Forges 101

[Mikey98118]

Okay, what I was hoping one of you would jump on was...although we have got smart enough to slow fast flames down much more than previously (over the distance that the combusted gas takes to be ejected out the exhaust port)..this brings up a new limiting factor. Yum! a fresh target to attack.

If we could brake gas acceleration of the flame front completely, there is still heat expansion of combustion products to account for. No way are we going to get around that one. So, the less combustion needed to achieve a given equipment temperature, the slower the rate of energy loss through the exhaust port.

Wee! Around and around the merry-go-round. We have worked hard to get higher combustion temperatures out of gas flames, to avoid the expense of hotter fuel or oxygen enrichment.

Now we can deal with efficiency, by seeking less flame through hotter fuel, rather than using hotter fuel to overcome weak burners. Thus, the importance of a hotter fuel that allows us to turn down gas pressure. Fuel cost versus flame temperature cancels further expense, leaving smaller flames to reduce heat less through the exhaust port to provide savings

MAYBE. Or maybe yes, maybe no...

[Frosty]

I thought that was a give, I even experimented when I was trying to figure out how fuel air behaved in the burner. I made a smoke generator to supply visible "gas" through the jet and another to supply smoke in the combustion air.

Without the heat of combustion a NA burner has surprisingly anemic output. 

I believe the bulk of the velocity and volume is the superheated water vapor combustion byproduct. For those of you out there following Mike and my musings and aren't aware of the phase change volume increase of H2O I'll run this ratio, with the understanding that the higher the temp the greater the expansion.

1 gallon of 212f water, heated to  make 212f steam makes 1,600 gallons of steam. Yes, water expands One THOUSAND Six HUNDRED times.

The expansion between water and 2,600f steam is REALLY big but I'm not going to search out a chart.

Another thought to consider is that is maybe hot enough to degenerate water into H and O2 which will re-combust in the forge. Maybe? I don't know and my eye is too dry to do a bunch of looking online.

So, there is the inherent problem multiplier for producing a low velocity flame. 

The easy solution is to use a hydro carbon fuel with a greater ratio of carbon to hydrogen. The expansion ratio of CO2 at 2,600f is a small fraction of water's. Gaseous C is also MUCH more energy dense with a higher max flame temp than typical hydrocarbon fuels. Acetylene is C2H2 for example. Does that qualify as a CarboHydrate fuel?

And there you have my past musings about alternative fuels, exhaust pressure and hag time. As always, once we've determined the optimum burner for the home builder we can start designing forges to take the advantages and mitigate disadvantages.

Were you thinking of something like this Mike? 

Frosty The Lucky.

[Mikey98118]

See there; I knew you would have lots of good stuff to say.

The more we muse about it, the more ideas all those lurkers out there will consider, too. All the factors you brought up,  are worth mulling over, since just what fuel is eventually picked, will circle around them. Not that I would ever try to coax others down to the deep end of the pool--honest  

[Kliff]

Ok.  After quite a bit of reading, and research, mostly here, I think I'm ready to build my first forge.  I'm hoping for a little feedback on my design before I start.  I'm planning a double burner forge, using two Frosty T burners.  I've attached a photo, with calculations, and descriptions if anyone is willing to take the time to review it, it would be greatly appreciated.  It's basically:

-a mailbox design, 16" L, 12" W, 11" H (Outside)

-1/6" (16 guage) shell.  This will allow me to weld burner ports & feet on, yet keep it light enough to easily move

-(2) layers of ceramic fiber blanket rated to 2400 degrees F

-rigidized using cabosil fumed silica (one layer at a time)

-coated with ITC 100 HT (rated to 5000 degrees F)

-high alumina kiln shelf 3/4" thick, cut to fit bottom

-two frosty Tee burners (3/4") placed at about 10:15, placed .3*L (about 4 3/4") in from each end.  Mounted in drilled & tapped ports that will allow for angle adjustment.

One question I have:  Is the end of the burner tube mounted so it is flush with the final inner layer of the coating, or is it held back a bit?  

