Gavin Stairs on wed 15 sep 99
At 12:34 10/9/99 EDT, Candise wrote:
>----------------------------Original message----------------------------
>Hi all!
>
>I'm sure I've lost my mind, but I have seen a couple of things lately
>that make me wonder if it is possible (or reasonable) to build an
>electric kiln.
Hi Candise,
Now that Nikom has broken the ice, I'll say this: I'm planning on posting
a design for a small electric test kiln, as soon as I've had a chance to
run it for a while. I'm also speaking to Euclid's to see if they can offer
a kit of parts. However, this will take a little while to get together.
Actually, it will be several designs, from a really cheap, not terribly
wonderful design, to a super fabuloso, big as a house fibre design, all
intended to operate on a standard 15A@115VAC house circuit.
These designs bypass the most difficult part of a do-it-yourself electric,
apart from the calculation of refractory liners and elements: that is, the
mechanical metalwork. In other words, these will be a pile of bricks
and/or fibre, on a very simple stand.
The first crack at it uses a 2mm diameter element wire, which is still too
small, but about as big as is practical in a very small kiln. This should
last quite well, even when operated to cone 10. However, I won't know how
long it really lasts until I try it.
I'm still trying to get the design work done, which is why I haven't leapt
in to announce the project. I expect that early next year I'll be ready to
post stuff on a web site.
If you want to make your very own design of a kiln, especially a front
loader, you have a number of hurdles to get over.
1: First, the refractory design. This is the prime determinant of cost,
and kiln performance. There are lots of choices. The main thing is this:
refractories are expensive, and the better the performance, the more the
cost. Simple. But you can go way off scale if you don't know a little bit
about calculating the expected performance, or have some experience in such
designs. Unless you do this right, you will end up spending more on the
liner than it would cost to buy a whole ready made kiln. You can do a lot
better than that in increased performance, but you are unlikely to beat the
manufacturers on cost of materials. Factors: service temperature
(interior, and at each interface where the liner material changes),
exterior temperature (determines how close to flammables you can install,
how likely you are to burn yourself, etc.), self heat load (heat required
to heat up the liner itself), steady state heat loss rate (amount of power
required for steady temperature after heating up), heating and cooling
rates, element support, durability, structural design.
2: Element design. The elements have to supply the heat, so they are
intimately connected to the refractory design. The better the insulation,
the less power required. If you compare the capital cost of the refractory
against the power cost, you can find a minimum. But that may not be the
only factor of importance. Higher grade refractories may last longer,
lower power requirement will mean lower outside temperature. Factors:
refractory heat load and loss rate, element cost, durability, life time,
power source, number of elements, heating zones, atmosphere, coating,
support system, wall loading, termination, service temperature.
3. Mechanical design: In a small, top loader, this can be very simple,
just a stack of bricks with some cement and some straps. The terminals of
the elements must be protected by some sort of box, and this must be
according to code. If you do this yourself, it will be wise to have it
inspected by your power supplier, or by an accredited code inspector who is
UL or CSA accredited. This has implications for your insurance.
In a front loader, or a larger top loader, there will be some metalwork.
There may need to be a cover, and there may be need for hinges and clamps.
Hinges and clamps need a metal frame. The controller may also need a
special box, with suitable heat shielding. This can be surprisingly
expensive, or dirt cheap, depending on your resources, and what you want.
4. Controllers, furniture, etc. All the other details.
Certainly, you can take all of this at a gallop, and just put something
together. It may do the job, but if it doesn't, you may be in for a
frustrating time trying to get it right. The people who will sell you the
refractories and elements can help you, but without putting the whole thing
in their hands, they cannot be held responsible for the final performance.
They can only make suggestions based on their experience, and what they
think you are trying to do.
The reasons this is more difficult with electric than fuel are simple:
most self made fuel fired kilns are built outdoors, where it doesn't so
much matter if the outside of the kiln reaches 400C: just crank 'er up, and
see how far you get. If you misjudge, well, add more burners, add another
layer of mud, etc. Also, there are good guides available. In an electric
kiln, things are usually more difficult to alter. It can be expensive if
you have to change your elements and then your electrical service because
you need more power than you thought. It can also be harder to add more
insulation, and the terminals, and what if the controller gets too hot?
But really, if you want a kiln which does just what you expect it to do,
then the design job is very similar, except for the heating method and gas
flow. Large industrial gas kilns are not built without extensive design
and computer simulation, because the buyers want the kiln to behave itself
from the outset. The same is true of electric kilns.
Bottom line: Designing a kiln for either fuel firing or electric can be
done successfully by rules of thumb and intuition, especially if you don't
stray too far from designs with which you are familiar and which work.
