Burn Notice

This post is really just a means to an end. There was no specific chemistry that I had in mind, but rather a step in creating a furnace for melting higher temperature metals, smelting ores, etc. I need a gas jet to heat the thing sufficiently.

I was thinking of Bernoulli's Law, the Venturi effect, and other elements of fluid dynamics mostly. But a funny thing happened on the way to this post. It occurred to me that I needed some chemistry for perfect combustion of propane:

C3H8 + 5O2 -> 4H2O + 3CO2

This tells me that I need a molar ratio of 5:1 between oxygen and propane. I don't plan on using pure oxygen, so I'll assume that it makes up about 21% of what we call air. 

If you don't get enough oxygen in the mix, you start getting soot and carbon monoxide and other emissions that are often undesirable. This is known as a reducing flame, with less energy (heat), and can be preferable when you want to avoid creating oxides of your metals or want to add some carbon. Here is a simple example where exactly one less oxygen pair generates soot in the exhaust (carbon). 

C3H8 + 4O2 -> 4H2O + 2CO2 + C

It is essentially impossible to keep one perfectly balanced reaction going. Here is another that can happen with almost but not exactly the right amount of oxygen in the mix:

6C3H8 + 29O2 -> 24H2O + 16CO2 + 2CO

Note that 29/30ths of the perfect oxygen amount creates carbon monoxide. That will happen just from minor variability in atmospheric air pressure. Yikes.

An oxidizing flame has more oxygen than needed. It is very hot as it fully combusts the fuel. But we don't really want extra oxygen unless we are trying to create oxides during smelting. If I ever use my graphite crucible, oxidation can turn it to gas. It seems like some sort of terrible disappearing magic, but the reaction is as simple as it gets.

C + O2 -> CO2

The bottom line is you generally want to design a sufficiently large air intake and then add an adjustable "choke" which reduces air intake so you can adjust the mixture you want. You may recall chokes from older cars, lawnmowers, tractors, and so forth. Or maybe that is just me. I had no idea what it did at the time. I knew to open it when the engine was flooded.

The general idea of the gas jet is to shoot a very thin and fast stream of gas out of very small hole into a larger pipe. Directly behind this, you have air vents that allow air to be pulled into the pipe according the the suction provided by the gas stream. Cars and airplanes have used similar mechanisms, but sometimes rely on "forced air" which gets scooped into the mix from the velocity of the vehicle travelling through air. My gas torch should not be moving (hopefully). In automobiles, the methods of obtaining more oxygen are numerous: turbochangers, superchargers, etc. 

At the end of the exit pipe, we have a flare that allows the mixed gas and air to expand outward and slow down so that we can light it. 

I looked at a few designs online, before deciding to use one similar to TKOR (God rest his soul). While his design seemed to meet my needs, I could not find a couple of the parts anywhere, and I do not have the tools he has to be able to drill and tap thick curved stainless steel.

(Picture: 3/8" brass flare to 1/4" MIP, 1/4" ball joint valve, 1/4" MIP to MIP, a couple of 1/2" washers that were probably not needed, a 1/4" female to female galvanized steel pipe coupling, a 1/4" brass plug, and a .6mm welding tip that I screwed into the endcap by drilling and tapping a hole into the brass.)

I am used to working around a lack of tools, and have found over the years that many applications can use a carved wooden replacement. I realize that sounds insane. Wood can do some amazing things. I even sailed a boat for years with the mast inserted into the hull by carved wood. It never failed me. In this case you may be thinking that wood will get hot and burn, but the heat is at the end where the flame begins. In the areas where gas escapes and pressure reduces, frost can actually form.

So I carved an interposer between my brass endcap and the 1.25" pipe reducer. I literally screwed it into the threading (like a reverse tap). Note that the hex-shaped endcap fits into the wood, which then has a slightly smaller  hole drilled through it for the welding tip and air to pass through.

(Picture: 6" 3/4" steel pipe with 1" reducer for flare and 1.25" reducer where my wooden venturi "screws" in. The hardware described and shown above is inserted into the wooden venturi.)

Initially I thought the welding tip was a bit of unnecessary artistic flare. Indeed, in the TKOR design he says it is optional but cool. But because I don't have an open space behind my endcap, I need the outlet positioned further into the pipe. So it is necessary with my design. This allowed me to drill 12 holes into the wood past the endcap but before the end of the welding tip. These are my air vents. My plan was to drill more holes if needed and fill some in with putty or plugs if it was too much. It just happened that my 12 holes give a good amount of air and don't need any further tuning at this time. As a future project, I may add more holes and an adjustable collar for the choke. 

Finally, I added a regulator and pressure gauge to my tank. This way I can keep the system at 30psi for safety and consistent results.

Now I was ready to light this candle! I tested it in daylight in case there were any issues. It made a nice loud jet noise, which is a good sign that my venturi was working. Then I tried to light it and it took immediately and easily, so there is sufficient oxygen and my flare works too. I tested the components and none of them were hot except near the end. Being daylight, I could not see the flame well, so that would be test #2 at night.

I should note again, always, not to use a device like this indoors. You don't want the fire risk or exhaust gasses in an unventilated area. Think of it like running a car in a garage.

Time for test #2 at dusk. Wow, what a huge (18" or so) nice blue flame. Not too oxidizing or too reducing. I'll keep the tuning as is. This flame will completely fill any kiln I build. I declare success!

(I will blog again after building my furnace. For now this jet is not much use to me. I need something to contain the flame and retain the heat.)

Thanks for reading,

Paul