If you prefer, you can e-mail me directly:  note email removed (use PM)

Edited December 7, 2021 by Mod30
email address removed

[ThomasPowers]

What is the proposed volume of the inside of the lined forge?  The outside dimensions are not of any use.

No hardshell layer for flame impingement/abrasion resistance?

Plistix is much better than ITC100 and MUCH cheaper.

[Buzzkill]

Here's my 2 cents.

The internal dimensions are the most important.  You didn't indicate what you plan to forge so it's hard for us to evaluate whether your plan is suitable.  Generally speaking, sixteen inches is longer than needed for most things if you can pass the stock through both ends.  You are paying for fuel to heat the entirety of your forge chamber, so the bigger you make that chamber the more it costs you to heat it.  For me 10 to 12 inches in length is about right.  The height is also a couple inches more than I would use, but it's not crazy big.

You don't appear to have any plan to line the fiber blanket with a hard coating that can take some physical abuse of stock rubbing or stabbing into it.  There are some people who build their forges that way, but you will most likely have to reline your forge more often if you go that route.  ITC 100 is expensive and is not better than a couple other coatings for this application, such as Plistex or Matrikote.

It appears as though you plan to aim your burners at the center of the floor.  Getting the flames to swirl is preferred in a gas forge and that is better accomplished by aiming the flames at the near side of the floor or at the opposite wall at an angle rather than at the middle of the floor.  For a gas forge ideally you won't have direct flame impingement on the stock.  The flames heat up the internal surfaces and those radiate the heat back into the forge interior to heat the stock.  You *can* heat your steel by aiming the burners directly at it, but if combustion is not complete before the flames reach the stock you can get some scaling and/or super-heated spots that can have detrimental effects.  That's a bigger concern with high carbon steel though.

To answer your question, we recommend that the end of the burners be just inside the shell - about 1/4 to 3/8".  So compared to the surface of the forge chamber the ends of the burners are recessed 1.5 to 2 inches, depending on the thickness of the lining and coatings.

[Kliff]

I guess I didn't include all of the specs in my summary, but they should be in the drawing.  Interior volume is 600 cu in.  My primary use will be forging my own tooling and knives. Don't see myself ever selling anything.

I came about my sizing this way:  Started with ROUGHLY 5" HJ * 7" W * 12" L.  I started here, thinking the 5*7 cross section is bigger than I need, but big enough to put stock in without banging on the lining too much.  At this size, it's 420 cu. in.  Too big for single burner.  So I decided double burner, then worked sizing backwards.  Keeping ROUGHLY the same cross sectional area, I started with 600 cu. in. (300 per burner).  That's how I got my 16" Length.  Then starting with mailbox shape, where W= 2*R and H=`1/2R.  I used these ratios for two reasons:  They seem reasonable, and they make the calculations simpler.  That's where I derived all of my interior dimensions.  Exterior, I added 2" in each direction, and rounded up a bit to account for the hard liner.

I was planning to use ITC-100 as a liner over the wool blanket, followed by a kiln wash, not yet mentioned.  Haven't found an available source for Plistix 900, still looking for Matrikote.  I'm looking at needing 1 pint of ITC-100, so $25 isn't a deal breaker.  I was also looking at Kast O Lite 30.  Might be cheaper per pound, but I don't need a 50 lb sack.

I was planning to aim the burners at about 1/3 of the way in from the edge, not the center of the floor.  In looking at my own drawing, I can see why it looks that way.  That line is used to indicate the penetration of the burner ports is not Top Dead Center, but offset to one side at about 10:15 (using clock angles).

Thanks for your input

 

[Irondragon Forge ClayWorks]

Look up in the blue banner for this.

https://www.iforgeiron.com/forum/202-store/

Everything you need for the forge is in there.

[Buzzkill]

33 minutes ago, Kliff said:

I was planning to aim the burners at about 1/3 of the way in from the edge, not the center of the floor.  In looking at my own drawing, I can see why it looks that way.

