In my previous blog Is Brazing Welding, I explained why brazing is not welding. This included the definitions for welding, brazing and soldering. Now I want to focus on some process examples which include some hidden and key variables.

Common HVAC Pressure Applications with Refrigerant or Water

Arc Welding is used for the main chambers of heat exchangers and steel compressors. Main components are called shells, tube sheets, nozzles, couplings and piping (usually 3 inch diameter and larger).

There are two common types of Silver Brazing filler metals for interconnecting tubing/piping/fittings(usually under 3 inch diameter) between heat exchangers and compressors on commercial equipment; 1) Copper to copper joints can be silver brazed with "low percent silver" filler metals and no flux is needed if phosphorous is part of the alloy which provides the necessary fluxing action. An example filler metal is AWS BCuP-6(2% silver). 2) Dissimilar metals like copper to steel can be silver brazed with "large percent silver" filler metals and a flux is required. An example filler metal is AWS BAg-28(40% silver).

Brazing, Silver Brazing and Silver Soldering are common on coils, headers, interconnecting tubing/piping/fittings on light commercial and residential AC equipment. Brazing copper U-bends (not subject to vibration) have been done with "no percent silver" filler metal, for example AWS BCuP-2. The silver brazing examples in the previous paragraph also apply here. A Silver Soldering example is using 96% Tin - 4% Silver filer metal for small diameter copper to copper joints on residential lines.

Key Variables - All 3 Processes

1) Requirements of state/local jurisdictions for codes and standards

2) Design meets service conditions

3) Compatibility of base metals, filler metals, fluxes, atmospheres, as applicable.

4) Safety

5) Process Qualification

6) Performance Qualification of workers

7) Cleanliness of base metals and filler metals

8) Fixturing of parts

9) Inspection and Testing

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Hidden Variables

• Thermal expansion of joints(at brazing temperature) between dissimilar metals may require larger or smaller diametral gap calculations.

• External environments such as sulfur in the air around paper mills corrode from the outside of joints if the correct filler metal is not selected.

• Water hammer is quite a dynamic shock to any joint, but especially to brazed or soldered joints. The same is true with refrigerant lines on some compressor startups on residential equipment. Brazed joints would definitely be better than soldered joints in this case.

• Purchased fittings in hardware stores have almost always been designed for soldering because of the long joint length(typically 10 times the thickness of the thinner part). Therefore, if brazing is used the safety factor will be very high, which is a good thing. A negative is the added cost of brazing filler metal in each joint.

In upcoming posts we'll talk about the main causes of braze leaks. Does your company have any braze leak problems? Stay tuned for concise answers to your braze quality problems.

Editor's Note: CR4 would like to thank Bill Spietz of GEA Consulting for contributing this blog entry.