A Vacuum pump is basically a pump that is used to remove air or gas from a sealed volume or system and leaves a partial vacuum.
A vacuum pump in refrigeration and A/C units is used for removing any moisture or air from a system. Modern Rferigeration and A/C systems are being built more tighter and charges are very important. This means the systems are more sensitive to substances like moisture and contaminants and this makes evacuation of a system very important task.
When Moisture is present in a system, it will directly or indirectly cause of most problems and complaints. Moisture causes freeze-up in a refrigeration system. because it is carried by the refrigerant and circulates round the refrigerant line as a fine mist and causes ice crystals to form at the point of expansion.
Moisture in a system also causes corrossion
Moisture also combines with the refrigerants and can form chlorine, hydrochloric acids. These in the long run will increase chances of corrosion of metals. The Refrigerant oil will also absorb some of the moisture in the system and will result in creating waterformed acid thus forming a mixture of globules. This is also called SLUDGING which reduces the quality of the lubricant.