A newly installed or opened refrigeration circuit contains air and moisture. Air does not condense, collects in the condenser and raises the condensing pressure; moisture freezes in the expansion valve and forms acid with the oil and refrigerant, which destroys the compressor winding. Evacuation uses a two-stage vacuum pump to bring the system below 500 microns (0.5 mmHg), expelling the air and boiling the water off at low pressure so it is drawn out.
Correct practice: large-diameter hoses or copper connections, vacuum drawn from both the low and high pressure sides at the same time, a digital vacuum gauge (a manifold gauge does not show microns), target 500 microns or below. If the pressure does not rise above 1,000 microns within 15 minutes after the pump is shut off, the system is dry and tight; if it rises quickly there is a leak or moisture. In cold weather and on large systems a nitrogen sweep and triple evacuation (vacuum–nitrogen–vacuum) are applied.
Pressure test with nitrogen (leak check) before evacuation, liquid charge under the closed vacuum after it — that is the sequence. Monoblock units are evacuated and charged at the factory, so this procedure is not needed on site; in split systems the pipework and evaporator must always be evacuated. Good evacuation directly determines the life of the filter drier and of the compressor.