Wood fired cocoa dryer (Nica)
In two tests of 125
pounds of apples and
tomatoes, the dryer used
one pound of wood to dry
one pound of wet
produce.The dryer uses
about 10 pounds of wood
per hour to keep at 130F
a space 4' by 10' by 4' full
of screens of sliced fruit,
etc. Full of cacao is
about 500 pounds.
The Dr Winiarski Rocket Stove
Insulated low mass
combustion chamber
Internal shelf allows sticks to
form a grate. Stick/air/stick/air
Small amount of high velocity
air is drawn under the coals
and the wood „grate‟ which
improves air to fuel mixture
Stove power is controlled by
regulating the fuel supply not
the air intake
Horizontal feed chamber is
convenient
X
Basic Rocket
Stove Geometry
1.5-2X
Total Height =
X + (1.5Xor2X) + 5 cm
X
Min 2x
“Whenever possible, insulate
around the fire using lightweight,
heat-resistant materials.”
Insulation around the fire keeps
it hot which reduces smoke
Insulation around the fire keeps
the heat from going into the
stove body instead of pot
Insulation is light and full of
small pockets of air
Ten Design Principles: Principle
One
If possible, avoid heavy
materials like sand, clay, and
cement.
Metal is better than heavy
material above (less thermal
mass) but does not last very
long near hot fire.
Heat resistant insulated
material are best
Ten Design Principles: Principle
Two
“Place an insulated short chimney right above the
fire.”
a short insulated chimney right above fire
forces mixing of flames and smoke which
burns up the harmful smoke.
The short chimney above the fire increases
the speed of the air drawn into the fire which
helps the fire to burn hotter.
Forcing the hot gases to scrape past the pot
at a high speed helps to heat up the food
“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.)
Low Heat High Heat
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.
The wind passing through
the coals helps to raise
the temperature of the fire
so that all the gases
become flame.
Ten Design Principles: Principle
Five
“Maintain a good fast draft
through the burning fuel.”
The air should be aimed at
the coals and not above the
sticks into the flame.
The wind that is drawn into
the coals heats up the fire.
Blowing air into the flames
can do the opposite and
cool the fire.
“Too little draft being pulled into
the fire will result in smoke
and excess charcoal.”
If a lot of charcoal is being
made by the fire then there
is too little air entering the
combustion chamber.
A fire that makes a lot of
charcoal is producing too
much harmful carbon
monoxide.
A hot clean burning fire will
not make much charcoal as
it is being used. Make sure
that enough air is freely
flowing under the fire into
the coals
Optimising heat transfer
3 rules for maximizing heat
transfer:
Maximize the surface area
where the hot flue gases
touch the pot
Maximize velocity of hot
flue gases to disturb
boundary layer around the
pot
Maximize temperature
difference between the hot
flue gases and the pot ( i.e.
make a high temp fire - With a heat exchanger,
1000C overall efficiency can be
improved by 50% or more
Rocket stove heat exchanger/skirt
Ensure the correct gap
between the pot and the
stove body (for average
household pots, 7-10 mm is
good rule of thumb)
Make the skirt as tall as
feasibly possible
•For a 10cm long
channel, the channel
efficiency drops from
46% for an 8mm gap to
26% for a 10mm gap.
“Maximize heat transfer to the pot with properly
sized gaps.”
Getting heat into pots or griddles is best done
with small channels.
gap too large: hot flue gases mostly stay in the middle
of the channel and do not pass their heat to the desired
cooking surface.
Gap too small: the draft diminishes, causing the fire to
be cooler, the emissions to go up, and less heat to enter
the pot.
“Maximize heat transfer to the pot with properly
sized gaps.”
Optimal Gap:
size of channel estimated by keeping the cross
sectional area constant
Using trial and error, start with a small gap, and
increase little by little till fire stays hot and vigorous
Gap ‘D’
Gap‘A’
Gap‘ B’
Gap ‘C’
Gap ‘D’
Gap‘A’
Gap‘ B’
Gap ‘C’
To calculate gap A ( between the top of the combustion gap A = Area of feed chamber
chamber and the pot) Perimeter of feed chamber
To calculate gap B ( between the pot and the gap B = Area of feed chamber
outer edge of the combustion chamber) Perimeter of outer feed chamber =
[(D+10 cm] *3.14)
To calculate gap C (under the outer edge of the pot gap C = Area of feed chamber
and the stove body) Circumference Pot
To calculate gap D (between the sides of the pot gap D = gap C * 0.75
and the stove body)
Rocket stoves with chimneys
Force heat to rub against heat exchanger
Insulate all parts of the stove body
Keep exit temperatures low (around 180)
Chimney can be ½ area of the stove entrance
Tapering the manifold
Exit temps should be 150-180 C
If lower what happens?
