Paul Vernier on wed 29 aug 07
I have been reading the threads on chimney sizing and have a related
question. I recently completed building a new kiln, a MFT. I initially
followed Nils guidelines on sizing the flue. But, I was warned that the
smaller flue size has contributed to stalling. So, I made the flue a little
larger thinking I could close it off some later as I experiment and learn to
fire my kiln.
OK, the question. Doesn't closing the damper effectively reduce the flue
size? Or is it more complicated and dependent on things like the chimney
base design and the distance between the flue and damper. I am sure there is
something to do with turbulence as well.
I really enjoy and appreciate the contribution people make to this forum.
Paul, in HOT Santa Cruz
om on thu 30 aug 07
On Aug 29, 2007, at 3:00 PM, Paul Vernier wrote:
> OK, the question. Doesn't closing the damper effectively reduce the
> flue
> size? Or is it more complicated and dependent on things like the
> chimney
> base design and the distance between the flue and damper. I am sure
> there is
> something to do with turbulence as well.
I'm no chimney expert by any means and what I'm about to say is
basically a regurgitation of things I've heard.
dimensions: wide + short = tall + narrow
Dampers:
Chimney's work because of the temperature differential between the
temperature inside the chimney and the temperature outside. When the
differential is high, the chimney pulls strongly. When it is low, it
doesn't. This is why a fire in a fireplace is smokey at first, but
clears up after everything gets warm -- the cold chimney could not
produce a draft because the inside and outside temperatures were at
equilibrium.
There are a couple damper types: passive and gate dampers. A
passive damper is a hole near the base of the chimney that allows
cold air into the chimney. This would do two things, first it would
cool the chimney to some degree and lower the temp. differential
between inside and out. It also makes the chimney draw from two
sources which reduces the suction applied to the kiln body and its
air inlets. A gate damper simply blocks the chimney to make it
harder for the gases in the kiln to escape. This also causes the
chimney to cool because it receives less energy while losing energy
out of the chimney mouth.
Closing a passive damper or opening a gate damper may not result in
immediate resumption of strong draft if the chimney has cooled too
much, because that differential between inside and outside temps must
be rebuilt.
In my kiln, I do not use dampers. I do keep a bit of kiln shelf over
the chimney to keep rain out when not in use, and early in the
firings I leave the "cap damper" mostly closed because I'm trying to
hold in a bit of the heat -- there's a lot of mass to warm up before
the chimney even gets a hint of it. When the firing is over, I cut
the draft by capping the chimney.
Now, considering all those things, it seems to me that there is
potential for a "cap damper" to be able to block draft, but maintain
a higher chimney temperature. Most gate dampers are down low where
they are easy to get at, but this deprives the chimney section above
the damper of fire. If the damper is placed on top of the chimney
mouth, the full chimney is exposed to fire, albeit at a lower rate
than when fully open, which may help it maintain temperature and
might allow for a quick turnaround from weak draft (cap on), to
strong draft (cap off).
Odin
PS: I suggest getting up close and personal with the chimney mouth.
It's a real kick!
http://anagama-west.com/anagama/firing/firing_04.php?page=3 (scroll
down three or four shots)
and some video closeup of the anagama chimney (toward the end):
http://www.anagama-west.com/firing_log/archives/29
Lee Love on thu 30 aug 07
It is much easier to make a flue exit big and then make it small than
it is to make a smaller one bigger.
--
Lee in Minneapolis, Minnesota USA
"Making pots should not be a struggle.
It should be like walking down a hill
in a gentle breeze." --Shoji Hamada
http://mashikopots.blogspot.com/
"For a democracy of excellence, the goal is not to reduce things to a
common denominator but to raise things to a shared worth."
--Paolo Soleri
Gary Navarre on fri 31 aug 07
On Thu, 30 Aug 2007 10:23:02 -0700, om wrote:
>Now, considering all those things, it seems to me that there is
>potential for a "cap damper" to be able to block draft, but maintain
>a higher chimney temperature. Most gate dampers are down low where
>they are easy to get at, but this deprives the chimney section above
>the damper of fire. If the damper is placed on top of the chimney
>mouth, the full chimney is exposed to fire, albeit at a lower rate
>than when fully open, which may help it maintain temperature and
>might allow for a quick turnaround from weak draft (cap on), to
>strong draft (cap off).
>
>Odin
Hay Crew, this is interesting in that I'll be working on the chimney soon. I
have thinking left over from the last kiln I built and I think some sort of
combination of the passive damper and a couple gate dampers, one part way up
the stack and one more or less a chimney cap.
First, when I start the fire the stack is open and I don't use the cap
until no more moisture forms on a mirror held over the chimney exit. Toward
the end I would manipulate the damper between stokes to hold heat in as the
stoke gets going or open to let out the smoke. Something I wondered is if I
could construct a lever system I could operate the damper with from my seat
at the firebox. I've seen a shot of a guy with a chain in his hand attached
to what must have been the stack and I think it went to the damper up the
stack a ways. I'm also putting in a small passive damper brick to see what
it does. It ain't vary big but I don't think it needs to be. Here is what it
looks like...
http://public.fotki.com/GindaUP/ca/kpap/hbgma06ac/passivedamperbrick.html
Ya, getting close to the fire is easy if you're careful...
http://public.fotki.com/GindaUP/ca/kpap/hobagama_1986-90/sf.html
I shot that with the Pentax K-1000 and not much camera experience. This new
digital stuff is a lot easier and I use a tripod for most shots. I'm glad I
did some documentation as I could back then as it makes it easier to
remember what I wanted to do in my next kiln. Just didn't expect it to take
almost 20 years to happen. Oh well, that's clay time for ya eh. Keep up
showin us what goes on out there at Anagama West and stay in there eh!
G in da U.P.
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