The open fireplace has for a long time served only as a source of pleasure and only to a small extent as a heat source. When the outside temperature is very low the open fireplace may even produce a cooling effect since very large quantities of cold air are drawn into the house to be transported out through the chimney in the form of heated space air. Whereas a normal fire may

3 require 30-60 m air per hour to sustain combustion as much as more than ten times this quantity of air may pass out through the chimney as thief air.

The old tile stove was not a subject to this kind of negative heat economy thanks to the possibility to con¬ trol the supply of air to the hearth by means of -the stove doors. The successor of the tile stove, the fire stove of today, has also controlled supply of primary air. The fire stove is furthermore frequently using preheated primary air which further improves the heat economy, see e.g. H. arsson, "Vedeldning geno tiderna" the-rapport No. 5, 1979, ("History of wood burning") .

Several attempts have been made to improve the heat economy for existing open fireplaces and similar furnaces Different kinds of attachments, "fire crows", have appeared on the market in these times of energy crisis. These means comprise heat exchanging means which are put up in the fire for heating of the space ^" air. The space air is drawn through the heat exchanger by natural convection or by means of a fan. In this way the open fireplace, which primarily delivers radiation heat to the room, may also deliver hot air which improves the efficiency from a level around 10% to the double. Forced circulation by means of a fan improves the efficiency

further. An attachment of this kind is described in the brochure "Braselement" (Fireplace element) fur¬ nished by Villaprodukter AB, Box 7061, S-30007, Hal - stad, Sverige, 1978. Fireplace attachments, however, do not solve the main problem with the open fireplace i.e. the negative heat economy at low outside tempera¬ tures - when the fire is most needed as has been dis¬ cussed above. In severe cases the attachment may even decrease the heat economy further compared to a fire- place without attachment. The hot air outlet from the attachment is in general located near the opening of the fireplace. Part of this frequently very hot air may then be drawn back to the fireplace, which accele¬ rates further the natural draft through the chimney and consequently also the intake of cold air to the house compared to the case that the "thief air" through the chimney consists of space air with normal temperature.

The primary purpose with the present invention is to supply an attachment for open fireplaces and similar furnaces which permits a controlled supply of combustion air to the fire and a reduction of the thief air through the chimney. Thereby a good total heat economy is ob¬ tained also during low outside temperatures instead of. the negative heat economy which has been discussed below.

The total efficiency may, thanks to the new fireplace attachment, be improved to a level which is approaching the level for the modern fire stoves without any expen¬ sive modifications of the fireplace.

It has also been demonstrated that the new attachment gives a number of other advantages which are partly un¬ expected. The feature that control of the combustion process may give a possibility to extend the burning

time for a given fuel charge is perhaps not an unexpected advantage. What is surprising, however, is that it is possible to provide such controlled combustion in an open fireplace thanks to the attachment according to the in- vention.

It is also completely unexpected and surprising that the combustion is taking place more- completely and more accept¬ able to the environment than with a fire in an open fire¬ place with a conventional fireplace attachment. The emissions of unburnt, tary substances, soot and ash with the flue gas may then be reduced to a substantial extent which is of quite large importance in the situation when many households in suburbs and similar more or less densely populated areas have to turn to wood heat because of the increased oil prices. The improved combustion can be explained by the special geometric conditions "which characterize the design and shape of the new attachment. The ^' invention also exhibits a number of other advantages, as will be apparent from the following description.

The invention thus comprises a new attachment for fire¬ places or open fireplaces and similar furnaces for burn¬ ing fuel in the form of wood logs, briquettes, pellets and related solid fuels whereby the attachment comprises a hearth chamber and a glow chamber which is inclining from its back towards its front separated by a grate whereby primary combustion air is supplied to said glow chamber characterized in that the hearth chamber is shaped in such a way that the fuel is contained within the limiting surfaces of the hearth chamber so that secondary air for the final combustion is supplied to the open upper part of the hearth chamber via its upper edge.

The invention will now be described in more detail by means of the drawing.

Figure 1 shows the principle for the attachment accord¬ ing to the invention.

Figure 2 shows an embodiment with a built in heat ex¬ changer for delivery of hot air to the room.

Figure 3 shows different sections of the hearth chamber.

Figure 4 shows an installation of the attachment in an open fireplace.

