Brazed VS Welded Fin To Tube Attachment
The heart of any extended surface heat exchanger is the finned tube. An older technology
for attachment of fins to tubes has been by a resistance welding process developed in the 1930's.
Although welding provides a strong attachment, it has many inherent downfalls. It is because of
the weaknesses listed below that controlled atmosphere brazed finned tubes should be utilized
in modern industrial heat exchangers.
An Example of Finned Tubing
- Brazing provides a 99.9% metallurgical bond between the fin and tube. A modern
controlled atmosphere furnance creates a gaseous cloud around the tube during the
brazing to keep impurities out.
- A nickel brazing compound should be preferred. Not only does it join the fin to
the tube, but it also melts over the entire outer surface area of the tube to provide a nickel
clad coating to the tube. This coating forms a protective layer on top of the tube for
additional corrosion resistance. The use of copper and other low melting point braze material shall be
avoided to limit exposure to any potential liquid film embrittlement.
- The melting braze compound forms fillets at the base of the fin and tube. Smooth fillets
provide two advantages over welding. First, canyons on the rough surface of the weld provides a
trap for impurities in the flue gas stream, the first step in the corrosion of a tube.
Also, the rough weld surface creates many stress risers, places where failures are much more likely
to get their start.
- By its own nature welding physically violates the tube wall and fin to accomplish its bond.
Brazing enhances the metal structure of the tube and fin.
- Brazing allows the joining of dissimilar metals and thicknesses. Don't be limited to carbon steel units when
a stainless steel or alloy tube unit is what you need. This means overly heavy tubes and fins do not
need to be used in fabrication to allow for these penetrations to occur.
- The brazing furnace tends to stress relieve the tube during the brazing process, reducing even
furthur the formation of stress risers. A stress relieved tube is much less likely to suffer stress
related failures than a welded tube.
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