Single Evacuation Process
A single evacuation involves connecting a vacuum pump to the system through the service ports, opening all valves to the system, and running the pump until the micron gauge reads 500 microns or below. At this deep vacuum, the boiling point of water drops below ambient temperature, causing any moisture in the system to boil off and be pulled out by the pump. The vacuum pump must be capable of reaching the required vacuum level, and its oil should be changed regularly to maintain efficiency.
A single evacuation pulls the system down to 500 microns or below, boiling off moisture at ambient temperature by lowering pressure.
Triple Evacuation Step-by-Step
The triple evacuation is the most thorough dehydration method and is used when a system has been open to the atmosphere. Step 1: Pull the first vacuum down to approximately 1,500 microns. Step 2: Break the vacuum with dry nitrogen to slightly above atmospheric pressure. Step 3: Pull a second vacuum down to approximately 1,500 microns. Step 4: Break with dry nitrogen again. Step 5: Pull the final deep vacuum to 500 microns or below. Each nitrogen break sweeps out moisture that the vacuum alone could not remove, dramatically improving dehydration effectiveness.
Triple evacuation: vacuum to ~1,500 microns, nitrogen break, vacuum, nitrogen break, final deep vacuum to 500 microns. Each nitrogen break sweeps out trapped moisture.
Vacuum Pump & Micron Gauge
The vacuum pump is the only tool that should be used to evacuate a system -- never use the system compressor as a vacuum pump, as it is not designed for deep vacuum and can be damaged. The vacuum pump oil should be changed when it appears cloudy or discolored, which indicates moisture contamination. A micron gauge (also called an electronic vacuum gauge) is the most accurate instrument for measuring deep vacuum. Compound gauges on manifold sets are not accurate enough below 1,000 microns.
Never use the system compressor as a vacuum pump. A micron gauge is the most accurate tool for measuring deep vacuum. Change pump oil when it appears cloudy.
Evacuation Procedures Quick Reference
Standing Vacuum Test & When Evacuation Is Not Required
After reaching 500 microns, isolate the vacuum pump by closing the valve between the pump and the system, then monitor the micron gauge. If the reading rises and stabilizes, it indicates remaining moisture boiling off. If it rises continuously without stabilizing, there is a leak in the system. A proper standing vacuum test should hold at or near 500 microns. Evacuation is not required for minor repairs that do not open the system to the atmosphere, such as replacing a service valve cap, tightening a flare fitting, or adding refrigerant through a service port.
In a standing vacuum test, rising and stabilizing = moisture. Rising continuously = leak. Evacuation is not needed for repairs that do not open the system to atmosphere.
Evacuation Procedures Review
- Single evacuation: pull the system to 500 microns or below using a vacuum pump.
- Triple evacuation: vacuum, nitrogen break, vacuum, nitrogen break, final deep vacuum to 500 microns -- most thorough method.
- Never use the system compressor as a vacuum pump. Change pump oil when cloudy.
- A micron gauge is the most accurate tool for measuring deep vacuum; compound gauges are not accurate enough.
- Larger diameter hoses reduce evacuation time significantly.
- Standing vacuum test: rising then stabilizing = moisture; continuously rising = leak. Evacuation is not required for repairs that do not open the system.