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Title:
A COMBUSTION AIR CASSETTE, A FIREPLACE WITH A COMBUSTION AIR CASSETTE, A METHOD FOR SUPPLYING COMBUSTION AIR, AND A KIT FOR RETROFITTING A COMBUSTION AIR CASSETTE
Document Type and Number:
WIPO Patent Application WO/2021/165578
Kind Code:
A1
Abstract:
The invention relates to a combustion air cassette (10) for conveying combustion air to different height levels of a furnace in a fireplace. The combustion air cassette (10) comprises a hollow primary air cassette (11) and at least one hollow secondary air cassette (12) with an air flow connection to the primary air cassette (11), and an air intake (35). Via the air intake (35), combustion air is arranged to flow into the primary air cassette (11) and further to the lower part of the furnace, below a wood charge to be placed on top of the primary air cassette (11), via primary air inlets (15); as well as into at least one secondary air cassette (12), to be preheated, and from which secondary air cassette (12) preheated secondary air is arranged to flow into the furnace, at least via secondary air inlets (16) provided at the height of the upper combustion space of the wood charge burning on the primary air cassette (11). The invention also relates to a fireplace (50) comprising a combustion air cassette (10), a method for introducing combustion air to a wood charge burning in the furnace of the fireplace, to different height levels of the furnace, by the combustion air cassette (10), as well as a retrofitting kit for a combustion air cassette.

Inventors:
NIELIKÄINEN JANNE (FI)
IMMONEN TUOMAS (FI)
Application Number:
PCT/FI2021/050117
Publication Date:
August 26, 2021
Filing Date:
February 18, 2021
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
NPI ENG OY (FI)
International Classes:
F24B5/02; F23B60/02; F23L1/02; F23L9/02; F23L15/00; F24B1/19; F24B1/191
Foreign References:
FI123547B2013-06-28
GB548026A1942-09-22
CH240716A1946-01-15
NL8303987A1985-06-17
Attorney, Agent or Firm:
BERGGREN OY (FI)
Download PDF:
Claims:
Claims:

1. A combustion air cassette for conveying combustion air to different height levels of a furnace in a fireplace, the combustion air cassette (10) comprising a hollow primary air cassette (11 ) and at least one hollow secondary air cas sette (12) with an air flow connection to the primary air cassette (11 ), and an air intake (35), via which combustion air is arranged to flow into the primary air cassette (11 ) and from the primary air cassette (11 ) to the lower part of the furnace via primary air inlets (15) in the top surface of the primary air cassette (11 ) as well as into at least one secondary air cassette (12) to be preheated, and from which secondary air cassette (12) preheated secondary air is ar ranged to flow into the furnace via at least secondary air inlets (16) at the height level of the upper combustion space of a wood charge burning on top of the primary air cassette (11 ), characterized in that the combustion air cassette (10) also comprises an air regulator (18) arranged to adjust the size of the primary air inlets (15) to decrease the amount of primary air to be introduced in the furnace and to close the primary air inlets (15) totally so that the com bustion air to be introduced into the furnace from the combustion air cassette (10) is only preheated secondary air.

2. The combustion air cassette according to claim 1 , wherein secondary air inlets (16) are provided on at least two different height levels in the secondary air cassette (12).

3. The combustion air cassette according to claim 1 , wherein secondary air inlets (16) are provided on several different height levels in the secondary air cassette (12).

4. The combustion air cassette according to claim 1 , wherein secondary air inlets (16) are provided on several different height levels in the secondary air cassette (12), from the bottom of the secondary air cassette (12) to the top of the secondary air cassette (12).

5. The combustion air cassette according to any of the claims 1 to 4, wherein the air intake (35) is provided under the primary air cassette (11 ).

6. The combustion air cassette according to any of the claims 1 to 5, wherein the air intake (35) comprises at least one limiter (36) for limiting the amount of air flowing into the primary air cassette (11 ).

7. The combustion air cassette according to any of the claims 1 to 6, compris ing two or three secondary air cassettes (12) with an air flow connection to the primary air cassette (11 ) so that combustion air will flow from the primary air cassette (11 ) into said two or three secondary air cassettes (12).

8. The combustion air cassette according to any of the claims 1 to 7, wherein the combustion air cassette (10) comprises a spacer (93) below the secondary air cassette (12).

9. The combustion air cassette according to any of the claims 1 to 7, wherein the combustion air cassette (10) comprises a spacer (93) at the bottom of the combustion air cassette (10), and the combustion air cassette (10) is arranged to be placed on the bottom of a fireplace.

10. A fireplace, characterized in that the fireplace comprises a combustion air cassette (10) according to any of the claims 1 to 9.

11. The fireplace according to claim 10, wherein the combustion air cassette (10) is arranged so that the secondary air cassette (12) is either on the side or rear wall of the fireplace.

12. A method for introducing combustion air for a wood charge burning in the furnace of a fireplace, to different height levels of a furnace, the method com prising receiving combustion air into a combustion air cassette (10) via an air intake (35) provided at the bottom of the combustion air cassette (10), wherein the combustion air is divided into primary air for a primary air cassette (11 ) and secondary air for a secondary air cassette (12), supplying the furnace with primary air via primary air inlets (15) in the top sur face of the primary air cassette (11 ), and preheating secondary air in the secondary air cassette (12), supplying the fireplace with preheated secondary air via at least secondary air inlets (16) arranged at the height of the upper combustion space of the burning wood charge, characterized in that the method further comprises controlling the size of the primary air inlets (15) by an air regulator (18) of the combustion air cassette (10) so that the amount of primary air to be supplied to the furnace is decreased, or so that the combustion air to be supplied to the furnace from the combustion air cassette (10) is only preheated secondary air. 13. The method according to claim 12, the method further comprising circulat ing secondary air around an air distributor (86) in the secondary air cassette (12) during preheating of secondary air before supplying preheated secondary air to the upper combustion space of the furnace. 14. The method according to claim 12 or 13, the method further comprising replacing air inlets (16) used for supplying preheated secondary air, with air inlets (16) provided at a height level corresponding to the respective upper combustion space of the wood charge burning in the furnace. 15. The method according to any of the claims 12 to 14, comprising continuing to supply the upper combustion space of the burning wood charge with sec ondary air only, after the primary air inlets (15) of the combustion air cassette (10) have been closed. 16. A retrofitting kit for a combustion air cassette, characterized in that the retrofitting kit for a combustion air cassette comprises a combustion air cas sette (10) according to any of the claims 1 to 9.

Description:
A COMBUSTION AIR CASSETTE, A FIREPLACE WITH A COMBUSTION AIR CASSETTE, A METHOD FOR SUPPLYING COMBUSTION AIR, AND A KIT FOR RETROFITTING A COMBUSTION AIR CASSETTE

Field of the invention

The present invention relates to a combustion air cassette for a fireplace, a fireplace comprising a combustion air cassette according to the invention, a method for supplying combustion air to a wood charge burning in the furnace of a fireplace, to different height levels within the furnace, as well as a kit for retrofitting a fireplace with a combustion air cassette according to the invention.

