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 a 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 will 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 in the same way that iron and oxygen react to create rust. When we heat copper over about 500° this begins to occur rapidly with more copper oxide scale building up the more heat and oxygen there is. We see this occur all the time on the OUTSIDE of the copper where oxygen is prevalent in the air and this can also occur inside the copper if there is air inside the system rather than dry nitrogen. This is made worse the hotter the joint and the longer the copper is left open 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 keeps line filter/driers, screens, compressor oil and valves (ever heard bad TXV?) free from contamination and prevents many issues not to mention that it ALSO helps speed up the evacuation process. Why Old Timers Say it Doesn't Matter Many techs who have done it 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 that it is very common for this cupric scale to clog screens pretty quickly after the changeover. The point being that mineral oil was forgiving on small amounts of cupric oxide on the walls of the tubing, POE isn't. The Process 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 on 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 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 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 prior to evacuation it will go much more smoothly and all in all can save you time in the process. Nitrogen is cheaper than callbacks and early component failure. Use it.