GB2081436A - Solid fuel burning stoves - Google Patents

Abstract

In a solid fuel burning stove which is designed to heat water or air which is passed through a heat exchanger suitably located within the stove casing (1), the fire grate comprises a relatively coarse grate (24) spaced above a relatively fine grate formed by a disc (19) which possesses an array of radially extending slots (20) and which is rotatably mounted on a plate (16) possessing a similar array of radially extending slots (17). Registry of the slots (17) and (20) can be adjusted between fully open and fully closed positions by rotation of the disc (19), although in either position holes (21) located in a central region of the disc (19) will remain in registry with holes (18) in the central region of the plate (16). Operation of the stove is controlled by means of an adjustable damper (32) in a flue outlet (31) leading from the top of the stove casing (1), a valve controlled air inlet (9) (preferably assisted by a thermostatically controlled blower (12)) leading into the ash collection space (3) below the plate (16) of the fine grate, and by rotation of the disc (19) to adjust the relative opening of the slots (17) and (20). <IMAGE>

Description

SPECIFICATION Stoves This invention relates to solid fuel stoves, particularly wood burning stoves. Primarily the invention is concerned with such stoves for domestic purposes, e.g. as a room heater or as a water heater for central heating and/or hot water systems, but is also applicable to larger heavy duty stoves.

A large number of different arrangements for heating air or water in solid fuel stoves are known, but the present invention is particularly concerned with arrangements comprising a casing which encloses a fire grate, a chamber above the grate for the combustion gases from the fire, and an ash collection space below the grate, a controlled air inlet below the level of the grate, and a flue outlet from the combustion gas chamber containing an adjustable damper, and in which one or more tubes form a heat exchanger in the combustion gas chamber and, preferably, also form part of the fire grate itself. With such arrangements, fluid which is passed through the tube or tubes is heated directly by the fire and the hot air and combustion gases immediately above the fire, the aim being to provide a stove which is simple in construction and which heats air or water quickly and efficiently.

With the object of improving the overall efficiency of such a stove, according to the present invention the fire grate of the stove comprises a relatively coarse grate and a relatively fine grate which is spaced below the coarse grate. The fire is built and fuelled by placing the fuel on top of the coarse grate.

However, as the fire burns, bits of the burning fuel fall through this coarse grate, but instead of falling directly into the ash collection space, fall onto the fine grate and form a fire bed which continues to burn below the coarse grate. Consequently the coarse grate becomes located in the heart of the fire, and when, as is preferred, the coarse grate is formed by one or more tubes forming a heat exchanger, preferably connected to the heat exchanger in the combustion gas chamber, much more heat is available for heating the fluid passed through the tube or tubes than when the fire grate heat exchanger tubes are located at the base of the fire as in the known arrangements. Preferably the spacing between the coarse and fine grates is in the region of 2.5 centimetres.

Preferably the fine grate is constructed so that at least some of its openings are adjustable in size. Not only does this make it possible to promote the discharge of ash through the fine grate to the ash collection space below, but more importantly it also enables the speed at which air is drawn through the fire to be varied in an advantageous manner. In a conventional stove the grate openings are fixed and the rate of air flow through the fire is controlled by means of the air inlet below the level of the grate and the adjustable damper in the flue. In a stove in accordance with the invention the amount of air allowed to flow through the stove is controlled in the same way, but by adjusting the area of the openings in the fine grate the velocity at which the air is caused to travel through the fire can be varied.For example, if the flue damper is opened and the size of the openings in the fine gate is reduced, the amount and velocity of the air drawn through the fire is increased, producing a dramatic increase in the heat output of the fire.

Normally however, the fire will be operated with the damper and the air inlet closed as much as possible so that the fire burns slowly and as much as possible of the heat generated is transferred to the fluid passing through the heat exchanger tubes.

