| Boiling Point No. 20- December 1989 |
by Manuel Solis Beltran, Energy Research
Technical University of Guayaquil, Ecuador. Present address Woodburning Stove Group, Eindhoven University of Technology, The Netherlands.
Ecuador is an undeveloped country located in the North of the South American continent with a population of 9 million inhabitants. Approximately 47 % of the total inhabitants consume wood and other biomass fuels for cooking tasks with a per capita consumption of 2.1 kg of wood every day. The rest uses for cooking activities LPG, kerosene and in a few groups electricity. Low efficiency open fires (4-5 %) along with other reasons have caused deforestation, erosion and general destruction of the eco system. Besides that some of them have been looking for better living conditions etc. All this has stimulated some wood consumers to change from biomass to kerosene and LPG as cooking fuels. LPG is a high cost fuel subsidized by the Ecuadorian government. Thus the LPG consumers pay only 280 sucres (0.56 US$) for 14 kg, instead of 1392 sucres (2.8 US$). This indicates that for 1989 the Ecuadorian Government will have to pay approximately 50 million US$. The yearly increase in LPG consumption is presently 16%. Reducing the increase to 6% per year, still doubles the consumption of LPG by the year 2000.
The Ecuadorian Government and the National Energy Institute in Ecuador have been working on projects intending to discourage the use of LPG. One of them is named "Proyecto BANDERAS", that includes the free distribution of low cost kerosene mainly to low economic classes. In the coming years the price of LPG will increase. Unfortunately people are against the idea to replace their LPG stove by kerosene due to bad experiences in the past. Some of these are:
(i) tediousness of starting a kerosene stove;
(ii) poor combustion quality; production of smoke, CO and other unburnt hydrocarbons;
(iii) bad smell and bad taste of foods; and
(iv) low power and low efficiency, resulting in long cooking times.
One of the objectives of the present work is to define the basic criteria for designing a high power, clean burning and odor free kerosene stove, adjusted to Ecuadorian cooking habits. The design has to be manufactured at local production centers. Presently an existing Colombian kerosene pressure stove from the World Bank is being tested. On the basis of this analysis and performance it should be possible to improve the design leading to a final model to suit the above terms of reference.
This paper reviews the work done till now. A detailed report will be available in December 1989.
Stove Performance Description
The Colombian stove is a pressure kerosene stove (vapor jet burner). Some of its characteristics are listed below:
overall dimensions: 275*300*110 mm
head burner diameter. 90 mm
weight: 2807 g
tank capacity: 1050.3 cm#
no. of burners beads: 1
burning fuel: kerosene
ignition fuel: alcohol
pressure source: stroking pump
pressure range: 0 - 200 kPa
price: 12 US$
trade mark: Superior
manufacturer: Industria Superior de Artefactos
The principal part is the combustion system which incorporates a nozzle, a tube (vaporizer), an interior spiral and a valve control with a long axle through the vaporizer tube. The axle is surrounded by the spiral and has a needle at its end. This is shown in figure 1.
Operation of the Stove.
The flame of alcohol heats the vaporizer tube. The kerosene in the tank is pressurized and starts flowing into the heated vaporizer tube. Here kerosene is vaporized and will pass through the nozzle acquiring a high speed forming a vapor jet that will flow (taking primary air) through the burner tube and finally comes out through the burner slits forming a large kerosene vapor jet that mixes with secondary air being able to burn with blue flames. The desired power level can be achieved through maintaining appropriate pressure in the tank by pumping and adjusting the flow area by means of the control valve.
Starting the Stove
By following the instructions provided by the manufacturer, the stove could not be lighted properly. It took about two weeks to develop a proper lighting procedure. Two facts appeared to be very important for proper lighting of the stove.
(i) The vaporizer tube should be sufficiently heated to assure evaporation of the kerosene. If the tube is insufficiently heated, the kerosene will collect in the bottom part of the burner and set the entire stove on fire.
(ii) Enough pressure should be built up in the fuel tank to assure a sufficient flow of kerosene through the vaporizer tube.
Apart from this the fouling of the system, especially at the end of the nozzle and in the slits in the burner head, can cause problems during ignition of the stove. The quality of the kerosene largely influences the rate of fouling. The kerosene from developing countries is known to be of less purity which stresses the need for regular cleaning of the stove.
