[Stoves] Servals natural draft wood stove

[rajan_jiby at dataone.in]

Dear Crispin,

>
> Message: 2
> Date: Fri, 20 Jul 2012 10:43:01 -0400
> From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> To: "'Discussion of biomass cooking stoves'"
> <stoves at lists.bioenergylists.org>
> Cc: 'Sujatha Srini' <sujatha.srini at gmail.com>, 'mukundan'
> <mukundanpa at gmail.com>, 'vijayaganapathi' <vijayaganapathi1 at gmail.com>
> Subject: Re: [Stoves] Servals natural draft wood stove
> Message-ID: <059f01cd6685$f7769640$e663c2c0$@gmail.com>
> 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