Burner for solid fuels, having means for feeding out ashes

TECHNICAL FIELD OF INVENTION

The invention refers to a burner arranged for the combustion of solid fuels, and especially biologic fuel, comprising a cassette which is insertable in a central heater to form a combustion chamber that opens towards the interior of the central heater, the cassette having a bottom with longitudinal sides that connect to upright and mutually parallel side walls between which a fire hearth can be supported on the bottom of the cassette, a fuel feed channel mouthing above the bottom of the cassette via a wall to a housing that is connected to the cassette and which is situated outside the central heater as the cassette is inserted therein, and air feed channels through which combustion air can be supplied from outside to the burning embers. More precisely, the invention refers to a burner of this type having a rake arranged for feeding out unburned fuel and ashes in accordance with the preamble of claim 1.

BACKGROUND AND PRIOR ART Burners of the subject kind are typically used for heat production on a smaller scale from singe family houses up to apartment blocks and small industries, and are available on the market in effects ranging from the order of 20 kW up to 300 kW which is mentioned here as examples merely with no intent to limit the invention to this use and size of burners.

The subject solid fuel burners typically use biofuel that is harvested from forests or fields, such as non-processed wastes from forestry or from the wood industry, or wastes processed into chips, briquettes, pellets etc., or agrifuel such as grain, peas, straw, elephant grass, Salix, or sieved out wastes or other suitable waste products from forestry, agriculture and gardening. The expression biofuel shall in this context be read to include all previously mentioned and other not mentioned biological fuels, fossil fuels included, which can be supplied to the burner from a close by fuel supply, typically by means of a mechanical fuel feeder. An example of prior art solid fuel burners referred to the same category of burners as the burner of the invention is disclosed in SE 514 364 C2. Although the efficiency of this burner results in comparatively small amounts of unburned fuel and ashes, a certain accumulation of ashes is formed over time on the bottom of the cassette from where the material needs to be removed at certain intervals in order to maintain an efficient combustion.

An attempt to solve this problem is disclosed in WO 01/84048 wherein a solid fuel burner is equipped with a rake arranged for feeding out unburned fuel and ashes. This rake is connected to the end of a rod which is movably supported in a housing and associated with a drive means that causes the rake to move linearly in the forward and backward directions. Since this rake also forms the bottom on which the fire hearth is supported, this structure cannot readily be applied to the burner of the present invention, having a stationary cassette bottom from which the material shall be removed in the length direction of the cassette.

When designing a rake for removing unburned fuel and ashes from the cassette of a burner having the structure as initially described, the skilled person faces the problem of forming the rake so as not to be hindered abruptly in its movement by getting caught by irregularities such as slag sticking firmly to the surfaces of the bottom or side walls of the cassette.

SUMMARY OF THE INVENTION

The present invention aims to solve this problem and avoid the risk of the rake getting caught in its feeding out movement, as well as in its return movement, by possible build ups in the surface of the cassette bottom or side walls.

This object is met in a burner for solid fuel having a rake as specified in claim 1, briefly characterized in tat the rake comprises a shield associated with a guiding member arranged close to the rear side of the shield, such that the guide member projects outside the edge of the shield, at least outside the bottom edge of the shield so as to contact the bottom of the cassette, the guiding member being an elastic element arranged at sufficient distance from the rear side of the shield so as to allow the guide member to flex in the movement directions of the rake, under the forward motion and under the return motion, to a position at least where the edge of the flexible element is aligned substantially with the edge of the shield.

The resiliency of the guiding member avoids abrupt stops in the movement of the rake caused from contact with build ups in the bottom of the cassette, which might otherwise cause jerks that could lead to damage or breakage in rake supports and drive means. The elastic property of the guiding member causes the rake to return to its normal position when the obstacle has been passed.

In a preferred embodiment, the guiding member may additionally project outside the side edges of the shield, respectively, so as to contact the side walls of the cassette, this way preventing also the rake from getting stuck to irregularities in the side wall surfaces of the cassette.

It is conceivable, naturally, to arrange the guiding member as a rigid member elastically supported on the shield. Preferably though, the guiding member is manufactured from an elastic material. In a highly preferred embodiment, the guiding member is a one piece element made from spring steel.

Among alternative embodiments one should be mentioned wherein the guiding member is formed with two or more tongues of spring steel having free ends that, in their unloaded condition, project outside the edge of the shield.

In another alternative embodiment the guiding member comprises multiple teeth made of spring steel that project outside the edge of the shield like a comb, in their unloaded condition.

Yet an alternative embodiment comprises a guiding member forming a brush of steel fibres that project outside the edge of the shield like a comb, in their unloaded condition.

The advantage achieved by forming the guiding member with tongues, teeth or brush fibres is mainly that the resistance and stopping effect is reduced when the guiding member is moved over an irregularity, since only a portion of the elastic element is effected, which results in a smooth movement that spares the connection, bearings and drive means concerned. Another advantage is that the rest of the elastic element is unaffected by any obstacle, and will maintain its contact with the subject surface of the cassette, this way also contributing to a cleaning of the cassette surfaces during the feeding out movement.

