402K05004
Design Principles for Wood
Burning Cook Stoves
              Aprovecho Research Center
                      Shell Foundation
            Partnership for Clean Indoor Air

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 The Partnership for Clean Indoor Air was launched by the U.S. Environmental Protection Agency (EPA)
 and other leading partners at the World Summit for Sustainable Development in Johannesburg in September
 2002 to improve health, livelihood, and quality of life by reducing exposure to indoor air pollution, primarily
 among women and children, from household energy use.  Over 80 organizations are working together to
 increase the use of clean, reliable, affordable,  efficient, and safe home cooking and heating practices that
 reduce people's exposure to indoor air pollution in developing countries.  For  more information, or to join the
 Partnership, visit www.PCIAonline.org.

 This document was developed by Aprovecho Research Center under a grant from the Shell  Foundation to
 provide technical support to household energy and health projects to ensure that their designs represent
 technical best practice. The principle authors of  this booklet include: Dr. Mark Bryden, Dean Still, Peter Scott,
 Geoff Hoffa, Damon Ogle, Rob Balis, and Ken Goyer.

 Indoor air pollution causes significant health problems for  the 2 billion people worldwide that rely on
 traditional biomass fuels for their cooking and heating needs. Over the last 30 years, awareness of the
 environmental and social costs of using traditional fuels and stoves and knowledge about how to reduce
 emissions from these stoves has grown. Yet the improved stoves currently available to poorer customers do
 not always represent best practice or an understanding of design based on modern engineering. The
 knowledge required to design cleaner burning stoves exists  in centers of  excellence in several locations
 around the world. Providing this  information to those involved in promoting improved stoves is a necessary
 first step to reducing indoor air pollution exposure for stove users.

 Aprovecho is a center for research, experimentation and education on alternative technologies that are
 ecologically sustainable and culturally responsive. The  Advanced Studies in Appropriate Technology
 laboratory at Aprovecho works to develop energy efficient, nonpolluting, renewable technologies that reflect
 current research but which are designed to be made  in most any country.  The center is located on a beautiful
 40-acre land trust near Eugene, Oregon. For more information on Aprovecho, visit www.Aprovecho.net.
Illustrations: Mike Van, Jayme Vineyard, and Ethan Hughes

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Design Principles for Wood Burning

Cook Stoves

Dr. Mark Bryden, Dean Still, Peter Scott, Geoff Hoffa, Damon Ogle,
Rob Bailis, Ken Goyer
Table of Contents
Introduction
Chapter 1 -Stove Theory	7-11
Chapter2-Ten Design Principles	12-16
Chapter 3-Designing Stoves with Baldwin and Winiarski	17-25
Chapter 4-Options For Combustion Chambers....	26-29
Chapter 5-In Field Water Boiling Test	30-35
Appendix-Glossary of Terms	37-38

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Design Principles for Wood Burning Cook Stoves
                                                                                          Introduction
Introduction
Proven Strategies
Indoor air pollution causes significant health
problems for the 2 billion people worldwide who
rely on biomass fuels for their cooking and heating
needs. Over the last 30 years awareness of the
environmental and social costs of using traditional
fuels and stoves has grown. At the same time,
studies of the problem have resulted in proven
strategies to reduce both fuel use and harmful
emissions. Unfortunately, the local stoves currently
available do not always represent the best designs
that modern engineering can offer. This booklet is
an attempt to address the problem by summarizing
some of the advances in stove theory and design.
Understanding these concepts would be useful to
administrators of stove projects, policy makers,
field workers, and cooks alike.

Although open fires are often used wastefully,
carefully operated open fires can be fuel efficient
and clean burning when tested in the lab. In many
situations, cooks are not overly concerned with  fuel
use, and studies have shown that when fuel is
plentiful three-stone fires can use an excessive
amount of wood to cook a small amount of food.
But in other places where fuel is scarce, open fires
can be carefully controlled so that fuel efficiency
rivals many first generation improved cook stoves.

How an operator controls the open fire makes the
difference, as in the use of other tools. In the
seventies and early eighties, open fires were
generally characterized as being basically
inefficient. But it was by analyzing the open fire
that researchers were able to develop truly
improved stoves. Dr. Grant Ballard-Tremeer and
Dr. Kirk Smith were foremost among those who
found that the three stone fire could be both more
fuel efficient and cleaner burning than some
"improved" cook stoves.

Respecting that indigenous technologies are
evolved from countless years of experimentation
and have great worth changes the perspective of
scientists who are trying to address the causes of
human suffering. Watching how experts operated
the open fire has taught engineers how to design
even better stoves. Modern cook stoves are
designed to clean up combustion first. Then the
hot gases can be forced to contact the pot
increasing efficiency without increasing harmful
emissions.

Fires can be clean burning when expert cooks push
the sticks of wood into the fire as they burn,
metering the fuel. The open fire can be a hot fire
useful when food or drink needs to be prepared
quickly. The
energy goes
into the pot,
not into the
cold body of
a stove. The
open fire can
burn wood
without
making a lot
of smoke; hot
fires burn smoke as it is released from the wood.
Unfortunately however, many fires used for
cooking are built emphasizing simplicity of use and
are wasteful and polluting.

Modern stoves score higher when tested than even
the most carefully operated fire in the laboratory.
Good stoves can offer many advantages. Stoves do
much more than save wood and reduce smoke.
How the stove cooks food is usually most
important to the users!

Improved stoves can make cooking with fire easier,
safer, faster, and can add to the beauty of the
kitchen. A good stove  is quicker to start, needs little
tending, and can meet the specific needs of a cook.
The successful design is appreciated as an addition
to the quality of life and usually these concerns far
outweigh scores on a test.
Figure 1 - Traditional Wood Fire

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 Design Principles for Wood Burning Cook Stoves
                                                                                           Introduction
 Decades of  Investigation
 Many investigators have contributed to a
 modern understanding of the thermodynamics
 of cooking stoves. The scientific study of wood
 burning stoves has reached the point where a
 great deal of consensus now exists about how
 stoves function. Dr. Larry Winiarski has
 studied combustion and wood burning cooking
 stoves for more than thirty years. He has
 helped organizations build thousands of stoves
 in countries around the world. Dr. Winiarski is
 the Technical Director of the Aprovecho
 Research Center, where stoves have been a
 major topic of study since 1976. The team at
 Eindhoven University, led by Dr. Krishna
 Prasad and including Dr. Peter Verhaart and Dr.
 Piet Visser, experimented with wood stoves for
 more than a decade and wrote pivotal books on the
 subject. Dr. Sam Baldwin summarized years of
 experience in West Africa and in the lab in his
 comprehensive book Biomass Cookstoves:
 Engineering Design,  Development and
 Dissemination (1987).

 Chapter One, Stove Theory, outlines the work of
 these leading researchers and offers strategies that a
 stove designer can use to improve a stove.

 Chapter Two, Ten Design Principles, details the
 synthesis of design created by Dr. Larry Winiarski.

 Chapters Three and Four, Designing Stoves with
 Baldwin and Winiarski, and Options for
 Combustion Chambers contain technical
 information to support the designer in charge of
 developing a stove project.

And lastly, chapter Five, In Field Water
 Boiling Test, provides designers with an in field
 method for measuring the performance of stove
prototypes as they are developed. The test does not
 require a computer or complicated calculations for
data analysis.
Respect for local knowledge
We hope that the following design principles add to
a project, highlighting the respect and inclusion of
local knowledge. A sensitivity and appreciation of
local knowledge supports a two-way information
exchange, learning from the expertise of local
people and their technology while sharing
knowledge.

Hopefully, sharing design principles is more
inclusive than promoting a static stove design.
The literature frequently points out that local
inventiveness has a place in  every part of a stove
project. Without information from the  community
that will be using the stove,  a project is starved for
the input needed for success.

All members of a design committee including
cooks, craftspeople,  administrators, promoters and
technical advisors can easily learn stove design
principles. The inventiveness and practical
experience of the whole team is essential to create a
product that suits local needs and 'tastes'.

The empowerment found in the design  process can
serve as motivation for locals to become trainers,
promoters, designers, and builders. Technical staff
frequently find valuable  input about design,
manufacture, and promotion from the users and
learn just as much as they teach. Perhaps the
conclusion that stove projects are more likely to
succeed when all concerned help to create the
design parallels the hope that better representation
will create solutions to larger problems.

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Design Principles for Wood Burning Cook Stoves
                                                                                         Stove Theory
Chapter 1

Stove  Theory

Even an open fire is often 90% efficient at the
work of turning wood into energy. But only a
small proportion, from 10% to 40%, of the
released energy makes it into the pot. Improving
combustion efficiency does not appreciably help
the stove to use less fuel. On the other hand,
improving heat transfer efficiency to the pot makes
a large difference.

Improving the combustion efficiency is necessary
to reduce smoke and harmful emissions that
damage health. Improving heat transfer efficiency
can significantly reduce fuel use. Fire is naturally
good at its job, but pots are not as good at
capturing heat because they are inefficient heat
exchangers. In order to reduce emissions and fuel
use, the stove designer's job is to first clean up the fire
and then force as much energy into the pot or griddle
as possible. Both of these functions can be
accomplished in a well engineered cooking stove.