Background

Attempts have been made to severely restrict emissions caused by burning wood in fireplaces in order to improve air quality. When a conventional grate is used in a fireplace, and a wood charge, for example chopped firewood, is placed on the grate for burning, the only combustion air is often primary air entering through the grate arranged under the wood charge. Gases evolved from this wood charge burn with the wood charge, which further increases the heat of the wood charge, which, in turn, increases gasification. Consequently, primary air does not suffice to burn all the gases evolved, so a large amount of hydrocarbons and soot will remain unburnt in the fireplace. Such combustion with insufficient oxygen supply may cause undesirably high emissions caused by burning of wood. The problem of emissions caused by burning of wood is even greater when a supplementary wood charge is added on the glowing em bers during the wood combustion process.

There are also solutions in which the combustion air comprises not only pri mary air but also secondary air which is introduced above the wood charge by separate piping or cassette arrangements. It is true that such solutions have been capable of reducing emissions of small particles and carbon monoxide as well as emissions of unburnt organic substances from fireplaces. For the climate, a further reduction in the amount of small particle emissions, carbon monoxide emissions as well as unburnt organic substances from fireplaces would be advantageous. Summary

Now, a combustion air cassette for a fireplace has been invented, for introduc ing combustion air into the fireplace; primary air is conveyed from below a wood charge to the lower part of the fireplace via a primary air cassette of the combustion air cassette, and secondary air preheated in a secondary air cas sette of the combustion air cassette is conveyed to an upper part of the furnace of the fireplace, to the height level of the respective upper combustion space, from one, two, or three sides of the fireplace. The introduction of secondary air to several different height levels according to the height of the upper combus tion space of the wood charge burning in the furnace enables a sufficient oxy gen supply onto the burning wood charge during the entire combustion pro cess; from the initial stage of wood burning, when the fire is ignited and the wood charge is high, up to the final stage of wood burning when the wood charge has burnt to low embers. The invention also relates to a method for supplying combustion air to a fireplace, a fireplace comprising a combustion air cassette, as well as a kit for retrofitting a combustion air cassette.

The invention relates to a combustion air cassette supplying combustion air to different height levels of a furnace in a fireplace. The combustion air cassette comprises a hollow primary air cassette and at least one hollow secondary air cassette connected to the primary air cassette with respect to the air flow. Via an air intake in the lower part of the combustion air cassette, combustion air is arranged to flow into the primary air cassette and further via primary air inlets to the lower part of the furnace, below a wood charge to be arranged on top of the primary air cassette, as well as to at least one secondary air cassette for preheating. From the secondary air cassette, preheated secondary air will flow into the furnace, at least via secondary air inlets provided at the height level of the upper combustion space of the wood charge burning on top of the primary air cassette.

In an embodiment, secondary air inlets are provided at at least two different height levels in the secondary air cassette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cas sette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cassette, from the lower part of the secondary air cassette up to the top of the secondary air cassette. In an embodiment, the air intake of the combustion air cassette is provided below or in the lower part of the primary air cassette. In an embodiment, the air intake of the combustion air cassette comprises at least one limiter for limiting the intake of air into the primary air cassette. In an embodiment, the combustion air cassette comprises two or three secondary air cassettes with an air flow connection to the primary air cassette so that combustion air will flow from the primary air cassette into said two or three secondary air cassettes. In an embodiment, the combustion air cassette is arranged to be placed on top of the grate of the fireplace. In an embodiment, the combustion air cassette comprises a spacer and is arranged to be placed on the bottom of the fireplace.

The invention further relates to a fireplace comprising a combustion air cas sette for conveying combustion air to different height levels of the furnace. The combustion air cassette comprises a hollow primary air cassette and at least one hollow secondary air cassette connected to the primary air cassette with respect to the air flow. Via an air intake in the lower part of the combustion air cassette, combustion air is arranged to flow into the primary air cassette and further via primary air inlets to the lower part of the furnace, below a wood charge to be arranged on top of the primary air cassette, as well as to at least one secondary air cassette for preheating. From the secondary air cassette, preheated secondary air will flow into the furnace, at least via secondary air inlets provided at the height level of the upper combustion space of the wood charge burning on top of the primary air cassette, the combustion air cassette being arranged in the fireplace so that the secondary air cassette is placed against either the side wall or the rear wall of the furnace.

In an embodiment, secondary air inlets are provided at at least two different height levels in the secondary air cassette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cas sette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cassette, from the lower part of the secondary air cassette up to the top of the secondary air cassette. In an embodiment, the air intake of the combustion air cassette is provided below or in the lower part of the primary air cassette. In an embodiment, the air intake of the combustion air cassette comprises at least one limiter for limiting the intake of air into the primary air cassette. In an embodiment, the combustion air cassette comprises two or three secondary air cassettes with an air flow connection to the primary air cassette so that combustion air will flow from the primary air cassette into said two or three secondary air cassettes. In an embodiment, the combustion air cassette is arranged to be placed on top of the grate of the fireplace. In an embodiment, the combustion air cassette comprises a spacer and is arranged to be placed on the bottom of the fireplace.

The invention further relates to a method for supplying combustion air to a wood charge burning in the furnace of a fireplace, at different height levels of the furnace. The method comprises following steps: receiving combustion air into a combustion air cassette, via an air intake provided in the lower part of the combustion air cassette. In the combustion air cassette, the combustion air is divided into primary air and secondary air; primary air is supplied below a wood charge via primary air inlets in a primary air cassette; secondary air is preheated in a secondary air cassette; and preheated secondary air is supplied to the fireplace via at least air inlets provided at the height of the upper com bustion space of the burning wood charge.

In an embodiment, the method further comprises recirculating secondary air around the air distributor of the secondary air cassette when secondary air is preheated in the secondary air cassette before the secondary air is supplied to the upper combustion space of the wood charge burning in the furnace. In an embodiment, the method further comprises changing of air inlets used for supplying preheated secondary air to air inlets provided at the height level cor responding to the respective upper combustion space of the wood charge burning in the furnace. In an embodiment, the method further comprises clos ing the primary air inlets of the combustion air cassette, and continuing the supply of secondary air only.

The invention further comprises a kit for retrofitting a combustion air cassette for a fireplace, the kit for retrofitting a combustion air cassette comprising a combustion air cassette comprising a hollow primary air cassette and at least one hollow secondary air cassette with an air flow connection to the primary air cassette. Via an air intake in the lower part of the combustion air cassette, combustion air is arranged to flow into the primary air cassette and further via primary air inlets to the lower part of the furnace, below a wood charge to be arranged on top of the primary air cassette, as well as to at least one secondary air cassette for preheating. From the secondary air cassette, preheated sec ondary air will flow into the furnace, at least via secondary air inlets provided at the height level of the upper combustion space of the wood charge burning on top of the primary air cassette. The combustion air cassette is arranged in the fireplace in such a way that the secondary air cassette is provided on either a side wall or the rear wall of the furnace.

