Short #81 – Air vs. Nitrogen vs. Oxygen
In this short podcast, Bryan discusses the differences between air, nitrogen, and oxygen. He also explains why we should only use nitrogen for purging, flowing, and pressurization.
You DON'T want to pressurize line sets with air because air contains water vapor and oxygen. Water acts as an oxidizer, and moisture can turn POE oil acidic via hydrolysis. You cannot dry out POE oil, and the acid can lead to compressor burnout.
Nitrogen is non-reactive (unlike oxygen) and does not contain water vapor (unlike air). It also does a good job of chasing water vapor out of the lines. Because nitrogen won't react with anything we put in the line sets, it is an ideal medium for purging, flowing, and pressurization. Nitrogen DOES, however, change pressure with temperature; it obeys the gas laws, and you can see it in action when the pressure changes at different parts of the day (with varying temperatures).
Oxidation can occur when oxygen reacts with copper to create a black scale called cupric (copper) oxide. It is similar to rust on iron; it is an undesirable form of corrosion. When the black scale comes off, it can get into screens on filter-driers and clog the system.
You purge nitrogen to chase all of the air out before brazing. When you've finished purging, you use a flow regulator to reduce the nitrogen pressure (2-5 SCFH) to flow it during brazing. When we pull the vacuum, we only want nitrogen to be in the system; exposure to air should be very short, and any air in the system should be temporary.
So, again, it's not a good idea to use air to pressurize the lines.
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