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A compressor in a refrigeration or air conditioning plant has to be provided with some safeties to protect it from operational faults. The three common safeties provided are the high pressure trip, the low pressure trip, and the low oil pressure trip among the others. A compressor has to be protected against high pressure that can cause structural failure therefore a high pressure cut out is provided, similarly any deficiency in the oil pressure can damage the bearings and a low oil pressure cut out has to be provided, a lower atmospheric in the pipe line can cause air ingress and therefore must be avoided. In this article we discuss the different safeties one by one.
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High Pressure Cut Out
High pressure can be caused in a refrigeration plant due to various causes like over charge, loss of cooling water, high ambient temperature, air, or other incompressible gases in the system, and obstruction in the discharge line of the compressor. For protecting the compressor from high pressure and subsequent failure, a high pressure cut out is provided that take a pressure tapping from the discharge line and when it detects an over pressure, it stops the compressor. The HP cut out is not resettable automatically but has to be reset manually by the operator. This is because the high pressure is a serious fault and the cause must be investigated and corrected before the plant is started again.
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Operation of a High Pressure Cut Out
The high pressure cut out as shown in the diagram is of a simple construction. It has a bellows that is set against a spring. The nut at the end of the spring is used to adjust the cut out pressure. When the high pressure gas enters the bellow, the bellow expands and presses the spring. At the cut off pressure the movement of the bellow against the spring releases the catch and the contact is broken and the compressor cuts off.
The switch arm can be pressed and the cut out reset after the cause of the over pressure has been found and rectified.
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Low Pressure Cut Out
To protect the compressor against low pressure in the system and to avoid the ingress of air into the system if a vacuum is generated in the lines a low pressure cut out is provided. Also when the refrigerated compartments are cut off by the solenoids and there is no return gas, the low pressure cut out is activated. When the solenoid of the refrigerated compartments open, the return gas comes in the inlet of the compressor and the suction pressure rises, and then the low pressure switch cuts in the compressor.
Unlike the high pressure cut out, the low pressure cut out is self-resettable and does not need to be reset manually.
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Low Oil Pressure Cut Out
The oil is pumped under pressure by an attached oil pump that supplies oil to the bearings for lubrication. Any problem in the lube oil pressure can jeopardize the bearings and therefore a tapping is taken from the pump outlet and fed to the oil pressure switch. Any fall in the pressure will activate the cut out which will stop the compressor.
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As oil is miscible with the refrigerant and often goes out of the compressor with it, it can go to the evaporator where it can cause a decrease in heat transfer. To avoid the oil from going to the evaporator where it can form a layer inside or cause obstruction an oil separator is used. It basically consists of baffle plates that separate the oil from the refrigerant and feed it back to the compressor. A float valve is provided so that short circuiting of the refrigerant should not take place.
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The refrigeration plant compressor has to be protected against unnatural working conditions by safety devices and controls. The high pressure cut out, the low pressure cut out, and the low oil pressure cut out are some of the basic protective devices provided. In large complex circuits other additional safety devices are provided according to the complexity of the circuit.