Atmospheric air will enter a refrigeration system any time it is opened for service or any time a new system is being assembled. Atmospheric air contains nitrogen, oxygen, and water vapor, all of which are harmful to a refrigeration system.
Nitrogen is a noncondensable. It will not condense in the condenser, but it will occupy space that is needed to condense refrigerant. This will cause an increase in the high-side pressure, resulting in higher discharge temperatures and higher compression ratios at the compressor. The oxygen and water vapor can cause a chemical reaction that will produce acids in the system. These acids cause deterioration of the system’s parts, copper plating, and a breakdown of the motor’s insulation, leading to premature compressor failure.