GB2116697A - Solid fuel-burning stoves - Google Patents

Abstract

A solid fuel-burning stove comprises a grate 9 for a combustion bed 3 of solid fuel, a flue 6, a hopper 1 for the introduction of the solid fuel into the combustion bed, means such as a damper 12 controlled inlet 11 for introducing primary air into the combustion chamber and thermostatically regulatable means such as an access door 2 for introducing secondary air so that it passes through the hopper to purge it of volatiles and then through ducting to the region where flue gas leaves the combustion bed. The secondary air flow to that region is regulatable so as to enable the flow to be eliminated or reduced when a low combustion level is required and to enable the flow to be increased continuously or progressively as the combustion level is increased. <IMAGE>

Description

SPECIFICATION Improvements in solid fuel-burning stoves The present invention relates to solid fuel-burning stoves, particularly those which are fuelled with bituminous coal products. It has relevance both to central-heating boilers and to room heaters of closed construction, and also to stoves utilising downdraught combustion techniques as well as to stoves utilising cross-draught combustion techniques.

Coal may contain large amounts of bituminous materials (up to 30% by weight). When coal is burnt, the bituminous materials are volatilised by the heat and if the temperature is high enough and there is sufficient air, the volatile gases and other materials will ignite and burn as a flame. If the temperature is insufficiently high, then these materials enter the flue and leave it as a more or less dense smoke. In orderto reduce the production of smoke and produce relatively clean emission when burning coal, down-draught burning has been developed by which air supporting combustion (primary air) passes downwards through the bed of fuel, burning taking place at the bottom of the bed. The flue gas then turns though 1800 to pass up the flue to the atmosphere.Means are usually provided to introduce secondary air at the point at which the flue gas leave the combustion bed in orderto promote secondary combustion and, since this is a very hot area, more complete combustion takes place than if the gases leave the top of the fuel bed, as in conventional up-draught combustion. The secondary air is conventionally supplied through ducts and holes in the firebrick at the location at which the gas leaves the combustion bed and in the past this secondary air has usually entered the firebrick through orifices at each side of the stove.

The fuel which is above the area of combustion will be heated by radiation from the fuel burning below it and if the primary air entering the appliance is reduced to a low level, or cut off altogether, then unburnt fuel above the hot combustion area will reach a temperature at which volatiles are produced which may fill the space above the fuel.

If the primary air inlet is then opened, this mixture of combustible materials, gases and air, will pass down the fuel bed until they reach a point where the temperature is such that they ignite. The 'gas'/air mixture may be such that this ignition causes a minor explosion, producing a blow-back up the hopper with the emission of flames from the primary air inlet. If the admission of air has been caused by opening the loading door, then a very dangerous situation may occur with the emission of flames and fuel dust from the door to the danger of the operator.

An alternative to a down-draught appliance is a cross-draught appliance, in which primary air is introduced at approximately the same level as the point at which the gas leaves the combustion bed and thus merely passes across the bed of fuel. In such an appliance the void space in the hopper above the fuel bed tends to become filled with volatiles passing upwardly through the fuel and, as this space is not purged by the introduction of primary air, the appliance is even more liable to blow-back problems, with the possibility of minor explosions emitting smoke, fumes and sometimes flames when the hopper is opened even if the primary air inlet damper is already open.

In order to overcome the problem of explosions, various methods have been used in the industry to ventilate the upper part of the hopper so as to reduce the concentration of volatiles to below the limit at which the explosions will occur. One technique that is known is for air to be introduced into the top of the hopper and drawn directly into the flue, by-passing the fuel bed. This method has the disadvantage that air entering the hopper and being purged directly to the flue does not all necessarily bypass the fuel bed.

Some will certainly pass through the fuel bed to the flue and encourage burning, so that low levels of combustion are not possible unless the hopper air inlet is closed, and then, of course, a potentially dangerous situation again exists. Venting the top of the hopper directly to the flue has another disadvantage in that if air leaks into the base of the appliance for example through the ash can door - then burning may take place up the fuel bed, the appliance converting itself into a conventional burning system.