Reducing the size of the fine grate openings to increase the velocity of the air flow through the fire helps the fire to stay alight when the stove is damped right down. The air inlet preferably comprises an opening in the portion of the casing which encloses the ash collection space, and may be provided with a manually operated slide valve for adjusting the area of the opening. In addition, the air inlet opening may be provided with a booster fan which is thermostatically controlled to operate when the temperature in the combustion gas chamber falls below a predetermined value. Furthermore, the air inlet opening may have a flap valve which is arranged to open when the booster fan operates and which closes to shut the air inlet opening when the temperature in the combustion gas chamber rises above the predetermined value and the booster fan switches off.The flap valve may be arranged to blow open under the action of the flow of air delivered by the fan, or alternatively it may be opened by a solenoid which is energised when the fan is switched on. In a particularly advantageous arrangement, when the booster fan is switched on the flue damper is opened automatically by means of a solenoid which is energised simultaneously with the booster fan, the damper closing automatically to a predetermined damping position when the fan is switched off and the solenoid is de-energised. In this way the fire is caused to flare up quickly to quickly raise the temperature in the stove. It also helps to prevent tar build up around the damper from causing the damper to stick.

It goes without saying, of course, that for the stove to be controlled efficiently by means of the air inlet, the fine grate adjustment and the flue damper, the casing must be substantially sealed against any other inflow an outflow of air, for example around the doors which provide access to the fire chamber and to the ash collection space.

The adjustable fine grate may comprise a pair of plates which are located one on top of the other and each of which is provided with a series of openings, at least one of the plates being movable to vary the degree by which the openings of one plate register with openings of the other plate and thereby to vary the size of the effective air passages through the grate. Preferably the plates are circular and one, preferably the top plate, is arranged to be rotatable concentrically relative to the other.

In a particular preferred form of grate, all of the fine grate openings are adjustable between fully closed and fully opened positions except for a group of relatively small openings which are located in a central region of the grate and are permanently open, or at least are open when the rest of the openings are closed. This arrangement ensures that the centre of the fire will remain alight and hot irrespective of how much the stove is damped down.

With such a fine grate, the coarse grate probably has an open central region above that of the fine grate containing the group of permanently open openings, the open central region of the coarse grate permitting the establishment fairly quickly of a fire core which assists in keeping the fire burning across the whole of the grate.

A particularly suitable form of coarse grate comprises a continuous tube which is spirally wound to form a flat or slightly dished coil. Because of limitations of the tightness to which a tube can be coiled, there will be a hole formed automatically at the centre of the coil. Preferably the heat exchanger in the combustion gas chamber is also formed by a continuous tube, the tube being wound in the form of a helical coil above the heat exchanging coarse grate and adjacent the inner wall of the combustion gas chamber. The tubes forming the two heat exchangers are preferably connected, and may even comprise a single continuous tube, so that fluid entering the coarse grate at one end of its spiral coil passes through the tubes and leaves the helical heat exchanger coil at its upper end.

This arrangement is primarily intended for heating water, the inlet and outlet ends of the connected heat exchanger coils, in use, being connected in a domestic central heating and/or hot water system in a conventional manner. There will of course be a thermosyphonic action with such a continuous water heating tube, but because the water heating efficiency of the stove can be extremely high, it will be necessary to include a circulating pump in the system. This pump may be thermostatically controlled in response to the temperature of the water at a suitable point in the system, having a bypass to allow thermosyphonic circulation when the pump is not operating.

The connected heat exchanger arrangement may also be used in a stove designed to heat the air in a room in which the stove is situated, the opposite ends of the connected heat exchangers in this case being open and the stove including a fan for blowing ambient air through the tubes. In this situation however, provided the heat exchanger tube or tubes rise continuously and are open at opposite ends the fan is not essential, although its provision will greatly increase the speed with which a room can be warmed. Also, in such a room heater it is not essential for the coarse grate to be formed by the heat exchanger tu be or tubes.

The stove casing may be any suitable shape, but usually it will have a cylindrical or cuboid lower portion in which the fire is located, and preferably has an upper portion forming a conical or pyramidal hood which surrounds the combustion gas chamber and its heat exchanger and which leads to the flue outlet. In this case the door through which fuel is fed to the fire will normally be located in the front of the lower portion of the casing above the level of the fire grate, the arrangement being such that the heat exchanging tubes of the stove do not hinder the feeding of fuel through the door.

Two examples of a stove in accordance with the invention will now be described with reference to the accompanying schematic drawings, in which; Figure lisa sectional side view of one of the examples which is intended for use as a water heater; Figure 2 is an exploded perspective view of part of the stove shown in Figure 1; Figure 3 is a front sectional view of the second example which is designed for use as an air space heater; and, Figure 4 is an exploded perspective view of part of the stove shown in Figure 3.

