Oil-flooded rotary compressors

By 22 May 2018

Rotary compressors require lubrication of the beatings, the step-up gears and the timing gears where fitted (although in most rotary screw compressors, the screws drive each other and no timing gears are necessary). The oil-flooded types use the same oil for both lubrication and cooling injected into the compression chamber. Usually the oil is circulated by an externally mounted gear pump, drawing oil from the oil separator. The oil in the separator is at supply pressure, which is high enough in itself to circulate the oil without a pump. Some manufacturers dispense with a pump and rely solely on pressure circulation. This is bound to be a compromise for two reasons: at start-up, when the pressure is low the circulation flow is lower than desirable; and at varying speeds the oil flow is constant rather than proportional to speed. For mobile units this seems to work well, but industrial units usually include a pump. Where the pump is driven off the main drive, circulation stops when the rotors stop. But where the circulation is done by pressure alone, oil flow would persist as long as air pressure was present in the reclaimer, with the result that oil would be discharged back into the compression space. This is clearly undesirable so this kind of pumpless circulation has to have a non-return oil-stop valve to prevent it; such a valve is pilot operated from a pressure signal in the delivery casing.

Figure 3 shows a typical oil circulation system

The heat of compression, absorbed by the oil in the compression chamber has to be removed by an oil cooler, usually of an air-blast type. The same cooling fan draws air through the oil cooler and the air aftercooler (where necessary).

The oil separator is one of the critical items in an oil flooded type of compressor. Its operation is shown in Figure 4. The method of separation is partly through mechanical impingement and partly through agglomeration in the filter pack.