Stuart Altmann on fri 18 jul 97
Liz Dodge asked about capturing kiln heat for indoor spaces without also
capturing the fumes. I have done this in our WV farm house, except that
instead of a kiln, the heat comes from a Finnish fireplace. This fireplace
is like a downdraft kiln: the hot flue gases must go down below the firebox
to get into the chimney. It is also like a kiln in that it burns extremely
hot, so much so that ordinary fireplace firebricks can not be used to line
the firebox. The fireplace heats a medium-size, 2-story house very
effectively. The following suggestions make use of the same principles that
we used in designing our house's system.
Efficiently capturing some of the otherwise-lost heat without getting the
fumes can be done by taking advantage of several physical properties. The
first is that when material is heated it expands and it therefore has a
lower density. So, the simplest way to capture heat but not fumes from the
kiln and move it into, say, your studio, is to have a closed and sealed loop
of duct, one end of which is next to (or built into) the wall of the kiln
and/or its chimney, the other of which is in the space that you want heated.
One part of the loop must be higher than (e.g. above) the other; if so, the
air will flow 'round and 'round the duct loop without any fan! Like this:
/----------------------------------------- / > > > flow > > > then | /---------------------------------------\down|
hot | | | v |
|^ | in- | | side
hot | | your | v | studio
|^ | | |
hot | | | v |
|^ \_______________________________________/ |
\ up < < < flow < < < /
\--------------------------------------------/
How can you increase the amount of heat that is transferred by the loop?
First, get good heat pickup, in several ways. (i) Make the loop from a
material that will not be damaged by heat and that conducts and radiates
heat well. Steel stovepipes come to mind, or the sheet metal ducts that are
used to carry heat from a furnace to the various rooms of a house, with the
joints sealed with a high-temperature tape. (ii) Place the pickup portion
of the loop in the hottest available place, e.g. against the chimney, and
trap heat against that part of the loop, e.g. by enclosing the other three
sides with insulation batting. (iii) Increase the heat-exposed surface area
as much as practicable, e.g. by branching the loop, with several parallel
pipes that are exposed to the heat and that then rejoin at the delivery
pipe. (iv) Have a large, unimpeded flow of air in the loop. This means, for
example, using large tubes with a minimum of bends or other impediments.
(v) If possible, take advantage of counter-current flow (explained below) to
improve transfer of heat to the loop.
Second, minimize loss of heat from the loop between where it picks up heat
and where it delivers it. If the loop just goes through a wall from the
kiln room to the studio, you probably don't need to do anything, but if it
goes through some other space first (e.g. if your kiln is outside the
studio), then that part of the loop should be insulated. Again, minimize
bends or other obstructions, particularly in this area.
Third, maximize transfer of heat from the loop into your studio. Increase
the surface area of the loop within the studio by using as many branches as
practicable. Place them in an area that has good, unobstructed air flow.
If, when the kiln is at its hottest, the loop delivers too much heat (lucky
you!), you can store heat by embedding the delivery tubing in a lot of
thermal mass, e.g. bricks. And if your ducting is really sealed well, you
don't need to have any of it in your studio, just the delivery pipe, open at
the end, coming in or with as much extra piping as you want to carry the
heat to where you want it, and a low (e.g. floor level) return duct carrying
it out. Your studio is now part of the loop. However, for good flow
through the studio, the open ends of the inlet and outlet should be on
opposite sides of the room.
Now, what happens when the kiln is not being fired? If the kiln room is the
same temperature as the studio and the loop does not go through a cold area
between the two, the flow will simply stop. But if the kiln is in an area
colder than the studio, or the insulation is inadequate, the loop will run
in reverse and transfer heat out of your studio! To avoid this, at a
minimum you should have baffles where the loop enters and leaves the studio.
These are standard items in house ducts. However, they are not air tight,
and so you might want to supplement them with, say, a wad of insulating
material shoved into the intake and outlet tubes.
Countercurrent flow heat transfer.--Every biologist (which is what I am when
I'm not wearing my ceramics hat) knows about this because countercurrent
flow systems occur in many organisms. Ever wonder how a wading bird can
stand in ice cold water without freezing? In its legs, the arteries run
immediately next to the veins, and because the blood is flowing in opposite
directions in the two ("countercurrent" flow), heat is efficiently
transferred out of the artery as it flows down the leg into the blood
ascending from the foot into the body. Thus the artery has lost much of its
heat when it enters the foot, so little is lost there.
Why does it work so well? Imagine that, instead, the blood in the vein that
is next to the artery was (say, as a result of a loop in the vessels) moving
in the same direction as in the adjacent artery. As before, the blood in
the artery is hottest at the top of the leg but as it flows down, it gets
colder, loosing heat to the air and to the surrounding tissues, including
the artery. The colder the artery gets, the less effectively it transfers
heat to the vein as the two approach a common temperature at the coolest
(lowest) part of the artery. Thus, the blood being carried back to the body
in the vein would not be very warm: little heat would be recovered.
However, in the nature's countercurrent system, with blood flowing in
opposite directions in the two pipes, the blood ascending in the vein is
gradually warmed as it ascends, yet as it does so, it encounters
progressively warmer blood closer to the body, and so heat continues to be
transferred to the vein and so recovered.
The same principle of countercurrent heat transfer is used in our house to
pick up heat from the walls of the chimney (about 25 ft. high) and firebox,
then deliver it to the rest of the house (details on request, large SASE).
On that scale, it involves construction along the walls of the firebox and
chimney that are too massive to be warranted for a kiln that is fired
intermittently. However, the principle can be applied in the following way.
If you have a downdraft kiln and the downdraft flows along an outer wall,
then that wall will transfer heat more effectively to your ducts, with their
rising, warming air, than will a wall of the same temperature but with an
updraft on its inner surface.
Stuart Altmann
1507 E. 56th St.
Chicago IL 60637
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