It is always best practice to add a chimney to any
wood burning cooking or heating stove.
Additionally, it is preferable to use a cleaner
 burning stove to protect air quality in and outside
 of the house. Chimneys that take smoke and other
 emissions out of the living space protect the family
 by reducing exposure to pollutants and health risks.
 Even cleaner burning stoves without a chimney can
 create unhealthy levels of indoor air pollution.

 Unvented stoves should be used outdoors or in
 open areas. When chimneys are not affordable or
 practical using a hood over the fire, or opening
 windows, or making vents in the roof under the
 eaves are all ways to decrease the levels of harmful
 pollution. The use of a cleaner burning stove can
 also be helpful in this regard but, if possible, all
 wood burning stoves should always be fitted with a
 functional chimney!

 How can we design a stove that improves upon the
 open fire? First, let's list the advantages of the three-
 stone fire when compared to some stoves:
  No energy is absorbed into the mass of a stove
  body. High-mass stoves can absorb energy that
  could have gone into the pot. The three stone
  fire can boil water fairly quickly.
  Fire hits the bottom and sometimes the sides of
  the pot, exposing a lot of the pot to the hot
  gases.
  Sticks can be fed in at the appropriate rate as the
  tips burn, assisting complete combustion.
  A hot open fire can burn relatively cleanly.
  Every stove suffers because it has some mass that
  absorbs heat. But an improved stove can still
  achieve better combustion and fuel efficiency
  than an open fire.
How to improve  combustion
(make less harmful pollution compared to an open
fire)
>•  Make sure there is good draft into the fire.
*•  Insulate around the fire to help it burn hotter. A
   hotter fire burns up more of the combustible
   gases and produces less smoke.
*  Avoid using heavy, cold materials like earth and
   sand around the combustion chamber.
*  Lift the burning sticks up off the ground so that
   air can scrape under the sticks and through the
   charcoal.
*  Placing an insulated short chimney above the
   fire helps to increase draft and gives smoke, air,
   and fire a place to combine, reducing emissions.
   This is a popular strategy used in many stoves
   such as the Z stove, the Vesto, the Wood Gas
   Camp Stove, the Rocket  stove, the Tso-Tso
   stove, etc. The Eindhoven group used a
   chimney above the fire in their cleanest burning
   downdraft stove. Micuta built stoves
   incorporating this idea as well (Modern Stoves for
   All, 1981). Winiarski developed the concept in
   the early 1980s creating a stove that cleaned up

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 Design Principles for Wood Burning Cook Stoves
                                                                                         Stove Theory
   combustion and improved heat transfer
   efficiency (Capturing Heat One,  1996).

   Meter the sticks of wood into the combustion
   chamber to make a hot, fierce, jumpy looking
   fire that does not make much charcoal. This type
   of fire will make  less dangerous emissions,
   chimney clogging soot, and creosote. Heat only
   the burning part of the wood. Do not encourage
   the non-burning wood to make smoke.
   Limit the cold air entering the fire by using as
   small  an opening as possible. Small openings into
   trie fire also force the cook to use less wood,
   which can be burnt more  efficiently.

   A certain amount of excess air is necessary for
   complete combustion. Preheating the air helps to
   maintain clean combustion.
How to improve  fuel
efficiency
(get more heat into the pot)
>  Increase the temperature  of the gas/flame
   contacting the pot, having the hot air scrape
   against both the bottom and sides of the pot in a
   narrow channel, using a pot skirt.
*  Increase  the speed of the hot flue gases that scrape
   against the pot. The fast gases punch through a
   boundary layer of still air that keeps slower
   moving gases from scraping against the surface of
   the pot (or
   griddle.) Air is a
   poor heat transfer
   medium. It takes
   a lot of hot air to
   bring heat to the
   pot.
*•  Use metal rather
   than clay pots
Pot Skirt
  because metal
  conducts heat
  better than clay.

  The size of the
  fire determines
  the size of the
Figure 2 - Appropriate Use
of Pot Skirt
    channel gap in the pot skirt and the maximum
    efficiency of heat transfer. Smaller fires that can
    still please cooks but are not too big will be
    considerably more fuel efficient.

  >•  Use wide pots with large diameters. Using a wide
    pot creates more surface area to increase the
    transfer of heat. Make sure that the top  of the
    stove slopes up toward the outer perimeter of
    the pot, as shown in Figure 2.

 Sam Baldwin's Biomass Stoves: Engineering
 Design, Development, and Dissemination (1987) is a
 very good summary of how to make improved
 stoves. It is highly recommended. Dr. Baldwin
 figured out how the channel size between pot and
 skirt, firepower and efficiency are related. Here are
 a few examples using a family sized pot:

 1.) A 1.7 kW fire with a channel gap of 6 mm that
    forces hot flue gases to scrape against the pot
    for 15 cm will be about 47% efficient.
 2.) 4 kW fire with a channel gap of 10 mm that
    forces heat to scrape against the pot for 15 cm
    will be about  35% efficient.

 3.) A 6 kW fire with a channel gap of 12 mm that
    forces heat to scrape against the pot for 15 cm
    will be about  30% efficient.

 4.) A 8 kW fire with a channel gap of 14 mm that
    forces heat to  scrape against the pot for 15 cm
    will be about  26% efficient.

As an approximate, general rule of thumb,
 Baldwin recommends that a family sized stove that
 burns less than one kg of wood per hour can use a
channel gap between pot skirt and pot of 11 mm.
If the stove burns  1.5 kgs per hour the gap needs to
be 13 mm. If 2 kilos of wood are burnt  per hour
then the gap should be 15mm. Please refer to
Biomass Stoves for complete information.

In wood burning stoves a lot of the heat is
transferred to the  pot or griddle by convection.
The amount of wood burnt per hour and channel
gap are related. If the pot skirt gap is made too
narrow, there is insufficient draft and smoke backs
up into the room.

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Design Principles for Wood Burning Cook Stoves
                                                                                           Stove Theory
Increase heat transfer to the pot by keeping the
temperature of the hot flue gases as high as possible.
Insulate everywhere the heat goes except to the pot
or griddle. If there is enough surface area in the
   o                    o
stove for the hot flue gases to scrape against, the
flue gases will be much colder by the time they exit
out of the chimney. If exit temperatures in the
chimney are above 200° C, add more surface area
to make use of the heat. Secondary pots or griddles
placed near the chimney may never boil water but
they can help preheat cooking water and warm
food and dishwashing or bathing water.

Using a pot skirt also  forces more heat into the pot
by forcing the hot flue gases to continue scraping
against the pot all along its sides in addition to its
bottom.

A haybox makes even more efficient use of
captured heat. Placing the boiling pot of food in an
airtight box filled with insulation holds the heat in
the pot, and food cooks without using added fuel
(See Figure 4).

Once the food has boiled, the fire can be
extinguished. Placing the pot of food in an
insulated cooking box most effectively uses the heat
to accomplish the task of cooking. The haybox
does all simmering without using extra fuel. This
technique saves tremendous amounts of wood. And
using a retained heat  cooker saves time for the cook
who lets the haybox do the simmering!
Figure 3 - Top Down
View of Haybox
Figure 4 - Placing
Boiling Pot in the
Insulation
Figure 5 - Putting
Insulating Lid on the
Haybox
                                                                      Figure 6 - Food
                                                                      Continues to Cook
                                                                      Inside the Insulated
                                                                      Haybox

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 Design Principles for Wood Burning Cook Stoves
                                                                                        Stove Theory
 Common  Misconceptions

 1. Retained energy in the stove body helps to
 cook food.
 FALSE

 Experiments by Baldwin have shown that retained
 energy is mostly lost. Leftover charcoal can heat
 food after the fire has been extinguished but
 retained energy in the stove body is usually too
 cold to effectively heat pots.

 Note that retained energy in a stove can be
 advantageous if the stove is used for space heating.

 2. Keeping energy in the stove by decreasing the
 draft will help to cook food. Lowering the exit
 temperatures in the chimney means that the cook
 stove is operating well.
 FALSE

 As stated, slowing down the draft hurts both
 combustion and heat transfer. Hot flue gases need
 increased velocity to achieve good heat transfer.

 3. Using a damper in the chimney helps to make
 a stove work better.
 FALSE

Again, slowing down the draft in a cooking stove is
 usually detrimental. Dampers should not be used
 in a well designed cooking stove.
4. Packed earth or stone acts like insulation.
FALSE

Dense materials absorb energy rather quickly while
insulation slows the passage of heat. Insulation is
made of pockets of air separated by a light weight
less conductive material.

Insulation is light and airy. Heavy materials are
better examples of thermal mass. Insulation helps a
stove to boil water quickly; thermal mass robs
energy from the pot which makes water take longer
to boil.

