WO1999036731A1

WO1999036731A1 - A furnace structure, a method for burning wood as well as a method for retrieving combustion heat

Info

Publication number: WO1999036731A1
Application number: PCT/FI1999/000018
Authority: WO
Inventors: Heikki HYYTIÄINEN; Michael Struschka
Original assignee: Puulämpö Suomi Oy
Priority date: 1998-01-13
Filing date: 1999-01-13
Publication date: 1999-07-22
Also published as: FI104345B; FI104345B1; FI980847A0; AU1970599A

Abstract

The invention relates to a furnace structure for burning of wood, said structure comprising a combustion chamber (1) including a grate (2) and an inlet (7, 23) for secondary air (12, 22), an after combustion space (9, 14), located above said combustion chamber, and a smoke conduit (30, 32) connected thereto. The structure comprises a combustion conduit (14, 25) which is designed in compliance with an essentially uniform laminar velocity of flow for said combustion gas (10, 15), said combustion conduit (14, 25) constituting an integral after combustion space and smoke box having guiding and heat transferring means (16, 17, 29) of a low heat capacity, said means changing the orientation of said combustion gases first in a horizontal direction and, thereafter, in a vertical direction. The invention further relates to a method for burning of wood, wherein exhausting combustion gases (10, 15) are retained in an integral hot zone (14, 25) in a furnace by controlling the amout of air (2, 12, 22) fed into a combustion chamber (1), while internally stirring said combustion gases (10, 15) by changing the essential orientation of the combustion gases/flames (10, 15, 8) immediately above said combustion chamber (1). The invention further relates to a method for recovering heat from hot combustion gases (15), which gases are initially conducted downwards in a first conduit (25) and upwards in a conduit (30) on the outside thereof.

Description

A furnace structure, a method for burning wood as well as a method for retrieving combustion heat

The invention relates to a furnace structure especially for burning of wood or the like, especially to a furnace core cassette having a combustion chamber including a grate and a secondary air inlet, an aftercombustion space and a smoke box located above said combustion chamber, as well as a smoke conduit connected to said smoke box. The invention also relates to a method for burning of wood or the like in a furnace structure comprising a combustion chamber having a grate arranged underneath, as well as a secondary air inlet, an aftercombustion space located above said combustion chamber, as well as a smoke conduit connected to said after- combustion space. The invention further relates to a method for recovering heat from combustion gases exiting from the upper portion of the combustion chamber in said fireplace.

Traditionally a wood burning furnace structure has comprised a combustion chamber having a grate underneath, through which an essential portion of the oxygenous air needed for the combustion process is introduced into the combustion chamber. A smoke conduit starts at the upper portion of said combustion chamber and in more developed furnace structures said smoke conduit comprises a smoke box wherein smoke gases and any residual air are mixed, thus facilitating an aftercombustion of any still unburned combustible wood distillate gases, after which the smoke gases thus generated will exhaust to a chimney or the like conduit. In modern furnace structures there is, between said combustion chamber and an upper combustion chamber or aftercombustion space connected to said smoke box, also a throat structure which is rather narrow in relation to the further smoke conduit, and with this arrangement one has attempted to bring about a turbulence in the smoke gases and thus improve the mixing of air and combustible gases rising to said aftercombustion space . In order to improve the combustive efficiency of the described basic design furnace structure different kinds of surplus air feeding systems have been developed, through which secondary air is fed especially to such portions where an oxygen deficiency is assumed to occur during the combustion. In this way one can also care for a sufficient presence of combustion air for a possible aftercombustion of the combustion gases.

In tile stoves of old type the hot smoke gases are conducted, from the upper portion of a fireplace arranged as an essentially open space, along both sides of the stove downwards to a collecting conduit located under said fireplace, from which conduit the gases rise behind the fireplace up to a chimney. The purpose of such a structure is that the hot smoke gases should deliver their heat to the essentially large mass of stone which constitutes the actual stove structure. However, the effect is slow due to the slow warming up of the stone material and the delivery of heat is not in all cases in any way efficient, so that a traditional stove initially will be cold, and hot only after a long period of heating. In connection with a very effective heating there still will occur the danger of heating the stone material unevenly and even too much and thus there is a susceptibility for damages.

