When Is Evacuation Process Complete?

July 2, 2007
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Any time a refrigeration system is opened to the atmosphere, it must be completely evacuated before refrigerant can be added back into it. Proper evacuation of a system is an important part of the overall repair process. It ensures that no atmospheric air remains in the system.

Atmospheric air contains oxygen, nitrogen, and water vapor; all three of which can be harmful to a system’s operation. If oxygen and water vapor remain in a system, they could react with the system’s oil and the heat generated in the system and produce acids. Nitrogen left in the system will collect in the condenser and cause the system’s high pressure to operate at elevated pressures.

How does a technician determine when the evacuation process is complete? Is it by time or the sound of the vacuum pump or by a reading on the compound gauge?

Actually none of these are correct. A technician needs to use a quality vacuum gauge that reads pressures in microns. Most manufacturers recommend the system’s pressure be reduced to a vacuum level of between 300 to 500 microns as part of the evacuation process.

However, reducing the pressure in a system to this level is only half the process. The system must also hold the vacuum with the vacuum pump off and isolated from the system. This will determine if the process is completed and the system is properly evacuated. Once a vacuum level of 500 microns or below has been achieved, the vacuum pump is turned off and isolated from the system. The vacuum gauge must remain connected to the system and the system’s vacuum level monitored for approximately 10 to 15 minutes. If the system holds a vacuum below 1,000 microns, the entire system is properly evacuated and refrigerant can then be added.

If the vacuum level rapidly rises above 1,000 microns and approaches ATM (atmospheric pressure) on the vacuum gauge, it is usually an indication of a leak within the system. The location of the leak should be identified and repaired before continuing with the repair process.

If the vacuum level of the system slowly rises above 1,000 microns, but remains below 5,000 microns, the system may still contain some moisture. The vacuum pump will need to be turned back on and opened to the system. Allow the pump to again reduce the pressure in the system to below 500 microns and repeat the process.

Continue the process until a vacuum level of 1,000 microns or below is maintained for approximately 10 to 15 minutes. On each attempt the vacuum level should rise to a lower value. This is a good indication that the vacuum pump is doing its job, but more time is needed to allow the moisture in the system to boil off.

Evacuating a system can be a time-consuming process, and to do the job properly, it should not be rushed. Proper planning will allow a technician to handle other tasks during the evacuation process.

If the evacuation process is the only thing left to do, a technician might become impatient and shorten the process in order to get the job done. This may lead to future problems for the system and the technician.

Publication Date: 07/02/2007

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