5. Anything is better than an open fire.
FALSE

An open fire can boil water faster than many heavy
stoves. The three stone fire can be clean burning
and relatively fuel efficient. While the open fire can
be wasteful when used carelessly, the early estimates
that any stove was better has been replaced with a
new respect for this ancient technology. Engineers
have learned how to design improved cooking
stoves by learning what is great about the three
stone fire.
10

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Design Principles for Wood Burning Cook Stoves
                                                                                         Stove Theory
Testing  is essential
Dr. Baldwin includes a remarkably thorough
chapter on stove testing in Biomass Stoves. He
points out that the testing of prototypes is necessary
while the stove is being developed. Testing stoves
also helps determine if the model is marketable,
whether production costs are as low as possible, and
if improvements are needed. Testing should happen
during the entire life of a stove project.

Baldwin includes tests to determine whether
consumers are happy with the product, if firewood
is being saved, and how lifestyle issues are affected.
Without continual testing, a stove project operates
in the dark;  it lacks essential technological,
sociological, and business information. Reading the
stove testing chapter in Biomass Cookstoves is highly
recommended.

Careful testing of stoves has resulted in a more
accurate understanding of how to make better
stoves. Without experimentation and testing, the
development of a stove is based on conjecture.
Careful investigation can quickly separate truth
from opinion. Testing has a twofold function: to
identify problems and to point out solutions. It is
an essential  ingredient for progress. A simple water
boiling test  is included in Chapter 5 on page 30.
Make  stoves safe!
Preventing burns is quite possibly one of the
most important functions of an improved stove.
Burns are quite common in homes using fire and
can be fatal or horribly disfiguring. To protect the
family the stove body should not be hot enough to
cause harm. Stoves and pots should be stable.
Surround the fire with the stove body so that
children cannot be burnt. Injuries from fire are a
major problem that stoves can remedy.

Chimneys or smoke hoods can be used to get
smoke out of the kitchen. According to recent
estimates by the World Health Organization, up  to
1.6 million women and children die every year
from breathing polluted air in their houses.
Pneumonia and other respiratory diseases in
children are caused by breathing smoke. Unvented
stoves can be used outdoors, under a roof, or at
least near  a large window. Operational chimneys
and airtight stoves can remove essentially all
pollution  from the indoor environment. Chimneys
are used in industrialized countries and are
required for protecting families from dangerous
emissions. Shouldn't people in poorer countries be
provided with the same protection?
                                                                                                 11

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 Design Principles for Wood Burning Cook Stoves
                                                                                    Ten Design Principles
 Chapter 2
 Ten
Principles
 Dr. Larry Winiarski's design principles have been used by many organizations to create successful stoves. The
 HELPS plancha stove in Guatemala, the PROLENA EcoStove in Nicaragua, the Trees, Water and People
 Justa stove in Honduras, the ProBec stoves in South Africa, the new generation of GTZ cooking stoves in
 Africa, and the famous Rocket stove are all designed using his principles. Winiarski's design approach
 combines both clean burning and optimized heat transfer characteristics. Any type of intermittently fed
 wood burning stove can first be designed by locals to meet their needs and then finished by adapting these
 principles.

 Batch fed and fan assisted stoves operate differently. These alternative stove design methods can be used as
 successfully to improve wood burning stoves. While many experts are working on these two approaches,
 both  Crispin Pemberton-Pigott and Dr. Tom Reed have developed excellent working models, both of which
 are for sale. For more information on the Vesto stove please contact: Crispin Pemberton-Pigott at
 vesto@newdawn.sz or VESTO, P.O. Box 85274 Emmarentia, Republic of South Africa 2029. Dr. Tom
 Reed has spent  decades experimenting with wood burning. His fan-assisted stoves are wonderful inventions.
 He currently markets them under the name "Wood Gas Camp Stoves." Dr. Reed can be reached through
 the Biomass Energy Foundation Press or at tombreed@comcast.net.
PRINCIPLE ONE:
Whenever possible, insulate around the fire
using lightweight, heat-resistant materials. If
possible, do not use heavy materials like sand and
clay; insulation should be light and full of small
pockets of air. Natural examples of insulation
include pumice rock, vermiculite, perlite, and
wood ash. Lightweight refractory brick (brick that
has been fired and is resistant to cracking at high
temperatures) can be made from locally available
sources (for recipes see Chapter 4, Option #2:
Insulative Ceramics,  page 27).

Insulation around the fire keeps it hot, which helps
to reduce smoke and harmful emissions. Also,
insulation around the fire keeps the heat from
going into the stove body instead of into the pot.
                         Unfortunately, metal does not last very long
                         near a hot fire. However, locally made ceramic tiles
                         can be found that are durable when used as walls for a
                         combustion chamber. Loose insulation can surround
                         this type of construction. (See Chapter 4, Option #1:
                         Floor Tiles, page 26.)
                                                   Insulative brick
                                                   Pockets of air which slow the
                                                   transfer of heat to the brick
                                                   Figure 7 - Insulation around the fire
12

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Design Principles for Wood Burning Cook Stoves
                                                                                  Ten Design Principles
PRINCIPLE TWO:
Place an insulated short chimney right above the fire. The combustion
chamber chimney should be about three times taller than its diameter.
Placing a short chimney above the fire increases draft and helps the fire
burn hot and fierce.  Smoke will contact flame in the chimney and
combust, reducing emissions. Pots or surfaces to be heated are placed above
the short chimney A taller combustion chamber chimney, more than three
times the width, will clean up more smoke, but a shorter chimney will
bring hotter gases to the pot. The very tall combustion chamber chimney
can develop too much draft bringing in too much cold air that will
decrease heat transfer.
                                                                         Figure 8 - An insulated
                                                                         short chimney above the fire
 PRINCIPLE THREE:
 Heat and burn the tips of the sticks as they enter the fire. If only the wood that is burning is hot there will
 be much less smoke. Try to keep the rest of the stick cold enough that it does not smolder and make smoke.
 The goal is to make the proper amount of gas so that it can be cleanly burned without making charcoal or
 smoke. Smoke is un-burnt gas! It is harmful  to breathe. Even cleaner looking combustion contains harmful
 emissions.
                Figure 9 - Cleaner Burning
            Figure 10 - Smoldering Wood Makes Smoke
 PRINCIPLE FOUR:
 High and low heat are
 created by how many sticks
 are pushed into the fire.
 Adjust the amount of gas made
 and fire created to suit the
 cooking task. (Wood gets hot
 and releases gas. The gas
 catches fire and makes heat.)
Figure 11 - Low Heat
Figure 12 - High Heat
                                                                                               13

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 Design Principles for Wood Burning Cook Stoves
                                                                                    Ten Design Principles
 PRINCIPLE FIVE:
 Maintain a good fast draft through the burning
 fuel. Just as blowing on a fire and charcoal can
 make it hotter, having the proper amount of draft
 will help to keep high temperatures in your stove.
 A hot fire is a clean fire.
 PRINCIPLE SIX:
 Too little draft being pulled into the fire will
 result in smoke and excess charcoal. But too
 much air just cools the fire and is not helpful.
 Smaller openings into the fire help to reduce excess
 air. Improving heat transfer to the pot or griddle is
 the most important factor that will reduce fuel use
 in a cooking stove. Improving combustion
 efficiency reduces pollution but is less important
 when trying to save firewood.
PRINCIPLE SEVEN:
The opening into the fire, the size of the spaces
within the stove through which hot air flows,
and the chimney should all be about the same
size. This is called maintaining constant cross
sectional area, and helps to keep good draft
throughout the stove. Good draft not only keeps
the fire hot; it is also essential so that the hot air
created by the fire can effectively  transfer its heat
into the pot. Air does not carry very much energy,
so a lot of it needs to go  through  the stove in order
to accomplish the task of heating food or water.
                                                                  Figure 13 - Maintaining a Good Draft
                                                     Figure 14 - Balancing the air flow in a multipot stove
The size of the openings is larger in more powerful
stoves that burn more wood and make more heat.
As a general rule, a door into the fire with a
square opening of twelve centimeters per side
and equally sized chimney and tunnels in the
stove will result in a fire suited to family
cooking. Commercial stoves need bigger openings,
tunnels, and chimneys because bigger fires require
more air. For more information, please see the
chapter Designing Stoves with Baldwin and
Winiarski on page 17.
  Figure 15 - Maintaining Constant Cross-Sectional Area
14

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Design Principles for Wood Burning Cook Stoves
                                Ten Design Principles
PRINCIPLE EIGHT:
Use a grate under the fire. Do not put the sticks on
the floor of the combustion chamber. Air needs to
pass under the burning sticks, up through the
charcoal, and into the fire. A shelf in the stove
opening also lifts up sticks so air can pass
underneath them. When burning sticks, it is best to
have them close together and flat on the shelf, with
an air space in between each stick. The burning
sticks keep the fire hot, each fire reinforcing the
other to burn more completely. It is optimum if the
air passes under the shelf and through the coals so
that when it reaches the fire it is preheated to help
the gases reach complete combustion. Air that passes
above the sticks is not as helpful because it is colder
and cools the fire. A hot raging fire is clean, but a
cold fire can be very dirty.
                  Shelf for
                  Wood
           Figure 16 - Use of a Grate Under the Fire
PRINCIPLE NINE:
Insulate the heat flow path. Cooks tend to like
stoves that boil water quickly. This can be especially
important in the morning when family members
need to get to work. If heat goes into the body of
the stove, the pot boils less quickly. Why heat up
fifty or one hundred kilograms of stove each
morning when the desired result is to heat up a
kilogram of food or a liter of water? Using
insulative materials in the stove keeps the flue gases
hot so that they can more effectively heat the pan
or griddle. Insulation is full of air holes and is very
light. Clay and sand or other dense materials are
not insulation. Dense materials soak up heat and
divert it from cooking food.
 PRINCIPLE TEN:
 Maximize heat transfer to the pot with properly
 sized gaps. Getting heat into pots or griddles is
 best done with small channels. The hot flue gases
 from the fire are forced through these narrow
 channels, or gaps, where it is forced to scrape
 against the pot or griddle. If the gap is too large the
 hot flue gases mostly stay in the middle of the
 channel and do not pass their heat to the desired
 cooking surface. If the gaps are too small, the draft
 diminishes, causing the fire to be cooler, the
 emissions to go up, and less heat to enter the pot.
When designing a stove, it is possible to decrease
the gap in the channel next to the pot or griddle
until the fire becomes "lazy." Using trial and error,
open up the gap little by little until the fire stays
hot and vigorous.
                                                                                                  15

