Dear Crispin,
Message: 2
Date: Fri, 20 Jul 2012 10:43:01 -0400
From: "Crispin Pemberton-Pigott" <[email protected]>
To: "'Discussion of biomass cooking stoves'"
<[email protected]>
Cc: 'Sujatha Srini' <[email protected]>, 'mukundan'
<[email protected]>, 'vijayaganapathi' <[email protected]>
Subject: Re: [Stoves] Servals natural draft wood stove
Message-ID: <[email protected]>
Content-Type: text/plain; charset="US-ASCII"
Dear Rajan
The fuel-wood stand is height-adjustable. One of the purposes here is to
reduce/regulate the excess air - also encouraging air-flow at the bottom
of the stand.
It is very likely, looking at het stove, that the excess air is
surprisingly
high. I understand your point that the primary/secondary air split can be
modified by the fuel shelf level (I am glad to see that it is a sheet of
steel, not a ladder-type support). However if you were to put a combustion
analyser on the gases coming out, I am really sure you would find that the
quantity of unneeded air passing through is much more than you need. If
you
use a couple of damp rags you can experiment with limiting the air
entering
above and below. At some point, even without instrumentation, it becomes
obvious that there is more smoke, not less. That is too little air. A
little
bit more and you will notice no smoke (or very little) and a much improved
time-to-boil.
At 2 KW level it is low-smoke. Around 2.5 KW, the smoke level increases.
The opening at the "flame concentrator" decides the power level.
The average requirement of fire-power ( used by people ) seems to be in the
range of around 1 to 1.5 KW. So we made a 2 KW stove.
The "sheet metal support" design was purposely brought in so that the
combustion mostly takes place in the grate area and not earlier. It guides
the air below towards bottom of the grate.
The opening at the "flame concentrator" ( just below the vessel support )
is also tuned for 2 KW fire-power.
It is an off-the-shelf part?
The part is from market. The opening has to be decided/made by us.
I have been toying with the idea of a drop plate also. But I am worried
whether the user will find it convenient.
Toy with it and watch the performance. There are various ways to make it
convenient.
The stove boils 3 kgs water in 18 minutes using around 200 grams of
firewood ( ambient temperature around 33 C ).
That is roughly a 25% efficiency. Now I am really sure you have too much
air. Try blocking the lower air perhaps 80-90% and the upper air perhaps
75%
(air going in with the fuel). Check the temperature of the gases as they
emerge between the pot and the stove body. Do not be surprised if the
temperature goes up. That does not mean the efficiency went down, not at
all. If the airflow is less, the temperature may go up and the efficiency
too because the volume of gases at that temp is much lower. Losses are a
combination of the two + unburned CO.
The fuelwood we used is casuarina. The moisture content of the wood will be
around 20 % - when used. Wood available in the market contains much more
moisture. We do a litle bit of sun-drying before use.
We do not do too much of drying - just because this is what people are
going to use. Finally the stove has to serve the people.
The dry weight of the stove is around 7 kgs.
That is surprisingly high. What is inside that it weighs so much?
Some simple low-cost insulation - which we find very effective. This also
makes the stove quite sturdy.
Another area of great worry is that people sometimes stuff the fuel-window
with fuelwood. This is habit probably picked up from using a "three stone
fire". In such a case, smoke is the result.
Regards,
Rajan
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