Air doesn’t belong in a refrigeration system, and if it gets in, it will cause problems. Air can enter a refrigeration system in many ways, including through tube, gasket, or flange leaks; poor charging procedures; poor recovery or recycling procedures; or by forgetting to purge hoses when accessing systems.
If air gets into a system, it will collect in the top of the condenser and be trapped. Air is a non-condensable and cannot be condensed like refrigerant vapors. The liquid seal (subcooled liquid) at the bottom of the condenser will prevent air from leaving the condenser. Air will cause a reduction of condensing surface area and cause high condensing (head) pressures.