|Boiling Point No. 22 - August 1990 (ITDG - ITDG, 1990, 44 p.)|
Better Bread Ovens
The following letter from Dr S Joseph should be read in conjunction with his article on page 19 of BP 21 and relates to the comments made by E Schutte of the Woodburning Stove Group, Eindhoven, University of Technology.
"Due to some poor communication on our part the article that you printed on bread ovens was a draft and the final publication did not reach you. I would like to make certain clarifications and amendments to the article as follows:
1) We have now developed a number of very simple computer programmes to allow us to estimate the size of the grate, combustion volume and heat transfer area of the oven walls. These models certainly assist in the initial design phase but for ovens it is necessary to carry out experimental work to determine much more precisely the required heat transfer areas.
2) The actual heat transferred from the hot gases to the rods holding the trays is small (approximately 0.3% of the heat transfer efficiency). In the production model the rods remain inside the stove.
3) Fig 1 Schematic of the Bread Oven. The walls and the floors are insulated. Ernst Schutte recommends insulation of the bakery department door, which would reduce heat loss by 6.5%. This particular oven was designed to meet the very strict regulations applying to commercial food preparation in Papua New Guinea. For instance all cooking surfaces must be made from stainless steel. The criteria were developed by VIRTU and thus I feel that the comments on the complexity and the costs are not really relevant to the PNG situation. Since the oven is used at a sawmill where there is plenty of timber, fuelwood use was not one of the criteria laid down by the owner.
However, it should be noted that we have designed the oven to use as little wood as possible. The oven has been designed to bake white bread as well as scones and small cakes.
Construction details are not given as we feel that this oven may not be appropriate to other countries and the design can be adapted to meet other food regulations and other sociocultural settings. We are more than welcome to assist people to undertake this task.
Textual Corrections (italics by E Schutte and agreed by S Joseph)
Equation 3. The equation is made from the latent heat of evaporation and the specific heat required to bring the water vapour from 100°C to 230°C. The latent heat of evaporation is 2260kJ/kg and the amount of energy required to raise the water vapour temperature is the specific heat (2.0kJ/kg) multiplied by the temperature difference (230-100°C).
It should be noted that although most bakers will cook at lower temperatures, we have sized the oven conservatively. If more heat is available than is necessary the fire can be damped down. This extra heat may be needed if the baker wants to use a heavier mix. We have also made the grate area 1.2 times bigger than necessary, both for the above reason and to allow wetter or less dense wood to be burnt without affecting the baking time.
The statement that the insulation is 25cm should read 25mm; there should be a decimal point in front of .162 sqm, the .003 square metres should read .003 cubic metres and the 538cm should read 538mm. In equation 8 we have actually used a more conservative figure for the amount of heat to be transferred into the oven than would be obtained from the initial theoretical analysis. If we assume heat into the oven of 13.65 MJ/kg over 20 minutes the surface area will be 2.85sqm. We have opted for a larger figure of 3.41 sq metres both to accommodate the pan sizes and to ensure that we do have enough heat transfer area if the conditions of operation are changed eg. if wet wood is used.
Equation 8. The surface area required on the oven walls is found as follows: divide the amount of heat needed to bake the bread i.e. 1,3650 kJ (including 5% heat losses) by the baking time of 20*60 seconds. This figure, the amount of heat, 11.4 kJ per second (kW) is then divided by the heat transfer rate of 4 kW/m² to obtain the surface area of 2.84 sq m.
To clarify the construction of the oven. The oven is designed so that 32 tins can fit in each oven. Small tins with a size of 25.4cm by 10cm are to be used. From previous experimental 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 2 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 20mm from the wail of the oven and leave a 5mm spacing between tins. To allow for the bread to rise it was decided to space the shelves at 75mm. To accommodate the baffles and the gap between tins the overall width and height of the oven was set at 570mm and the depth 538mm. To ensure that the bottoms of the lowest loaves were not burnt, a distance of 50mm was left from the bottom of the oven. This was sufficient space to place a baffle if necessary. For ease of construction we decided to reduce the area from 162sq m (as calculated) to .158sq m.
Equation 9. Thermal Efficiency =
(Ww* 4. 184* (100-Ti) + 2240*We)/Wf*NCV (as fired)
where: Ww = total weight of water
We = total weight of water evaporated
Wf = weight of fuel for heating the oven and baking
NCV = net calorific value of the fuel
Most people found the graph so I am providing the information on evaporation rate in tabular form.
Table 1 The amount of water evaporation the different pans in the left and right oven
Comments by Alex Bush, ITDG, on points not explained above.
"Steve seems to be giving a spurious gloss of technical sophistication to the design of bread ovens - why 30% efficiency? why 5% heat losses? does bread bake at 100°C?
Thermal efficiencies and power outputs of bread ovens are spurious data. There is no commonly agreed standard for bread oven efficiency and water boiling tests certainly do not provide a good test. Power output quoted is actually power input and bears no relation to the baking power of the oven.
The interesting information on a bread oven is how much bread can it bake, how controllable is it and how much wood does it use per loaf. Even more interesting would be some comparison to existing technology."
Other points raised by E Schutte of Woodburning Stove Group - Eindhoven University, The Netherlands
The specific energy consumption is expressed in kg sawdust/kg of dough. Usually specific energy consumption is expressed in MJ/kg of flour. The weight of the flour instead of the dough is used as a comparable basis to avoid different results due to different water contents for different dough recipes. Since the calorific value of the fuel depends on the kind of fuel, chemical composition and moisture content, for reasons of comparison it is common to use MJ instead of quantity of sawdust.
Ed - We are most grateful to Stephen Joseph, Ernst Schutte and Alex Bush for their corrections and comments.
From: Ms Anna Pearce, Box Aid, Orchard Cottage, 11 Hill Top Lane, Saffron Walden, Essex. CB11 4AS
Your letter on page 39 of your April issue addressed to 'Dear Reader" needs urgent correction.
The Wonderbox is in no sense a solar cooker and has never been claimed to be one. It is simply a fuel-saving cooking aid which can be used in any kitchen or on any camping site. Properly used it can save 2/3 of normal cooking fuel. It is currently being produced and sold by Africans in southern Africa at the rate of over 1000 per week.
The Wonderbox helps to conserve wood (trees) and fossil fuels and so to reduce production of carbon dioxide, the principle gas responsible for the "greenhouse effect".
Wonderboxes need no sun I personally use a Wonderbox every day, rather than leaving food simmering on a stove. There are others in every continent doing the same, having obtained their Wonderboxes through churches, organisations or over the counter.
We apologise for referring to the Wonderbox as a solar cooker, such as the "Solar Box". The Wonderbox was described and illustrated in BP20.
The Original Wonderbox (as seen in BP20Dec 1989)
Boiling Point is the journal of the Intermediate Technology Development Group's Fuel for Food Programme and the GATE/GTZ Programme. It is printed on 100% recycled paper by Rugby Community Printworks (affiliated to the Rugby Youth Promotion Programme) founded in 1979 to serve the local Community.
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