Nitrogen Facts and Tips
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Nitrogen makes up about 78% of the air we breathe, but we want to keep it out of HVAC refrigerant lines because it is a non-condensable gas. It doesn’t condense inside the HVAC system under normal conditions, so it takes up space, doesn’t move through the system very well, and increases the head pressure. Nitrogen in the system also affects the pressure-temperature relationship.
However, nitrogen is also an inert gas, meaning that it doesn’t react with other substances readily. As a result, it’s a great medium for pressure testing and to displace oxygen while brazing.
Nitrogen also displaces water, which is highly reactive and can cause corrosion if left in the system. Nowadays, we also use highly hygroscopic POE and PVE oils, meaning that water is attracted to them; if POE oil comes into contact with water, hydrolysis occurs and produces acid.
In retrofit situations, we can actually reuse the line sets of systems that previously had mineral oil, as mineral oil and POE don’t create a toxic mixture or anything. However, mineral oil systems fared a lot better with moisture, so we need to make sure that we remove oil and moisture from the line sets if we decide to reuse them. When we flush the lines, we typically use foam pigs and send them through the lines with nitrogen.
To prevent corrosion, we also purge nitrogen to displace any air or moisture in the lines, flow nitrogen while brazing to keep reactive fluids out of the lines, and pull a deep vacuum on the system. In our Central Florida market, we notice that corrosion is most common in places that run copper underground and irrigate with reclaimed water (which is high in chlorine and other reactive chemicals) or buildings with water softeners near the chase.
We also use nitrogen for the pressure test. When doing a pressure test, you can pressurize the system to the highest safe pressure. You put nitrogen in on the high side and watch the pressure on both the high and low sides rise. If the pressure on the low side stops rising, then you likely have a hard shutoff TXV. To do leak detection during a pressure test, the pressures need to be equalized. For best results, we also recommend using a bubble solution on joints, valves, and other common leak points so that you don’t have to rely on the pressure test alone to find leaks.
When brazing, we need to know the difference between purging (or sweeping) and flowing nitrogen. Purging the lines requires you to send nitrogen through at a relatively high velocity to displace everything that’s already in the lines, including oxygen. In the case of existing line sets, the goal of purging is to create turbulence that releases refrigerant and moisture from the system. Flowing nitrogen occurs AFTER you get all the air out, and it happens at a much lower velocity, only 2-5 standard cubic feet per hour.
A deep vacuum is what ensures that a system is clean, tight, and dry at the end of an installation, after nitrogen has been purged, flowed, and used for a pressure test.