I.Lewis on sun 7 nov 99
------------------
The original inquiry of Joanne Van Bezooyen in which she asked reasonable
questions has, to my satisfaction, been answered satisfactorily since she =
was
given practical advice on how to solve her problem. But the strand had =
turned
into a debate about the behaviour of cold and hot gases as they pass through=
the
passage ways and up out of the chimney. Very interesting and often =
contradictory
opinions have been voiced about what happens as the temperature rises. All =
seem
to draw on a good knowledge of high school physics. Unfortunately, the
generalisations known as Boyle=92s Law and Charles=92 Law only apply to =
enclosed
volumes, or am I wrong about .
Would those who insist that the pressures are higher or lower in this place =
or
that in a kiln please illustrate their opinions with their experimental =
results,
reading from their manometers and thermometers and so on . Then we can all =
trust
these opinions. I love reading these technical postings but please, please,
remember when you write that assumptions, assertions and conjectures, though
they may lead to thinking out a good answer only have value when they are
qualified by observations and readings from instruments.
Now I recall that there is an excellent article in a Ceramics Monthly from =
round
about 1981 which describes how to tune a kiln. And B. Leach gives some very =
good
guidelines about the relationships between the parts of a kiln which would =
give
anyone without knowledge or experience to basics from which to start =
building a
kiln.
To those who gave good practical advice, thank you
Regards,
Ivor. Who has been luting handles on a batch of mugs. And not a scratch tool=
in
sight=21=21
Gavin Stairs on mon 8 nov 99
At 05:23 PM 07/11/99 , you wrote:
>----------------------------Original message----------------------------
>------------------
>The original inquiry of Joanne Van Bezooyen in which she asked reasonable
>questions has, to my satisfaction, been answered satisfactorily since she was
>given practical advice on how to solve her problem. But the strand had turned
>into a debate about the behaviour of cold and hot gases as they pass
>through the
>passage ways and up out of the chimney. Very interesting and often
>contradictory
>opinions have been voiced about what happens as the temperature rises. All
>seem
>to draw on a good knowledge of high school physics. Unfortunately, the
>generalisations known as Boyle's Law and Charles' Law only apply to enclosed
>volumes, or am I wrong about .
Dear Ivor,
My Goodness, dear old Boyle and Charles and all the others must be fairly
twirling in their graves! What they discussed was the behaviour of gasses,
not of enclosed volumes. The same relationships are quite generally
applied, usually in differential form, to the behaviour of gas flows in
everything from pipes to the atmosphere. There are many complications,
especially when the flow velocity reaches some phase transition, such as a
flame front, or a laminar to turbulent transition, or a sonic shock front,
or a liquid surface, etc., etc. Nevertheless, the relationships I outlined
are useful for describing many gas and liquid flows in the subsonic
region. We were taught in engineering school to calculate with these
things, and we measured real flows in our labs to verify them. The only
way to practically measure the energy flow in a kiln, if anyone would want
to do that, is to monitor the pressures, temperatures, etc., and apply the
gas laws. It's done every day in the combustion engineering field, and
also in the HVAC industries.
These things are called Ideal Gas Laws simply because they do not take
account of the finite size of gas molecules, nor of the very slight van der
Waals forces between them. Since practical gasses in our temperature
regime are about 500-2000 times the volumes of their condensed (liquid)
volume, these effects are well below the 1% level. In other words,
practical gasses in kilns behave quite like ideal gasses.
I don't have an adequate demo unit to satisfy you. I'd like to, as I
mumbled at the end of one of my messages. But I don't have time and money
to satisfy my every whim, worse luck. Perhaps someday I will be able to do
this. Sorry.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Gavin Stairs on mon 8 nov 99
At 05:23 PM 07/11/99 , you wrote:
>...
>Would those who insist that the pressures are higher or lower in this place or
>that in a kiln please illustrate their opinions with their experimental
>results,
>reading from their manometers and thermometers and so on . Then we can all
>trust
>these opinions. I love reading these technical postings but please, please,
>remember when you write that assumptions, assertions and conjectures, though
>they may lead to thinking out a good answer only have value when they are
>qualified by observations and readings from instruments.
...
Dear Ivor,
I should have included a couple of observations on real kilns in my last
post. I got carried away in defense of Mssrs. Boyle and Charles , et al.
Observation #1: Every kiln I have ever seen (with natural draft) sucks air
in at the firing ports during firing, except some wood kilns which have not
yet established draft, and some wood firing ports located high on the kiln
wall. I've never actually seen a burner stall in a gas kiln. So all kilns
that I know about have pressure below atmospheric at the lower levels.
Observation #2: Kilns set up to gently reduce blow slightly during
reduction at peeps located fairly high on the side of the kiln. I have
observed a nice, gentle flow out of such a peep, and have observed charring
and not burning of test splints, etc. I have also observed puffing dragons
blowing sheets of flame out of such ports. These kilns have been in
reduction. The ports are above atmospheric pressure.
Observation #3: Kilns set up with the dampers wide open suck at every
port. No flame is observed, and when viewing in the port, smoke can
sometimes be observed to be blown by the draft inside the kiln. These
kilns are never in reliable reduction, as the sucking ports are letting
oxygen in. The ports are below atmospheric pressure. A well sealed kiln
could be operated in reduction in this pressure regime. Some industrial
kilns do just that, with sealed burner ports both high and low. But even
so, most such high efficiency kilns are pure oxidation.
