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How (and Why) to Flow Nitrogen While Brazing
When I started in the trade, the idea of flowing nitrogen while brazing was nothing more than the punchline of a joke. Like pulling a vacuum with a micron gauge or proper recovery, it was a wink and nod proposition rather than a real practice. I've had to unlearn many bad habits since those early days, and the practice of flowing nitrogen while brazing is one that I still hear being mocked as “overkill” by old-timers. It isn't hard, it isn't overkill, and it makes a BIG difference. If you just keep reading instead of rolling your eyes, I will tell you the reasons why.
What Flowing Nitrogen Accomplishes
Copper and oxygen react to create “cupric oxide,” or copper oxide, similar to how iron and oxygen react to create rust. When we heat copper over about 500°, this begins to occur rapidly. As heat and oxygen increase, you'll see more copper oxide scale building up. We see this occur all the time on the OUTSIDE of the copper, where oxygen is prevalent in the air, though this can also occur inside the copper if there is air inside the system rather than dry nitrogen. It's made worse when the joint is hotter and when you leave the copper open to the air for longer periods while working. When we prevent air from entering the lines in the first place by keeping them sealed as much as possible and THEN flow nitrogen while brazing, we can prevent copper oxide from forming—this practice keeps line filter/driers, screens, compressor oil, and valves (ever heard of a bad TXV?) free from contamination and prevents many issues. Not to mention, it ALSO helps speed up the evacuation process.
Why Old-Timers Say it Doesn't Matter
Many techs who have done this work for a long time haven't flowed nitrogen EVER and have gotten away with it because of mineral oil. Older CFC and HCFC refrigerants used mineral oil rather than POE/PVE or PAG oils that we see today, which have higher solvent properties that “scrub” the oxide from the walls and deposit it into driers and screens. Techs in the grocery industry know that when a system is converted from mineral or AB oil over to POE, it is very common for this cupric scale to clog screens pretty quickly after the changeover. The point here is that mineral oil was forgiving on small amounts of cupric oxide on the tubing walls, but POE isn't.
Keeping copper oxide out of the system is really quite easy with a common-sense approach in place, starting with just pulling the nitrogen tank and flow regulator off of the truck, along with the torches, FIRST THING. There are several great flow regulators on the market—a few built right into the regulator and some that you attach to the outlet of the regulator. Keep in mind that you need to FLOW nitrogen, not PRESSURIZE with nitrogen while brazing. Otherwise, you never get the joints to hold.
- REMOVE CORES – When you set up, you first need to remove your cores in the inlet and outlet of the brazing path so that you can get full flow with minimum backpressure.
- PURGE OUT AIR – Purge nitrogen at fairly high pressure in the direction of the refrigerant flow to help “chase” the air out of the circuit and fill it with nitrogen.
- FLOW WITH 2-5 SCFH – Flow with a VERY LOW flow of 2-5 standard cubic feet per hour of flow, which is just a whisper out the end.
- DON'T FREAK OUT – The very last joint can be tough on small systems. It's OK if you shut off the flow or reduce it down to next to nothing for that last joint. Don't make being “perfect” the enemy of getting started flowing nitrogen while brazing. Don't overthink it, and stop making excuses for not doing it.
If you follow this process and then pressure test with nitrogen before evacuation, it will go much more smoothly. All in all, it can save you time in the process. Nitrogen is cheaper than callbacks and early component failure. Use it.