The point of my comment regarding the burners is where they are aimed, not their placement.  Your drawing indicates you have them aimed at the center line of the kiln shelf. What I was suggesting is to either aim the burners near the edge of the kiln shelf closest to them or at the opposite wall slightly higher than the floor.  Either of those options will create more swirl than aiming the burners at the center line of the floor.

Kastolite is not something you use *instead* of ITC, Plistex, or Matrikote.  Typically we put a layer of Kastolite over rigidized fiber blanket and then apply one or more coats of the aforementioned products to the Kastolite layer. Adding a kiln wash layer after the ITC layer would mostly defeat the purpose of the ITC I believe.

For the last couple D-shaped (or mailbox shaped if you prefer that term) forges I have built I just use insulating firebricks to block off the forge openings, which means that the length of the forge (unlike the other 2 dimensions) remains the same as the shell length - or very close to it anyway.

I find it easier both in building the forge and in use if it has uniform openings at both ends.   I can just block off all or part of an opening as needed and am not limited by a permanent design feature which restricts stock shape and size to a greater degree.  Your forge and your choice though.

[Mikey98118]

4 hours ago, ThomasPowers said:

Plistix is much better than ITC100 and MUCH cheaper.

Thomas is certainly right about the cost, and probably right about the product too. This isn't the one I used, but everything I've read from customers concerning ITC 100, after the change in ownership, is quite negative. Customer evaluations of Plixtex 900 have all been very positive.

If you want to look through the the last two years of posts, there are better coatings that guys have come up with than Plistex, but not as economical, effective, and simple--over all.

I am proud to be a perfectionist picky butt to the max. But Plistex 900 is still what I advise for anyone who isn't 

[Frosty]

Welcome aboard Kliff, glad to have you. That's not to shabby for not really knowing what you need. Rather than reverse design a thing I much prefer to start with what I need and work up.

Soooo, What kinds of tools do you wish to make? Knives don't require much forge size, 4 1/2" x 4 1/2" x 9" driven by a single 1/2" NA burner is making a number of local blade smiths a decent living here. I'll post pic of the current popular local forge a bit later.

The thing that's going to be the main determining size factor is most likely the tools. 

Whatcha wanna make?

Frosty The Lucky.

[Mikey98118]

1 hour ago, Frosty said:

ll post pic of the current popular local forge a bit later.

I'm looking forward to that. We need more photos around here.

[Kliff]

Thanks for the feedback guys.  Mikey, I have read many posts under this topic in the last couple months.  So much so, that I ignored some of your advice early on:  keep it small, etc.  I initially picked ITC 100 based on posts here.  Apparently I confused it as an alternative to Kastolite, and not as a kiln wash.  Without this thread, I'd be no where near as close to building a forge as I am.  Just glad I asked for feedback BEFORE I ordered all the stuff.  Although I do have all of the fittings for a two burner design.  Oh well.  I'll save most of it for forge #2.  I'm pretty sure #1 will not be final.

 I'm reworking my design, and I won't bore you with pictures.  It's basically going to be about 4.5H by 7W by 10L.  "D" shaped.  Single burner.  port about 20 degrees off Top Dead Center.  Aimed at approximately 1/3 of the way in from the near edge. 

Open on both ends, using kiln shelf pieces to close off as needed.

2 layers ceramic wool rigidized

Kastolite (approx. 1/4" thick walls/ceiling, 1/2" thick floor

Plistix kiln wash

High alumina kiln shelf floor.

Probably the biggest tool I can think of that I'll be making would be a series of draw knives.  Oh, and a gigantic sword of course  (no sword).  For the most part, I guess I'll be doing a few knives, and other items approximately the same size.  

[Buzzkill]

I think it's clear to all of us that you have done a fair amount of homework before diving into the deep end.  Kudos to you for that.  I wish I had when I was starting.  Your modified plan seems pretty sound to me.  I'm looking forward to seeing your work.  If the initiative you've shown on getting your forge built right is an indication of how you operate in general  then I have high expectations about your quality of work.

[Frosty]

Oooh, you're going to fit right in here Kliff! I like your second forge, I think it has a lot of staying power while your skills grow. While maybe not ideal, you've already realized you'll build more. BINGO! I don't think any of us who've been playing with propane forges for a while doesn't have a couple few old models collecting dust under a bench or in a back corner. 

