Stoves Digest V1 #17
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Ron, Ken Bryben Etienne et al:
Let me second Ron's interest in Ken's modelling whole tree combustion; received
and noted. Interesting use of whole trees. (Aren't they routinely too wet?)
Concerning the "three stone" stove and Etienne's comments: Is it possible that
we have been overlooking the possibility of making significant improvements in
the TSS? Feeding the sticks in supplies the volatiles and makes charcoal. The
charcoal then sustains further pyrolysis. This could be called the "whole
stick" stove.
How about a small pit under the burning area to collect the charcoal (which
burns much slower than the volatiles). If the pit is deep, the charcoal would
extinguish itself for later use.
How about an air tunnel under a shallow pit and a crude wire grate? How about
always using <20% moisture, uniform sticks. Then as the sticks are pushed into
the fire zone they are able to jet the pyrolysis gases into the burning char
gases for added combustion - and give high heat rates - less wood feed gives
low heat rates.
Top burning of charcoal: See my earlier comments that I couldn't get a top
burning charcoal fire lit! I was very surprised. Any experience out there?
Comments? TOM REED
(These are Comments on Ron's comments on Ken's modelling papers.
I. Modeling
A. To Ken Bryden - thanks - I enjoyed your papers ("Numerical Modeling
of a Deep, Fixed Bed Combustor" and "Combustion of Thermally Thick Woody
Biomass"), which show excellent agreement with experiment. Although the
papers do not relate directly to stoves, I think others in the stoves
group will enjoy them and get some new ideas.
B. To other stovers:
I took advantage of Ken Bryden's offer of reprints, received
these two a few days ago and thought I would reply publicly because there
are some modeling and pyrolysis topics here of interest to stove
research.
Ken's work concerns the whole tree combustor - generating many MW
per square meter of plan area - a very different regime fron that of
stoves. My reading leads to these questions for Ken:
a) Can you tell us more about the early development of this
geometry. It produces a fair amount of gas but is a combustor - not a
gasifier. (There is secondary or "overfire" air - but apparently less
than half of the total?) Are there any notable weaknesses for this geometry?
b) Might a pyrolyzer (charcoal by-product) make technical and economic
sense? I am especially thinking of developing countries with a stronger
need for charcoal than here.
c) You describe gasification updraft modeling by Kayal of
vertical sticks. Might his work be of use in the pyrolyzing stove? (I
haven't yet found Bioresource Technology 1994, 49, pp61-73.) Was this
top-lit or bottom lit? You don't mention this vertical orientation for
the whole tree combustor - is there a reason?
d) You state (p E) these Kayal sticks "are parallel to the air
flow vice in cross flow as might be expected." Could you explain this
use of the word "vice"?
e) Your model assumes a constant-with-height (65%) void
fraction. This seems high and even difficult to achieve; any comment?
Do you have any data for Kayal's void fraction and the relative air-flow
resistances (or pressure drops) of the two fuel orientations?
f) You report that the ash is blown upward. Might this be true
for a much lower primary airflow (pyrolysis) situation as well? (where
the air flow is much less). I have presumed that not seeing ash meant
that I was not consuming much charcoal; when do you think that might not
be true?
g) Your seven-gas analysis seems amenable to being used for a
charcoal-making stove geometry also, with reasonably small
modifications. Among them are
1) top lighting with charcoal on top rather than the bottom
2) a need for a very different variation in particle size with
height (being only a small decrease as the pyrolysis zone moves past).
I hope that you will find time and/or funding to do such modeling.
II. Chimneys
Some of you will recall that I have felt it important to focus on
the internal pressures, flows, and velocities due to the several thermal
buoyancies and the chimney. Etienne recommended that chimneys might be
in the book by Gebhart entitled "Buoyancy Induced Flows". I need to go
back, but think I found something closer to my needs in Chapter 26
(Chimneys) of the 1988 (probably other editions also) ASHRAE handbook on
"Equipment". There are several nice nomograms to accompany the usual
equations. I am in the process of trying to simplify this for the
charcoal-making stove - but will be busy this next week. I hope others
will look up this reference and see if it helps you also. My problem
now is not yet knowing how to specify the resistances of the fuel bed and
of the secondary air inlet.
I also found an 1899 book on chimneys - not very useful, since it
started with 50 footers and went larger. But this was based mostly on
papers in the ASME Transactions - mostly vol XI from a few years
earlier. These I found quite fascinating - as the chimney paper
reviewers then held nothing back in their dislike of each other and the
others' theories. These review comments and author responses were much
more interesting than the original papers. Because everything was based
on several feet of coal fuel bed and only primary air - I don't think I
will go back to these. There is a lot more generality in the ASHRAE
work. I later found much of the same heuristic 19th century chimney
tabulations in several old ME handbooks.
