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Forge from an air tank


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I'm getting ready to dive into this build. The tank I have is 12" wide X 24" long with 18" of straight cylinder. 

I successfully evacuated all the air without serious incident. I'm going to take one of the ends off so I can get in and work without trying to stuff everything through a small hole. Anyone have a can of Weld-B-Gone I can borrow? I may just cut it front of the weld, won't make much difference.

b2YPFog.jpg

It's a nice solid tank. It was used to pressurize microwave transmission lines. We were tearing the site down and it was in the dumpster. It has nice feet too.

Anyway, it's a start to a project I'm sure you all will be absolutely captivated with. All comments and advice are welcome. 

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That tank will make a forge that is way to large.  Better to find an old 20# Propane tank or Freon bottle.  A forge that is to large for the work you are doing will waste fuel, be more expensive to build and operate, cause excess scaling, decarbonation , grain growth.  You can only forge about 6" in a heat anyway.

Let me know if I can help you.

Wayne Coe

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As Wayne says, that would be much too large a forge -- IF you use the standard thickness of insulation. You could conceivably use extra layers of ceramic fiber to make a smaller, more standard-size chamber, which would also help keep the heat in a little bit better. Probably not enough to make a huge difference, and with some additional expense (unless you're using salvaged materials).

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Thanks Wayne, you're right. I decided to make it into a heat treatment oven instead. I have all the electronics and other parts ready to go. I wound the heating element last night and will start assembly this weekend. 

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A finished inside diameter of 9" is about as large as is recommended for a large forge. 6" is better for a first forge. However, circumstances alters cases, and if someone handed me such a nice forge shell, I would be loath to let it go...so the thing to do is change the terms of the equation; what term? Change tunnel to oval or D forge; suddenly its size becomes convenient, rather than over-large :D

By this I mean that you can reconfigure the internal shape more easily; not that you want to end up with  a large internal volume.

Also change ceramic fiber to K26 hard insulating firebricks, to increase strength of the internal structures and ease of controlling the finished internal shape (well it will last much longer too, but who's counting, right?).

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I'm back to making this a forge again. I'm going to cast the chamber outside the forge, since I am integrating the burner into it. I have a couple of large PVC pipes I'll be using for the forms. The inner diameter will be around 5 1/2" with about an inch wall thickness. I'm putting a ribbon burner in it with a single line of holes. These will come in tangent to the radius of the chamber. I'm still trying to figure out a few details, but this may be a little different than your standard forge. 

I have plenty of wool and bricks to fill up the rest of the cylinder. 

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I modified my ribbon burner by inserting a refractory brick over the tubes.

Here's how it started out:

PF4PvHw.jpg

I pounded a brick on to the tubes to get an impression, then I drilled the holes out about 2/3 the way into the brick. Since these tubes are 1/4" ID, I drilled the rest of the way through 1/4".

There is a good 3 inches of brick between the end of the tubes and the final outlet. This provides lots of insulation so the tubes don't get hot and deteriorate. I also modified the input to accept a 2" pipe

Here's the burner:

pkl9N2J.jpg

I originally had straight holes coming out but that configuration wouldn't hold a flame. So I reamed out the holes to provide a widening taper, and also chamfered the end for a wider flare. This slowed the mix down enough to hold the flame close even with the blower wide open.

This is full tilt...

EBqEXfA.jpg

I have not tried this in the forge since my top opening isn't big enough. Nevertheless, I think this method can be developed as an alternative to casting. Further testing needs to be done, but it's promising. I achieved a good tight fit around the tubes, and I put the torch to every seam and found no leaks. The joint between the brick and the plenum could certainly be sealed easily enough if there were any doubt. 

 

Ted

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I got the center cylinder cast. I built an acrylic "brick" to act as a form for the burner. The burner is going in at an angle to allow the flame to create a swirl in the chamber.

Here it is all done and filled with cement:

ZM2LnQT.jpg

I allowed it to cure for 24 hours and pulled the forms off. Even though I cut a lengthwise slit in the inner PVC pipe, I still had to pound it out. Cement always shrinks towards the center and makes getting the center piece out a xxxxx. I put a couple of hose clamps around the ends of the cylinder as that part has a tendency to want to crack and break during form extraction. 

Edited for Language.

Here's all the pieces afterwards. I built an acrylic ring to seal the bottom between the two forms. I also used lots of plumbers putty and Gorilla tape to seal the cracks where the "brick" went through the forms. The actual brick fits in the hole nice and snug. 

IaQy7wm.jpg

Q1ZzArc.jpg

Once I fix the angle the burner is mounted at (probably horizontal) I'll cast a level platform for the work within the cylinder.

 

Ted

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I'm not real happy about the way I built the burner. I think the basic design is fine, but the steel tubes are too long (and not exactly true) and stress the brick as they are inserted. I would just go ahead and cast it, which is physically stronger, but I found that it works so much better with a taper starting at the end of the tubes. The taper slows the gas down enough to keep the flame burning right at the brick, even at full air pressure. That taper would be a challenge to cast. 

I may try a hybrid: half cast, half brick. I can try a number of variations since the burner is removable. 

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Alternatively you could wet some fiber blanket with rigidizer and poke the holes where needed to fit the tubes.  Just keep stacking the blanket on until you get to the end of the tubes.  If you have small cones of the right shape you should be able to easily form the wet blanket to your desired shape.  You might have to coat the final layer with something to hold up to the heat better though.