Along with a growing environmental consciousness still more stringent demands are set on emissions also from wood burning furnace structures . It has also been found that there still is left much to be desired regarding the efficiency rates of known furnace structures, so that a considerable amount of components which are still, as such, combustible are discharged with the smoke. A portion of the amount of heat included in the smoke will also pass as waste heat through the chimney. In spite of experiments made at a vast number of places there has, however, been found no ways through which the efficiency of conventional wood burning furnace designs could any more be considerably raised while simultaneously taking care of keeping the emissions to the atmosphere of certain harmful components emanating from the combustion process, like certain hydrocarbons, particles and the like, at as low a level as possible. Thus furnace structures known for the time being still can be considered as rather unsatisfactory. This is regrettable, taking in consideration that wood as such is a renewable source of energy, the utilization of which also in small scale households should be a target for improvement without, however, causing further environmental problems.

The new type of furnace structure according to the present invention emanates from the problem discussed above and provides a new and, in relation to prior art, considerably more efficient comprehensive solution in order to improve the operating efficiency for the combustion process in a wood burning furnace and for the reduction of harmful emissions. In the same connection there has arisen a need for improving the heat recovery in the furnace arrangement in question as well as in connection with conventional furnaces, and thus one object of the present application is to improve the ^' utilization of the heat in the combustion gases.

The characteristics of the invention are evident from the appended claims. Thus, the furnace structure according to the invention is characterized in that it comprises a combustion conduit which is designed in compliance with an essentially uniform laminar velocity of flow for the combustion gases, said combustion conduit constituting, in direct connection to the upper portion of said combustion chamber, an integral aftercombustion space and smoke box having guiding and heat transferring means of low heat capacity, which means change the orientation of said combustion gases initially in a horizontal direction and, thereafter, in a vertical direction. Especially, the directional guiding is implemented so that said aftercombustion space and said smoke box constitute an integral conduit, the dimensions of which are calculated for the velocity of an essentially uniform laminar combustion gas flow. In said conduit thin guiding means having a low heat capacity are arranged for internally stirring the combustion gases, said means changing the general orientation of said conduit at least 75°. Said guiding means bring about a mixing of hot combustion gases and secondary air, in practice approximately in the direct vicinity of the upper part of the combustion chamber's flame zone. In practice, a furnace in accordance with the invention is favorably implemented as a furnace cassette which is manufactured of sheet metal or the like material having a low heat capacity and, correspondingly, good heat conductance, said cassette being arranged in a furnace of essentially free external design in accordance with one ' s own taste . Due to the thin intermediate structures in the furnace cassette according to the invention the heat transferal from one furnace portion to another will be especially effective and thus an evenly distributed high temperature can be maintained in the furnace's integral aftercombustion spaces and corresponding smoke conduits, resulting in an effective energy utilization.

The method for burning of wood according to the invention is characterized by retaining the exhausting combustion gases in an aftercombustion space / smoke box which constitutes an integral hot zone of the furnace, in a flow having an essentially uniform velocity, on one hand, by controlling the amount of air fed to said combustion chamber and, on the other hand, by guiding said gases through an aftercombustion space of essentially uniform thickness and, correspondingly, through a smoke box of uniform thickness, simultaneously mixing said combustion gases internally by changing, in several subsequent operations, the essential orientation of said combustion gases / flames immediately above the combustion chamber and, especially, in immediate vicinity of a flame zone accompanying the wood charge .

Favorably, in a furnace arrangement according to the present invention said smoke conduit is at the same time designed as a heat exchanger so that it comprises, in the flowing direction of the hot combustion gases, at least one first essentially vertically downwards extending descent conduit, adjoined by a combustion gas ascent conduit at least partly surrounding said descent conduit so that between said conduits is at least one essentially thin common wall portion.

Further, the heat recovering method according to the invention is characterized by conducting the hot combustion gases emanating from the furnace structure initially in a first descent conduit essentially downwards and then in an opposite direction upwards in a second conduit located on the outside of said first conduit and in contact with said first conduit, whereby the temperature of the outer surface of the outer conduit will remain, in the vertical direction, essentially uniform as an essential amount of heat will be transferred from one conduit to the other through one or several thin walls common to said conduits.

The general concept of the invention differs essentially from the furnace designs so far considered to be the most effective, in that there is no clear choking throat between the combustion chamber and said aftercombustion space, in which throat one would aim at a mixing achieved by means of a raised velocity and turbulence of the combustion gases. Quite surprisingly, and deviating from the general course of development in the prior art, it has been found that one can achieve a still better final result if the smoke conduit as such is dimensioned in compliance with a laminar flow, but that there is made at least one sharp bend along one or several guiding means, which are thin and thus have a low heat capacity, and favorably even around the outer edge thereof into the opposite direction. Thus, due to the arrangement of said aftercombustion space relatively near the flames, secondary air introduced especially at the upper level of the wood charge will mix with the combustion gases immediately in connection with the upper portion of the flame zone which in turn will bring about a final combustion of the gases which is clearly more complete than what has been known until now.

