First forge: will this do? Am I getting the math right?

[Alwayslearning]

It has been a long time and a few head injuries since I had to do geometry, so please, be kind..  I just want to order my supplies and not find out I screwed up my calculations!

i was given a stainless steel fire extinguisher from a commercial kitchen for use as a forge body.  The base of the cylinder is 7" diameter, and the height is 20" (20.5" if you count the threaded part at the tapered end where trigger and output were formerly attached).

if I wanted to line this tube with two inches of inswool, some rigidizer, and some ITC 100, how much inswool would I need to buy?  I'd be buying from High Temp Tools, and they sell in lineal feet.  From an online cylinder calculator, I got 440 sq. inches for lateral area of the cylinder,  so about 3 sq. feet for the first layer.  Subtracting 1" from the original radius gives me 2.5" for radius of cylinder with 1" of inswool, so plugging THAT into the cylinder calculator gives me 2.1825 sq. feet for the second layer of inswool, right?  That makes my total just over 5 square feet of inswool to line my cylinder.   I notice (now) that I didn't include both ENDS of the cylinder's area.  Ugh.

I'd love this to be a versatile and efficient forge (of course!) but I understand that this shell might be limiting after a while.  I want to start with making some tooling and bladed items, but would eventually like to do some small decorative  work (not making a wrought fence gate).  I also have some SS chimney that is a little larger in diameter and about 23" long.  Another thing, I've got a bunch of soft kiln bricks, so that is yet another option.  Oh, and a Frosty T burner almost finished.

Should I make this a vertical or horizontal forge to be most versatile?   With a 3" diameter hole to put stock in (after it is all lined), has this already become a "silly forge I should not make"?

i realize similar individual questions like these have been posed and answered on this forum, but you as you can see, all the questions together kind of tangle/link the variables into the mess I am having trouble with.  I get thinking too much and I don't get to hit hot metal!

As always, thank you for your kind and thoughtful tutelage.

[Glenn]

28 minutes ago, Alwayslearning said:

The base of the cylinder is 7" diameter,

Is that ID or OD?

Let us say Inside Diameter is 7 inches, minus two inches on the left and minus two inches on the right leaves 7-2-2 = 3 inches Inside Diameter after insulation. Get a piece of 3 inch plastic pipe and play with the size to see if it fits your forging needs. Remember that you can not bump the sides of the pipe or in the forge it will tear the insulation.

[Frosty]

Welcome aboard, glad to have you. If you'll put your general location in the header you might be surprised how many IFI members live within visiting distance.

Rather than worrying about how many sq/ft you'll need stick to linear measurements. it's easier. Lining a cylinder 7" x 20" Ball park 7"x 3.15" I know that's more than the real circumference but for this it's just fine. Ball park 22" x 20. Buying Kaowool off the roll is 24" wide x whatever you buy.

If you leave it just a little wider than fits compression will hold it in the shell.

The trick getting that shell to work well is the shape of the chamber @ 3"dia. x 20". To get even temps you're going to need to install several small burners. I get around 140 cu/in so a single 1/2" T is about right for the volume number. Unfortunately it's darned narrow and really long so a single burner is going to have maybe 10" just SCREAMING HOT and the temperature will fall off as the distance from the burner increases. A 3/4" T probably isn't going to work worth spit in that narrow a space, back pressure will be a serious issue.

You might be able to make that shape work with two or maybe three 1/2" Ts. That will be 2 or 3 x the burner to volume but you can turn the pressure down. You're definitely going to need to align the burners tangentially. This will cause the flame to swirl, (induce a vortex) doing 2 things. First it'll help even the temperature in the forge chamber and secondly it'l relieve the back pressure against the flame. If you aimed the burners directly at the far wall it will impact it and be driven back against the flame from the nozzle. Too much back pressure and NA burners don't work so well.

Personally I'd find something else, just because you found something doesn't mean you HAVE TO use it. We all fall into that trap now and again, don't sweat it. I'd use stove pipe were I to want to make a cylindrical forge. SS is my favorite but black works a treat too. Avoid galvanized it's not something you want near a forge's direct heat.

Frosty The Lucky.

 

[WayneCoeArtistBlacksmith]

Read the attachments at the Forge Supplies page at www.WayneCoeArtistBlacksmith.com.  This will tell you how I like to build a good, efficient, long lasting forge.  You may decide that you are better off getting an old useless 20# Propane bottle and starting form there.  Do you want to build it with what you have and then start over when you decide that was not the best way to start.  Plistix or Metrikote are way cheaper than ITC-100 and do the same job. 

Let me know if I can help you.  You can send an e-mail or PM.

[Alwayslearning]

Thank you Glenn, Frosty, and Wayne -for your helpful replies!

will probably go with an old propane tank, I've carefully "decommissioned" a few for previous musical projects ("Hank drum"), so I'll probably give that a go after checking Wayne's plan as well.

Onward and Upward!