Means may therefore be provided to close off the opening between the hopper and the flue so that it is only open when venting is required. This mechanical device must be sited in the stove where heavy bituminous deposits are likely to occur, and it is possible that it could fail to operate.

Another known alternative is to vent the top of the hopper via a duct to the firebrick at the bottom of the fire bed. Thus, the air purging the hopper also provides all or part of the secondary air. Again, part of the air may pass through the fuel bed and it is difficult to maintain low rates of combustion.

Appliances such as those described are usually fitted with a thermostat to control the primary air inlet damper, the activating element of which may be inserted into the water jacket or into the air surrounding the appliance. Secondary thermostats are also known, also controlling the primary air inlet and sometimes activated by the.flue gas temperature. However, we are not aware of any prior proposal to apply thermostatic control to the secondary air inlet.

We have now developed an improved construction, suitable both for cross-draught and for downdraught appliances, which enables the problem of blow-back to be overcome and permits very low slumbering rates together with an improvement in efficiency.

According to the present invention, a solid fuelburning stove comprises a grate for a combustion bed of solid fuel, a flue, a hopper for introduction of the fuel into the combustion bed, means for introducing primary air into the combustion bed in a down-draught or cross-draught arrangement and thermostatically regulatable means for introducing secondary air so that it passes through thehopper to purge it of volatiles and then through ducting to the region where flue gas leaves the combustion bed, said secondary airflow being regulatable so as to enable the flow to be eliminated or reduced when a low combustion level is required and to enable the flow to be increased continuously or progressively as the combustion level is increased.

In preferred embodiments means are also provided for preventing access to the hopper for the purpose of loading more fuel until the secondary air passing through the hopper has had time to purge it of potentially explosive volatiles.

Various different constructions are possible within the overall scope of the invention. For example with down-draught appliances the secondary air, like the primary air, will be introduced into the upper part of the hopper and the thermostatic regulation can be provided in the ducts leading the secondary air from the hopper to the region where flue gas leaves the combustion bed.

In one embodiment when the appliance is of cross-draught construction the access door through which fuel is loaded into the hopper or part of this door can be adapted to form an inlet damper which can be opened to a limited regulatable extentforthe introduction ofthe secondary air. The opening of this damper is controlled by means of a thermostat from high to low air inlet flow levels, either continuously or progressively, to enable controlled purging and introduction of secondary air. Preferably the door is fitted with means to prevent it being fully opened until the hopper has been properly purged of volatiles.This may be achieved by use of a time delay mechanism which enforces a time delay between opening of the access doorto its full extent as a damper and its opening for loading more fuel into the hopper.

In a second embodiment of a cross-draught appliance the introduction of primary and secondary air into the appliance can be regulated by a single thermostatically-controlled damper, the single air stream subsequently split into two separate streams and at least the primary air stream being separately controlled by a further damper. The secondary air stream is led up into the hopper to purge it of volatiles and means are preferably provided to prevent the access door to the hopper being opened until the primary air stream has been shut off and purging of the hopper satisfactorily completed.

In a third embodiment of a cross-draught appliance the primary and secondary air streams are introduced separately into the appliance each through an independent thermostatically controlled damper.

Preferably, the thermostats are linked with means controlling opening of the access door to the hopper so that the access door cannot be opened until the primary air damper is closed and the secondary air damper has been fully open long enough to purge the hopper satisfactorily.

It will be appreciated that these are not the only possible arrangements for introduction and control of secondary air. It should also be pointed outthat not all the air introduced into the hopper to purge it need pass through the ducting to the region where flue gas leaves the combustion bed. Preferably, however, the supply of secondary air to that region is regulatable in such a way as to provide an approximately correct amount of secondary air in that region for efficient combustion to occur.A fixed orifice to supply the secondary air has the disadvantage that at low burning levels more secondary combustion air than necessary is supplied which lowers the temperature in the combustion area, making the combustion less efficient, as well as reducing the temperature of the flue gases with further losses of efficiency in the heat exchanger, and also carrying more heat up the flue. Alternatively, too little secondary air will not allow complete combustion, again with loss of efficiency.

