Evacuation
Also known as: pulling a vacuum, system evacuation, vacuuming
Short definitionEmptying the air and moisture inside the pipes with a vacuum pump before refrigerant is charged. The target is to get below 500 microns (0.67 mbar); a skipped vacuum comes back as acid, an iced-up valve and high pressure.
This is the job that gets rushed on the last day of installation, yet it is where the life of the compressor is decided. On site the order does not change: a pressure test with nitrogen, then vacuum, and finally liquid charging into the circuit under vacuum. A two-stage pump pulls the pressure below 500 microns; at this pressure water boils off and leaves with the air. The reading is taken from a digital vacuum gauge; a manifold gauge does not show microns. If the value has not passed 1,000 microns 15 minutes after the pump is isolated, the circuit is dry and tight.
Air left inside does not condense; it takes up space in the condenser and pushes the discharge pressure up. Moisture is more insidious: it turns to ice in the expansion valve and combines with oil and refrigerant to make acid, which eats the windings. If the reading keeps climbing, moisture is left in the circuit or there is a leak. In cold weather and on large plants, the vacuum is pulled three times with nitrogen in between. A split system’s pipes and evaporator are evacuated on site; a monoblock unit arrives evacuated and charged from the factory.