ITC-100 is a legacy kiln wash for decades, much longer than I've been smithing. There are much better available but ITC-100 is stuck in people and organization's minds and probably won't go away. 

I've posted pics of the Association of Alaskan Blacksmiths burner/forge clinic "no weld forge" before but it's still worth taking a look. I'm not suggesting this one instead of what you've drawn up it's just to show how uncomplicated a forge can be.

The club built IIRC 37 of these, burners included in a 2 day build clinic. Buying materials and components in quantity meant they cost just under $100 each. Some of the professional bladesmiths here have all but switched over completely, 2"x2"x6" pattern welded billets are no sweat in it.

This pic was take less than 5 mins. after lighting, the bar is 1/2" round hot rolled. It's heated by a 1/2" T burner and will melt your work if you don't pay attention.

The IFBs (Insulating Fire Bricks) are Morgan Ceramics, K-26.

Frosty The Lucky.

 

[Kliff]

I like the "door" design.  I was just going to stack bricks.  That would keep my fat fingered clumsy hands from knocking them all over the place while I'm working.

[Mikey98118]

22 hours ago, Kliff said:

Oh well.  I'll save most of it for forge #2.  I'm pretty sure #1 will not be final.

Some  guys have gone on to "have their cake and eat it too." One guy even started out deliberately to make on oversize  two burner forge, with a movable (and removable) internal baffle, which two people can use at one time, or a burner can be shut off on one side and only half the forge used with the other burner.

Is this "the best of all possible worlds"? I don't know. Probably this will turn out to be neither right or wrong, but just another case of personal choice. I'm inclined to like the idea, but is that personal bias? It may take a couple of years to decide

Us old-timers preach that smaller is better. But, we came to that conclusion based on experience. Experience has to make room for new ideas...or become fossilized. I have suggested internal movable baffles as a fix for people who built oversize forges; maybe they aren't really oversize, now.

BUT, if someone is going to choose this path, a kiln shelf floor, and a hinged and latched door on one end of the forge, will smooth their way...

[Kliff]

Yeah, Mikey, I thought about modifying my initial design much like you outlined, to accommodate 2 people using it, shut off 1/2, etc.  Then I decided not to pursue the "holy grail" of forges, and start with a simple one, just big enough to do what I envision doing in the next 6 months.  Worst case, I build another one in 6 months, with a lot more than WAG's to base it on.

[Mikey98118]

That really isn't worse case, because, the beauty of miniature forges is that you will continue to use them, to have fuel and keep your shop cooler in warm weather. Since there is no such thing as THE PERFECT FORGE, you will end up with more than one, no matter how cleverly you design your first; they will accumulate like hammers.

I can't build a forge without think up several improvements on whatever design a choose; usually just after it is to late to implement them

[Mikey98118]

Why small burners?

Naturally aspirated burners have large turn-down ranges. So, it would seem that a wide variety of burner sizes isn’t needed. But heat management is about more than how hot fuel burns.

    The reason flames, whenever possible, are aimed on a tangent, is to cause their combustion gasses to swirl around equipment interiors; creating a longer distance from flame tip to exhaust opening. Obviously, a lengthened exhaust path increases the amount of hang time needed to deposit combustion energy on internal surfaces. What isn't so clear is that the heat gained isn't added by hot gases blowing an extra foot or two at high speed; it’s due to the continuing drop in velocity over that added distance.

    Combustion gases begin to slow as soon as they leave the flame envelope, but small flames decelerate in shorter distances than large flames. The smaller flames of two 1/2" burners will use the same amount of fuel to produce an equal level of heat as a single 3/4” burner; but will drop velocity much faster in a five-gallon propane cylinder forge, greatly increasing efficiency; because they can burn faster/hotter without creating a wasteful tongue of fire out the exhaust port.