In an embodiment, secondary air inlets are provided at at least two different height levels in the secondary air cassette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cas sette. In an embodiment, secondary air inlets are provided at several different height levels in the secondary air cassette, from the lower part of the secondary air cassette up to the top of the secondary air cassette. In an embodiment, the air intake of the combustion air cassette is provided below or in the lower part of the primary air cassette. In an embodiment, the air intake of the combustion air cassette comprises at least one limiter for limiting the intake of air into the primary air cassette. In an embodiment, the combustion air cassette comprises two or three secondary air cassettes with an air flow connection to the primary air cassette so that combustion air will flow from the primary air cassette into said two or three secondary air cassettes. In an embodiment, the combustion air cassette is arranged to be placed on top of the grate of the fireplace. In an embodiment, the combustion air cassette comprises a spacer and is arranged to be placed on the bottom of the fireplace.

Brief description of the drawings

In the following, various embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Fig 1 a shows a combustion air cassette according to an embodiment of the invention, seen in a perspective diagonal view from above, Fig. 1b shows a combustion air cassette according to an embodiment of the invention, seen in a perspective diagonal view from above,

Fig. 2a shows a perspective top view of a combustion air cassette accord- ing to an embodiment, with the primary air inlets open,

Fig. 2b shows the combustion air cassette according to Fig. 2a, with the primary air inlets closed,

Fig. 3a shows a top view of a combustion air cassette according to an embodiment of the invention,

Fig. 3b shows a cross-sectional view of the combustion air cassette ac cording to Fig. 3a at the line A-A,

Fig. 3c shows a cross-sectional view of the combustion air cassette ac cording to Fig. 3a at the line B-B,

Fig. 3d shows an embodiment of the combustion air cassette of Fig. 3af- rom below,

Fig. 3e shows a cross-sectional view of the combustion air cassette of Fig. 3d at line C-C,

Fig. 3f shows another alternative embodiment of the combustion air cas sette according to Fig. 3a from below,

Figs. 4a to f each show a combustion air cassette according to an embodiment of the invention, in a perspective diagonal view from above,

Fig. 5a shows a front view of a fireplace comprising a combustion air cas sette according to an embodiment of the invention,

Fig. 5b shows a cross-section of the fireplace of the invention along line D-D in Fig. 5a, Figs. 6a to d each show a cross-sectional side view of the process of burning of a wood charge in connection with a combustion air cassette according to an embodiment of the invention; Fig. 7a shows a front view of a combustion air cassette according to one embodiment of the invention, and air flows therein,

Fig. 7b shows a top view of the combustion air cassette of Fig. 7a, and air flows therein,

Fig. 8 shows a cross-sectional view of a combustion air cassette accord ing to one embodiment of the invention at a location correspond ing to line B-B in Fig. 3a, Fig. 9a shows a perspective diagonal front view of a combustion air cas sette according to an embodiment of the invention, intended for a fireplace without a grate,

Fig. 9b shows a perspective diagonal front view of a combustion air cas sette according to an embodiment of the invention, intended for a fireplace without a grate,

Fig. 9c shows a perspective diagonal front view of a combustion air cas sette according to the embodiment shown in Fig. 9a,

Fig. 9d shows a perspective diagonal front view of a combustion air cas sette according to the embodiment shown in Fig. 9c,

Fig. 10 illustrates a method for supplying combustion air to different height levels in a fireplace,

Fig. 11 a shows a combustion air cassette according to one embodiment of the invention for a sauna stove, in a perspective diagonal view from above, Fig. 11 b shows a combustion air cassette according to one embodiment of the invention for a sauna stove, in a perspective diagonal view from above, and

Fig. 11 c shows a combustion air cassette according to one embodiment of the invention for a sauna stove, in a perspective diagonal view from above.

Detailed description

The fireplace may refer to conventional fireplaces, heat retaining fireplaces, iron stoves, fireplace hearths, open fireplaces, and sauna stoves used for heat ing private homes or other buildings or spaces. When wood is burnt in fire places, however, emissions and particles are evolved by combustion of wood, sometimes even to a significant extent, if a sufficient amount of air, that is, oxygen, is not available for the wood charge.

In view of the climate, the emissions and particles evolved by combustion of wood, particularly when incomplete, as well as black carbon accelerating the climate change, are undesired side products of processes of burning wood in fireplaces. For example, when a conventional iron bar grate is used in a fire place and a wood charge, for example, chopped firewood, is placed on that grate, primary air flowing through the grate arranged below the wood charge may be the only combustion air for the wood combustion process. Thus, gases evolved from the wood charge will burn together with the wood charge, which will further increase the heat of the wood charge, which will, in turn, increase gasification. Normally, primary air is not sufficient for burning all these evolved gases. The problem of emissions is aggravated further when supplementary firewood is added on the hot embers of the wood charge during the combustion process.

By providing a sufficient amount of air for burning the wood charge, not only below the wood charge (primary air) but also particularly above the wood charge, that is, in the so-called upper combustion space of fire where the flames are and where the burning of gasified substances of wood takes place (secondary air), it is possible to reduce the amount of particles evolved by combustion of wood by up to 90%. Such introduction of combustion air to the space where it is mostly needed each time can be implemented by a combus tion air cassette according to embodiments of the present invention, supplying combustion air consisting of both primary and secondary air to different height levels in a furnace. The combustion air cassette comprises a horizontal hollow primary air cassette to be placed on e.g. an iron bar grate and to which com bustion air is supplied from below the combustion air cassette, for example through the slits of the iron bar grate and via and air intake/air intakes of the combustion air cassette, provided below and/or in the lower part of the primary air cassette. Of all the combustion air entered in the combustion air cassette, the share of primary air will exit the primary air cassette via primary air inlets provided in the upper surface of the primary air cassette, on top of which the wood charge is burnt. No primary air will flow from the primary air cassette into the furnace via inlets other than the air inlets in the upper surface, not for ex ample from the sides or the front part of the primary air cassette. In addition to the horizontal primary air cassette, the combustion air cassette comprises at least one hollow secondary air cassette which is normally vertical and to which the share of secondary air of the combustion air will flow to be preheated, for example through the primary air cassette. The secondary air cassette or cas settes may also be placed at an angle to the primary air cassette so that the cassette or cassettes are tilted away from the primary air cassette, that is, from the central part of the combustion air cassette. The secondary air cassette comprises secondary air inlets at different height levels in the so-called front surface of the secondary air cassette which faces the furnace when the com bustion air cassette is placed in the furnace, and in this way the secondary air cassette can always convey a turbulent flow of air/oxygen to the level of the upper combustion space of fire of the wood charge, the height level being de pendent on the stage of the wood combustion process and/or the size of the firewood charge on the primary air cassette. Preheated secondary air is supplied to secure that the combustion of gases evolved by the burning wood charge is as complete as possible, by supplying a correct/sufficient amount of oxygen for the combustion of the gases in the upper combustion space. The more turbulent the secondary airflow is, the better the gases and the combus tion air are blended in the furnace, and the more complete the burning is. Con sequently, the combustion air cassette according to the embodiments of the present invention supplies oxygen to the wood combustion process more effi ciently than before, whereby less emissions are particles are evolved than be fore, because the wood charge burns more efficiently and more completely when air is supplied not only from below the wood charge but also to the flames in the upper combustion space of the fire. In both the primary and the second ary air cassettes, the diameter of the air inlets may be between, for example, 2 and 30 mm, such as 10 mm, 15 mm or 20 mm. In some embodiments, the air inlets may also be provided with collars extending away from the surface of the cassettes. Furthermore, in some embodiments, the air inlets in the cas settes may have a diameter different from those in other air inlets or in air inlets at other height levels in the secondary air cassette. The surface area of the air inlets of the secondary air cassette should be smaller than or equal to the sur face area of the air intake in the lower part of the secondary air cassette, through which air enters the secondary air cassette, in order to achieve blend ing of combustion air and gases in the furnace, efficient preheating of second ary air in the secondary air cassette, as well as a sufficiently high volume flow entering the secondary air cassette. In this way, the air flow through the sec ondary air cassette picks up speed, and a sufficient amount of secondary air is supplied to the correct location in the furnace at each moment of the com bustion process.

