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.
During the initial stage of any evacuation process, a vacuum pump will exhaust relatively large quantities of water vapor from the system. This water vapor will be pulled into the vacuum pump and condensed within the pump before it is exhausted. Since the ability of a vacuum pump to achieve the required micron level will depend on the cleanliness and dryness of its oil, any water vapor that is condensed will dissolve into the vacuum pump's oil and will retard the pump's ability to achieve the required micron level.
Many two-stage rotary vane vacuum pumps incorporate a valve known as the gas ballast. The gas ballast is designed to retard the process of water vapor condensing within the vacuum pump. The gas ballast is a valve arrangement which, when opened, will allow relatively dry atmospheric air to enter the second stage chamber of the vacuum pump. The introduction of this atmospheric air will inhibit water vapor from condensing before it is exhausted from the vacuum pump.
During the initial part of the evacuation process the gas ballast should be opened, allowing relatively dry atmospheric air to enter the vacuum pump. Once the system’s micron level approaches about 3,000 microns, it should be closed for the remainder of the evacuation process.
Using this feature of the vacuum pump does help to retard the contamination of a vacuum pump’s oil, but it does not prevent it. The pump’s oil should still be changed on a regular basis to ensure good service life and the ability of the pump to achieve the required micron level during the evacuation process. Always follow the manufacturer’s recommended procedure for changing the vacuum pump’s oil as well as the frequency.
Periodically remove the gas ballast valve needle, clean or replace its O-ring and clean its mating surfaces. Then lightly coat with vacuum pump oil before securely retightening to ensure a good seal when closed.
Using the gas ballast feature of a vacuum pump will aid with the overall evacuation process while servicing and installing refrigeration systems, making good technicians better technicians.