Oil-lubricated compressors inject oil for lubrication and cooling, while oil-free compressors use timing gears and require external lubrication for those gears.
Two types of rotary screw compressors
There are two main types of screw compressors:
- Oil-lubricated compressors
- Dry (oil-free) compressors
We offer only oil-lubricated compressors, but for educational purposes, we’ll also explain oil-free screw compressors.
Oil-free screw compressors
These compressors operate without lubrication between the screws (hence their name ‘oil-free’). The compression action takes place in either one or two stages, each housed separately. This type of compressor requires external synchronization gears, which must be continuously lubricated to ensure proper rotor operation.
Oil-lubricated screw compressors
These compressors use oil as a lubricant between rotating gears and don’t require any external timing gear since oil is injected between the screws. They are more efficient and widely used in industrial applications.
Working principle of a rotary screw compressor
Oil-lubricated compressors have two separate circuits, air circuit and oil circuit.
Air circuit explained
Stage 1 - Air intake: Atmospheric air enters through the filter and inlet valve.
Stage 2 - Compression: Air is drawn into the compressor where compression begins.
Stage 3 - Mixing stage: Compressed air mixes with oil and flows into the oil separator and air receiver via a check valve.
Stage 4 - Air outlet: Air exits through minimum pressure valve (maintains lubrication pressure) and air cooler (reduces temperature)
Key Components:
- Minimum pressure valve: Ensures separator tank pressure stays above lubrication level.
- Check valve: Prevents compressed air from escaping.
- Inlet valve closure: Stops air from flowing back into the filter when the compressor halts.
Oil circuit explained
Stage 1 - Oil injection: Oil is injected into valves for lubrication.
Stage 2 - Oil separation: Most oil is removed in the air receiver/oil separator using centrifugal action.
Stage 3 - Oil collection: Oil gathers in a receptacle within the separator.
Stage 4 - Bypass valve: Stops oil cooling when temperature is below set point.
Stage 5- Oil flow:
- Oil moves through the oil filter and stops valve under air pressure.
- If the temperature exceeds the set point, oil flows into the cooler.
- Stop Valve prevents flooding when the compressor stops.
FAQs
It ensures proper lubrication by maintaining pressure in the separator tank during operation.
Through centrifugal action, oil particles are separated from compressed air in the oil separator.
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