According to the invention the attachment is preferably made of fire resistant steel and may be described as a box without cover, a firebox, placed in the furnace. The front (1) of the box which is directed toward the opening of the furnace is lower than the back. The outer sides (3) of the box are therefore inclining to¬ wards the front of the firebox, see Figure 1. The inner wall of the furnace may also constitute the back (2) . The fuel which consists of wood logs, briquettes, rods of extruded chips, pellets etc. is arranged on the in- clined grate (4) which at the bottom is finished off with a bend (5) to prevent the fuel to fall off. The space below the grate (4) constitutes a glow chamber (6) , The dimension of the firebox is such that a normal fuel charge consisting of e.g. seven middle sized logs practically fills the hearth chamber (7) which is formed in the firebox above the grate (4) .

The front (1) is equipped with stove doors or similar means to provide for supply of primary air to the glow chamber (6) . Figure 1 shows simple turnable doors (8) of the same kind as are used for tile stoves etc. Such

cι.

doors can also be placed on the outer sides (3) and possibly on the back (2) for the supply of primary air from the surroundings to the glow chamber (6) . The invention is not, of course, thereby limited to this specific embodiment of the stove doors . There are great many state of art suitable constructions for manual or automatic regulation of the supply of primary air such as simple adjustable doors, rosette valves, automatically operating devices with bimetallic or other means which control the adjustment of the stove door(s) . An especially suitable embodiment in this connection is to design the entire front (1) in the form of a sliding lid made of toughened glass which can be pushed up in its frame so that a slot forms at the bottom to permit supply of primary air. This special embodiment also gives a better release of radiant heat from the hearth to the room and is of considerable value for the sense of well-being since the entire fire can be -viewed from the room.

The technical effect of the invention depends on several cooperating ^' constructive measures:

The design of the hearth chamber which comprises and contains the fire so that the secondary air for the final combustion is supplied to the open upper part of the hearth chamber across its upper side.

The inclined grate which permits a successive trans¬ port of the fuel towards the front side of the fire¬ box whereby the upper side of the fire for the main part will lean against the opening of the fireplace.

The regulated supply of primary air to the glow chamber which provides heating of the primary air

and final combustion of the char, which gives radiant heat for pyrolysis and gasification reactions of the f el on the grate .

These seemingly simple constructive measures give at first hand the unexpected effect that the departure of thief air to the chimney is substantially reduced. It is difficult to fully explain this ^' technical effect but an essential contributing factor is probably the quite different flow conditions at the fireplace attachment according to the invention compared to an open fire¬ place without attachment or an open fireplace with state of art attachments . The supply of primary air via the stove doors (8) or similar arrangements influences also the supply of secondary air for final combustion above the upper edge of the firebox into the fire. An al¬ most laminar flow of combustion gases takes place from the firebox up to the smoke sack and the chimney in the furnace. .It is surprising that this flow pattern of the secondary air can be attained without special provisions other than the geometry of the firebox, and its fuel charge and the stove doors for the primary air.

The inclination of the fuel charge gives a certain amount of counter-current flow during the combustion which seems to contribute to the efficient combustion. The most intense glow combustion takes place at the front of the fuel charge from where the hot gases move partly in the bed towards the back. New, often raw fuel is added on from the top so that the fuel is subject to drying and pre-pyrolysis, pyrolysis, gasification and final combustion successively during the movement from the back of the firebox towards the front. Gases emitted at the back are mixed with very hot com-

bustion gas and combustion air which gives an efficient combustion also of these gases which otherwise would leave the chimney unburnt.

It should also be added that the inclination of the bed permits more radiant heat to enter the room so that even a fire with a less amount of logs gives the same feel¬ ings of well-being and warmth as a conventional fire.

The seemingly simple and unsought design of the new fireplace attachment consequently gives a number of un- expected partly essentially different, partly connected technical effects which all combine to a surprising total result which is manifested by considerably lower fuel consumption and better controlled combustion in an open fireplace which is modified in this simple _^ way.

Figure 2 shows a special embodiment where the firebox in its entirety is designed as a heat exchanger for ^' delivery of warm air to the room. For the sake of simplicity the grate is not shown.