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 Design Principles for Wood Burning Cook Stoves
                                                                                       Ten Design Principles
 The two most important factors for getting large
 amounts of heat into a pot or griddle are: 1) keep
 the flue gases that touch the pot or griddle as hot as
 possible; and, 2) force the hot gases to scrape against
 the surface quickly, not slowly. Air does not hold
 much heat. Faster hot flue gases scraping against the
 pot or griddle will transfer much more heat than
 slow-moving cooler air.
The size of the channel can be estimated by
keeping the cross sectional area constant
throughout the stove. When using an external
chimney that provides greater draft, channel gaps
can be reduced. For more information on gaps,
please see the next chapter.
       Figure 17 - A proper sized gap
       optimizes heat transfer to the pot
   Figure 18 - Too large a gap will reduce
   heat transfer to the pot
16

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Design Principles for Wood Burning Cook Stoves
                 Designing Stoves with Baldwin & Winiarski
Chapter 3
Designing  Stoves with  Baldwin  & Winiarski
Forcing hot flue gases to flow past the surface area of
a pot or griddle in a narrow channel is a stove
design strategy popularized by both Dr. Samuel
Baldwin and Dr. Larry Winiarski. In 1982 Dr.
Winiarski created the pot skirt, a cylinder of sheet
metal that surrounded the pot, which formed a
narrow channel increasing heat transfer efficiency.
Dr. Baldwin studied stoves in Africa and in 1987
wrote his seminal book Biomass Stoves: Engineering
Design, Development, and Dissemination in which he
also stresses the importance of using narrow
channels  to deliver more heat to the pot.

In general, there are three ways to increase
convective heat transfer:

^  The flue gases scraping the surface to be heated,
   should be as hot as possible.

*  The surface area of the heat exchanger should be
   as large as possible.

*  The velocity of the hot flue gases should be
   increased as much as possible. A faster flow over
   the exterior of the pot disturbs the stagnant
   boundary layer of air that slows effective heating.

The narrow channels formed close to the pot by an
insulated skirt (see Figure 19) can help to optimize
the three principles simply and inexpensively.
Although narrowing the gap increases heat transfer
efficiency, doing so also decreases the flow of air
through the stove. The size of the gap must
therefore be in relation to the firepower. As more
wood is burned  per minute, more air is needed to
                Narrow Channel
 Pot Skirt
support both the combustion and the necessary
flow to avoid back drafting into the room. If too
small a gap is used the fire may burn well while
simmering but will be short of air when operated at
high power. On the other hand, very large channel
gaps will sustain a large fire but unnecessary
amounts of heat will be lost due to poor heat
transfer.

Design  Strategies
The two stove designers approach the problem of
sizing the channel  gap differently. Winiarski in
Rocket Stove Design Principles (1997), advises
technicians to start designing stoves by maintaining
constant cross sectional area throughout the stove.
He sets the cross sectional area at the opening into
the fire, or fuel magazine, and then creates
appropriate gaps around the pots based on
maintaining the same cross sectional area. Baldwin's
method requires a designer to pick a maximum
high power for the stove design. Starting from a
fixed firepower the size of the channel gap is then
determined. In one case, Winiarski chooses the size
of the fuel magazine first while Baldwin uses
firepower as the starting point. The spaces within
the stove are determined by either of these two
primary choices.

                                 m
                                   t
 Figure 19 - The narrow channel close to the
 pot increases convective heat transfer
Figure 20 - Hot flue gases are forced to flow past the
surface of the pots in a narrow channel
                                           17

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Design Principles for Wood Burning Cook Stoves
                                                                     Designing Stoves with Baldwin & Winiarski
Winiarski  Method
The following stove diagram and tables (see pages
19-20) show how the size of the channels near to
the pot or griddle change as the opening into the
fire is expanded.  Dr. Winiarski suggests that a
12 cm by 12 cm opening is usually sufficient for a
family sized cooking stove. Larger openings that
allow more wood into the fire result in higher
power and larger channel gaps.

Establishing the same cross sectional area
everywhere in a cooking stove ensures sufficient
draft for good combustion while resulting in
channel gaps that increase heat transfer efficiency.
This means that the opening into the combustion
chamber, the combustion chamber, the air gap
under the pot or griddle, and the chimney are the
same size (equal number of square centimeters)
while having different shapes. Winiarski advises
designers to create prototype cooking stoves that
maintain the cross sectional area to keep the draft
flowing at an optimal rate.  Slowing down the draft
hurts both combustion and heat transfer efficiency
to the pot.
            Pot
      Pot Skirt
     Short
     insulated
     chimney
     above fire
     Fuel
     Entrance
                 Figure 21 - A Typical Winiarki Stove
                 (Use this diagram along with the calculations found on pages 19-25 to
                 determine proper gap size)
18

-------
Designing Stoves with Baldwin SWiniarski
CROSS SECTIONAL AREA FOR SQUARE COMBUSTION CHAMBERS
Use these tables to create stoves with constant cross sectional area
Table 1
12 cm X 12 cm
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
14 cm X 14 cm
Pot Size (cm)
GAP A (cm)
GAP B (cm)
; GAP C (cm)
GAP D (cm)
16 cm X 16 cm
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
18 cm X 18 cm
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
20 cm X 20 cm
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)

Square Combustion
20
3
2.5
2.3
2.1
Square Combustion
20
3.5
3.1
3.1
2.7
Square Combustion
20
NA
NA
NA
NA
Square Combustion
20
NA
NA
NA
NA
Square Combustion
20
NA
NA
NA
NA

Chamber
30 40
3 3
2.5 2.5
1.5 1.1
1.5 1.1
Chamber
30 40
3.5 3.5
3.1 3.1
2.1 1.6
2 1.5
Chamber
30 40
4 4
3.7 3.7
2.7 2
2.5 1.9
Chamber
30 40
4.5 4.5
4.3 4.3
3.4 2.6
3.1 2.4
Chamber
30 40
5 5
4.9 4.9
4.2 3.2
3.7 3


50
3
2.5
0.9
0.9

50
3.5
3.1
1.2
1.2

50
4
3.7
1.6
1.6

50
4.5
4.3
2.1
2

50
5
4.9
2.5
2.4
                                    19

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  Design Principles for Wood Burning Cook Stoves
CROSS SECTIONAL AREA FOR CIRCULAR COMBUSTION
Table 2


CHAMBERS



12 cm Diameter Circular Combustion Chamber
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
14 cm Diameter Circular
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
16 cm Diameter Circular
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
18 cm Diameter Circular
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
20 cm Diameter Circular
Pot Size (cm)
GAP A (cm)
GAP B (cm)
GAP C (cm)
GAP D (cm)
20
3
2
1.8
1.6
Combustion Chamber
20
3.5
2.4
2.4
2.2
Combustion Chamber
20
NA
NA
NA
NA
Combustion Chamber
20
NA
NA
NA
NA
Combustion Chamber
20
NA
NA
NA
NA
30
3
2
1.2
1.2

30
3.5
2.4
1.6
1.5

30
4
2.9
2.1
2

30
4.5
3.4
2.7
2.5

30
5
3.8
3.3
3
40
3
2
0.9
0.9

40
3.5
2.4
1.2
1.2

40
4
2.9
1.6
1.5

40
4.5
3.4
2
1.9

40
5
3.8
2.5
2.4
50
3
2
0.7
0.7

50
3.5
2.4
0.9
0.9

50
4
2.9
1.3
1.3

50
4.5
3.4
1.6
1.6

50
5
3.8
2
1.9
20

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Design Principles for Wood Burning Cook Stoves	____^_	Designing Stoves with Baldwin & Winiarski

Baldwin:  Firepower Determines Channel Size
As can be seen in the chart below, Baldwin and Winiarski's methods seem to create similar sized gaps. These
values are derived from charts found in Biomass Stoves which summarize Baldwin's findings. The chart is an
approximation meant to serve as a guide to the relationship between firepower, wood use per hour, length
and width of gap size, and stove efficiency.