More in about a week - as this week is devoted to local solar politics.
Sorry I have been so quiet.
Ron Larson
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Charcoal, Xcoal, ...
Etienne:
I stand corrected on charcoal heat of combustion. I took it from graphite,
/\Hc=96 kcal/mol, being on the road. Obviously the difference between 24 and 33
is the amount of volatile material left in the charcoal.
However, I think your statement that a 25% yield being due to unpyrolysed or
less pyrolysed wood is too strong. The Lambiotte (Belgian? still manufactured?)
charcoal process regularly gave yields over 30% from hardwoods. As mentioned
before, the presence of minerals greatly influences the Xcoal yield. Also,
pressure can take us up to 45% cooking charcoal and I hope to see the pilot
plant in Hawaii this June. More later. It is in part a question of how much of
the volatiles can be caused to char before they exit the wood - or redeposit on
colder portions.
Do you agree that there is room here for a few basic theses? I'd love to come
to Twente for a few years, but my wife would veto. Visit?
Your 24-31% charcoal yield sounds fine, but I'd like to re-fine it further. I
made comments earlier on Xcoal, X being the degree of charring (water removal)
and I also suggested that "cooking charcoal" production is homeostatic. Once
you reach about 300 C the reaction becomes exothermic and carries itself to 450C
and end point that is hard to exceed. Do you agree? And what is our best heat
content at that point? Probably your 24 kJ/g.
Regarding kindling/tinder: I don't think the African continent should be our
only focus. China and India have a high level of culture - and still cook on
biomass. If it takes a preformed "fire starter" costing a penny to make a
working fire, they'll distribute it. (I''m not saying it does.)
Too much preoccupation with ZERO COST too early limits our thinking too much.
Weds:
I just received "WOOD HEAT FOR COOKING". Thanks so much. Clearly this is the
"Old Testament of Stoves". Let's hope we may write a New Testament here, though
so far we are probably only apocryphal.
I'm confused by the relationships between Twente University (Nederlands) and TU,
Eindhoven (Germany?). None? Nearby? Also between Moerman, Prasad and Verhaart
- - all old friends? What's the history here?
Forward.. TOM REED
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Re: Hobo Stove
Etienne:
Your reference to the "Homo Stove" is very interesting, but didn't have his
complete address or any description. Top lighting?
The battery operated stove if VERY superior to the hobo stove, made by Fred
Hottenroth and has inspired Ron and me. It is truly elegant and a 10c battery
(in India) lasts 30 hours. A variation is being commercialized in Nepal. (We
don't have many Hobos in the US any more, but lots of backpackers.)
The Girl Scouts also had an interesting sawdust stove/candle which provided a
steady flow of pyrolysis gas for 15-30 minutes. I'll try and reproduce it.
The real problem of hobos, Hottenroth and most others is not making gas, but
getting it to mix with air in suitable proportions and burn steadily, cleanly
and controllable.
It is very useful for us stovers to have a very wide view of all previous
manifestations.
Note that I am back on my main PC and no more double letters.
TOM (I'm running out of cheery greetings.)
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NEW TEST, NEW MODEL
Stovers all:
Here is a digest of a recent top-ignited gasifier test that I ran and a simple
model constructed from it. I would appreciate comments on its utility and what
you believe should be added to a final model. (Because of my double letter
probleem, it was coomposed on MSWORD aand EEXCEL, then transferred.)
Hi Tom,
Thrilled to see you are modeling in excel. I have excel so can
probably open a file from you. Have you posted an excel file? I would
like to open an excel file from you. So if you havn't posted one
please do if you can. You can send it as an attached file to your
message. I am able to open attached files.
I have been to Walt Disney World doing some imagineering. So I am a
little behind on my e-mail. However I am benefiting from exposure to
Disney's approach to inovation. So expect to be able to make more
significant contribution in the days to come.
I model in excel also. Charts work good. Can change input by draging
chart bars. See output visually. Also can draw sketches in excel.
Tom Duke
4363 Hunt Road
Burlington IA 52601-8917
The Renewable Energy Research Center & Farm (319)754-7384
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Charcoal, Xcoal, ...
Tom Reed wrote:
Do you agree that there is room here for a few basic theses? I'd love to
come to Twente for a few years, but my wife would veto. Visit?
Etienne:
For Twente you have to contact other people (Huub Stassen, Piet Visser), but
as far as I am concerned you are welcome at Eindhoven. You better check with
Prasad though, he might be able to arrange a temporary office and lab.