The circulation air is taken through an intake (9) and is fed to the bottom of the glow box (12) by means of the fan (10) via the inlet channel. The bottom of the glow box consists of heat exchanging elements, slots or channels in the same way as the other parts of the glow box. In the embodiment according to Figure 2 the air, which is now preheated, is passing from the bottom of the glow box (12) to the back of the glow box (13) where additional heating is taking place. The hot air stream is then distributed between the two sidewalls (14) resp. (15) which are connected to the outlet channels (16) resp. (17) at the front. It is of ad¬ vantage to furnish the two outlet channels with the dam-

pers (19) , resp. (20) , for distribution of the air between the two outlets in a useful way. It is of ad¬ vantage to enlarge the back upwards so as to take care of as much as possible of the heat from the fire. Tem- p'erature indicators (21) resp. (22) may also be dis¬ posed in the outlets for controlling the speed of the fan so as to prevent that the outgoing air in any of the two channels does not have a temperature above a preset maximum temperature, e.g. 70 C. (Preheated air may also be led to the glow box from the outlet.)

The furnace attachment according to Figure 2 is also furnished with a front comprising a oveable door of toughened glass (23) which is moveable along the slots (24) . The door can be moved up and down by means of the eccentric disks (25) which are moved by means of the rods (26) with the knobs (27) disposed at the out¬ let channels.

There are of course many other possibilities to take care of a larger or smaller share of the heating power of the fire in the form of hot air for delivery to the room using known design elements and design principles from this field.

These devices may comprise simple means relying on self- circulation or more complicated complete systems with hot air ducts with special safety dampers with thermostatic control, means for using part of the hot air as_pre- heated primary air, etc.

The furnace attachment is designed so that the grate is inclining towards the horizontal plane with an angle in the range 5-45°, preferably 10-30°, so as to get a suitable fuel movement along the grate and to get the useful pattern for the gas flow with counter flow between

fuel charge and gas. The part of the firebox which is above the inclining grate plane has a height, h, which is at least about 1/10 of the width of the glow box so as to contain a least one layer of wood logs within the limiting surfaces of the firebox.

Figure 3 shows examples of suitable profiles for the inclining grate. Figure 3a shows an adjustable grate which can be adjusted for different angles of the incli¬ nation in relation to the horizontal plane. The support- ing rod (28) can be rotated at (29) and (30) and is pivoted towards the hooks (31) . The grate is a self- contained unit. Figure 3b shows a profile where the .height (h) above the grate plane is increasing towards the back of the firebox in order to compensate for the volume reduction of the fuel during the combustion and transport to the front of the firebox. This grate is built into the firebox which it is a part of. Figure 3c shows a profile which is characterized by that the angle tov/ards the horizontal plane is diminishing to- wards the backside of the firebox against its front with the angle at the front almost falling into the horizontal plane. This profile is frequently of ad¬ vantage compared _, to a constant angle of inclination along the whole grate since it gives a more even distri- bution of the glowing char layer in the glow chamber. This grate is a separate unit which is put into the firebox.

It is frequently practical to make the front (1) as shown in Figure 2 to consist of a moveable door of toughened glass which is projecting above the firebox to serve as protection against sparks. It is of course also possible to use a protection for sparks consisting of a net or foraminous steel plate which may be part of the furnace attachment. The front of the firebox is

here counted at the level where the sides are connected to the front, whereby the part of the front above this level is serving as protection against sparks .

The technical effect of the inventions is not influenced to a significant degree if the front of the firebox is also serving 'as a protection against sparks according to Figure 2. The transport of secondary air to the open surface of the firebox above the front of the fire¬ box is reduced to some extent but this is compensated for by an increased transport of secondary air above the edges of the outer walls. A higher front may also some¬ what improve the picture of laminar gas flow for the dis¬ charge of flov; gas from the inclining firebox.

Figure 4 shows an attachment (32) according to _f igure 1 disposed in an open fireplace (33) with a chimney (34) comprising the smoke sack (35) , the damper (36) and the • heat exchanger (37) . The furnace is equipped with glass doors (38) . The damper (36) can be put in different positions by means of the damper rod (39) and the pins (40) . This possibility to adjust the damper in differ¬ ent positions depending on the supply of primary air to the attachment gives a possibility to further reduce the discharge of thief air in the use of the attachment according to the invention.

A certain freedom exists also in the dimensionir-g of the firebox in particular its hearth chamber. The im¬ portant feature is of course that the hearth chamber is inclining tov/ards the front of the firebox and -hat the walls have such a height that the fuel charge is con- tained within the hearth chamber below a hypothetical lid resting on the upper edges of the hearth chamber.

