Fw: Constant area

["Dean Still" <dstill@epud.net>]

Dear Stovers,

I've been working on how to even the temperatures on a griddle (comal) so
that tortillas don't burn in one place and stay raw in another. When using a
large open fire the heat is generalized over a large surface. But placing a
stove's smaller diameter opening under the griddle can create an unwanted
hot spot. (If we're boiling beans on top of the griddle you want to put the
pot over as hot a spot as can be created and hope that the rest of the
griddle is at room temperature but making tortillas is different.)

My mentor, Larry Winiarski, discovered a long time ago that tapering the gap
under a pot, decreasing towards the rim, assists better heat transfer and
therefore more even temperatures. My recent experiments show this to be so.
I asked Ken Goyer, Aprovecho consultant, to write up for Emma in Uganda how
to calculate the cross sectional area under a pot. Larry starts a design out
by keeping the same cross sectional area throughout the whole stove allowing
the hot flue gases to flow unimpeded. This seems to be a good rule of thumb.

But, Emma is using an external chimney in her design which adds more draft
so gaps can be decreased a bit which helps heat transfer all the more.
Following is Ken Goyer's explaination of one of the important Rocket design
principles:

The way to calculate the height of the gap under the pot in
order to maintain a constant cross section is as follows:
    First, figure out the area of the combustion chamber. If the
combustion chamber is five inches in diameter then the area is A=pie r
squared, or 3.14 x 2.5in. x 2.5in. = 19.6 square inches. 19.6 square
inches of gasses are proceeding up the rocket elbow and we want to
maintain the draft and not change it's velocity so we want to maintain
this cross section under the bottom of the pot as the gasses progress
outward toward the edge of the pot. At the top edge of the rocket elbow
the gasses are going to turn and follow the bottom of the pot. The five
inch diameter has a circumference  of  C = pie d or 3.14 x 5 inches =
15.7 inches. If this space were one inch high it would only be 15.7
square inches. But we need 19.6 square inches. So it needs to be higher
than one inch or 19.6 / 15.7 =  1.25 inches high over  the edge of the
combustion chamber where the gasses are turning and flowing radially
under the pot. Now as the gasses go outward the circumference gets
bigger so the gap must get smaller in order to maintain the same cross
section. At 7 inches the circumference is C = pie x d or 3.14 x 7 = 22
inches. So the gap is 19.6 inches divided by 22 inches or .9 inches. At
9 inches the circumference is C = pie x d or 3.14 x 9 = 28 inches. 19.6
divided by 28 inches = .7 inches.
    You can figure this height at any diameter under the pot. For
example if the pot was 20 inches in diameter circumference would be C =
pie x d or 3.14 x 20 = 63 inches and the gap would be 19.6 divided by 63
inches or .3 inches.  You can figure this as precisely as you wish. You
can plot it for every inch of the diameter, or even every half inch. But
with three or four points you can get a good approximation of the cross
section at various diameters. I hope this helps to explain how to keep
the cross section equal under the pot so the combustion gasses can
maintain their velocity and scrub the pot at closely as possible.

 Best
regards, Ken (and Dean)






-
Stoves List Archives and Website:
http://www.crest.org/discussion/stoves/200204/
http://crest.org/discussiongroups/resources/stoves/
>
Stoves List Moderators:
Ron Larson, ronallarson@qwest.net
Elsen L. Karstad, elk@wananchi.com www.chardust.com
>
List-Post: <mailto:stoves@crest.org>
List-Help: <mailto:stoves-help@crest.org>
List-Unsubscribe: <mailto:stoves-unsubscribe@crest.org>
List-Subscribe: <mailto:stoves-subscribe@crest.org>
>
Sponsor the Stoves List: http://www.crest.org/discuss3.html
-
Other Biomass Stoves Events and Information:
http://www.bioenergy2002.org
http://www.crest.org/articles/static/1/1010424940_7.html Bioenergy
http://www.crest.org/articles/static/1/1011975339_7.html Gasification
http://www.crest.org/articles/static/1/1011975672_7.html Carbon
>
For information about CHAMBERS STOVES
>http://www.repp.org/discussiongroups/resources/stoves/Chambers/Chambers.htm