Recommendation? Forge Insulation Liner Query

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swan_king

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Dec 9, 2011
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Hello everyone!

I have moved into a larger space here and I am building a larger forge. This will be a blown unit, (which I have never built or used previously.) Naturally, I have a few questions.

First, I know Wayne Coe always is comfortable with 1" insulation. If I use 1" of maybe Mizzou or Kastolite 3000 over the wool layer, can I get away with an 8 cu. ft./lbs. 1" insulation that is a 2300° max? Or is that temp wool too low if I want to get to forge welding temps?

Secondly, my forge skin is a 16-gal barrel. The chamber volume with an inch of insulation and another of castable refractory comes in at 1145 in³. At that volume, I would need several atmospheric burners, but I don't know about a blown system. Would a single, tangentially-set blown burner be enough for that volume? If so, what size is recommended? 16-18" seems deep for a single unit, (but what do I know? ). Longitudinally, where would it need to be in relation to the midpoint?

Finally, without getting into the "burner wars" , would I be better served with a ribbon burner? I don't necessarily want to go that route, because it seems to add a layer of complexity to a possibly finicky system. I believe I would rather have two blown burners than a ribbon, but I am open to being convinced.

The satisfaction of getting a new shop space is lessened by all of the new benches I need to build, woodworking and other equipment to rearrange, and a 2x72" to construct. I am even stressing to get this new forge done so I can move on to other things. I appreciate all of the knowledge advice you folks have for me, especially if you have experience with blown forges. Thanks!

- Chris
 
Assuming your drum is 14X25" I would suggest you either have two burners, or shorten the barrel to 20". a 20X14" shell is perfect. It will give you a 16" long by 8" chamber.

1) A cast liner is great. Make it 1" thick. They will take a while to heat up, buy have much more even heat.
2) Use 1" thick #8 wool. Wrap two layers around the liner. Buy a roll (25') and you will have plenty.
3) With the above info, you will have a 8" chamber around 16" length, which is more than big enough, but will run on one blown burner. My quick head math says that is around 800 cu.in.
4) A ribbon burner is nice, but more complex than you need if you haven't built forges. Make a normal blown burner with a good size blower and a 1.25" burner tube.

Other tips:
Use insboard for the ends. One piece of 1" thick is sufficient. No need for wool or castolite liner unless it is a welding forge.
Use a mixing chamber in the burner manifold.
 
Huah, sir. Much thanks for the reply. I have previously built a small bucket gas forge with a single venturi burner, so a blown forge is new for me. And I wasn't sure what the rate of burners to cubic inches would be...besides more than the 350ci for a single draft burner.

I have 2" of wool and about ½" refractory, so it seems like you would go ahead with 2" of wool and double the castable over what I have now. Is the 8# 2300° max temp stuff good enough for forge welding temps, if protected by a 1" thick refractory liner, then?

The drum I have can be cut down to accommodate the length of chamber you suggested. In all of the various forums and posts on blown forges, I have seen burner pipe with everything from ¾" on up. With the proposed chamber volume, insulation, and the desire to be able to forge weld, the single 1.25" burner is sufficient?

As for a mixing chamber, I will go back over one of the how-to threads in the stickies where you made suggestions about how best to introduce the propane for the most thorough mixing.

Thanks again for the time and answers.
 
The standard 8# wool will be fine as long as there is a refractory protecting it from the flames. 1" of castable will be plenty of protection!

A 1" burner will be enough, but if you are going to weld, a 1.25" burner will allow more heat. You can make two burner tubes - 1" and 1.25"- and change them as needed at the elbow after the expansion chamber.

An expansion chamber is a place in the manifold after the gas enters the airstream where the pipe size increases by around 50%. This slows the airstream and allows mixing of the gas and air. After the chamber, the pope goes back to its original size and then reduces to the burner tube size. A 4" section of 3" pipe is usually what is used for the expansion chamber. This is a typical forced air blower setup:
Blower with 3" pipe coming out - reduced to a 1.5" Tee with gas line entering from side by 1/4" pipe - 1.5" short nipple - 4" section of 3" pipe with 1.5X3" bell reduces on each end - short piece of 1.5" pipe - 1.5" elbow - short section of 1.5" pipe as needed - reduce to burner tube size - enter forge.

