Cameron Harman on sat 25 oct 97
Craig,
You are correct. It is the continual expansion and
contraction that is moving the brick. Although there is some
pressure inside the kiln, it is highest at the top of the
kiln and is being at least partially if not completely
relieved by the flue. Even if you built up a pressure of 0.1
inch water column of pressure (very high by kiln standards)
it would not push the outer brick apart. That would exert a
total force of about 13 pounds on a wall measuring 5 feet by
5 feet. If you lean against the wall while loading or
unloading, I am sure you put more force than that on it. The
force exerted by an expanding brick is awesome, I have seen
8" I Beams bent by such force, so there is enough force
there to slide a brick, but not much force to slide it back.
In time the wall will separate, unless it is held in place.
A proper kiln is built with expansion joints which allow
room for the brick to expand into. However, the outside of
the kiln must be contained or it will still move apart. Just
for passing information most bricks will move a little less
than one quarter of an inch per foot of length or height
for 2300 degrees F of hot face temperature.
The flat top will move the same amount if it is brick, but
if it is fiber it will compress and move a whole lot less. A
sprung arch must be confined or it will generate enough side
pressure to collapse itself. The pressure exerted on side
supports of a sprung arch is tremendous, but your flat arch
is great ... not much pressure.
If your teacher wants more of an explanation have that
person contact me, I will be glad to discuss it.
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
see our web site at http://www.kilnman.com
THE place for solutions to ALL your kiln and drier problems
**********************************************************
Marcia Selsor on mon 27 oct 97
I am just joining in to this thread becasue I have just replaced my
computer.
For kiln expansion problems I have always used a car valve spring with
large washers
on threaded rods that keep a sprung arch under pressure. This allows for
the expansion and contraction. All kilns breathe.
I have found that it is the heat more than pressure that eventually
bends angle iron. I get
100's of firings out of kilns and metal frames.
Marcia in Montana
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