equipment service

Dehydration of Wet Systems

Remove moisture from a system that has been opened or contaminated.

9 steps3 criticalLimit 60 min

Steps

1Make sure the refrigerant has been recovered to the required level (Table 1, or the leak and minor-repair allowances in 82.156), and verified, before the system is opened; venting is prohibited, so dehydration is never done by venting
2Find out how the moisture got in: building or opening the system, a suction-side leak while under vacuum, wet oil or refrigerant, or a water-cooled condenser leak
3Critical stepRepair the leak or fix the cause so moisture cannot get back in
4Replace the filter-drier whenever the system has been opened or a main component replaced; never reuse a used drier
5Connect a vacuum pump with a blank-off (isolation) valve through vacuum-rated lines to both sides, and a micron gauge that reads the system
6Pull a deep vacuum with the pump until the micron gauge reaches the target from the manufacturer or your procedure (sources differ: JB Industries calls a system that holds under 500 microns complete)
7Critical stepIf your procedure calls for a staged evacuation, break the vacuum with dry nitrogen between pulls and pull again, as that procedure directs (EPA GreenChill's commercial procedure does this)
8Close the isolation valve and watch the micron gauge: a slow rise points to moisture still in the system, a rapid rise toward atmospheric pressure points to a leak
9Repeat until the vacuum holds, then break it with the refrigerant to be charged and charge as the manufacturer specifies

Tips

Moisture in a refrigeration system can block the expansion device with ice, corrode metal parts, damage the insulation in hermetic compressors and break the oil down into acid
A filter-drier removes moisture left after evacuation or that enters later; Danfoss says never to reuse a used drier, because it can give moisture back off
A deep vacuum measured on a micron gauge is the way to tell whether a system is dry and free of leaks; a compound gauge cannot show it

Starting before recovery is complete, which means venting refrigerant.
Reusing the old filter-drier, leaving saturated desiccant in the circuit.
Charging before checking that the vacuum holds, so a leak or leftover moisture is sealed in.
Stopping the vacuum early because the pump has run a while, rather than because the micron reading is low and steady.

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Sources and how we check them: How we verify.