Table 3 - Baldwin's Suggested Gap Sizes
Wood burned per
hour (kg)
0.50
0.75
1.00
1.25
1.50
1.75
Skirt gap (mm)
8
10
11
12
13
14
Length of gap (cm)
20
20
20
20
20
20
Thermal efficiency
of stove (%)
40
35
30
28
26
25
Firepower (kW)
2.8
4.1
5.5
6.9
8.3
9.6
A typical Winiarski designed stove with a square, 12 cm x 12 cm combustion chamber burns wood at
approximately the rate of 1.5 kg/hr at high power. In his computer program Baldwin uses a 30 cm diameter
pot as "family sized." Given this size of pot, the gap at the perimeter using the Winianski model would be
calculated by dividing the area (A = 12 cm x 12 cm = 144 square cm for a square combustion chamber) by
the perimeter at the edge of the pot (P = pi (d), the circumference, or 3.14 x 30 = 94 cm). The resulting gap
is 144 cm/94 cm = 1.5 cm (15 mm). Following Baldwin's chart, we see that a stove burning wood at a rate
of 1.5 kg/hr. would call  for a gap of 13 mm for maximum efficiency, a difference of 2 mm from Winiarski's
model.
                                                                                            21

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 Design Principles for Wood Burning Cook Stoves     ____ __          Designing Stoves with Baldwin &Winiarski

 Calculations

 To use Wmiarski's method of maintaining a constant cross sectional area under the pot, you will need to
 calculate the correct height of the gap under the pot. This height will vary as you move from the center of
 the combustion chamber out to the edge of the pot. To do this, calculate the needed gap at the edge of the
 combustion chamber and at the edge of the pot. Although this sounds complicated it is relatively straight
 forward. There are 5 steps to make this calculation:

    1.  Determine the area of the combustion chamber, which will be continued throughout the stove. If
        the combustion chamber is cylindrical, the area is calculated using the formula
    where AC is the area, n = 3.14, and r is the radius. The radius is one-half the diameter. If the com-
    bustion chamber is square or rectangular, the area is calculated as

                                           Ac=l-w

    where / is the height and w is the width.

 2.  At the edge of the insulated  chimney above the fire, the gasses turn and follow the bottom of the pot.
    To determine the needed gap at the edge of the combustion chamber, first determine the circumfer-
    ence of the area that the hot gasses will pass through. To do this measure from the center of the
    combustion chamber outlet  to the farthest edge, r . In a circular combustion chamber this will be the
    radius. In a square or rectangular chamber this will be  from the center to one of the corners. Deter-
    mine the circumference associated with this distance. This is

                                           Cc =2-n-rc

3.  Next, divide the cross sectional area, A^ determined in Step  1 by the C determined in Step 2. This i
                                                                                                   s
       where (7 is the needed gap between the bottom of the pot and the top edge of the combustion
       chamber.

    4.  Now determine the optimal gap at the edge of the pot. Measure the circumference, C , of the pot.
       This is the distance all the way around the pot. The circumference can be measured two ways. The
       easiest is to take a piece of string, wrap it around the pot and measure the length of the string.
       Alternately, you can determine the circumference from the diameter, r .
    5.  As in Step 3, divide the cross sectional area, Af determined in Step 1 by the C determined in Step 4
       to calculate the needed gap at the edge of the pot, G . This is
22

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Design Principles for Wood Burning Cook Stoves             	     	    Designing Stoves with Baldwin & Winiarski

As noted above, the area under the pot will need to be slowly decreased moving from the edge of the com-
bustion chamber to the edge of the pot. Careful readers will note that this thinning of the gap is not linear.
However, using the constant area thumb rule as an approximation is the easiest way to handle this. Smoothly
match the gap distance from the edge of the combustion chamber to the edge of the pot by hand in a linear
fashion.

After creating the prototype with a constant cross sectional area, the cooking stove will need to be fine-tuned
by reducing the channel gap while watching the fire at high power. Set the gap as small as possible while
making sure that the draft is sufficient for clean combustion. It is good practice to remember that the stoves
will often be operated at very high power; therefore, the careful designer does not tighten gaps below the
maximum possible firepower. Widening the distance beyond the theoretical best gap also provides some
degree of protection against clogging by products of incomplete combustion.
Example 1
Consider the case of a stove with a cylindrical combustion chamber 12 cm in diameter with a 30 cm
cooking pot.

The first step is to calculate the cross sectional area of the combustion chamber. This is
Next calculate the gap needed at the edge of the combustion chamber. First we find the circumference of the
area that the hot gasses will pass through. This is

                                    Cc =2-n -6 = 71 -12 = 37.7cm

From this you can find the needed gap at the edge of the combustion chamber as
                                       Gc =113/37.7 = 3.
 If this space were only two centimeters high, the cross sectional area at Gap A would only be 75.4 cm2,
 reducing the draft and increasing the production of smoke. If the space at Gap A were 5 centimeters, the
 cross sectional area would be 188.5 cm2. This area is so large that even though flow rate is maintained, the
 velocity of hot gases is decreased and gases are not forced to scrape against the pot and so cannot effectively
 deliver their energy to it.

 At the edge of the pot, the circumference that the hot gasses need to pass through is

                                   C/,=2-n-15=7c-30 = 94.4cm

 The needed gap at the edge of the pot is
                                        G= 113/94.4 = 1.2cm
 We need to remember that this is an approximation and that the gap will need to be field tuned at the
 highest power setting of the stove. In addition, we will need to smoothly thin the gap from 3.0 cm at the
 edge of the combustion chamber to 1.2 cm at the edge of the pot.
                                                                                                 23

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 Design Principles for Wood Burning Cook Stoves


 Example 2

 Often it is less expensive to build square or rectangular combustion chambers. Consider the case of a 12 cm
 x 10 cm rectangular combustion chamber with a 30 cm diameter cooking pot.

 The first step is to calculate the cross sectional area of the combustion chamber. This is

                                       Ac =12-10 = 120 cm2

 Next we calculate the gap needed at the edge of the combustion chamber. First we find the circumference of
 the area that the hot gasses will pass through. The distance from the center of the combustion chamber to the
corner is
                                  = -Vl02-122 =-V244 = 7.
We know this looks complicated but remember in the field you will be using a tape measure not geometry.
With this we can find the circumference of the area at that point. This is

                                 Cc=2-Tt -7.8-Ti -15.6 = 48.0cm

From this we can find the needed gap at the edge of the combustion chamber
                                      Gc= 113/48.0 -2.4 cm
At the edge of the pot, the circumference that the hot gasses need to pass through is

                                  Cp = 2-7i -15 = 71 -30 = 94.4cm

The needed gap at the edge of the pot is
                                      G  = 113/94.4 = 1.2 cm
Again we need to remember that this is an approximation and that the gap will need to be field tuned at the
highest power setting of the stove. In addition, we will need to smoothly thin the gap from 2.4 cm at the
edge of the combustion chamber to 1.2 cm at the edge of the pot.
Example 3

Another application of the constant area thumb rule is determining the gap needed between the pot and an
insulated pot skirt. An insulated pot skirt is a band of metal insulated on the outside that goes around the
cook pot, forcing the hot gases to run along the sides of the pot. Consider the cook stove with the 12 cm
cylindrical combustion chamber  and the 30 cm pot examined in Example 1.

To calculate the gap between the pot and the skirt along the side walls, or Gap D in the diagram on page 18,
start with the area of the cooking chamber found in Example 1.

                                    A = n-62=7C'36
24

-------
Design Principles for Wood Burning Cook Stoves
Divide this by the circumference around the pot.
The gap needed becomes
                                  G w,= ,4/C= 113/94.4 = 1.2 cm
                                    skirt    c/  p      /
Note that this is the same gap as between the edge of the pot and stove surface. Also the careful reader will
have noted that this is an approximation. But it is a very good approximation. Also remember that this is
only a starting point and should be tuned at the high power setting in the field.


Conclusions
Both Winiarskis and Baldwins methods result in workable solutions that seem to be closely related.
Creating small channels to increase heat transfer efficiency is a common strategy engineers use to optimize
heat transfer. Applying the practice to cooking stoves has been shown to effectively improve fuel efficiency.
Even an open fire is often 90% efficient at the work of turning wood into heat. But only a small
proportion, from 10% to 40% of the released heat makes it into the pot. Improving combustion efficiency
can have little appreciable effect on overall system efficiency; i.e., decreased fuel use. On the other hand,
improving heat transfer efficiency to the pot can make a large difference, saving significant amounts of
firewood.
Stoves have to use gaps that are large enough to support the airflow at high power. Much less firepower is
required to simmer food. But the efficiency of heat transfer suffers because the channels  are larger than
needed at this reduced rate of flow. For this reason, without adjustable gaps, stoves tend to display better heat
transfer efficiency at high power. A pot skirt with adjustable gaps solves this problem.