Tom R.:
homeostatic. Once you reach about 300 C the reaction becomes exothermic
and carries itself to 450C and end point that is hard to exceed. Do you
agree? And what is our best heat content at that point? Probably your 24
Etienne:
Yes, very likely.
Tom R.:
I'm confused by the relationships between Twente University (Nederlands)
and TU, Eindhoven (Germany?). None? Nearby? Also between Moerman, Prasad
and Verhaart - all old friends? What's the history here?
Etienne:
TU Eindhoven is also in The Netherlands. We have contacts with Twente.
Distance about 250km. Piet Verhaart, Piet Visser, Prasad and I all worked at
WSG Eindhoven (TU) at one time or another.
Etienne
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Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands
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Modeling and chimneys
Hello Stovers all,
At 09:51 PM 4/7/96 -0600, Ron wrote:
To Ken Bryden - thanks - I enjoyed your papers
Thanks for reading them. I am always eager for comments and suggestions.
Ken's work concerns the whole tree combustor - generating many MW
per square meter of plan area - a very different regime from that of
stoves. My reading leads to these questions for Ken:
Comment:
In this paper I work on WTE (whole tree energy) and a portion of my research
has been funded to work on this model, but the core of my research is the
development of engineering models of woody biomass combustion. Because of
this I am very interested all types of wood combustion.
Question:
a) Can you tell us more about the early development of this
geometry. It produces a fair amount of gas but is a combustor - not a
gasifier. (There is secondary or "overfire" air - but apparently less
than half of the total?) Are there any notable weaknesses for this
geometry?
Answer:
With the caveat that no one has yet built a Whole Tree combustor, I am not
aware of any particular weakness. Yes, the overfire is generally less then
half of the total air, rather than thinking of it as a gasifier I would
think of it as deep fixed bed combustor. As a comment most stoves are also
"deep" combustors or gasifiers and most are both to some degree.
Question:
b) Might a pyrolyzer (charcoal by-product) make technical and economic
sense? I am especially thinking of developing countries with a stronger
need for charcoal than here.
Answer:
probably not. The WTE combustor is designed to be operated in steady state
and as a consequence trying to get the charcoal out from a 3 or 4 m deep
pile of wood would be very difficult. think of the weight of the wood above
the charcoal.
Question:
c) You describe gasification updraft modeling by Kayal of
vertical sticks. Might his work be of use in the pyrolyzing stove? (I
haven't yet found Bioresource Technology 1994, 49, pp61-73.) Was this
top-lit or bottom lit? You don't mention this vertical orientation for
the whole tree combustor - is there a reason?
Answer:
It may be but it was steady state not transient. His experiment was similar
to the WTE combustor ... steady state, up draft, with char combustion on the
bottom and drying and gasification on the top.
Question:
d) You state (p E) these Kayal sticks "are parallel to the air
flow vice in cross flow as might be expected." Could you explain this
use of the word "vice"?
Answer:
In most of the work I am familar with the fuel is placed in the combustor in
cross flow not in parallel with the flow. To place the fuel in cross flow
all one has to do is fling it in. To place it in parallel flow it has to be
gathered or bundled and carefully put in and even then it seems that in a
high volume high heat release operation jamming could occur. Even in the
small combustor of Kayal he reports jamming and clogging of air passages by
tar which had to be relieved by tapping on the unit.
Question:
e) Your model assumes a constant-with-height (65%) void
fraction. This seems high and even difficult to achieve; any comment?
Do you have any data for Kayal's void fraction and the relative air-flow
resistances (or pressure drops) of the two fuel orientations?
Answer:
A constant void fraction was chosen for two reasons 1) there was no
experimental evidence to support any assumption 2) if the original fuel
geometry is retained ie. cylinders vice breakup, it is easy enough to show
that the void fraction remains constant. I agree that the void fraction
seems high but it is a measured value for whole trees, not logs, stacked up
over 100 ft high also void fractions in the 50-60% range are common. Also
it gave good agreement with the single data point.
Kayal doesn't report the pressure drop. As a starting place for this I
would suggest looking at non-reacting packed beds. There is a large body of
work wich addresses pressure drop for this case. I suspect the reacting
part doesn't add much to the pressure drop in the bed.
Question:
f. You report that the ash is blown upward. Might this be true
for a much lower primary airflow (pyrolysis) situation as well? (where
the air flow is much less). I have presumed that not seeing ash meant
that I was not consuming much charcoal; when do you think that might not
be true?
I'm not sure, but you can measure you flow rate and then perform a force
balance and determine the maximum size that will blow out. I would bet that
you will find the ash that is released (and there won't be much) will blow
out. If you need help with this let me know I have a simple EES (this is a
commercial solver...like math cad) routine that solves this problem given
particle shape and flow rate.
