the main unit to reduce internal leakage.
The atomized oil injected into the compressor mixes with the air and is compressed, then re-enters the oil-gas separator through the exhaust check valve.
1. Oil Cooler The cooling method of the oil cooler is identical to that of the air cooler, categorized into air-cooled and water-cooled types. If environmental conditions are poor, the fins of the air-cooled cooler are prone to dust accumulation, which impairs the cooling effect and can severely lead to automatic shutdown caused by excessively high oil and gas temperatures. Therefore, low-pressure air should be used regularly to blow away dust accumulated on the fin surfaces. If it cannot be cleaned by blowing, solvent must be used for washing to ensure the heat dissipation surface of the cooler remains clean. When the tubes of a water-cooled cooler are clogged, they must be soaked in solvent, and the scale blocked inside the tubes must be mechanically removed to ensure thorough cleaning.
2. Oil Filter Equipped with a differential pressure transmitter, the function of the oil filter is to remove impurities from the oil to keep the lubricating oil clean, thereby protecting the operation of the screw compressor's main unit. If the filter is clogged, it will lead to insufficient oil supply to the main unit, causing the oil-gas temperature to rise and thus affecting the service life of various moving parts in the main unit. When the oil filter is clogged, the differential pressure transmitter gives an indication and the signal light turns on; the machine should be shut down promptly for inspection or replacement. Whether to replace the filter element should be determined based on actual conditions.
3. Shut-off Valve The shut-off valve is mainly composed of a valve body, valve spool, floating plug, spring, and other components. It is one of the important components in a compressor. Its operating principle is: the moment the machine starts, the high-pressure chamber of the main unit supplies air to the end of the shut-off valve. The piston overcomes the spring pressure to push open the floating plug, which in turn opens the shut-off valve spool to start oil supply. While the unit is running, the shut-off valve remains open; after the unit shuts down, the shut-off valve should close promptly to prevent oil from backflowing into the main unit.
4. Thermal Control Valve (Thermostatic Valve) Water-cooled compressors are equipped with a thermal control valve at the oil inlet of the cooler. Its function is to control the bypass flow of lubricating oil passing through the cooler, ensuring that the oil-gas temperature during load operation remains higher than the pressure dew point temperature. This is because a lower oil injection temperature would cause the oil-gas temperature in the main unit to be too low, leading to the condensation of water in the oil-gas separator and cooler which is difficult to be carried away by the air circuit system. This would otherwise degrade the quality of the lubricating oil and shorten its service life. The operating principle of the thermal control valve is as follows: Initially, when the lubricating oil temperature is low, the lubricating oil bypasses the cooler and flows directly into the main unit via the oil filter and shut-off valve. When the oil-gas temperature rises to 71°C, the temperature-sensing element inside the thermal control valve expands, pushing the valve spool to move within the valve body. It begins to partially close the bypass channel while gradually opening the channel leading to the oil cooler. The ratio of the flow areas of the two channels is determined by the oil-gas temperature. When the oil-gas temperature rises to 75°C, the bypass port closes, and all the lubricating oil flows to the oil cooler for cooling circulation.