Everyone can observe such things in any operating kiln. There is no need
for instruments. Just observe the direction of gas flow at all ports in
the kiln. That will tell you whether the local pressure is above or below
atmospheric. The pressure gradients are quite small, and these
observations can be quite sensitive. If you are unsure which way the flow
is going, dangle a thread or a hair or smolder a stick or a cigarette below
it, and see which way the smoke blows.You can even measure the velocity
this way.
I want to set up an instrumented kiln not really because it is necessary to
have numbers to tell what is going on, but because with accurate
measurements you can tell not only what is happening, but also how
fast. Very useful for all sorts of fundamental experiments. Not really
required for practical kiln tuning.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Louis H.. Katz on tue 9 nov 99
Dear Gavin,
People who sell to appliance and heating professionals foten sell very accurate
draft gauges that measure in small fractions of an inch water column. My Granger
( an international catalog based compamy focused on wholesale industrial and
commercial equipment and supplies ) lists a Dwyer Mark II Manometer for $21.50
accurate to within 3 % of 3 " water column scale.
Louis
> I want to ......have numbers to tell what is going on, but because with accura
>
> measurements you can tell not only what is happening, but also how
> fast. Very useful for all sorts of fundamental experiments. Not really
> required for practical kiln tuning.
>
> Gavin
>
>
stephen baxter on tue 9 nov 99
Gentleman:
I am not an engineer but it seems to me that you are making things very
complicated for the average clay person to follow. Please correct me if I'm
wrong.
1. the kiln is surrounded by air at atmospheric pressure (14 psi or so)
2. if you open the peephole and gas comes out the pressure is greater than
14 psi in the kiln
3. if you open the peep and air goes in the pressure is less than 14 psi in
the kiln
Did I miss something?
respectfully,
Stephen Baxter
----------
> From: Gavin Stairs
> To: CLAYART@LSV.UKY.EDU
> Subject: Re: Kiln conversion and the ways of burning gas
> Date: Monday, November 08, 1999 3:17 PM
>
> ----------------------------Original message----------------------------
> At 05:23 PM 07/11/99 , you wrote:
> >----------------------------Original message----------------------------
> >------------------
> >The original inquiry of Joanne Van Bezooyen in which she asked
reasonable
> >questions has, to my satisfaction, been answered satisfactorily since
she was
> >given practical advice on how to solve her problem. But the strand had
turned
> >into a debate about the behaviour of cold and hot gases as they pass
> >through the
> >passage ways and up out of the chimney. Very interesting and often
> >contradictory
> >opinions have been voiced about what happens as the temperature rises.
All
> >seem
> >to draw on a good knowledge of high school physics. Unfortunately, the
> >generalisations known as Boyle's Law and Charles' Law only apply to
enclosed
> >volumes, or am I wrong about .
>
> Dear Ivor,
>
> My Goodness, dear old Boyle and Charles and all the others must be fairly
> twirling in their graves! What they discussed was the behaviour of
gasses,
> not of enclosed volumes. The same relationships are quite generally
> applied, usually in differential form, to the behaviour of gas flows in
> everything from pipes to the atmosphere. There are many complications,
> especially when the flow velocity reaches some phase transition, such as
a
> flame front, or a laminar to turbulent transition, or a sonic shock
front,
> or a liquid surface, etc., etc. Nevertheless, the relationships I
outlined
> are useful for describing many gas and liquid flows in the subsonic
> region. We were taught in engineering school to calculate with these
> things, and we measured real flows in our labs to verify them. The only
> way to practically measure the energy flow in a kiln, if anyone would
want
> to do that, is to monitor the pressures, temperatures, etc., and apply
the
> gas laws. It's done every day in the combustion engineering field, and
> also in the HVAC industries.
>
> These things are called Ideal Gas Laws simply because they do not take
> account of the finite size of gas molecules, nor of the very slight van
der
> Waals forces between them. Since practical gasses in our temperature
> regime are about 500-2000 times the volumes of their condensed (liquid)
> volume, these effects are well below the 1% level. In other words,
> practical gasses in kilns behave quite like ideal gasses.
>
> I don't have an adequate demo unit to satisfy you. I'd like to, as I
> mumbled at the end of one of my messages. But I don't have time and
money
> to satisfy my every whim, worse luck. Perhaps someday I will be able to
do
> this. Sorry.
>
> Gavin
>
>
> Gavin Stairs
> Stairs Small Systems
> 921 College St., # 1-A
> Toronto, Ontario, Canada M6H 1A1
> phone: (416)530-0419 stairs@stairs.on.ca
Gavin Stairs on wed 10 nov 99
At 12:47 PM 11/9/99 -0500, you wrote:
>----------------------------Original message----------------------------
>Dear Gavin,
>People who sell to appliance and heating professionals foten sell very
accurate
>draft gauges that measure in small fractions of an inch water column. My
>Granger
>( an international catalog based compamy focused on wholesale industrial and
>commercial equipment and supplies ) lists a Dwyer Mark II Manometer for
>$21.50
>accurate to within 3 % of 3 " water column scale.
>Louis
Hi Louis,
Yes, furnace techs are very interested in getting the draft just right,
because it affects the furnace fuel efficiency. What I meant by not
necessary for practical kiln tuning is that most studio kilns are not
operated for maximum efficiency. Their operators generally have other
things on their minds. Industrial ceramics kilns are a different matter.
They are generally set up for efficiency, or at least for a particular
operating point. I have been reading about high efficiency tile furnaces
lately. They are about 40% fuel efficient (about 40% of the fuel energy
goes to heating the tile, and driving off its volatiles). By comparison, a
standard studio kiln is probably only a couple percent efficient.
And yes, I am familiar with Grainger'.
Gavin
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