A rotary screw compressor draws air in. The air first travels through a filter, then through an open inlet valve into the compressor element. This inlet valve is effectively a one-way valve. When the compressor is turned off, the valve closes, stopping air and oil from heading into the filter.
The air is compressed in the compressor element as described above.
In an oil-free screw compressor, the compressed air is effectively able to be moved from the compressor to the outlet. But, for an oil-injected screw compressor, oil inevitably ends up mixed in with the compressed air. Since it needs to be removed, the compressed air is passed from the compressor element into an air receiver/oil separator via a check valve. This check valve stops the air and oil from travelling backwards when the compressor is turned off.
When the rotary screw compressor is operating, a minimum pressure valve keeps the pressure in the separator tank above a minimum value. This is required for lubrication. Further downstream, another valve stops compressed air from being vented into the atmosphere when the compressor is turned off.
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