Comment:
g. Your seven-gas analysis seems amenable to being used for a
charcoal-making stove geometry also, with reasonably small
modifications. Among them are
1) top lighting with charcoal on top rather than the bottom
2) a need for a very different variation in particle size with
height (being only a small decrease as the pyrolysis zone moves past)
Answer:
both of these are easy to incorporate, a larger issue is the for transient
analysis instead of steady state analysis. it may be possible to assume
quasi-steady operation and then step in time but right now the solver finds
the steady state solution. I suspect that this is a couple of months of
work. As a comment WTE has good hope that a pilot plant will be built, If
this is the case, I'll probably have to build the transient model could then
be applied to this stove.
I hope that you will find time and/or funding to do such modeling.
As you both are always in short supply! but I will eventually (maybe not /
probably not this year) get around to it.
As a final comment in support of combustion modelling let me quote Prasad
and Verhaart from the forward to "Wood Heat for Cooking" 1983.
"Another feature of the woodstove for domestic cooking is its small size.
This is its strength as well as its weakness -- strength because testing can
be carried out with relative ease and at a small cost on full scale
prototypes, and weakness because modelling -- the forte of engineering
science -- is presumed unnecessary for design and development of small
devices. It is the editors' contention that modelling is the precisely what
is required if one is to contemplate the availability, in a period of twenty
years, of stoves with superior performance to a billion people or so living
in diverse places with varying local resource situations."
WELL SAID!
I would be interested to hear from Prasad and Verhaart how they think we
(the engineering world) is doing at making the twenty year goal mentioned
above. I wonder if we are "behind schedule" and need to catch up.
Keep burning!
Mark Bryden
UW-Madison
Re: "Whole Stick Stoves"etc.
Tom,
At 10:00 AM 4/10/96 EDT, you wrote:
Let me second Ron's interest in Ken's modelling whole tree combustion;
received and noted. Interesting use of whole trees. (Aren't they routinely too wet?)
Thanks for the interest. They aren't too wet to burn but if left wet the
efficiency drops dramatically and the bed height becomes too high ... hence
the heat output drops. Because of this plan is to use waste heat from the
combustor to dry the trees to ~23% moisture (as rec'd) prior to use as
fuel.
Mark Bryden
UW-Madison
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Re: Modeling and chimneys
Mark Bry wrote:
Kayal doesn't report the pressure drop. As a starting place for this I
would suggest looking at non-reacting packed beds. There is a large body of
work wich addresses pressure drop for this case. I suspect the reacting
part doesn't add much to the pressure drop in the bed.
Etienne:
This is not true. The reacting part is causing the greatest pressure drop in
the fuelbed. This can be concluded from measurements I did on this (I will
send you a copy of the article). The large contribution of the reacting part
of the fuelbed is most likely to be caused by the increase in air
temperature in the zone. This results in a lower air density and a larger
gas velocity. The larger velocity means probably more drag. For very large
combustors the situation might be a bit different due to a transition from a
laminar to a turbulent flow.
Etienne
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Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands
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Re: The Whole Stick Stove
Hi Mark and all:
The other day I speculated on an improved "3 stick stove" (after reading good
things about three stone stoves in Prasad et al). As you say, easy to test
small scale ideas, easier than whole tree combustors.
APPARATUS:
I have a sheet of insulating refractory board 2 inches thick from
which I cut a 5 inch circle. At the center I drilled a 1 inch hole 1 1/2 in
deep to form an insulated furnace/crucible. At the periphery I drilled 3 1 inch
holes 1 1/4 below the top intersecting the central hole so that I could feed
sticks into the central crucible at any rate I pleased. I drilled 4 3/16 in
holes in the bottom of the crucible to let in limited air.(Construction time
10 minutes).
EXPERIMENT:
I placed three irregular, very dry, nearly 1 inch sticks in the
horizontal feed holes. I ignited the intersection with a propane torch.
Initially they burned quite brightly and steadily with a luminous flame
(volatiles burning). After about 5 minutes the flame began to waver and dim but
I could revive it by rotating or feeding the sticks to the center. After about
10 minutes I got a beautiful violet (CO) flame, quite transparent, for a minute
or so. Then the fire went out and no amount of torch or feed or fiddling would
relite.
CONCLUSIONS:
Charcoal formed at the ends of the sticks and charcoal burns so
much more slowly than volatiles in wood that this geometry would never be
practical with any charcoal forming wood. I had hoped that the charcoal would
fall into the crucible-pit and add its heat to the fire. Not this time. Maybe
someone can suggest a modification.
