Vacuum Practices for Large Jobs
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After performing a nitrogen pressure test, you’ll vent the nitrogen and prepare for your evacuation (deep vacuum). A vacuum pump creates an area of low pressure relative to the atmosphere; since molecules move from areas of higher pressure to lower pressure, unwanted air and water molecules will move out of the system and into the vacuum pump. Liquid water also boils off during evacuation. We use a very tiny unit of measure for evacuation, the micron (one-millionth of a meter of mercury column), and we need a gauge that can read microns.
You should use a vacuum pump that can pull a vacuum as low as 50 microns when isolated from the system (micron gauge fastened to the pump). The pump should be sealed during storage to prevent oil contamination, and then you should use large-diameter hoses (more than 1/4″) when pulling a vacuum. When you’re dealing with a wet system, you can leave the gas ballast open to keep moisture from condensing in the vacuum pump oil. Using the gas ballast will cause smoke to leave the pump, which could fill the room and cause you to drain oil; it is also far less efficient to run with the gas ballast open for the full length of every evacuation.
The vacuum pump oil should be clean and at the proper level for EVERY evacuation, and you may have to change the oil several times during a large market-refrigeration evacuation job. You can check the sight glass to see your oil level and cleanliness. If the level is low or the oil appears milky, you should drain out the old oil and replace it with clean oil.
To pull a deep vacuum as deeply as possible, use a core removal tool with large hoses (not a manifold) and remove Schrader cores. To get the most accurate measure of your vacuum, connect your micron gauge as far away from the pump as possible. Then, you’ll want to pull a vacuum below the manufacturer’s recommended pressure (often 500 microns, but it can be as low as 200 microns for new systems). Once you’ve pulled a deep vacuum, valve off the pump and perform the decay test. The decay test will let you know if your system is clean, dry, and tight, and it can last anywhere from 24 hours to 30 days on large commercial HVAC/R equipment. During the decay test, the vacuum shouldn’t “decay” past a certain threshold (usually 500 microns for new systems and 1000 for repairs on commercial HVAC/R equipment, though the threshold will vary by application).
When you pull a deep vacuum, you will experience very quick gains at the beginning of the evacuation, and you will pull down a lot more slowly when you get closer to the target. You may have to valve off the vacuum and let the system stabilize before pulling down again, which is a best practice that descended from triple evacuation (we’re just not breaking the vacuum with refrigerant or nitrogen). After valving off the vacuum pump, we can monitor the decay on apps like measureQuick.
Once a system passes the vacuum test, you can charge it with refrigerant.