My forge design.

[Ross_E]

UPDATE: I'm not building a burner, I am not adding a secondary blower and I will use a standard typical design.

I am leaning towards purchasing a T Rex 3/4, it would be nicer then anything I could build and someone else has put a lot of thought into it. Does anyone have one?

I am still considering different methods and designs for my forge.

Again, my goals start with tools I can use (tongs and so on), from there I want to move to blades and eventually weld patterned blades so I don't need anything too large.

The two basic designs I am interested in are the tank style, where a steel tank is lined with insulation , and the box style which I would build with fire brick or cast it and possibly line with a second layer of ceramic or wool or both.

Is there a preference when it comes to casting material? I guess I shouldn't be too concerned about the out layer insulation if I am going to use a wool, how is cast-o-lite?
Are all ceramic coatings comparable?
Thanks,
Ross

Edited October 18, 2008 by Ross_E

[Ross_E]

bttt


Thanks

[Ross_E]

No advice on even brand names????

[Ice Czar]

Personal preference
your basic Insulated Firebrick
(precluding a cylinder unless you miter the bricks by hand)
and Plastech 85P as a hardface
(a high alumina phosphate bonded plastic, max temp 3100F, rated for direct metal contact, Thermal conductivity 2.12 W/m C, modulus of rupture after 650F 2000 lb/in2
after 2550F 2700 lb/in2, at 2550F 500 lb/in2) Runs $80 per 55lb bag


get the best of both worlds, a highly insulative barrier with low thermal mass and a thin strong and chemically resistant hardface with a high thermal mass. that will withstand considerable abuse. But then my experience is architectural where the forges where beat on daily by the stock in them. It would have an acceptable thermal inertia, and excellent durability.

But it wouldnt be cheap, and youd have enough hardface for a few forges
Given its shelf life hard to not see some of it go to waste. Upside its resistance to flux should be good.

Edited October 19, 2008 by Ice Czar

[Frosty]

Ross:

I'm curious about your drawing. Can you explain the reasoning behind the open bottom?

As for refractories, there are quite a few castables and rammables that will fill the bill. Putting an insulating layer around the outside is a good idea but you'll need a ceramic blanket, just anything won't do.

A coat of ITC-100 IR reflective coating will increase efficiency quite a bit.

As Ice Czar says a high phosphate or phosphate bonded refractory will last a long time in contact with welding fluxes where ceramic blankets, soft fire brick and most castable and rammable refractories won't.

High alumina refractories are more resistant to fluxes but not as resistant as phosphate refractories.

Brand names change all the time, your best bet is to talk to a local furnace company or supplier. Those guys LIKE fire and will usually be enthusiastic helping a blacksmith design a forge liner. The guys I go to E.J. Bartell give me significant discounts and usually give me drops of Kaowool, brick and samples to test to destruction.

Frosty

[Ross_E]

Frosty, believe it or not, the image you are looking at is of the other axis. I know it's kind of hard to make out as it's done using MS Paint. The overlap of the wall would be to allow a secondary blower to create a an atmosphere that would allow what you probably call reducing without fire outside the door.

I now know that this is not a typical design, and whether it would work efficiently or not, I am going to just basically lean on others experience and go with a proven idea.

[Ross_E]

Personal preference
your basic Insulated Firebrick
(precluding a cylinder unless you miter the bricks by hand)
and Plastech 85P as a hardface
(a high alumina phosphate bonded plastic, max temp 3100F, rated for direct metal contact, Thermal conductivity 2.12 W/m C, modulus of rupture after 650F 2000 lb/in2
after 2550F 2700 lb/in2, at 2550F 500 lb/in2) Runs $80 per 55lb bag


get the best of both worlds, a highly insulative barrier with low thermal mass and a thin strong and chemically resistant hardface with a high thermal mass. that will withstand considerable abuse. But then my experience is architectural where the forges where beat on daily by the stock in them. It would have an acceptable thermal inertia, and excellent durability.

But it wouldnt be cheap, and youd have enough hardface for a few forges
Given its shelf life hard to not see some of it go to waste. Upside its resistance to flux should be good.

I like the idea of superior durability. It's my first forge and I was never good at "Operation".

[Ice Czar]

my personal preference derives from repairing the remains of what 3 to 5 smiths stuffing large architectural stock in on around over and under each others, does to a forge

but could equally apply to the odd oversight (oops too far) slip (xxxx support) accident ($#@#!$# hot) or premeditated chemical attack can do to a forge.

Ive seen most refractories go tits up one way or another, wool just dont last in high traffic, IFB (Insulated Firebrick) varies from busted straight off to progressively gouged to death, average hardface over wool, death by a thousand cuts, and full blow hard kiln firebricks, and cast refractories while durable and resistant to chemicals cost alot to heat up and transfer a whole hell off alot of energy away from usable work.

my suggestion can also be applied on the cheap by digging a hole, filling it with ashes or vermiculite and adding a ceramic pot or a lining of clay. Same basic thermal design, not as durable but easily replaceablerepairable

same kind of thermal inertia. might be a good place to start to "sort" your design features

if you do go with a high alumina phosphate plastic, figure out a way to use it all inside of 6 months, maybe a crucible furnace, some molten metal molds, more than one forge, an annealing kiln ect ;)

Edited October 20, 2008 by Ice Czar

[Ross_E]

UPDATE: I've acquired the materials to begin the forge.