On the other hand, the heat recovering solution according to the invention essentially differs from arrangements related to prior art furnace designs therein that the combustion gases are conducted through an effective heat exchanger immediately adjoining the furnace structure and functioning according to the counterflow principle, the outer surfaces of said heat exchanger achieving a uniform high temperature due to said arrangement . Though this heat recovering arrangement favorably essentially immediately connects to the aforesaid furnace structure, said arrangement can be adapted also for other furnace designs known per se .

The present invention will now be described in more detail, with reference to one favorable embodiment thereof and to the appended drawings , wherein

Figure 1 as a sectional drawing discloses a schematic example of one furnace design according to prior art,

Figure 2, correspondingly, schematically discloses a solution in principle of a furnace design according to the present invention,

Figure 3 shows a vertical section, along line B-B in Figure 4, of a furnace design according to one favorable embodiment of the invention, and

Figure 4, correspondingly, shows a horizontal section, along line A-A, of the furnace design according to Figure 3.

A traditional effective furnace intended for burning of wood, as shown in Figure 1, comprises a combustion chamber 1, the bottom of which constitutes a grate 2 under which an ash box 3 is located. A wood charge 4 to be burnt is arranged in said combustion chamber 1 and set to fire in a manner known per se . Oxygenous primary air 5 needed for the combustion of the wood will primarily flow through said grate 2 into the lower portion of said combustion chamber 1 and usually also, through air intakes 7 arranged in the lower half of a combustion chamber feed door 6, chiefly to the upper part of said combustion chamber 1. After ignition said wood charge 4 will burn, under consumption of oxygen in the air, so that the heat building up in the combustion chamber 1 will bring about a volatilization of combustible gases from said wood charge 4. Said gases will ignite and burn especially above said charge 4 and thus the radiation heat from the burning flame 8 will continue the impact upon said charge 4. Simultaneously, due to the carbonization caused by the heat, the coal material in said wood charge 4 will change to ash which gradually falls down into said ash box 3. The heat generated from the combustion process is recovered either directly in a heat recovering arrangement directly arranged at the furnace, e.g. a cooking table, as heat bound to the mass of the furnace structure, or in separate heat recovering arrangements. For ages wood has been burnt traditionally in this manner for benefit purposes .

In the most modern furnace structures the space located over said combustion chamber 1 has further been detached into a separate aftercombustion space 9, i.e. a secondary combustion chamber. The combustion gases 10 which still contain combustible gases like carbon monoxide will then be conducted to said aftercombustion space 9 through a separate narrow throat 11 where one has tried, utilizing the increase in flow velocity and a desired turbulence emanating therefrom, to achieve an effective mixing of said combustion gases 10 and secondary air 12 coming especially through said air inlets 7 in said feed door 6. In said aftercombustion space 9 one tries to burn said combustible gases as fully as possible so that the smoke gases 13 leaving said aftercombustion space 9 mainly would comprise only incombustible components and especially carbon dioxide and water steam, which usually can be conducted straight to a chimney 32.

One problem with these known furnace structures, however, has until now been that the smoke gases, in spite of all efforts, still comprise also a significant portion of combustible components like carbon monoxide or coal gas as well as still combustible harmful hydrocarbons and other components which, as such, contain energy and often also constitute a health hazard. The discharge of such components in uncombusted state in the smoke causes, on one hand, direct health and environmental problems, and, on the other hand, also a loss of energy.

In order to eliminate the deficiency discussed above, the comprehensive furnace solution according to the present invention has been developed. It is a characteristic of this solution that the combustion gases discharged from the upper end of said combustion chamber 1 are retained within an integrally designed furnace structure as long as possible and as hot as possible. Heat will transfer from different portions of the furnace to adjacent portions without significant bonding to the inner structures of the furnace, which guarantees that a high temperature will be very quickly achieved inside the furnace, and this temperature will be especially evenly distributed between different portions of the furnace. This arrangement facilitates an especially efficient after- combustion of the combustion gases, after which the gases are cooled down by the furnace structure which effectively transfers heat and especially by a heat recovering arrangement integrated with the aftercombustion space and the inlet of secondary air, so that as high an amount of energy as possible can be gained for benefit. Thus a maximal energy gain and minimal environmental stress can be achieved with the overall arrangement according to the invention. Regarding the general furnace structure according to the present invention a primary reference is made to Figure 2 disclosing the general principle of the present invention. Said Figure discloses a first aspect of the invention, according to which the furnace structure does not comprise any narrow throat between the combustion chamber 1 and the aftercom- bustion space, and thus the combustion gases are not accelerated in order to achieve a turbulence for mixing combustion gases and air. Instead, a gas conduit, which generally is indicated with reference numeral 14 and which is designed conforming to a laminar flow, will continue at the top of the combustion chamber. Said gas conduit 14 interconnects said combustion chamber 1 and a favorably essentially vertically downwards directed descent conduit 25 acting as an extension of said gas conduit, which in turn functions as a primary aftercombustion space, According to one embodiment of the invention said gas conduit 14 comprises guiding means 16, 17 for the hot combustion gases 15, which effect the orientation of the gas flow to be changed at least 75°, favorably initially about 90° and immediately thereafter a further amount of about 180° .

