Liquid knock in compressor during start-up
1 Compressor discharge valve letting by (refrigerant condenses in the discharge line to enter the compressor).
2 Low ambient temperature in condensing unit location (refrigerant condenses in the compressor).
When refrigerant vapour condenses in the discharge line, which is common in equipment located outside and during winter operation, or with low ambient temperature combined with long ‘off’ cycles, liquid refrigerant can enter the cylinders if discharge valves are not seating correctly. This condition can be attributed to poorly designed pipework (see ‘Pipework and oil traps’ in Chapter 10).
3 Suction line passing through low ambient temperature, or not insulated when installed in a low temperature coldroom (refrigerant condenses in suction line).
When condensation occurs in the suction line, refrigerant will be drawn into the cylinders when the compressor starts.
4 Suction line has a free fall to the compressor.
This result of poor pipework design can be overcome by installing an oil trap in the suction line.
5 Expansion valve bulb mounting gives poor thermal contact, or the bulb is located in a warm place.
The expansion valve thermal element will respond to temperature. If it is incorrectly located it can produce overfilling of the evaporator during operation, which may allow liquid refrigerant to enter the suction line and compressor. This is more likely to occur with short suction line runs.
Liquid knock in the compressor during operation
Evaporator pressure too low, or suction superheat too narrow Suction pressure too low, or suction superheat too wide
A low pressure reading on the compound gauge during operation does not necessarily indicate malfunction. The pressure reading must be related to the design temperature and the temperature difference recommended to the product being stored.
Single phase/three phase motor compressors and remote-drive motors
Compressor will not run
1 Fuse blown.
2 Main isolator open.
3 No supply at motor terminals.
4 Overload open circuit.
5 Control circuit open.
6 Burnt-out motor windings.
Compressor hums but does not start
1 Incorrect wiring to motor winding (3 ph).
2 Low line voltage.
3 Start capacitor open circuit.
4 Relay inoperative.
5 Motor winding open circuit.
6 Seized compressor.
7 Piston or impellor jammed by broken component.
Compressor does not attain design speed
1 Incorrectly wired motor winding (3 ph).
2 Low line voltage (3 ph).
3 Defective relay.
4 Start capacitor defective.
5 High discharge pressure.
6 Star/delta motor compressor not unloaded (3 ph).
Compressor short cycles
1 Control differential too narrow.
2 Valve plate letting by.
3 Motor overloading.
4 Shortage of refrigerant.
5 Expansion valve defective.
6 Restriction in system.
7 High pressure cut-out set too low (switch operating too soon).