Nils Lou on thu 14 aug 08
Jon from Auburn wrote raising the question of whether the damper might =
actually function as a venturi in the double-venturi flue box design. =
It's a frequently raised question and seems logical enough. In the MFT =
design the exit flue functions to do two things. Paradoxically they seem =
counterproductive=97first, being comparatively small in cross sectional =
area, only 30 to 35 square inches, exhaust gases are restricted =
creating turbulence and at the same time speed up in the drafting as =
they expand into the flue box. Many kilns have a 9"x9" exit flue with 81 =
sq. inches. I think that size allows the exhaust gases to flow out too =
freely, cooling the the kiln in the bottom typically. Simply blocking =
the exit with a brick can have a beneficial effect if the kiln has this =
problem.=20
At the top of the flue box supporting the stack sits the second venturi, =
also 30 to 35 square inches, which amplifies the flow of gases as it =
expands into the larger volume of the stack.
In between the two venturi orifices we have the the damper used to =
modify the internal pressure of the kiln atmosphere. This has the effect =
of controlling the secondary combustion air entering the burner ports. =
With an oxygen probe we can see first-hand the effect of the damper as =
1/4" movement can have a profound effect on the atmosphere in the kiln. =
The reason the damper does not function as a venturi and cannot replace =
the second venturi orifice is that it is variable and most of the time =
presents only about half the area of the fixed venturis. For instance, =
it usually is open about 2" which is only 18 to 20 square inches in =
cross section. While it may have some venturi effect during a firing it =
does not replace the function of the two fixed orifices. I hope this =
helps in understanding how the double venturi flue box functions. nils
nils lou, professor of art
AAVC department
www.linfield.edu/~nlou
503.883.2274
"Play is the essence of creativity", Carl Jung
jonathan byler on thu 14 aug 08
Nils,
thanks for the explanation. I am going to try narrowing the flue =20
entrance a bit in one of our kilns that has been consistently cold in =20=
the bottom.
by the way, I am an ex student of ted vogel at lewis and clark, just =20
down the road from you. say hello for me if you run into him in your =20=
travels.
best,
jon byler
jon byler
3-D Building Coordinator
Art Department
Auburn University, AL 36849
On Aug 14, 2008, at 12:06 PM, Nils Lou wrote:
> Jon from Auburn wrote raising the question of whether the damper =20
> might actually function as a venturi in the double-venturi flue box =20=
> design. It's a frequently raised question and seems logical enough. =20=
> In the MFT design the exit flue functions to do two things. =20
> Paradoxically they seem counterproductive=97first, being =20
> comparatively small in cross sectional area, only 30 to 35 square =20
> inches, exhaust gases are restricted creating turbulence and at the =20=
> same time speed up in the drafting as they expand into the flue =20
> box. Many kilns have a 9"x9" exit flue with 81 sq. inches. I think =20
> that size allows the exhaust gases to flow out too freely, cooling =20
> the the kiln in the bottom typically. Simply blocking the exit with =20=
> a brick can have a beneficial effect if the kiln has this problem.
> At the top of the flue box supporting the stack sits the second =20
> venturi, also 30 to 35 square inches, which amplifies the flow of =20
> gases as it expands into the larger volume of the stack.
> In between the two venturi orifices we have the the damper used to =20
> modify the internal pressure of the kiln atmosphere. This has the =20
> effect of controlling the secondary combustion air entering the =20
> burner ports. With an oxygen probe we can see first-hand the effect =20=
> of the damper as 1/4" movement can have a profound effect on the =20
> atmosphere in the kiln. The reason the damper does not function as =20
> a venturi and cannot replace the second venturi orifice is that it =20
> is variable and most of the time presents only about half the area =20
> of the fixed venturis. For instance, it usually is open about 2" =20
> which is only 18 to 20 square inches in cross section. While it =20
> may have some venturi effect during a firing it does not replace =20
> the function of the two fixed orifices. I hope this helps in =20
> understanding how the double venturi flue box functions. nils
>
> nils lou, professor of art
> AAVC department
> www.linfield.edu/~nlou
> 503.883.2274
> "Play is the essence of creativity", Carl Jung
DRB Seattle on thu 14 aug 08
Maybe I'm just slow on the uptake...
To me your statement-
=A0"Many kilns have a 9"x9" exit flue with 81 sq. inches. I think that
size allows the exhaust gases to flow out too freely, cooling the the kiln =
in
the bottom typically."=20
says that the chimney is too big.=A0{Its one of my pet theories (yes Anne, =
we all have our theories) that many chimnies are sized too large being base=
d on Leach without paying attention to the fact that those rules are for DF=
B kilns not IFB.}
If the flue size was a constant 30-35=A0sq. in.=A0e.g.6.5" x 6.5" in the st=
ack and 5" x 7.5" at the exit wouldn't you get the same results?
DRB
Seattle
--- On Thu, 8/14/08, Nils Lou wrote:
From: Nils Lou
Subject: damper as venturi question
To: CLAYART@LSV.CERAMICS.ORG
Date: Thursday, August 14, 2008, 10:06 AM
Jon from Auburn wrote raising the question of whether the damper might actu=
ally
function as a venturi in the double-venturi flue box design. It's a
frequently raised question and seems logical enough. In the MFT design the =
exit
flue functions to do two things. Paradoxically they seem
counterproductive=97first, being comparatively small in cross sectional are=
a,
only 30 to 35 square inches, exhaust gases are restricted creating turbule=
nce
and at the same time speed up in the drafting as they expand into the flue =
box.
Many kilns have a 9"x9" exit flue with 81 sq. inches. I think that
size allows the exhaust gases to flow out too freely, cooling the the kiln =
in
the bottom typically. Simply blocking the exit with a brick can have a
beneficial effect if the kiln has this problem.=20
At the top of the flue box supporting the stack sits the second venturi, al=
so
30 to 35 square inches, which amplifies the flow of gases as it expands int=
o the
larger volume of the stack.
In between the two venturi orifices we have the the damper used to modify t=
he
internal pressure of the kiln atmosphere. This has the effect of controllin=
g the
secondary combustion air entering the burner ports. With an oxygen probe we=
can
see first-hand the effect of the damper as 1/4" movement can have a
profound effect on the atmosphere in the kiln. The reason the damper does n=
ot
function as a venturi and cannot replace the second venturi orifice is that=
it
is variable and most of the time presents only about half the area of the f=
ixed
venturis. For instance, it usually is open about 2" which is only 18 to 20
square inches in cross section. While it may have some venturi effect duri=
ng a
firing it does not replace the function of the two fixed orifices. I hope t=
his
helps in understanding how the double venturi flue box functions. nils
nils lou, professor of art
AAVC department
www.linfield.edu/~nlou
503.883.2274
"Play is the essence of creativity", Carl Jung
=0A=0A=0A
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