Unexpectedly it has turned out that this change of the flow direction brings about a clearly better mixing of the gases than the traditional narrow throat structure. Said gas conduit is dimensioned so that the velocity of flow for the combustion gases in theory remains linear, the structural width of said conduit 14 having a dimension which generally is about the functional inner width of the furnace structure or mainly the width of the grate. In a wood charge according to the DIN-standard the length of a piece of firewood is 33 centimeters and the functional width for a furnace structure dimensioned for such firewood is in the order of 38 centimeters.

According to another aspect of the invention said descent conduit 25 for hot gases disclosed above as being a suitably essentially vertical downwards directed extension of said combustion gas conduit 14, is connected to said gas conduit 14 and further constitutes a part of the heat recovering arrangement according to the present invention. According to a favorable aspect of the invention an ascent conduit 30 at least partly extends around said descent conduit 25, in which case a wall 29 of a favorably relatively thin structure and common for both said conduits is arranged between said conduits for facilitating the heat transferal.

The functional cross sectional dimension of said gas conduit 14 acting as an aftercombustion space, i.e. the mutual minimum distance between said guiding means 16, 17 for the combustion gases, as well as the mutual minimum distance between other wall surfaces restricting said conduit, is of an essentially uniform dimension. For a DIN-standard wood charge this distance suitably is in the order of 80 to 120 millimeters, favorably about 90 millimeters. In relation to the grate area the cross sectional area of said gas conduit is favorably in the order of 0.6:1 to 0.3:1, most favorably about 0.45:1, the cross section suitably being approximately rectangular in shape, which facilitates the manufacture of the furnace body e.g. of steel sheet.

The hot combustion gases will flow through this conduit 14 acting as an aftercombustion space and there they will burn essentially out so that they do not any more contain any significant amounts of combustible components. According to an especially favorable embodiment of the present invention said hot gases 15 will hereafter flow along a descent conduit 25 which is designed to have an essentially triangular cross section. Said triangular descent conduit 25 is favorably designed so that one side of the triangle simultaneously constitutes a back wall 19 of said combustion chamber 1, in which case heat can transfer through said wall 19 into said descent conduit 25 also directly from the uppermost portion of said combustion chamber 1. The warm gases will then descend in said conduit 25 downwards, in practice almost to the bottom 35 of the furnace structure. At said bottom 35 said descent conduit 25 will end so that there is formed a gas passage gap 36 opening into said ascent conduit 30, in which gap the orientation of the gas flow will change into the opposite direction. Now the smoke gases, having partially delivered their heat, will be distributed into said ascent conduit 30 favorably having a rectangular cross section and surrounding said triangular descent conduit 25, in which conduit 30 said gases will rise upwards. Thus the hotter gases flowing in said descent conduit 25 will deliver, along the whole way, heat to the relatively colder gases which in said conduit 30 rise in a counterflow direction behind said thin wall 29.

The gases flowing in said ascent conduit 30 will, again, deliver heat, on one hand, via outer surfaces 34, 34a, 34b of said ascent conduit and, one the other hand, especially via heat exchanger tubes 31 arranged in said conduit 30, to the open air, to a separate heat transfer system based on e.g. a fluidum and/or to a mantle surrounding the furnace structure and manufactured e.g. of bricks. At the same time said gases will all the time be in contact also with the relatively hotter surface 29 of said descent conduit 25 and thus the transferal of heat from said descent conduit 25 to the surroundings is as effective as possible. Heat will partly also leave through the end surfaces of said ascent conduit, e.g. through bottom 35 and the surfaces of a collector chamber 37 favorably arranged at the upper end of said conduit 30.

It is typical for the solution according to the present invention and contrary to prior art that the temperature of the heat transferring surfaces, e.g. surfaces adjoining the open air and/or a heat transferring intermediate agent, is essentially uniform and unitarily high which facilitates the use of the furnace and the adapting of it for different purposes of use, without any ineffective surfaces having a low temperature or the danger caused by too hot surfaces. In the arrangement according to the present invention these properties are achieved also at a very high degree of load for the furnace.

