John Baymore on mon 26 may 97
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=3Csnip=3E
From: Nikom Chimnok =3Ckoratpot=40loxinfo.co.th=3E
No sir, heat doesn't rise. Heat is omni-directional, radiating at a rate
in proportion to the cube of the temperature differential in all
directions. But hot air rises ....=3Cclip=3E .......... The idea of heat
rising is precisely what the original writer called it: a wive's tale. The
seemingly common sense observation of a person insufficiently trained in
how to observe. .....=3Ccut=3E
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Way to go..... about time someone mentioned some of the the basics of
thermodynamics=21
As a kiln designer and troubleshooter for some 20 years, I have seen that
this =22common understanding=22 that =22heat rises=22 has resulted in =
thousands of
kilns being built with terribly underinsulated floors=21 Rhodes =
=22Kilns=22
book probably had a great influence in this...... most kilns pictured in
that have 5=22 thick HFB floors.
The way the thought goes in getting to this situation usually is that you
can save some money on the floor of the (already expensive) kiln by using
HFB instead of IFB and only make it two courses thick (5=22) cause everybody
knows........... heat rises=21=21=21=21
I have seen a recent (1996-97) piece on kiln construction published showing
a kiln that has a floor that appears in the photographs to be only two
layers (2.5=22 each) of hard firebrick. At the best (since I can't see the
lower course under the actual chamber) the area under the hardbrick placed
on the hot face of the interior chamber MIGHT be some insulating brick =
2.5=22
thick. But the edges that show under the perimeter walls appear to be HFB.
So the =22myth=22 continues to be promoted even today.
If you decide that you need 7=22 or 9=22 or 12=22 of insulating material in =
the
walls and/or arch to get the cold face temperature to the point you'd like
(and with that the accompanying heat loss figure in BTU/sq. ft./ hr. ),
then the floor needs to be commensurate in insulating value. As already
stated, heat moves from areas of higher concentration to areas of lower
concentration without respect for =22direction=22. Down is as good as up or
sideways.
Right under the fireboxes in gas kilns (particularly those with blower type
burners capable of running 100=25 aeration) is of particular concern.......
the heat differential here is often one of the higher in the interior for
the longest time. So the heat loss through a given refractory will be
higher (although it is given that the total area --sq. ft.-- of the firebox
floors is small).
And people wonder why the bottom tends to fire cold and it costs so much to
fire the kiln =3Cg=3E.
Best,
........................john
PS: There are some other minor factors such as higher heat transfer off
the cold face surface due to convection cooling (increases the differential
between hot face temp and cold face temp.....the driving force in the
equation) that =22ups=22 the heat loss on the walls slightly (and a little
less so, the arch)....... but these factors are pretty small in magnitude.
Also, the positive pressure (with reference to atmospheric) that often is
the case in the upper parts of the kiln (hot gases rising) can drive gases
out any cracks or pores in the upper kiln structure...... so the =22common
sense=22 approach to solving this resulted in these areas often being
thicker...... continuing to justify the underinsulated floors.
Additionally, with a thinly constructed floor the possibility of air
infiltration through the cracks in the refractories (in this typical zone
of negative pressure) is another justification for better construction than
that which is commonly seen in site-built kilns.
PPS: BTW, as a sideline here...... the title -----=22old wives =
tale=22------
used by many of us in this fashion is a somewhat sexist comment, if you
think about it. How about just =22myth=22? =7BSorry.....just had to =
comment on
that. =3Cg=3E)
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
JBaymore=40Compuserve.com
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