Thermal Mass

[Mikey98118]

Thermal mass; good or bad idea, and why.

Historically, thermal mass was embraced for heating metals, glass, and pottery, because neither available fuels, nor available insulation (dirt) where quite up to the job at hand.; this gradually changed, as by trial and error man discovered better techniques, such as bellows to replace natural drafts near cliff faces, and coke instead of wood fuel. Ashes are far better insulation than dirt, but whether or not our ancestors ever built double wall ash lined furnaces is unknown. Certainly, the Romans used conical shaped lidded crucibles placed within fuel lined furnaces, which naturally benefited from the formation of ash to help conserve heat , as the fuel was consumed; but it's unlikely the idea was ever taken further than that.

Sskipping forward to the age of iron (nineteenth century) Fan-blown brick lined furnaces heated massive amounts of cast iron, and the idea of thermal mass became popular; this was partly due to poor available choices for insulation, compared to what is available today, and partly too the sheer volume of high melting temperature alloy desired for industrial purposes; ditto for glass.

Along comes the twentieth century, and with it hotter fuel gases. Also cheaper hot-face refractories and metal shell materials make asbestos insulation practical; yet thermal mass remains a popular idea, because for most of the twentieth century low tech (cheap) construction and high overhead (fuel costs) trumped common sense every time as a business model; every rising fuel costs overturned this idea during the second half of that century.

Then comes the space race with its technological spin offs; also, during the fifties propane replaced natural gas as  the fuel of choice.

Industrial refractories in general have been rapidly evolving since the end of the nineteenth century, so should we revisit thermal mass loading as a practical necessity, or even a relevant idea in the materials age?

Well, first we need to ask the question "have our choices for thermal mass loading materials stood still while every other refractory product moved on? The answer is a resounding NO! In fact the material of choice, started replacing hard firebricks about a century ago, and has been improved on every since then; it is high alumina refractory, available as castable, hard brick, and kiln shelving. So, what is the difference between hard refractory brick and high alumina products for thermal loading? typical hard refractory brick mostly consists of clay, which has very little resistance to heat transfer; high alumina products have up to seven times the resistance to heat transfer. There we go with that phase "up to" again! But in this case the use is legitimate, rather than double talk; the difference being found in the purity of the product involved.  high alumina kiln shelves are a full seven times more resistant to heat transfer than clay based products. 85% pure alumina refractories are going to be closer to six times more resistant to heat transfer, which is still quite respectable.

Finally, radiant heat transfer within a forge or furnace depends on resistance to heat transfer away from the hot-face and through your thermal mass; using both ideas working together trumps thermal loading alone.

[bird]

So I don't have a technical material science background and I'll ask you to excuse me in advance if, in attempts at precision, I butcher the technical language. 

It seems like you're saying that thermal mass is still a good thing in our forges, but not all masses are created equal? We like high alumina refractory because it keeps more heat in our forges, or at least slows it's loss down much more than fire clay based products. 

I tend to think of forge mass as a reservoir of heat I can dip into to heat and reheat my steel. This is why forges I build tend heavy as I favor faster heats over initial start up time. Is this wrong?  I have no empirical evidence but I swear I've worked in light mostly frax forges that visibly cool when I put steel in and I've always expected this to be reflected in longer heating times. Common sense suggests that unless we just use our burners like torches, we'd need hot mass to heat mass. But if my observation and reasoning are right, aren't we in a catch 22 where we need and rely on heat transfer for our heats?

[Mikey98118]

No Catch 22 (truly hated that movie). Your worries don't apply because of radiant heat transfer with a low heat transfer material, such a high-alumina castable refractory and/or kiln shelves. The best place to employ thermal loading from thicker hot-face layers is in the  forge floor, so thinly coated rigidized ceramic fiber wall and end enclosures remain practical, for the best performance. Have your cake, and eat it too.

[bird]

So I should cast thicker floors and use lighter walls for best performance?

What do you think is an optimal wall/arch construction for balance of performance and durability?

[Charlotte]

My experience, and with the obliging help of an engineer from a refractory manufacturer,   I learned that  High mass refractories are essentially transparent when looking at heat conduction.  That is if one side is at 1400 deg F  the other side will be at the same temperature. 

There is a benefit in having hard brick or cast refractory as the floor and else where in a forge.  The primary benefit is durability  and stability as well as resistance to welding fluxes.

The benefit of Hard brick mass happens this way:   When cold metal is placed in a hot forge the temperature of the forge drops.  The metal absorbs the radiated heat heating it and some of the convection heat surrounding it.    The Hard refractory continues to radiate heat and the convection of the hot gas picks heat from the brick  so the total temperature drop is less and the temperature recovers more quickly. 

My conclusion was>>>>  Surround the hard refractory with layers of toughened blanket. Only use Hard brick and cast refractory  in areas where direct flame  or welding flux will cause problems. The Insulating blanket and the Hard Refractory should be sealed with a coating to prevent the High temperature carbon monoxide  present in the forge from attacking the material.  ( note:  Reading technical literature in refractory publications indicates that Carbon Monoxide at high temperatures shortens life expectancy of some materials)

Final word:  This is my understanding and my belief.  You will have to take the advice of others and follow your own thinking.

