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?
- AIt 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.
- It keeps oxygen away from the hot copper so black oxide scale does not form inside the tubing
- CIt 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.
- DIt 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.
Why B is correct
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.
What this question is testing
The candidate must state the purpose of a brazing purge and distinguish it from cooling, from pressurizing, and from leak testing, all of which sound plausible to someone who has only seen the nitrogen bottle at the job.
On the job
The setup is a regulator turned down very low, a hose into one end of the tubing, and an open end somewhere downstream so the nitrogen has a path out. Techs who cap both ends end up with pressure that blows molten filler out of the joint, which is dangerous and ruins the work. It is one of the clearest quality markers on an installation: cut open a drier on a no-purge job and the black flakes tell the story.
Memory technique
Flow it, do not blow it. The nitrogen is there to chase out the oxygen, not to push on anything.
Exam tip
Purge nitrogen flows, it does not push. Small flow, open path out, no pressure behind the joint.
Where to look it up
Flowing dry nitrogen through tubing during brazing prevents internal cupric oxide scale formation. An open vent path is required.