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Ed Caffrey's Workshop Talk to Ed Caffrey ... The Montana Bladesmith! Tips, tricks and more from an ABS Mastersmith.

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Old 08-18-2005, 06:22 AM
trozzo51 trozzo51 is offline
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Forge burner design

HI All, I'm starting my second attempt at a knife forge capable of reaching welding heat at 2200 foot elevation. I've learned about the more efficient vertical round tube design with burner entering tangentially at the bottom, KOA wool refractory liner, small 4x4 opening at the top, hinged lid for cleaning etc. Am I missing anything critical here?

My 2 questions are: Would a back opening be advisable for longer stock assuming a 12" diameter body?

Is burner design really that critical? I see guys putting needle valves on for fine tuning and air valves for air flow control. Is this over kill? I'm thinking of a 2" elbow with a 1/4 NPT nipple welded into it with a .035 mig tip. THe other end will connect directly to the propane regulator. Can adequate adjustment made from the regulator alone or will a needle valve work gooder?

I understand that down line from the mig tip a reducer will be more efficient, say 1 1/2" pipes that ends at the venturi burner made by welding 3 equally spaced small pipe sleeves into the 1 1/2" tube.

On the air end I plan to attach a small blower. Is a pipe valve really needed to adjust air flow from the fan?

Is this rocket science or is it an age old efficient design that we can just screw up by getting too complex?
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Old 08-18-2005, 10:52 AM
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Ed Caffrey Ed Caffrey is offline
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Sounds like your on the right track. I'm at 3500 ft, so something like I've built should work even better at lower elevations.

Here's what I built:

150 cfm blower, mounted on a 2" pipe flange. Pipe flange screwed to a 12" long piece of 2" pipe. This screws into a 90 degree reducing elbow (2" to 1 1/2"). Next, 12" X 1 1/2" pipe for the burner tube. I also have the concentric pipes for about 3" inside the end of the burner tube.
The gas inlet is mounted about 3" below the blower (in the 2" pipe). It's a 3" X 1/4" pipe nipple threaded into a hole int he 2" air pipe. I don't have an orfice in the pipe nipple, because I use a propane needle valve, mounted on the outboard side of the pipe nipple, to control the fuel input. (with the needle valve you don't need an orfice) You mentioned a .035 orfice...that MIGHT be a bit small. I would suggest going to approx. an .059 (this is a #53 drill bit) Keep in mind that the further away from the blower you place the gas inlet, the less the fuel/air will mix, and you could get erratic combustion.

I have my blower wired into a ceiling fan speed controller, but a simple sheet metal air gate at the blower intake will work just fine.
I also think that the regulator you spoke of will work fine too.

Keeping it simple is the best idea for gas forges. Too many times I have had people contact me all flustered because the can't get their forge(s) to operate correctly. More often than not it turns out that they have more plumbing and gadgets included than you can shake a stick at. I generally have them strip all the "extras" off, and then the forge works fine.
As far as gas forges are concerned......The more you overtake the plumbing, the easier it is to stop up the drain!


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Old 08-19-2005, 05:14 AM
winstonknives winstonknives is offline
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Ed, how is your fuel consumption with this arrangement? For example if you were going to forge for 8 hours.


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Old 08-19-2005, 08:01 AM
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Fuel consumption is very economical. I can't tell you exactly how much fuel per hour it uses because of my setup. I have a 500 gallon propane tank that is located about 30 feet behind my shop. I run three forges, the forced air mentioned above, and two ventrui type forges. I fill the tank about once every 10-11 months.

In others' shops I have seen a 100lb bottle last about 2-3 days with the forge running for 8-10 hours a day with similar setups.


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