The air we breathe is composed primarily of nitrogen and oxygen. Both elements remain in gaseous state at the temperatures and pressures found in commercial refrigeration systems.
Since the air is not condensable, it will normally be trapped in the upper part of the condenser and liquid tank. During compressor operation, the discharge pressure will be a combination of the condensing pressure of the refrigerant plus the pressure exerted by nitrogen and oxygen. When a condensation pressure rise occurs in a system, the first suspicion is of air in the system.
Nitrogen is an inert gas and does not readily produce chemical reactions. Oxygen is just the opposite, and it may, at the least opportunity, be combined with other elements. Rust, corrosion and burning are common processes of oxidation. In the refrigeration system, moisture and oxygen can attack refrigerant and oil, causing corrosion, copper deposition, acid formation, sedimentation, and other harmful reactions. The tests performed have shown that in the presence of heat the air-moisture combination is more apt to cause decomposition of the refrigerant and oil mixture than large amounts of moisture alone.
Since it is not moisture but air, there is no filter that can eliminate this residual air to a level small enough to ensure perfect operation of the system. Evacuation of the system is the only safe and effective way to remove air from the system.
