Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to an improve-
ment in stoves for combustion of solid fuel for
example wood, coke, coal etc., comprising a primary
combustion chamber and a secondary combustion chamber,
both chambers with separate supply of combustion air.
Attempts have been made with previously known
stoves of this type to achieve an after combustion
of incompletely burned gases by supplying secondary
air to same in the combustion chamber. Tests have,
however, shown that the effect aimed at very rarely
is obtained in small stoves and that if the result is
obtained, it is due to particularly advantageous
circumstances.
Due to the signs of a future shortage of energy
recently, the interest has increased in utilization of
forest products as a source for domestic heating.
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There will then be a demand for a stove which without
pollution of the environments, is able to burn forest
products and similar fuels with the highest possible
efficiency and the best possible degree of controlled
combustion.
It is a feature of this type of stoves that
the fuel undergoes an incomplete (pyrolytic) combus-
tion at a comparatively low temperature in a primary
chamber. In this process inflammable gases (pyro-
lytic gases) are liberated at a rate which is depen-
dent on the temperature and the form and composition
of the fuel. The temperature in the primary chamber
is dependent on the rate of combustion which takes
place therein, and consequently dependent on the
supply of primary air and the heat insulation of
the primary chamber. The total potential energy
dissipation will consequently be the sum of the
intensity of the primary combustion and the heating
value of the pyrolytic gases. A controlled dissi-
pation of energy requires at all times an adjustment
of the quantity of primary air and the insulation
of the primary chamber to the fuel which is in use.
In order to secure complete combustion of
the pyrolytic gases and thereby obtain a high
efficiency it will be necessary to supply secondary
air to the gases under controlled conditions
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concerning quantity, temperature and resting tirne.
The combustion can in an effective manner take
place in a separate secondary chamber with suitable
heat insulation and supply of air.
The purpose of the present invention is
consequently to provide a stove for combustion of
solid fuel in which the combustion takes place in
two stages, viz. firstly in a primary chamber and
thereafter in a secondary chamber in such a manner
that a high efficiency of the stove is obtained, and
the stove should further be so designed that a con-
trolled combustion is obtained.
According to the invention this is achieved
by an improvement in stoves of the previously men-
tioned kind, which is characterized in that both
chambers are thermally insulated in such a manner that
the production of pyrolytic gases in the primary cham-
ber takes place within a predetermined space, and
that the temperature in the secondary chamber is
sufficiently high to cause a complete combustion of
all pyolytic gases. Usually the highest temperature
is required in the secondary chamber, and this
suggests a better heat insulation for this chamber
than in primary chamber.
Another feature of the invention is that the
secondary chamber is designed as a two stage combustion
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chamber. The size of an optimal secondary chamber
will be denendent on the energy dissipation of the
stove. A two-stage secondary chamber will thereby
ensure good secondary combustion over a wide range
of adjustment, as the combustion with low dissipations
will take place in the first stage and at higher dis-
sipations the combustion will be completed in the
second stage.
A further feature of the invention consists
in the provision of a heat exchanger surface between
an inlet passage for secondary air, and an outlet
passage for the hot combustion gases from the
secondary chamber in order to preheat the combustion
air admitted to the secondary chamber.
Another feature of the invention is that
the secondary air passage opens out in the space
between the primary and the secondary chamber.
A preferred embodiment of the invention
is characterized in that the secondary air passage
lS located in the partition between the primary
chamber and the secondary chamber and in that the
passage is insulated against the primary chamber and
has a heat exchanger surface towards the secondary
chamber. The result is a pre-heating of the secondary
air.
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Still another feature of the invention is
that a source of heat is located in the secondary
chamber or in the air supply passage to the secondary
chamber, the source of heat being supplied with
external energy and may, for example be an electric
heating element. The energy dissipated by this source
of heat can in accordance with the invention, be
controlled by
a) the temperature in the secondary chamber,
or,
b) the temperature rise in the secondary
chamber, or
c) the temperature in the primary chamber or
d) a combination of the parameters (a,b,c).
In order to lower the ignition temperature for
the gases in the secondary chamber it is a feature of
the present invention that the secondary chamber can
be provided with a catalyst. An alternative to this
feature is that the secondary chamber is provided with
plates, rods or similar means having a large thermal
inertia so that ignition of combustion gases will
take place easier than with a free gas atmosphere.
