Are you are missing blowpipe

February 14, 2010

The information bellow is of experimental nature. No testing as far as safety of modification, has been done. Use you own judgement whether or not it is something you want to experiment with. The information provided "AS IS AND WITHOUT ANY WARRANTY WHATSOEVER".

Whenever there is a discussion about torches, somehow it is always on how quickly it can melt a ton of gold, and is it hotter than the sun. For my money, I want a flame just barely hot enough to melt solder; I want my torch to be as light and as small as possible; and I want to be able to adjust flame to different operational envelopes. In short, I am missing blowpipe. But, I am not missing blowing into it. Is it possible to eat proverbial cake without ever buying one?

I have been using Little Torch for many years. It is not perfect, but it was as close to my requirements, as I could manage. Still, even adjusted to produce reducing flame, the torch is a bit harsh. I have been modifying tips trying to improve operational characteristics with some degree of success. However, the type of flame that blowpipe can produce, had remained elusive, until recently.

Why blowpipe flame is different ?

Chemists differentiate two types of flame - premixed and diffusion. Premixed type of flame is exactly what the name says - fuel an oxygen were premixed prior to burning. All torches produce premixed type of flame. An example of diffusion type is the flame resulted from burning candle. In diffusion burning, oxygen is separate from fuel prior to burning, and when burning starts, oxygen diffuses though fuel. Such flame is irregular, with different temperature zones, but from jeweler's point of view, diffusion flame is very desirable.

Another characteristic of flame is the flow of burning gases. If we adjust oxygen just right, we could observe a highly organized blue cone, surrounded by yellow envelope, and transparent outer shell. The reason that we could observe this arrangement, is because torches designed to produce laminar flow of premixed gases. The flame produced by blowpipe is very much different.

It has no obvious structure, because the flow of gases is turbulent. In real life, any flame will be a combination of both types of flow, and flames would differ only in percentages of each component.

Design

All right, that was helpful. Once I understand the nature of flame, I know how to design torch tip. What needs to be realized from the very start that it would be impossible to imitate blowpipe flame completely. But, it should be possible to reduce temperature of the flame and increase the size of reducing zone. Turbulence of the flow will also be enhanced.

To reduce temperature of the flame, some oxygen must be replaced with air, and to increase the size of reducing zone, flame diameter must be increased. My theory is that it should be possible to adjust a torch so there is only enough oxygen to combine with 80% of available propane.  Remaining 20% should combine with surrounding air and burn with much cooler flame reminiscent of blowpipe. It is also possible to achieve with regular torch, but commercial tip design makes it very difficult. That makes perfect sense, because designer was striving for maximum efficiency, which is to ensure complete burning of propane with almost any knob setting. My design goal is quite different.

Implementation

I started by taking one of the tips with sapphire nozzle and cutting of sapphire end. Next step was to make copper tube out of copper sheet. Take a look at the drawing bellow.