[Stoves] Understanding Stoves

Sun Dec 11 05:20:43 CST 2011

This is a great summary that everyone designing stoves should be well aware
of, thanks so much Sai!

Kind regards and best of luck!
Rok Oblak

On Fri, Dec 9, 2011 at 7:46 PM, Sai Bhaskar Reddy Nakka <
saibhaskarnakka at gmail.com> wrote:

> 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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>

-- 
*Rok Oblak, MAA Design*

rok.stoves at gmail.com
www.holeyroket.com

Gregorciceva ulica 5
4224 Gorenja vas
Slovenia
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