Ivor and Olive Lewis on tue 22 nov 05
Dear Tony,
Using your observations is all I can do.
First thing however is to be clear in you mind about what "Heat" is and =
what "Temperature" is.
Lets start with "Heat" Heat is an energy form that can be moved from =
place to place. It can do this by Convection, Conduction and Radiation. =
All three happen in a kiln, but mostly conduction.
Temperature is the quality of "Stuff" or "Space" that shows heat is on =
the move. As Energy moves the place it moves from cools and the place it =
goes to heats up, provided the destination is cooler than the departure =
point. Now all things have heat energy in them of one sort or another. =
When there is a difference in temperature heat always flows from stuff =
with the high temperature to where the temperature is cooler. To make it =
happen the other way you need an engine, something to drive the motion. =
In a Refrigerator you have a motor and a compressor to push heat from =
inside the fridge out into the atmosphere. In a kiln it is the fire on =
the hobs or the burners or the electric elements that act as that =
driving motor to get the heat moving
You burn wood in your firebox and release chemical energy by combustion. =
I have difficulty in understanding the next step but the way it appears =
to me is that the flames represent gas in a very agitated state, the =
greater the agitation the higher the flame temperature. This is as hot =
as things can be in that system. As the heated gas moves through your =
cold kiln the kinetic energy of the gas is transferred into things that =
are at a lower temperature. As heat transfers from the hot stuff to the =
cool stuff the cool stuff hots up and the hot stuff cools down, all the =
way to the stack.
As you get to the end of a firing the fire box and the early chambers =
are as hot as they can get at the damper setting you use and the rate at =
which you feed the wood. Open the damper to draw the hot stuff into the =
cooler chambers, to draw cool air in to calm the fire down and feed fuel =
at a slower rate (Guessing !). Keeping the early part of the kiln at a =
constant consistent heat means that less energy is syphoned away into =
the hot bricks and maturing ware (So you hope !). That heat will only do =
its work when it gets into a colder or cooler place, where the =
temperature is lower.=20
Right ! Back to the idea of Green Wood, damp or wet wood. Here the =
active ingredient is Water but there are two systems working, Physical =
and Chemical. At the beginning of the firing the physical one is in =
operation, at the end, above 1100=BA Celsius both are in operation.
You treat your water to four operational phases.
a) heat is needed to raise the temperature of the water to its boiling =
point. This is its specific heat and can be measured in Joules per =
degree C per 18 grams (gram molecular weight of water). Then there is b) =
the heat which converts water into steam, Latent heat of Vaporisation. =
You can't get any useful work from this fella, no temperature change, no =
work done but energy is needed. That's a few more Joules per 18 Grams of =
water. Now phase c). Your fire starts to warm the steam up. This =
requires a bit of heat since Water vapour has its own value for Specific =
Heat but it still needed for every degree rise in temperature. There's a =
few more Joules per degree C per 18 grams of water. So instead of =
heating the kiln your bursts of energy are adsorbed by water in your =
wood. Add to this the bit of heat c+) to change dry wood into a gas, =
which is only partly successful. These things need energy so things keep =
cool and you get a lot of dead coals clogging the system. Do you see the =
three operational phases, using energy to get gas of one form or =
another.
Now to the top of the firing where it seems as though things are =
stalling and the last chamber will not get up there
=20
Above 1100=BA Celsius you start to add wet wood again. What has happened =
in Phase a. b.and c plus c+ to get the wood burning still has to happen. =
An equal amount of cooling takes place to change every gram of water =
into steam. But now your Coals start a huge love affair with superheated =
steam. Marriage and Divorce. Carbon has the ability to separate the =
bonds that bind Hydrogen to Oxygen. Seems that the bond between Oxygen =
and Carbon is the stronger so there is an interchange and you start to =
flush a mixture of two lively gases, Carbon Monoxide and Hydrogen, into =
your kiln. The price to pay is that the temperature tends to fall =
because the heat has to come from somewhere. You will cool the firebox =
unless there is a stead pull from the stack, re-energising the coals. =
Now you have two energy rich gases entering the kiln from the firebox =
and they are hot and they have energy to spare. All they need is a few =
puffs of oxygen. If they get this then you get a flame front moving =
through the kiln and those flames are at about 1550=BA Celsius. The =
back of the kiln is down near a thousand degrees. Pots and those cool =
bricks are a heat sponge, eager and ready to soak up all those =
kilojoules from burning Hydrogen and Carbon Monoxide.
So friend, your theory that green wood prevents a blast off early in the =
firing and gives a blast off in the final hot stages of a firing would =
seem to have a sound basis founded on Scientific Facts.
I'll leave the Calcs to Bruce.
Anyway, its only about Economics when you think about it.
All the best, and thanks for the accolade.
Ivor
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