According to one favorable embodiment of the present invention the furnace structure thus is constituted of a sheet metal cassette which as such can be bedded in into a fireplace or the like furnace of any desired design and material. Figures 3 and 4 disclose an example of such a cassette. Therein said guiding means 16 and 17 as well as said conduit walls 19, 26, 29 and 34, 34a, 34b favorably are made of relatively thin metal plates or sheet metal whose heat capacity is low. Due to the low heat capacity the inner parts of the furnace will heat up very quickly and without causing any disturbances in the furnace's operation even in the initial stage, and through said structures the heat of the gases will effectively be transferred between the separate channels and away from the furnace structure .

According to one aspect of the invention a first guiding plate 16 is arranged at the back wall 19 of said combustion chamber 1 in such a manner that the combustion gases 10, 15 rising from said combustion chamber 1 due the impact of said plate will be guided at an angle of at least 75°, favorably about 90° in relation to a vertical plane, i.e. approximately straight forwards. At the outer edge 21 of said guiding plate 16 the orientation of said gas conduit 14 functioning as an af ercombustion space again will favorably change, in this case suitably about 180°, i.e. the flow of said combustion gases 15 will now be directed about straight backwards . For this purpose said gas conduit 14 favorably comprises a heat insulated front wall 38. All the time the velocity of flow for said combustion gases 15 in this "folded" aftercombustion space, i.e. said combustion conduit 14, will remain relatively slow, typically about 0.8 to 2 meters/second, favorably about 1.2 meters/second, so that they will remain a sufficiently long time in a hot zone in order to guarantee a good after- combustion. Favorably another guiding means 17 is located prior to said first guiding plate 16, in the direction of the gas flow, said other guiding means 17 also being an essentially thin fast heating metal plate which extends at the upper portion of said combustion chamber 1 approximately from a frame 18 for said feed door favorably obliquely upwards towards said back wall

19 of said combustion chamber 1 so that said plate 17 will end at an aforesaid minimum distance from said back wall 19 of said combustion chamber 1 or from a possible fireproof layer

20 arranged there. Correspondingly, said guiding plate 17 will end at a distance from the lower level of said first guiding plate 16, preferably so that said guiding plates 16 and 17 slightly interlace and so that between them remains a distance which meets the requirement regarding said minimum dimension for said gas conduit 14. The inclination angle for this guiding plate in relation to the horizontal plane is suitably about 15 to 45°, favorably about 25°. Said other guiding plate 17 will direct combustion gases 10 rising from said combustion chamber 1 so that they will be guided into a corner constituted by said first guiding plate 16 and said back wall 19 of said combustion chamber 1 and especially they will meet said guiding plate 16 itself at an approximately straight angle, from which they further will be guided, at an approximately perpendicular direction away from said back wall 19 of said combustion chamber 1, into said gas conduit 14 which functions as said aftercombustion space.

According to another favorable embodiment of the present invention, so as to keep a sufficient amount of combustion air present in said aftercombustion space, i.e. in said combustion conduit 14 and for effectively mixing it with said combustion gases, irrespective of the fact that there will occur no turbulence caused by any throat structure, secondary air 22 is fed into the combustion chamber 1 through a narrow slit 23 or a row of openings located in said back wall 19 and to said flame zone 8 located above said wood charge 4, as shown in Figures 2 to 4. Favorably this additional air supply 23 is arranged approximately at the level of the upper surface of said wood charge 4, generally at a mid level of the actual combustion chamber 1. According to an especially favorable embodiment of the present invention a feeding duct 24 for the secondary air 22 is arranged to run at least partly between said back wall 19 of the combustion chamber and the front wall 26 of said combustion conduit extension extending downwards behind the furnace structure, i.e. said descent conduit 25, which arrangement guarantees an effective preheating of said secondary air 22.

According to an especially favorable embodiment of the present invention secondary air 12 is fed also to the front portion of said flame zone 8 through an air slit or air openings 27 arranged in the suitably glazed front door 6, favorably at the upper edge thereof, so that this air stream 12 initially will be directed downwards along the inner surface of said door 6 and thus keeps the glass surface clean. Also this feed of secondary air guarantees that the combustion gases, in spite of the laminar flow taking place in said combustion conduit 14, will come, in a sufficient manner, into contact with oxygen rich air, so that there occurs a mixing of said gases 15 and air streams 12, 22 especially at said edges 21 and, correspondingly, 28 of said guiding plates 16, 17. In this embodiment the relation between the cross-sectional areas of the air openings located in said back wall and, correspondingly, the air openings in said door favorably is in the order of 0.3:1 to 0.8:1.

