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Flowing nitrogen while brazing and pressuring with nitrogen are great practices. What about putting nitrogen in with the refrigerant? Not so much. Nitrogen is a “non-condensable” gas because it cannot be condensed (under normal conditions), but nitrogen isn't the only non-condensable.
First, let's talk about what a non-condensable gas is.
Any gas that does not condense (change from vapor to liquid) under the normal compression refrigeration conditions is called a non-condensable gas or NCG. These would commonly be air, nitrogen, carbon dioxide, argon, and oxygen.
Non-condensable in the system will result in high head pressure/condensing temperature, occasionally high side pressure fluctuations, and decreased cooling capacity and efficiency due to higher compression ratios.
The only way to remove non-condensables COMPLETELY in a small air conditioning or refrigeration system is to recover the entire charge and recharge with virgin refrigerant. You can recover the charge, let it sit in the tank for a while, and then recover the vapor from the top into another tank and recharge with liquid only to remove most of the non-condensables, but it's a pretty inexact science.
You can't remove non-condensables with a line drier, and while you do remove air with a vacuum pump, you only remove the air that entered the system once you open it. The vacuum does nothing for the refrigerant you already pumped down or recovered, as the non-condensables remain mixed with the refrigerant unless you are dealing with large volumes where they can actually be separated and the NCGs removed.
Non-Condensables Don't Cause Restrictions
“Non-condensables” is often a term used by techs to mean ANYTHING in the refrigerant that shouldn't be there, such as moisture, solid contaminants, and other refrigerants.
Carbon buildup from brazing is a solid contaminant, not a non-condensable. Moisture in the system is moisture in the system, not a non-condensable. A high glide refrigerant blend (such as R-407c) charged in a vapor instead of liquid is a fractionated charge—not non-condensables.
I think you get the point.
When we use a term like “non-condensable” as a replacement for “anything weird going on in the system we can't explain,” then it becomes a useless phrase, like saying a compressor is “bad” rather than explaining the actual fault.