[Mikey98118]

Bird asks,

"So I should cast thicker floors and use lighter walls for best performance?"

Answer: Not exactly--for best balance between performance and durability. If you are looking for performance alone, thermal loading is still an open subject, because you haven't included what size and how  much material you are heating, and how fast you can forge or weld  the parts you are heating. A properly built gas forge with a hot running burner will heat parts faster than most smiths can keep up, without thermal loading, beyond a high- alumina kiln shelf for a floor.

"What do you think is an optimal wall/arch construction for balance of performance and durability?"

Answer: Since you included durability in your second question; an inner hot-face layer of Kast-O-lite 30 1" thick shaped in a "D" pattern, with the flat side on the bottom to create a forge floor. After the hot-face layer is cast and heat cured, wrap it in ceramic fiber; compress the spring fiber in twin so you come up with a denser outer wall of insulation. Install this in a Freon or propane cilinder (depending on if you want a general size or knife maker size gas forge). Cut the cylinder lengthwise and install a piano hinge on one side, and threaded latches on the other side, As Wayne recommends in his forge build  method, because it will make installation of the ceramics much simpler, and accesse to the forge enterior for maintaince as well. You will also want to include a folded "pillow" of ceramic fiber underneath the floor area.

End enclosures of ceramic fiberboard can be installed the most easily in this construction scheme, and will hold up better than 2" of ceramic blanket, but cost a lot more to use. some extra struggle followed by rigidizer, and then coated with whatever homemade "kiln wash or other heat reflective coating you desire on the cast hot-face inner layer, will also do wonders to preserve ceramic fiber blanket enclosures too.

[Mikey98118]

Charlotte,

Your engineer was being sloppy in his thinking. Standard clay based hard firebrick is practically transparent to heat transfer. With high alumina based brick, castable refractory, or kiln shelves, this just ain't so; it especially ain't so with an insulating alumina based semi insulating refractory like Kast-0-lite 30, which is rated at 4.54 degrees of heat loss per hour, per inch at 2000 F; that is hardly transparent!!! Nevertheless  a 1" thick hot-face layer of this stuff will also provide as much thermal loading as you can use in a gas forge. or home built casting furnace. Of course there "ain't no free lunch." So what is the down side? About ten extra minutes reaching yellow-white heat (with a high-emissive coating, such as ITC #100, and an effective burner). The upside is that you quickly recoup those lost ten minutes, if you're doing more than an hour's worth of forging or casting

[Mikey98118]

I think even heavy weight Mizzou castables are high alumina based refractories these days, so the seven times more resistant to heat fransfer than clay based refractories still applies.

[bird]

So granted, I'm picky about my tools and made some aesthetic decisions as well as a few technical ones about limiting ceramic fiber exposure, but maybe the type of forge build I posted notes on a bit ago is pretty ok. I like it, but I'm always interested in improving.

34 minutes ago, Mikey98118 said:

If you are looking for performance alone, thermal loading is still an open subject, because you haven't included what size and how  much material you are heating, and how fast you can forge or weld  the parts you are heating. A properly built gas forge with a hot running burner will heat parts faster than most smiths can keep up, without thermal loading, beyond a high- alumina kiln shelf for a floor.

Fair point. I use that forge for anything from 3/8" up to 1 1/2" square when it's smallish volume works. For production forging and large heats I have a much bigger forge built similarly but proportionally more massive. I only run it when I must. Whether or not the forges can keep up with me or I with them depends entirely on how many parts I'm working at once. Maybe I'm just lazy (sometimes it seems hopelessly so) but there are plenty of moments in the process when I can only focus well enough on one or two parts and that's part of why I like to set up my forges for quick reheats. I can always dial the burner back and more or less match my number of parts to heating and working times, but waiting on heats is unprofitable. Very. 

[Mikey98118]

Why isn't everyone familiar with heat transfer blocking versus heat insulating refractory products? Because the only information you get about resisting heat transfer comes by reading the numbers given (sometimes) for heat transfer ratings. If you want to know the how and why of those figures, you must research crucible materials and coatings; that's where I came across the "seven times" rule of thumb, and also where I learned about high-emissive coatings.

[Mikey98118]

So, if clay is a combination of aluminum oxide, and natural glass, why would high-alumina be so different from it? High-alumina is short for high purity alumina a manufactured form of aluminum oxide used as the refractory concrete, with the same very pure aluminum oxide as grog, and with a very small amount of calcium alumina as binder, instead of a large amount of glass as the binder. Whether you are considering fine china or a lump of clay from your backyard, it is the glass content in the ceramic that is mainly responsible for heat transfer. To demonstrate the difference, purchase a small high-alumina crucible from eBay, and line it up with a ceramic coffee cup and small glass container; pour hot water into all three containers and touch them; no contest! the culprit in high heat transfer is the glass content.

