Don Kopyscinski on mon 14 oct 96
Hi everyone,
Speaking of math...........
In an effort to improve the performance of my own kilns, and to gather some
information on an expanded version of my recent "electric kilns and element
life" post, I have hit one stumbling block. I recently acquired the book
"Calibrating and Calculating the Electric Kiln" by M. Wikey (a stellar
presentation of the subject, acquired from "The Potters Shop" [plug for a great
place] ). This book has helped me bridge the gaps missing (those of my of
understanding) in Fourniers "Electric Kiln Construction for the Potter" and The
Kanthal Corp. "The Kanthal Handbook". I have now calculated the input in
amperage required for my 10 cubic foot electric kilns and take exception to Mr.
Wikey's numbers (not a reflection on the work, it's truely outstanding! If you
want to understand how an electric kiln works, this is a wonderful resource. Not
light, casual reading ) I think that this handbook may be used to calculate
kilns, that are in my opinion, are underpowered (in my opinion, as derived by my
results, with kilns outfitted with the suggested power requirements in my
circumstances). and that many suffer compromised performance as a result of it.
Fornier does take exceptions with Mr. Wikey's numbers as well and offers two
scales which appear to be more practical in a production situation of regular
use. In an effort to compare the suggested power requirements I would like to
see BTU requirements from some kiln builders out there for the following
configuration.
12 sided kiln 29" X 26 3/4" with 3" thick K-23 insulation, having a lid and
floor 35" in dia. with a thickness of 3".
Total inside wall area in sq. in. - 3568.57
Total outside wall area in sq. in. - 4814.00
Mean sq. inches in sq. in. - 4191.29
Total mean sq. feet - 29.11
exterior to interior proportion (X to 1) 1.35
Hotface temp. in *F - 2400.00
Coldface temp. in *F (interpolated from Mr. Wikey's graph -chart 9T P. 125) -
380.00
Mean temp *F 1390.00
Hot face temp. in *F 2400.00
K-Factor (K-23 @ 2400*F hotface derived from K-factor graph) - 1.60
Wall thickness 3.00
******It is the following portion of the calculation I am questioning******
BTU's needed/sq. ft.(calculated by the given formula-Hot face temp. x K-factor
of brick /wall thickness in inches) - 1280.00
BTU's needed for this kiln fired in ambient air temp. of 68*F (calculated by the
given formula - BTU's per sq. ft. x Mean wall sq. footage - 37255.87
Watts needed this kiln (@ 3.413 BTU's per Watt) 10915.87)
Voltage available (not a typo, adjusted for line drop) - 230.00
Amps required 47.46
This puts it right at the 48 amp. threshold I spoke of in my last post.
I have found that this level of power in a kiln causes it to reach equilibrium
at too low a temp. contributing to the type of stalling that was recently noted
by Corrina Null in her "temp. rise and venting" query. I am proposing a minimum
Amperage of 55 amps. with a more reasonable one aproaching 60 amps (@ 230V).
This would convert to 43,174 BTU's and 47,099 BTU's or 4317/sq. ft and 4710/sq.
ft. (rounding to nearest whole unit) respectively. How does this figure look to
you?
{{{DO NOT MAKE MODIFICATIONS BASED ON THESE CALCULATIONS !!! I AM A NOVICE AND
HAVE BEEN CALCULATING A GRAND TOTAL OF THREE DAYS (having had the book for a
week, and already questioning the author's recommendation). THIS IS AN EFFORT TO
LEARN MORE ABOUT THE REQUIREMENTS, NOT A SETTING OF ANY RATIONAL STANDARD (yet).
}}}
My questions:
1- Regarding the wall thickness figure. Since the elements grooves are cut into
the wall of the kiln, should the thickness between the outside wall and closest
touching element component be used? (This would put the calculations within my
range).
2- Regarding the BTU's/ sq. ft. figure. How does that compare with your usage
figures? If you were calculating the BTU requirements for a gas kiln, what would
you propose as a functional figure?
3- When calculating the power (any) requirements for a kiln, what rate of climb
(in *F/hr.) might you consider would you reasonable at firing peak (lets
consider 2350*F as an endpoint)?
4- Apparently Mr. Winkey's power requirement figure is determined with some
respect to the kiln's reaching equilibrium (in an ambient air temp. of 68*F)
just above the end point so as to not cause severe overfiring detrimental to the
kiln. 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?
5- Given that the thermal source (elements in case of electric) will diminish in
it's intensity with use, what % of deterioration might one expect before the
coils would no longer be able to maintain a functional temperature rise?
6- Does the formula, and do the calculations appear sound?
7- Wanting to stay "fiber free", what might be the choices of safe back-up
material available? (castable with alumina bubbles?)
8- 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......
Ed, Bill, David, Frank, John, Jonathan, Karl, Marc, Nils, Ron, Tom, Tony,
Anyone???
Inquiring potters want to know,
Don Kopyscinski
Bear Hills Pottery
Newtown, CT
74134.2672@compuserve.com
or
DonKopy@aol.com
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