|Boiling Point No. 21 - April 1990 (ITDG Boiling Point, 1990)|
by Dr. Stephen Joseph, Dr. Richard Bunon, Stephen Errey Of Biomass Energy Services and Technology (BEST) and Dr. B Lawson of University of New South Wales; Australia
The development and testing of a low mass bread oven with the village industries research and training unit of the North Solomons province of Papua New Guinea (note the oven is designed to burn sawdust).
Bread is now a major part of the diet for many people in developing countries. In some countries bread is baked in large electric or gas fired ovens in urban areas and transported to the rural areas. Many programmes are now underway to introduce low cost' efficient wood burning ovens to rural womens groups as a way of generating income.
In other countries bread is baked in very simple wood fired ovens. Much of the wood used for balking is purchased from the local market and its cost can account for 30% of the cost of bread. As wood becomes scarce and price increases are much greater than the inflation rate, these bakeries are not able to make a profit and are closing. Very little research has been undertaken to develop low cost efficient ovens to meet the needs of these small industries.
Development of the Bread Oven
Over the past four years, working in collaboration with researchers, entrepreneurs and extension officers in developing countries and with Dr Bill Lawson at the University of New South Wales, BEST has developed a range of ovens to meet the needs of small food processing industries. The designs derive from extensive theoretical and experimental research by ITDG and Reading University and at the BEST research station at Wamberal, Australia. Computer models help to determine the performance of different designs and test rigs help to determine their operating characteristics using different fuels with a range of moisture contents. A number of prototypes have been built and tested at BEST's research centre at Wamberal.
For a number of years the Village Industries Research and Training Unit (VIRTU), a special project within the provincial government of the North Solomons, had been promoting the development of more appropriate bakeries for urban and rural centres. Previously baking ovens had been imported and either used electricity or oil as a source of fuel and thus were expensive to operate. In 1988 VIRT1J was approached by an indigenous saw milling company to set up a bakery to use residues from this sawmill. The organization then approached Bill Lawson and BEST to assist in the development of an oven for small scale enterprises.
The bakery was to be established with mechanically driven mixing equipment, clean working areas and an oven that could produce 250 loaves and 1600 scones in 4 hours. The capital cost of the bakery was expected to be $125,000 and it was expected that the owners would pay back the investment during the second year of operation. The following design criteria were developed for the oven:
· It was to be as small as possible so that the building costs could be minimised.
· It should have a very short start up time so that the bakers did not have to get up too early and that baking could be carried out within a four hour period.
· It should be able to be built in a local workshop and should not cost more than $US 5,000.
· It should be aesthetically pleasing.
Description of the Oven
The bread oven has 3 distinct components: a well insulated firebox that has a grate and primary and secondary air control, two baking chambers and a series of passage ways that separate the baking chambers from the outer insulated walls. The flue gases produced from the burning wood pass from the firebox around all three walls of the baking chamber and out of the chimney. To increase the rate of heat transfer from the hot gases to the bread, a series of steel rods are placed through the baking chambers into the flue passageways. These rods are also used to support the bread inside the baking chamber.
The baking chamber is made from stainless steel (to comply with local food regulations) and the outer walls are made from mild steel. In this oven 25cm of alumina insulating blanket (Durablanket) was placed between the inner and the outer wall. The doors are made from steel plate and a thermometer was situated in the middle of the door. The
door/oven section is made in one piece and can be removed from the main frame to allow for periodic cleaning and repair. The fire box is insulated with cement and vermiculite mixture and lined with fire bricks. Having used high grade materials it is expected that this oven will have a lifetime of 10 years.
Designing the Oven
The following procedure was used to design the oven:
· The grate area and the combustion volume of the tirel>ox were estimated.
· The size of the gas passageways was fixed and the surface area of the walls of the baking ovens was estimated.
· The dimensions of the fireboxand the baking oven were calculated.
· The overall design was then drawn for manufacturing.
a) Estimating the Grate Area and Combustion Chamber Volume
To determine the size of the firebox it is necessary to determine the approximate heat requirements for converting the dough to bread. The amount of heat required in kilojoules if the oven is 100% efficient is the sum of:
1) the amount of heat to bring the water (approx 45% of the weight of the dough) in the flour to 100°C.
