Air in the system may be caused by one of two reasons:
By carelessness in servicing or charging methods permitting air to enter in the system.
By a leak in the lowside when all refrigerant leaks out allowing air to be drawn into the system when operating
under a vacuum.
Air is non-condensable under the temperature and pressure conditions existing in refrigerating compressors. Therefore,
the air becomes an inert substance which remains in the compressor head and condenser, resulting in: (a) increased
discharge pressures and (b) reduced condenser efficiency.
If sufficient air is present, the discharge pressure will become so high that the compressor operation will be overworked
forcing the motor overload device to cut out.
To distinguish whether air in system or an overcharge of the refrigerant is causing the trouble it will be relatively easy to
determine this by shutting off the unit and allowing it to cool to room temperature. In the case of a refrigerant overcharge,
the discharge pressure will return to the normal figure which corresponds the pressure, that is normal for the existing room
temperature. However, if air is present, it will be seen that the head pressure does not return to the normal figure when
the unit has cooled down to the existing room temperature.
An excess of refrigerant or the presence of air in the system will necessitate a purging operation. This is accomplished as
follows: Disconnect the pressure gauge at discharge service valve, and by opening the discharge valve so that air is
allowed to escape slowly,. the service man at this discretion can purge the required amount. This is entirely a matter of
judgement and experience.