Thus, the operation of the combustion air cassette according to the embodi ments of the present invention is based on supplying oxygen to the combustion process according to the stage of burning, that is, on the staging of the com bustion, the turbulent mixing of combustion air and gases, the efficient supply of air to the upper combustion space, and the preheating of secondary air. Properly optimized, the combination of these features reduces the total emis sions, the amount of gases, particles, and black carbon. The staging of the combustion refers to the ratio between the primary and secondary air flows supplied to the combustion space, and it can be implemented by means of the primary air cassette of the combustion air cassette and at least one secondary air cassette, in which secondary air is heated by the combustion process be fore supplying secondary air to the furnace. The preheated secondary air con tributes to the reduction of emissions evolved by the burning of wood. Thermal energy released from the combustion process on top of the primary air cas sette heats the mass of air within the secondary air cassettes by radiation heat as well as by convection. A greater and faster flow of air into the secondary air cassette is due to the thermal expansion of air volume in the secondary air cassette, an underpressure caused by the chimney effect in the whole furnace, as well as the combined effect of these, and this is why the ratio between the area of the inlets in the secondary air cassette and the area of the air intake in the lower part of the secondary air cassette has a significant effect on the air flow rate within the secondary air cassette. This creates a pressure difference between the furnace and the secondary air cassette, as well as between the secondary air cassette and its air intake, i.e. the lower part of the secondary air cassette. By adjusting the ratio between the area of the air intake in the secondary air cassette, i.e. the opening in the lower part of the secondary air cassette, and the air intake of the primary air cassette, i.e. the air intake of the combustion air cassette, and by adjusting the ratio between these areas and the change in the temperatures of the secondary air cassette and the furnace, it is possible to optimize the combustion process to reduce the emissions even further. The adjustment of air flow rates for the combustion process is easier when a primary air cassette according to the invention is used instead of a bar grate.

The turbulence of the air flow is increased more at the reduction points of the combustion air cassettes, which also speed up the air flow. By means of and within the combustion air cassette, the air flow can be choked in the air intake channel, i.e. at the air intake of the primary air cassette, by making its area smaller than the cross-sectional area of the primary air cassette. Air can be choked further by both the primary and the secondary air inlets. By means of choking, it is thus possible to increase the turbulence of the air flow and to increase the air flow rate, which facilitates the blending of gases and combus tion air with each other in the furnace and thereby reduces the evolution of emissions.

Air is supplied to the furnace via the primary and secondary air inlets, by which a sufficient amount of oxygen can be supplied to several single combustion sites in the furnace. The air inlets in the combustion air cassette are located along the X and Y axes, whereby air can be supplied throughout the area of the wood charge. In other words, the combustion air cassette conveys air to the lower part of the furnace of the fireplace, below the wood charge, via air inlets in the primary air cassette, and to different height levels in the furnace via air inlets provided at different height levels in at least one secondary air cassette. The capacity to introduce primary air below the wood charge within the fireplace, i.e. the furnace, makes sufficient air and oxygen supply possible when the wood charge is ignited. And the capacity to introduce secondary air to different height levels within the fireplace, i.e. the furnace, makes sufficient air and oxygen supply possible in the upper combustion space, i.e. on top of the of stack of firewood throughout the wood burning stage, i.e. the combustion process. By the secondary air cassette, it is thus possible to introduce air on top of the wood charge in the upper combustion space, from the very beginning of the wood burning stage, when the fire is kindled and the stack is high, to the final wood burning stage when the wood charge has burnt almost down to embers. The secondary air cassette may also supply air over the height of the firewood charge if the charge is not placed against the secondary air inlets, although the most efficient air flow is always to the upper combustion space. The combustion air cassette according to the embodiments of the present in vention may also be a retrofitting or renovating kit for a fireplace, when the combustion air cassette is put on top of an existing grate so that the primary air cassette is placed on top of the grate and the secondary air cassette or cassettes are placed against the walls on one, two or three sides of the fur nace. The combustion space for the wood charge will remain aligned with the grate but on top of the primary air cassette instead of the grate. The combus tion air cassette according to the embodiments of the invention may be a ret rofitting or renovating kit for a fireplace without a grate as well. In such a case, the combustion air cassette is equipped with spacers, and the cassette is placed at the bottom of the furnace. However, it should be noted that the com bustion air cassette according to the embodiments of the invention may also be an integrated part of the fireplace.

As already mentioned above, the amount of particles evolved upon burning firewood can be reduced by up to 90% when a sufficient amount of air is sup plied right above the wood charge. Even though the secondary air cassette comprises several air inlets which are not covered by the wood charge, the upper combustion space is the main zone to which secondary air is primarily introduced, because the vortex of fire in the upper combustion space sucks the secondary air to flow through exactly those secondary air inlets which are located at that height level of the furnace. Therefore, the structure of the sec ondary air cassette with several air inlets arranged at different height levels is suitable for supplying air to the upper combustion space at all stages of wood burning, from beginning to end, making the combustion process more efficient and cleaner. The diameter of the secondary air inlets may be, for example, between 2 and 30 mm, for example 10 mm, so they are quite point-like. In a case where the air inlets are very large or elongated in the vertical direction, secondary air will not necessarily flow to the exactly right height level in the furnace, and/or be sufficiently turbulent and/or quick, and/or have a desired temperature, to make the combustion process as clean as possible, even if the combustion process evolved less emissions when larger or elongated inlets are used.

In the following, embodiments of the invention will be discussed with reference to Figures 1 to 8. However, it should be noted that the invention is not limited solely to the presented embodiments. In fact, different embodiments can be combined and applied in any environment where the combustion air cassette can be used, for example in a sauna stove or a barbecue. Part of the combus tion air cassettes in the figures are shown as cross-sectional views to illustrate the location of the air intakes and air inlets in the combustion air cassettes more clearly.

