Dave Finkelnburg on fri 27 feb 04
Earl,
Regarding dampers, let's refer to a "passive" rather than "dilution"
damper. The term passive damper has been in use for quite a while.
You ask why use a passive damper. The answer is simple--it's all about
the degree of control one can achieve.
An "obstruction" damper is a control valve. It controls the flow of gas
passing the damper. Without getting into Bernoulli's theorem in detail, the
control of flow is proportional to the square root of the pressure drop
across the damper.
Following this square root function, most of the control exerted by the
damper occurs as the damper approaches being fully closed. This is true for
any control valve. At half closed the valve changes the flow very little.
At three quarters closed, the valve begins to have an effect. Most of the
control is between that 75% closed point and having the valve fully closed
(100% control).
What potter has not encountered a gas kiln with a too-sensitive damper?
Touch the darn thing a hair and the kiln leaps from oxidation to reduction,
or vice versa. This is very common with an oversized flue. You need to
have the damper 97% closed, or so it seems, to get the kiln into reduction.
Fred Olsen, in "The Kiln Book," recommended flues that are oversized. After
all, better too big than too small! Nils Lou tried to fine tune dampers, as
described in "The Art of Fire," by taking some pressure drop across an
obstruction in the flue ahead of the "obstruction" damper.
The problem is the wide temperature range over which we use our
fuel-fired kilns. We start out trying to keep the kiln temperature below
100 C (212F) to dry the ware, then want the same flue and damper to work at
1,300C (2,372F)!
A passive damper can really take the pressure off the obstruction damper
early in the firing. Wood-firers are very adept at pulling bricks out of
the base of the stack to lower flow through the stack as they start firing.
As they get above cone 10, though, they have all the passive damper shut and
want the most flow possible through the fire box.
So yes, the passive, active damper combination allows more control in
changing weather.
Hope all is well with you on the wet coast,
Dave Finkelnburg, in the semi-arid Intermountain West of the US
----- Original Message -----
From: "Earl Krueger"
Sent: Thursday, February 26, 2004 1:36 AM
> So, I'm trying to _understand_ about kiln chimneys and the
> types and effects of dampers.
> The kind of damper I am familiar with, from stoves not kilns,
> restricts the flow through the kiln by partially blocking
> the path the gasses must take. Not knowing a better or more
> proper terminology I shall call this an "obstruction" damper.
> From what I have gleaned by reading this thread there is
> another method used. This method I shall call the "dilution"
> damper as the effect of reducing the gas flow through the
> kiln is accomplished by diluting the hot gases in the chimney
> with cool ambient air thereby reducing their buoyancy. This
> reduced buoyancy then produces less "draw" on the kiln proper.
> I have read in other posts that reduction levels in a kiln are
> very sensitive to damper adjustments. I assume that
> obstruction dampers were being referred to. From my
> limited knowledge of physics I can see why this would be
> true.
>
> Question #1. When would you use an obstruction damper
> versus a dilution damper? Would there be an advantage
> of having both available? When would you adjust one
> as opposed to the other?
>
> Question #2. I have read of kilns stalling due to low atmospheric
> pressure, as when a storm is approaching. Would a taller
> chimney, with a dilution damper allow better control in situations
> like this ( i.e. ports open when barometric pressure is high and
> closed when pressure is low)?
Ivor and Olive Lewis on sat 28 feb 04
Dear Dave Finkelnburg,
Having both devices incorporated into a fuel fed kiln makes a lot of
sense..
Best regards,
Ivor Lewis. Redhill, South Australia
Earl Krueger on sat 28 feb 04
Thanks Dave. Your well written and complete
answer cleared up a lot of the confusion I had
over terminology and use of dampers.
But, I have to agree with Bruce. "Dilution"
damper still seems less vague than
"Passive" or "Atmospheric". Perhaps it is
time to change the lexicon.
Think: DILUTION, DILUTION, DILUTION! :-)
Earl K...
Bothell, WA, USA
Mark & Sylvia Mondloch on sat 28 feb 04
one more thought for the discussion...
After a firing, when I close the "obstruction" damper and burner ports, I
also pull out the brick to open the "dilution/passive" damper. The chimney
cools very rapidly as it is drawing lots of room air and I can go to bed
without worrying about a build up of heat by the rafters near the chimney.
Sylvia
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