[Stoves] Jeb's clay stoves with at twist

Crispin Pemberton-Pigott crispinpigott at gmail.com
Tue Oct 29 20:03:41 MDT 2013

Dear Jeb


I owe you a comment promised some time ago. Things are slackening slightly
so here goes.



The design includes (for the first time?) some secondary air holes through
the upper body to create a secondary air supply with a significant element
of preheating.  There is no obvious preheater but the holes being deeper
than they are in diameter means the heat is added to the air.


I am not sure how long people cook at a time on these stoves but the total
mass looks large so the preheating will not work until the clay, at least on
the inner portion of the thickness, will have to get hot first.


Regarding the tangential entrance - this is a good way to do it on two
conditions: that there is enough flame space above the air entrances to
allow the flames to burn completely before hitting the pot, and provided it
does not already have too much air in the gas path from below. Stoves with
this layout - hole at the bottom side and a vertical fire channel above,
tend to have far too much excess air. All the nitrogen in the air that
accompanies the unneeded oxygen cools everything results in heat loss,
creating of CO (chemical loss of heat) and poorer heat transfer efficiency
(because the gas stream has been lowered in temperature).


There are several ways to improve the situation, the first and cheapest and
most effective is to lower the pot rests. These pot rests above are huge by
any standard and they allow the wind to rush through the stove warming the


So Jeb, please do some experiments to convince yourself of the important
role placed by the pot as an essential part of the stove - used for closing
the system and restraining the gas flow to a desirable rate.


If you have a thermocouple place it in the gas stream well below the pot and
see what temperature you get. Then fill in the gaps (or shorten the top) to
reduce the space. Then measure again. The temperature at some point will
start to go up. It is not really going up because you are getting nearer to
the pot, it is going up mostly because the air flow is being restricted
through the stove and this leads to lower excess air and less cooling and
higher temperatures. As the gap reaches 7 to 8 mm you will notice a
significant change in the way the flames appear (if you can see them). 


You can't tell how a stove is working with the pot off - it is part of the
stove. There is no point looking at the flames of a stove with no pot in
places because it is such an important part of the 'enclosure'.  As the
pot-stove gap starts to become a serious limitation on the gas flow, you
will notice the flames lose much of their 'rushing' and become more 'lazy'.
You will see the secondary air jets clearly as sources of flame, though in
fact the air is just entering the hot gases which burn around a clear finger
of air entering through the holes. This is the ideal appearance.


Under these conditions, the temperature at the pot will start to rise
significantly. This is the effect of low excess air. The great majority of
heat is transferred to the pot by convection, not radiation. If the gases
are hotter, the effectiveness of this transfer is much larger. 


There is a stove from Indonesia which is a traditional bucket with a very
small exhaust gap. Here it is:


Notice how the pot rests are actually most of the rim with some of it cut
away. There is not much gas-space. There is a huge hole in the front for
fuel and hopelessly large amounts of air and enter, but because of the
restricted exit, this stove is quite good (for being so simple). Instead of
making pot rests like this:



It has the opposite. The result is air control without having to restrict
the entrance way.


Now here is the ultimate example of traditional air control:



This one has only 4 small slits for gases to exit, and it will be perhaps a
bit too effective. However this stove has a dual function which is to roast
meat. The sticks with small bits of meat skewered on them are placed
(sideways) across the lower part of the stove. Some of the how gases and
flames are directed to the meat, plus the coals are raked back from the fire
onto the shelf. The stove does exactly what the designer wants. The
usefulness today is to show that control of the gas exit can make it do
something you want.


In the case of the Roket stoves, it is to reduce the flow of air to that
which is needed.


Now there are drawbacks to having the air controlled in this manner. The
main one is that of the pressure inside the stove builds to the point that
gases leave the secondary entrances, you have a real problem. You need the
section above the secondary entrances to be low enough so that the
combustion above is hot enough to draw in cold air, not push hot gases out.
At some point it will work well. You may lower the tangential holes to get a
hotter chimney effect above, and this will simultaneously allow you to run
'closer to the line' in terms of lowering the excess air - hopefully into
the 80-120% range. If you have an oxygen meter the reading would be about
9-11% O2 in the gases coming out.


There is a photo of a grate with stones on it in the PDF you sent around.
Does the stove indeed have a grate and are there stones on it?


This is a really good idea and I am going to try it on some stoves. If the
pebbles are sitting on the grate, the grate hole size is effectively reduced
without having to pay for a lot of material, and the steel (probably mild
steel bar) will last a lot longer if it is not heated much.


Experimenting with a traditional Keren stove in Java last year and this we
found that having a grate in it (tilted actually) burned nearly all the
charcoal which is otherwise wasted (about 10% of dry fuel consumed). The
trick to getting it to burn well was to make the holes quite small. We are
using a 5mm gap between steel bars. Using larger spacing and pebbles might
cause the same effect. It will also limit the tendency of this stove type to
pull too much air through the fuel bed.


A downside of getting your air supply sorted out and having low EA and lots
of high temps is that the clay will start to crack! This is inevitable
because when you get it really hot on one side, it will expand and crack the
cold side because clay is much stronger in compression than it is in
tension.  The only way out of this 'success' is to use a clay with a lower
thermal expansion and high heat shock tolerance. This was explored at length
with the POCA stove in Mozambique.  There are documents available on that.


So you are well on your way to getting your amazingly beautiful stoves to
work even better. Please keep us updated with progress and experiments that
fail and work (both). We will learn and adjust out thinking (or unthinking)


Crispin back in the land of cold.

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