In developing countries, and particularly in the numerus refugee camps thereof, there exists a persisting need for low-cost and simple devices applicable to the cooking of food with a high efficiency using the available supplies of charcoal or other similar fuel. However, the fact is that even today half of the mankind cooks its food using a so-called three-stone stove system in which the combus- tion efficiency available from a biomass fuel is only about 8 %, which means that the world-wide consumption of fuelwood in cooking exceeds the amount of wood used in the industry. According to experts, the result is an extensive conversion of fertile land into desert expand- ing presently at an annual rate of 20 million hectares and expected to gain in strength in the future. It must be borne in mind that the industry receives its wood deliveries mostly from commercial forests adhering to the rules of sustainable growth, whereby the progress of desert wasteland formation is chiefly related to the rapid growth of population in the areas of the world inhabited by the poorest people (characterized by 4 W<BR> nativity meaning a 2 k annual net increase of popula- tion), in which areas the sustainable carrying capacity of the nature is already exceeded. Eating is one of the most primitive basic needs of man and its satisfaction is impossible without a facility of cooking the food, which requires the application of thermal energy. When the

basic need of preparing food is considered in conjunction with the diminishing resources of available energy, a logical conclusion must be made that a stove with an improved energy-conversion efficiency is urgently required for coking, possibly also serving for heating.

Growing and burning of cultivated wood is considered to produce energy in a sustainable manner, because the growth of wood binds C02 from the atmosphere into carbon which is circulated in firing with wood. By contrast, firing with fossil fuels such as kerosene continuously increases the C02 load of the atmosphere. Hence, the advancement of food preparation techniques employing fuelwood must be preferred over those employing fossil fuels in the interest of protecting the environment and the atmosphere.

In addition to exhibiting a maximally good efficiency in cooking and possibly also in heating premises, a stove must also have a simple structure, and thereby, a con- struction easy to assembly and service inasmuch the availability of functional tools and other accessories or services is scant in the primitive circumstances of de- veloping countries. Moreover, a stove should have a dur- able construction combine with small size, multifunc- tionality and ease of use.

Several different approches are known in the art to fulfill the above-described requirements. In US Pat. No.

5,024,208, for instance, is disclosed a small-size port- able stove having its vertical wall made from an insulat- ing material. The bottom edge of the insulative stove wall is provided with air inlet apertures 96 permitting the introduction of the ambient air into space under the stove, wherefrom the air preheated by a reflector plate can flow into the fuel chamber. The construction of such a stove is rather complicated, and its assembly requires a plurality of different materials, separate parts and

fastening means holding them together, as well as the use of tools. The assembly of the stove must be carried out at the factory. Moreover, the stove does not support a combustion process of satisfactory efficiency.

US Pat. No. 4,915,091 discloses a stove with a casing comprising an inner shell and an outer shell spaced apart from each other. However, this system has a continuous inner shell, whereby the lack of secondary combustion air results in a combustion efficiency far from satisfactory.

Moreover, also this stove construction is assemble from separate parts requiring the use of tools and separate fastening means. Hence, the stove must be factory- assemble.

Conventional stove constructions are generally made from thin sheet steel by welding. Such a material, however, is difficult to weld and exhibits a strong thermal expan- sion. In reality, thin sheet steel is very difficult to weld and the stiffness of weld seams are impractical in a product subject to continuous thermal expansion and con- traction.

It is an object of the present invention to provide a stove featuring a small size, easy assembly and good combustion efficiency for cooking and heating purposes, said stove being free from problems hampering the prior- art constructions. The stove according to the invention is characterized in that at least a portion of the stove inner shell is provided with holes opening into the air gap of the intershell space and said holes serving for the introduction of secondary combustion air into the fuel chamber and that the air gap has a closed upper end.

Then, the preheated combustion air is forced to enter the fuel chamber via said holes under a positive head, where- by the combustion process in the fuel chamber becomes very efficient. The positive head in the intershell space

is achieved by virtue of the thermal expansion of the rising air and the blocked upright passageway thereof at the closed upper end of the annular preheating space.

Then, under the draught induced in the stove, the air flows naturally upward in the intershell passageway. The fire itself generates a negative head in the stove, that is, a draught which is further amplifie by the positive head prevailing in the preheating passageway formed between the shells.

A preferred embodiment of the stove according to the in- vention is characterized in that the holes of the inner shell are made at least to the upper portion of the stove inner shell. Then, the secondary air flow is introduced under the cooking utensil thus optimally supporting the combustion process.

Another preferred embodiment of the stove according to the invention is characterized in that the grate is per- forated with holes and above or under said grate, in parallel therewith, is placed a second grate plate serving as a damper which is also perforated and adapted to rotate about the vertical axis of the stove thus per- mitting the control of the open area of the perforated holes, thereby adjusting the flow rate of the primary combustion air.

A still another preferred embodiment of the grate accord- ing to the invention is characterized in that the stove is assemble from a template which is prepunched or precut into an essentially solid sheet of metal by manual folding without the need for using any tools. This tech- nique of assembly by manual folding is a cost-efficient and flexible method of making a stove. As compare to welding, folding offers an additional benefit of giving the stove a longer life. Moreover, such a manually folded stove is easy to service.

The stove according to the invention functions so that the combustion air first passes to the underside of the stove, is preheated there and rises therefrom as the primary combustion air via the holes of the grate into the fuel chamber undergoing a continuous heating during its passage. It must be noted that the combustion air passes over a large contact surface with the grate as there are provided two grates separated by an empty space. The secondary air is sucked in the above-described manner first into the intershell heating space and there- from via the holes of the inner shell into the fuel chamber.

