Brazing with a Nitrogen Purge: Why, How Much Flow, and Setup

Copper heated to brazing temperature in air forms a black oxide on the inside of the tube. When it cools, that oxide flakes off as scale. The refrigerant and POE oil carry the flakes to the TXV, screens, filter-drier and compressor. Flowing dry nitrogen through the tube during brazing keeps oxygen away from the hot copper, so no scale forms. Most equipment installation instructions require it.

What you need

• Nitrogen cylinder with a regulator.
• A flow regulator or flowmeter that reads in SCFH (standard cubic feet per hour). Pressure (psig) and flow (SCFH) aren't the same thing; a standard regulator set "low" can still push far too much gas.
• Hoses, and a core removal tool for the service ports.

Setup

1. Remove the cores from the service ports at both ends of the brazing path so the gas flows freely and can escape. A small vent hole or open end must let nitrogen out; you want flow, not pressure.
2. Connect nitrogen to one port (often the liquid line service port) and let it exit at the other end of the piping.
3. Point the flame away from service valves and wrap them in a wet rag or heat paste to protect the seals.

Step 1: Purge the air out first

Before you light the torch, push nitrogen through at a high flow to chase the air out of the piping. HVAC School's guidance is to purge at a fairly high rate in the direction of refrigerant flow. A low brazing flow alone takes a long time to clear a line that started full of air; at 3 SCFH, moving the volume that a 25 to 35 SCFH purge moves in about 20 minutes takes hours.

Step 2: Turn it down and braze

• Reduce to a very low flow. HVAC School recommends 2 to 5 SCFH, "just a whisper" out the end. Uniweld's purge-and-braze regulator specifies a brazing range of 3 to 5 SCFH.
• Lennox installation instructions describe the same idea in pressure terms: flow regulated nitrogen at 1 to 2 psig through the gauge set into the liquid line service port, with the core removed on the vapor side so it can escape.
• Keep nitrogen flowing until the joint has cooled. Harris recommends starting the flow before heating and continuing until the part cools.

Common mistakes

• Pressurizing instead of flowing. Too much pressure pushes the molten alloy out of the joint and you'll chase leaks. Too much flow also cools the tube and makes it hard to get the joint to temperature.
• No purge first. Starting the low flow at the same moment you light the torch leaves air in the line.
• Cores left in. Back-pressure stops the flow, or worse, builds pressure.
• Leaving lines open for hours before brazing. Moisture and air get in. Keep line set ends capped until you're ready.
• Stopping the flow the moment the alloy sets. The copper is still hot enough to oxidize.

The last joint

On small systems the final joint can be tricky because the gas has nowhere to go. HVAC School's advice: it's fine to reduce the flow to almost nothing, or shut it off, for that last joint. Don't let a perfect setup be the reason you skip nitrogen on the rest.

After brazing

• Reinstall the cores, pressure test with nitrogen to the manufacturer's test pressure, and check every joint.
• Pull a deep vacuum and do a decay test: how to pull a deep vacuum.

FAQ

Sources

• HVAC School: How (and why) to flow nitrogen while brazing
• HVAC School: Flowing nitrogen, purge first
• Lennox XC21 installation instructions (flow nitrogen at 1 to 2 psig)
• Harris Products Group: Nitrogen purge and brazing
• ACHR News: Ignoring advice on nitrogen purging