However, if you misjudge your adaptations, you can also end up with an
expensive white elephant, not at all what you intended. And there's no one
to blame but yourself. It is much more reasonable of you can do some
refractory calculation, and are familiar with electric power calculations,
and maybe have a bit of experience in mechanical design.
So, you have to judge for yourself how much of this applies to you, and how
much is just baloney. If you have confidence in your own abilities, then
don't let me discourage you. I have been a mechanical and electrical
design engineer for many years, which is why I feel confident in designing
these things. If I were to have tried this thirty years ago, I doubt if my
confidence would have been justified.
Nine cubic feet (usable) is about a 27" cube. The finished kiln will be
approximately a I yd cube. A reasonable design will require about 3.5 to
4.5 kW of power, or about 16 to 20A, depending on the insulation and other
foactors. Maybe a bit more. I tend to the well insulated designs, so my
power estimates tend to be lower than others. I'll be glad to help out if
you want to try this. Don't expect me to execute the design for you. It's
too much work, and too much responsibility if it doesn't work out the way
you wanted it. But I can look at what you intend, and tell you if I think
it will work out. Some of the suppliers of kiln materials may be willing
to do the same. If you can find someone local willing to sit down with you
for a cup of coffee, you might be able to get some good advice. Also, bug
your local rep for refractories. Ask for any design guides or programs
they may have.
But don't think this will be a cheap kiln, except in the amount of money
you spend, if even that.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
Candise Flippin on thu 16 sep 99
Gavin,
Thank you for the informative, if not daunting email. I'm fairly daring
and somewhat experienced in the areas needed, but would need a lot of
advice. Here's my real objective: I do sculptural work that is next to
impossible to load from the top. I hold my breath, pray to the kiln
gods and nearly have a panic attack every time I do this. The other day
I saw a picture of the traditional kiln with a pulley system that raised
the kiln for loading and unloading. Maybe it would be easier to build
one for and existing kiln than build the kiln itself. What do you
think?
Thanks, Candise in San Diego
Gavin Stairs on fri 17 sep 99
At 07:37 AM 16/09/99 , Candise wrote:
>... Here's my real objective: I do sculptural work that is next to
>impossible to load from the top. I hold my breath, pray to the kiln
>gods and nearly have a panic attack every time I do this. The other day
>I saw a picture of the traditional kiln with a pulley system that raised
>the kiln for loading and unloading. Maybe it would be easier to build
>one for and existing kiln than build the kiln itself. What do you
>think?
Hi Candise,
Yes, I think this would definitely be easier. Also, I have seen advertised
by Bailey and others, kilns that work this way. Others still, including
PSH Canada (the parent of Euclid's Elements), would be happy to build you a
kiln like this. Or, if you want to do it yourself, you could probably just
take the round, top loader of your choice, detach the lid, turn it all on
its head, and rig a pulley system above it to lift the kiln up and
down. The lid, now the bottom, would have to be raised up off the floor on
the stand, but that is probably ok. The element slots and the controller
are now the wrong way around, which is the worst problem. The elements may
tend to fall out of the groove, and the kiln sitter (if it is used) is now
upside down. So that has to be fixed up. Better would be to get a kiln
with detachable rings, and simply detach the bottom, or the bottom and any
dead rings that are there. Then lift the rest of the kiln by attaching
your pulleys to the bottommost firing ring.
You can be as elaborate or as crafty as you like with the pulley
system. You can buy a winch to run it electrically. You can make a stand
of steel rails to make it go up and down straight. Or you can just use
some pulleys from the hardware store with sash cord, and live with the
inconvenience. If you use sash cord or any combustible rope, make sure you
detach it and store it well away from the kiln when you fire. And check
for fraying regularly. The simplest would be to use the sort of garage
hoist that people use to store their boats above their cars in their
garages: usually just a pipe with a chain wheel attached to the end, and
drop cables that wind on the pipe, all set in bearings. The worst part of
a simple system like that is controlling the drop. If you let it all
freewheel, down it comes in a hurry. The are all sorts of ways to handle
this, like worm drives or counterweights or springs.
Aside from making sure the whole thing doesn't fall on your head, this
would bypass all the worries about the kiln itself. Of course, if you
really want a front loader, but don't want the worries of trying to design
it yourself, any of the kiln firms you may be familiar with can probably
build it for you. Front loaders tend to be more expensive than top
loaders. They are generally build more to industrial norms, and the
framework to swing the door costs more as well. So, cheapest, but not
necessarily most trouble free, would likely be to rig an old top loader on
a sling, and make the necessary mods to it. More expensive, but
essentially off the shelf, would be to get some kiln manufacturer to adapt
one of their designs to your purpose: you're probably not the first person
to ask for this. And most expensive would be to ask some firm to design
and build a front loader to your specifications.
Gavin
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