Patent Publication Number: US-6213116-B1

Title: Wood and multi-fuel burning stove

Description:
TECHNICAL FIELD 
     This invention relates to a wood and multi-fuel burning stove which will burn smoke. 
     BACKGROUND ART 
     Burning wood and related fuels efficiently in a domestic stove poses the dilemma of trying, on the one hand, to keep the burning temperature of the fire in the stove as high as possible to maximise the combustion efficiency, whilst upon the other hand, trying to get as much heat as possible from the stove into the room. A further challenge for a stove designer has been to provide for a practical and controllable method of burning combustible components carried in the smoke. Such a process is referred to as ‘burning its own smoke’ and such components in some fuels can constitute up to fifty percent of the calorific value energy value of fuel burnt in the stove. 
     In a traditional burning arrangement, with smoke rising from the embers (wood or coal), the temperature of the smoke is not raised sufficiently high to reach the combustion temperature of at least some of the combustible components without the help of a catalyst in the smoke flow path. Such a catalyst is easily damaged irrevocably by components in the smoke arising from burning an unsuitable fuel such as, for example, painted wood. 
     Stoves have been produced and designed using what is termed a ‘down draughter’ arrangement, whereby smoke is made to pass downwards, through glowing embers (such as of wood or coal) in a fire bed in the stove, before passing back to the flue. In this way the temperature of the glowing embers is sufficiently high for combustible components of the smoke to be burned. Few, if any, of these down draughter designs have proved both practicable and controllable and as a consequence such stoves have not been popular in the market place. This is despite their promise of much greater burning efficiency, with combustible elements in the smoke energy being converted to heat, and the concomitant benefit of the chimney not becoming coated with condensation products such as soot or tar. 
     U.S. Pat. No. 4,677,965 (Duerichen) shows a wood and coal burning heater which includes a primary combustion chamber for the controlled burning of a solid fuel positioned above a secondary combustion chamber for the subsequent combustion of combustible gases and pollutants which pass downwardly from the primary combustion chamber. An independent air supply is provided for each of the combustion chambers and air flowing to the primary combustion chamber is controlled to govern the rate of burn. A separate air supply is provided in the secondary combustion chamber and this air upon contact with the combustible gases and pollutants passing downwardly from the primary combustion chamber to cause further or secondary combustion to cleanse the smoke and gas of pollutants prior to discharge. Smoke and exhaust gases leaving the secondary combustion chamber pass first rearwardly and then upwardly along the back of the heater and then forwardly beneath the top surface of the heater prior to discharge to provide increased heat exchange contact between surfaces of the heater and exhaust smoke and gases. 
     The heater proposed and described by Duerichen is a complicated structure and does not make full use of air flows to promote effective operation of the stove when in use. In addition the flow passages described by Duerichen for the mixed exhaust gasses and what are referred to as ‘pollutants’ would tend to be readily blocked so serving to adversely affect efficient operation of the stove. 
     DISCLOSURE OF INVENTION 
     According to the present invention there is provided a wood or multi fuel stove comprising a primary combustion chamber, a flue duct extending from an upper part of the primary combustion chamber; a grate defining a lower boundary for the primary combustion chamber, the grate extending from the front towards the back of the stove interior and having a front edge juxtaposed with the front of the stove; a secondary combustion chamber located beneath the grate linked to the primary combustion chamber; an inlet path for supplying fresh air to the secondary combustion chamber, the inlet path by-passing the primary combustion chamber; and an outlet path for conveying combustion products from the secondary combustion chamber to the flue, the outlet channel by-passing the primary chamber; the stove being characterised in that: 
     the primary combustion chamber (X) is defined for the major part of its perimeter (grate  1 , its rear side and sides ( 5 ) and top ( 12 )) by insulated members; 
     the grate ( 1 ) having: 
     i a rear edge off set from the insulated rear side ( 5 ) to leave a gap (G) extending over all, or part of, the width of the grate ( 1 ), 
     ii a front edge set higher than the rear edge (A) so that the grate ( 1 ) slopes down from the front towards the gap (G); and 
     a secondary combustion chamber C) located beneath the grate ( 1 ) and linked to the primary combustion chamber (X) solely by way of the gap (G). 
     According to a first preferred version of the present invention the stove is characterised in that the gap (G) is of a size sufficient to allow for ashes and some solids during combustion to fall from the primary combustion chamber(X) downwards through the gap (G), and smoke to pass through gap (G). 
     According to a second preferred version of the present invention or of the first preferred version thereof is characterised in that the grate ( 1 ) provides for extra air for burning to be channelled by the grate configuration to pass into the gap (G) so as to mix with the descending smoke, and so enhance the combustion of the smoke. 