It is interesting that Baldwin was impressed by the improvements made possible by placing a short insulated
chimney above the fire, which is the defining characteristic of Winiarski's Rocket stove.  By reconfiguring the
combustion chamber in this way Baldwin reports an increase in velocity of hot flue gases due to the height
of the chimney, which results in clean burning and good fuel efficiency (Page 43, Biomass Stoves). In practice
installing a short insulated chimney above the fire seems to help clean up combustion. Forcing the cleaner
hot flue gasses to scrape against the pot or griddle in narrow spaces can increase heat transfer efficiency
without significantly increasing harmful emissions.
                                                                                                  25

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 Design Principles for Wood Burning Cook Stoves
                                                                         Options for Combustion Chambers
 Chapter 4
 Options  for  Combustion  Chambers*
 Multiple tests of the sand and clay Lorena stove,
 beginning in 1983, showed that placing materials
 with high thermal mass near the fire can have a
 negative effect on the responsiveness, fuel
 efficiency, and emissions of a cooking stove because
 they absorb the heat from the fire. Examples of
 high thermal mass materials are mud, sand, and
 clay. When stoves are built from high thermal mass
 materials, their efficiency (when tested in the
 laboratory) can be worse than that of the three-
 stone fire.

 So what other materials can be used? Cleaner
 burning stoves can produce such high temperatures
 in the combustion chamber (where the fire burns)
 that metal, even stainless steel, can be destroyed.
 Cast iron combustion chambers, though longer
 lasting,  are expensive.

 While mud, sand, and clay are high in thermal
 mass, they do have certain benefits. They are
 locally available, cheap, easy to work with, and are
 often long lasting because they don't burn out
 under the intense heat produced by a fire.
 Creativity and good engineering allow a stove
 designer to use these materials advantageously
 without allowing their high thermal mass to
 degrade the quality of the stove.

 Stove makers have been using ceramic parts for
 many years. The Thai Bucket Stove uses a ceramic
 combustion chamber. The Kenyan Jiko Stove also
 uses  a ceramic liner to protect  the sheet metal stove
 body. Books have been written describing how to
 make clay combustion chambers that will last for
 several years.** A women's co-operative in
 Honduras called Nueva Esperansa makes long-
 lasting refractory ceramic stove parts from a
 mixture  of clay, sand, horse manure, and tree gum.
 These combustion chambers are  used in the Dona
Justa and Eco Stoves now popular in Central
America.
The benefit to using ceramic combustion chambers
in these instances is their longevity. As we shall see
in the example below, the key to minimizing the
drawback of ceramic material, which is its high
thermal mass, is to use the least amount possible
without compromising its strength and by
surrounding it with an insulative material.

Option #1: Floor Tiles
Don O'Neal (HELPS International) and Dr.
Winiarski located an alternative material in
Guatemala, an inexpensive ceramic floor tile called
a baldosa. The baldosa is about an inch thick and
can be cut or molded into appropriate shapes to
make a combustion chamber. Loose insulation fills
in between the combustion chamber and the inside
of the stove body. Wood ash, pumice rock,
vermiculite, and perlite are all good natural heat
resistant sources of loose insulation. The baldosa is
inexpensive and has lasted four years in the
insulated HELPS and Trees, Water and People
stoves built in Central America.
Figure 22 -  Ceramic Floor Tile
* First published in Boiling Point #49
"A good book on the subject is The Kenya Ceramic Jiko: A Manual for Stovemaken (Hugh A\kri, 1991).

26

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Design Principles for Wood Burning Cook Stoves
                       Options for Combustion Chambers
The baldosa floor tile is tested by placing it in a fire
until it is red hot. Then the tile is removed and
quickly dipped into a bucket of cold water. If the
tile doesn't crack, it will probably last in the
combustion chamber. Baldosa are usually made
with red clay and are fired in a kiln at around 900°
- 1000°C. They are somewhat porous and ring
when struck with a knuckle. Using baldosa in a
combustion chamber surrounded by loose
insulation adds one more material option for the
stove designer.

Option  #2: Insulative
Ceramics
These recipes are intended to assist stove promoters
in making insulative ceramics for use in improved
wood burning cook stoves. Each of these materials
incorporates clay, which acts as a binder. The clay
forms a matrix around a filler, which provides
insulation. The filler can be a lightweight fireproof
material (such as pumice, perlite, or vermiculite),
or an organic material (charcoal or sawdust). The
organic material burns away, leaving insulative air
spaces in the clay matrix. In all cases, the clay and
filler  are mixed with a predetermined amount of
water and pressed into forms (molds) to create
bricks. The damp bricks are allowed to dry, which
may take several weeks, and then fired at
temperatures commonly obtained in pottery or
brick kilns in Central America.

Our test samples were made using low-fired "raku"
clay obtained from a local potter's supply store. In

Table 4 - Insulative Ceramics
other countries, the best source of clay would be
the kind used by local potters or brick makers.
Almost everywhere, people have discovered clay
mixes and firing techniques, which create sturdy
ceramics. Insulative ceramics need to be lightweight
(low density) to provide insulation and low thermal
mass. At the same time, they need to be physically
durable to resist breakage and abrasion due to
wood being forced into the back of the stove.
These two requirements are in opposition; adding
more filler to the mix will make the brick lighter
and more insulative, but will also make  it weaker.
Adding clay will usually increase strength but
makes the brick heavier. We feel that a good
compromise is achieved in a brick having a density
between 0.8 gm/cc and 0.4 gm/cc.

The recipes in Table 4 indicate the proportions, by
weight, of various materials. We recommend these
recipes as a starting point for making insulative
ceramics. Variations in locally available clays and
fillers will probably require adjusting these
proportions to obtain the most desirable results.

Insulative ceramics used in stoves undergo
repeated heating and cooling (thermal cycling),
which may eventually produce tiny cracks that
cause the material to crumble or break. All of these
recipes seem to hold up well to thermal cycling.
The only true test, however, is to install them in a
stove and use them for a long period of time under
actual cooking  conditions.
Type
Sawdust
Charcoal
Vermiculite
Perlite Mix
Pumice Mix
Filler
Wt. (Grams)
490
500
300
807
1013
Clay (damp)
Wt. (Grams)
900
900
900
900
480
Water
Wt. (Grams)
1300
800
740
1833
750
Fired at
(degrees C)
1050
1050
1050
1050
950
Density
gr/cc
0.426
0.671
0.732
0.612
0.770
                                                                                                 27

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 Design Principles for Wood Burning Cook Stoves
                                                                            Options for Combustion Chambers
 Sawdust/Clay:
 In this formulation, fine sawdust was obtained by
 running coarse sawdust (from a construction site)
 through a #8 (2.36-mm) screen. Clay was added to
 the water and mixed by hand to form thick mud.
 Sawdust was then added, and the resulting material
 was pressed into rectangular molds. Excellent
 insulative ceramics can be made using sawdust or
 other fine organic materials such as ground
 coconut husks or horse manure. The problem with
 this method is obtaining large volumes of suitable
 material for a commercial operation. Crop residues
 can be very difficult to break down into particles
 small enough to use in brick makine.
          o                     t>

 This method would be a good approach in
 locations where there are sawmills or woodworking
                                              t>
 shops that produce large amounts of waste sawdust.

 Charcoal/Clay:
 In this formulation,  raw charcoal (not briquettes)
 was reduced to a fine powder using a hammer and
 grinder. The resulting powder was passed through a
 #8 screen. Clay was hand mixed into water and the
 charcoal was added last. A rather runny slurry was
 poured into molds and allowed to dry. It was
 necessary to wait several days before the material
 dried enough that the mold could be removed.
 Dried bricks were fired at  1050°C. Charcoal can be
 found virtually everywhere, and can be used when
 and where other filler materials are not available.
 Charcoal is much easier to reduce in size than other
 organic materials. Most of the charcoal will burn
 out of the matrix of the brick. Any charcoal that
 remains is both lightweight and insulative.

 Charcoal/clay bricks tend to shrink more than
 other materials during both drying and firing.
The final product seems to be lightweight and
 fairly durable, although full tests have not yet
 been run on this material.
 Vermiculite/Clay:
 In this formulation, commercial vermiculite (a soil
 additive), which can pass easily through a #8 (2.36
 mm) screen, is mixed directly with water and clay
 and pressed into molds. Material is dried and fired
 at 1050°C.

 Vermiculite is a lightweight, cheap, fireproof
 material produced from natural mineral deposits in
 many parts of the world. It can be made into
 strong, lightweight insulative ceramics with very
 little effort. The flat, plate-like structure of
 vermiculite particles makes them both strong and
 very resistant to heat.

 Vermiculite appears to be one of the best possible
 choices for making insulative ceramics.

 Perlite Mix/Clay:
 For best results, perlite must be made into a
 graded mix before it can be combined with
 clay to form a brick. To prepare this mix,  first
 separate the raw perlite into three component
 sizes: 3/81 to #4 (9.5 mm to 4.75 mm), #4 to #8
 (4.75 mm to 2.36 mm), and #8 (2.36 mm and
 finer). Recombine (by volume) two parts  of the
 largest size, one part of the midsize, and seven parts
 of the smallest size to form the perlite mix. This
 mix can now be combined with clay and water and
 formed into a brick, which is dried and fired.

 Perlite is the mineral obsidian, which has been
heated up until it expands and becomes light. It is
 used as a soil additive and insulating material.
 Perlite mineral deposits occur in  many countries of
the world, but the expanded product is only
available in countries that have commercial
"expanding" plants. Where it is available, it is both
inexpensive and plentiful.

