Fault finding guide for vapour compression systems:Visual fault finding

This checklist will assist in the quick diagnosis of faults on vapour compression systems.

Visual fault finding
Compressor not running

1 Main supply isolator open.

2 Fuse blown.

3 Overload open circuit.

4 Control circuit open (low pressure switch, high pressure switch or thermostat).

Fixture temperature too high

1 High evaporator load.

2 Defrost heater(s) energized.

3 Condenser blocked with dirt.

4 Restricted air flow over the condenser or restricted water flow.

5 Condenser fan inoperative.

6 Evaporator blocked with ice.

7 Evaporator fan inoperative.

8 Vapour bubbles in sight glass (possible shortage of refrigerant).

9 Frosting liquid line (blocked drier).

10 Broken or loose drive belts.

11 Expansion valve bulb loose on suction line or broken capillary.

System noisy

1 Evaporator/condenser fan blades touching fan guards or loose on the motor shaft.

2 Compressor oil sight glass empty (shortage of oil or oil entrainment).

3 Compressor head frosted (compressor pumping liquid refrigerant).

4 Loose motor pulley, compressor flywheel, compressor mountings.

Pressures

If the fault is not obvious after visual inspection:

1 Attach compound and pressure gauges.

2 Check that the compressor is pumping.

3 Observe operating pressures.

High discharge pressure

1 Overcharge of refrigerant.

2 Air in the system.

3 Dirty condenser, poor air supply.

4 Inadequate water flow.

5 High load imposed on the evaporator.

Low discharge pressure

1 Shortage of refrigerant.

2 Compressor inefficient.

High suction pressure

1 Compressor inefficient.

2 Overcharge of refrigerant (capillary systems only).

3 Expansion valve defective.

4 High evaporator load.

5 Defrost system operating when compressor is running.

Low suction pressure

1 Shortage of refrigerant.

2 Blocked or defective expansion valve.

3 Blockage in liquid line, drier solenoid valve or shut-off valve.

4 Blocked evaporator (excessive ice build-up).

5 Inoperative evaporator fan.

6 Defective water pump or blocked water filter on a chiller system.

Note: a temperature difference across any component in the liquid line is an indication of a partial restriction.

Advanced diagnosis
Expansion valve capacity too small

1 No subcooling of the refrigerant liquid.

2 The pressure drop across the expansion valve is less than that for which it was dimensioned.

3 Incorrect expansion valve bulb location (too cold or being cooled).

4 Large pressure drop across the evaporator.

5 Expansion valve blocked by ice or foreign objects.

6 Incorrect expansion valve external equalizer location.

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