Here is what I went with:

-Superwool - Rated for 2300F
-Soft fire brick
-ITC-100 HT

These are the two designs I am trying to decide between.
-Cylindrical Design
-Simple Brick Design

Of course I would not build these exact forges. If I went with the cylinder, I would look for something heavier then a metal bucket, and if I build the brick I would build a little but larger. I don't have any way to weld, so I am stuck using other methods.

Is there a preference amongst smiths when it comes to the shape of the chamber?

Note: Heat source with be a 3/4" TREX propane burner, volume of the chamber whichever it will be will be based on his websites data sheet.

Thanks!

[Frosty]

You don't need anything heavy for the forge shell, a steel bucket is more than strong enough unless you're planning on putting pieces in it that weigh more than a couple hundred lbs.

I'm rapidly losing my feelings for soft fire brick, they're just too fragile and I'm easy on a forge. Just a few heat cycles and they begin cracking.

A 3/4" will heat 350-400 cu/in volume to welding temp, a Rex is good for the upper limit easily. Shape will effect this number. If the chamber is long and narrow a single burner won't heat the whole thing evenly.

Frosty

[Ross_E]

Frosty:

I'm glad you mentioned this about the soft brick because I was thinking about applying my pint of ITC onto that brink, which would be a waste of the ITC. I actually cracked one of the bricks as I removed them from the box today.

What would be your ideal order of material?

I was thinking chamber > ITC > cast > wool> shell.

I have bought the soft brick, but after examining it, breaking it and speaking to people I'm wondering where it will fit into my forge as I am leaning towards a cylindrical.

[Frosty]

A cylinder works pretty well for bladesmithing, it allows you to aim the burner tangentially for a vortex and more even heating throughout the chamber. Even heat in the chamber is more important for blades than general stuff.

I'd say you have the order right. Ceramic blankets aren't usually rated high enough to be in contact with the fire though lots of forges are lined with it. Even the 2,600f rated blanket will degrade over time and welding flux goes through it like hot water through cotton candy.

I use Sonotubes to form up the hard refractory. One tube has an OD the same dia as the inside of the chamber and the second tube has an ID from 1 1/2" - 2" larger. Center one tube inside the other and ram/cast the refractory into the annulus. (gap)

Once it sets, peel the Sonotube off so it'll dry more quickly. Before it's dried though drill the hole for the burner port. I've had the best luck using a sacrificial hole saw and doing it before I peel the Sonotube, timing it just as the refractory begins to set.

When the inner liner is set wrap it with Kaowool, etc. and slip it into the shell. If it doesn't want to go use a sheet of newspaper to wrap it tightly compressing the blanket, (Just like a piston ring compressor) don't worry about getting the paper off.

It's a good idea to use either a ceramic blanket stabilizer or something else to encapsulate the blanket where it's exposed on the ends. I used hard refractory on my old forge and it worked okay.

Coating the inside with ITC-100 is the last step. If you made steel ends to contain the ceramic blanket coating it with ITC-100 will help it live longer.

Have you checked out Don Fogg's blade forge? forge

Frosty

[Ross_E]

I see. I am familiar somewhat with those forms, but I am a little lost.

Are you saying you cast two shells, wrap the smaller shell with wool and lower it into the larger one? If so I understand completely.

Seeing how the inner shell will be floating, is it recommended to compress the wool?

Also, I notice you left out any mention of a outer steel housing. Is it recommended to use the large cast as the outer most layer?

Thanks a bunch!

[Frosty]

Just one hard refractory liner cast BETWEEN the Sonotube forms. It should be at least 3/4" thick but doesn't need to be much more than 1" thick. The OD of the small sonotube forms the ID of the refractory liner. The ID of the larger sonotube forms the OD of the refractory liner.

Light weight shells work just fine, my preference in a pipe forge is SS stove pipe. My old pipe forge was made with a piece of 10 1/2" x 1/4" wall steel pipe and with liners it's a real bear to move around and it's chamber is only 6" x 12". The weight to useful volume is WAY too high, it's not like I'm going to put a dozer in it..

Compressing the wool blanket is just to get it in the shell if necessary.

Frosty

[Ice Czar]

addressing the last question, from the point of thermodynamics

if the level of insulation between the interior and the exterior is perfect, it wouldn't matter at all how large a thermal mass is on the outside.

generally the denser and thicker the material the more thermal mass it will have. What that implies is that since your insulation isn't perfect a larger mass will wick away more heat than a thinner.

the 2nd law thermodynamics covers that.

you would use a thicker exterior if you anticipated a thinner one eventually getting enough heat to oxidize and corrode. Ideally you wouldn't have a "burn through" your insulation and could have a thin shell for greater efficiency (not having to replenish heat to a larger thermal mass even at a greatly reduced rate). Or accept that if you do get a burn through you'd patch the thinner shell and replace the insulation.

