[Stoves] FW: pot skirt efficiency estimates, scale-up

Tue Sep 18 19:24:23 CDT 2012

Dear Jed

Great set of drawings! You asked for comments:

There are a few things you can do to improve the combustion and heat
transfer efficiencies (which are two separate things). 

First, put in a grate which is what we have just been discussing. You can
make it out of anything you want but cast iron if possible. If you have to
make it, use reinforcing steel bars with the bumps on them. They have a
higher carbon content and resist the heat better.  It has to be loose and
replaceable in the sense that you should not cement it into position. It is
best if you can insert it through the fuel hole but based on your drawing
there probably isn't enough room. Give it 4 legs to stand at the right
height.

Although there are no dimensions on the drawing it seems to be reasonably to
scale. The position of the secondary air holes it probably too high.
Basically, the flames should never touch the pot if you want to get good
combustion. It is widely believed that when flames are running along the
surface of a pot the heat transfer is good. In fact, it is killing the
combustion as the gases can't burn when they are cold and the whole surface
of the vessel is killing combustion that should have been completed before
the that point. Try to achieve 'no flames toughing the pot' combustion.

There is a long yellow brick above the fuel entrance. The top of that brick
is a good place to all secondary air to enter, preferably from the front and
the back in equal amounts as you have it now.

An important consideration for this application is that the excess air level
in the stove must be kept low. You have a very large area for heat transfer
which is really good.  The heat transfer efficiency is strongly affected by
the flame temperature, and the flame temperature is reduced by the presence
of unnecessary air (which is full of cold, useless Nitrogen).

If you want, you can improve the burning slightly and reduce wear and tear
on the combustion chamber in general by putting the lower air supply (on the
bottom of the drawing) from either the side or the back. This brings in air
that is not aligned with the air entering with the fuel. Anything that you
can do to break up the alignment helps. What it does is bring the flames
into the central space of the combustion chamber instead of driving all the
flame to the back wall. A Lion Stove
http://www.newdawnengineering.com/website/library/Stoves/lionstove  is built
that way as is the Esperanza Stove and also the similar sized stove made by
Andrew (Not AJH). This change in the air entry increases the life of the
combustion chamber by removing some portion of the thermal shock experienced
when flames touch the walls.

You might consider putting a hard brick at the back of the combustion
chamber to better resist the wood when it is shoved to the back by the
stoker. The back surface of the combustion chamber is a frequent problem
spot on a Rocket Stove if the bricks are soft. In large units people push in
large and long pieces of wood until it hits the back with a whack. Whatever
you teach, people will bash the heck out of it. The brick arrangement of the
Lion Stove is an example of how to resist the forces with the structure
available. They have held up well (over 5 years) without maintenance.

If you want to have some lower conduction bricks behind it, no problem, but
make sure they are cemented well. The cement joint will separate because of
differential thermal expansion, but at least they will be the same shape and
transfer the impact evenly.

The addition of a fish smoker is very interesting. That is a good use of
waste heat.

To know if you have enough air or too much getting into the fire, you need
an Oxygen meter at a minimum. A combustion analyser is best of course. Try
to get an oxygen reading of about 8% in the chimney.  If you have nothing,
try limiting the air getting into the fire until you get a visible increase
in smoke then add air again. 

The most important thing to change is the secondary air entrance which is
too high, creating flames around the pot. The flames should be completed by
then and they cannot if there is a lack of air (from below). If there is
enough air from below (through the fuel and under the fuel shelf) there is
no need at all for the secondary air above - that just cools the fire and
reduces the heat transferred to the big jar by whooshing in unneeded air.
Because you have a chimney on it, it is very likely you have more than the
required air getting into the stove. Remember to put in a grate flush with
the top of the fuel shelf to keep the charcoal up off the bottom to let it
burn completely.

Regards

Crispin

Dear All

With all respects, i wish to join in the discussion with my own concern
similar to that of Josh Kearns. 

Im at present building an improved wine distillery in favor of the small
nipa wine producers in my hometown. The constructin is now in full scale and
so im beginning to really get worried about the integrity of my design.

I do not have enough engineering background on this and do not have much
equipments. So i would really appreciate it if you could also guide me
through with this project. 

I attached some photos and drawings of the project. 

It was organized by a Japanese  JICA volunteer and will help more than 200
wine producers not to mention their health and the immediate environment. 

Best regards

JEd Guinto

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