Field of the Invention

This invention relates to cooking stoves, in particular, to lightweight, efficient, and readily portable outdoor cooking stoves for use primarily by those undertaking general camping or other outdoor leisure pursuits such as backpacking, hiking or boating. The cooking stoves are designed to collapse or dismantle to an easily storable flat pack, to be robust, to make efficient use of solid-fuel blocks and to provide the burning fuel a level of protection from the wind.

Background to the Invention

Outdoor enthusiasts and military personnel that carry their own equipment, often for extended periods of time, need their equipment to be lightweight (under 150 grams) and suitable for compact storage. Environments that do not offer a source of fuel, such as dried wood, are frequently encountered, requiring the prudent person to also carry their own fuel. To assist with keeping weight down, solid-fuel blocks can be used, such as hexamine blocks, trioxane blocks, solidified methyl decanoate blocks or gelled alcohol packs. These blocks prevent the need for extra containers (as with gas or liquid fuels) or regulation equipment (pressure regulators or valves) and reduce the risk of fuel spillage or other accidental release. Certain solid fuels are waterproof, and further waterproof protection can be provided by means of a thin plastic wrapper.

To use such a portable cooking stove with a solid-fuel block, the fuel block is placed in the combustion chamber of the stove and a cooking vessel is placed on top of the stove. The solid fuel is set alight and the flames heat the cooking vessel. The problem encountered with solid-fuel blocks is that the characteristically aggressive burn leads to tall flames. These tall flames often spill out of the top of the combustion chamber and spread out around the sides of the cooking vessel, resulting in lost heat energy. This lost heat energy must be accounted for by burning additional fuel blocks, meaning that a greater number of fuel blocks must be carried by the operator. As such, it is of key importance to consider the efficiency with which any given stove arrangement can transfer the energy of the burning solid-fuel block into the cooking vessel. As well as simply heating food and drink, portable cooking stoves provide the critical ability to sterilise food and water. With no consumer standards of performance set for portable stoves, the only benchmarks available are those used by military organisations. These tough standards and performance requirements specify that the stove must be lightweight, robust and capable of raising water from near freezing point to boiling and then to hold the boil for at least 5 minutes. For heating ready-to- eat, pre-packed meals, the stove must be able to raise a standard meal pack of 500 grams in weight plus 250 ml water to a minimum of 65°C. These standards clearly require that the stove plus fuel combination must be highly efficient.

Lightweight portable cooking stoves that can be dismantled and packed in a flat configuration are known in the prior art. The prior art stoves have generally been designed to utilise a number of fuels, most often to utilise burning wood. The efficiency with which prior art stoves can transfer heat from burning solid-fuel blocks to a cooking vessel is generally not optimal. The present invention aims to provide a stove that exhibits improved efficiency.

Summary of the Invention

According to a first aspect, the present invention provides a portable cooking stove comprising at least three sheet-like members, wherein each sheet-like member has two faces, a top edge, a bottom edge and two side edges, wherein the sheet-like members are coupled together at their side edges, wherein the sheet-like members have at least one exhaust opening at or close to the top edge, and wherein the portable cooking stove is characterised in that each sheet-like member comprises a lattice of perforations which covers at least 60% of the surface area of the faces of the sheet-like member.

The provision of a lattice of perforations provides a portable cooking stove that can, surprisingly, increase the efficiency with which energy is transferred from burning fuel to a cooking vessel. The perforations are effectively air inlets, and allow air into the stove. Using a lattice of perforations was found to restrict airflow into the stove in a manner that reduces the intensity with which fuel, such as a solid-fuel block, burns. This solves the problem of the intense burning of solid-fuel blocks leading to tall flames that do not remain underneath the cooking vessel. Careful investigation was required to ensure that the burn rate was lowered without leading to volatile organics to not combusting at all (causing white smoke) or only partially combusting (causing black smoke). Furthermore, providing air inlets in the form of a lattice of perforations provides more protection to the burning fuel from gusts of wind than would be provided by fewer larger openings, as in prior art stoves.