Fig. 1 a shows a combustion air cassette 10 according to an embodiment of the invention in a diagonal perspective view from above. The combustion air cassette 10 has a hollow body and it consists at least of a horizontal primary air cassette 11 and three vertical secondary air cassettes 12 with an air flow connection to it. The combustion air cassette 10 has an air intake (not shown) in a first part of the cassette 10, which is the lower part of the cassette 10, for taking air into the hollow body of the combustion air cassette 10 so that air will flow into the primary air cassette 11 as well as to the secondary air cassette 12 connected to it, via the primary air cassette 11. Examples of air intakes are shown, for example, in Figs. 3d and 3f. The combustion air cassette 10 has a plurality of primary air inlets 15 for conveying primary air out from the body of the primary air cassette 11 for a wood charge placed on top of the primary air cassette 11 , as well as secondary air inlets 16 for conveying secondary air out of the body of the secondary air cassette 12 for the wood charge placed on top of the primary air cassette 11. The primary air inlets 15 are provided on the top surface of the primary air cassette 11 , and the secondary air inlets 16 are pro vided on the front surface of the secondary air cassette 12 so that the inlets 16 of the cassette 12 are facing the furnace. The cassettes 11 and 12 are con nected to each other so that the surfaces of the primary air cassette 11 and the secondary air cassette 12 provided with the air inlets 15, 16 and facing the furnace are arranged at an angle of about 90 degrees to each other. However, it is possible to also provide secondary air inlets on e.g. such sides of the sec ondary air cassette that are not facing the furnace and the burning wood charge. The number and the locations/height levels of the air inlets 15, 16 may be selected freely, for example according to the fireplace, but substantially they are provided at several different locations on one surface of the cassette 11 or 12.

The combustion air cassette 10 is arranged to be installed in a fireplace so that its secondary air cassettes 12 are placed against/close to/to face the walls of the fireplace - in this embodiment of three secondary air cassettes 12 against the rear and side walls of the furnace of the fireplace - and the primary air cassette 11 is placed on top of the grate of the fireplace, with the air intake downward so that air can flow from the air intake to the combustion air cassette 10 and further through the cassettes 11 , 12 via the air inlets 15, 16 into the furnace. The primary air inlets 15 of the primary air cassette 11 are arranged to be adjustable in size, to be closed entirely, or their opening can be limited by using an air regulator 18 of the combustion air cassette 10, whose position can be adjusted for moving a metal plate partly or entirely to the front of the inlets 15 in the primary air cassette 11. Closing off the flow of the primary air may be necessary, for example, when the combustion air or the combustion process is to be restricted by the intake of primary air via the lower part after the wood charge has already been ignited.

Within the hollow body of the secondary air cassette 12, the air is preheated when a wood charge is burning on top of the primary air cassette 11. Via sec ondary air inlets 16 in the secondary air cassette 12, preheated secondary air is arranged to flow from the secondary air cassette 12 into the furnace, above the burning wood charge placed on top of the primary air cassette 11 , and to the upper combustion space. The idea of the air inlets 16 provided at different height levels from the bottom of the fireplace is that preheated secondary air can always be provided to the height level of the flames above the wood charge, irrespective of the stage of burning and/or the height of the wood charge. It is possible that preheated secondary air is also conveyed to other height levels but primarily at least to the height of the top of the wood charge. The combustion air cassette 10 as well as its secondary and primary air cas settes 12, 11 may be made of, for example, stainless steel or black iron. The material thickness may be, for example, between 1 and 10 mm.

It is also possible that the combustion air cassette 10 comprises more than one air intake, the air intake is divided into parts, or that at least one air intake is provided in another part than the lower part of the cassette 10. The cassette 10 may comprise, for example, at least one air intake in the rear part of the cassette 10, or at one or two edges of the cassette 10, for example in the lower part of the cassette 10. Furthermore, it is possible to introduce air into the cas sette 10 from the outside of the space in which the fireplace is located.

Fig. 1 b shows a combustion air cassette 19 according to an embodiment of the invention in a diagonal perspective view from above. The combustion air cassette 19 of this embodiment differs from the combustion air cassette 10 of Fig. 1a in that its secondary air inlets 16 are elongated, i.e. longitudinal and vertical, in which case the combustion air cassette 19 can provide secondary air to different height levels even though inlets are not provided at several dif ferent height levels. Because of their shape, the secondary air inlets 16 of Fig. 1 b may have a larger size than the secondary air inlets 16 of Fig. 1a alto gether, which may slow down the flow of secondary air into the furnace and reduce turbulence with respect to the embodiment of Fig. 1 a, because choking of air is reduced with larger inlets.

In this embodiment of Fig. 1b, the primary air inlets 15 of the primary air cas sette 11 are elongated as well, but they could also be similar to those in the combustion air cassette 10 of Fig. 1 a. The primary air inlets 15 of the combus tion air cassette 19 can also be totally closed after the igniting of the wood charge, or their opening can be limited by using an air regulator 18 of the com bustion air cassette 19, whose position can be adjusted for moving a metal plate partly or entirely to the front of the inlets 15 in the primary air cassette 19. Figure 2a shows a combustion air cassette 20 according to an embodiment in a perspective view from above, when primary air inlets 21 are open, and Fig. 2b shows the combustion air cassette 20 of Fig. 2a when the primary air inlets 21 are closed to prevent the flow of primary air, whereby the combustion air flowing from the combustion air cassette 20 consists of secondary air only. The closing is performed by moving a damper with holes/openings within the primary air cassette of the combustion air cassette 20 by means of an air reg ulator 22. The air inlets 21 can be reduced in size, or they can be totally closed. When the holes of the damper are fully aligned with the air inlets 21 , the inlets 21 are fully open; when the holes of the damper are partly aligned with the air inlets 21, the holes 21 are partly open/closed and the amount of primary air flowing into the furnace is reduced; and when the holes of the damper are not at all aligned with the air inlets 21 , the inlets 21 are fully closed and no primary air will flow into the furnace. It is also possible to apply the air regulator 22 and the damper to close/adjust only part of the inlets, for example inlets 21 which are located in the middle of the primary air cassette 23 and on top of which the wood charge is primarily placed, or not to close/adjust he inlets in the middle. Figure 3a shows a top view of a combustion air cassette 30 according to an embodiment. The combustion air cassette 30 comprises one primary air cas sette 31 and three secondary air cassettes 32 separate from each other and extending vertically upwards from the primary air cassette 31. Air will not flow from one secondary air cassette 32 to another but only from the air intake (35 in Figs. 3b to 3f) of the combustion air cassette 30 into the primary air cassette 31 and from the primary air cassette 31 via primary air inlets 33 to the furnace, as well as from the primary air cassette 31 into the secondary air cassettes 32 and from the secondary air cassettes 32 further via secondary air inlets 34 into the furnace, to the respective upper combustion space of the wood charge.

Fig. 3b shows a cross-sectional view of the combustion air cassette 30 of Fig. 3a at line A-A. In this view, the air intake 35 is at the bottom. The second ary air cassettes 32 intended to be placed against the side walls of the furnace rise vertically upwards from the edge areas of the primary air cassette 31. Some of the primary air inlets 33 and secondary air inlets 34 are also shown in this Fig. 3b. Figure 3c shows a cross-sectional view of the combustion air cassette 30 of Fig. 3a at line B-B. In this view, too, the air intake 35 is at the bottom. The secondary air cassette 32 intended for the rear wall of the furnace extends vertically upwards from the primary air cassette 31. Some of the primary air inlets 33 and the secondary air inlets 34 are also shown in this Fig. 3c.

