A technician runs a small flow of dry nitrogen through the tubing while brazing new line set into a refrigeration system. What does that flow accomplish?
Correct Answer
B) It keeps oxygen away from the hot copper so black oxide scale does not form inside the tubing
Copper heated in air grows a layer of black oxide on the inside surface. That scale flakes off as the system runs and travels with the oil until it lodges in a filter drier, a screen or a metering device, and it also contaminates the joint area. A slow flow of dry nitrogen displaces the oxygen inside the tube so the scale never forms. The flow has to be small and there must be an open path out of the tubing, so the gas moves through rather than building pressure behind the joint.
Why This Is the Correct Answer
Oxide scale requires oxygen and heat. Removing the oxygen from inside the tube by displacing it with an inert gas prevents the reaction entirely, which keeps the inside of the new tubing clean and keeps loose scale out of the system's metering devices and screens.
Why the Other Options Are Wrong
Option A: It cools the joint so the filler metal sets up faster after the torch is pulled away from the copper
The flow is far too small to cool anything, and quick cooling of a brazed joint is undesirable in any case. The nitrogen is there to keep air off the inside of the copper.
Option C: It raises pressure inside the tubing so the filler metal is pushed into the joint gap
Filler metal is drawn into a properly heated, properly fitted gap by capillary action. Pressure behind the joint would blow molten filler back out of it rather than pushing it in.
Option D: It tests the joint for leaks at the same moment the joint is being made and heated
Leak testing is a separate step, done after the joint is complete and cool, with the system brought to a controlled test pressure. A purge flow tells the technician nothing about tightness.
Memory Technique
Flow it, do not blow it. The nitrogen is there to chase out the oxygen, not to push on anything.
Reference Hint
Flowing dry nitrogen through tubing during brazing prevents internal cupric oxide scale formation. An open vent path is required.
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