A new compressor is being brazed into a household refrigerator after the charge was recovered. Why do technicians run a slow trickle of dry nitrogen through the tubing while the joints are being heated?
Correct Answer
C) It keeps oxygen away from the hot copper so no flaking oxide scale forms inside
Copper heated to brazing temperature in the presence of air grows a black scale on the inside of the tube. That scale flakes loose once the system runs, and in a small appliance it travels straight to the capillary tube, where a few flakes are enough to block the metering and kill the unit. A slow flow of dry nitrogen through the tubing displaces the air, so the inside of the joint comes out bright instead of black. The flow is kept low, just enough to sweep the tube, and it continues until the joint has cooled.
Why This Is the Correct Answer
Hot copper and oxygen make cupric oxide, a loose black scale that forms on the inside surface where nobody can clean it. In a capillary tube system that scale is a system killer because the metering device is a tiny orifice with no filter ahead of it beyond the drier. Purging with dry nitrogen removes the oxygen from inside the tube, so the joint is made in an inert atmosphere and the bore stays clean.
Why the Other Options Are Wrong
Option A: It raises the pressure enough to keep the joint from collapsing while the alloy is molten
Nitrogen during brazing is run at a trickle, only ounces of pressure. Copper tubing does not collapse under its own weight at brazing heat, so there is nothing to hold up.
Option B: It cools the tubing on either side of the joint so the capillary tube is not annealed
The nitrogen flow is far too small to cool anything, and cooling the tube next to a joint would work against the brazing rather than help it.
Option D: It carries the flux vapor out of the system before the joint is allowed to cool
Flux is used on the outside of the joint where the alloy flows, and its vapor rises into the room. A purge inside the tube does not touch it.
Memory Technique
No oxygen inside, no black flakes inside. The purge is a shield, not a coolant.
Reference Hint
Flow dry nitrogen through the tubing while brazing so no oxide scale forms on the inside of the joint.
More epa-608-type-1 Questions
An under-counter ice maker is one factory-sealed cabinet, while a modular ice machine head is joined to a remote condenser by field-run tubing. What certification does servicing the modular machine's circuit require?
A technician recovers refrigerant from a small appliance using a recovery machine that was manufactured in 1991. What recovery level must that older equipment achieve?
Two technicians recover from identical refrigerators with working compressors, one using a 1991 recovery machine and the other a 2015 machine. Why do their required recovery levels differ?
System-dependent recovery equipment may be used only on appliances that normally hold up to what amount of refrigerant?
What feature defines a self-contained recovery unit and sets it apart from the system-dependent kind?
When system-dependent recovery equipment is used on a small appliance whose compressor still runs, how can that working compressor be put to use?
Recovery from a small appliance has slowed down and the pressure is barely moving. What accepted technique helps drive the remaining refrigerant out?
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A sealed small appliance offers no service port at all. What has to happen before recovery equipment can be connected to it?
As refrigerant from a string of small appliances collects in a recovery cylinder, what fill limit applies to that cylinder?
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