Most HVAC installation instructions require flowing nitrogen through the copper tube during brazing. This is an important step in producing a quality HVAC system.
Why nitrogen purge?
Oxygen in the air combines with copper to form surface copper oxide. We see this on copper tube as a light to dark brown discoloration. You've probably seen ACR/medical gas copper tube supplied from the tube mill nitrogen charged and capped. This is designed to prevent this oxide formation inside the tube. Once the caps are removed and the tube is cut for installation, the nitrogen protection is lost.
At high brazing temperature a heavier black oxide forms (cupric oxide). On cooling this oxide flakes off to form "scale".
Figure 1: copper tube brazing scale
While mostly cosmetic on the tube exterior, inside the tube the oxide flakes are carried by the refrigerant through the system. This contaminant can restrict flow through small orifices such as metering devices or or the pilot valve capillary tube in a reversing valve.
Figure 2: inside oxidation scale from 5/8" tube
This problem has long been an issue in brazing HVAC tube. It has become more important with the change from HCFC refrigerants like R-22 that use mineral oil to the new HFC refrigerants (410a) using POE oils. Due to their polar nature, POE oils have a solvent effect and can "scrub" the copper tube walls. Oxide from tube walls and loose scale can circulate through the system.
What's the procedure?
To prevent oxidation, flow dry nitrogen through the tube during brazing. Nitrogen is inert, (non- reactive), and will displace the oxygen to prevent scale formation.
Figure 3: nitrogen purged tubes at top, no nitrogen purge at bottom
Nitrogen is typically introduced into the system throught the Schrader valve (after removing the core), or other system opening.
Figure 4: Harris nitrogen regulator
Connect a hose or tube from the nitrogen cylinder to one end of the pipe. The cylinder will be equipped with a regulator or flow control valve. There is no universal requirement for the delivery pressure setting, but the goal is to use low volume/pressure to displace the oxygen. A suggested starting point is 2 -3 CFH or 1.5 - 2 PSI. Some users will set pressure until they feel a slight flow at the exit point on the back of their hand. It's good practice to initiate flow before heating and continue to flow nitrogen until the part has cooled.
Avoid an excessive flow rate that builds pressure inside the tube. A high flow rate will tend to cool the tube reducing brazing heat efficiency. Excess nitrogen pressure can build up inside the tube and reduce braze alloy penetration. A small hole in a cap at the end of the line will allow the nitrogen to escape.
It's a good idea to experiment with flow rates by test brazing parts on the bench. Section the finished assemblies and inspect for a clean inner tube wall.
Take this step during installation and eliminate problems down the road.