Prior art stoves have typically been designed to be compatible with a variety of fuels, primarily wood. As such, optimisation of portable cooking stoves specifically for burning solid-fuel blocks has not been investigated. Typical constructions of these prior art stoves have involved using tall structures. While this creates a large combustion chamber that goes some way to maintain more of the flame under the cooking vessel, the stoves are heavy and have a larger surface area from which to radiate heat.

In a preferred embodiment of the present invention, each sheet-like member defines a substantially square, rectangular or trapezium shape so that the assembled stove, which is the sheet-like members coupled together, defines a substantially cuboid, prism or pyramidal frustum shape. The coupled sheet-like members define the combustion chamber and also provide the structural support upon which to place the cooking vessel. The use of a pyramidal-frustum shaped cooking stove allows the heat from the burning fuel to be concentrated more centrally underneath the cooking vessel.

An important feature of the portable cooking stove is that it is modular. The stove may be assembled from a minimum of three sheet-like members. Should the cooking vessel be larger, the stove can easily be assembled from more sheet-like members, for example four, five or six sheets, to accommodate both more fuel and the larger cooking vessel. The stove is designed such that the increased number of solid-fuel blocks in the larger version of the stove maintains the optimal burn rate. The sheet-like members have at least one exhaust opening at or close to the top edge. This is an important feature of the stove of the present invention as controlling the rate at which the exhaust fumes exit the stove assists with controlling the burn rate of the fuel. Allowing the exhaust fumes to vent too freely will allow the fuel to burn vigorously. Preventing the exhaust fumes from venting freely enough will extinguish the burning fuel. The inventor has found that providing at least one exhaust opening at or close to the top edge of each sheet-like member can optimise the air flow through the stove, and hence the efficiency. Preferably, the combined surface area of all the exhaust openings on a single sheet-like member is up to 4 cm ² , more preferably up to 2 cm ² . In a preferred embodiment, the total exhaust opening area per sheet-like member comprises two exhaust openings per sheet-like member, wherein each exhaust opening has a surface area of up to 2 cm ² , preferably around 1 cm ² .

It has been found that the optimal airflow into the stove can be achieved when a lattice of perforations covers at least 60% of the surface area of the faces of the sheet-like member. It is preferred that the lattice of perforations covers at least 70% or even 80% of the surface area of the faces of the sheet-like member.

As mentioned above, using a lattice of perforations, rather than a small number of relatively large air inlets, has the advantage of giving better control of fuel burn rate, and better wind protection. By using a lattice of perforations, a large number of relatively small air inlets is provided, in contrast to prior art stoves.

It has been found that the optimal airflow into the stove is achieved when the lattice of perforations has a grid size of at least 2 by 3 perforations, preferably at least 10 by 10 perforations, more preferably 15 to 25 by 15 to 25 perforations, as in preferred embodiments of the present invention. The size of the perforations and the density of perforations is also important. It has further been found that the optimal airflow into the stove is achieved when each perforation has a surface area of 0.7 to 13 mm ² , preferably wherein each perforation is circular with a diameter of 1 to 4 mm, most preferably 1 to 2.5 mm. It has further been found that the optimal airflow into the stove is achieved when the perforations of each sheet-like member have a total surface area of 10 to 55 mm ² per cm ² , preferably 20 to 40 mm ² per cm ² in the region of the lattice.

In a preferred embodiment of the present invention, the coupling between side edges of the sheet-like members is reversible, so that the stove can be assembled and disassembled during use. Preferably when the stove is disassembled, the sheet-like members can be arranged to lie flat on top of one another. These, embodiments allow the portable cooking stove to be stored and transported as a flat-pack, which is space- economic and can be conveniently stowed and carried. Such stoves can be rapidly and conveniently assembled, used and then returned to the flat-pack configuration after use.

According to second aspect, the present invention provides a sheet-like member which has two faces, a top edge, a bottom edge and two side edges, wherein the side edges of the sheet-like member are adapted to be coupled to another sheet-like member, and wherein the sheet-like member is characterised in that it comprises a lattice of perforations which covers at least 60% of the surface area of the faces of the sheet-like member. These sheet-like members can be assembled to form a portable cooking stove according to the first aspect of the present invention.