     According to a third preferred version of the present invention or any preceding preferred version thereof the stove is characterised in that the stove ( 10 ) provides for smoke to pass downwards into the secondary combustion chamber (C) and subsequently back to the flue duct ( 11 ) by way of a route up either side of the stove ( 10 ) through a passage ( 6 ) defined between an inner fire box ( 5 ) and an outside casing (M) of the stove ( 10 ). 
     According to a fourth preferred version of the present invention or any preceding preferred version thereof the stove is characterised by means ( 3 ) provided to enable a primary flow of air to enter the stove-box and pass through a hollow top ( 7 ), to pass, or wash, down the inside of a glass panel ( 4 ) incorporated in an access door to the interior of the stove ( 10 ). 
     According to a fifth preferred version of the present invention or any preceding preferred versions thereof the stove is characterised by means providing for an additional flow of air to enter the stove ( 10 ) and follow a route ( 1 A) under the grate ( 1 ), to pass into the gap (G). 
     According to a sixth preferred version of the present invention or of any preceding preferred version thereof the stove is characterised by a grate (G) of a hollow sandwich type construction. 
     According to a seventh preferred version of the present invention the stove is characterised in that the primary combustion chamber (X) is bounded by insulation material ( 5 ,  12 ), apart from the front ( 4 ), where a glazed panel or access door ( 4 ) is located. 
     According to an eighth preferred version of the present invention or any preceding preferred version thereof the stove is characterised by a window ( 2 ) positioned so as to offer a view of burning smoke beneath the grate in the secondary combustion chamber (C). Typically the window ( 2 ) is double glazed. Additionally the window ( 2 ) can lie in an air flow path enabling an air wash to be directed past the window ( 2 ) on its inner side. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     An exemplary embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: 
     FIG. 1 shows a cross section of the stove from the front to the back; and 
     FIG. 2 shows the front view of the stove. 
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     A wood or multi-fuel stove  10  has a primary combustion chamber X of which the lowest section is formed by a grate  1 . The grate  1  has a solid upper surface and hollow middle  1 A, and slopes from the front to back B of the stove  10  at a fixed angle A. The grate  1  does not extend right to the back B of the stove but stops short of the back B to form a gap G, running at least part, if not fully across the width of rear side of the grate ( 1 ). The gap G opens into a secondary combustion chamber C beneath the grate  1 . Fuel  13 , in this case lengths of wood, burns in primary combustion chamber X which is in the form of an insulated box  5  (except for the forward facing section) within metal body M of the stove  10 . 
     At top T of the metal body M of the stove is of a hollow channel  7 , of sandwich type construction. 
     The stove  10  is configured so as to work in two modes. 
     In the first, traditional, mode smoke rises from the fuel  13  in the primary combustion chamber X and passes from thence under and round the baffle  12  and exits to flue  11  by way of flap  3 . The flap  3  is shown in an open position pointing towards door  4  to allow the smoke to pass between the door  4  and baffle  12 . The combustion products following this path exchanges heat with primary air drawn in through the sandwich type construction lid  7 , so pre-heating the primary air. This pre-heated primary air passes into the front part of the primary combustion chamber X through slit  9  to wash over inner surface S of glazed door  4 . This air wash flow W serves to keep clean the inner side of the glazed door  4 . Smoke from the burning fuel  13  rises and passing under and round the baffle  12  exits to flue  11  via the flap  3 . 
     In the second, down draught, mode the flap  3  is set in the closed position away form the door  4  and touching the front edge of the baffle  12 . As a consequence smoke from burning fuel  13  is blocked from following a direct upward route round the baffle  12 , to the flue  11 , and the stove functions to bum in its down draught mode. In this mode smoke is drawn downwards through burning fuel F and is subsequently mixed with additional pre-heated air, which burns the smoke at region  8  in secondary combustion chamber C located beneath grate  1 . This pre-heated air is drawn in via a path through either side of the sandwich type lid  7  down either side of the stove case  10  through a pair of vertical channels to open underneath the grate  1  into hollow passage  1 A, from which it emerges into region  8 . 
     The exhaust gasses from the burning at region  8 , returns to the flue pipe  11 , via a route in the space  6  (FIG. 2) at either side of the stove  10 , formed between the outer metal body M and side walls  5  of insulated primary combustion chamber X. 
     A window  2 , is located in the lower part of the stove S to allow a view beneath the grate  1 , of the burning smoke in the secondary combustion chamber C, in particular at and around region  8 . This window  2  is kept clean by an air-wash system which is a similar to, but a smaller scale version of, the air-wash over door  4 . 
     Typically, the window ( 2 ) is double glazed. Additionally the window ( 2 ) can lie in an air flow path enabling an air wash to be directed past the window ( 2 ) on its inner side. 
     INDUSTRIAL APPLICABILITY 
     The present invention provides for a stove of relatively simple construction to be readily operated with high thermal efficiency with a range of readily obtainable fuel without a need for catalytic devices.