As evident from the above said descent conduit 25 constituting an extension of said combustion conduit 14 functioning as an aftercombustion space favorably is directed downwards behind the combustion chamber 1 and in direct connection with said feed duct 24 for secondary air, which duct is narrow but essentially has a width which favorably corresponds essentially to the width of the furnace structure. According to sectional Figure 4 the downwards orientated extension of said combustion conduit 14, i.e. said descent conduit 25 is of a favorably triangular cross section so that said back wall 19 of ^"the combustion chamber 1, and at a lower level said wall 26 of said secondary air feed duct constitutes one side thereof, both other sides 29 thereof directly adjoining said ascent conduit 30 ascending from the lower portion 35 of the furnace structure back up towards the chimney 32. According to one embodiment of the present invention one or several heat exchanger surfaces 19, 26 and 29 further comprise formations facilitating the heat transfer, such as ribs, ridges or the like formations which enlarge the effective surface. Favorably, said ascent conduit further comprises heat exchanger tubes 31 for heating the indoor air or e.g. a heat transfer fluid. For heating the indoor air said tubes 31 extend open ended favorably along the whole extent of said ascent conduit 30, i.e. they are open to the space outside said conduit 30.

When said conduit 25 functioning as an extension of the combustion conduit is located between, on one hand, the hot combustion chamber 1 and, on the other hand, the hot smoke conduits 30, the temperature of the combustive gases remain sufficiently high as long as possible which makes an efficient final combustion of the gases possible prior to their exhaust through the chimney 32. Simultaneously, the hot surface 26 conducts heat to said secondary air duct 24 for preheating the secondary air, which contributes to the improvement of the combustion in the beginning end of the process . Surprisingly it has turned out that this arrangement due to the especially high heat exchange efficiency permits a combustion at very high effect without a too high increase of temperature in the smoke gases 13 exhausted through the chimney 32. Thus, and differing from prior art, the structure according to the invention in most cases permits a combustion with fully loaded combustion chamber and full draught without any danger or disadvantage emanating therefrom. From Figure 4 it also will be evident that the combustion chamber itself favorably is lined with layers of shock resistant fire proof material 20, 33. Insulating material, indicated with reference 38, is favorably arranged also at the outer surfaces of said conduit 14 which functions as an after- combustion space. In said aftercombustion space this layer of insulating material 38 receives the hot combustion gases 15 and guides them backwards, as well as contributes to the keeping of said hot combustion gases 15 as hot as possible during the aftercombustion which guarantees the purity of the smoke gases .

Figure 4 further shows that said grate 2 for feeding primary air and for letting the ashes down favorably has the width of the whole bottom of the combustion chamber, so that no un- combusted brands will remain at the edges and impair the final end of the combustion. The relation between the cross-areas of the grate 2 and said openings 7, 23 for secondary air is favorably in the order of 0.4:1 to 1.4:1. The combustion chamber itself is favorably only slightly wider than the length (330 mm ±10%) of the normal piece of wood defined by said DIN-standard, i.e. suitably about 38 centimeters, so that there will be no empty volume in the combustion chamber.

A furnace structure according to the present invention achieves, in relation to prior art designs, a considerably more uniform and complete combustion and thus a more effective gain of energy. In relation to prior art the portion of harmful compounds such as carbon monoxide, harmful hydrocarbons and particles are clearly lower in the disclosed solution. At the same time the temperature of the smoke gases is clearly lower than in prior art solutions, and thus the gain of the amount of heat in the wood is more effective. As evident from Figure 2 the solution according to the present invention further provides a far more compact furnace structure than prior art . Above some favorable features of the present invention have been disclosed as an example with reference to one embodiment, but for the person skilled in the art it will be obvious that the structure can be modified, within the scope of the appended claims, also in many other ways without diverging from the basic idea of the invention. It is, for example, clear that the integrated aftercombustion and smoke box arrangement as well as the heat exchange arrangement according to the invention can be utilized also in connection with other types of furnaces than the above described.

Claims

1. A furnace structure, in particular intended for burning of wood or the like, especially a furnace core cassette having a combustion chamber (1) comprising a grate (2) and an inlet (7, 23) for secondary air (12, 22), an aftercombustion space (9, 14) and a smoke box, located above said combustion chamber, as well as a smoke conduit (30, 32) connected to said smoke box, c h a r a c t e r i z e d in that said furnace comprises a combustion conduit (14, 25) which is designed in compliance with an essentially uniform laminar velocity of flow for said combustion gas (10, 15), said combustion conduit (14, 25) constituting, in immediate connection to an upper end of said combustion chamber (1) , an integral aftercombustion space and smoke box having guiding and heat transferring means (16, 17, 29) of low heat capacity, said means changing the orientation of said combustion gases initially in a horizontal direction and, thereafter, in a vertical direction.