According to a third aspect, the present invention provides a kit comprising at least three sheet-like members of the second aspect of the invention and pre-made solid- fuel blocks. Examples of suitable solid-fuel blocks are hexamine blocks (e.g. as supplied by ESBIT Gmbh, Germany), trioxane blocks (e.g. as supplied to the USA military), solidifed methyl decanoate blocks (as supplied by Zip Military Fuels, UK) or gelled alcohol packs. According to a fourth aspect, the present invention provides a method of heating a cooking vessel using the cooking stove of the first aspect of the invention, wherein a solid-fuel block is placed in the portable cooking stove and set alight, and a cooking vessel is placed onto the portable cooking stove. Brief Description of the Drawings

Figure 1 relates to a preferred embodiment of the present invention, a kit of six sheetlike members that can be used to assemble a three, four, five or six-sided stove.

Figure 2 relates to a preferred embodiment of the present invention, an assembled four-sided stove shown without fuel or cooking vessel.

Figure 3 relates to a preferred embodiment of the present invention, an assembled six- sided stove shown without fuel or cooking vessel.

Figure 4 relates to a preferred embodiment of the present invention, an assembled six- sided stove shown without fuel but with a cooking vessel in the cooking configuration. Figures 5 to 6 relate to experimental results as explained in the Examples.

Description

This invention relates to a portable cooking stove. By portable we mean that the stove can easily be carried by one person. As explained above, it is envisaged that the stove will primarily be used by those undertaking camping or other outdoor leisure pursuits. The portable cooking stove comprises sheet-like members. By sheet-like member we mean a sheet of material that can be either planar or curved, and is of a shape that can be defined as having a top edge, a bottom edge, two side edges and two faces. Typically this would be achieved by the sheet-like member having a substantially square, rectangular or trapezium shape when viewed face-on. When a rectangular sheet-like member is used, either the long edge or the short edge would define the bottom edge. When a trapezium-shaped sheet-like member, either of the parallel edges would define the bottom edge. It is intended that, during use, the bottom edge will be lowermost, typically on the ground, or other support, and the top edge will be uppermost. In a preferred embodiment the sheet-like members are constructed from a fire-proof material, preferably wherein the material is a metal or alloy such as aluminium, titanium, nickel, copper, mild steel, stainless steel or brass. This ensures that the stove is able to withstand the heat of the burning fuel without burning or substantially deforming while being strong enough to support a cooking vessel and robust enough for repeated use.

The stove comprises sheet-like members coupled together at their side edges. By coupled at their side edges, we mean that the side edges must be aligned such that the length of the side edge of one sheet-like member is substantially positioned along the length of the side edge of the other sheet-like member. The side edges include a coupling mechanism that holds the two side edges together. The coupling mechanism can be the shape of the side edge, or the region close to the side edge, or can be a hinge.

