After the recovery process is complete and all available refrigerant has been removed from the system, it's ready for evacuation. When a system is severely low on refrigerant or has been opened for repair, moisture and air enter the system. Moisture and air are removed from the system with a vacuum pump to draw the system into a vacuum.
Moisture in an R-134a system mixes and reacts with the refrigerant and oil in the system. It forms a corrosive hydrochloric acid that corrodes lines, clogs screens, and damages other components in the system. It blends with the hygroscopic oils used in today's R-134a systems, reducing their ability to lubricate components like the compressor. Performing an evacuation does not remove metal shavings from a system though, this requires a flush.
A vacuum pump is used to draw the system to a vacuum of 29.75 "hg or greater. By bringing a closed system into a vacuum, the boiling point is greatly reduced, and any moisture boils into a vapor that's pulled out through the vacuum pump.
If a system has been opened for repair, a vacuum should be pulled for at least 30 minutes. Check with manufacturer's specifications. If a system is left open and exposed to outside air and humidity for a long time, the pump on-time should be increased significantly.