|Boiling Point No. 18 - April 1989 (Intermediate Technology Development Group , 1989)|
by Dr S. Joseph & Dr W Lawson of BEST
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 women's 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 baking is purchased from the local market and its cost can account for 30% of the cost of the loaf of bread. As wood becomes scarce and cost 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.
Over the past four years, working in collaboration with researchers, entrepeneurs and extension officers in developing countries and 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 at Reading University and at the BEST research station at Wamberal, Australia.
Computer models helped to determine the performance of different designs and test rigs helped to determine consumption of wood fuel with a range of moisture contents. A number of prototypes have been built and tested at Wamberal and one design is now being built and tested in Indonesia and PNG.
Description and Performance of the Oven
The bread oven has three distinct components:
1. A well insulated firebox that has a grate and primary and secondary air control
2. Two baking chambers
3. A series of passageways that separate the baking chambers from the outer insulated walls.
The flue gases produced from the burning wood pass from the firebox around all four walls of the baking chamber and then out of the chimney. To increase the rate of heat transfer from the hot gases to the bread, steel bars are placed through the baking chambers into the flue passageways. These bars are also used to support the bread tins inside the baking chamber.
The baking chamber is made from steel sheet. The outer walls can be made from steel and insulated with ash, vermiculite or diatomite or from bricks made with ash, husks and a high quality clay. If steel is used for the walls the oven is light enough to be carried from village to village. The oven can be made in a range of sizes with a minimum capacity of forty loaves and a maximum of two hundred loaves. The cost and the lifetime depend on the quality and type of materials used.
The portable oven takes 20 minutes to warm up. During that time the bread can be raised on top of the baking Chamber. The bread is then baked at 240°C for 20 to 25 minutes. By then it has a golden crusty surface and can be removed.
It takes 10 minutes to warm up the oven again to baking temperature before the second batch can be put into the oven. On average 0.13 to 0.2 kg of wood is required to bake one loaf of bread. A unit capable of baking 500 loaves in a day has now been installed in Papua New Guinea and local manufacture is to begin in the first half of 1989.
BEST- Biomass Energy Services and Technology Pty Ltd. 5 Kenneth Avenue, Saratoga, NSW 2250, Australia