Q(1) = 4. f84 (specific heat H20) *weight dough *.45*
2) the amount of heat to bring the flour to I OOC
Q(2j = 1.8 (specific hHeat of flour) *weight dough*.55
3) the amount of heat required to evaporate water which accounts for approx 10% of the weight of the dough and heat it to approx 230°C)
4)the amount of heat required to obtain the necessary chemical reactions
5)the amount of heat to form a crust
The heat required for reactions 4 & 5 are negligible. For this oven the maximum amount of dough that could be baked is 29 kilogrammes. The water temperature is assumed to be 30 degrees celcius therefore:
Q1 = 4.784*29*45* (100-30) = 3,822kJ
Q2 = 1.8*29*55* (100-30) = 2,010kJ
Q3 = 2519*29*.1 = 7,305kJ
Qtotal = 13, 137kJ
Let us assume that the oven is approximately 30% efficient and the baking time will be in the order of 20 minutes. The amount of heat that is needed over a 20 minute period is:
Q4 = Qtotal/.3 = 4S, 790 kJ
The output of the fire in kilowatts is then:
Q5 = Q41 baking time (sees) = 43,790/20*60 = 36.5kW
The heat losses will include heat radiated and convected from the air, the retained heat stored in the ovenwalls that is not used in the baking, the hot air that escapes from the oven, the heat in the flue gases and the heat lost through incomplete combustion.
For most small ovens the amount of heat produced is approximately 200-250kW per square metre of grate area. Thus the grate area required is approximately:
Q6 = Q51225 = .162 sq meters
It has been found that for every kilowatt of heat that can be produced, a firebox volume, above the burning wood of .003 square meters is required. Therefore the combustion volume in this oven should be:
Q 7 = Qua *. 003 - . 1062 cubic meters
b) Estimating the Surface Area of the Oven Walls
Heat transfer in bread ovens is extremely complex (fig 1) and as yet there has been no reliable mathematical models developed. Thus designing must be carried out using more empirical methods.
Experience from an earlier prototype oven indicated that a distance of 20mm should be left between the oven wall and the outer wall of the gas passageway. This spacing ensures that the flue gas easily passed around the oven, without having to build a very large chimney, lout at sufficient speed to ensure that there is a reasonably high rate of heat transfer. Keeping the gap at this size also means that the oven is easier to build and maintain.
If the gap is maintained at 20 mm then the average heat transfer rate is approximately 4 kW /sqm. We have already determined that to bake the bread approximately 13,û00 kJ of heat input is required over a period of 20 minutes (10.7kW). Heat will be lost when steam escapes through the oven doors and when heat is radiated from the surface of the doors. Let us assume that 54~G of the heat coming into the oven will be lost. Thus the total amount of heat that must be transferred from the hot flue gases anci the radiating charcoal bed and the flames is approximately 13*1.05 = 13.65
The surface area is thus:
Q8 = Qoven/4 = 13.65/4 = 3.41 sq meters.
c) Calculating the Dimensions of the Ovens and the Firebox
The oven is designed so that 32 loaf tins can fit in each oven. Small tins with a size of 25.4cm by 10cm are to be used. From previous work it has been found that distribution of heat in the oven is a problem when more than 4 shelves are used and more than 4 tins are placed side by side. Thus to get 32 tins in each shelf it was necessary to place ~ tins end to end. It has also been found that baffles need to be placed at the side of the oven and gaps left between the tins to ensure that there is some circulation of air. It was decided to place a baffle 20 mm from the wall of the oven and leave 130 mm between shelves which are spaced at 7~ mm. To accommodate the baffles and the gap between tins the overall width and height of the oven was set at 570 cm and the depth 538 cm. To ensure that the bottom of the lowest loaves were not burnt a distance of 50 mm was left from the bottom of the oven. T his was sufficient space to place a baffle if necessary.