Troubleshooting a Stove
Too much wood
Smoke in combustion
• Encourage
users to reduce
coming chamber
fuel
The wood is wet
out of the The gap is too • Encourage
small between pot users to store
top of the and stove body fuel for drying
stove? which is resulting
in a reduction of • Check the pot
air flow through gaps
the system
Troubleshooting a Stove
• Smoke •The gap between the pot
• Check the pot
coming and the sides of stove are
too too small or uneven gaps
out of the •The gap between the • Clean
bottom of the pot and the combustion
front of combustion chamber is too chamber
the stove? small
•The combustion chamber
is clogged with ash
Troubleshooting a Stove
• Flames •The gap between the pot
• We want a
coming and the sides of stove are
too too small or uneven vigorous draft
out of the •The gap between the that draws the
bottom of the pot and the flames up into
front of combustion chamber is too the combustion
the stove? small zone
•The combustion chamber
is clogged with ash
Troubleshooting a Stove
Things that can go wrong with a stove
• Slow •Is the stove wet? A wet
• Keep the stove
cooking stove will produce a lot of
smoke and take a long indoors when
times time to heat up . not in use
•Black soot will appear on • Dry the stove
the inside of the bricks if before testing it
the stove is wet or damp
• Check gaps
•The gap could be wrong
(too big or too small)
Improvements for the bread oven
Rocket Stove Combustion
Chamber options for Lesotho
Material and Design options
X
Hard fired bricks used at entrance
and at the back of the combustion
chamber . Ground and graded
Pumice (use excel guide) and
cement fondue at other points 1.5-2X
Mortar firedbricks together
with a mixture of 20% cement
fondue and 80% < 1mm ground
pumice
Use min water for max.
strength
Thinnest joint possible 3-5
mm Cures in 48 hours
Mortar should NOT be
insulative
Min 2x
Insulation around baking compartment (Do
not use vermiculite)
Fiberglass wool Good to 450C 12-96 Kg
m³ 50Rand per m³
Rock Wool Good to 850C 60Kg/ m³
40 Rand per m³
Contact www.owenscorning.co.za
Auckey or Frickie 02711 360 8200. In
Gauteng
Material options (clay tiles )
The Baldosa clay tile or clay pipe
Very durable: 4 years of success in
Central America
Inexpensive: less than 1US$ per
combustion chamber
Not monolithic: individual parts
„float‟ so they can withstand
greater thermal shock.
Low mass: needs insulation
Can be cut from pre-existing tiles or
made from moulds
Possibly surround with Rock
wool insulation!
Heat loss of various materials
Material Density Specific Heat Thermal Estimated heat
(kg/m^3) (J/kg-C) Conductivity loss
(W/m-C) (MJ)
Ordinary Brick 1600 840 0.7 5.7
Guatamalan 1691 812 0.219 3.1
Baldosa
85% pearlite 439 921 0.128 1.4
15% clay
1400 F firing
50/50 729 701 0.081 1.2
sawdust/clay
85% 559 698 0.12 1.4
vermiculite
15% clay
El Coco 1328 835 (estimated) 0.181 2.5
Baldosa
Pumice brick 770 835 (estimated) 0.107 1.5
Glass wool 40 700 0.038 0.37
Pumice insulation
Can calculate density (g/cc) or test to see if the
mixture floats . Yes? Than less than 1 g/cc
A very light insulative mixture 4 g/cc (ideal for
top plate)
.6 -.8g/cc ideal for combustion chamber above
abrasion points ( i.e above feed chamber)
Using Excel to grade pumice
must be done each time a new batch of pumice is ground!