The width of the hearth chamber should be somewhat larger

than the length of the wood logs, briquettes etc. to be burnt in the attachment. A v/idth of 30 centimeter or slightly above is suitable for the smallest logs of wood used in practive. Larger open furnaces may pre- ferably be fuelled with wood logs e.g. of deciduous trees, which may be e.g. 50-60 centimeter long. Those who can use v/ood from their own land of course like a fairly large length for practical reasons . The trans¬ port of fuel towards the lower part of the grate is also easier with roundwood instead of split logs.

The depth of the firebox which is the same as the length of the outer walls, can be varied within certain limits. It is frequently of advantage to use a square section which makes the depth of the firebox equal to its width. The depth should not, if possible, be below half the width and should not be above twice the width in order to produce the conditions for gas flow which is a characteristic feature of the invention.

The inclination of the hearth chamber is mainly obtained by using an inclining grate as shov/n in Figure 3. The average angle of inclination should be above about 5 . A particular suitable value is the range 10-30 but it is also possible to use higher ^■ angles up to 45 and slightly above.

The upper edges of the hearth chamber should in general follow the inclination of the grate even if it may be of advantage to increase the height (h) at the back of the hearth chamber as shov/n in Figure 3 b. The height (h) should be above at least one tenth of the v/idth of the hearth chamber so as to give room for at least one layer of wood logs. It is, however, of advantage to make the height (h) to correspond to 2-3 tenths of the width of the hearth chamber in order to give room for

several layers of wood logs. The height should not, however, be above half of the width of the hearth chamber in order also to permit efficient final combustion of the fuel charge during the conditions which characterize the invention.

The dimensioning of the glow chamber is of less importance for the technical effect of the attachment. The glow chamber should, however, have such a height at the front that there is sufficient space for char and ash from at least tv/o or three fuel charges without reducing passage of primary air to the char layer. A frequently useful value of the height at the front of the glow chamber is between 5-10 centimeter.

The attachment is preferably made of welded steel plate v/ith a thickness of e.g. 4 millimeter. When the walls are made double in order to serve as heat exchanger for delivery of hot air to the room one may alse use rectan¬ gular steel tubes or elements which are welded together, e.g. according to Figure 2. It is important to control such heat exchanging elements that they are completely tight and they should be subjected to leakage test at 2 atmospheres.

The grate is preferably manufactured by rods, steel plates, or other suitable profiles preferably in heat resistant steel. The design of the grate of course depends on what kind of fuel the attachment is intended for. If the fuel comprises wood logs, roundwood, briquettes etc. the slots between the grate rods can be comparatively large from one to a couple of centi- meters up to five centimeters or slightly above. If the attachment is to be used with pellets i.e. small pieces of compacted chips, peat, or similar fuels, it has to be designed so that the pellets are kept on the

grate until they have been converted to glow. In such cases the grate could comprise a perforated plate of fire resistant steel or a sheet of expounded metal etc. There are pellets of different dimensions on the market with diameters from a few millimeters up to several centi¬ meters . The openings in a grate intended for pellets should be smaller than the pellet dimensions in at least one direction. A suitable value for the smallest pellets is a maximum opening amounting to a few milli- meter whereas a maximum of 5-10 millimeter is suitable for large pellets.

A furnace attachment according to Figure 2 and used in an open furnace according to Figure 4 was loaded with a normal charge comprising 7 average wood logs with a total weight of 2 1/2 kilogram. The supply of primary air was regulated so that the fire burnt down in 4 hours

* whereby the average heating power to the room was about 1 kW corresponding to an efficiency near 50%. When the same fuel charge was burnt directly in the open furnace it burnt dov/n in two hours and delivered heat mainly as heat of radiation corresponding to about 200 W.

The total delivery of hot air to the surroundings via

3 the chimney was almost 2000 m v/ith a conventional firre to be compared with a corresponding value around 400 π for the total escape of gas, thief air plus combustion gases, using the attachment.

Deposits of soot and so-called creosote in the stack were much smaller when the attachment was used for combustion compared to a conventional open fire. Final combustion in the flame zone which is formed above the fuel charge was taking place quite evenly, efficiently and completely as was evident from the σuite small formation of smoke.

The description has emphasized the special charac¬ teristic features for the attachment for furnaces according to the invention. A few preferred embodi¬ ments have also been indicated so as to give the artisan guidance in the application of the invention. The scope of the invention is, however, not limited to the special embodiments which have been brought up in the description. ^• - ^• -