    What about people who want to build a two-gallon forge from a non-refillable helium or Freon cylinder? They will need two 3/8" burners to gain the same advantage. Someone who wants to forge hand tools in a one-gallon paint-can, #10 food can, or three Lb. coffee can, will want two 1/4" burners to run their equipment with maximum efficiency; a 1/4" burner is also ideal for the so called “one-brick” forge (which actually ranges from a single drilled brick, up to five half bricks).

    Box forge/furnaces can employ small burners, mounted high up on one side wall, to great effect.

    Do multiple flame burners (Giberson ceramic burner heads, or homemade ribbon burners) take deceleration even further? Certainly; unfortunately, the burners themselves tend to be somewhat large. Over time, compact multi-flame burners will be perfected, but first there must be a lot more interest in doing so; there isn’t much, because they provide the most advantage when heating large equipment.

[localsmith]

I never understood the recommendations for smaller forges. A larger forge will only use slightly more fuel compared to a smaller forge in my experience. I had a small forge I got rid of because it was using practically the same amount of fuel as a forge I had that was twice its size. 

[Mikey98118]

You are speaking from personal experience, and that is fine, but it is also subjective. I speak from over twenty years building burners and the equipment they heat. A 1/2" Mikey burner will cost one-half the fuel to run at any given gas pressure that a 3/4" Mikey burner does. If we start comparing 1/2" Mikey burners to 3/4"  "T" burners, science just left out the back door. Then, there is the differences in forges, and various burners in various forges...

This is why subjective experience, while excellent for judging shop practice, is a shaky foundation for making sweeping conclusions. Its like math formulas; they can be quite correct, but misapplied.

In general, a smaller forge will cost less to heat than a larger forge. But what small forge design, compared to what large forge design? How efficient was the burner in each forge? What kind of insulation, and how thick in each? What kind of hard flame face layer and floor in each?

[localsmith]

I acknowledge that you and frosty are experts in the field and I have learned a TON from both of you and am very thankful for the educational content you both have provided on here. I thought that most heat loss was from the size stock you put in the forge?

I was running a forge that was 4X4X14" long with a 3/4" T burner and it seemed like overkill so I switched it over to a forge that is 6X6X12 and the larger forge seems to only consume slightly more fuel than the smaller forge. Do you think it could have been because the 3/4" burner was overkill for the smaller forge? The insulation is the same in both forges using castable over 2" Kaowool. 

[Frosty]

The smaller forge is around 192 cu/in volume about right for a 1/2" T burner unfortunately it's long and narrow so heat won't be very even and it will suffer greater back pressure for no other reason than friction.

The problem running a 3/4" NA burner in it is back pressure, that's 2x the burner necessary and close to 75% the smallest healthy size for a 3/4" T. And you still have the same problems of shape and friction. 

The larger forge is over the generally accepted max volume for a 3/4" NA burner but it's a better shape, not ideal but better. Three 1/2" NA burners would be sweet.

The smaller forge is a non-starter in my experience except maybe as a mediocre heat treat oven. That one would need a string of 3/8" NA or a really long thin ribbon to even temp out. Even then back pressure against the center burners or ribbon outlets would probably make them poor performers if at all.

Frosty The Lucky.

[localsmith]

That makes a lot of sense Frosty. I noticed that the smaller forge does not heat up very evenly.

Is there a point of diminishing returns in regards to how small you could build a forge in relation to the size stock you place in it with the sole purpose to lower fuel consumption? As an example, lets say someone builds a forge only for forging hammers that was 3"X3"X7"L which comes out a whopping 63" SQ. Inches.

Would the 3"X3"X7" forge use around 1/10 as much fuel as a forge that was 8"X8"X10"L (640 SQ. Inches) to bring a piece of 1 1/2" Dia. X 5" SQ. Stock up to the same temperature forging heat all other forge design considerations being equal (such as 2" Kaowool and same thickness castable in both forges)?

I assumed (potentially falsely) that a larger forge could contain, store, and distribute more heat (compared to a smaller forge) to any given size stock placed into it because of it's larger mass. I thought that this was one of the reasons why Ron Reil suggested a minimum forge size when it comes to forge welding but I could be mistaken.