A joint has to be brazed on a system that still holds its full operating charge. What has to happen before the torch is lit?
- ACrack a service valve and bleed the system down to atmospheric pressure before applying heat to the joint
Bleeding the system to atmosphere is deliberate venting and it empties the whole charge into the work area. It trades a pressure problem for a legal violation and a breathing hazard.
- BWrap the line in a wet rag so the charge stays liquid while the joint is heated with a torch
A wet rag does not stop pressure rising elsewhere in the circuit, and it does nothing about refrigerant escaping into the flame. Cooling one spot is no substitute for removing the charge.
- CPump the charge into the low side and braze only on the high side of the system instead
Pumping the charge to one side still leaves refrigerant under pressure in a connected circuit. Heat anywhere raises that pressure, and any opening still feeds vapor to the flame.
- Recover the charge into approved equipment, then braze with dry nitrogen flowing through the lines
Why D is correct
Recovery removes both hazards at once: there is no pressure to rupture the hot joint and no refrigerant left to decompose over the flame. Purging with dry nitrogen during brazing then displaces the oxygen inside the tubing, preventing the black flaking scale that forms on hot copper and later contaminates the system.
What this question is testing
This item combines a safety judgment with a regulatory one. The candidate has to reject venting as a shortcut, recognize that a charged system cannot be safely heated, and know that nitrogen purging during brazing is about keeping oxygen out of the tubing rather than about pressure testing.
On the job
Nitrogen purging while brazing is standard practice on any system a technician expects to run reliably. Set a regulator for a few psig, run a low flow through the tubing with a vent open at the far end, and braze. Skip it and the copper builds an internal oxide layer that flakes off after startup and lands in the metering device or the compressor. The pressure hazard is the more urgent reason to recover first, but the scale is what generates the callback three months later.
Memory technique
Empty it, then purge it. Recovery handles the pressure and the law, nitrogen handles the scale.
Exam tip
Two different nitrogen uses show up on this exam: pressurizing to find leaks and low flow purging while brazing. Read which one the question is describing.
Where to look it up
Recover before brazing. Never heat a charged system, and never bleed a charge to atmosphere to depressurize it.