The sheet-like members may, for example, be coupled using a slit substantially parallel to and within 10 mm of the side edges. By this, we mean that a substantially rectangular section, with two long edges and two short edges, is missing from the sheet-like member, the long edges of the slit running substantially parallel to the side edge, and the short edges defining a width that is at least that of the thickness of the sheet-like member. The slit may also be cut into a protruding section of the side edge. The slit may extend from either the top or the bottom edge, or the top or the bottom edge of a protruding section of the side edge, such that three edges of the slit are defined by the sheet-like member and the final edge is a short edge that is open. The slit may also be positioned such that the slit is closed on all four edges. The slit of one sheet-like member can couple with a complementary slit of another sheet-like member. By complementary, we mean that where one slit has an open edge facing the bottom edge of the sheet-like member, the other slit has an open edge facing the top edge of the sheet-like member, and the slits are both positioned such that they can be used to couple the members. Where a first slit is closed on all four edges, the complementary second slit is a slit with an open narrow edge facing either the top or bottom edge of the sheet-like member, and having a closed narrow edge at substantially the same height as either the upper or lower closed narrow edge of the first slit. Two complementary edges can be coupled by inserting the complementary slits into each other. The coupled sheet-like members can also be readily disassembled. By this we mean that after use, each sheet-like member of the portable cooking stove can be separated. The sheet-like members can then be stored in a flat- packed configuration. By flat-packed configuration, we mean that the sheet-like members can be arranged such that they are stacked with their faces aligned, thus taking up minimal space. A further option is that the sheet-like members can be coupled using hinges. By this we mean that the mechanism that holds the two side edges together is a pivoting hinge that pivots along the same axis as the side edge. Using this mechanism, the sheet-like members can be folded into the stove configuration for use, and after use can be folded into a flat-pack configuration for storage. The stove comprises at least three sheet-like members, and preferably comprises four to six sheet-like members, more preferably five sheet-like members. Using three sheet-like members as an example, a first side-edge of a first sheet-like member is coupled to a first side-edge of a second sheet-like member. The second side-edge of the second sheet-like member is coupled to a first-side edge of a third sheet-like member. The second side-edge of the third sheet-like member is coupled to the second side-edge of the first sheet-like member, thus creating a closed loop of sheetlike members. Optionally, the last coupling may be left open. To create a stove with more sheet-like members, the same principle is applied to create a loop of sheet-like members, coupled at their side-edges. Where a sheet-like member has a square or rectangular shape, the assembled stove will resemble a prism. Where three sheet-like members are used, the assembled stove will resemble a triangular-based prism. Where four sheet-like members are used, the assembled stove will resemble a square-based prism, or a cuboid. Where five sheetlike members are used, the assembled stove will resemble a pentagon-based prism. Where six sheet-like members are used, the assembled stove will resemble a hexagonal-based prism, and so on.

Where a sheet like-member has a trapezium-shaped sheet-like member, and the long parallel edge defines the bottom edge, the assembled stove will resemble a pyramidal frustum. Where a sheet-like member has a trapezium-shaped sheet-like member, and the short parallel edge defines the bottom edge, the assembled stove will resemble an inverted pyramidal frustum. Where three sheet-like members are used, the assembled stove will resemble a triangular-based pyramidal frustum. Where four sheet-like members are used, the assembled stove will resemble a square-based pyramidal frustum. Where five sheet-like members are used, the assembled stove will resemble a pentagon-based pyramidal frustum. Where six sheet-like members are used, the assembled stove will resemble a hexagon-based pyramidal frustum, and so on.

The portable cooking stove of the present invention includes sheet-like members which each comprise a lattice of perforations which covers at least 60% of the surface area of the faces of the sheet-like member. By perforations, we mean holes that extend through the sheet-like member from face to face. As explained above, these holes act as air inlets.

By lattice, we mean that the perforation are arranged in a grid. In other words, there is a plurality of perforations arranged in a series of lines, one above the other. The lattice of perforations provides regular spacing of the perforations. In the lattice there are at least 2 lines of perforations with at least 3 perforations in each line, i.e. a grid size of 2 by 3, with a total of at least six perforations. Preferably the lattice comprises a grid of at least 10 by 10, and most preferably between 15 to 25 by 15 to 25.

As noted above, the inventor has surprisingly found that using a lattice of perforations can have the effect of controlling the air flow into the stove which can increase the efficiency of the fuel. The lattice of perforations also gives protection against gusts of wind. Prior art stoves which have used fewer but larger air inlets do not provide as much protection from the wind, or as much control over the burn rate.

The lattice of perforations covers a substantial proportion of the face of the sheet-like member. In particular, it covers at least 60% of the surface area of the faces of the sheet-like member. In a preferred embodiment, the lattice of perforations covers at least 70%, or preferably at least 80% of the surface area of the faces of the sheet-like member. In the present invention, the size of the perforations and the density of the perforations is also important. In particular, the inventor has found that the performance can be optimised when each perforation has a surface area of 0.7 to 13 mm ² . Preferably each perforation is circular with a diameter of between 1 and 4 mm, preferably between 1 and 2.5 mm. The density of perforations is preferably such that, in the region of the lattice, the perforations have a total surface area of 10 to 55 mm ² per cm ² . Preferably the perforations have a total surface area of 20 to 40 mm ² per cm ² in the region of the lattice.

