Purpose — Evacuation removes air and moisture trapped in a refrigeration system by the use o f a vacuum pump. Air occupies space in the refrigeration system, reduces heat transfer, and causes erratic operation. Moisture creates problems o f freeze-up that can result in blocked refrigerant flow. Moisture also combines with oil and refrigerant to form corrosive acid and sludge. All of these conditions can cause system damage or failure.

The vacuum pump creates a pressure differential between the system and the pump. This causes air and moisture vapor trapped in the system at a higher pressure to move into a lower pressure (vacuum) area created in the vacuum pump. When the vacuum pump lowers the pressure (vacuum) in the system enough, as determined by the ambient temperature of the system, liquid moisture trapped in the system will boil and change into vapor. Water will boil at room temperature if it is in a deep enough vacuum. Like free air, this water vapor is then pulled out of the system, processed through the vacuum pump, and exhausted to the atmosphere.

The deep vacuum method and triple evacuation method of system evacuation and dehydration are both used frequently. The deep vacuum method is typically used after a repair was made that required the system charge to be recovered and the system opened. Use of the triple evacuation method is recommended when a system has been especially wet. This can be determined by performing an acid/moisture test on the system refrigerant (SP-5) before or during recovery of the refrigerant. Generally, evacuation and dehydration of a system is performed in the following circumstances:
• After assembly and prior to charging a new field-piped split system, evacuate/dehydrate the low-pressure side and piping. When complete, open the service valves and add refrigerant charge to the system as needed.
• After assembly of a field-piped system, evacuate/dehydrate
the entire system before charging.
• After an installed system is opened to the atmosphere as a result of parts replacement or leak repair.
• When an acid/moisture test (SP-5) shows moisture or noncondensible gas in the system.

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