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Re: Modeling and chimneys
At 03:31 PM 4/13/96 +0100, you wrote:
Mark Bryden wrote:
Kayal doesn't report the pressure drop. As a starting place for this I
would suggest looking at non-reacting packed beds. There is a large body of
work wich addresses pressure drop for this case. I suspect the reacting
part doesn't add much to the pressure drop in the bed.
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Etienne:
This is not true. The reacting part is causing the greatest pressure drop in
the fuelbed. This can be concluded from measurements I did on this (I will
send you a copy of the article). The large contribution of the reacting part
of the fuelbed is most likely to be caused by the increase in air
temperature in the zone. This results in a lower air density and a larger
gas velocity. The larger velocity means probably more drag. For very large
combustors the situation might be a bit different due to a transition from a
laminar to a turbulent flow.
Mark:
I look forward to reading the article.
I agree with Etienne to a point. What he has written is true but can be
accomodated by using the nonreacting data and correlations as local data
with quasi-steady state assumptions and integrating top to bottom knowing
the temperature and mass flux as a function of height. This is what makes
the natural convection numerical model yet more difficult to achieve
convergence on. The old proverb that everything we want to use is atttached
to everything else and is this case numerically stiff and tightly coupled.
Purnomo and several others used this in their investigations of reacting
packed beds (wood and other) with forced convection.
Mark Bryden
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Re: Modeling and chimneys
Mark Bryden wrote:
I agree with Etienne to a point. What he has written is true but can be
accomodated by using the nonreacting data and correlations as local data
with quasi-steady state assumptions and integrating top to bottom knowing
the temperature and mass flux as a function of height. This is what makes
the natural convection numerical model yet more difficult to achieve
convergence on. The old proverb that everything we want to use is atttached
to everything else and is this case numerically stiff and tightly coupled.
Purnomo and several others used this in their investigations of reacting
packed beds (wood and other) with forced convection.
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Etienne:
For a cold fixed fuelbed the resulting airflow is surpisingly simple. It can
be represented as an exponential or an inverse linear function of the weight
of the fuelbed. For a burning bed the situation is more complicated, but for
a large cold section and a small reaction zone this dependence still holds.
For larger reaction zones you have to resort to temperature profiles and
mass production profiles in the fuelbed as described by Mark.
Etienne
Mark Bryden
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Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands
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Re: Draft in top burning
Moerman, Bryden et al:
Having made many natural draft runs on the top burning (inverted downdraft)
gasifier using balance to follow the rate of fuel consumption, I can say it is
quite linear.
This is at first surprising, since initially the only resistance is that of the
cold fuel, while toward the end the gases pass through many inches of hot
charcoal.
However, there is about a 50% shrinkage in the fuel-charcoal conversion, so I
suppose that compensates for the higher resistance of hot vs cold.
OK? TOM REED
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Re: Draft in top burning
Tom Reed wrote:
Having made many natural draft runs on the top burning (inverted downdraft)
gasifier using balance to follow the rate of fuel consumption, I can say it
is quite linear.
This is at first surprising, since initially the only resistance is that of
the cold fuel, while toward the end the gases pass through many inches of
hot charcoal.
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Etienne:
I thought there was only a small layer of hot and burning char. If there was
a large layer the char would burn away. I understand from you, Ron and Tom
Duke that this is not the case. Also the sticks have the same direction as
the airflow. Under these circumstances the exponential and the inverse
linear dependance reduces to a linear dependance.
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Tom R.:
However, there is about a 50% shrinkage in the fuel-charcoal conversion, so
I suppose that compensates for the higher resistance of hot vs cold.
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Etienne:
This is only one of the reasons.
Etienne
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Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands
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Re: Draft in top burning
Etienne:
Well, in the run I reported last week in the small, un-insulated, 4 in diameter
can the reaction only reached 400C and declined quite rapidly in the layer of
charcoal above the zone as it grew from 0 to final dept, 3-4 in.
In the run I made last night I inserted a riser sleeve (3 in ID) and it made a
world of difference giving a 6 in fuel bed.. The reaction peaked at 492C at
thermocouple # 1, 3 in below the top of the can (and starting point of fuel) and
at 538C at thermocouple #2 5 in below the top. The TC at 3 in had only cooled
to 448C (from 492) at the end of the experiment.
So with good insulation there is very little cooling of the charcoal; without
it cools very rapidly. (This is a very small diameter - it would be less
dramatic in larger diameters, but similar near edges.) INSULATIN IS IMPORTANT.
Tom
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End of stoves-digest V1 #17
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Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands
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