As well as inlets, which are provided by the perforations, it is important that the portable cooking stove has an air outlet. This is provided by at least one exhaust opening in each sheet-like member which is at or close to the top edge. B close to the top edge, we mean that it is provided within 2 cm of the top edge. The exhaust opening is simply a hole in the sheet-like member, or notch cut into the otherwise substantially straight the top edge. Preferably the total surface area of the exhaust opening or exhaust openings is up to 4 cm ² , preferably up to 2 cm ² . In a preferred embodiment the exhaust openings are notches in the top edge of the sheet-like members. In the most preferred embodiment the top edge of the sheet-like member is castellated to provide two notches in the top edge which are exhaust openings and allow exhaust gases to escape the stove, when a cooking vessel is placed on top of the stove. The two notches can have a surface area of around 1cm ² each.

According to second aspect, the present invention provides a sheet-like member which has two faces, a top edge, a bottom edge and two side edges, wherein the side edges of the sheet-like member are adapted to be coupled to another sheet-like member, and wherein the sheet-like member is characterised in that it comprises a lattice of perforations which covers at least 60% of the surface area of the faces of the sheet-like member. These sheet-like members can be assembled to form a portable cooking stove according to the first aspect of the present invention.

According to a third aspect, the present invention provides a kit comprising at least three sheet-like members of the second aspect of the invention and pre-made solid- fuel blocks. Examples of suitable solid-fuel blocks are hexamine blocks (e.g. as supplied by ESBIT Gmbh, Germany), trioxane blocks (e.g. as supplied to the USA military), solidifed methyl decanoate blocks (as supplied by Zip Military Fuels, UK) or gelled alcohol packs.

According to a fourth aspect, the present invention provides a method of heating a cooking vessel using the cooking stove of the first aspect of the invention, wherein a solid-fuel block is placed in the portable cooking stove and set alight, and a cooking vessel is placed onto the portable cooking stove. As the solid-fuel block burns, air is drawn in through the lattice of perforations, maintaining combustion. The coverage of a substantial area of the sheets by a lattice of perforations restricts the airflow, resulting in a slower rate of combustion. As the solid-fuel block burns, air also exits from the exhaust openings. Careful control of airflow in and out of the combustion chamber ensures that, even though combustion is slowed, there is enough airflow to ensure full combustion occurs. This means that, primarily, the efficiency of heat transfer to the cooking vessel is high, but also that there is no black or white smoke generated, and that the burning fuel doesn't extinguish.

Detailed Description of the Drawings

Figure 1 shows a preferred embodiment of the sheet-like members, 1, that can be assembled to form the portable cooking stove of the present invention. Figure 1 shows the sheet-like members, 1, having two faces, 2, a top edge, 3, a bottom edge, 4 and two side edges, 5. The sheet -like members, 1, comprise two exhaust openings, 6, which are in the form of a castellated top edge, 3. A lattice of perforations, 7, covers a substantial portion of the faces, 2. A protruding edge, 8, extends from one side edge, 5, which has slits, 9. The opposite side edge, 5, of the sheet-like members, 1, have one open slit, 10 and one closed slit, 11, for coupling individual sheet-like members, 1.

The sheet-like members, 1, shown in figure 1 can be coupled at their side edges, 5, to form a four-member portable cooking stove, 12, as shown in figure 2, or to form a six- member portable cooking stove, 13, as shown in figure 3. Figure 4 shows the portable cooking stove, 13, of figure 3 in use with a cooking vessel, 14, placed in the cooking configuration. The cooking vessel, 14, has a flat bottom that lies flush with the top edge, 3, of the sheet-like member, 1. This causes the exhaust openings, 6, to be closed on all sides.

Examples

The following examples provide details of the performance of specific embodiments of the present invention. The portable cooking stove of the present invention used in the examples is that shown in figures 1 , 2, 3 and 4 as described above.