The stove illustrated in Figure 1 comprises a casing 1 having a rectangular base section 2 housing, an ash collecting compartment 3 located below an opening 4 in the top of the base section 2 and communicating with a rectangular intermediate section 5 of the casing 1. The ash compartment 3 contains an ash collecting tray 6 which can be removed to empty the tray by means of a door 7 at the front of the base section 2. At the rear of the ash compartment 3 the floor 8 of the compartment has an air inlet opening 9 the area of which can be adjusted by means of a slide valve 10 which is controlled manually by means of a rod 11 which extends out through the front of the base section 2.

Mounted below the air inlet opening 9 is an electrically driven blower 12 which is arranged to take in air through an opening 13 in the rear of the base section 2 and to blow the air through the inlet 9 into the ash compartment 3. If desired, the air inlet 9 may also be provided with a flap valve 14 which is arranged so that when the blower 12 is in operation the valve 14 opens to allow air into the ash compartment 3, but closes under the action of gravity to shut the inlet 9 when the blower 12 is switched off.

The rectangular intermediate section 5 of the casing 1 houses a circular fire box 15 which rests on the top of the base section 2 covering the opening 4.

The base 16 of the fire box 15 has a series of angularly spaced radially extending slots 17 positioned as shown in Figure 2 around a central region in which there are three relatively small equiangularly spaced circumferentially extending slots 18. Supported concentricaliy on the base 16 of the fire box 15 is a circular disc 19 having an arrangement of angularly spaced radially extending slots 20 similar to the slots 17 in the base 16. In its central region however, the disc 19 has three equiangularly spaced holes 21 which register with the small central slots - 18 of the base 16. The disc 19 is rotatable about a central pivot 16a on the base 16 by means of a rod 22 (Figure 3 only) which engages with a lug 23 depending from the underside of the disc 19 through an aperture (not shown) in the base 16, the rod 22 projecting out through the side of the base section 2 of the casing. The base 16 and the superimposed disc 19 together constitute a fine grate, and rotation of the disc 19 on the base 16 adjusts the area of the effective openings through the grate between a minimum in which the slots 20 are totally out of register with the slots 17, and a maximum in which the slots 20 are in full register with the slots 17. In either position the central holes 21 of the disc 19 register with the central slots 18 in the base 16 so that the holes 21 constitute permanently open openings through the fine grate.In operation the disc 19 will be rotated to adjust the fine grate openings to suit the fuel and the burning rate required, or vigorously to effect riddling of the grate.

In addition to the fine grate, the fire box 15 also contains a coarse grate 24 which is supported (by means not shown) approximately 2.5 centimetres above the surface of the disc 19. In the present example the coarse grate 24 is formed as a heat exchanger, comprising a continuous water heating tube 25 which is wound into a spiral as shown in Figure 2 and which has its inlet end 26 extending from the centre of the spiral radially beneath the coil outwards through the fire box 15 and the back of the casing 1. The front of the intermediate section 5 of the casing is provided with a door 27 which can be opened to permit fuel to be fed into the stove, the front of the fire box 15 being cut away as shown at 28 to facilitate the insertion of the fuel.

Above the intermediate section 5 the casing 1 has a top section 29 in the form of a pyramidal hood which defines a chamber 30 for the combustion gases from the fire in the intermediate section 5 and which leads at its upper end into a flue outlet 31 containing a manually adjustable damper plate 32.

Mounted within the chamber 30 is a heat exchanger 33 formed by a continuous water heating tube which is wound into a helical coil which tapers upwardly following the taper of the hood 29. At its lower end the heat exchanger tube 33 is connected to the outlet end 34 of the coarse grate heat exchanging coil 24 by a riser 35 at one side of the intermediate section 5. At its upper end the heat exchanger coil 33 leads out through the back of the hood 29 as shown at 36. To ensure that the combustion gases from the fire pass up through the hood in contact with the heat exchanger coil 33 a deflector plate 37 is positioned near the top of the fire chamber defined by the intermediate section 5 of the casing and centrally above the fire box 15.

