Operating instructions

TM 11574A-OI
3-11
(3) Evacuation and dehydration. Moisture is the deadly enemy of refrigeration systems. The
presence of moisture in a refrigeration system can have many undesirable effects. The most
common are copper plating, acid sludge formation, “freezing-up” of metering devices by free
water, and formation of acids, resulting in metal corrosion.
(a) Preparation. To prepare the refrigeration system for evacuation and dehydration, follow
the steps below:
1
Evacuate and dehydrate only after pressure leak test (refer Refrigerant leak check
paragraph 3-10b(2)(e)).
2
A vacuum pump is essential to properly evacuate and dehydrate any system.
3
If possible, keep the ambient temperature above 60 F (15.6 C) to speed evaporation
of moisture. If the ambient temperature is lower than 60 F (15.6 0C), ice might form
before moisture removal is complete. Heat lamps or alternate sources of heat may
be used to raise the system temperature.
4
Additional time may be saved during a complete system pump down by replacing the
filter-drier with a new filter drier. This temporary filter drier will be disposed of and
replaced after total system evacuation.
(b) Complete system procedure. To perform the evacuation and dehydration cycle on the
complete refrigeration system, follow the steps below:
1
Remove all refrigerant using a refrigerant recovery system.
2
The recommended method to evacuate and dehydrate the system is to connect three
evacuation hoses (Figure 3-6) to the vacuum pump and refrigeration unit. Be sure
the service hoses are suited for evacuation purposes.
3
Test the evacuation setup for leaks by back seating the unit service valves and
drawing a deep vacuum with the vacuum pump and gauge valves open. Shut off the
pump and check to see if the vacuum holds. Repair leaks if necessary.
4
Mid seat the refrigerant system service valves.
5
Open the vacuum pump unit. Start the vacuum pump. Evacuate unit for 3 hours.
Close vacuum pump valve. Shut off the vacuum pump. Wait a 15-20 minutes to be
sure the vacuum holds.
6
Break the vacuum with nitrogen. Raise system pressure to approximately 2 psig @
(0.2kg/- cm), monitoring it with the compound gauge.
7
Remove nitrogen from system.
8
Repeat steps 5 and 6 one time.
9
Remove the temporary filter drier and replace with a new filter-drier. Evacuate using
vacuum pump for 24 hours. Shut off the vacuum pump. Wait 10 minutes to see if
vacuum holds. This procedure checks for residual moisture and/or leaks.
10
With a vacuum still in the unit, the refrigerant charge may be drawn into the system
from a refrigerant container on weight scales. Continue to Refrigeration Charge
paragraph 3-10b(4).