Before an engineer can repair a refrigerant leak in the system it has to be located and for this reason leak detection is the first step. If the system has not lost all its refrigerant charge, leak detection may proceed as described in Chapter 3 provided that there is an adequate test pressure within the system. When the entire refrigerant charge has been lost, the system has to be pressurized.
If the leak is found to be minor and on the low side of the system, ‘pumping down’ will be necessary before any attempt is made to repair the leak. Leakage from the high side of the system will require the removal of all the refrigerant before repairs are undertaken.
Pumping down
This procedure transfers the refrigerant circulating around the system from the liquid shut-off valve at the liquid receiver to the inlet of the compressor for storage in the condenser and the receiver, thereby making it possible for other tasks to be performed.
The method is as follows:
1 Fit gauges, set the service valves to operating positions and operate the plant.
2 Close the liquid shut-off valve at the receiver.
3 Allow the compressor to operate until a pressure slightly above atmospheric pressure is registered on the compound gauge (3 psi or 0.2 bar). If the system employs a low pressure cut-out switch then the range will need to be altered; note the cut-out pressure beforehand. Modern switches have a small lever which can be moved to override the cut-out point, thus keeping the compressor running.
4 Stop the plant and front seat the suction service valve. The system can now be worked on for a number of repairs.
When it is required to close down a plant for an extended period, the discharge service valve should also be front seated after the gauges have been removed.
Adding refrigerant
When it is necessary to add refrigerant to a system after a loss it indicates that there is a leak in the system. The leak must be located, repaired and the system leak tested. If the leak is found to be on the low side of the system the repair can be made after the system has been ‘pumped down’ to a balance in pressure. Do not draw in any air which contains moisture, especially where hygroscopic ester oils are concerned, (for example R134a). Some oils are not affected and accept the ingress of air and moisture, recovering after a vacuum is drawn with a recovery unit.