Note: Descriptions are shown in the official language in which they were submitted.
CA 02293779 1999-12-02
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Overfeed burner for solid fuel and method of.its operation
The present invention refers to a top-feed burner according to the
introduction to the claim 1.
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
II-II 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 lire 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
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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 fiuel 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 sta -rt. Such momentum-detecting
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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 2~ 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 2~ 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 S. 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
2~ 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 ever;
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
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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) IS
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
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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.
1~ 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 islmachined 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
2~ 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 t~~
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
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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.