On the other hand accepting less effciency and a more robust sidewall means you can ignore a burnthrough for a longer period of time, but at that point the forge will have a large thermal inertia (take more energy to heat up or cool down)


avoid crushing insulation, the actual insulator is the air in the wool, not the wool iteself. How effectively the wool traps air, how much air, and how little air moves in and out of the insulation determines its insulative value.

Wool is pretty open to air moving in and out of it, whereas Insulated Firebrick (IFB) is very good at holding air inside the brick.
BUT
IFB is much better at conducting heat through it because its far more dense than wool

a few other bits of thermodynamics to consider

the Arrhenius Equation basically it say the hotter something is the faster it eventually goes to hell in a handbasket. (the more chemical activity occurs)
this is ultimately why all refractory materials fail. A rule of thumb being for every 10C increase, the lifetime is cut in half.

Differential Temperature: The difference between two temperatures. Conduction through a solid and convection between a solid and liquid depends on temperature differential. If a given solid has a temperature on one side of 25�C and 35�C on the other, the heat transferred will be identical if the temperatures become 45�C and 55�C.

Conduction Coefficient: A measure of how efficiently a solid conducts heat. Metal has a xxxx fine ability to transfer heat, whereas IFB isnt nearly as good.

Conduction: conduction is the transfer of heat through a solid material. It follows a simple equation that is worth empirically understanding. (and is why I gave you ther terms above)

Q=kL(T1-T2)

Q (Heat flow) = k (conduction coefficient) divided by L (the thickness of the material) times T1 minus T2 (or the temperature differential from one side of the material to the other.)

"We can draw some interesting conclusions from this equation. First, heat transfer is proportional to the temperature difference on the object. If the temperature differential doubles, the heat transferred doubles. Second, the conduction coefficient "k" is proportional to heat transfer. If the conduction coefficient doubles, the heat transfer doubles. Alternatively, for the same differential temperature, twice as much heat may be transferred. The final observation is "L". As thickness increases, heat transfer decreases. Alternatively, to maintain the same heat transfer through a material twice as thick requires twice the temperature differential. "

Newton's Law of Cooling (interactive demo) pretty much describes the same but for convection instead of conduction. (The heat transfer to the air that escapes the insulation and the heat transferred off the exterior of the forge to the shop's air)


summary: the denser the material the less of it you want to use, less mass it has the more you want to use.
BUT
the bigger the space your using for all these materials the more space ultimately has to be heated. And ultimately how efficient a forge is, has a fairly large amount to do with the ambient temperature in the shop itself.


just like any physics its tradeoffs,
like in racing, more down force the faster you can go around a corner,
but the greater the drag the slower you go on a straight away. A winning race car strikes a balance. Edited June 22, 2012 by Glenn

[Ross_E]

I'm having trouble finding an outer shell to house the refractory. I've been to ever hardware and paint store in my area and they have all said the same thing, no one sells metal anymore.

Here are my option if I want to use a cylindrical steel housing.
-Just buy the steel bucket with acetone or whatever in it.
-Find an old one.
-Go to a machine shop and just have something made.

I've also thought about building a rectangular housing and casting a rectangular refractory but still have the chamber as a cylinder. This would be easier then finding something to cast the chamber with (no luck finding Sonotube in my area)

Another issue I am having is deciding wheter to "float" the hard cast shell in the 2" wool, or to cast mounts on it. I'm just wondering "legs" on the shell would create a heat sink effect because the surface area would be greatly increased.

Any ideas?

[Frosty]

Buy SS stove pipe. If you can't find the diameter you need off the shelf you can make it by snapping different dia pieces of stove pipe together along the seam.

A big plus of using stove pipe is you don't need any special tools to work with it, tin snips, hand drill, hole saw, pop rivets or sheet metal screws, etc.

Another plus is you won't have to do anything special to get a Kaowool wrapped hard liner in it, just close the last seam with it in.

Lastly, SS is a poor conductor of heat and being shiny reflects more IR back so it's more efficient.

Frosty

[Ross_E]

I see.

Well, the 2nd cast is done. It's not as nice as the first one but it's one piece!

Now about the burner placement. I see it is recommended to be placed at an angle.

Question, should the burner be near the middle or closer to one end?

[mcraigl]

Ross,
I just finished my gasser last night and will post pix in the next few days. I'm using a blown ribbon burner so there are some differences, but I put my burner in the middle as that's how I've always seen them done. My forge is a piece of well casing pipe 12" in diameter by 18" long lined with 2"thick kaowool coated with itc100. The burner is level, but placed above center on the side of the pipe and it is generating a real nice vortex inside the forge. Again, I'll post pix later. maybe I'll get a couple of quick pix and post them durring intermission durring blue prints tonight. When my kaowool gives up the ghost I think I'll cast a liner about 2" smaller od than the id of my pipe and then wrap with 1" kaowool and newspaper like frosty was talking about. I expect to get several years out of the current setup though. Anyway, to answer you question put the burner in the middle. sorry for the crappy cell phone pix...
ML

Edited November 5, 2008 by mcraigl
added pix