Step A Grind 2 kg of pumice
Step B Stack a 4mm and a 2mm screen. Use these to prepare 3 grades
of pumice. Be sure to sift very thoroughly
Step 1 Sift these piles with the full range of screens. Then Weigh each
screen and insert to excel
Step 2 Change proportions to find true readings . If possible For larger
sieve sizes (4-19mm) choose closer to the „least‟ , for 0-4 sieves
choose closer to the most
If proportions cant be met , additional grinding may be necessary
Top plate insulation
Insulate with graded pumice
(excel) and cement fondue
OR
Central American Pumice recipe
A 12.5-4.75
B 4.75-2.36
C 2.36 –
Recombine 2A + 1B+ 4C
Combine with sufficient cement
fondue to bind (10-20%)
needs experimentation
Other potential insulative recipe
( will probably be heavy)
By Weight
35% cement fondue
10% 4-6 mm
22% 2-4 mm
22% 1-2 mm
11 % 0-1 mm 0-1
Heat diffuser above combustion chamber
Use a 300 by 300 mm by 3-5
mm ‘diffuser’ above the
combustion chamber. The
diffuser plate should be
supported by sturdy 50 mm legs
( 15 mm round bar or
equivalent).
Legs must not restrict air flow.
This will help prevent lower
loaves from burning.
Add mass inside the baking compartment
To help stabilize temperatures
inside the oven more mass is
neccesary . 50 kgs should be
placed in the stoves once the
stove has been
Painting the baking compartment
The inside of the baking
compartment can be painted
with high temp paint
Purchased from Herbert Evans
0027 11 614 0000
Plascothermsilicon HRA6
Good to 540 C
Must be heated to 200 C for it t
bond to the steel . Fires silver
5 Litres 384 Rand will cover 7-
8 sqmetres ( 3 stoves).
Metal options
For proofing chamber . Use
lightest material possible .
Experiment with galvanized
For all areas not exposed to high
temp or direct flame use 1.2 mm
or galvanized . Otherwise use
1.6 mm
Use 1.2 mm 3CR12 for bottom
plate of baking compatment .
Similar to stainless. Can with
stand high temp and doesn’t
need to be painted . Low
conductivity. Can replace 3 mm
mild steel. Can purchase in SA
Design options for next bread oven (32 loaves)
Two separate baking
compartments each containing
16 loaves. 10 mm gap between
the ovens
180 gap between shelves to
allow easy removal of bread .
Leave 50 mm gap between front
and back baking pans.
50 mm gap between lower
baking tray and ovven floor
Bolt-on optional proofing
chamber
Wood shelf and
wood support
Build a small , 3-5 mm
thick wood shelf . Shelf
MUST include a limiter
to prevent it entering
to far into the stove
And a hinged wood
support to keep wood
level !
Basic Rocket The Shelf
Stove Geometry
Shelf can not
enter beyond 2X=34
this point
TH= 56
Tile thickness
(5mm)
X= 17
56 0.3X ( 0.3*170= 56) 59?
Fire the oven, not the cooks !
The ovens need to be tested t ofind the optimal baking protocol
( length of tiem to preheat ? Maybe only place first 16 loaves on top rack
for ten minutes then switch to bottom racks. Then place second 16 inside
of oven on top racks When , if at all do the loaves need to switched
between the lower and the upper racks
Cooks then need to be trained how to use the oven. Quantites of wood ,
when is it placed in the chamber Baking is an art an is a new skill for the
people using this type of oven
One test to perform: Heat oven to 300C 30-60 minutes of firing .
(Research this with new oven . Oven should bake bread in 30-45 minutes.
Especially the 16 loaf oven if its going to have commercial application
Include temperature gauge that can be read from outside of the oven new
oven Increase mass under the first rack
Geyser/ heater
Uses 210 L used oil drum.
Pressure release
pipe. Must be
the same height
as storage tank
Roof
Hot outlet Cold intake
15 mm gap between
drum and insulated jacket
Double walled Insulated
jacket. Inner jacket made
from 1.6 mm mild steel.
Outer jacket made from 210 L oil drum
1 mm mild or galvanized
steel jacket. Insulated
with sawdust.
Inset
Three 50X50X50 mm
pot supports . Placed
under the drum lip
18X18 cm fuel
magazine
clay pipe
Not monolithic: difficult to construct
refractory pipe that can withstand
thermal shock.
Low mass: needs insulation
Can be thrown on a wheel or extruded
Can be insulated with loose medium or
fine grade vermiculite or wit ha
small amount of cement if greater
stability is desired
Could be surrounded with Rock Wool
Good to850C
Mortar recipes sift Everything! < 1mm
South African Recipe
Mortar / liner basics
by volume By weight
2-3 parts stable 66% grog powder
aggregate grog/ 12% fine clay plastic
mullite (can be non refractory
1 part plastic binder or refractory)
e.g.high temp cement 21.5% sodium
/high temp plastic silicate liquid
clay .o3 %muti (magic)
Rocket Stove Combustion
Chamber
Mortar Options