Non-condensable gases
Also known as: non-condensables, air in the system
Short definitionAir or nitrogen left in the circuit that does not liquefy in the condenser. It takes up condenser surface and raises discharge pressure and power consumption; it is usually left over from poor evacuation.
Refrigerant turns to liquid in the condenser, while air and nitrogen stay as gas. They are carried to the condenser by the flow and collect there because they cannot get past the liquid at the outlet. They block part of the tubes and shrink the condensing surface, and they add their own pressure to the total. The gauge shows a value above the saturation pressure of the refrigerant.
The source is almost always installation workmanship: a short vacuum, test nitrogen not fully released, charging through hoses that were not purged. Each 1 K rise in condensing temperature increases compressor consumption by 2–3%. To check, the system is stopped, the fans bring the condenser to ambient temperature and the pressure is compared with the saturation value for that temperature. If they differ, the refrigerant is recovered and the circuit is evacuated below 500 microns and recharged.