Pump nitrogen through the refrigerant tubing. Set the regulator on the nitrogen tank for low pressure. The target is 1 or 2 pounds per square inch (psi).
Insert the regulator hose into the valve pressure tap port on the air conditioner unit. Turn the valve on the nitrogen tank, and purge the system by allowing the gas to flow through the tubes and coil and out the exit port. The nitrogen will not hurt the environment.
Vacuum the system after the purge to ensure that all traces of moisture are gone. Hook up a vacuum pump to the vapor line. Close off all the other ports. Plug in the vacuum pump.
Set the vacuum at 100 microns of pressure. The exact time to evacuate the system will vary by unit type and size. Archie Nalbandian, a field service engineer, recommends leaving it on for five hours.
Check the micron meter on the vacuum pump after five hours have elapsed.Turn off the pump when it reaches 350 microns. Watch the micron meter. If it does not rise above 500 microns in one minute, then the vacuuming process is finished.