The water to be heated by the stove is supplied to the inlet end 26 of the coarse grate heat exchanger coil 24 and is extracted from the outlet 36 at the top end of the heat exchanger coil 33 in the hood 29.

While the water will flow through the heat exchanger coils by a thermosyphonic action, it is preferably pumped through.

In use the fire is initially built on the coarse grate 24, and while the fire catches, the damper 32 and the slide valve 10 are opened fully and the blower 12 is operated to promote maximum flow of air through the stove. Rotation of the disc 19 by means of the rod 22 adjusts the openings through the fine grate, and therefore the velocity of the air flow through the fire, as required according to the fuel being burned.

When the fire is well alight the air flow controls can be adjusted to reduce the draught as desired. As the fire becomes established the coarse grate 24 becomes embedded in the heart of the fire, thereby ensuring a good transfer of heat to the water which flows through the tube 25 of the grate. Normally the stove will be operated damped down as far as possible, i.e. with the damper 32 almost closed, and the fine grate openings closed or almost closed, and the blower 12 being operated only when the temperature in the combustion gas chamber 30 falls below a predetermined minimum value. For this purpose the chamber 30 includes a temperature sensing element 38 of a thermostat which is arranged to control the switching on and off of the blower 12.If preferred the element 38 may be located eisewhere, e.g. in the flue 1, so long as this permits suitable control of the air supply to keep the fire alight. As mentioned previously, the damper 32 may be controlled automatically by a solenoid (not shown) which is energised to open the damper and deenergised to close it simultaneously with switching on and offofthe blower 12. In this way the temperature of the fire is raised quickly whenever it dies down too far, and following that the airflow through the stove is kept to a minimum in order to retain the heat of the combustion gases in the chamber 30 for as long as possible to enable the majority of the heat to be transferred to the water which flows from the grate heat exchanger 24 and through the heat exchanger coil 33 to the outlet 36.

For the stove to work efficiently in this manner it is necessary to ensure that very little air can enter or escape from the stove around the closed doors 7 and 27 or around the entry point of the control rod 22 into the base section 2 of the casing.

The second example illustrated in Figures 3 and 4 is similar to the first example except that instead of heating water it is designed to heat air which is drawn from the room in which the stove is located.

Parts of the second example which are identical to those of the first example have therefore been given identical reference numerals and will not be described again.

In the second example the coarse grate which is supported in the fire box 15 above the fine grate formed by the plates 16 and 19 is not formed by a heat exchanger coil, but simply comprises a grid 39 formed as shown in Figure 4 by inner and outer rings 40 and 41 connected by radially extending spokes 42. Furthermore, the heat exchanger in the hood 29 is not formed by a helically coiled continuous tube, but instead comprises two pairs of tubes 43 and 44 (only one tube of each pair being shown) which zig-zag across the combustion gas chamber 30 from opposite sides as shown in Figure 3. The lower ends of one pair of tubes 43 lead from the top of a manifold 45 which covers one side of the intermediate section 5 of the casing housing the fire chamber and which is open at its lower end to a compartment 46 in the base section 2 of the casing. The compartment 46 contains an electrically driven blower 47 which entrains air from the room via an opening 48 at the back of the base section 2 and blows the air upwards through the manifold 45 and into the tubes 43. The upper ends of the tubes 43 open through the side of the hood 29 into a cowl 49 which disperses the air back into the room. Similarly, the lower ends of the tubes 44 open into a manifold 50 which is identical to the manifold 45 but on the opposite side of the casing section 5, and their upper ends open into a cowl 51 on the opposite side of the hood 29 from the cowl 49. The lower end of the manifold 50 opens into a compartment 52 in the base section 2 through which air is blown by an electrically driven blower 53 which entrains air from the room via an opening 54. The two tubes 44 interleave with the tubes 43 in the chamber 30 so that, in operation, the gases from the fire rising upwards through the chamber 30 to the flue outlet 31 are forced to flow around the tubes to effect efficient heat exchange with the air flowing through the tubes, thereby heating the air prior to it re-entering the room via the cowls 49 and 51. Because of this arrangement the deflector plate 38 which is incorporated in the first example is dispensed with in the second example.

Otherwise the operation and control of the fire is the same in both examples.

