Refrigerant condenses in the condenser and becomes liquid; air and nitrogen, however, remain gas at this pressure and temperature. Once they have entered the circuit they are carried to the condenser with the flow and are trapped there, because they cannot pass the liquid seal at the outlet. They occupy part of the condenser tubes and reduce the condensing surface. What is more, they add their own pressure to the total pressure: the gauge shows a value higher than the saturation pressure of the refrigerant. As a result the condenser behaves like a smaller condenser than it really is.
The source is almost always workmanship: inadequate evacuation or none at all, the test nitrogen not being fully released, charging with manifold hoses that have not been purged of air, air being drawn in through a leak on a low-pressure side running below atmospheric pressure. The result is measurable: every 1 K rise in condensing temperature increases compressor consumption by 2–3%. Air can lead to several bar of excess discharge pressure; on a summer day this difference is enough to trip the high-pressure switch.
Diagnosis is a simple comparison. The system is stopped and the condenser fans are run until the condenser has come to ambient temperature. Then the discharge side pressure is read and compared with the saturation pressure at ambient temperature. Example: if the ambient is 30 °C and the refrigerant is R404A, the expected value is about 13.2 bar g; if the gauge shows 14.5 bar g the difference is non-condensable gas. For zeotropic refrigerants the bubble column is taken as the basis. You can read the saturation values from our refrigerant pressure–temperature chart. One must be sure of the accuracy of the gauge and the thermometer.
The symptoms seen while running can be confused with other faults: high discharge pressure, high discharge temperature, high subcooling, a flickering gauge needle. A dirty condenser and overcharge give a similar picture; that is why no decision is made without carrying out the standstill test. In small and medium systems the correct solution is to recover the refrigerant, evacuate the circuit to below 500 microns and recharge. In large ammonia plants there are automatic air purgers for this job.