Why nitrogen and what flow rate
Heated to brazing temperature (approx. 600-800 °C), copper rapidly oxidises on the inside, forming black flaking scale (copper oxide). These particles break loose and circulate in the system, plug capillaries/expansion valves, score bearings and clog filter-driers. Nitrogen (OFN — oxygen-free nitrogen, grade 4.0 or cleaner) displaces oxygen inside the pipe and eliminates scale completely.
Set the bottle through a regulator to 0.02-0.05 MPa (0.2-0.5 bar) and a very low flow — roughly 3-5 l/min, just enough to feel a gentle bleed at the open end. Too much flow chills the joint and blows the filler out. Never braze with nitrogen at working pressure — the flow must be free, with an open outlet.
Choosing filler and flux
Copper-to-copper: phosphor-copper hard filler (CuP, e.g. L-CuP6 / B-Cu93P) — it is self-fluxing, use no flux. Copper-to-brass or copper-to-steel: silver filler (e.g. 30-45 % Ag) ALWAYS with flux, because phosphorus on ferrous metal gives brittle, cracking joints. Capillary gap 0.05-0.2 mm — the pipe should enter the cup with light resistance, not loose.
CuP melting range is approx. 710-820 °C; heat pipe and cup evenly and introduce the filler only when the copper is a dull cherry-red and the alloy draws itself capillary-wise into the joint. Do not melt the filler with the flame — the hot metal should do that.
Step-by-step procedure
1) Cut the pipe with a tube cutter, deburr it (reamer), keeping swarf out of the bore. 2) Clean the ends and the cup to bright metal. 3) Connect nitrogen to one end of the system, leave the other open, purge for 30-60 s to clear the air. 4) Keeping the flow on, heat with a torch (propane-butane/MAP or oxy-acetylene), rotating the flame around the joint.
5) Feed the filler around the circumference until it forms a full ring. 6) After it solidifies, keep nitrogen flowing until it cools below ~100 °C, otherwise hot copper still oxidises. 7) Do not quench the joint with water — thermal shock causes micro-cracks. Protect nearby valves, sensors and driers from overheating (wet rag, heat shield).
Quality control and common mistakes
Cut a test joint: the inside must be bright copper — black soot means no or too weak a nitrogen flow. The fillet should be an even, smooth ring with no porosity or unfilled gaps. Most common mistakes: skipping nitrogen "because it's just one joint", excessive flow blowing the filler out, overheating (filler oxidises, goes dull and brittle), dirt/moisture in the joint, phosphorus on steel.
After brazing always pressure-test with nitrogen to test pressure (per refrigerant, typically 1.1× max working pressure, e.g. R410A up to ~42 bar on the high side), then deep evacuation to <500 µmHg (microns) with a rise test, and only then charge the refrigerant. Mind leak-tightness per EU Reg. 2024/573 (F-gas).