[Stoves] Servals Wood Stove

[Crispin Pemberton-Pigott]

Dear Rajan

 

>We want the grate to burn as much charcoal as possible and in that process
keep the combustion chamber as hot as possible  -  to get an overall cleaner
combustion.

 

And that is exactly how to achieve it. Well done.

 

>The flame seen has around 2 KW fire-power. This is the maximum rating 

for this stove. For the average "South Indian Domestic Cooking", the average


fire-power used is around 1.5 KW. At around 1.5 KW fire-power, the flame is 

slightly shorter and also looks better.

 

That is helpful information. It seems you have a pretty good balance at the
moment.

 

>The normal LPG stoves used here have a maximum fire-power rating of around
2 

KW per burner, and people do not complain.

 

I agree - we tested an Indian gas burner and it was 2.2 kW. The thermal
efficiency was about 65%.

 

>But with a wood stove, people are likely to push in more wood to get a 

higher fire-power initially ( while boiling water for cooking rice, etc. ). 

>Once water boils, the fire-power is immediately brought down.

 

As you infer, this may be acceptable to people. A bit more smoke to start on
high and then a pretty clean burn when burning at reduced power may be a
good overall answer (or an acceptable answer).

 

>The height of the stove is kept low for a particular reason. In India at 

many places, ladies sit close to the ground for cooking. Here the stove has 

to be essentially short in height. It is a matter of user-friendliness.

 

This is pretty well known and discussed here. There is one thing you can do
which is to try to limit the entrance of secondary air (making the fuel hole
smaller, for example) and allowing secondary air to enter (possibly
preheated) through tangential ports which are low to the fire, and which are
angled so as to swirl the flame like a tornado. This lengthens the flame
path without raising the height of the stove. The increased mixing also
reduces the flame length. It might be a good solution for India.  Having all
the secondary air enter from one side through one large hole is actually not
a very good way to get optimal combustion.

 

>In this stove, the diameter of the "flame-concentrator hole" also decides 

the fire-power.

 

It is performing a choking function. This is quite acceptable. It can also
be done by using a tight fitting pot skirt or by limiting air entry.
Generally speaking it is always better to limit air entry than gas exit
because in the latter case there is a chance of increasing emissions near
the cook.

 

>> Reducing the total amount of air in the chamber as also increased the

>> temperature in there! This further accelerates the production of wood gas

>> creating even worse combustion conditions.

 

>True in normal conditions. Here the design of the fuelwood stand ( sheet 

>metal design ) helps a little bit. The fuelwood does not easily burn until 

>pushed nearer to the grate.

 

Good news. Perhaps you could try cutting the primary air hole area in half
(put some clay over 1/2 of them) and put a damp rag over the fuel entry hole
to mostly close it. If you get an adequate burn with much lower excess air,
the heat transfer efficiency will increase immediately and you may get the
same performance from 1.2 kW instead of 1.5.

 

>>...It is possible to get a balanced combustion at a range 

>>of power levels without getting high excess air in the

>> low power condition.

 

>This is a very important point. In a well-designed FD stove, the ratio 

between primary and secondary air is comparatively well-managed. In ND 

stove, it is slightly difficult.

 

Especially at different power levels. The 'in the bottom out the top' air
path is not very good for getting it right at more than one power level.

 

>...Although drop-down door is a simple idea, we do not know whether 

this will be user-friendly. People are highly conscious about convenience.

 

Perhaps it needs to be an 'L' shaped door with a lip facing the user so it
can be popped open with the tip of a new stick. We found that using such a
system tends to extinguish sticks that burn until the flame is outside the
stove. Sometimes people forget the stove and the burning stick falls out
after burning back to the point that the flame is no longer going into the
combustion chamber. The 'guillotine' type of door chills the hot coal and
usually extinguishes the flame if it burns 'out of the stove'.

 

Here is an example of a multi-lever air-blocking fuel-chilling device, not
quite a guillotine but very effective for all three tasks.

 



Regards

Crispin

 

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