JJHerb@aol.com on mon 6 jan 97
Gavin Stairs posted something about heat loss not varying with ambient
temperature. Calculations of heat loss include factors for material
insulating value (or heat transmission rate), material thickness, and the
difference in temperature across the material. This last is the famous Delta
T and is expressed in terms of absolute temperature. At normal human living
temperatures, 100 degrees is a lot. The difference between 450 and 550
degrees absolute (Rankin) is about 20 percent. At firing temperatures, the
difference between 2400 and 2500 degrees, on the other hand, is about 4
percent. If all else is constant, the increased thickness of material, 16
percent for the half inch from 2.5 to 3 inches, should be much more important
that any variations in the temperature of the air surrounding the kiln.
Joseph Herbert
JJHerb@aol.com
Gavin Stairs on tue 7 jan 97
>Gavin Stairs posted something about heat loss not varying with ambient
>temperature. ...
This surprised me so much that I went back to my original post to see just
what I said that gave this impression. What I wrote is correct. However,
heat loss is a quantity which varies over the whole firing interval, and
which depends on several factors which are not strictly linear. Most of
these are relatively insignificant relative to the major factor of
insulation. In order to get the ware to a given temperature, you must
supply heat to do the following:
1. Evapourate all the remaining water in the ware.
2. Heat the ware itself to temperature.
3. Heat the furniture and all the kiln hardware, including the insulation.
4. Change the various phase in the clay body, for it to mature.
5. Heat the air which passes through the kiln.
6. Replace heat which is conducted away via the frame, wiring, ducting,
etc. which are attached to the kiln.
7. Replace heat which is removed from the outside of the kiln by air
convection or draft.
8. Replace heat which is radiated away from the exterior skin, or the view
holes and various other apertures.
I don't propose to go through these in detail. Items 1, 2 and 4 are
irreducible (except by predrying). Items 2 and 3 help to keep the kiln from
cooling too rapidly, and even out the temperature fluctuations when you are
ramping less slowly than the full power rate, by switching the power on and
off, for example, as with an electronic controller. Item 5 I referred to
in my original post. It is to some degree under your control: make sure
your kiln is tight! Item 6 is a function of kiln design, and is probably
not in your control if you are buying as opposed to building a kiln. Items
7 and 8 are both highly non-linear, and greatly affected by the amount of
insulation you have. By non-linear, I mean that the amount of heat you lose
from the kiln rises much faster than the temperature at the outer skin of
the kiln. Item 8 is greatly affected by the skin of the kiln: a shiny
metal skin (clean!), or a clean white skin will reduce the amount of
radiative heat loss. Similarly, a clean exterior, without lots of fins and
things sticking out, will reduce the convective loss. Both of these things
will increase the external temperature of the kiln for the same heat loss,
which means that the temperature difference across the insulation will be
less, resulting in a smaller total heat loss. Notice that the kiln controls
and wiring will be designed to have a high rate of heat loss in order to
keep them from overheating.
For all of the factors but the insulation remaining constant, the effect of
changing the insulation is approximately linear with insulation thickness.
This is, in most designs, the dominant effect, so it is approximately
correct to say that the effect of doubling the insulation will be to halve
the heat loss. Approximately correct. In fact, you will not realize quite
the halving of the power required to fire a given constant temperature
profile, but close enough to make it worthwhile if your energy costs are
high. If you can fire at a faster rate than possible with the thinner
insulation, you may save more than this. A detailed calculation of heat
loss is quite complicated, and until recently most kiln designers would not
have attempted it. They would rather have made a couple of kilns, and
measured as much as they could. Today, there are computer programs which
can do a reasonable job of such calculations, but they are expensive, and
require considerable expertise to run.
So, back down to earth: Consult the kiln manufacturer or someone familiar
with that particular kiln about the savings you may expect. Ask about
firing rates, and use your own judgement about what rate you will want to
fire at. Compare what you hear to your expectations based on a linear
estimate. Work out the power savings relative to the increased kiln cost,
and buy what suits you best. When you get your kiln, keep it clean,
especially on the outside and around the controls and wiring. And make sure
that the lid/door closes tightly, using a gasket if necessary. Chink the
holes unless you need them open. Vent when you need to remove volatiles and
water, and turn it off when you need to stretch out for the last few degrees.
Bye, Gavin
PS: In my previous post I seem to have slipped from F to C without
translating. I meant to indicate a high cone temperature around 2400 -
2500F, rather than around 1200 - 1300F.
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