[Stoves] Understanding Stoves

Sai Bhaskar Reddy Nakka saibhaskarnakka at gmail.com
Fri Dec 9 12:46:12 CST 2011


Dear Charlie Sellers, Crispin and All,

Testing the stoves is one of the ways of understanding stoves. I am
thankful to this group especially Crispin, for sharing so much regarding
the stoves. I started writing a  small reply, but it ended up so length.

Although there are various methods of testing the stoves, the most
important testers are the users. The multiple factors for a successfully
adopted stove in time and space also vary.  In the last two months, I have
done about 100 tests for some of my stoves, followed WBT 4.1.2 methodology.
Since last 7 years, had been designing stoves, testing them, training on
stoves, getting the community (user) feedback etc., some of my experiences,
observations and insights into the stoves are as such.

Testing the stoves involves not only following the standard procedures but
also the tester having the experience. Apart from the values, the testers
experience would help a lot in designing good stoves.
 Fuel wood

Size of fuel wood: It was observed that for fuel many testers chose thin
sticks. In our region people chose wood of 1.5 inches to 2.5 inches
diameter. The wood is never split perfectly into square cross sections, as
usually one uses during the testing of stoves. Splitting the wood into thin
size say 2 cms x 2 cms is very difficult and consumes energy and time. The
length of the wood is usually about 1.5 feet to 4 feet. Bundling and
transportation of the cut wood is not convenient if the wood length is too
short.  Sometimes the wood carried by head loads is very long say about 5
to 8 feet in length. The women / men balance the wood on their heads, to
carry it. In a tribal area, people use trunks of wood without cutting as
fuel wood, they are often 6 to 12 feet in length and nearly 4  inches to 12
inches in diameter. The trunks of wood as fuel is a common practice for
many institutional stoves. Such usages are commonly found in the
habitations located in the forest areas / in forest fringe areas.

Type of wood: Different types of wood are available such as Neem, Accacia
Nilotica, Prosopis Juliflora, etc.

Wood moisture: In many tropical and semi-arid environments on a dry day the
wood moisture varies between 12 to 18%.

Very thin wood <1 cm is used for kindling. Sometimes kerosene is used
(about 5 to 10 ml). Thin wood is not preferred by communities for cooking
(except for kindling). Thin wood burns conveniently with very less smoke,
but sometimes they yield soot. The stove users does not like soot deposit
on the utensils and on their walls. The bark of the wood although good for
fire but sometimes leads to excess sooth emissions. The kerosene added for
starting the fire, leads to conspicuous soot emission.
 Size of the stove:

In our region, 8 inches height of the stove is ideal. Any stove too high
about 12 inches or more is inconvenient. Women prefer sitting comfortably
on the floor and cook. Many of the activities done by women in the rural
areas are by sitting. Especially while making rotis, they prefer the low
height of the stove. Scientifically the chimney effect would help the smoke
to burn in tall stoves. But, as per the ergonomics and local practices, it
is not convenient to the user. Sometimes a scientist is successful in the
lab, but fails at the community level. Often the pan for making rotis is
about 10 to 12 inches in diameter. In a 4 inch diameter stove, the stove
should be operated in high power for the flames to spread underneath the
pan for complete burning of a roti. The diameter of the stoves about 7
inches is ideal for different types of the utensils used in the region.

Two of my stove designs were a failure due to small diameter of the stove
and also being tall. As a designer there is a need to balance between the
science and user requirements (including ergonomics and many other factors).
 Stove life and efficiency



The thermal efficiency is also dependent on the mass of the stove. The
light weight metal stoves are thermally more efficient, especially during
the cold start phase. During the hot start phase they show slight
improvements in thermal efficiency. Heavy stoves are thermally less
efficient during the cold start phase. During the hot start phase of
testing, heavy stoves having the retained heat, show better efficiencies. A
compact stove is more adoptable then, the stove with more movable or
removable parts. It costs more to make stoves with two or more materials.

