Any time the internal piping or components of a refrigeration system are exposed to atmospheric air, they must be completely evacuated before refrigerant can be added into the system. Proper evacuation of a system is an important part of the overall repair or installation process — it ensures that no atmospheric air remains in the system and mixes with the refrigerant.
Atmospheric air contains oxygen, nitrogen, and water vapor — all of which can be harmful to a system’s operation. If oxygen and water vapor remain in a system, it could lead to multiple system failures such as a restricted TXV due to water vapor freezing at its outlet. The water and oxygen could react with the system’s oil and the heat generated in the system and produce acids, leading to a failed compressor. Nitrogen left in the system will collect in the condenser and cause the system’s high pressure to operate at elevated pressures, which can also lead to system inefficiencies and failures.