Figure 3d shows an alternative embodiment of the lower part of the combus tion air cassette 30 of Fig. 3a, in a view from below, i.e. from the bottom. Through the air intake 35 at the bottom of the primary air cassette 31 , primary air inlets 33 are also visible from below. The surface area of the air intake 35 of the combustion air cassette 30 is smaller than the cross-sectional area of the primary air cassette 31. Figure 3e shows a cross-sectional view of the combustion air cassette 30 of Fig. 3d at line C-C. In this view, the air intake 35 is shown at the top, and the secondary air cassettes 32 extend vertically downwards from the primary air cassette 31. Some of the primary air inlets 33 and secondary air inlets 34 are also shown in this Fig. 3e.

Figure 3f shows an alternative embodiment of the lower part of the combustion air cassette 30 of Fig. 3a from below, that is, from the bottom. In this embodi ment, the size of the area of the air intake 34 of the primary air cassette 31 is limited by closing the corner areas of the air intake 35, that is, by preventing the flow of air into the combustion air cassette 30 at the corners of the air intake 35. These metal pieces covering part or parts of the air intake 35 may be called air intake limiters 36. Through the air intake 35, primary air inlets 33 are also visible. In this embodiment, the area of the air intake 35 of the combustion air cassette 30 is even smaller than the cross-sectional area of the primary air cassette 31, in comparison with Fig. 3d. The number of limiters 36 may be different, and their locations in the air intake 35 may be different from that shown in this embodiment.

Figures 4a to 4e each show a combustion air cassette according to an embod- iment of the invention in a diagonal perspective view from above. A combustion air cassette 40 shown in Fig. 4a comprises a horizontal primary air cassette and two vertical secondary air cassettes with an air flow connection to the pri mary air cassette, one intended for a side wall and the other for the rear wall of the furnace. Between the corners of the secondary air cassettes, an angular guide 47 is provided to prevent a flow of secondary air away from between the cassettes or from the area between them, e.g. to the rear side of the combus tion air cassette 40. The structure of the guide 47 is not hollow, so that no air will flow through it from the cassettes to each other or to the guide 47. The guide 47 may also be different in shape; in other words, it may connect the cassettes to each other without an angle.

A combustion air cassette 41 shown in Fig. 4b comprises a horizontal primary air cassette and two vertical secondary air cassettes connected to the primary air cassette and intended for opposite side walls of the furnace. In this embod iment, no guide is provided between the secondary air cassettes, but the sec ondary air cassettes are separate from each other and only connected and fastened to the primary air cassette to allow an air flow from the primary air cassette to the secondary air cassettes.

A combustion air cassette 42 shown in Fig. 4c comprises a horizontal primary air cassette and one vertical secondary air cassette connected to the primary air cassette. In this embodiment, the secondary air cassette is intended for the (right hand) side wall of the furnace.

A combustion air cassette 43 shown in Fig. 4d comprises a horizontal primary air cassette and one vertical secondary air cassette connected to the primary air cassette. In this embodiment, the secondary air cassette is intended to be placed against the rear wall of the furnace.

A combustion air cassette 44 shown in Fig. 4e comprises a horizontal primary air cassette and two secondary air cassettes connected to the primary air cas sette and intended to be placed against opposite side walls of the furnace. Between the secondary air cassettes, a guide 48 having a size almost equal to the size of the rear wall is provided, also intended to prevent the flow of secondary air away from between the cassettes or from the area between the cassettes, e.g. to the rear side of the combustion air cassette 44, and to guide air to the actual combustion process. The structure of this guide 48 is not hol low either, so that no air will flow via it from the secondary air cassettes to each other or into the guide 48. The guide 48 may also have a shape different from that shown in the figure; in other words, it may connect the cassettes to each other by a gentle arc, for example.

A combustion air cassette 45 shown in Fig. 4f comprises a horizontal primary air cassette and three vertical secondary air cassettes connected to the pri mary air cassette and intended to be placed against opposite side walls and the rear wall of the furnace. In this embodiment, no guide is provided between the secondary air cassettes, but the secondary air cassettes are separate from each other and only connected and fastened to the primary air cassette to allow an air flow from the primary air cassette to the secondary air cassettes.

Figure 5a shows a front view of a fireplace 50 comprising a combustion air cassette 51 according to an embodiment. The combustion air cassette 51 is arranged in the furnace 53 of the fireplace 50 so that its secondary air cas settes 57 are against or in the vicinity of the walls of the furnace 53. A primary air cassette 55 is arranged on top of the bar grate (not shown) of the furnace 53. On the side with the door 59 of the furnace 53 (shown in Fig. 5b), no sec ondary air cassette is provided but a wood charge is fed and ignited on top of the primary air inlets of the primary air cassette 55 (shown in Fig. 5b). Com bustion air may flow into the combustion air cassette 51 from the surrounding room or directly from the outside of the building, for example via an ash box 52. Within the combustion air cassette 51 , part of the combustion air flown into the combustion air cassette 51 via the openings/slots in the grate, i.e. second ary air, is preheated in the secondary air cassettes 57 before flowing via the secondary air inlets 54 of the combustion air cassette 51 to the flames and the respective upper combustion space. The rest of the combustion air, i.e. primary air, in turn, will flow, without being significantly heated, via primary air inlets 58 of the primary air cassette 55 (shown in Fig. 5b) to several locations in the lower part of the furnace 53, below the wood charge, when the wood charge is ignited. Figure 5b shows a horizontal cross-section of the fireplace 50 of Fig. 5a and the combustion air cassette 51 along the line D-D. Figure 5b shows the pri mary air cassette 55 and the primary air inlets 58 therein. In this embodiment, the inlets 58 are circular, like the air inlets 54 of the secondary air cassettes 57, but they may also be, for example, oval, elongated or rectangular. The inlets 54, 58 may also have different sizes with respect to each other, or the same cassette may comprise inlets of different sizes, or elongated inlets may be provided at only one height level (or several height levels) of the secondary air cassette.

It should also be noted that the height and width of the combustion air cassette 51 , as well as the number of secondary air cassettes 57, may be freely se lected, for example on the basis of the size and/or the shape of the furnace 53 of the fireplace 50. Furthermore, the secondary air cassettes 57 may have dif ferent sizes with respect to each other.

Figures 6a to 6d show the process of burning of a wood charge in connection with a combustion air cassette 60 according to an embodiment of the invention. The combustion air cassette 60 is arranged on top of the grate (not shown) in the furnace, and a stack of firewood 61 is provided as a wood charge on top of the primary air cassette 62 of the combustion air cassette 60. There are flames 63 on top of the firewood 61 . For clarity, Figs. 6a to 6d show the cross- sectional view of one secondary air cassette 64 only, although two or three secondary air cassettes might be provided as well. The introduction of com bustion air into the combustion air cassette 60 is illustrated with arrows 67 in Figs. 6a to 6d.