In all embodiments it is of further advantage when the guiding member is arranged to extend in parallel with the edge of the shield and substantially for the total length of the shield's edge, in the unloaded condition of the guiding member.

It would also be conceivable to arrange the guiding member so as to be supported on the rear side of the shield. Preferably, and in order to ensure that adequate space is provided for the guiding member to flex in both directions of movement of the rake, the guiding member is mounted on the rod that carries the rake, with a spacer element inserted between the shield and the guiding member. In this embodiment is also ensured the provision of a gap for cooling between the guiding member and the shield, which is exposed to the fire hearth.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be more closely explained below with reference made to the accompanying schematic drawings. In the drawings

Fig. 1 is a schematic sectional view of a central heater with a burner according to the invention connected thereto;

Fig. 2 is a schematic sectional view on a larger scale showing the burner having means for feeding out ashes in accordance with the invention,

Fig. 3 is perspective view showing the burner according to the invention on a further enlarged scale, showing a rake for feeding out ashes in alternative working positions, and

Figs. 4a-4d are partly broken away end views showing the burner's cassette with a rake, illustrated in four different embodiments, all of which are seen in a direction towards the rear side of the rake, i.e. in the feeding out direction. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS For purpose of explaining the invention, a burner for solid fuel to which the invention is implemented will initially be described. However, it should be pointed out that the detailed design of the illustrated burner is not necessarily required in order to benefit from the invention, and that the invention may likewise be implemented in burners that depart from the illustrated burner in aspects which are insignificant for the invention.

In fig. 1 , reference number 1 denotes a central heater having an interior combustion space 2 and an outlet 3 for flue gases. A burner, commonly referred to by reference number 6, is mounted at an opening 4 through a front wall 5 of the central heater. With reference to figs. 1 and 3, this burner 6 comprises a cassette 7 that is insertable into the central heater, said cassette defining a combustion chamber 8 which is open towards the interior of the central heater, and which is assembled from an upper part 9 and a lower part 10. The lower part 10 of the cassette includes a boxlike structure which is open outwardly from the central heater but closed at its inner end. The boxlike structure has centrally a horizontal bottom 11 and two side walls 12 rising from the bottom, the side walls comprising holes for supply of air to the combustion chamber. In addition to the bottom 11 and side walls 12, the boxlike structure 10 includes a secondary bottom 13 situated below the first bottom 11. Along the opposite longitudinal sides thereof the secondary bottom 13 adjoins two vertical side pieces 14, the upper margins of which adjoin planar and horizontal sections 15 which on the other hand by their inner margins adjoin the side walls 12. As seen in Fig. 3, the side walls 12 may advantageously be inclined (upwards /outwards) so as to form a trough- or channel shaped configuration. The sections 11, 12, 13, 14 and 15 together define a substantially U- shaped cavity which is closed by means of an end wall 16 at its inner end. At its opposite end, the cavity opens into the interior of a housing 17 to be explained below.

Accordingly, the cassette 7 is connected to a housing 17 which in the mounted position is situated on the exterior of the central heater. More precisely, the lower part 10 of the cassette is inserted through a first opening 18 formed through a front wall 19 of the housing 17. Outwardly from the front wall 19, a cover of four- sided section extends with two vertical side walls 20 and two horizontal, upper and bottom walls 21, 22. The said walls are all connected to an outer vertical wall 23 carrying a fan 24 effective for blowing air into the interior of the housing. A pipe 25 reaches through the interior of the housing, forming a fuel supply pipe the upper end of which forms into a vertical pipe socket 26 for connection of a suitable fuel supply line, such as a flexible tube. The opposite end of the fuel supply pipe mouths at an opening 27 through the front wall 19 of the housing 17. More precisely, the opening 27 is an inlet opening which is centrally located just above the opening 18 into which the lower part 10 of the cassette is inserted. The pipe 25, running inclined at a suitable angle such as at 45 between its ends, may be circular or rectangular in section to provide a shaft via which solid fuel bodies, such as pellets, can slide or fall non-frictionally towards the bottom 11 of the cassette. It should be pointed out that the pipe 25 is connected airtight towards the walls 19 and 21 at its two ends, respectively.

In the illustrated burner, air supply holes arranged in the lower part 10 of the cassette may be formed as elongate slots 28 that are located in the transition region between the central bottom plate 11 and each side wall 12. More precisely, each singular slot 28 comprises a first portion formed in the bottom plate 11 and a second portion formed in the adjacent side wall 12. As is clearly seen in fig. 3, each slot 28 has an angular orientation relative to the longitudinal direction of the cassette. When the lower part of the cassette is mounted in the opening 18, the opening is closed and air is prevented to pass axially along the outwardly open channel that is formed between the side walls 12 and the bottom plate 11. Air to be passed via the slots 28 is thus forced to pass via the cavity 33 inside the cassette lower part 10. As fig. 3 also clearly shows, slots 28 are only formed for a limited length of the bottom plate 11, and more precisely in a region adjacent the housing 17. In the embodiment of a burner as heretofore described in order to illustrate the invention, a hole realized as a piece of pipe 29 is arranged on each side of the fuel supply opening 27 and effective for supplying air to the combustion chamber 8. By operation of the fan 24, an overpressure is generated inside the housing 17 by which air is discharged via the pipes 29 as substantially axial air beams into the combustion chamber.