In order to ensure a self-controlled combus-
tion process with high efficiency under all conditions
independent of the type of fuel or form or varying
draft conditions, one feature of the invention may
be that-a fan is provided to ensure a uniform air
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upply to the combustion. In an advantageous
embodiment the fan is driven by a thermoelectric
element positioned on one of the walls in one of
the combustion chambers, preferably the secondary
chamber. The fan can be controlled by a thermo-
stat.
The invention will in the following be
explained with reference to the diagrammatic
drawing which shows one embodiment of the invention.
The stove shown as an example, consists of
a primary chamber 1, in which at least the top wall
2 is insulated. It is, however, possible to insulate
also the side walls if this should prove necessary
in order to increase the temperature in the primary
chamber when a predetermined rate of combustion is
desired. Combustion air to the primary chamber 1 is
drawn through draft openings 4 in the front of the
stove.
The combustion gases from the primary chamber
1 is conveyed through an opening 5 at the rear wall
3 into the secondary chamber 6. This chamber is
constructed as a two-stage chamber in which the
combustion when the energy dissipation is low, takes
place in stage 1, while the combustion will be com-
pleted in stage 2 when the enerqy dissipation is high.
Combustion air to the secondary chamber is drawn
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in through a draft opening 7 in the front of the
stove to a passage 8 in the top wall of the
primary chamber 2. The supply of combustion air
is so adjusted that the primary air and secondary
air is controlled simultaneously. The passage 8 is
insulated against the primary chamber as shown on
the drawing, but is provided with a heat exchanger
surface 9 towards the outlet passage 10 from the
secondary chamber. The secondary chamber has a
good thermal insulation, so that sufficiently-high
temperatures (750-1000C) are obtained therein.
To ensure ignition with a complete combustion of the
combustion gases in the secondary chamber, viz. that
the temperature will be higher than the ignition
temperature for the gases, a heating element 12
with external supply of energy can be used. The
heating element 12 can be an electric heating element
as in the shown embodiment, or the heating element
can be an oil burner, gas burner etc. The heating
element can be located in the supply passage 8 or
the secondary air instead of in the secondary
chamber proper. As mentioned above the secondary
chamber may have a catalytic surface, for example a
platinum or rhodium compound so that the ignition
temperature for the combustion gases is lowered.
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The secondary chamber can also be provided with
plates, rods or similar means having a certain
thermal inertia, so that the ignition of the
combustion gases will take place easier than in a
free gas atmosphere. It should be pointed out
that the provision of heating element, catalyst
or rod or similar means with thermal inertia can be
used alone or in combination.
The hot gases from the secondary chamber
6 is conveyed out through the outlet 10, re-enters
at the forward edge of the top wall 1 and flows
through a passage 13 on the upper side of the top
wall 11 to the flue 14. From the outlet of the
passage 10 to the flue the combustion gases dissipate
the largest possible quantity of heat, and the passage
13 is therefore provided with a heat exchanger for
transfer of the heat to the environments.
The flue 14 may be provided with a fan 15
which could be supplied with electric current from
a thermocouple 16 located on the hot wall surface
of the passage 13. The thermocouple will generate
an electric current which is dependent on the tempe-
rature difference between the hot side of the wall
and the environments. The fan 15 can be controlled
by a termostat so that the supply of combustion air
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is made dependent on a desired room temperature.
The starting of a cold stove can be made with a
booster battery as the thermocouple does not gene-
rate any current, or natural draft through the
flue can be utilized.
The external supply of energy to the
secondary chamber will not be required at all time.
To control this supply of energy the following
parameters should be taken into account:
a) the temperature in the secondary chamber.
If this temperature falls below 800C, supply of
external energy is required of combustion gas still
is flowing from the primary chamber. A temperature
sensor could then be provided in the secondary
chamber.
b) Temperature rise from inlet to outlet
in the secondary combustion zone. A positive value
which is not due to supply of energy, indicates that
combustion gas to be burned still is flowing,
c) The temperature in the primary chamber.
A low limiting value indicates that the stove has no
more fuel and therefore there is no need for external
supply of energy.
By combination of these parameters in the
form of for example electrical signals from thermi-
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stors or similar means it is possible to achieve a
fully automatic and complete combustion of allinflammable gas.
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