Perlite/clay bricks are some of the lightest usable
ceramic materials we have produced so far.
28

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Design Principles for Wood Burning Cook Stoves
                        Options for Combustion Chambers
Pumice Mix/Clay:
Pumice, like perlite, produces the best results when
it is made into a graded mix. Care should
be taken to obtain the lightest possible pumice for
the mix. Naturally occurring volcanic sand,
which is often found with pumice, may be quite
heavy and unsuitable for use in insulative
ceramics. It may be necessary to crush down larger
pieces of pumice to obtain the necessary
small sizes. The mix is prepared by separating
pumice into three sizes: 0.5 inch to #4 (12.5 mm
to 4.75 mm), #4 to #8 (4.75 mm to 2.36 mm),
and #8 (2.36 mm) and smaller. In this case, the
components are recombined (by volume) in the
proportion of two parts of the largest size, one
part of the midsize, and four parts of the smallest
size. Clay is added to water and mixed to form
thin mud. The pumice mix is then added and the
material is pressed into molds. Considerable
tamping or pressing may be necessary to work out
the air and form a solid brick. The mold can be
removed immediately and the brick allowed to dry
for several days before firing.
Pumice is widely available in many parts of the
world and is cheap and abundant. Close attention
to quality control is required, and this could be a
problem in many locations. It is very easy to turn a
lightweight insulative brick into a heavy non-
 O     D                           '
insulating one through inattention to detail.
Pumice (and perlite as well) is sensitive to high heat
(above 1100°C). Over-firing will cause the pumice
particles to shrink and turn red, resulting in an
inferior product. Despite these concerns, pumice
provides a great opportunity to supply large
numbers of very inexpensive insulative ceramics in
many areas of the world.

There are many viable recipes to make lightweight
refractory ceramic combustion chambers. Using
insulation around the fire helps to boil water more
quickly, makes the stove easier to light, and saves
firewood. It is necessary to create very high
temperatures in a combustion chamber in order to
clean up dangerous emissions. Unfortunately these
high temperatures quickly degrade metals,
including stainless steel. Refractory insulative
ceramics provide a material that is both long lasting
and does not lower combustion temperatures as do
materials with a higher thermal mass.
                                                                                                   29

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 Design Principles for Wood Burning Cook Stoves
                                                                               In Field Water Boiling Test
 Chapter 5
 In  Field  Water  Boiling  Test  (WBT)
This test provides the stove designer with reliable
information about the performance of wood
burning stove models. The test consists of three
phases that determine the stoves ability to:
(1) bring water to a boil from a cold start;
(2) bring water to a boil when the stove is hot; and,
(3) maintain the water at simmering temperatures.
It is used to evaluate a series of stoves as they are
being developed. The test cannot be used to
compare stoves from different places because the
different pots and wood used change the results.

The test is a simplified version of the University of
California Berkeley (UCB)/Shell Foundation
revision of the 1985 VITA International Standard
Water Boiling Test. The wood used for boiling and
simmering, and the time to boil are found by
simple subtraction. All calculation can be done by
hand in the field.

By using a standard pot, taking into account the
moisture content of the wood, steam generated and
other factors the complete UCB/Shell  Foundation
Water Boiling Test makes comparison of stoves
from different places possible.

Before starting the tests...

1. Collect at least 30 kg of air-dried fuel for each
   stove to be tested in order to ensure that there
   is enough fuel to complete three tests for each
   stove. Massive multi-pot stoves may require
   more fuel. Use equally dry wood that is the
   same size. Do not use green wood.

2. Put 5 liters of water in the testing pot and bring
   it to a rolling boil. Make sure that the fire is very
   powerful, and that the water is furiously boiling!
   Use an accurate digital thermometer, accurate to
   1/10 of a degree, to measure the local boiling
   temperature. Put the thermometer probe  in the
   center of the testing pot, 5 cm above the pot
   bottom. Record the local boiling point on the
   data sheet (see page 34).
3. Do the tests in a place that is completely
   protected from the wind.

4. Record all results on the data sheet.
  Equipment used for the In Field
  Water Boiling Test:

  •  Scale of at least 6 kg capacity and 1 gram
     accuracy

  •  Heat resistant pad to protect scale
  •  Digital thermometer, accurate to 1/10 of a
     degree, with thermocouple probes that can
     be in liquids
  •  Timer
  •  Testing pot(s)

  •  Wood  fixture  for holding thermometer
     probe in water

  •  Small shovel/spatula to remove charcoal
     from stove
  •  Tongs for  handling charcoal
  •  Dust pan for transferring charcoal
  •  Metal tray to  hold charcoal for weighing
  •  Heat resistant gloves
  •  3 bundles of air-dried fuel  wood. One, used
     for simmering, weighs around 5 kgs. The
     other two  bundles, used for cold and hot
     start boiling, weigh about 2 kgs each.
30

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Design Principles for Wood Burning Cook Stoves
                                                                                In Field Water Boiling Test
Beginning  of Test

a.  Record the air temperature.

b.  Record weight of commonly used pot without
   lid. If more than one pot is used, record the
   weight of each pot. If the weights differ, be sure
   not to confuse the pots as the test proceeds. Do
   not use pot lids for this, or any other phase of
   theWBT.

c.  Record weight of container for charcoal.

d. Prepare 2 bundles of fuel wood that weigh about
   2 kgs each for the cold and hot start high power
   tests. Prepare 1 bundle of fuel wood that weighs
   about 5 kgs to be used in the simmering test.
   Use sticks of wood roughly the same size for all
   tests. Record approximate dimensions of the
   fuel wood. Weigh and Record weights in spaces
   marked # on the  attached data sheet. Identify
   each bundle and keep them  separate.

High Power (Cold Start) Phase:
The stove should be at room temperature.

 1. Fill each pot with 5 L of clean water (-20°).
   Record the weight of pot(s)  plus the water.

2. Using the wooden fixtures, place a thermometer
   probe in each pot so that water temperature may
   be measured in the center, 5 cm from the
   bottom. Make sure a digital thermometer is
   used. Record water temperatures.

 3. Record the weight of the starting materials.
   Always use the same amount and material.

 4. Start the fire using the wood from the first 2 kg
   bundle.

 5. Once the fire has caught, start the timer and
   Record "0". If using a watch Record the starting
   time. Bring the first pot rapidly to a boil
   without being excessively wasteful of fuel.
6. When the water in the first pot reaches the local
  boiling temperature as shown by the digital
  thermometer, rapidly do the following:
  a. Record the time at which the water in the
     primary pot (Pot #1) reaches the local
     boiling point of water. Record the water
     temperature for other pots as well.
  b. Remove all wood from the stove and put out
     the flames. Knock all loose charcoal from the
     ends of the wood into the tray for weighing
     charcoal.
   c. Weigh the unburned wood from the stove
     together with the remaining wood from the
     pre-weighed bundle. Record the result.
   d. Weigh each pot, with its water. Record
     weight.
   e. Remove all the charcoal from the stove, place
     it with the charcoal that was  knocked off the
     sticks and weigh it. Record the weight of the
     charcoal and container.

This completes the high power (cold start) phase.
Continue without pause to the high power (hot
start) portion of the test. Do not allow the stove to
cool.

High Power (Hot Start) Phase

1. Refill the pot(s) with 5 L of fresh cold water.
   Weigh pot(s) (with water) and measure the
   initial water temperatures; Record both
   measurements.

2. Start the fire using kindling and wood from the
   second 2 kg bundle. Record weight of any
   additional starting materials.

3. Record the time when the fire starts and bring
   the first pot rapidly to a boil without being
   excessively wasteful  of fuel.

4. Record the time at which the first pot reaches
   the local boiling point. Record the temperature
   of all pots.
                                                                                                31

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 Design Principles for Wood Burning Cook Stoves
                                                                                 In Field Water Boiling Test
 5. After reaching the boiling temperature, rapidly
    do the following:
    a. Remove all wood from the stove and knock
      off any loose charcoal into the charcoal
      container. Weigh the wood removed from
      the stove, together with the unused wood
      from the second bundle. Record the result.
    b. Weigh each pot, with its water and Record
      these weights.

 6. Remove all remaining charcoal from the stove
    and weigh it (including charcoal which was
    knocked off the sticks). Record the weight of
    the charcoal plus container.

 Without pause, proceed directly with the
 simmering test.

 Low Power (Simmering) Test
 This phase is designed to test the ability of the stove
 to simmer water using as little wood as possible.
 Use the 5 kg bundle of wood to bring the water to
 boil. Then record the weight of the remaining
 wood and simmer the water for an additional 45
 minutes.

 Only the primary pot will be tested for
 simmering performance.

 Start of Low  Power test:

 1. Record the weight  of the 5 kg bundle of fuel.

 2. Refill the pot with 5 L of cold water. Weigh the
   pot (with water). Record weight. Record
   temperature.

 3. Rekindle the fire using kindling and wood from
   the weighed bundle. Record the weight of any
   additional starting materials. Replace the pot on
   the stove and Record the start time when the
   fire starts.
 4. Bring the pot rapidly to a boil without being
   excessively wasteful of fuel. As soon as local
   boiling temperature is reached, do the following
   steps quickly and carefully:

 5. Record the boiling time and temperature.
   Quickly weigh the water in the primary pot and
   return it to the stove. Record the weight of the
   pot with water. Record the weight of remaining
   wood in 5 kg bundle. Replace the thermometer
   in the pot and continue with the simmer test by
   reducing the fire. Keep the water as close to 3°C
   below the boiling point as possible.