A rake, commonly denoted 30, is configured for feeding out ashes and unburned fuel in the length direction of the cassette. In fig. 2, the rake is shown in its returned resting position in which the rake 30 is located in that end of the cassette which is close to the housing 17. The rake 30 is carried in the end of a rod 32 which is supported in the housing for forward and backward movement, driven by a drive means 33, such as linear drive, acting on the rod. The drive means is controllable to be activated electrically, or pneumatically or hydraulically. The drive means and the rake may be arranged for manual or/ and automatic activation through control means and regulators (not shown) installed for controlling the operation of the burner and the central heater.

The rake 30 comprises a plate-shaped shield 31 having a height adapted to the vertical depth at which the bottom of the cassette is located, and a width adapted to the horizontal width between the cassette's side walls 12. The shield is defined by an upper edge, a bottom edge u and two side edges s. Preferably, the shield is dimensioned substantially in correspondence with the sectional shape of the cassette defined by the bottom 11 and the side walls 12, or slightly undersized with respect to the sectional shape.

Fig. 3 shows the rake 30, on one hand in its returned position wherein the shield 31 is illustrated by continuous lines adjacent to the wall 19 of the housing, and on the other hand in a forward projected position wherein the shield 31 is positioned close to the free end of the cassette 7. For purpose of visibility, the shield 31 is illustrated with dash-dot lines and the rod 32 is only shown in part in the projected position of the rake. On that side of the shield 31 which faces the housing 17, defined below as the rear side of the shield, a guiding member 33 is supported. The guiding member 34 can be realized in various ways, as will be explained in the following. In all embodiments it is a common feature that the guiding member projects for a portion outside the margin of the shield, at least outside the bottom edge u so as to contact the bottom 11 of the cassette. Further in all embodiments, the guiding member 34 is an elastic element which is arranged at such distance from the rear side of the shield by which the guiding member is permitted to flex in the shield's directions of movement F, both during the forward movement and during the return movement. Preferably, the spacing between the shield and the guiding member 34 is set such that the guiding member can bend elastically to a position wherein the edge of the guiding member is substantially aligned with the bottom edge u of the shield 31, in a view perpendicular to the plane of the shield. To this purpose, the size of said spacing may be in the order of 5 mm or more, and should at least not be smaller than 2 mm, but can also be substantially larger and amount to the order of 2-50 mm depending on the other structure and dimensions of the subject burner.

As illustrated in fig. 4a, the guiding member 34 may further be configured as a one piece disk 34A made of spring steel, having inherent flexibility, and which advantageously can have a shape that substantially corresponds to the sectional shape of the cassette and the shape of the shield 31 , while at least the bottom edge thereof projects outside the bottom edge u of the shield by a portion having a width Wu ranging from 0.5 to 5 mm.

As illustrated in fig. 4b, the guiding member 34 may alternatively be configured to have two or more tongues 34B made of spring steel, the free ends of which project outside the edge of the shield in the unloaded condition of the tongues.

A third alternative embodiment comprises a guiding member 34 including multiple teeth 34C made by spring steel, the teeth together in similarity to a comb projecting outside the edge of the shield in the unloaded condition, as illustrated in fig. 4c.

It is also conceivable to arrange the guiding member 34 as illustrated in fig. 4d, realized as a brush 34D including steel fibres that project outside the edge of the shield in their unloaded condition.

In all embodiments it is advantageous, in order to ensure the functionality, when the guiding member 34 is arranged in the unloaded condition to project in parallel with the edge of the shield, and additionally for the total length of the shield edge.

In all embodiments it is further of advantage and likewise preferred when the guiding member 34 is configured also to project outside the side edges s of the shield 31, with a portion if corresponding width of 0.5 to 50 mm, so as to contact the side walls 12 of the cassette.

For a complete description of the invention reference is also made to reference number 35, defining a spacer element 35 that is inserted between the shield 31 and the guiding member 34, and which provides the guiding member adequate flexing space for an unhindered flexing movement towards the shield. It should be realized that in practise the required flexing space not need to be longer than to permit the guiding member to bend to position wherein the edge of the guiding member is aligned with the edge of the shield. The gap formed by the spacing member between the shield and the guiding member also provides improved cooling of the guiding member, which increases the option of suitable materials for a guiding member 34. The shield 31, which may be manufactured from stainless steel or other high quality steel, also acts in conjunction with the gap as a heat shield that protects the guiding member 34 during operation of the burner.