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40 minutes ago, JustAnotherViking said:

Would it be feasible to cast a solid block and then just drill and profile, rather than trying to cast around a form for the holes? 

The Kast o lite sets up pretty hard, although it may be possible to machine it within the first 24 hours of curing. The drilling would probably be dicey, but tapering might work. I'll cast a test piece and see how it goes. 

1 hour ago, Buzzkill said:

Alternatively you could wet some fiber blanket with rigidizer and poke the holes where needed to fit the tubes.  Just keep stacking the blanket on until you get to the end of the tubes.  If you have small cones of the right shape you should be able to easily form the wet blanket to your desired shape.  You might have to coat the final layer with something to hold up to the heat better though.

If I have to I can machine a bunch of tapered cones to mold around, but it's a lot of lathe work. If I can run the taper through some semi cured cement that would be optimal. 

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Nice build Ted. I see you discovered the problem with using a hard inner form casting a cylinder. I wrapped a piece of sono tube with a couple layers of news paper but the refractory saturated the paper and it never left the liner. Once the refractory sets slit the sonotube with a utility knife and it peals right out. The outside form doesn't really matter the refractory shrinks and Sonotube sometimes slips off but it peals easily.

An issue I discovered with multi outlet burners is heat build up in the block to the ignition temp or increasing the rate of propagation till it burns back, either way it's not a good thing. 

I experimented and tested by drilling holes in short 2" x 4"s and screwing it to the plenum. I built Naturally Aspirated (NA) rather than blown (gun) multi outlet burners so finding a balance of back pressure to the T inducer and outlet velocity was the main issue. I thought about casting or grinding tapered outlets but discovered that adding or subtracting the number of outlets was WAY easier.

I use crayons (approx 3/8" dia.) as lost wax cores to make the outlets. The mold shell is 3 part: the base is a piece of 2" x 6", sanded and painted to allow the refractory to separate more easily. I have to use a release agent or Kastolite bonds to every-darned-THING! I found vegetable shortning like Crisco applied liberally worked best. There are holes drilled in the pattern I found works, to hold crayon outlet cores. The sides of the flask are 2" x 2" sections screwed in 2 "L" sections that screw into the base and together to form the mold. The "L" side sections ID is the important dimension.

I cast the burner blocks with Kastolite 30 and once it's filled to the desired level I simply press the plenum into the refractory and let it set and cure. 

I make the plenum by sawing one side from a piece of 2" x 2" square steel tubing and weld cut pieces of the drop onto the ends to seal it. To make sure it won't separate from the refractory with the thermal cycling I weld a SHORT piece of 3/8" expanded steel to the long edges. It allows the refractory to "grip" the plenum and it acts like rebar.

That's it, as simple as I could make it. 

Frosty The Lucky.

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3 minutes ago, Frosty said:

Once the refractory sets slit the sonotube with a utility knife and it peals right out.

I just let it dry for a few days and then burn the sonotube out. I figure I'm less likely to damage the casting that way.  No matter how you do it, if it works it's good.

However, the first time I did that I found out that a dry sonotube will suck enough water out of the Kastolite to cause cracking in thin castings.  The second time around I made sure the sonotube was good and damp before applying the Kastolite.  And, per your advice, I covered the whole thing with a thoroughly dampened blanket and left it that way for a few days until both the blanket and the sonotube appeared to be dry. Much better results that way.

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I burned it out of the first one I made and it worked fin. I'm not sure about Kastolite though but if it's had time to set completely it won't matter if you get fire on it. Heck, fire the burner in it.

Perhaps wrap the sonotube with visquine so it slips out. We used to do that for concrete forms that had to come off or to prevent the ground from gripping and letting freeze thaw jack the "pier" out of the ground.

Frosty The Lucky.

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2 hours ago, Ted Ewert said:

The Kast o lite sets up pretty hard, although it may be possible to machine it within the first 24 hours of curing. The drilling would probably be dicey, but tapering might work

Fair enough. It's not something I have tried to work with before, it was just a curiosity that crossed my mind. If we can drill steel, bricks, etc, how hard could it actually be? 

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I've found that no stick BBQ grill spray works pretty good, which is what I used for the PVC pipe. Wax paper is also good in some places. 

3 minutes ago, JustAnotherViking said:

If we can drill steel, bricks, etc, how hard could it actually be?

The problem is cracking and splitting. If it holds together, a masonry bit in a hammer drill will make short work of it. Nevertheless, you also want straight holes that line up. That's almost impossible with a hammer drill. Casting is still the most accurate method.

I've done a fair amount of casting, and I have a pretty good idea how I'll solve this one. I've got some more Kast-o-lite coming in tomorrow (the tube took about 18 lbs out of the 20 I had).

Thanks for all the helpful suggestions!

2 hours ago, Frosty said:

An issue I discovered with multi outlet burners is heat build up in the block to the ignition temp or increasing the rate of propagation till it burns back, either way it's not a good thing. 

I've thought about this, and it is a concern. I really won't know until I fire it up for a good long test, but I'm hoping that the tubes are long enough, and the air velocity fast enough, to keep the flame from anywhere near the plenum. If the plenum starts smoking you got trouble. 

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5 hours ago, Ted Ewert said:

The taper slows the gas down enough to keep the flame burning right at the brick, even at full air pressure. That taper would be a challenge to cast. 

You are over thinking things. Taper or step will give nearly the same effect.

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