Oil-filled rotary screw air compressors are considered positive-displacement compression units. They comprise a pair of identical helical screws with lubricant filled in the gap to provide an air-tight hydraulic seal and facilitate motion transfer between the two. As the screws rotate and mesh with each other, the air in the threads is forced through the compressor and delivered to the application.
How the System Works
During the compression stage, oil is injected into the compressor to lubricate the rotors, gears, and bearings, cool the compressed air to approximately 40 degrees Celsius above ambient temperature, and assist in sealing clearances in the screw rotor housing.
As the lubricant is conveyed through the compressor, it mixes with the air and gets discharged in an oil/air separator. Here, most of the oil is eliminated due to change in velocity and impingement.
The air then passes through an air/oil separator located at the top of the sump to get rid of the remaining oil. The separated oil drops to the bottom and is carried back to the system via an oil-return-line.
Standard units come with two oil filters:
- The primary filter – All oil must pass through it
- The secondary filter – Used to remove impurities from gear and bearing oil
For oil-flooded air compressors, ensuring that there is sufficient, high quality oil in the system is critical for proper function. Most manufacturers recommend the use of lubricants with ISO viscosity grades 32, 46, or 68.
You may also consider using synthetic lubricants that provide longer oil-drain intervals of up to 8,000 hours, compared to the recommended 1,000-hour change intervals for mineral oil-based compressor oils for comprehensive maintenance programs.
If you choose synthetic lubricant, make sure that you also change the oil filters and air/oil coalescer elements at the recommended change intervals for those lubricants and for your specific equipment.
When changing the oil, keep in mind that not all lubricants that are applicable to oil-flooded rotary screw units are compatible. Check the lubricant compatibility, and if you plan on changing the lubricant, make sure you flush the system first to avoid any problems.
For professional routine maintenance and repair work, consider working with a professional distributor and service company