2. A structure as defined in claim 1, c h a r a c t e r i z e d in that an initial change of orientation for said combustion gases (10, 15) is arranged, for stirring said combustion gases, to extend to and fro in a horizontal direction and another change of orientation, correspondingly, is arranged, especially for a transferal of heat, to extend in a vertical direction, said guiding means (16, 17) for horizontally directing said gases (10, 15) favorably being arranged immediately above said combustion chamber (1) so that the orientation of said hot combustion gases (10, 15) and/or flames (8) and any secondary air (12, 22) being mixed therein approximately immediately at the upper portion of a flame zone

(8) will change at least 75┬░, favorably about 90┬░, suitably so that the orientation of said gases (15) approximately immediately after said initial change of orientation further changes essentially 180┬░, after which said combustion gases

(15) essentially immediately are guided downwards to a combustion duct (25) located immediately behind said combustion chamber (1) , preferably so that a first portion of a horizontal orientation brings said combustion gases (10, 15) forwards with respect to a vertical plane constituted by a back wall (19) of said combustion chamber (1) and a second portion, correspondingly, backwards, after which a first portion of a vertical orientation directs said combustion gases in a descending combustion duct (25) downwards directly along said back wall (19) of said combustion chamber (1) and a second portion brings said gases, correspondingly, upwards while surrounding said descending combustion duct (25) at least at two sides at the location of a common wall portion (29) made of an essentially thin material, suitably of sheet metal or the like which material has good heat conductivity, said wall portion (29) favorably comprising protrusions which facilitate a heat exchange .

3. A structure as defined in claim 1 or 2 , c h a r a c t e r i z e d in that said thin guiding means (16, 17) for directing said gases (10, 15) comprise at least a first guiding plate (16) made suitably of metal, said plate extending essentially from the surface of the favorably low heat capacity back wall (19) of said combustion chamber (1) forwards an amount which approximately corresponds to the depth of a vertical portion of said combustion conduit (14), and suitably a second guiding plate (17) which, located before said first guiding plate (16) , seen in the direction of flow for said combustion gases (10, 15), extends from the front portion of said combustion chamber (1) towards said back wall

(19) approximately to the level of a front edge (21) of said first guiding plate (16) , favorably to some extent past said edge (21) .

4. A structure as defined in any one of claims 1 to 3 , c h a - r a c t e r i z e d in that guiding plates (16, 17) are arranged alternating successively in an essentially vertical combustion conduit (14), the cross-sectional area of which, in relation to the area of the actual combustion chamber portion, i.e. of said grate (2), favorably is of an order of 0.6:1 to 0.3:1, most favorably about 0.45:1, in which case the width of said combustion conduit (14) suitably is essentially the same as the width of said grate (2), i.e. favorably about 38 centimeters for standard pieces of fire wood having a length of 33 centimeters, and the height of the horizontal portions and, correspondingly, the depth of the vertical portions of said combustion conduit (14) being about 9 centimeters, while the cross section of said conduit (14) is essentially rectangular.

5. A structure as defined in any one of claims 1 to 4 , c h a - r a c t e r i z e d in that at least one feeding point (23) for preheated secondary air (22) is formed by a narrow slit which opens into said combustion chamber (1) essentially immediately above a charge (4) of fire wood, i.e. generally at middle height in said combustion chamber (1) , a second feeding slit (7) or the like for secondary air (12) favorably also being arranged at an upper edge of a front door (6) for said combustion chamber (1) .

6. A structure as defined in any one of claims 1 to 5 , c h a - r a c t e r i z e d in that said combustion chamber (1) back wall (19) , which preferably is manufactured of sheet metal or the like thin material, as such constitutes an upper portion of one wall in a descent conduit (25) constituting an extension of said combustion conduit (14), and favorably also constitutes a wall for at least one portion of a secondary air duct (24) , while a low heat capacity lower wall portion of said descent conduit (25) favorably having a triangular cross- section and descending downwards through a heat exchange chamber (30-31) constitutes a portion of an opposite wall (26) of said secondary air duct (24) so that said secondary air duct (24) at least partially extends between said combustion chamber (1) and said extension (25) of said combustion conduit (14) and immediately adjoining thereto in order to constitute a preheating arrangement for said secondary air (22) .