In the stove according to the invention, the air heated in the intershell space receives thermal energy from the heat absorbe in the shell walls thus utilizing such a portion of the fuel heat content that is already wasted as regards to the cooking process itself. When the. air passing via the intershell heating space recovers a por- tion of the heat absorbe in the shell walls, it can thus return the combustion energy once already lost back to the fuel chamber.

The stove according to the invention also offers a number of other benefits making it vastly superior over conven- tional stoves. A particular advantageous property is the possibility of delivering the stove to its final destina- tion of use in the form of a plate having thereon pre- punched or precut a template from which the stove can be assemble by manual folding. Thence, it is possible to provide, e. g., a refugee camp with bundled stacks of such prepunched/precut blank sheets from which the people living in the camps can themselves prepare the stoves by manual folding.

The sheet metal blanks of the stove can be made with automate machines in an extremely short time in large

series, stored in sheet format and even transported in sheet format, whereby also the transport costs per stove will become more advantageous than for ready-assembled stoves.

In the following the invention will be examine in greater detail with reference to the appende drawings, in which Figure 1 shows a sheet having the template of a stove according to the invention prepunched or precut thereon; Figure 2 shows a stove assemble by manual folding from a sheet illustrated in the first drawing; and Figure 3 shows the stove of Fig. 2 in a bottom view.

Referring to Fig. 1, therein is shown a prepunched stain- less-steel sheet from which the stove according to the invention is folded into its final form. All the elements required in the assembly of the stove are included in the sheet. The major elements of the stove comprise an outer shell 1, an inner shell 2, as well as a lower grate 3 and an upper grate 4. Both grates are made detachable from the sheet for a later insertion in place into the stove.

Also detachable are the legs 5 of the stove that are later inserted in compatible slots 6 made to the outer shell of the stove. The top edge of the outer shell is provided with a number of different fastening means 7,8, 9 for fixing the inner and the outer shell to each other by bending said means in an essentially inwardly radial manner. Furthermore, some of these fastening means 7,9 are foldable into supports for utensils used in cooking food on the stove. These fastening/support means 7,8,9 also close the upper end of the annular intershell space formed between the outer shell 1 and the inner shell 2.

As can be seen from Fig. 1, the inner shell 2 of the stove is provided a plurality of holes 10 which are also shown in Fig. 2. The holes serve for introducing the jets of the secondary combustion air into the fuel chamber in the manner described above.

In Fig. 3, the stove is illustrated in a bottom view.

Therein is visible the intershell space 11 which is formed between the outer shell 1 and the inner shell 2 so that the air gap is open at its lower end but closed at its upper end as described above. Fig. 3 also illustrates the lower, fixed grate 3 which is perforated with holes 12. Over the fixed grate 3, in a parallel plane there- with, is mounted an upper grate 4 which is made rotatable in its horizontal plane by means of a lever 4 adapted thereto (refer to Figs. 2 and 3). Also the upper grate is provided with holes 13 adapted to coincide with the holes 12 of the lower, fixed grate when the upper grate is rot- ated into one of its extreme positions. When the upper grate is rotated away from this extreme position, the open area of the hole passageways is diminished until the holes are fully closed in the other extreme position.

Thus, a provision is offered for controlling the flow rate of the primary combustion air.

Starting from a prefabricated sheet blank shown in Fig. 1, the assembly of the stove is commence by first detaching the grates 3 and 4, as well as the legs 5, from the sheet blank. Next, the outer shell 1 and the inner shell 2 are folded and interlocked into the final shape of essentially coaxially cylindrical configuration shown in Fig. 2 so that the inner shell 2 remains in the inte- rior of the outer shell 1 and thereinbetween is formed an essentially cylindrically annular air gap 11 (refer to Fig. 3). Subsequently, the fastening means 7,8 and 9 are folded into place, whereby the upper portion of the stove receives its final shape in which supports are provided

for a cooking utensil, the upper end of the intershell space is closed and the exhaust openings are left for flue gas. Finally, the legs 5 and the grates 3 and 4 of the stove are inserted in place.

To the lower edge of the stove inner shell 2, below the grate level, are made holes 15 of a relatively large diameter serving to improve the passage of the combustion air into the space between the shells 1,2. Advantageous- ly, the air is first passed via the air intake opening 16 of the stove front wall to the underside of the grate, where the air is heated under the glowing grate and therefrom passed via the holes 15 of the inner shell into the fuel chamber. The draft in the stove is induced by virtue of the negative head of the stove upper portion caused by the upward flow of the flue gas.

Thus, it is possible to prepare a stainless-steel pre- punched sheet into a fully functional dual-shell stove, whose assembly takes place by merely manual means without any need for tools (obviously not excluding the use of tools such as pliers). Combustion in the stove is effi- cient and controlled by virtue of the primary and second- ary air system according to the invention. The steplessly ajustable dual-plane grate permits control of primary air introduction into the fuel chamber and the inner shell with holes perforated on the upper portion of the inner shell wall allows passage of preheated secondary air into the combustion space. An empty space is provided below the grates for ash collection, fire ignition and air intake.

To those versed in the art it is obvious that the inven- tion is not limited by the exemplifying embodiments described above. Therefore, there is no need to give a detailed description of each stove element as they may be implemented in a variety of different ways according to the teachings of the inventions. Alternative materials for the stove are, e. g., black sheet steel and others.

The fuels primarily suited for firing the stove are charcoal, wood and other readily available solid fuels.