Evacuation and Dehydration

Evacuation removes air, nitrogen and moisture from a refrigerant system before charging. Moisture left inside combines with refrigerant and oil to form acids, and non condensable gases raise head pressure and distort every pressure reading you take afterward.

A compound gauge cannot read a vacuum accurately. It will sit against the stop long before the system is actually dry. Only a micron gauge tells you whether the moisture has boiled off and whether the system holds.

• Moisture freezes at the metering device and causes intermittent restriction.
• Moisture plus refrigerant and heat produces acid that attacks windings and oil.
• Non condensables raise discharge pressure and make subcooling readings misleading.

1. 1Confirm the system is empty and any existing refrigerant has been recovered to an approved cylinder.
2. 2Pressure test with dry nitrogen to the pressure the manufacturer specifies and confirm the system holds before evacuating.
3. 3Release the nitrogen, replace the filter drier, and connect with large diameter hoses or a core removal tool on both service ports.
4. 4Remove the valve cores to eliminate the biggest restriction to vacuum.
5. 5Connect the micron gauge away from the pump, as close to the system as practical.
6. 6Run the pump with fresh oil until the gauge reaches the target, commonly 500 microns or the manufacturer value.
7. 7Isolate the pump and watch the gauge. A rise that levels off in the low hundreds usually means remaining moisture. A rise that continues without levelling means a leak.
8. 8Once the system holds a stable vacuum, break the vacuum with the charge itself, never with air.

Refrigerant handling, recovery, record keeping and technician certification requirements vary by jurisdiction and by the regulations in force where you work.

• Two stage vacuum pump with clean oil
• Micron gauge
• Core removal tools and large diameter hoses
• Nitrogen cylinder with regulator
• Recovery machine and approved cylinder