Homegrown Chip Bed Forge

[Ian Wille]

This is an idea I've toyed around with for two years now, and due to losing my coke supplier (yep, I'm a coke addict ) and a recent thread about these forges elsewhere on this site, I've begun pursuing this project in earnest. Note that I have never actually seen one running, and that I have built this prototype going off of pictures with what knowledge I could glean from said images and applying common sense/logic. Naturally, I have run into a few problems.

The setup is pretty basic. Its no more that a ribbon burner head mounted to shoot the flame upwards, and is powered by a Rex/Porter style hybrid burner. All of this is built on my coke forge (just made an adapter to fit the firepot hole). I am using firebricks to form the hearth, and broken up graphite (scrap) crucible from a local foundry for the refractory chips. See attached photos.

The burner itself has 25 5/16" dia. holes, counterbored to 7/16" by 3/4" depth to create a step to hold the flame. The face of the burner is mild steel, 1 1/2" thick. I did this so I could easily change burner port size.

The burner/mixer is based on Rex Price' 1 1/4 foundry burner. I am using a .045 mig tip for the orifice. The main tube is approx. 1 1/4" i.d.

The chips are approximately nickel/nut sized.

Set up as is, the unit will run for 15 to 20 minutes fine, running the burner full out (i do not have a pressure gauge on my regulator). One oddity that occurs once the burner is burning, is that is "whistles," that is it produces a low frequency, low decible sound not unlike a deep steam whistle. Also, as the chip bed grows hotter, more flame becomes visible/seems to pass through the bed. On ignition, no flame is visible coming from the bed. At the end of 10 minutes, however, there is considerable dragon's breath coming through the bed.

However, after this warm up period, the burner likes to flash back and burn in the mixing tube. This also happens when I insert a piece of stock into or take it out of the chips. Or even if the chips are disturbed by moving them with a fire rake. The forge is a very bright orange, near yellow at this point, but not hotter, and I know that the chips are not fusing together to block off the ports. At this temperature, the forge seems to easily flash back, even doing it on its own at times.

I've tried lower fuel gas pressures, changing air intake amount/changing mixes, all with the same results (flashback in to the burner/mixer tube).

I am at a loss for what's going wrong here. Any thoughts?

Ian W.

P.S. I tried this setup two years ago, same burner head, and a blown burner body. At that time I used broken up firebrick for the chips. I still ran into the flashback problem, but I believe that was because the firebrick chips were fusing together.

[Sam Salvati]

Because it is an atmospheric burner, you might not be getting adequate pressure for the ribbon burner, especially with the chips on top. Try making a blown burner and going with that.

[Sweany]

I'm guesssing, so... on the flashback that the burner is getting hot enough to ingnite inside the burner.

just for fun, put a grate over burner so there is an air space between the burner and the chips. nothing fancy just some scrap round bar or what have you for now, to hold the chips up off the burner

i think that might give the gas's some expansion room.

and yeah i agree blown burner sounds better for this application

[Charlotte]

Flash back in most systems stems from static pressure behind the burner being less than the resistance of the hot end. As gases get hotter there is increased resistance to mass flow. Essentially the hot gas is tries harder to go in every direction instead of the you want it to.

You don't need anything fancy for a blower because you already have some static pressure created by the burner.

I would buy one of the 30-50 dollar blowers from WW. Grainger or similar.

I just bought one on sale for $35. run some flex duct to burner and arrange it so that it surrounds and seals to burner.

Edited April 15, 2009 by Charlotte
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[Ian Wille]

Thanks all for the response. A blown burner for this application does make more sense, and I realized that from the start. I chose to go the atmospheric route first out of curiosity after reading what Frosty had said in this thread:

http://www.iforgeiron.com/forum/f7/flamefast-ds430-ceramic-chip-forge-10624/.

In it, he mentioned a naturally aspirated chip bed forge that Mike Porter and he worked out together. That was the inspiration for me to try the same idea. However, I am no Frosty or Mike Porter, and went wrong/missed something somewhere.

All other forges of this type I have seen via the internet use a blower, and as I stated in my first post, I had tried that before with this same burner head, and still ran into flashback problems.

The grate idea sounds promising, Sweany, and I think would be in line with what Charlotte said about the behavior of hot gasses.

