John Baymore on fri 23 apr 99
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This is a long-standing question of mine... why do we put kilns on
stands? Why not just put fiber down (on a concrete floor) and place the
kiln directly on the floor? It sure would make loading it easier and
seems it would help in heat retention. Don't know about the fumes....
but since I don't have an envirovent, ???
(snip)
Tena,
Electric kilns of the type most seen in ceramics studios (as opposed to
industry) are VERY poorly insulated. Particularly the high fire models.
(They are actually VERY wasteful of natural resources in this regard.......
and are expensive in terms of energy usage, not necessarily firing cost. )
The floors of these kilns are no exception to this. So the refractories
pass a lot of heat through themselves.
There is one concept that is important to talk about here before we get
right to the question. Unlike the so-called =22common sense=22 =
understanding
that =22heat rises=22 (which is totally incorrect), heat energy moves =
equally
in all directions from areas of higher concentration to areas of lower
concentration (3rd Law of Thermodynamics....entropy). So for all intents
and purposes, heat energy basically moves through the floor refractories of
an electric kiln (any kiln) at the same rate as it moves through the walls
and the lid (or arch).
Another important concept is the difference between =22temperature=22 and
=22heat=22. Heat is a form of energy. Change in temperature is the result=
of
the application of heat energy to some form of matter.
The amount of heat energy going through the wall of a kiln is predominantly
a function of a combination of the insulating rating (per inch) of the
brick itself (or other refractories), the thickness and composition
(layering of refractories) of the wall, the hot face (inside) temperature,
and the cold face (outside) temperature.
(I would be remiss here in not mentioning it is also affected by something
called the emissivity of the surfaces on both the hot and cold faces,and
also the exchange rate of the heat energy off of the cold face into the
air. BTW ......My best guess as to the ITC 100 product's function is in
affecting this emissivity factor on the hot face surface along with sealing
(gas leakage and air infiltration through) the refractories. But I
digress..........)
This overall heat loss rate is measured (here in the US) in British Thermal
Units per square foot per hour, (BTU / sg. ft. / hr.). As the kiln heats
up, the total heat loss figure goes up, relating to the change in hot face
temperature. They are directly related. The greater the differential
between hot and cold face temperatures, the greater the =22driving force=22 =
for
the transfer of heat energy.
The cold face temperature is directly related to the heat loss per hour
figure above and the disipation of the energy from the cold face surface.
As heat loss goes up the resultant cold face temperature goes up. As heat
dissipation goes up, the cold face temperature goes down. There is some
minor interplay: as the temperature of the cold face goes up it is able to
transfer more heat to the surrounding environment. And as that temperature
goes up the differential temperature goes down. But let's not go there in
this case.
The outer surface of the electric kiln remains relatively cool (still very
darn hot) because it is open to continually radiate heat energy into the
nearby environment (radiant heat transfer), and because cool air flows
across the kiln surface thereby being warmed (conductive heat transfer)
and taking heat energy with it (convective heat transfer).
So now we get more directly to the answer:
Restrict this exchange of heat energy off the outside surface and you get
what happens when you apply heat energy to any material....... a rise in
temperature.
Try this experiment. Place a thermocouple tightly against the center of
the outer surface of the lid of a firing electric kiln (don't weigh it down
with anything on top of it). Hook it to the appropriate meter, and monitor
the temperature of the top of the outside lid surface at the peak of the
firing just when the kiln sitter shuts off. Record this measurement.
Now, on the next firing to the same temperature over the same time cycle
place the same thermocouple against the same exact spot, but this time
cover the entire lid (and thermocouple) with a layer of 2 1/2 inches of
additional insulating firebrick. Check the temperature again at the peak
of the firing and record it..
Compare the readings. Interesting, huh? The hot face (inside the kiln)
was not any higher. You fired to the same cone.
Now while concrete is not as good an insulator as IFB, can you imagine what
the temperature of the interface between the outside of the electric kiln
floor and the top of the concrete floor surface would be? Concrete
deteriorates at high temperatures. (If it is moist with ground water it
can cause a steam explosion if it is contained enough.) So that's clearly
out as an option.
Now if you place a fiber blanket down first under the kiln on the concrete
floor, that will provide some added insulation factor for sure. One thing
to remember here though is that fiber insulates by creating many tiny,
teeny, itsy, bitsy dead air spaces between the fibers. Compress a lot of
these spaces out, and you lose some of the fiber's ability to insulate.
So how heavy do you think a loaded electric kiln is?
But let's assume that the fiber is still providing significant insulation.
Is it enough to protect the concrete floor?