In Fig. 6a, the combustion process is just about to begin, at the initial stage, the wood charge is being ignited, and the stack of firewood 61 used as the wood charge is high. The flame 63 is still really small and low. Hardly any sec ondary air will flow at this stage from the secondary air cassette 64 because the burning wood charge has not heated the air of the secondary air cassette 64 to a significant extent yet. Thin arrows from the air inlets 66 of the secondary air cassette 64 indicate a low secondary air flow to the area of the whole wood charge. Since the combustion process has just started, primary air is supplied to several different sites below the wood charge from the primary air cassette 62, via the air inlets 62a of the primary air cassette 62. With several different primary air supply sites, it is possible to secure sufficient oxygen supply at the igniting stage and the igniting of the whole wood charge at the very beginning of the combustion process. If primary air is conveyed to be only supplied from the vicinity of the door of the furnace, i.e. from the front part of the furnace, then the whole area of the wood charge will not necessarily be provided with a sufficient amount of primary air.

The amount of primary air can be adjusted by the air regulator 69 of the primary air cassette 62 so as to limit its supply to the wood charge as the combustion process proceeds, and thus prevent the burning of gases discharged by the wood charge, which will further increase the heat of the wood charge, which in turn will increase gasification, whereby the combustion air available for the fire is not sufficient to burn all the gases discharged from the burning wood charge, and thereby a large amount of hydrocarbons and soot will remain unburnt and undesired high amounts of emissions may be evolved by burning of wood. The supply of primary air, i.e. the air inlets 62a of the primary air cassette 62, can be closed by an air regulator 69, whereby a perforated metal plate 69a can be moved within the primary air cassette 62 to the front of the air inlets 62a after preheated secondary air has started to flow from the secondary air cassette 64 into the furnace. The supply of primary air can be reduced by adjusting, i.e. reducing the size of the air inlets 62a of the primary air cassette 62 with the air regulator 69. In this case, the burning is more complete and cleaner during the course of the combustion process, when it is carried out at least primarily with secondary air preheated via the secondary air cassette 64, because all the combustion air entered in the combustion air cassette 60 will then be supplied as preheated secondary air to the combustion process.

In Fig. 6b, the combustion process has proceeded and is at the stage of full burning. Secondary air has been preheated and is flowing into the furnace. This major site of secondary air supply is indicated with an arrow 65; smaller arrows from the air inlets 66 of the secondary air cassette 64 indicate a smaller flow of secondary air which can be supplied to the whole area of the wood charge. The supply of primary air has already been decreased by adjusting i.e. reducing the size of the air inlets 62a of the primary air cassette 62 with the air regulator 69, by which the perforated metal plate 69a can be moved inside the primary air cassette 62 partly to the front of the air inlets 62a.

In Fig. 6c, the combustion process has proceeded to the middle stage, and the wood charge is lower. The combustion process is at the middle stage. Here, combustion air is introduced as secondary air to the upper combustion space, and via a secondary air inlet 66 lower than that shown in Fig. 6b. This greatest secondary air flow is indicated with an arrow 65 again. The supply of primary air has already been totally blocked by closing the air inlets 62a of the primary air cassette 62 with the air regulator 69, by which the perforated metal plate 69a can be moved completely to the front of the air inlets 62a within the primary air cassette 62.

In Fig. 6d, the combustion process has proceeded further, and the wood charge has almost burnt down. The combustion process is at the final stage. Combustion air still consists of preheated secondary air, and most of it is sup plied to the upper combustion space also in this case, but via a lower second ary air inlet 66 than before. This secondary air flow is indicated with an arrow 65 again.

Figure 7a shows a front view of a combustion air cassette 70 according to an embodiment of the invention and air flows therein. The combustion air cassette 70 comprises a primary air cassette 72 and three secondary air cassettes 71 . Combustion air 75a will flow into the combustion air cassette 70 via an air in take 75, i.e. an air intake channel. Combustion air may flow into the combustion air cassette 70 from the surrounding room or directly from the outside of the building, for example via an ash box. In the primary air cassette72 of the com bustion air cassette 70, combustion air 75a is divided into primary air 74a flow ing into the furnace via the primary air inlets 74 below the wood charge (not shown) to be arranged on top of the primary cassette 74, and secondary air 73a to be preheated in the secondary air cassette 71 and from there via sec ondary air inlets 73 to the upper combustion space of the wood charge in the furnace. In this embodiment, the arrows indicating primary air 74a and the ar rows indicating secondary air 73a are equally thick and long, with respect to each other, even though the ratios and quantities of primary and secondary air flows may vary at different stages of the combustion process and in different locations of the cassettes, depending on e.g. the location of the wood charge, the stage of the combustion process, the height of the wood charge, the tem perature, etc. For example, at the initial stage of the combustion process, after the kindling stage, the amount of primary air 74a may be 80% or more of the combustion air (the primary air inlets 74 being fully open), whereas during the most intensive burning stage, the amount of secondary air 73a increases and may be e.g. 50 to 90% or even more, if primary air inlets 74 have been reduced in size or closed by a regulator 76 (shown in Fig. 7b); and at the end of the burning stage the amount of secondary air 73a is 100% if the primary air inlets 74 are closed. The percentages at different stages of the combustion process may vary to a great extent, depending on the above-mentioned factors. Fig ure 7b shows a top view of the combustion air cassette 70 of Fig. 7a and the air flows therein.

Figure 8 shows a cross-sectional view of a combustion air cassette 80 accord ing to an embodiment of the invention, at a location corresponding to the line B-B shown in Fig. 3a. Air distributors 86 within the secondary air cassettes 82 are illustrated in this combustion air cassette 80. Such an air distributor 86 divides the secondary air cassette 82 into two parts, wherein air entering the cassette 82 will travel a longer distance in them and thereby also be subjected to more heating, the secondary air having so-called double circulation in the secondary air cassettes 82. In this case, too, after entering the primary air cas sette 81 via the air intake 85, air will flow further into the secondary air cas settes 82. In the cassettes 82, secondary air will flow up along the outer edge of the cassettes 82, between the outer edge and the air distributor 86, i.e. the edge without secondary air inlets 84. Flaving reached the end of the air distrib utor 86 at the top of the secondary air cassette 82, the flow of secondary air will make a turn down the cassette 82 and exit the secondary air cassette 82 via air inlets 84 suitably located in view of the combustion process, into the furnace. It is possible that all or only some of the secondary air cassettes 82 of the combustion air cassette 80 are provided with an air distributor.