I shall try a blown burner setup again (as I still have that blower and the necessary plumbing), and see if the chip material this time around makes a difference. Last time the brick chips fused together. I'm afraid the graphite chunks may do the same.

Ian W.

[Frosty]

Mike's idea was to put one of his burners into a plenum chamber below a refractory grate supporting the chips. The plenum and the gaps in the grate have to be large enough to provide a free flow through the chips or back pressure will do exactly what you experienced.

The one experimental chip bed forge built like this melted the 3,000f kiln shelf grate and the FlameFast chips purchased for the forge. As far as I know the experiment wasn't repeated. Then again the guy who did try it would probably tell Mike rather than me.

Anyway, it sounds like back pressure causing the backfires. Charlotte's explanation about covers it. It's what the plenum is for, to provide expansion room for the mix and a chance for the pressure to equalize under the entire grate. The needs to be plenty of space in the grate openings to allow it to flow freely.

Unfortunately this set up using a neutrally tuned burner resulted in the burn taking place IN the plenum and concentrating the heat there and in the grate rather than the chip bed. Mike's thought was to run it rich so it couldn't burn in the plenum. Like I said I don't know if any more was done.

Frosty

Frosty

[Ian Wille]

I have a question: would not the ribbon burner head itself serve as a plenum with a grate? After all, the burner head consists of 3" square tube, 10 1/2" long capped on each end, with an inlet for the gas/air mix on one long side, and the grate (plate with holes in it) on the opposite side? The tubing would provide space for the air/gas to expand and equalize under the grate/port plate.

If that logic is correct, then perhaps the port holes on the burner face are too small to allow the mix to flow freely enough. Or perhaps the plenum needs to be larger.

Knowing about the plenum now helps to explain the forge pictured below, which is sold by Johannes Angele in Germany. Here's the link to the webpage with more info:

- ANGELE Schmiedetechnik - ANGELE-SHOP

Note that the images are from Angele's website.

Ian W.

[Charlotte]

Thanks for the Link it is very interesting!

One thing that the site tells us is that they are using serious direct drive centrifical blowers.

That is, a blacksmith blower on hormones. These radial fan blower produce high static pressures. New, with out the one horse motor, they run around $200 and produce something like 200 cfm at a 7"wc Pressure (wc= water column) ( price and volumn approx based on similarity to WW. Grainger cat. Items.


This is some information that I was looking for my self. It will greatly influence my own design thinking for a chip bed blower

[Frosty]

To go with the air volume it burns 1.5kg or 3.3lbs or better than 3/4 gl. of propane an hour heating 25kg of chips.

That's one bad boy forge.

Frosty

[Sweany]

I just built a ribbon burner, using the method described in the hammers blow.

square tubing with a rammable refrac for the burner head, same pattern as yours, using crayons as the orifice's and then burning out the crayons as I calcified the rammable material.
However it is mounted in the top of my forge, and it is blown.
I get a very even heat from that burner.

I really like playing with burner designs. :)

[David E.]

Interesting thread, I run a DS 230 Flamefast chip forge alongside a Swan Mother both use Propane, I have not gone into the workings of the chip forge too much apart from changing the jet from Natural Gas to propane. There are inbuilt solenoids and valves controlling volume and pressure of air/gas which I dont know the figures of. The mixing is done before the plenum as far as I can see and the holes in the grate are about 3 mm dia, there are about 25 approx over an area of approx 6" dia.BTW I think a full load of ceramic chips from Flamefast is about 12kgs.
My perception is the grate does offer a restriction to the free flow of the air/gas mix and this keeps a positive pressure inside the plenum and also keeps the fire on the chip side of the grate. I stress this is only my perception and may be totally wrong.
I enjoy my forge which easily gets hot enough for welding ( tho' I keep my attempts to the Swan) My only reservation is the blower does encourage a lot of scaling, my fear is that if I cut back the air volume to give a richer burn, I might poison myself:o I need to speak to the very helpful technicians at Flamefast for reassurance.
There were several negative remarks about these forges on the other thread which I think are misplaced, perhaps they were powered by Natural gas at a lower pressure IMO. Anyway best of luck to the pioneers amongst you who are experimenting and building your own versions:)