Load up your kiln again. Cover the whole lid of the electric kiln with a 1
inch layer of fiber blanket. (The dust from fiber is HIGHLY toxic to
breathe. Do this with extreme caution and appropriate peotection. ) Put
that same thermocouple back on the lid of the electric kiln, but this time
on the center of the TOP surface of the ceramic fiber layer, not on the lid
top surface. Cover the whole lid (over the thermocouple too) with a layer
of 4 1/2 to 5 inches of hard brick (as a reasonable stand-in for concrete).
Fire the kiln to the same temperature as before with the same cycle.
Measure again.
Pretty hot isn't it=21
That's why. You need the airspace under the kiln to disapate into the
natural air circulation the heat energy leaking out of the kiln. Otherwise
it gets way too hot.
Wood is a better insulator per inch than concrete. Picture the kiln
directly on a combustable wood floor. Not a pretty sight.
It IS possible to insulate the kiln floor well enough so that you could
place it directly on the concrete floor. But it would take substantial
thickness of brick refractories, or something like a bunch of fiber with
some structure that kept the weight of the kiln from compressing the fiber
too much. Certainly do'able, with a little thought, understanding, and
planning. But you'd still raise the kiln up off the floor a bit and if
that is a loading issue, not really solve it.
There is some complex math that allows you to predict the cold face and
interface temperatures of any combination of insulating materials if you
know the insulating value (per inch) of the mateials, the hot face
temperature, and the cold face temperature. This is what engineers use in
one aspect of kiln design.
So hopefully this answers your question.
BEst,
............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Vince Pitelka on sat 24 apr 99
A>Electric kilns of the type most seen in ceramics studios (as opposed to
>industry) are VERY poorly insulated. Particularly the high fire models.
>(They are actually VERY wasteful of natural resources in this regard.......
>and are expensive in terms of energy usage, not necessarily firing cost. )
>The floors of these kilns are no exception to this. So the refractories
>pass a lot of heat through themselves.
I seem to be especially talkative tonight. It is because my son and his
fiancee just left, after a three-day visit.
I was glad to see John Baymore's post above. I have thought about this so
much. I really believe that electric kiln manufacturers are being very
irresponsible in making kilns which reach temperature by virtue of their
POWER rather than their INSULATION!! It seems so obvious. AXNER has a kiln
which has three inches of softbrick PLUS a layer of fiber board. This
should be the standard, at the least. I really like L&L Econokilns, but in
midrange firings, the radiated heat from the outside of the kiln is obscene.
I wish there was an agency licensing electric kilns to guarantee a minimum
level of EFFICIENCY and not just SAFETY. We really need that.
Best wishes -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
john a gibson on sun 25 apr 99
Can a fiber blanket be safely added to an electric kiln to increase the
insulation value and efficiency of the kiln?.
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Jonathan Kaplan on sun 25 apr 99
>----------------------------Original message----------------------------
>A>Electric kilns of the type most seen in ceramics studios (as opposed to
>>industry) are VERY poorly insulated. Particularly the high fire models.
>>(They are actually VERY wasteful of natural resources in this regard.......
>>and are expensive in terms of energy usage, not necessarily firing cost. )
>>The floors of these kilns are no exception to this. So the refractories
>>pass a lot of heat through themselves.
>
>I seem to be especially talkative tonight. It is because my son and his
>fiancee just left, after a three-day visit.
>
>I was glad to see John Baymore's post above. I have thought about this so
>much. I really believe that electric kiln manufacturers are being very
>irresponsible in making kilns which reach temperature by virtue of their
>POWER rather than their INSULATION!! It seems so obvious. AXNER has a kiln
>which has three inches of softbrick PLUS a layer of fiber board. This
>should be the standard, at the least. I really like L&L Econokilns, but in
>midrange firings, the radiated heat from the outside of the kiln is obscene.
>I wish there was an agency licensing electric kilns to guarantee a minimum
>level of EFFICIENCY and not just SAFETY. We really need that.
>Best wishes -
>- Vince
>
>
Both VInce and John make some excellent and totally relevant points. I
think where the differences between the "hobby" type kilns ( perhaps with
the exception of ConeArt and Axner) (see my last post...Electric kiln
maintencance....a rant) and those that are classified as"industrial" kilns
are:
1. the thickness of the insulation brick
2.what the insulation brick is backed up with and the steel frame
3. the quality of the elements
4. the electronics
5 and finally, the amount of power needed.
Don't get me wrong here. I think that the standard hobby kilns that are
available from most of the manufacturers are fine pieces of equipment in
the low temperature ranges, say up to cone 1, maybe.
Most of them need somewhere in the neighborhood of 40-50 amps, single phase
power at 240 VAC.
This is fine for most of us to bisque and for low temperature glaze work.
Once we need to get into elevated temperatures above cone 1, these kilns
are seriously defficient
in both insulation, power needs, and construction.
I think these kilns stem from a market driven need, including potters.
