Karl P. Platt on wed 16 oct 96
Don --
Wilkey's book is, indeed, a gem. It also must've been a labor of love to
have typed all of that back in analog days.
In short, the heat needed to bring a kiln to any temperature is the sum
of the heat required by the load to reach a given temperature; the heat
required to heat the walls to that temperature, and wall losses at the
highest temperature the kiln will experience. Then one needs to decide
how fast they want it to get hot.
The load, assuming pots and clay based kiln furniture requires roughly
600 BTU/Lb. to get to 2,300F. Wall losses are fixed for any given type
of construction and depend greatly on whether or not there's any wind
blowing on the kiln.
Since everything else is a given, the time to temperature you want to
achieve greatly determines the net cost of the installation. I recently
built a large-ish glass fusing kiln (12" x 4" x 1") that can dance its
way to 1,500F in about 30 minutes. This requires 200 amps @ 240 VAC/3
phase. Not cheap stuff to wire or control.
-- by the way, my experience with BVC components on this project has
been disasterous, but that's another story. Suffice it to say that I'll
not be buying BVC components any more in favor of those offered by other
purveyors who understand high powered appliances and stand behind their
products.
>>Since the elements grooves are cut into the wall of the kiln, should the thick
Assuming this worst case is reasonable. You're much better off over
powered than under-powered. Your post suggests you prefer 60 amps to 50
amps -- a 20% increase over the calculations. This provides for a lot of
contingencies and is a good number to go with.
>> Regarding the BTU's/ sq. ft. figure. How does that compare with your usage fi
These numbers are in the ballpark. However, I'd want to point out that
when determining the net requirements for a gas kiln one needs to
account for the diminished amount of heat available from a flame as
temperature within the kiln increases. Assuming natural gas as your
fuel, at 2,300F only 35% of the heat released in the flame is available
to do any work. This means that the burner capacity needs to be 65%
above what the calculated thermal requirements call for.
This is not to suggest that gas is in some way less efficient than
electricity. It is not. Electrical generation is a process which on its
best days is 25% efficient -- put 1,000 tons of coal into a boiler and
you get 250 tons worth of electricity out -- along with a lot of
byproduct, like the "global _cooling_" sulphur compounds. Popular
suggestions that electricity is a somehow more pure form of power are
disingenuous.
>>3- When calculating the power (any) requirements for a kiln, what rate of clim
Depends on the type of load, uniformity of heating, nature of the clay
body, etc. For pots, which are of varying cross-section, possibly thick,
made of clay rich in organic materials, etc., a rate of rise from 1-5
:F/min is pretty typical. Gres Porcelainatto tile, as made by Crossville
Ceramics in TN or at Eliane in Criciuma, SC, Brasil, are fired to 1,200C
or so, cold to cold in about 40 Mins.
>>When calculating thermal requirements for a comparable fuel fired kiln,
are you seeking equilibrium? If so, at what point above the end point
would you consider it prudent? In a relatively unheated kilnroom with
lower ambient temp.of the stated 68* in the winter, what % factor would
you increase the power requrements?<<
I'm never seeking thermal equilibrium and want to ensure that the kiln
is overpowered to some extent -- you can always throttle down the
fuel/air or switch the electricity. I learned this lesson early on when
I build a ceramic kiln that pooped-out as a consequence of having been
built to overly-narrow design criteria. What you want to avoid is
running up the capitalization expenses to unreasonably. As noted above,
switching gear and wiring costs increase markedly with increasing
current carrying capacity.
Ceramic kilns are subject to widely variable loads -- sometimes being
filled with teacups and at others with heavy sculpture. At a minimum it
needs to deal with the worst case with respect to the load. However,
wall losses are fixed by the type of construction used and are a
function of the difference between the the temperature inside and
outside the kiln. Wall loss is a linear function of the temperature
difference. The increase in input required by a change in ambient
temperature of 40F ( 70 to 30F) is actually quite small -- on the order
of a couple %.
>>Given that the thermal source (elements in case of electric) will diminish in
Depends on how much excess capacity they had in the first place and what
is operating to diminish their performance.
>>Wanting to stay "fiber free", what might be the choices of safe back-up
material available? (castable with alumina bubbles?)<<
Why fiber free? I'm a great fan of vacuum formed ceramic textile boards.
These are magnificent materials for both hot-face use in certain
applications and as a back-up insulation. Consider that the 12 Lb/ft^3
board is only about 25% the weight of IFB and the savings in wieght is
enormous. When treated with rigidizer these materials are quite durable
and dust-free. I have one installation that's been operating for a
couple years which doubled capacity and lowered firing costs by 40%
through the use of these materials. As the application involves firing
on bright gold decoration, dusting is bad juju. These have never been
losses owing to this.
Mineral wool board, too, works well behind IFB, and it's real cheap.
However, over time it tends to crumble and should not play any sort of
structural role.
>>And while I'm at it...........Where might one source high temp kiln
components (preferably not plain vanilla borderline) such as box wiring,
porcelain insulators, porcelain wire nuts, high temp. connectors,
various
(nickle?), switches, heat resistant supply cord, refractories, cements,
castables, insulative coatings, etc......<<
Box Wiring -- Glass clad Cu wire can be had at appliance suppliers.
Porcelain Insulators are tough to find, but there's a place in LA that
makes them.
All the porcelain wire nuts in the world are made at Curran-Pfeiff in
Edison, New Jersey. This shop is _amazing_. They routinely fire to ^13
with oil and make a wide variety of highly interesting ceramic products.
Ask George Pfeiff about other porcelain wiring components.
You'll notice most kiln makers use proprietary type element/line
connectors. The screw fastened silicon bronze types seem to work pretty
well and are probably the best thing going.
Switches are a Johnstone Supply item.
Refractories you'll have to find locally.
Anyway, go for 60 amps.
Regards,
KPP
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