Figure 9a shows a perspective diagonal front view of a combustion air cassette 90 according to an embodiment of the invention for a fireplace without a grate, such as an open fireplace or the like. This combustion air cassette 90 com prises a spacer 93 in addition to the primary air cassette 91 and the secondary air cassettes 92. The combustion air cassette 90 according to the embodiment is designed to be placed particularly at the bottom of a fireplace without a grate. The spacer 93 will allow the supply of combustion air via the lower end of the combustion air cassette 90, from below the primary air cassette 91. In this em bodiment, a separate spacer is provided below each secondary air cassette, but the spacer 93 may also be uniform and common to all the secondary air cassettes 92; this kind of a spacer is shown in Fig. 9b. In Fig. 9a, the walls in front of the spacers, i.e. facing the center of the cassette 90, do not extend all the way up, and in this way combustion air will flow into the combustion air cassette 90 not only via the air intake (not shown in Fig. 9a) below the primary air cassette 91 but also via air intakes 94 of the spacer 96, at an angle of for example 45°. The angle may be smaller as well. When air does not enter the combustion air cassette 90 at an angle of 90°, it will flow more easily into the cassette 90 and the flow will be more efficient. If the combustion air flowing into the combustion air cassette 90 hits the wall of the cassette 90 at an angle of 90°, a counterpressure may be caused and result in inadequate functioning or inoperability of the cassette 90. Other parts of the spacer 93 may also be provided with an air intake, in addition to or instead of the air intakes 94 pre sented above.

Figure 9b shows a perspective diagonal front view of a combustion air cassette 90a according to an embodiment of the invention for a fireplace without a grate, for example an open fireplace. This combustion air cassette 90a comprises a primary air cassette 91 , secondary air cassettes 92, and a spacer 95. In this embodiment, all the combustion air will enter the combustion air cassette 90a from the direction where an opening is provided in the spacer 95, in this em bodiment via the front of the combustion air cassette 90a. In this way, the flow of combustion air into the combustion air cassette 90a can be facilitated by making the spacer 95 so high that combustion air can flow at a gentle angle into the cassette 90a. If the spacer 95 is very low, for exam pie lower than 3 cm , combustion air will not flow into cassette 90a as smoothly as in the embodi ment of the preceding Fig. 9a or with a higher spacer, and the combustion air cassette 90a will not necessary operate as efficiently.

Figure 9c shows a perspective diagonal front view of the combustion air cas sette 90 according to the embodiment of Fig. 9a. This combustion air cassette 90 comprises not only a primary air cassette 91 , secondary air cassettes 92 and a spacer 93 but also a glass pane 96 on the side without a secondary air cassette, i.e. the open side, of the combustion air cassette 90. The purpose of the glass pane 96 is to increase the underpressure forming in the combustion air cassette 90, compared with the environment of the cassette 90, and thereby to enable the combustion air cassette 90 to operate as efficiently as possible. Instead of tempered glass, it is possible to use any other suitable material, but glass as the material makes it possible to watch the combustion process.

Figure 9d shows a perspective diagonal front view of the combustion air cas sette 90 according to the embodiment of Fig. 9c. This combustion air cassette 90 comprises not only a primary air cassette 91 , secondary air cassettes 92, a spacer 93, and a glass pane 96 on the side without a secondary air cassette, but also a hood 97 on top of the combustion air cassette 90. The purpose of the hood 97 is to further increase the underpressure formed in the combustion air cassette 90, compared with the environment, and thereby to enable the combustion air cassette 90 to operate as efficiently as possible. The hood 97 may also be used without the glass pane 96, in the same way as the glass pane 96 may be used without the hood 97.

The air inlets of the primary air cassettes 91 of the combustion air cassettes 90, 90a in Figs. 9a to 9d are also adjustable for restricting or blocking the sup ply or primary air when it is desired to use secondary air as the main or only combustion air. The spacers shown in Figs. 9c to 9d may have shapes different from those shown in the figures, as long as they enable the supply of combus tion air into the combustion air cassette. Further, the hood 97 of Fig. 9d is only an example of a possible hood design. A glass pane 96 and/or a hood 97 may be used with the combustion air cassette 90 of Fig. 9b as well.

Figure 10 shows a method 100 for supplying combustion air to different height levels of a fireplace via air inlets of the primary air cassette of the combustion air cassette as well as air inlets provided at different height levels in at least one secondary air cassette. The height level of the secondary air supplied will depend on the height level of the wood charge burning in the fireplace, and secondary air will be supplied to the upper combustion space of the burning wood charge. Primary air will be supplied below the wood charge. In step 101 , combustion air enters the combustion air cassette via an air intake provided in the lower part of the combustion air cassette, in which the combustion air is divided into primary air and secondary air flowing from the primary air cassette to a secondary air cassette. In other words, in step 101 , combustion air is re ceived in the combustion air cassette. In step 102, primary air is supplied be low the wood charge via air inlets on the top surface of the primary air cassette. In step 103, secondary air is preheated in the secondary air cassette. The method may also comprise a step 103a in which secondary air circulates/is circulated around an air distributor in the secondary air cassette, if one is pro vided in the secondary air cassette; that is, preheating of secondary air in the secondary air cassette involves so-called double circulation of secondary air. In step 104, preheated secondary air is supplied into the fireplace via at least those secondary air inlets that are located at the height of the upper combus tion space of the burning wood charge. In further steps of the method 100, as the wood charge is burning and thereby becoming lower, the air inlets used for supplying preheated secondary air are gradually replaced by air inlets located at a height level corresponding to the respective upper combustion space of the wood charge burning in the furnace. In other words, the secondary air inlets are replaced by inlets provided at a lower level of the secondary air cassette as the height of the wood charge becomes lower during the combustion pro cess. At least some of the steps of the method 100 may also take place sim ultaneously during the combustion process. When supplementary firewood is added on the burning wood charge and the height of the wood charge in creases, air inlets at an upper level of the secondary air cassette are taken to use again, for supplying secondary air to the upper combustion space placed higher.

Figure 11 a shows a perspective diagonal top view of a combustion air cassette 110 according to an embodiment of the invention, for use in a sauna stove, and Fig. 11 b shows a perspective diagonal top view of a combustion air cas sette 111 according to another embodiment of the invention, for a sauna stove. In Fig. 11b, the secondary air cassettes intended to be placed against the side walls of the furnace of a sauna stove are provided with a bend. In Fig. 11 c, the secondary air cassettes of the combustion air cassette 112 have a structure without a bend but extend diagonally outward from the primary air cassette, that is, not vertically upwards, whereby the cross-section of the combustion air cassette becomes substantially V-shaped. In these combustion air cassettes 110, 111, 112 for a sauna stove, too, the flow of primary air to be supplied to the furnace via the air inlets on top of the primary air cassettes can be adjusted or totally blocked by air regulators 113. Combustion air cassettes for sauna stoves, as well as combustion air cassettes for other fireplaces, if necessary, may be modular, i.e. they can be assembled from two or more elements to the shape of the combustion air cassette. This may be necessary in sauna stoves for the reason that the doors of furnaces in sauna stoves or other fireplaces may be relatively small and it may be difficult to install a ready-to-use combus- tion air cassette in the furnace.

It will be obvious that the present invention is not limited solely to the above- presented embodiments, but it can be modified within the scope of the ap pended claims. Also, the embodiments of the combustion air cassettes shown in the different figures may be combined. For example, one combustion air cassette may comprise 1 to 3 secondary air cassettes of different shapes and/or sizes, air inlets of different shapes and/or sizes in the secondary air cassettes and/or the primary air cassette, a limited or unlimited air intake, i.e. air intake channel, one or more guides, no guides, a spacer with or without and air intake(s), even though it were used in a fireplace with a grate, such as an open fireplace or a sauna stove, etc.