Shaping the top of the single pot cooking stove

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

Dear Friends,

For many years, Larry has been reminding his students at Aprovecho that hot
flue gases need to scrape against pot surfaces for efficient heat transfer.
This is true when using one pot on top of a fire. Larry likes to taper the
gap between the top of the stove and the pot, keeping the same cross
sectional area. In other words, if there is an inch gap directly under the
center of the pot then the gap gradually decreases until it is very small
under the outer rim, say around 3/16" or so, depending on pot size... This
is a useful design characteristic!!!

If the gap under the pot stays constant a lot of heat escapes that could be
heating food. Today, I did a couple of experiments using a 12" in diameter
griddle called a comal used for making tortillas in Mexico. Three type k
thermocouples were placed in the middle (1) and the two opposite and
furthermost edges (2,3) of the steel comal. When the gap was correctly
formed into a diminishing taper to the outer edges there was about a 70
degree F. difference (at 440 degrees F) between the warmest (middle) and
coldest sensor. But when the taper was removed and the gap increased to
7/16" under the edges of the comal the difference in temperature more than
doubled. I chose to measure the difference at 440 degrees F. because I've
read that tortillas cook best at between 440 to 480 F.

I'll fool around with the gap some more but for now it seems that Larry's
rule of keeping the same cross sectional area under the pot and creating a
gradual, smooth curve from vertical to horizontal flow is quite beneficial.

How much fuel does it take to make tortillas in a Rocket stove? Well, I made
thin corn tortillas like we use for tacos. It took about a pound of wood to
make about a kilo of finished tortillas, burning about four 1/2" in diameter
sticks at a time, pushed into the fire as they were consumed. Four sticks
kept the center of the comal around the optimal temperature and eventually I
was making three at a time using the whole surface. I'll continue playing
around with comals, tortillas, and gaps and present a paper at the next
ETHOS meeting. Heck, I'll make everybody tacos!

Best,

Dean




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