Experimental work has indicated that the walls from the grate should be sloped at around 60 degrees to improve heat transfer and charcoal bed formation. The back of the firebox will have a row of refractory bricks and insulation behind these bricks. To ensure that the surface area was a minimum of 1.58 sq metres it vas found that the dimensions of the grate should be 440 mm wide and 470 mm deep.
Testing the Bread Oven
A simple calorimetric test was developed to determine the heat transfer efficiency, time temperature profile and the distribution of heat in the oven.
Four hundred grams of water were charged into each tray. The oven was then preheated to 250° C. The tins were then loaded into the oven and the oven was run for a total of 40 minutes. Initially the oven temperature dropped and thus fuel rate was increased. Once an oven temperature of 220°C was attained the feed rate was reduced. Wood was occasionally added to maintain this temperature. At the end of the test the pans were removed for weighing.
The amount of wood used, the oven, wall and stack temperatures and the amount of water evaporated from each tin were recorded. The thermal efficiency was then calculated using the following formula:
It:79 = Thermal Efficiency = Ww*4.184*(100-Ti)+2240*Ww
Wf * NGV (as fired)
Ww = tote/ weight of water
We = total weight of water evaporated
Wf = weight of fuel
NCV = net calorific value of the fuel
The Power output of the oven was calculated as:
Q10 = Wf*NCV time (sees)
A pot of water evaporated and position of each tray was then drawn (fig 2 & 31. This indicated that there was more heat going into the right hand oven and that more heat was going into the trays on the right side of the left oven and the left hand of the right oven. Also more heat was going into the back pans than the front pans.
The time to heat up the ovens to 300°C was 18 minutes. Once the water was placed in the ovens the temperature decreased to 100°C and did not rise to 200°C for 15 minutes. The left hand oven was 150°C while the right hand oven was 200°C. When the test was completed the right hand oven was 216°C and the left was 162°C. The average stack temperature was 250°C, the efficiency was 23% and the average power was 51 kW. The oven was pulled out to reveal that there had been some misalignment during assembly such that more flue gas was passing around the right hand oven. It was also apparent that baffles should be placed inside the baking compartment on the top and the bottom walls to reduce the amount of heat going to the top and bottom pans.
The necessary changes were made and the oven retested. The efficiency increased to 289ó the average power to 41 kW and the average temperature difference between the ovens was now 5°C. The plot of water evaporation versus pan position revealed that more heat was going into the right hand oven and that more heat went into the top and bottom pans.
Before making any other changes it was decided to carry out a baking test to see what other problems may arise. The bread tins were filled with approximately 3901cg of dough. The oven was heated to 250 C before putting in the bread. A trial run was carried out to determine how baking should be carried out, before the accurate tests were undertaken. The first load of bread was baked at 230°C. The upper and lower pans started to burn after 13 minutes and the middle pans took 2û minutes to cook properly. The output of the fire was 50kW.
It was apparent that the output of the oven needed to be reduced and that the bread on the middle 2 shelves should be cooked slightly longer than the bread on the other 2 shelves. Two lots of 64 loaves were then baked at an average temperature of 200°C. The total amount of dough baked was 53.05kg and the wood used was 7.118 kg. The average baking time was 17.5 minutes and the amount of time required to unload the first batch and load the second batch was S minutes. The specific consumption was:
Specific Fuel Consumption - .133kg sawdusti kg dough
Steve Layton of VIRTU felt that a more indepth test programme could now be undertaken in PNG. The oven was then shipped to PNG and Steve Errey went out to help in the testing and modification of the oven.
The people at VIRTU said that larger, lkg loaves should be baked and therefore the oven should be redesigned. The oven dimensions were then altered and a stainless steel liner was placed rem inside the mild steel baking oven. This further improved the distribution of heat.
Ovens have now been installed in bakeries and marketing of the technology is being undertaken.
Further development work to reduce the cost of the oven and improve the distribution of heat is being carried out at VIRTU.
Ed Note: Boiling Point hat camed several articles on bread ovens (see Boiling Point 10, 16) all of which have been for small village capacity' '0- 100 loaves a day and have been much simpler, cheaper and necessarily less encient .
BEST has since been reconstituted under the title 'Energy for Sustainable Development Ltd. London UK