Stove Behaviour at Different Settings
Before testing and measuring the characteristics of the stove (power ranges, combustion quality and efficiency) work was carried out to obtain a general picture of its performance and behaviour for different combinations of pressure and valve settings. Two different pressure controls were used in the experiments:
(i) a constant pressure by means of an external pressure source;
(ii) the pump on the stove.
In both cases a manometer was installed to read the tank pressure. With the first set of tests it was possible to map out the general characteristics of the combustion in the stove. This is shown in figure 2. The results in this figure should be treated as tentative. They were obtained without a pan and only one stove was tested, since it is not clear at the moment of writing whether the manufacturing inaccuracies will change this graph. The following things can be seen from the figure:
(i) 6 regions show the different flame characteristics. The working blue flame area is the best reference for improvement of the design;
(ii) blue flames can be obtained within a power range from 0.5 kW to 5.0 kW;
(iii) the power output is much more influenced by a change in flow area (valve setting) than by a change of pressure.
As pointed out earlier, the results of figure 2 were obtained without a pan. The pan changes the performance of the stove in two respects:
(a) the maximum power is reduced from 5 kW to 3.12 kW; and
(b) the stove makes an unpleasant high frequency whistling noise when using pans with a diameter larger or equal to 28 cm.
This is however the minimum size required by Ecuadorian families. The power outputs corresponding to various constant pressures for 3 different valve settings are shown in figure 3. The curves through the data points are drawn by the computer and seem to be proportional to the square root of the pressure. It is however necessary to carry out more tests, especially for the middle curve to confirm this. Figure 3 can be used to predict the power loss due to the pressure loss in case no external pressure source is connected. For example: the top curve shows that an initial pressure of 154kPa corresponds to a power output of 3kW. If the burning is started with + a tank full (525cm#), what would be the change in power output in case 262.5cm# ( half a tank full) is consumed and how much time is required for this? The computation have shown that the consumption of 262cm# of kerosene takes approximately 56 minutes, during which the tank pressure drops from 154kPa to 102.5kPa.
This new pressure corresponds to a power output of 2.45 kW (see figure 3). Thus we can state that to maintain a power close enough to 3 kW, the stove need not be pumped that frequently - probably once in 20 minutes should be more than adequate.
A number of efficiency tests was carried out. They showed for example that with a 5kW power and a pan with a diameter of 24cm, the flames remained blue and steady. The efficiency measured was approximately 50 %. Applying the same power for a pan with a diameter of 28cm produced yellow flames and a high frequency whistle. Thus the power had to be reduced to almost 3kW to obtain again steady blue flames. The efficiency in that case was about 60%. To avoid the whistling sound when using a pan with a 28cm diameter, the distance between the pan bottom and the burner head was varied. It appeared that increasing the distance from 48mm (standard distance) to 66mm and a power output of 3.7kW only slightly reduced the sound. However the efficiency dropped by almost 10 percentage points and because of the increased gap, the flame became less stable due to air movements.
(i) When the stove is operated at a power output of 3.1kW, using the standard pan support (48mm), any pan diameter can be used without the appearance of a whistle. The flames are stable and blue. The efficiency will vary between 50 and 60% depending on the pan diameter. Power outputs up to 5kW can only be applied for small pans (24cm diameter). Larger pan diameters cause a whistling sound and the flames turn yellow and become unstable.
(ii) Considerable care is essential to start the stove. The vaporizer tube should be sufficiently preheated, and the burner should contain no kerosene from previous cooking sessions.
Some of The Disadvantages of The Stove
(i) Two types of fuel are required to operate the stove:
(a) alcohol for preheating of the vaporizer tube and
(b) kerosene for the actual burning.
(ii) The quality of kerosene largely influences the operational behaviour of the stove. The stove gets fouled quite easily.
(iii) The quantities of food cooked in Ecuador require pans with a diameter larger than 28 cm, maximum power outputs around 4.5 to 5 kW and minimum power outputs of 1 kW. The present
design of the Colombian stove does not permit this because it will start to produce unpleasant sounds and unstable blue flames.
CAMARTEC CENTRE FOR AGRICULTURAL MECHANIZATION AND RURAL TECHNOLOGY KITUO CHA ZANA ZA KILIMO NA UFUNDI VIJIJINI