7. A structure as defined in any one of claims 1 to 6, c h a r a c t e r i z e d in that a smoke conduit (30) extends essentially vertically and constitutes a heat exchange chamber

(30-31) which suitably at two sides surrounds said suitably triangular descent conduit (25) arranged as an extension of said combustion conduit (14) , said conduit (30) favorably having an essentially rectangular cross-section, while preferably several heat exchange elements are arranged in at least one of said conduits (30, 25) or in an immediate contact thereto, said heat exchange elements favorably being heating ducts (31) for fresh air or the like intermediate medium, said ducts (31) extending generally vertically through said structure from one end thereof to the other.

8. A method for burning of wood or the like in a furnace structure comprising a combustion chamber (1) comprising a grate (2) located underneath said chamber (1) and feeding means (7, 23) for secondary air (12, 22), an aftercombustion space (9) and a smoke box, arranged above said combustion chamber (1), and as well as smoke conduit(s) (30, 32) connected to said smoke box, c h a r a c t e r i z e d by retaining exhausting combustion gases (10, 15) in an after- combustion space / smoke box constituting an integral hot zone (14, 25) in said furnace, at a flow having an essentially uniform velocity, by controlling, on one hand, the amount of air (2, 12, 22) fed into said combustion chamber (1), and guiding, on the other hand, said combustion gases (10, 15) through a combustion conduit (14, 25) established by an integral aftercombustion space and, correspondingly, a smoke box, both of essentially uniform thickness, while simultaneously internally stirring said combustion gases (10, 15) by changing the essential orientation of the combustion gases/flames (10, 15, 8) immediately above said combustion chamber (1) and in several successive operations.

9. A method as defined in claim 8, c h a r a c t e r i z e d by changing the orientation of said combustion gases (10, 15), using at least one thin guiding means (16, 17) having a low heat capacity, said means being connected to the upper portion of said combustion chamber (1) favorably directly to the upper portion of a flame zone (8) , said changing being initially at least 75┬░, suitably essentially 90┬░ forwards and immediately thereafter essentially 180┬░ backwards and thereafter downwards into a descending conduit (25) made as an extension of said combustion conduit (14), favorably so that said combustion gases (10, 15) are kept in said aftercombustion space / smoke box constituting an integral folded hot combustion conduit (14, 25) for as long as possibly, suitably by controlling the velocity of flow for said gases (15) so that it lies in the region of 0.8 to 2 meters/second, favorably about 1.2 meters/second.

10. A method as defined in claim 8 or 9, c h a r a c t e r i z e d in maintaining the relation between the cross- sectional areas of the air slits in said grate (2) and feeding openings (7, 23) for said secondary air (12, 22) in the range of 0.4:1 to 1.4:1, favorably so that said secondary air (12, 22) is fed through two openings (7, 23), i.e., on one hand, approximately at the centre of a back wall (19, 20) of said combustion chamber (1) essentially at the level of the upper surface of a fire wood charge (4) , and, on the other hand, through the upper portion of a front door (6), while favorably maintaining the relation between the cross-sectional areas of said openings (7, 23) in the range of 0.3:1 to 0.8:1.

11. A method for recovering heat from hot combustion gases (15) exiting at the upper portions of a combustion chamber (1) of a furnace structure, c h a r a c t e r i z e d by conducting said hot gases (15) initially in a first conduit (25) essentially downwards, said conduit (25) being separated from said combustion chamber (1) by a back wall (19) having a low heat capacity, and thereafter in an opposite direction in an outer second conduit (30) in contact with said first conduit (25) and located at the outer side of said first conduit (25) , the outer surface (34, 34a, 34b) temperature of said outer conduit (30) remaining essentially uniform as an essential amount of heat transfers through one or several essentially thin walls (29) common to said conduits (25, 30) from one conduit (25) to the other (30) .

12. A method as defined in claim 11, c h a r a c t e r i z e d by bringing said hot gases (15) downwards in a descent conduit

(25) approximately to the level of a bottom (35) of said furnace structure, said conduit (25) being arranged in an immediate contact with said back wall (19) of said combustion chamber (1) and having an essentially triangular cross- section, from which location said gases (15) are brought in an ascending conduit (30) in contact with outer walls (29) of said triangular descent conduit (25) upwards, suitably so as to pass a combustion conduit (14) arranged for hot combustion gases (15) above said combustion chamber (1) and suitably leading therefrom.

13. A method as defined in claim 11 or 12, c h a r a c t e r i z e d by bringing a heatable intermediate medium, suitably air, in separate heat exchange tubes (31) along the essential direction of flow of said hot gases in one or several of said conduits (25, 30) .