Condenser subcooling ensures that there is a liquid seal at the condenser’s bottom so the liquid line or receiver will not be fed with vapors. This condition prevents any noncondensables, like refrigerant vapor or air, from leaving the condenser’s bottom and entering the receiver or liquid line. On systems without receivers, condenser subcooling can be an indicator of how much refrigerant charge is in a system. See the manufacturer’s specifications for condenser subcooling amounts when charging a system using these criteria.
Another important reason why condenser subcooling must be present is to prevent liquid line flash gas (vapor bubbles) from forming in the liquid line. Liquid line flash gas is primarily caused from pressure drop in the liquid line. Pressure drop in the liquid line can be caused from any restrictive device, including filter dryers, sight glasses, solenoid valves, shutoff valves, kinked or undersized liquid lines, large liquid line runs, and large vertical lift in the liquid line.