 6. Record temperature of the water.

 7. Record the time. For the next 45 minutes
   maintain the fire at a level that keeps the water
   temperature as close as possible to 3°C below the
   boiling point.

 8. After 45 minutes rapidly do the following:

   a.  Record the finish time of the test (this should
      be 45 minutes).

   b.  Record the temperature of the water at end
      of test.

   c.  Remove all wood from the stove and knock
      any loose charcoal into the charcoal weighing
        '                                 O    O
      pan. Weigh the remaining wood, including
      the unused wood from the preweighed
      bundle. Record the weight of wood.
   d.  Weigh the pot with the remaining water.
      Record the weight.

   e.  Extract all remaining charcoal from the stove
      and weigh it (including charcoal which was
      knocked off the sticks). Record the weight of
      pan plus charcoal.

This completes the full water boiling test. The full
test should be done at least three times for each
stove for accurate results.
32

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Design Principles for Wood Burning Cook Stoves
                                 Analysis of Results
  It is ok if temperatures vary up and down, but:
  1. The tester must try to keep the simmering water as close as possible to 3°C below the local
     boiling point.
  2. The test is invalid if the temperature in the pot drops more than 6°C below the boiling
     temperature.
  3. The tester should not further split the fuel wood into smaller pieces to try to reduce power.
ANALYSIS  of RESULTS:

•  Figure out the time to boil for cold start, hot
   start, and for the boiling phase of the simmer
   test.

•  Calculate the wood use by subtracting the wood
   left at the end of each phase from the starting
   weight. Do this for cold start high power, hot
   start high power, boiling phase of the simmer
   test, and simmering.

•  Calculate the water lost to steam for each of the
   four phases by subtracting the remaining weight
   from the starting weight of the water.

•  Do the same for the charcoal produced.

•  Use these numbers to evaluate stove
   performance. Change the stove design to reduce
   wood use and to create less charcoal. Making a
   lot of charcoal indicates poor combustion.

•  Calculating the steam lost is a valuable method
   to check that performance is similar in all
   phases. Usually the hot start high power phase
   uses substantially less fuel, and time to boil is
   faster compared to the cold start high power
   phase. If there are  significant differences
   between the recorded weights for wood use,
   time to boil, and steam lost between phase 2 and
   3 it is recommended to repeat the testing
   procedure being careful to feed the fire without
   as much variation.
•  Steam lost during the simmering phase is also a
   good indicator of the stoves ability to perform
   well during low power use. It is difficult to
   design a stove that can boil water quickly and
   simmer well without using a lot of fuel.
   However, since the majority of cooking time
   often occurs at low power (simmering),  the
   greatest fuel savings can be made with a stove
   that saves fuel during this time. Producing large
   amounts of steam while simmering is an
   indicator that the stove is having a difficult time
   transitioning from the high power needed to
   boil water quickly to the low power needed for
   simmering food efficiently. Try changing the
   design so that the stove easily maintains a low
   simmer while keeping cooks happy with rapid
   boiling.

Remember that results from this test cannot be
used to compare stoves tested in other places. The
complete UCB/Shell Foundation test should be
used for those purposes.

For more information, visit Aprovecho's web site at
www.Aprovecho.net or contact us at:
Aprovecho Research Center
80574 Hazelton Rd.
Cottage Grove, OR 97424
(541)942-8198
                                                                                               33

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to
     Data Sheet
     DATE
TEST NUMBER
                                                                             STOVE  |    |
     local boiling point

     air temperature

     wood dimensions

     weight pot one

     weight pot two

     weight charcoal container
notes:    IN FIELD WBT DATA AND CALCULATION SHEET
All spaces should be filled in.
Results from TWO and THREE should be similar.
Better stoves use less wood and make less charcoal.
Rapid boiling is usually appreciated by cooks.
        BUNDLE 1 - 2kg  cold start hi power  2 - 2kg hot start hi power 3 - 5kg bring to boil
                        begin     end            begin    end          beain   en
     time
     weight wood
     water temp pot one
     water temp pot two
weight pot one plus water  [N
     weight pot two plus water
     weight fire starter
     weight charcoal and container
A
B

#G
H







N
O




r


U
c
D

#l
J







p
Q







V
                                                                          begin    end end
                                       4  simmer 45 minutes
                                          begin	end
                     #K
                                                                                                    M
                                                                          R
                                                    T
                                                    w

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Design Principles for Wood Burning Cook Stoves	_	Calculation Sheet


Calculation Sheet


Time to Boil:

	 = B - A = Time to boil for cold start hi power phase

	 = D - C = Time to boil for hot start hi power phase

	= F - E = Time to boil for boiling phase of simmering


Wood Use:

	= G - H = Wood use for cold start hi power phase

	= I - J = Wood use for hot start hi power phase

	= K - L = Wood use for boiling phase of simmering

	= L - M = Wood use for simmering phase


Water Converted to Steam:

	= N - 0 = Water lost to steam for cold start hi power phase

	= p - Q = Water lost to steam for hot start hi power phase

	= R - S = Water lost to steam for boiling phase of simmering

	= S - T = Water lost to steam during simmering phase


Charcoal Created:

	= U - Y = Charcoal made in cold start hi power phase

	= V - Y = Charcoal made in hot start hi power phase

	= W - V = Charcoal either made or consumed during the simmering phase.
                  (If this number is positive, then additional charcoal was created during
                  simmering, and if negative, then charcoal was consumed during the
                  simmering phase.)
                                                                                   35

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Design Principles for Wood Burning Cook Stoves
                          Appendix: Glossary of Terms
Appendix
Glossary  of  Terms
Baldosa—Inexpensive ceramic floor tile about one
    inch thick that can be cut or molded into
    appropriate shapes to make a combustion
    chamber.

Boundary layer—The very thin layer of slow
    moving air immediately adjacent to a pot
    surface; insulates the pot from hot flue gases
    and diminishes the amount of heat that enters
    the pot.

Charcoal—The black, porous material that
    contains mostly carbon that is produced by
    burning of wood or other biomass.

Convection—The heat transfer in a gas or liquid
    by movement of the air or water.

Combustion chamber—The region of the stove
    where the fuel is burned.

Combustion efficiency—The percentage of the
    fuel's heat energy that is released during
    combustion. Combustion efficiency refers to
    the amount of the energy from the biomass
    that is turned into heat energy.

Draft—The movement of air through a stove and
    up a chimney.

Emissions—The byproducts from the combustion
    process that are discharged into the air.

Excess air—The amount of air used in excess of
    the amount for complete combustion.

Firepower—The rate of fuel consumption, usually
    in- kg-fuel per hour.
Flue Gas—The hot gases that flow from the
   combustion chamber and out the chimney (if a
   chimney is present).

Fuel efficiency—The percentage of the fuel's heat
   energy that is utilized to heat food or water.

Grate—A framework of bars or mesh used to hold
   fuel or food in a stove, furnace, or fireplace.

Haybox—A relatively airtight insulated enclosure
   that maintains the temperature of the pot
   enabling food to be cooked to completion after
   the pot is removed from the stove.

Heat transfer efficiency—The percentage of heat
   released from combustion that enters a pot.

High mass stove—A stove made of uninsulated
   earth, clay, cast iron, or other heavy material
   that requires significant energy to be warmed
   during stove operation.

High power—A mode of stove operation where
   the objective is to boil water as quickly as
   possible; the highest power at which a stove can
   operate.

Low power—A mode of stove operation where the
   objective is to simmer the water or food
   product; the lowest power at which a stove can
   operate and still maintain a flame and simmer
   food.

Pot skirt—A tube, usually made of sheet steel, that
   surrounds a pot creating a narrow space so that
   more of the heat in the flue gases enter the pot.

Retained heat—Heat energy that warms the
   enclosures around the fire that does not escape
   to the surroundings; can be used for space
   heating.
                                                                                              37

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 Design Principles for Wood Burning Cook Stoves
                                                                                   Appendix: Glossary of Terms
 Vermiculite—A lightweight, cheap, fireproof
    material produced from natural mineral
    deposits in many parts of the world.
    Vermiculite can be made into strong,
    lightweight, insulative ceramics with very little
    effort. It is very strong and resistant to heat,
    and appears to be one of the best possible
    choices for making insulative ceramics.
Water Boiling Test (WBT)—A test used to
   measure the overall performance of a
   cookstove. There are several versions of the
   water boiling test. In general the test consists of
   three phases. These are: (1) bringing water to a
   boil from a cold start;  (2) bringing water to a
   boil when the stove is hot; and, (3) maintaining
   the water at simmering temperatures.
38

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Aprovecho Research Center
 Advanced Studies m Appropriate Tcchnologv
SHELL
FOUNDATION
vvEPA
L             United States
             Environmental Protection
             Agency
Office of Air & Radiation    EPA-402-K-05-004
(6609J)

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