Claims (23)

1. A solid fuel stove comprising a casing which encloses a fire grate, a chamber above the grate for the combustion gases from the fire, and an ash collection space below the grate, a controlled air inlet below the level of the grate, a flue outlet from the combustion gas chamber and containing an adjustable damper, and one or more tubes forming a heat exchanger in the combustion gas chamber for heating a fluid which, in use, is passed through the tube or tubes, the fire grate comprising a relatively coarse grate and a relatively fine grate which is spaced below the coarse grate.

2. A stove according to claim 1, in which the coarse grate is formed as a heat exchanger, comprising one or more tubes for the passage of the fluid to be heated.

3. A stove according to claim 2, in which the heat exchanging coarse grate is connected to the heat exchanger in the combustion gas chamber so that, in use, the fluid to be heated passes through both heat exchangers.

4. A stove according to claim 3, in which the heat exchanging coarse grate is formed by a continuous tube which is spirally wound to form a flat or dished coil, and the heat exchanger in the combustion gas chamber is formed by a continuous tube which is substantially helically wound adjacent the inner wall of the combustion gas chamber.

5. A stove according to claim 4, in which the two heat exchangers are formed by a single continuous tube.

6. A stove according to claim 4 or claim 5, in which the part of the casing surrounding the combustion gas chamber and its heat exchanger forms a conical or pyramidal hood which leads to the flue outlet.

7. A stove according to any one of claims 4 to 6, in which a deflector plate is positioned substantially centrally in the lower region of the combustion gas chamber for deflecting the combustion gases to the peripheral region of the chamber where the heat exchanger is located.

8. A stove according to any one of claims 2 to 7, in which the heat exchangers are connected in a circulatory water system for the purpose of heating the water, the system including a circulating pump.

9. A stove according to claim 1, in which the tube or tubes of the heat exchanger in the combustion gas chamber rise continuously and are open at opposite ends to allow ambient air to be drawn through and heated.

10. A stove according to claim 9, in which the heat exchanger comprises two groups of tubes which zig-zag across the combustion gas chamber in opposite directions and which interleave with each other.

11. A stove according to claim 9 or claim 10, including a fan for blowing air through the tube or tubes of the heat exchanger.

12. A stove according to any one of claims 3 to 7, in which opposite ends of the connected heat exchangers are open, and the stove includes a fan for blowing ambient air through the heat exchangers.

13. Stove according to any one of the preceding claims, in which the spacing between the coarse and the fine grates is substantially 2.5 centimetres.

14. Astove according to any one of the preceding claims, in which at least some of the openings of the fine grate are adjustable in size.

15. A stove according to claim 14, in which the fine grate comprises a pair of plates which are located one on top of the other and each of which is provided with a series of openings, at least one of the plates being movable to vary the degree by which the openings of one plate register with the openings of the other and thereby adjust the effective size of the fine grate openings.

16. A stove according to claim 15, in which the plates are circular and are rotatable relative to each other.

17. A stove according to any one of claims 14 to 16, in which all of the fine grate openings are adjustable between fully closed and fully opened positions except for some which are grouped in a central region of the grate and which are relatively small and are permanently open.

18. A stove according to claim 17, in which the coarse grate has an open central region above that of the fine grate containing the group of permanently open openings.

19. A stove according to any one of the preceding claims, in which the controlled air inlet comprises an opening in the part of the casing enclosing the ash collection space and a manually operable slide valve for adjusting the area of the opening.

20. A stove according to claim 19, in which the air inlet opening is provided with a booster fan which is thermostatically controlled to operate when the temperature in the combustion gas chamberfalls below a predetermined value, and a flap valve which opens when the booster fan operates and which closes to shut the air inlet opening when the temperature in the combustion gas chamber rises above the predetermined value and the booster fan switches off.

21. A stove according to claim 20, in which the flap valve is opened by means of a solenoid which is energised when the booster fan is switched on.

22. A stove according to claim 20 or claim 21, in which the flue damper is opened automatically when the boosterfan is switched on by means of a solenoid which is energised simultaneously with the booster fan, the damper closing automatically to a predetermined damping position when the fan is switched off and the solenoid is de-energised.

23. A stove according to claim 1, substantially as described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.