[Clayart] Kiln Flue question

Sun Jun 2 05:10:18 UTC 2024

Joel,
   As you know, the goal of a well-designed fuel-fired kiln flue with
damper is to provide smooth control of the flow of combustion air into the
kiln (and simultaneously the flow of kiln gases out through the flue) over
a wide temperature range and a wide range of damper positions. The trouble
with very oversized flues is that a small change in damper position
produces a large change in combustion air flow.
   If you look at Euler's derivation of what's commonly called Bernoulli's
equation, the velocity of the flowing fluid is typically the significant
factor in controlling pressure loss through a flue system. The elevation
and density factors are important, of course, but the shape of the entrance
to the flue and the dimensions of the dampered section determine the flow
rate and velocity of the kiln gases to the flue exit.
   You are right in saying, "the flu only needs to be as large as the
widest the damper will be open." Ideally, maximum air flow and fuel
efficiency occurs with the damper wide open.
   The turbulence is very important in a heavily dampered, oversized
flue. Wall turbulence in the stack is usually a minor influence on flow up
the flue.
       Regards,
             Dave Finkelnburg

On Sat, Jun 1, 2024 at 8:10 PM joel joelfink.net via Clayart <
clayart at lists.clayartforum.com> wrote:

> This is a curiosity to me.
>
> I can't do Bernoulli's math, so there is that. But, I do conceptually
> understand the dynamics of fluid flow to some usable degree.
>
> If we remove turbulence from the equation, just to look at other variables
> unimpeded, can someone tell me what the difference is between a wide flu
> and an open damper, and then of a small flu and an equally closed damper?
>
> In my imagination, speaking specifically of a pressurized system
> (gas-fired) the venturi effect of a gradual narrowing of the orifice is
> negated by the damper opening dimensions, if they are less than the upper
> point of the final diameter of the venturi. In other words; In a gas kiln,
> the flu only needs to be as large as the widest the damper will be open.
> After that, the pressure differential is set by the interior pressure and
> the damper aperture, and not by the flu itself.
>
> Isn't the pressure differential the actual number in this equation?
>
> Jeff is speaking about a Guil kiln with the damper being roughly the
> height of the kiln and not a venturi, so my comments go to that example.
>
> Note: I'm asking in curiosity. I would very much like to hear anyone's
> knowledge as to what happens in a gas-fired kiln's flu, if the flu size is
> greater than the maximum damper aperture.
>
> As to turbulence, in my mind this will create myriad minor eddy currents
> along the imperfections in the wall, such that a slightly larger flu would
> compensate for any backpressure.
>
> Joel Fink
> Ficklefink Pottery.
> ________________________________
> From: Clayart <clayart-bounces at lists.clayartforum.com> on behalf of
> ronroy--- via Clayart <clayart at lists.clayartforum.com>
> Sent: Wednesday, May 29, 2024 11:12 AM
> To: Clayart international pottery discussion forum <
> clayart at lists.clayartforum.com>
> Cc: ronroy at ca.inter.net <ronroy at ca.inter.net>; Jeff D. Walker <
> jdwalker at silverdollarcity.com>
> Subject: Re: [Clayart] Kiln Flue question
>
> An oversized flue opening can always be made smaller - or maybe just
> close your damper more. Care should be taken to make sure dampers
> close properly to control cooling.
>
> Getting cracking during bisque cooling? Look to see if your damper is
> closing completely.
>
> RR
>
>
> Quoting David Woof via Clayart <clayart at lists.clayartforum.com>:
>
> > Hi Jeff,
> > Have you considered corbeling the main IFB wall just around and
> > above where you need your exit flue opening and then building a hard
> > brick catenary arch for the flue?  One could then "chink up" and use
> > the castable to fill in the gap spaces between the IFB and exit flue
> > arch.
> >
> > I use term "exit flue" to avoid confusion and attempt clarification
> > regarding the "stack" or "chimney" as some refer also as flue.
> >
> > Also often overlooked is that building the size of exit flue
> > openings and chimney volume, and propensity for "stalling" are
> > influenced and dependent on geographic altitude as well as other
> > physical or human caused landscape features and sudden weather and
> > barometric changes during a firing. One size does not fit all under
> > all conditions.
> > Some may get their panties skid marked, or in a bunch over my saying
> > this, but the proof lies in the solutions.............
> >
> > Misneach,
> >
> > David
> >
> Woof...........................................................................................................................................................
> >
> *****************************************************************************************************
> > ________________________________
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> Ron Roy
> ronroy at ca.inter.net
> Web page ronroy.net
>
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