Insulation and refractory are two important factors in a stove design.
Getting a material which has both the properties is of great value. The
refractory material available in the markets (aluminum and iron based
bricks) are heavy and bad insulators. But the life of the stove is
guaranteed for even 10 years. The thermal efficiency goes down. The cost of
the stove goes up because of the cost of refractory bricks and excess
weight of the stove. Heavy stoves are difficult to facilitate. The volume
of the stove and the surface area of the stove increases relatively for
more durable stoves. There ought to be loss of heat due to radiation for
durable stoves.



Air is the best insulation material. Even by using the thin metal sheets
for combustion chamber (with air holes - side air / secondary air hole
features), the life of the combustion chamber can be increased. For safety
another thin sheet can be used as external wall. The surface area of
contact between the hot parts and other parts of the stove should be least.
By using pointed or thin screws, ceramic watchers, the heat transfer to the
outer body can be reduced. Often between the inner and outer walls
insulation material is filled. The contact between the grate and the stove
body should be least. This can be achieved through pointed supports for the
grate.



For facilitation to wider geographies, the convenient weight of the stove
is around 2 to 4 kgs. The stoves weighing more than 10 kgs, are the most
difficult stoves to sustain as an enterprise. It is even more difficult to
sustain the facilitating team, if the stoves weigh more. The stoves
facilitated through subsidies / stoves facilitated through the schemes are
often heavy. Majority of the stoves available in the market and sold on a
large scale (although inefficient) are always light. These stoves being
light also cost less and are also less durable.

In heavy stoves, it is convenient to sustain the simmering test as compared
to light weight stoves.


 Removable grate:

The holes in the grate should be small and at the same time should not
affect the life of the grate. Too much primary air / too little air
underneath the grate affects the stoves performance.


 Capacity of the domestic stove:

A domestic stove often serves on an average about 5 members in a family.
The same stove should also support if there are more number of guests on
occasions say 10 members. It is very difficult for many families if they
can’t have this flexibility of cooking. The range of cooking pots used are
of 1 to 5 liters capacity.
 Charcoal in the stoves



Many stoves achieve higher efficiencies because of continuing to
consumption of charcoal in the stoves (including the TLUD stoves). However
during the testing of stoves, one of the most difficult tasks is weighing
the charcoal as the water reaches boiling point. Dousing the flames is
convenient when the wood is pushed into the sand. Scrapping all the
charcoal from the wood is a difficult task. Some time is lost in the
process of weighing the charcoal, weighing the wood and the water in the
pot after boiling.


 Charcoal yield



 The charcoal yield from Magh 3G stove is around 15% by weight. For TLUDs
the yield is about 25%.


 Cooking conditions



In the semi-arid region, cooking is often done is semi-ventilated
conditions. The cooking pots are closed with the lid while cooking.
Sometimes for stirring / during simmering of rice, etc. the lid is opened
or semi-closed. While making Rotis the pan is completely exposed. Utensils
used for cooking vary: i.e., pots are cylindrical, taper from mid way of
the pot, semi-spherical, or bowl shape, etc. For the WBT test, cylindrical
utensil of about 3 to 3.5 liters capacity is used.


 Time of testing



If the tests are conducted at the transition phase of sun rise / sun set,
the air temperature changes are very fast. Sometimes chill effect will be
there on the stove and the pot. The places selected for testing are not
completely closed, so there could be influence of the local weather and
other environmental conditions. The ideal time for testing the stove could
be other than those two timings.
 Flame

The observation of the flame is very important, for which the photographs
would be more useful then the videos. In a dark room the observations about
the stove performance can be more convenient. Blue flame is best visible in
the dark conditions.


 Rushing and dancing flame

There are two types of flames, rushing flame (as in rocket stoves) and
dancing flame (say as in Magh 3G stove). Rushing flames are common where
there is chimney effect and also due to chimneys. In the rushing flame
stoves, although the flames appear dynamic and impressive, but the
conduction of the heat to the pot is low. Where as in dancing flame stove,
there is through mix of air with the combustible gases, and the flame takes
its time without rushing to combust the gases underneath the pot. Therefore
the heat conduction to the pot is high.


