The invention, as is evident from the characteristics section of claim 1, is a completely new design of a burner with an output adjustable all the way down to below 10% (excluding, of course, when the burner is completely shut down) of the normal output.

The invention will therefore be described in greater detail in the form of examples with reference to the drawings, where Fig. 1 shows a schematic cross- sectional view a burner according to the invention, Fig. 2 shows a section along line 11-11 in Fig. 1 and Fig. 3 shows schematically an embodiment of a fire grate viewed in section from the side.

The burner according to the invention consists of a combustion chamber housing 1 surrounding a combustion chamber 2. The combustion chamber 2 is circular when viewed from above and is limited at the bottom by a fire grate 3. The fire grate 3 is supported by a telescopic casing 4 and driven by a axle 4'in the form of a splined fitting, for example, via angled gearing 5 of a motor 6 that can be operated at intervals and that simultaneously drives an ash screw 7. A scraping device 8 in the form of vanes can also be arranged on axle 4'. The fire grate 3, which in Fig, 1 is shown in its lower position, can be raised and lowered with the help of, for example, a linkage as indicated schematically with the designation 9. A number of ceramic spheres 10 are loosely distributed on top of the fire grate. The spheres 10 can lay in several layers on the fire grate 3, which can also support a stirrer 11.

In the gas outlet 12 from the circular combustion chamber, there is an adjustable draught vent 13 arranged, which by being raised or lowered respectively restricts or opens the gas outlet, which is connected to a burner duct 14. The adjustable draught vent 13 is arranged so that it can be displaced to restrict the gas outlet 12 and is even equipped with one (or more) channels 15, through which even secondary air can be introduced into the gas outlet. Secondary air can even be introduced via a channel 16 (or channels) arranged over the top of the draught vent

13. The high speed of gas flow in the gas outlet 12 generates turbulence and thereby a good mixing of gases in all positions of the draught vent 13. Channel 15 (or channels) in draught vent 13 open at the side of the draught vent, so that when they close, the channels will be reduced due to the channels'inlet being cut-off by the draught vent's 13 outer guide 15'. The upper channel 16 (or channels) can be restricted or closed by a draught vent 17.

The combustion chamber housing 1 is surrounded by an outer and an inner shell, 18 and 19 respectively. Air for combustion is drawn in between shells 18 and 19 through an opening 20 in the outer shell 18 by a blowing fan 21 and is led on further between the inner shell 19 and the combustion chamber housing 1 to the area of channels 15 and 16. The air is even led through a channel 22 to the underside of the fire grate 3 through which the air passes as primary air. A scavenging air draught vent 23 with a nozzle connects the combustion chamber 2 with the combustion chamber housing scavenged with air for combustion.

A fuel duct 24 opens centrally above the combustion chamber 2 and is supplied with fuel such as chippings or wood pellets via a feed duct 25 from, for example, a larger store (not shown). A slowly rotating geared motor 26 turns a suitably formed stirrer of steel wire 27 that extends down into the fuel duct 24. A momentum-sensing device 28, including a drive motor 26 for the stirrer, detects the load on the stirrer 27, which is intended to rotate slowly in the fuel duct 24 to prevent the fuel fastening in the fuel duct 24 and forming aggregates, which can be a problem, especially when chippings are used. When the level of fuel in fuel duct'24 rises, the resistance to the turning of the stirrer 27 increases, which causes the motor 26 mounted on ball bearings to attempt to turn itself, against the action of a spring, against a stop bolt. In contrast, when the level of fuel in the fuel duct 24 falls, the resistance to the turning of the stirrer 27 decreases, and the motor 26 is influenced by the spring to turn in the opposite direction against another stop bolt. These tendencies to movement of the motor are detected and influence a micro-switch, for example, in such a way that when the momentum of the stirrer 27 increases, the motor 26 stops, while a reduced momentum causes the motor to start. Such momentum-detecting

arrangements can be executed in many different ways and are well known within the area of technology. For these reasons, they do not form part of the actual invention and will not be described in greater detail here.

A shunt duct 29 connects feed duct 25 with the pressurised side of the blowing fan 21, which means that over-pressure occurs upstream in the opening of fuel duct 24 into the combustion chamber 2. This impedes smoke rising up fuel duct 24.

In Fig. 1,30 designates a sealable ignition and inspection hole for the combustion chamber 2 and 31 designates a container for ash, which is fed by the ash screw 7.

According to the invention, the functions of the burner are as follows.

Fuel in the form of wood pellets, chippings or similar is fed through feed duct 25 and down into the vertical fuel duct 24 where the steel wire formed and suitably shaped stirrer 27 slowly rotates and prevents the fuel from fastening in the duct.

Fuel from fuel duct 24 fills combustion chamber 2 and, according to the rate at which the fuel is consumed in the combustion chamber, will sink down through fuel duct 24 and maintain a certain level in combustion chamber 2. The fire grate 3 rotates at a suitable speed driven by the motor 6 operated at intervals and that also drives the ash screw 7 via gearing 5. The speed of motor 6 is adjusted to match the intended output from the burner. A stirrer 11 arranged on the on the fire grate distributes the fuel in the combustion chamber 2. When fire grate 3, which is perforated, for example, turns, the glowing bed provides a certain resistance and the spheres 10 currently on the fire grate and lying in a sufficiently thick layer develop an uneven movement, whereupon a slow"boiling"movement occurs that keeps the glowing bed sufficiently aerated but that which prevents blow through. The ash falls through the bed of spheres and any glow remaining burns out. The spheres even protect against heat radiation in a downwards direction and the heat taken up by the spheres is passed on to the primary air streaming up from below.

