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Sunday, January 19, 2014

Venturi Forge Build Part II - Burners

Continued from Part I Here

The first things I wanted to build were the burners. This is the only part that really needs to be precise. Namely, the drilling of the orifice where the propane enters the pipe to the forge. There are two basic types of burners, Venturi and Blown burners. Blown burners use a fan to mix the fuel and air and propel it into the forge. They are great, but I prefer to avoid the limitation of needing a power source. Venturi burners use the nature of gas exiting a small hole in a stream to create a vacuum which draws atmospheric air with it down the burner into the forge. They might not be considered to be as efficient or controllable as blown burners, but they are completely self sufficient as long as there is a gaseous fuel source being pushed out of the orifice.

So, to begin the build.

This is the layout for a simple venturi burner. The propane will flow in through the T fitting in the top middle, exiting through holes that will be drilled in the brass pipes. The gas will then shoot down the 3/4 to 1 1/4 bell into the black iron pipes (3/4") into the forge, drawing with it the air. All you really need to do this is a drill and a bit of patience.

First I marked the holes to drill in the bells for the brass pipe to sit. The hole will go all the way through both sides so the cap can seal off the outside ends of the brass. There are several variations on how actually to build a venturi burner, but this is how I will be doing mine.

Pilot holes drilled, an arbitrary size that had a sharp drill bit. I held these to the platen of the drill press by sliding a dowel rod through the bell and clamping that down. I had to support the 3/4" end of the bell so it was not at an angle.

Drilled to the final size. This was where the first snag hit. The largest drill bit the chuck could hold was just under the outer diameter of the brass pipe, so I had to file it out by hand until I could fit the pipe through it.

I have learned the hard way that it is important to always fit things as they will be when at the final state of assembly, else there will be errors and shortcomings. Here I fitted the caps to the pipe and slid the lot through the two holes I drilled in the bells. This is a good representation of where they will be when all assembled, so I marked the centre of the 3/4" hole on the bell where the orifice will point.

As marked, I centre punched the pipe so I can drill it without wobble. This is critical because the drill bit is so small (size #57) and would easily break if forced onto the convex surface. If nowhere else, this is the place to take your time.

I held the pipe in place with a piece of scrap lumber sawed in half and clamped to the platen. By minute adjustments I aligned it just so.

And in the final stages of the second hole, the bit snapped inside the hole. This is why I bought two of the bits. A few minutes of surgical extraction later and I had the bit freed. Fortunately the damage was contained and I was able to eliminate its effects within the radius of the finished hole.

Now onto assembly. It is necessary to ensure a tight fit of all the fittings, so I used nylon plumbers tape to wrap the threads. You could solder them or use some sort of liquid binder, but I had neither of those around and I used this on my old forge with no problems. A combination of the tape and tightening them to death seems to do the trick. However, ALWAYS check fittings for leaks when working with flammable/explosive gasses. One good way to do that is take a spray bottle and fill it with soapy water. Turn on the gas and spray all the connections. If you see bubbles from the joints, there is a leak.

Before I sealed the ends, I took a full round file and removed the burrs from where I drilled the orifice. Flow is a touchy thing, and those burrs could be enough to make the stream out of the orifice more turbulent than strictly necessary. At this stage I also took a hammer and slightly dented the pipes just before where they would enter the inside (non capped) side of the bell. I did this because it will prevent the bells from rotating around the brass pipes, which is not a good thing (the orifice needs to be pointed as close to down the centre of the black iron pipe as possible).

Together the two bells join with the T fitting, again sealed nice and tight. It is critical that you check to see the orifice is still pointing down the centre of the 3/4" (smaller) side of the bell. If not, just be careful to tighten one of the pipes or fittings until they are aligned with the bell that the two bells are aligned with one another.

Because I could not find a gas rated 1/4" valve that had male connections on both side, I had to join the T to the valve and the valve to the hose with two 1/4" male-male pipe fittings. An extra step but not catastrophic. Just be sure to seal the fittings!

And here it is with the hose and regulator attached. I scavenged the regulator from my old forge's burner, as I could not find them locally and didn't want to wait to order one. The regulator allow you to control the pressure of the gas as it enters the orifices. Since there are two, each will theoretically receive half the pressure. That means I would need to burn twice the amount of gas for the same intensity flame as my old forge, but I have faith that this one will be better insulated so it will not need to run at as high a pressure to hold a temperature.

Next step, building the housing.

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