Technically, it is best for the blower to be higher than the forge, but 99% of forges put them below the forge or level with it.
 
What's your plan for such a large forge? My first gas forge was out of 7 gallon propane bottle and ever forge since became smaller than the previous one.
 
I was going with the size he gave and assuming that he would do larger knives and welding in this forge. I agree most folks make way too big a forge. A chamber of 3" diameter and 12" length will suffice for most knife work. The standard build I suggest to people asking for suggestions onan all around forge is a 4" chamber and 16" length.
 
I wanted a chamber wide enough for some structural, artistic type things or oddball shapes -- scrolls and the like, not necessarily for knife blades. Same for the length, but especially if I do decide to try doing longer blades.

It wasn't necessarily appealing for me to have to build one, only to build a larger one later. (I also confess that the narrow barrel I had seemed to be ideal when I initially thought of doing 3-4 atmospheric burners and adjust the length of my chamber.) No one size fits everything, but I was looking for compromise since I imagine I will continue to have more call for items that just narrow knives.

There are some smaller steel buckets lying about here. I could always try a smaller unit and see if and when I need to grow from that. ‍
 
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Yeah, you're right. I guess my thinking is not in the right dimension. The length of my chamber is not as big of a concern, since I likely won't be making swords and the like, but I could widen it near the floor.

I think then, given your advice, I will trim my barrel length down a few inches to have a more manageable chamber depth. The floor can built up a bit underneath by welding in a plate, then wool, then the refractory. Anything will seem larger than my little steel pail forge!

Thanks again.
 
That works.
When building a vertical welding forge with a wool and satanite liner, it is easiest to just squish the shell into an oval shape.
 
I can see that it would. I am not familiar with the ins-and-outs of a vertical forge. While I have seen a lot of positive response for them, I am not sure I would want to go that direction. It would be interesting to see one in action. Perhaps that should be my YouTube homework this weekend.

In a post from a few years ago, you mentioned 1" of mizzou, and a bubble alumina floor for obvious flux protection. Would you do anything different from that time? Most write-ups seem to go that same direction, more or less.
 
A verticle forge is most useful for damascus welding. The floor on a horizontal forge should be covered with bubble alumina or another refractory that is flux resistant. A verticle forge has a couple inches of kitty litter in the botttom to catch the flux.
 
Good. Bubble alumina floor was what I had planned, even if I don't plan on slopping Borax all over the inside of my chamber!

I ordered wool this evening, along with a 2hp, 3ph TEFC ~3600RPM motor. Tomorrow I'll be out in the shop, deciding if I should cut down my barrel and try to shrink some interior space, or just root around for a useable steel 5-gal bucket to use instead. Methinks that might be the easier route for a more manageable chamber than reconfiguring the larger barrel. Time and effort on making the proverbial "square peg in a round hole" might be wasted when a simpler solution could be found in the barn or at the hardware store.

Thanks also for your advice on plumbing the blower works. I have plans downloaded, so now I need to see if I can make sure they fit the parameters you suggested and vista the hardware store. My plan when deciding to go with a blown forge over my existing atmospheric burner was to have a frame to keep the blower up above the forge body. A power outage and the possible grenade-like "boom" from pooling propane gas would not be good for my nerves.
 
A simple $15 solenoid on the gas line at the needle valve stop the concern about propane pooling if there is a power failure.
 
I have seen this before in a post; perhaps one of yours. I may consider that, though I have the room to put the blower anyway. I also have my HVAC guy looking to see if he can scrounge a suitable blower to fit the parameters laid out on here.

My shop opens into my barn at the back, and only one room is climate controlled. My forge will be back in the darker barn area. The smaller pail forge I had with a NA burner worked well enough, but the draft through their played havoc with my flame on some days. Having a blown forge will be nice if for nothing else just giving me a consistent flame.
 
The often forgotten concern of larger forges is propane cost. A larger forge, like a 3 burner majestic, can be up to $7 per hour to operate. That's the reason a lot of smiths will have a couple different forges, one big and one small. Use the smaller forge except when absolutely necessary due to size.
 
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