 Color of flame

The reddish color flame yields more soot. The yellowish-red color flame is
common. The bluish-yellowish-reddish flame is good. (Observing the flames
is the best experience, one could spend hours just watching the flames,
because the color is energetic and flames are dynamic change their form
continuously),


 Air for combustion

There are three sources of air possible for a stove : - Primary air, Side
air and Secondary air.


 Primary air



This air is mainly from the bottom of the grate / underneath the wood.
Excess of air flow will, lead to excess / uncontrolled combustion. Primary
air is always required in less quantities. If this air is preheated before
reaching the wood it will make the stove perform better. The embers at the
bottom of the grate, sometime after preliminary combustion are very much
useful for preheating the primary air.


 Side air:



In majority of the traditional stoves this is the only source of air. From
the fuel feed opening this air gushes. In three stone stoves from all the
three sides this air gushes. Too much of this air in open air conditions
has dampening effect on the flames. Controlling gushing side air is also
important. If the mouth of the fuel feed is completely closed due to over
feeding of the stoves by wood in traditional stoves, sometimes leads to
extinguishing of the flame and release of excess smoke. Using side air
shutters is a good idea. Using simple small holes on the sides, as given in
Magh 3G, would also help. In efficient stoves the fuel feed opening should
be reduced, as already primary air facility is given.


 Secondary air



For all TLUDs this is an important feature. This feature can also be given
to the domestic stoves (as compared to TLUDs, few holes can be given),
which helps in complete combustion of any smoke otherwise left. In the
efficient stoves without chimney effect, this is a very useful feature.



Note: It is convenient to use a stove in open air conditions with all the
three air features (primary, side air and secondary air), as compared to
other stoves.
 Gap between the grate and bottom of the stove



In the domestic stoves, the gap between the grate and the bottom of the
stove could be around 1.5 to 2 inches. Seldom people remove ash while
cooking, and ash being a good insulator, it protects the bottom of the
stoves. The embers falling beneath the grate helps in preheating the
primary air. In any case the movement of the primary air should not be
blocked.
 Who can test the stoves?





Anyone can test the stoves. One need to have some experience of preparing
the kindling wood, lighting the stove, tending the fire, taming the flames,
choosing the wood, feeding the wood, observing the fire, all that is needed
apart from cooking which means nearly 10 ways of preparing the food.
(Roasting, frying, boiling, steaming, mashing, simmering, etc.). For WBT
tests one should know only Boiling and simmering. Biomass stoves are
different as compared to other stoves. The most difficult stoves to learn
and operate are biomass stoves. Sometimes cooking on biomass stoves is like
caring babies.

Best way to learn cooking is to start with the three stone stoves. Anyone
who can reach thermal efficiencies of around 30% with a three stone stove
(in closed conditions) is an expert. Everyone need not be an expert for
testing stoves. Patience and interest are the two most important factors in
stove testing. Best way to learn cooking is by observing and learning from
the experienced people. Operating a stove professionally for the first time
using a manual is difficult.
Highest thermal efficiencies are achieved in lab conditions and only by the
experts. Most often highest thermal efficiencies achieved are reported by
the stove facilitators. But thermal efficiencies achieved by operators with
different experiences and conditions are not reported. Sometimes, stoves
reported by a producer are so high, but they fail to pass through a
countries specific test. Rating the stoves by “stars” is important, because
people buy different types of stoves for various reasons.

-- 
Dr. N. Sai Bhaskar Reddy
CEO GEO <http://www.e-geo.org> | GOOD STOVES <http://goodstove.com> | BIOCHAR
INDIA <http://biocharindia.com> | GEO
Resources<https://sites.google.com/site/geoweblinks/>
Mobile No. (+91) 9246352018 or 9505760669
saibhaskar at facebook.com | skype: saibhaskarnakka
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