Refrigeration System Evacuation - 2012-09

Refrigeration System Evacuation   

Whenever a refrigeration system is opened for service, proper evacuation is necessary before it can be safely resealed. It is critical that non-condensables and other contaminates be removed from the system.

A suitable vacuum pump, one capable of blank-off to at least 300 microns or lower, must be connected to both the high and low sides of the refrigeration system. The size of the connecting hoses should be such that they will not restrict the flow from the system to the vacuum pump. A vacuum gauge that reads in microns should be connected to the furthest point in the system away from the vacuum pump.

A triple evacuation process is strongly recommended. For triple evacuation, pump down the refrigeration system to 1,500 microns, and then break the vacuum using dry nitrogen. At 1,500 microns any moisture or ice trapped in the system will outgas. After backfilling with dry nitrogen to atmospheric pressure, operate the vacuum pump a second time to 1,500 microns and again backfill with dry nitrogen. Finally, operate the vacuum pump the third time to 300 microns, but no lower. Close all valves and isolate the vacuum pump, then turn the vacuum pump off.

Caution: At a pressure below 300 microns (mT) the POE oil in the compressors will start to degrade and begin losing its lubricating ability.

Watch the vacuum gauge to ensure vacuum is holding. If after five minutes there is a slight loss of vacuum, there could possibly be some residual outgassing in the system. Below 1,500 microns any remaining moisture is present as ice which will sublime. In this case the vacuum pump should be operated one more time to further dry the system. After a hold time of ten to fifteen minutes at 300 microns the system is considered successfully evacuated.

An inability to pump down to 1,500 microns indicates a system leak or a pump problem. A loss of vacuum to above 1,500 microns during the hold test indicates a system leak. System leaks must be repaired before the refrigeration system can be safely operated. Any system leak requires you go through the necessary steps to insure there has been no contamination of the refrigerant.

Something to remember: After you have finished using the vacuum pump, a good procedure is to change the oil. Any contamination in the refrigeration system is now in the vacuum pump oil. If you do not change the oil and the vacuum pump sits idle for any period of time, the contamination will start attacking its internal components.


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These technical tips are for use by qualified maintenance personnel who are familiar with their specific freeze drying equipment. These tips are intended only as general guidelines. Lyophilization equipment is frequently custom configured and some tips may not be appropriate for all freeze dryers. Always read and follow the directions of your equipment's maintenance manual. If you would like to discuss one of our tech tips, please contact Dave Clayton at 215-672-7800 ext-1376.


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