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 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 through the Schrader valve (after removing the core), or other system opening.
Figure 4: Nitrogen regulator
We connect a hose or tube from the nitrogen cylinder to one end of the pipe. The cylinder will be equipped with a regulator control valve. The goal is to use low volume/pressure to displace the oxygen. The practice is to initiate flow before heating and continue to flow nitrogen until the part has cooled.
At Air Care Today we take this step during installation and eliminate problems down the road.