The Importance of Deep Vacuum

The purpose of a vacuum pump is to remove air and moisture from an A/C system. 

Moisture in a refrigerant system, directly or indirectly, is the cause of most problems and complaints.  First, moisture can freeze-up in an A/C system.  Moisture is picked up by the refrigerant and transported through the refrigerant line as a fine mist with ice crystals forming at the point of expansion.

"Freeze-up" is not the only problem caused by moisture.  It can also cause corrosion, the effects of which are not apparent until the real damage has occurred.  Moisture alone in the form of water can cause corrosion. But when combined with refrigerants such as R-12 containing chlorine, hydrochloric acids can form which greatly increase the corrosion of metals.

Refrigeration oil has an attraction for moisture and will absorb it rapidly.  Water-formed acids combine with the refrigerant, forming a closely bonded mixture of fine globules.  The effect is called sludging and it greatly reduces the lubricating ability of the oil.

A vacuum pump removes this troublesome moisture by lowering the pressure within the system and vaporizes it, then exhausting it along with the air.