Karl P. Platt on sun 25 aug 96
Someone was wondering about flues and chimneys.........
Someone also mentioned that kilns with forced air combustion didn't require much
of a stack. They don't, but if the flue is too small the kiln will run under
pressure and more likely than not all of the metal holding it together will get
fried...fried. Alternatively, if you're using dinky blowers, like one sees on
Alpine kilns, it's not too hard to find that the pressure in the kiln caused by
undersized (or choked off) flues can actually exceed that of the gases (flame)
issuing from the burner, which sets the stage for a host of combustion
difficulties -- and fried burner tips.
The height of a stack makes a the difference between whether a kiln will work or
not in a natural draft kiln -- one which used "venturi" burners or solid fuel
like coal or more typically wood.
In designing the flue/stack there are three things to consider.
1. The size of the flue opening
2. The size/length of the breeching (the section between the flue and the stack.
3. The stack diameter and height.
In general the nominal size of any of these depends on the quantity of gas they
must handle -- which in the case of studio scale kilns is quite small relative
to what one would have needed for an old brickyard beehive of the early 20th
century. Most studio kilns operate at way less than 1 million BTU/Hr. input.
Taking the case of natural gas and a kiln that fires to 2,200 oF it is noted
that the volume of the cold air-gas mix input to the flame expands about 5x. So,
in a kiln using 400,000 BTU/Hr the volume of flue gases evolved is 24,200 cubic
feet/Hr. If the kiln is, say, 80 cubic feet, this means that the gases inside
the kiln change every 10 seconds or so -- this sort of relation is also the case
in smaller kilns which, in general, require more heat on a unit volume basis.
The numbers for LPG aren't all that different insofar as it still requires
roughly 10 cubic feet of air for every 1,000 BTU released and the preponderance
of gas being moved through the kiln was introduced as air.
The flue opening, while it is intended to furnish an exit for the flue gas, also
presents a resistance. The breeching also presents a resistance and there's
resistance to the flow of flue gas up the stack -- regardless of the type of
burner arrangement used. Since it's much worse to build a stack that's too small
than one that's too large, it's always better to build them oversized -- but not
too much. A fleu that's too large wll probably cause a cold spot in its'
proximity.
There's actually a formula for working out generally how many square inches of
flue are desirable for a given volume of flue gas, and the interested can go
look at pg. 226 of the North American Combustion Handbook for the gory details.
Generally speaking, however, considering that a studio kiln will operate at less
than 1 million BTU/Hr and seldom sees much more temperature than 2,200 degrees
F., 9" x 9" flues are a handy -- and modular -- size for kilns operating between
500,000 and 1 million BTU/Hr. Smaller than that, say down to 250,000 BTU/Hr., a
4.5" x 9" flue will do the trick quite nicely.
The size of the stack is usually determined by the setting of the bricks making
it up, but again, the 9" x 9" (81 in^2) is usually more than adequate for Studio
needs. There's nothing lost by making it bigger as draft depends on height --
where area comes into play is in determining the resistance to the flow of the
flue gases.
TO BE CONTINUED.........
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