light weight bricks, air placement, etc.

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

Dear Stovers:

Ken Goyer taught a class on refractory ceramic solutions to the June
Advanced Studies in AT class here at Aprovecho.  The students seemed to
enjoy the subject and have been doing some more experimenting on their own.
Damon Ogle and Jim Wilmes had made some refractory insulative bricks from
clay and perlite that seemed promising. Damon returned to the lab this last
week for two days and Ken and Larry, Damon and Jeff & I made up a new batch
of bricks that use less clay and are getting lighter and hopefully more
insulative. Perlite is readily available in South Africa and Uganda so
learning about this approach may be fruitful.

(At the same time we are looking into modern alternatives with help from
Thermal Ceramics, Foseco, etc. hoping to find a magic material that costs
very little, is highly insulative, and a bit more rugged that riser sleeves,
which are close to perfect...We want to join the Billion Stoves club, too!
Finding a great combustion chamber for sale would make it all so much
easier. We are with you, Tom!!)

The new bricks are based on road building experience. Damon Ogle, an
engineer, builds roads and brings this experience to creating light weight
ceramic bricks for use in stoves. The gravel for roads is made up of about 7
different sizes of material that have been found to fill in spaces best so
that less binder is necessary. Only about 5% asphalt is necessary if a best
mix of gravel is used. So Damon led us in investigating whether using the
same road building proportions might lessen the amount of clay needed as a
binder with pumice rock and perlite. Using less clay should result in light
bricks, more insulative. Our test bricks have from 5% to 30 % clay.

Ken, Apro ceramics expert, will be firing the bricks soon and we'll get back
with results.

Damon Ogle also built a wonderful Rocket stove experiment that has revealed
some great data on the effect of air placement in the combustion chamber on
exit temperatures, a good predictor of efficiency of heat transfer. The
following is his summary:

variations in the size and location of air openings in a modified “rocket
stove” and their effect on exhaust temperatures

Standard geometry for a “rocket elbow” recommends placing a shelf in the
mouth of the combustion chamber 1/3 of the way up from the bottom of the
feed tube.  A single layer of sticks (with air spaces between) is placed on
top of the shelf and some free air enters above the sticks.  Air entering
under the shelf is partially preheated and passes up through the burning
sticks.  As the sticks burn off, the “charcoal “ ends fall into the bottom
of the combustion chamber and a portion of the “under shelf” air passes
through this “charcoal” layer
The cross sectional area of a 5” feed tube is about 19 1/2 square inches
with about 71% above the shelf and 29% under.  The percentages of cross
sectional area for four components:  (1) sticks, (2), air “above sticks”,
(3) air between sticks and charcoal layer, and (4) air through the charcoal
layer, are unknown.
The purpose of this experiment was to construct a stove in which the cross
sectional area of each of the 4 components is measurable and adjustable.
The total cross sectional area was held constant at 19 ˝ square inches but
different percentages of air were directed to different areas of the stove.
The output temperature of the stove was measured for each of 9 air input
configurations to see if air input had an effect on exit temperatures..

RESULTS
Cross sectional area
Exit Temperature (deg F)
      (%)                sticks  above sticks   mid-level  under charcoal
Max T  Min T  Est Mode
  12             59             23            6
1429    921       1175
  12             50             23            15
1441    1050     1208
  12             40             23            25
1380    1040     1200
  12             30             23            35
1510    1025     1283
  12             20             23            45
1595    1148     1383
  12             10             23            55
1839    1311     1625
  12             10             15            63
1810    1379     1641
  12             10              7 ˝         70 ˝
1846    1439     1658
  12             10              0             78
1880    1481     1733

 These results are the average of three “runs” each at 9 different settings.
“Estimated mode” represents the averages of my best guess of the temperature
around which exit gases hovered during each period of observation.  Minimum
and Maximum Temperatures are actual measurements for the period.  The
highest temperature observed during the tests was 1921 F.

PROCEDURE
- Outside air temperature and weather conditions were recorded.
- The feed magazine was loaded and the fire was started using ˝ a sheet of
newspaper and a small amount of finer wood.  New sticks were added to the
magazine as fuel was consumed.
- The stove was allowed to run for 20+ minutes to allow a stable state to be
reached.
- After each new setting of adjustments was made, the stove was allowed to
run for 2 minutes before recording was begun.
- The temperature probe was placed 1” below the top of the rocket chimney in
the center of the tube
- Temperatures were recorded for a period of 8 minutes for each setting.
During this time the thermometer was observed and a visual estimate was made
of the mode temperature.  At the end of this period, the estimated mode T,
actual maximum T and actual minimum T were recorded.
- Adjustments were made to the damper settings and the procedure was
repeated until all 9 variations had been recorded.
- At the end of each run, the coals were allowed to burn off from the
screen.  Very little charcoal remained and the screen stayed fairly open and
clean.

The results of this experiment seem to indicate that location and size of
air inlets does have a noticeable effect on the exit temperatures of the
stove.  In general, the less air entering above the feed shelf and the more
air entering through the “charcoal” layer, the higher the exit gases
temperature.  The most noticeable jump in exit temperature took place when
the air space above the feed shelf was minimized at 1/8” above the level of
the sticks.  This represented 10% of the total cross sectional area (6%
between the sticks and 4% in the 1/8” gap above the sticks, which was needed
to allow for “kick-up” of the sticks as they were fed into the combustion
chamber).  This observation seems to confirm the possibility that excess air
entering the chimney above the feed level cools the exit gases more than it
produces heat by encouraging combustion of pyrolysis  products
No instruments were available to measure actual air flow, but it is
reasonable to suppose that cross sectional area and air flow are closely
related.  Even with a full load of coals on the bottom screen, there
appeared to be unimpeded flow of air at all times.
The stainless steel screen seemed to be structurally sound after some 10
hours at very high temperatures.  Perhaps this is because cool air is
continuously entering on the bottom of the screen.  Only time will tell how
long it will actually hold up, but I am encouraged by it’s performance so
far.
The thinner sheet metal in other parts of the stove was oxidized and almost
worn through in some places.  It will have to be replaced before further
experiments can be performed.
As one would expect, there were more and higher flames visible at the top of
the chimney when higher exit temperatures were obtained.  More smoke was
also observed at these higher temperatures.  This suggests that higher
Temperatures alone are not sufficient to clean up combustion.
I’m hoping that a combination of a higher chimney and /or limiting the draft
in some way will help obtain a cleaner burn.  At least, that’s what I’ll try
next.

These are the conclusions I came to after the experiment:


The exit gas temperatures in a “rocket” stove are strongly dependent on the
size and location of air inlets.
Air entering the rocket stove above the level of the sticks in the feed
opening tends to cool the exit gases of the stove.
Air entering at the “mid-level” (below the sticks but above the coals) is
less effective in increasing stove temperatures than air being forced up
through the coals.
Exit temperatures can be increased by minimizing the air which is allowed to
enter the stove through the area above the feed shelf and maximizing the air
which is forced to pass upward through the coals underneath the feed shelf.

WHAT A GREAT EXPERIMENT! A STUDENT/COLLEAGUE LIKE THIS MAKE ME A VERY
CONTENTED TEACHER!!....Dr. Winiarski is a great believer is directing air at
the coals and this experiment seems to put some numbers to the benefit. By
the way, the next Aprovecho stove class begins August 19 to 23 and we'll be
looking at reducing emissions, etc. Contact me for more info.


Best,

Dean


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