In the accompanying drawings: Figure 1 shows a cross-draught appliance according to the first embodiment mentioned above, Figure 2 shows a cross-draught appliance according to the second embodiment mentioned above, and Figure 3 shows a cross-draught appliance according to the third embodiment mentioned above.

In each appliance there is a fuel hopper 1 having an access door 2 for loading fuel 8 into a combustion bed 3 on a grate 9 and a firebrick 4 in the region 5 where flue gas leaves the combustion bed 3 to pass up a flue 6. Secondary air from the hopper 1 passes through ducting (not shown) to an outlet 7 in the firebrick 4.

In the appliance of Figure 1, primary air enters through an inlet 11 controlled by a damper 12 and secondary air enters through the access door 2 which can be opened to a limited small extent (indicated by the dashed line 13) for this purpose as compared to its full extent (indicated by the chain dashed line 14) for loading purposes.

In the appliance of Figure 2 primary and secondary air enter the appliance through a common inlet 21 controlled thermostatically by means of a damper 22 and the stream then splits into two parts, depending on the siting of a damper and/or the resistance to flow in the ducts, for the primary air to flow via duct 23 and the secondary air via duct 24. Flow of the primary air is further controlled by damper 25 in duct 23.

In the appliance of Figure 3 primary air is introduced through inlet 31 controlled thermostatically by a damper 32 and secondary air is introduced through inlet 33 controlled thermostatically by a damper 34.

Neither the form of the firebrick nor the arrangement of the ducting for introducing the secondary air into the region where flue gas leaves the combustion area is crucial to the inventive concept defined above.

Claims (8)

1. A solid fuel-burning stove comprising a grate for a combustion bed of solid fuel, a flue, a hopper for introduction of the solid fuel into the combustion bed, means for introducing primary air into the combustion bed and thermostatically regulatable means for introducing secondary air so that it passes through the hopper to purge it of volatiles and then through ducting to the region where flue gas leaves the combustion bed, said secondary air flow to the region where the flue gas leaves the combustion bed being regulatable so as to enable the flow to be eliminated or reduced when a low combustion level is required and to enable the flow to be increased continuously or progressively as the combustion level is increased.

2. A stove as claimed in claim 1 also including means for preventing access to the hopper for the purpose of loading more fuel until the secondary air passing through the hopper has had time to purge it of potentially hazardous volatiles.

3. A stove as claimed in claim 1 or 2 wherein the primary air is introduced in a down-draught manner and means is provided in the ducting between the hopper and the region where flue gas leaves the combustion bed for regulating the flow of secondary air to the latter region.

4. A stove as claimed in claim 1 or 2 wherein the primary air is introduced in a cross-draught manner.

5. A stove as claimed in claim 4 wherein the access door through which fuel is loaded into the hopper, or part of this door, forms an inlet damper which can be opened to a limited thermostatically regulatable extent for the introduction of the secondary air into the hopper and thence through the ducting to the region where fuel gas leaves the combustion bed.

6. A stove as claimed in claim 5 wherein full opening of the access door to load more fuel into the hopper is prevented by means of a time delay mechanism until after it has been opened to its full extent as an inlet damper and the air admitted thereby has purged the hopper of potentially explosive volatiles.

7. A stove as claimed in claim 4 wherein the secondary air stream is introduced into the appliance through a thermostatically controlled inlet separate from the access door through which fuel is loaded into the hopper and the primary air stream is introduced through the same or a different thermostatically controlled inlet.

8. A solid fuel-burning appliance substantially as illustrated in Figure 1 or Figure 2 or Figure 3 of the accompanying drawings and hereinbefoe described with reference thereto.