The air feed or the air of combustion is drawn in by the blowing fan 21 from the opening 20 and between the outer and inner shell, 18 and 19 respectively, to

later scavenge the combustion chamber housing 1. The air flows on through channel 22 as primary air in under the fire grate 3. Secondary air can be admitted at regular intervals through channels 15 and 16 into the areas for the adjustable draught vent 13 after the combustion chamber 2 but before the burner duct 14. The nozzle of scavenging air draught vent 23 blows a stream of air towards the wall of the combustion chamber above the glowing bed to prevent the build-up of a coating of ash. As the combustion air is drawn in between the outer and inner shell, 18 and 19 respectively, the outer shell is cooled at the same time as the combustion air is pre- heated. The combustion air is pre-heated further by it thereafter scavenging the combustion chamber housing 1.

Ignition of the fuel takes place through the sealable opening 30 with, for example, the help of an ignition spiral.

When the burner is at full output, blowing fan 21 operates with maximum rated output. By the use of the shunt duct 29, the blowing pressure can in principle be unlimited without smoke leaking out. When the output is to be reduced, the speed of the blowing fan is first reduced, and then stops, by means of the which the burner, at a lower output of about 60%, works by self-ventilation. The adjustable draught vent 13 begins to close and simultaneously restricts the channel (channels) 15 while the draught vent 17 restricts the channel (channels) 16 and the secondary air supply to the burner duct 14 diminishes. At the same time, the interval for the fire grate's motor 6 for the slow rotation of the fire grate 3 changes. At a lower output, the fire grate 3 can even be raised and, at the lowest output, less than 10%,'the scavenging air draught vent 23 closes. During the lowest output, the supply of secondary air is completely cut-off and the adjustable draught vent almost completely closed. The fire grate is raised so that the glowing bed only comprises a small area under the fuel duct 24. This means that part of the primary air flows past the fire grate and the combustion of the gas begins in the combustion chamber 2. At this time, draught vent 13 is shut-down to a level so that only a small hole is left, which causes all the gas to pass to the boiler pan (not shown), whereby a small concentrated hot flame occurs, which burns up any possible non-combusted gasses that pass by. As the

space in the combustion chamber 2 is now small, the temperature in the combustion chamber can be kept high, and similarly so in the entrance to the burner duct 14.

Even the primary air that is drawn in through duct 22 is pre-heated to a very high degree.

To maintain the minimum output without the flame going out or increasing, a temperature detector (not shown) is arranged somewhat above the burner duct's 14 outlet in the boiler pan (not shown). At this low output, the hot gasses travel upwards as soon as they have left the burner duct 14, but at higher outputs, the gasses blow out under the temperature detector that, if the flame becomes too hot, stops the fire grate motor completely, whereupon ash quickly begins to build- up in the glowing bed and the flame diminishes.

As the adjustable draught vent, which is of critical importance at low output, is responsible for restricting flow to the boiler pan, there is no risk of disturbing false draughts either in the combustion chamber or in the fuel duct 24.

Fig. 3 shows one embodiment of fire grate 3 that in this case comprises a centre section 32 plus an outer fire grate ring 33 and an inner fire grate ring 34.

Ring-shaped gaps occur between rings 33,34, and the centre section 32. The latter has a hole or perforations 35. The centre section is supported by the axle 4 that rotates the section, and rings 33 and 34 are supported by three extended axles 36 that are radially symmetrical in relation to one another and that are fastened to a central axle support 37 (casing 4, for example, see Fig. 1) as shown in Fig. 3 to allow the raising and lowering of the fire grate. A groove 38 is machined in each respective axle 36. Each respective groove 38 is machined off-centre in relation to the centre line of axle 36 and is done so that in each axle, the two grooves are machined with opposite off-centres. Rings 33 and 34 rest against supporting edges 39 in the grooves, as is evident from the figure. If the axles 36 are turned or manoeuvred in relation to one another with 1/3 of a turn, a skewed up and down movement occurs with opposite movement between the outer ring 33 and the inner ring 34. Due to this, the glowing bed is kept in motion and ash can easily fall down between the rings. The respective axles 36 are driven at the same speed via an angled gearing 40 and the

inner ring 34 will turn at a greater speed than the outer ring 33. It should, of course, be understood that more than two rings 33,34 can be used.

The complete regulation of the blowing fan 21, the adjustable draught vent 16, the draught vent 17, the turning of the fire grate 3 with the help of motor 6 and the raising and lowering with the help of the slewing bracket system 9, the scavenging air vent 23 and the stirrer 27 to achieve the desired combustion sequence with different levels of output takes place with the help of commands from a controlling computer. This is neither shown nor described here since the program- ming only involves an optimisation of the burner's operating conditions and is not in itself an invention but can be considered to be what is now commonly applied programming and computer technology.