How Energy Is Spread In An Electric Kiln

Thread Starter

Blue Crystal

Joined Mar 21, 2017
4
Heat from the coiled wire is transferred in three possible ways: conduction (through physical contact of the heating element and the material being heated); convection (through the air movement in the kiln); and by radiation (directly from the elements to the material being heated).

It seems to me that at some point the flux of thermal energy flowing from the resistors will become almost constant. But this thermal energy keeps accumulating in the air, in the walls and other objects inside the kiln and, as consequence, the temperature will keep increasing until the the heating elements melts down if a controller does not intervine or nobody turn the swtch off.

Is my view on this heating process correct ?
 

tcmtech

Joined Nov 4, 2013
2,868
For the most part yes.

In a theoretically perfectly insulated vessel the thermal energy would keep rising indefinitely as long as energy kept being put into it.

Realistically however no insulator is perfect so at some point energy loss out of the system will balance things out at some point assuming the materials don't break down and change states at some point like the heating elements or containment vessel melting some other such release of the energy occurs.

Also beyond the three energy transfer methods you mention there are others as well like induction heating and RF based energy absorption (microwave based) to name just two more commonly used ones in heating ovens, kilns and such which are usually only limited to whatever the material the containment vessel is made from can handle. Or if the media to be heated can be contained and manipulated with magnetic fields it can be taken to crazy high plasma state temperatures in the millions of degrees! :eek::cool:
 

wayneh

Joined Sep 9, 2010
16,128
Heat from the coiled wire is transferred in three possible ways: conduction (through physical contact of the heating element and the material being heated); convection (through the air movement in the kiln); and by radiation (directly from the elements to the material being heated).

It seems to me that at some point the flux of thermal energy flowing from the resistors will become almost constant. But this thermal energy keeps accumulating in the air, in the walls and other objects inside the kiln and, as consequence, the temperature will keep increasing until the the heating elements melts down if a controller does not intervine or nobody turn the swtch off.

Is my view on this heating process correct ?
Mostly. One factor that may come into play is that the heating element increases in resistance as the temperature rises, so the power dissipated into the kiln declines as temperature rises. Heat loss from the kiln also increases with temperature. So if you had a kiln with a small heating element, it could reach an equilibrium temperature below where the element melts.
 

#12

Joined Nov 30, 2010
18,167
Not only do we do allaboutcircuits here you can also get an allaboutthermodynamics education :D
What a multi-skilled group here..
In the case of a kiln, I would go at the thermodynamics first because that would lead me to the wattage required. Trying to start with some watts and design a kiln to use them will make you crazy.:D

And yes, it's mostly radiation in a kiln. The victims in the middle don't touch anything but air so there is some convection going on but the general principle is that a confined space reaches the same temperature everywhere, eventually.
 

tcmtech

Joined Nov 4, 2013
2,868
Back when I was married the not yet Ex wanted to get into ceramics as a hobby. I encouraged it until it became clear her hobby was going to become a huge amount of work and expense for me.

But anyway, One thing she needed was kiln that could run at ~2200 -2400 F for extended periods so I did get to doing the basic design for one. Common vermiculite insulation with a sodium silicate cementing agent and aluminum oxide coating made to about 12" thick was my choice being it can handle up to at least 2800 F without issue and at that thickness the thermal losses were pretty low so once the kiln was up to temperature the energy requirements to sustain its operating temperature would've been tolerable.

For heating I was going to use common commercial grade halogen bulbs on 5 sides of the interior (most even heating method I could come up with) being they can run at ambient temperatures over 3000F without issue and be easily controlled.

Down side was she wanted a commercial production capable kiln that could hold ~ 1 cubic meter of volume which put the build costs at ~ $2000 and the power consumption at ~15 KW minimal just to try her hand at making ceramic trinkets and mugs worth $5 - $10 profit each that might not even sell. :rolleyes:

Upside is I may still build it (but smaller at 24" cube) for personal use (aluminum, brass and copper smelting work) just to piss her off when she finds out I have one she but would now have to pay to use it! :D
 

#12

Joined Nov 30, 2010
18,167
Back when I was married the not yet Ex
I also have an Ex that thought about piddling around with a kiln.:(
I also escaped without a $2000 device dedicated to making $5 trinkets.:D

I have tried throwing clay, and it's fun, but not $2000 worth of fun.
That's why you rent space in *O.P.s kiln until you discover that you can't even recoup the cost of renting with your amateur trinkets.;)

(* O.P.s = Other Peoples')
 

#12

Joined Nov 30, 2010
18,167
Sounds a lot like 3D printing.
Yebbut...you can fondle the clay.:D

It's an organic feeling hobby. Art + skill = fun.:)
Very satisfying work. History across millennia. Useful, durable results.
The main problems are the price of equipment and the time required to develop skills to make really good products.
Clay is cheap. Hobby labor is cheap. Kiln is not cheap.:(

You could use the kiln to harden certain alloys of steel, and anneal some others. I've done that. Heat treating metal is a possible source of money to pay for the kiln, but you would have to be in business a long time to get enough customers to make a profit. More likely to pay for your hobby is good craftsmanship and a retail outlet.;)

Not that profit is the prime motivator, but it's one of the factors to consider. Some people can drop two grand for equipment and call it a hobby. I suspect this Thread exists as an attempt to avoid dropping two grand for a slow furnace.
 

tcmtech

Joined Nov 4, 2013
2,868
I also have an Ex that thought about piddling around with a kiln.:(
I also escaped without a $2000 device dedicated to making $5 trinkets.:D

I have tried throwing clay, and it's fun, but not $2000 worth of fun.
That's why you rent space in *O.P.s kiln until you discover that you can't even recoup the cost of renting with your amateur trinkets.;)

(* O.P.s = Other Peoples')
I can knock out the smaller kiln for about a $500 and I do have enough barely justifiable uses for it at that. The aluminum, brass and copper casting work I could do with it just for personal projects would justify that.

My Ex had this idea she was going to make $100+K a year doing the arts and crafts hobby thing in her spare time at a at best $10 an hour profit. My math said it was impossible, hers said I was a pessimist trying to ruin her success. :confused:
Every calculator and calendar I have found said I was probably more right than her given there are only 8766 hours in a year and she spends 2000+ of them at work, 2000+ more sleeping and another 2000+ on other life actions.

Admittedly she did make ~$6000 gross with ~ $1000 - $1500 profit on her own last year making custom painted coffee mugs and whatnot with I have to say is pretty good! Not my $2000+ profit on selling my junk on eBay last week good (I'm still an 'unemployed bum' by her standards just the same :rolleyes:) but at keeps her mind where it needs to be and that's priceless by everyone's standards! :cool:
 

tcmtech

Joined Nov 4, 2013
2,868
Sounds a lot like 3D printing.
It's like a lot of things. They sound incredibly profitable in theory but in reality the setup and getting established to the point of just being at a legitimate break even let alone actually profitable at all is way harder and more costly in time and money than anyone cares to think about.

It's like my eBay sales. The stuff I sell aren't items that just anyone can scrounge up for trading labor. It took me years to get into the resources I have now and even at that I have to attribute much of it to dumb luck than anything else for what I do get that's highly profitable. I do the work for fun and the challenge of helping people out who need my skills. The money is typically pretty low on the motivations list for doing any of it. ;)
 
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