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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Tech Tips Home
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.