One of the most common and simplest of causes of compressors failure is due to the plant electrics not having a compressor overload fitted. When large Semi-Hermetic or Hermetic Compressors are fitted to systems where the compressor contactor has no overload fitted, the compressor is left wide open to liquid washout, Valve breakages or even full blown Smash Ups. This problem can be caused by one of the components designed to save a compressor. This component is none other than the compressor motor’s KLIXON.
Most commercial refrigeration systems use a system PUMP Down control. This system offers good protection against Liquid Migration, and to some extent Liquid Slugging. When the thermostat calls for duty the thermostat contacts are made, which puts a live connection onto the liquid line solenoid valve. The solenoid becomes energised and opens to allow liquid refrigerant to pass through the solenoid valve, expansion valve and into the evaporator.
At this point, the pressure in the low side of the system rises, which causes the Low Pressure switch to make contact, allowing the compressor starter to energise and the compressor to run. When the thermostat reaches its cut-out temperature, the stat opens its contacts. This de-energises the solenoid valve which closes and stops any more refrigerant entering the expansion valve and evaporator. The pressure in the low side of the system drops as the compressor removes the vapour from the evaporator and eventually the pressure in the compressors suction line reaches the Low Pressure CUT OUT Point. As the LP Switch opens its contacts, the compressors starter is de-energised, and the compressor STOPS.
Now there is virtually no refrigerant left in the Evaporator. Therefore, during the compressor OFF Cycle there is little if any Liquid migration taking place. When the compressor is called to start up once again, the liquid refrigerant is injected under control from the expansion valve into the evaporator and normal safe superheat control is maintained. The problem starts when the compressor starts to either draw excessive amps or starts to run hot, causing the compressor to trip on its internal KLIXON.
High discharge pressures caused by high ambient temperatures or simply a dirty condenser block, can cause these problems. Also an iced up evaporator or choked drier can cause a low suction pressure which will cause the compressor to run extremely hot with a rise in amps drawn.
Remember, the KLIXON is the ultimate safety feature and is the last line of defence.
If there is no contactor overload fitted and the compressor sees one of the above problems, then the cold room may be frozen or chilled, but it will normally be colder than the compressor. The compressor starts up via the thermostat, solenoid valve and then the Low Pressure switch. The compressor runs OK.
When the compressor is subjected to one of the above problem scenarios, where the amps drawn rises, or the compressor temperature rises, the compressor will get to a point where it will stop on its internal KLIXON, which is at the Star point of the motor winding. The KLIXON is not in the compressor’s safety circuit as it’s in the motor, hence the thermostat sill closes its contacts, the solenoid valve is still energised and open, and the Low Pressure switch is still made calling for the compressor to run. Since the compressor can’t run due to it being stopped on its internal KLIXON, the liquid refrigerant continues to pour into the cold evaporator. The refrigerant vapour slowly migrates back into the compressor slowly diluting the compressors lubricant. Depending on how hot the compressor motor has become, it can be switched off for up to 8 hours.
When the KLIXON does eventually reset, the compressor restarts and either the lubricant is so heavily diluted with refrigerant that the bearings are washed out and they seize, or the liquid simply floods back into the compressor causing suction and discharge valves to break, as well as pistons and con rods to break and most likely smash the compressor.
Preventative measures required:
If an overload had been fitted and wired through the control circuit, providing the overload had been set to the correct setting, the compressor would have been stopped, breaking the feed to the solenoid coil and forcing it to close. The compressor motor would have been saved from running in an overloaded state, which weakens the motor insulation and reduces the compressors life-time, and the solenoid valve forced to shut, preventing liquid refrigerant filling the evaporator and causing fatal damage to an innocent machine. The cold room temperature alarm would give the plant operator chance to check the plant and rectify the problem, as against having to replace a costly compressor simply because of a dirty condenser or a faulty defrost control.
Saving the cost of an OVERLOAD could cost you a new very expensive compressor.
The liquid line solenoid valve must always be closed if the compressor is stopped by one of its safety controls. The compressors KLIXON is the last line of safety, when all else fails.