There is a huge industry that fires in the cone 06-04 range and for which
these kilns are perfect. As many potters fire hotter glaze kilns, these
kilns are not practical. They become practical from only a purchase point
of view, and I think that while the specifications on many of these kilns
do say that they are able to fire to cone 10, the necessary power needed is
way too small, the kilns under insulated, and the cost to fire big. IMHO, I
would not fire these kilns to cone 10. We have been able to get much better
service out of our TnF28-3 Paragon kilns by installing higher rated
elements. They now fire faster and evenly, and there has been no
appreciable rise in our electric bill, but then again, we are only paying
$.06 a kilowatt hour and no demand.
I researched electric kilns in depth over the past year to try and find
kilns that were suitable for higher temperatures and had the necessary
heavy duty construction in refractories, structural steel, and electrical
areas. WhaI found was that these units are indeed available, and at a
substantially higher cost in comparison to their low temperature relatives.
What was of importance to my company in selecting a new kiln was
1. cost per cubic foot of interior firing space
2. type of elements used
3. electrical componetry
4.structural steel
5. refractories
and finally
6. a front loading kiln
We have 2 limitations in our building: single phase power and 100 amp
maximum service to the kiln.
While 3 phase power is available from the street, the cost to bring it into
the building exceed the cost of the kiln. We needed to have a unit capable
of maximum cone 8 temperatures on a 100 amp service, single phase.
After researching with the help of many people and their expertise
including Kiln Ray Services, Euclids/Pottery Supply House, and more, we had
a 26 cubic foot kiln built to our specifications by Olympic Kilns. The cost
per cubic foot of interior friing space was reasonable incomparison to the
many others we researched including Alpine, Frederickson, Nabertherm, Gile,
L and L, and a host of others.
Our new kiln has a 3 zone controller. Other features are heavy duty Hoskins
elements including elements in the door as well as in the floor, a fiber
module roof, heavy duty electrical componentry(mercury displacement relays
etc), venting, and a very solid structural steel frame. What makes the
design even more appealing is that we wanted to use the same size kiln
shelves as our gas car kiln, and the kiln was built with interior
dimensions of 30" x 30" x 50".
The kiln was shipped last week. I'll keep the list posted on the
installation as well as the firings.
Jonathan
Jonathan Kaplan, president
Ceramic Design Group LTD/Production Services
PO Box 775112
Steamboat Springs CO 80477
plant location
1280 13th Street Unit 13
Steamboat Springs CO 80487
(970) 879-9139 voice and fax
jonathan@csn.net
http://www.sni.net/ceramicdesign/
Carolynn Palmer on mon 26 apr 99
Most of my Crusader electric kilns have had to have their blankets replaced
due to crystalization and loss of insulating properties of the blankets they
came with. When we do the replacements, we use a blanket of kaolwool that
was twice as thick as the original. They work much better - heating up
faster and cooling down slower.
Carolynn Palmer, Somerset Center, Michigan
Euclids on tue 27 apr 99
> I was glad to see John Baymore's post above. I have thought about this
so
> much. I really believe that electric kiln manufacturers are being very
> irresponsible in making kilns which reach temperature by virtue of their
> POWER rather than their INSULATION!! It seems so obvious. AXNER has a
kiln
> which has three inches of softbrick PLUS a layer of fiber board. This
> should be the standard, at the least. I really like L&L Econokilns, but
in
> midrange firings, the radiated heat from the outside of the kiln is
obscene.
> I wish there was an agency licensing electric kilns to guarantee a
minimum
> level of EFFICIENCY and not just SAFETY. We really need that.
> Best wishes -
> - Vince
Hi Vince,
I too have put alot of thought into this. I do not disagree. However,
working in the kiln department here for 12 years it becomes quite obvious
that a very high percentage of customers do not want to pay for the extra
insulation, as well as other useful upgrades such as type `S` thermocouples
& mercury contactors. Is is.
Our standard line of round, top-loading, hobby kilns have 3" brick & no
insulation. Our square top-loading kilns have 3" brick plus either 1 or 2"
of mineral board. At a guess we will sell 30-40 non-insulated kilns to one
insulated (in the pottery industry).
I admit that Euclids are powerful kilns in comparison to some of our
competitors, but they reach temperature on a regular basis.
We also build a lot of industrial furnaces for companies such as Ford,
Alcan, Inco. These customers do pay for the upgrades. So from my
perspective, quality kilns with plenty of insulation & all of the "bells &
whistles" are being made everyday (before my very eyes) but potters are not
the ones buying them.
If there is an industry standard, we will meet or exceed it.
If you would like to create a Clayart kiln standard, we are up to it.
I will gladly work with you to design what you would consider an effecient
& safe kiln.
Chris @
Euclids Kilns & Elements
1-800-296-5456
euclids.com
mail@euclids.com
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