The following conditions were measured. Wind speed is in miles per hour and was measured using a Testo-410-1 Handheld anemometer. Air pressure is in kPa and was measured using a wall-mounted digital barometer (Fischer Scientific). Dew point is given in degrees centigrade and was determined online using Acuweather.com. Relative humidity is given in percentage and was measured using a Picolog combination digital humidity probe/ambient temp. Ambient temperature is given in degrees centigrade and was measured using a Picolog combination digital humidity probe/ambient temp. Experiments were conducted in Moncarpacho, Portugal.

Example 1 - four-sided portable cooking stove (as shown in figure 2) comprising sheet-like members with a top edge of 70 mm in length, a bottom edge of 90 mm in length and the distance between the top and bottom edges being 65 mm. The experiment was conducted using one or two Zip (RTM) fuel blocks to heat 500ml of water in a standard issue UK military cup from ambient temperature to boiling. Zip (RTM) fuel blocks are available commercially. A comparison is made between stoves with circular perforations of different diameters. The different diameters and density of holes are shown below. lmm

0.79mm ² /perforation

5 x 6 perforations/cm ²

30 perforations/cm ² 23.57mm total perforation area / cm 2mm

3.14mm ² /perf oration

3 x 3 sperforations/cm ²

9 perforations/cm ²

28.28mm ² total perforation area/cm ² of plate 3mm

7.07mm ² /perforation

2 x 2.5 perforations/cm ²

5 perforations/cm"

35.35mm ² total perforation area cm ² of plate 4mm

12.56mm /perforation

2 x 2 perforations/cm ²

4 perforations/cm ²

50.27mm ² total perforation area/cm ² of plate

The results are shown in figures 5 and 6, and are tabulated below.

Table 1. 1 x Zip 26gm block

Perforation diameter 1mm 2mm 3mm 4mm

Time to boil (minrsec) 20min 30sec 15min 53sec 9min 3sec llmin 43sec

Boil length (min:sec) 9min 45sec llmin 3sec 13min llmin 5sec

Total burn time (min:sec) 28min 36sec 27minS7sec 22min 40sec 21min 54sec

Wind speed (mph) 0 - 1 0 - 1 0 - 1 0 - 1

Air pressure (kPa) 102.5 102.5 102.5 102.5

Ambient temperature (degC) 12.5 14 16 10

Dew point (degC) 12 12 12 12

Relative humidity (%) 80% 80% 80% 80%

Table 2. 2 x Zip 26gm block

Perforation diameter 1mm 2mm 3mm 4mm

Time to boil (mimsec) 20min 12sec 13min 20sec 9min Osec 8min 13sec

Boil length (min:sec) 29min 21min+ 15min 27sec 12min 55sec

Total burn time (min:sec) 50min lOsec 33mirt Osec 25min 22sec 21min 54sec

Wind speed (mph) 0 - 1 0 - 1 0 - 1 0 - 1

Air pressure (kPa) 102.5 102.5 102.5 102.5

Ambient temperature (degC) 12.5 14 16 10

Dew point (degC) 12 12 12 5

Relative humidity (%) 80% 75% 80% 68%

A comparison of the data in figures 5 and 6 and the conesponding data in tables 1 and 2 shows a clear trend. The larger 4 mm hole allows air to enter the stove more readily, resulting in a more vigorous burn. The time taken for the water to start boiling is relatively short, but the boil cannot be sustained for long. As the diameter of the hole is reduced from 4 mm to 1 mm, the burn rate of the fuel reduces. This can be seen in that the water takes longer to boil. Critically, however, the length of the boil becomes longer. Even though the increased time involved provides more time for heat to be lost through radiation, this is massively overcompensated by the increased efficiency with which energy is transferred from the fuel block to the cooking vessel. The effect is very pronounced when using 2 Zip fuel blocks, with the boil time more than doubled through simply using a 1 mm diameter rather than a 4 mm diameter. Critically, the relationship between the diameter of the perforation and the burn rate of the solid-fuel block can be exploited by tuning the perforation size to obtain the desired balance between time-to-boil and boil length required.