[Mikey98118]

BTW Charlotte,

I found most of what you had to say quite educational.

Thank you for that.

[Mikey98118]

Bird,

If you want a demonstration of what high-alumina kiln shelving for an all around hot face in a forge can do, have a look at http://www.chileforge.com/

[Mikey98118]

So, if we aren't sure how much thermal loading is optimal in our forge, what then? Who says all thermal mass must be built in? Nothing prevents you from including extra thermal mass on top of the forge floor, on a temporary basis, when heating large parts, and storing it when heating small parts? You can see this very trick used in one of the Chile Forge videos.

[bird]

Yep. Not to be a jerk but I'm pretty sure I'd prefer to work in mine . I didn't use kiln shelf but I did use high alumina refractories and 2" of fiber all around. I might have to get a bit of ITC100 or some such and see if I can tell a difference though. 

Good point on temporary or removable mass. 

[Charlotte]

Mickey   sorry ,  hard fire brick is essentially transparent when you run the equations.   I was not talking about insulating castable.  The point being that the use of Kast-o-Lite is not the subject of the discussion.  Hard fire brick as a floor is a heat storage thermal mass.   Oh and btw It as the refractory Manufacturer's Engineer that run the calculations and we looked at a several different compositions of Hard Brick and insulation configurations. When you look at heat capacity of brick the differences between insulating brick and hard brick vary with the weight not the area.

My personal build is hard brick floor with satanite coating over layers of inswool. Interior is insulating brick covered with Satanite  surrounded by layers of inswool.   Why Insulating brick?  It is easier to shape and easier to replace than cast in place refractory.  It lasts a while if protected from direct contact with hot gas by mortar. 

I have followed Frosty's suggestion of using kiln shelf over the floor and found it works well saving wear and tear on the whole operation,  And no I don't  do pattern welding so my gas forge rarely gets to that temperature.

[Mikey98118]

Charlotte,

If we completely agreed, one of us would be unnecessary to this conversation; nor do I desire for things to be done my way. It is my desire for readers to look at both sides of the coin and make up their own minds as to what they think. I enjoy your differing views, quite as much as Frosty's.

[Mikey98118]

So, if thermal mass is a great idea for heavy forge work, what about using temporary thermal mass to spiff up results in forges with underwhelming heat sources (ex. coffee-can forges being heated with a canister mounted propane or even butane torch), while the builder is trying to get a real burner constructed to replace such a temporary heat source?

[ThomasPowers]

You mean thermal mass that the underwhelmed burner will never bring up to forging temperature?   What are you trying to accomplish?  If you have a poor burner you would want to max out the insulation and minimize the volume if you want to try to get it hotter.

[Frosty]

Mike: You're going to get called on Strawman arguments and lose the regard you're knowledge and experience deserve.

Frosty The Lucky.

[Mikey98118]

Thomas,

Good point.

 

Frosty,

I take it you're assuming I'm arguing with someone, rather than examining another possible use for temporary thermal loading? If so, I would begin my post with that someone's name, like I did this one. But if we are down to "low regard" I'll just let this thread end and go on thinking further on the subject privately.

[Frosty]

I didn't mean to touch a nerve Mike, I apologize for that. I didn't use the term argument as a verb it's a noun meaning a point in a discussion as in the way debaters "argue" their cases. Make a statement, challenge said statement, defend the statement. I thought we were enjoying arguing some great subjects recently. I certainly don't use the term argue interchangeably with fight, they're two entirely different things.

Thomas simply called out a statement that was exaggerated to an 'eh hem' degree. Suggesting increasing the "thermal mass" in a volume with insufficient burner output is nothing more than an attempt to negate another person's position by attaching an obviously unworkable exaggeration to THEY'RE position.

Perhaps I'm over sensitive to receiving these arguments and having the opposition's exaggerations connected to my position as proof I'm wrong. So, yes I should've said something on the side rather on the public forum. I have no excuse and offer none.

I leave this thread with my sincere apology.

Jerry

 

[ThomasPowers]

Argument?  I thought this was abuse!  to misphrase Monty Python.  I took a class in speech/debating back in High School and once debated myself to rather wish washy results---half the class thought I had won and half the class thought I had lost...I rather like gedanken experiments and aristotelian discussions and as is quite evident; I like analogies to try to equate someone's ideas to something that is generally known.

[Frosty]

You're right Thomas, I stuck my foot in my mouth again. I don't know what sets me off sometimes but rereading what I wrote I can see I was mad about something or someone else. I wasn't even involved in the conversation. I'd take it back but it's there now. My second post was even worse behavior.

I do I sincerely apologize Mike I was way out of line. I took something out on you you didn't deserve nor were you involved with in the first place. I don't know why but it's no excuse. I'm sorry.

Jer

 

[Frozenforge]

A fine example of adults having a conversation, blood pressures are low, moderators didnt have to get involved and none of those words that are all Xs!