John Baymore on sun 22 jul 01
Again, I assume that this is not only due to the defect in his kiln lid
but also because of the heat rising inside. =
Lisa,
Just picked up a little minor detail in your post of questions..... but o=
ne
that seems to be somewhat universal...so I want to clarify it a bit so th=
at
it doesn't inadvertantly reinforce a common misunderstanding.
Heat does not rise. Heat energy moves in all directions from areas of
higher concentartion to areas of lower concentration. It knows no "up" (=
or
down or sideways for that matter). This is basic thermodynamics.
Heated gases (like air) are less dense than cooler gases. The less dense=
gas, if allowed to be free to do so, will rise when contained/surrounded =
by
cooler gas. The hot air baloon is a good graphic example of utilizing th=
is
concept.
We precieve this situation all the time with our human senses...... the
upper reaches of a house are typically warmer than the lower areas. So w=
e
tend to make the intuitive connection........ that "heat" rises. Hot AIR=
rises........ moving some heat energy by convective means. The hotter a=
ir
is upstairs... the cooler air is downstairs. This is why a celing fan ca=
n
help with heating and cooling costs....... it mixes the natural "dead air=
"
stratification that occurs. This makes the house "feel" warmer/cooler wi=
th
the same amount of energy (heating or cooling) expended. In the case of
heating, it also decreases the differential between the hot face (inside)=
of the ceiling insulation and the cold face (outside).... which is the
prime driving force for heat transfer through any substance....like house=
insulation . Ditto for cooling....but the hot face is then the
outside...and heat energy is moving IN.
(This is also one reason why industrial high velocity burners can improve=
how a kiln fires....... good mixing and heat distribution. Even a
"potter-technology" forced air burner can do this when compared to some l=
ow
pressure venturi-type burners.)
This misunderstanding about "heat" often leads to things like putting les=
s
insulation in the floor of a kiln because, of course, "heat rises".......=
so it is often thought that the kiln floor doesn't need insulation simila=
r
to the walls or particularly the roof/arch. NOT. If the hot face of a
brick is at 2300F and the cold face is at 100F, the heat energy being
transmitted through the brick doesn't really "care" if the brick is in th=
e
wall, floor, or arch. The same amount of energy goes through any of them=
.
(There are a few other minor factors that also affect this heat loss issu=
e
through floor, walls, and roof...... transmission of heat away from the
cold face surface, gas permeability causing air infiltration or leaking h=
ot
gases, and so on.)
I see site built kilns all the time with 9 inches of insulating firebrick=
in the walls, 9 inches of IFB in the arch, and only 5 inches of hardbrick=
for the floor!!!! (Thanks, Daniel Rhodes! ) Then the poor potter
wonders why there is a tendency for it to fire cooler at the floor level,=
and you have to adjust bag walls and such things to even it out much at
all.
So... to finally answer a couple of your questions ......... =
This is why the local pickup downdraft electric kiln vents DO make the
kilns fire more evenly. They counteract the tendency for more heat energ=
y
to be moved to the top of the static environment electric kiln by "normal=
"
convection assisted by the leakage of a small amount of gases from the se=
al
at the lid and upper spy ports and so on. So yes... they DO even out the=
kiln....and better yet, they help supply oxygen for chemical reactions th=
at
require that to go to completion.
If you need to work in the room with your kiln firing...... you are
ASSUMING that the local pickup vent is getting all the junk coming out of=
the kiln. If you are correct, there is no problem. If you are wrong....=
..
there MAY be a problem. Kiln fumes can contain many things. (No molds th=
at
I know of .) To quote Clint Eastwood, "Do you feel lucky today?" =
Seriously....... kilns are BEST installed in a separate location.... that=
's
why God invented the term "kiln room". =
If you are not able to isloate the kiln from your workspace, install not
only a local pick up kiln vent, but also some general dilution ventilatio=
n
for the general studio workspace (good idea anyway), and make sure that t=
he
clean make up air flow goes through your typical breathing zone BEFORE it=
reaches the kiln area on it's trip through and out of the room.
As to putting in two vents on an oval kiln...... call the manufacturer of=
the vent system you are considering for the best info. They know their
equipment designs. What is most important for the vent design is the tot=
al
volume of the kiln, not the shape. However, the distribution of the exit=
pickups could affect the circulation patterns in the kiln chamber.....so
the placement of the holes in the floor might be an issue.
Hope this is of help.
Best,
..............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 17-26,
2001"
L. P. Skeen on sun 22 jul 01
John,
Thanks much for that detailed answer! The kiln will actually be in a room
separate from my work room, but will still be indoors, thus the question
about the vent.
L
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