All compressors except the smallest have mechanical lubrication and will fail if the oil pressure falls because of a pump fault or oil shortage. Where an oil pump is used a cut-out, which will stop the compressor in the event of inadequate




Figure 9.4 The pump down circuit — the solenoid valve is thermostat controlled and compressor is low pressure controlled

Oil pressure, is provided. This takes the form of a differential pressure switch with a starting time delay.

Since the oil pump inlet is at sump (suction) pressure, a pressure gauge on the pump discharge will indicate the total pressure at that point above atmos­pheric, i. e. suction (gauge) plus pump head. Any detection element for true oil pump pressure must sense both suction and pump outlet pressures and trans­duce the difference. Oil pressure cut-outs have connections to both sides of the oil pump and two internal bellows are opposed to measure the difference.

Since there will be no oil pressure at the moment of starting, a time delay must be fitted to allow the oil pressure to build up. This timer may be thermal, mechanical or electrical.

Operation of the oil cut-out indicates an unsafe condition and such controls are made with hand reset switches. Contacts on the switch can be used to oper­ate an alarm to warn of the malfunction.

Several compressor manufacturers offer electronic oil protection systems which provide more functionality, whilst retaining the option of hand re-set.

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