Lily Krakowski on mon 20 jan 03
For a number of reasons I had to pull down the chimney of my little
woodburner. I now have a bunch of hard brick with cement on them.This will
be their second winter lying there on the frozen ground under snow, and I
hope that the frost will split off some cement. Does anyone have any ideas
easier than chipping the cement off to clean the brick?
Then: I do not want to rebuild the chimney as high as it was. I DO want to
have a length of chimney running horizontally before it goes up. I suppose
there are figures that tell one how much horizontal run replaces or is equal
to vertical...Yes? Maybe? Ideas?
Thanks
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Roger Korn on tue 21 jan 03
Vince Pitelka wrote:
Lili -
With greater horizontal run, you need considerably MORE vertical height.
Horizontal run slows down the draft, so you have to add more
vertical height
than the added horizontal run. Olson discusses the relationship in
the Kiln
Book. So the only way you are going to get away with a shorter
chimney is
if you had WAY more draft than you needed.
Good luck -
- Vince
Actually, the effects of height on chimney draft are not a nice
linear sort of thing (being a physicist doesn't help). There are
standing waves - think of sound, but lower, sub-sonic frequency -
of pressure in the firebox, with the length of the exhaust flue to
damper length, and damper to stack top length as "tuned pipes" that
serve to increase or decrease draft from the firebox/chamber.
I think it was Nils who said that if lengthening the chimney didn't work,
try shortening it instead.
The above is conjecture, but it accounts for what I've seen. The
propagation velocity of sound is a function of entrained gas velocity,
temperature, and pressure, all of which are changing as gases move
from the "bass enclosure" (the chamber) through the "tuned port" (the flue).
I'm too lazy to do the science right now. Where are the grad students
when you need 'em?
Roger
--
Roger Korn
McKay Creek Ceramics
In AZ: PO Box 463
4215 Culpepper Ranch Rd
Rimrock, AZ 86335
928-567-5699 <-
In OR: PO Box 436
31330 NW Pacific Ave.
North Plains, OR 97133
503-647-5464
Philip Poburka on tue 21 jan 03
Too...one may have a fan or blower comeing through a "Y" in
the horizontal or vertical run...and have that on a speed
controll or rheostat mechanical adjustment or other, and use
it to get the draft one wants...
Phil
Las Vegas
----- Original Message -----
From: "Vince Pitelka"
To:
Sent: Tuesday, January 21, 2003 6:00 PM
Subject: Re: chinney; bricks, cement; as well as stats
> Then: I do not want to rebuild the chimney as high as it
was. I DO want
to
> have a length of chimney running horizontally before it
goes up. I
suppose
> there are figures that tell one how much horizontal run
replaces or is
equal
> to vertical...Yes? Maybe? Ideas?
Lili -
With greater horizontal run, you need considerably MORE
vertical height.
Horizontal run slows down the draft, so you have to add more
vertical height
than the added horizontal run. Olson discusses the
relationship in the Kiln
Book. So the only way you are going to get away with a
shorter chimney is
if you had WAY more draft than you needed.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://iweb.tntech.edu/wpitelka/
____________________________________________________________
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Gavin Stairs on tue 21 jan 03
Hi Lili,
No real ideas about easy ways to split off the cement. Maybe a high
pressure water stream, as from a pressure washer, but if it would work, it
might also damage the brick.
As for the horizontal run of flue, it doesn't substitute for any chimney
rise. In fact, it may require a slightly higher chimney to get the same
draw, as the length of horizontal flue makes more flow resistance. You
might want to add a foot to the chimney. If you wanted a lower chimney,
subtract what you thought you needed to, and then add a bit back.
It is the rise of the whole flue which drives the draft. When gas (air,
kiln gas) is heated, it expands and therefore becomes buoyant. The
temperature rise in the kiln is about 1500K versus 230K room temp, so the
gas expands to about 6 times its volume. Thus, it is about as buoyant as
helium, more so than the gas in a hot air balloon. By the time it reaches
the flue, it has cooled considerably, but it is still a few of times less
dense than the surrounding air, so it wants to rise. Actually, it gets
pushed up by the pressure difference between that of the column of hot
(less dense) gas in the chimney, and the equivalent height of cool air
outside the kiln. The density of 15C air at standard atmospheric pressure
is 1.2250 kg/m^3. Disregarding the compressibility of the gas, the
pressure difference in the gas is equal to the height difference times the
pressure, so, the pressure at the top of a 2m chimney is 1.225 kg/m^3 * 2 m
= 24 Pascals less than that at the bottom. However, the hot gas is say 4
times less dense, so the pressure fall in the stack is just 6 Pa. The
difference, about 18 Pa, is what drives the chimney draft. No matter how
long the flue, or how convoluted the path, that is all the draft there can
be. So, adding length to the path, or adding bends, can only reduce the
flow by viscosity drag and flow restrictions. When calculating the height
of the stack, you should measure from the effective mouth of the
kiln. Usually this is the burner port openings, but if there are sizable
openings elsewhere, or a lot of leakage, that may contribute, and reduce
the draft. In a climbing kiln, like an anagama, the mouth may be quite a
lot lower than the chimney base, and this additional height will add to the
draft. Especially as the kiln cavity is usually hotter than the chimney.
