Karl P. Platt on sun 20 oct 96
I've quite honestly never seriously studied a potter's book on kilns,
but would make the following observations from my side of the street.
jrsydnor@iglobal.net was wondering about sizing the flue for two forced
air burners (about 375,000btu's a each).
Assuming on-ratio combustion and natural gas for fuel, the flue will
need
to deal with roughly 45,000 ft^3 gases every hour it is operating at
peak temperature. Here we're assuming 2,400 F. This volume of gas is
5.5x the volume of the ambient temperature air and gas passing through
the flame into the kiln. In the 50 ft^3 kiln described, this means the
gases within it change every 4 seconds while at full heat and that
262,500 BTU will be available to cover wall losses, heat the walls
and heat the load.
The minimum flue necessary to handle 45,000 cfh flue gas would be 9" x
9" -- this is even a bit tight for my comfort as this figure does not
account for friction losses through the flue opening or the stack
itself. I'd go up 30% to 12" x 9" to allow for these factors. A
kiln/furnace run under serious pressure gets burned-up pretty quickly
and the surgery required to upsize the flue once it's built is quite
complicated.
As forced draft burners are to be used, it really doesn't matter how
high the stack is as long as it guides the gases out of the firing room.
If a steel stack is to be used above the breeching, it should be a
minimum of 14" diameter and should incorporate a venturi arrangement to
induce cool air into the flue gas stream -- this keeps the metal from
getting fried prematurely.
If you want to go through the exersize of running these numbers
yourself, it's pretty direct even for the numerophobes. See the North
American Combustion Handbook, vol 1, pp 218-227.
If you do bother to do this also have a look at the passages relating to
wall losses. There you'll see that a 4.5" thickness of 2600 IFB will
emit about 1,200 BTU/ft^Hr -- which is to say that every square foot of
wall will be losing 1,200 BTU each hour.
If the 50 ft^3 kiln has 80^2 of exposed wall, this means every hour at
full heat close to 100,000 BTU will leave through the walls. Given that
the heat available from the flame at 2,400F is only 38% of what is input
to the kiln, we find that 260,000 Btu needs to be input to the burners
compensate this loss.
In California, where gas is $7.10/MCF, the cost of this is $1.84/Hr.
Adding another layer of IFB to the outside as a 4.5" layer costs about
$16.00/ft^2. Thus, covering roughly 80 ft^2 costs about $1,300.00 --
less labor.
The effect of adding this layer is to reduce the wall losses to about
600 Btu/Hr. The savings in wall loss is about 50% or $0.92/Hr. As such,
the wall loss is paid off by the "insulation" in 1,400 hours or 140 (10)
hour firings. If firing proceeds every two weeks, this adds up to almost
6 years.
Here I say "insulation" only because adding the layer also increases the
mass of material that needs to be heated for the kiln to make it to
temperature.
One might note here that 'frax is only $1.50/ft^2 and a much more
effective insulator.
Without going through the gory details, in this light, it would be more
sensible to construct the wall of 4.5" 2,600 IFB and 3" of 2,300 or
2,600 IFB. The wall losses are plausibly reduced and the costs are
within reason.
In very rough terms, below is shown the gross BTU input required to fire
of the kiln described above (with the 7" wall) loaded with 1,200 Lbs of
ware and kiln furniture in 8 hours.
260000 Load
150000 Losses
200000 Wall Storage
610,000 Btu/Hr
God, why am I doing this...........
KPP -- off to do something fun
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