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Blacksmithing and metalworking questions answered.

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Robert Bastow asks:

RE design of Propane Forge.
I am in the process of building a propane forge primarily for damascus blade making (up to 14" overall) There are several excellent designs available on the net but a little knowledge can be confusing if not dangerous! I would appreciate some expert input on several questions.

1. Vertical (a la Don Fogg) or horizontal?

I am not familiar with the Don Fogg arrangement but most vertical tube gas forges need to be lifted off the work (complete enclosure). The type of work you are going to do should determine your configuration. I would think horizontal would be best for knife work.

To avoid this problem I built my gas forge for loose brick. This allows me to restack the brick to any configuration. The last job I did was to cast some bronze. It only took a few minutes to rearrange the brick to heat a 10 lb. crucible. The only problem with the restackable forge is that burner assemblies work best for one volume and do poorly if the volume is larger or smaller. Of course the stacked brick helps here too. It is easily adjustable to correct design problems in the burner.

2. I have 8 1/2" dia x 17" steel tube ($5.00 from local scrap yard)...One burner or two?

One large burner (1-1/2" or 2" dia tube) would work. Sometimes two smaller burners give a more even heat.

3. Forced air (fan) or venturi?

Venturi burners work well but are difficult to design and manufacture. Blower type burners almost always work and are easy to build.

Blower type burners are noisier than venturi or "atomospheric" burners, but they also provide more BTU in most cases. Safety is more of an an issue with blowers than with venturi. If the blower loses power or quits the gas will flare out of the forge and may reach several feet above the forge! This is one of several reasons you NEVER leave a gas forge running unattended. On my current forge the gas is controlled by a normally closed solenoid valve. If the forge loses power both the air and gas are turned off. This is combined with electric arc ignition so that everything is more or less automatic.

4. Kao-wool or castable?

I have very little experience with either but I DO know that castable takes a good while to cure and most of us will not wait long enough!

Castable also has the dissadvantage of a short shelf life. You will most likely purchase extra and it will probably go bad before you get a chance to use it

5. Any advantage or problem in insulating the outer casing? (summer in GA is hot enough!)

This can be problematic if you are using a steel shell. The only thing keeping it from melting or burning up is the outside being cooled. I use sheet metal heat shields in these cases. A thin steel plate or enclosure set about 1" (2.54cm) from the case with ventilation (duct it into the exhaust or recoop some heat). I use galvanized flat roofing material and use two air gaps between motors and controls. The first air gap and shield converts most of the radiant heat to non radiant. The second finishes the job and is usualy cool to the touch.

6. What best insulation..would fibreglass or vermiculite with stainless foil wrap do the trick?

(Air, See No.5)

7. Industrial blast and open hearth furnaces go to extreme lengths to preheat the blast. I have an idea to conduct air from the blower(s) thru a heat exchanger labyrinth built around the forge casing, thence via a blast gate to an insulated plenum before entering the mixers and burners proper. Any advantage/problem/danger in this?

Preheat is very advantageous. It increases your upper temperatures and is more fuel efficient. Feed the blower or venturi intakes off the insulating shell I described above. Just be careful that your blower (if you use one) can take the heat. The heat will conduct up the shaft of a direct connect blower and many small blowers have the motor attached to the fan housing through rubber isolation gromets. A friend of mine uses preheat on his coal forge in the winter because it is difficult to get a welding heat at the very gentle blast he prefers.

Years ago the guys at SANDIA labs built a little atomospheric gas forge with air preheat with great success. It was written up in the Anvils Ring and at one time plans were available. Their problem was that at high altitude propane forges do not do very well. The system has been duplicated all over the world in high altitude areas such as in Peru. Preheat is also advantageous in cold climates AND for reaching forge welding heats with reduced scaling.

8. Can I direct exhaust heat through a water bath to prevent freeze-up of my 100# propane tank?

As long as you use water and hot hot air off the furnace there should be no problem. Freezing up is a big problem with small cylinders. I started with one 40lb cylinder, then manifolded it to another 40lb cylinder and when full they still freeze up after about 4 hours. When half full they freeze up sooner due to the lower mass. I once saw some guys cooling their beer in the washtub full of water they had set their propane cylinders in. . .

Your 100# propane cylinder may be heavy enough to run most of the day (or as long as you can stand it) without freezing up. I've see small farriers forges run all day on a 30# cylinder. The freeze up problem is strictly a ratio of (BTU/volume)/time. I don't have a specific number for this but then you don't have a specific number for BTU. I do know (based on gas usage) that my gas forge runs between 30,000 to 40,000 BTU and that two full 40lb cylinders will not keep up after 4 hours. AND a 150# tank running the same forge had no problem at all (the tank WAS sitting out in the sun).

Copyright © 1998 by Jock Dempsey, DEMPSEY'S FORGE!

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