2018-04-04

How to Tell Double Wall Bundy Tubes from Single Wall Bundy Tubes

Copper Coated Bundy Tube

Copper Coated Bundy Tube

Many buyers are confused by names of the double wall and single wall Bundy tubes. Once a while one of our client who needed double wall Bundy tube, but the tubes he purchased from somewhere else were actually single wall tubes coated with copper both inside and outside.

Buyers of Bundy tubes should firstly clearly indicate to the supplier if the tube is single walled or double walled.

This post will tell you how to tell the double wall from single wall  Bundy tubes.

Background Information

Bundy tube, originally stands for the special tube fabricated by rolling steel sheet 720 degrees and using copper to braze the double layer steel tubing. So Bundy tube is indeed the double wall copper brazed steel tube, as also written on this Wiki page: https://en.wikipedia.org/wiki/Bundy_tube.

This brazed steel tubing is mostly used as the steel tubing for vehicle hydraulic brake lines.

However, in some countries like China, and India, people began to use Bundy tube to call the single wall ERW steel tube which is just like any other welded steel tubes, having only one layer and one straight seam (not visually apparent). This welded tube of small diameter and thin wall is mostly used for refrigeration heat exchangers, sometimes fuel transfer lines on vehicles, and other less critical fluid transfer applications.

Double Wall Brazed Steel Tube Structure

Double Wall Brazed Steel Tube Structure

Both the single wall welded and double wall brazed steel tubes are having similar mechanical performances such as tensile and yield strength, elongation and flaring rates. The very important difference between them is, double wall brazed steel tubes are much more reliable than the single wall ERW  steel tubes thanks to the special structure. Two layers of steel sheet in 720 degrees mean that the seam is not a line along the tube, but approximately two times of the tube circle length.

How to Distinguish

The manufacture of double wall brazed steel tube uses steel sheets that have copper coated on both sides, so it has copper on both inside and outside surfaces as the original surface finish; while to manufacture the copper coated single wall ERW  steel tubes, steel sheets shall be coated with copper on the inside surface, rolled and welded, and then coated with copper on the outside surface finally.

So, if we can get the tube wall split into 2 layers, it will definitely be the double wall brazed steel tube; or, it’s the single wall ERW steel tube.

Single Wall ERW Welded Tube Structure

Single Wall ERW Tube Structure

  1. Pick a short section of the tube, cut the tube to two halves (the cross section of the tube would be two half-circles);
  2. Heat either half, to near 1000 degrees or even near 1100 degrees (the melting point of pure copper is 1083 degrees) for at least 1 minute;
  3. When the piece of half tube is taken out, try to use tools to stick the two sides of the tube piece and move the two sides towards opposite directions to slide them off (do this as quickly as possible upon taking out the tube piece from the heating stove before the copper coagulates in the air) ;
  4. If it’s double wall copper brazed steel tube, you will easily get at least two thin steel pieces (if it happenly is the one with the seam joint, there would be 3 pieces); or, it is single wall ERW steel tube coated with copper on inside and outside surface.

BE CAREFUL OF THE HEAT AND PREPARED WITH PROPER PROTECTION!

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