All the best, Gavin
At 05:28 PM 20/01/2003 -0500, Lili Krakowski wrote:
>...Then: I do not want to rebuild the chimney as high as it was. I DO
>want to
>have a length of chimney running horizontally before it goes up. I suppose
>there are figures that tell one how much horizontal run replaces or is equal
>to vertical...Yes? Maybe? Ideas?
Marcia Selsor on tue 21 jan 03
I seem to recall that to horizonal run does have an equation to the
height of the chimney. i think it is in either Leach's potters' book or
Rhodes kiln building book.
As for chipping off the concrete..it sucks! I hated doing that with a
chisel and smashing my fingers BUT..recently i bought a very cool chisel
from either Loew's or Home depot which came with a bright orange hand
protector. Where was this 30 years ago when I needed it?
Best wishes,
Marcia
Lily Krakowski wrote:
> For a number of reasons I had to pull down the chimney of my little
> woodburner. I now have a bunch of hard brick with cement on them.This will
> be their second winter lying there on the frozen ground under snow, and I
> hope that the frost will split off some cement. Does anyone have any ideas
> easier than chipping the cement off to clean the brick?
>
> Then: I do not want to rebuild the chimney as high as it was. I DO want to
> have a length of chimney running horizontally before it goes up. I suppose
> there are figures that tell one how much horizontal run replaces or is equal
> to vertical...Yes? Maybe? Ideas?
>
> Thanks
>
>
>
> Lili Krakowski
> P.O. Box #1
> Constableville, N.Y.
> (315) 942-5916/ 397-2389
>
> Be of good courage....
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
--
Tuscany in 2003
http://home.attbi.com/~m.selsor/Tuscany2003.html
Vince Pitelka on tue 21 jan 03
> Then: I do not want to rebuild the chimney as high as it was. I DO want
to
> have a length of chimney running horizontally before it goes up. I
suppose
> there are figures that tell one how much horizontal run replaces or is
equal
> to vertical...Yes? Maybe? Ideas?
Lili -
With greater horizontal run, you need considerably MORE vertical height.
Horizontal run slows down the draft, so you have to add more vertical height
than the added horizontal run. Olson discusses the relationship in the Kiln
Book. So the only way you are going to get away with a shorter chimney is
if you had WAY more draft than you needed.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://iweb.tntech.edu/wpitelka/
Gavin Stairs on wed 22 jan 03
At 08:00 PM 21/01/2003 -0600, Vince Pitelka wrote:
>Lili -
>With greater horizontal run, you need considerably MORE vertical height.
>Horizontal run slows down the draft, so you have to add more vertical height
>than the added horizontal run. Olson discusses the relationship in the Kiln
>Book. So the only way you are going to get away with a shorter chimney is
>if you had WAY more draft than you needed.
>Good luck -
>- Vince
So how does this work? Let's take the simple case of adding a horizontal
run before a chimney with constant cross-section in both. And let's say
that the previous chimney was about the right draw. Let it be X meters
high. The resistance to that draw is proportional to the old chimney
height, X, plus a constant K for the kiln itself. The new resistance will
be proportional to X plus the horizontal run Y, plus K. So if we make a
constant of proportionality k, the same for both the horizontal run and the
vertical chimney, which represents the resistance of both to flow of kiln
gasses, we get Flow = (k*(X+Y)+K)*R(T), where R(T) is the ratio of the
effective kiln temperature to the ambient temperature in absolute units
(Kelvins or Rankines).
Unfortunately, we know just a little too little about the values of K and k
to solve this. We can simplify by moving the K term to the left side, so
we get Flow/R(T) - K = k*(X+Y): now divide by k, and call the whole thing
F. F = (Flow/R(T)-K)/k = X + Y. So, since all the things in F are
constants or the value of the flow that we want to remain constant, we can
call F a constant, and now X = F - Y. Now, for the final trick; if Y = 0,
which it was in the initial case, then F = X(Y=0) =X0. Now the new X, or
the new height of the chimney, is just X = X0 + Y.
So, for the very simple case that the initial draft is just right, and we
add a simple horizontal flue exactly like the chimney flue, we need to add
as much vertical as we add horizontal. In the case Lili proposed, the
original chimney was a bit too high. So, she should make the adjustment as
she feels to get the right flow in the old configuration, and then add as
much vertical as she adds horizontal, on top of the initial correction.
If the cross-section of the horizontal run is greater than the vertical
chimney cross-section, the needed extra height will be less, by
approximately the ratio of the cross-section areas. The actual
relationship is a bit more complex, as it depends on wall effects which are
non-linear, but this is probably close enough.
One last comment: because the horizontal run will cool the flue gas by
some amount, it may be necessary to add even more to the chimney height.
After all this, I suspect that Lili will have decided not to use the
horizontal run after all, so she just needs to dock the chimney by whatever
she feels she can get away with. She may be able to lower the chimney a
bit more by widening its cross-section, but this can't be taken to extremes.
All the best, Gavin
Neil Fallon on wed 22 jan 03
This will be an interesting thread to watch. A number of years ago a community group I was involved in cleaned old brick street pavers from their cement "crustaceans" with the intention of having them re-laid into a historical street. With over 20 people working an entire summer thousands of bricks were cleaned ... and the only successful method we found was to chip, chisel and clean them using a wire brush.
Now if I learn their was an easier way.....
Neil Fallon
Rock Pond Pottery
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