Note: Descriptions are shown in the official language in which they were submitted.
-- 3
:
The present invention relates to a method of burning
solid fuel or combustible in a closed firebox or fuel chamber
which is locally shielded or separated from the surroundings
and which is provided with an air-supply passage and is intended
for heating air, the rate of combustion in said firebox
being controlled by means of at least one damper means arranged
in the air-supply passage, the temperature of the heated
circulating air being limited to a prescribed maximum value.
When firing a stove with wood and like solid fuel, such
as coke, the amount of heat given out by the firebox cannot be
controlled by continuously damping the supply of air thereto,
since the incomplete combustion resulting from such damping
results in an uneconomic surplus of non-combusted carbon monoxide
and in the formation of tar and soot, which might result in a
chimney fire and which would reduce the efficiency of any heat
exchanger arranged to operate in conjunction with the stove, as
a result of the insulating tar and soot coatings.
One object of the present invention is to provide such
a method of burnlng or firing solid fuel in which the aforesaid,
serious disadvantages are eliminated and which enables the stove
to be fired with a large quantity of fuel at one time without
this fuel being combusted at a higher rate than that required in
order to maintain a certain pre-set temperature in a heated
space, a hot-water apparatus or the like.
In the Swedish patent specification No. 110,379 there is
described a wood-fired boiler having an impulse-regulator for
closing and opening a damper associated with the boiler. The
arrangement is comparatively complicated and does not fulfil
,
`''''
:, .
. . .
~ t
-- 4 --
" ~
modern safety and reliability requirements.
Other similar arrangements are known from the German
patent specifications Nos. 235,019, 311,494, 352,252,
498,226, 647,750 and 1,230,957. None of these specifica-
tions, however, describes a solution which is satisfactory
from all points of view. For example, none of the arrange-
ments described therein can be used in those cases when it
is impossible to be sure that the electric current fed to
the thermostat for controlling the same will not be
interrupted.
In its widest aspect, the method according to the inven-
tion is mainly characterized by controlling by means of one
or more control means, such as a thermostat, time-relay or
the like the movement of the damper between either a fully
open position or a fully closed position in which the supply
of air to the firebox is cut-off, in a manner such that
combustion practically ceases but that the fuel maintains a
; sufficiently high temperature for the combustion process to
recommence when the damper is reopened whilst, at the same
time, maintaining a desired given temperature in a space
heated by the circulating air or an object heated by the
hot flue gases, such as an oven, a hot-plate, a hot-water
producing device or the like.
When the damper is open, the fuel will burn with the best
possible efficiency, the fuel being combusted without the
formation of soot and tar. In betweentime, the process of
combustion in the fireplace is restricted in a manner such
that only a small amount of heat is developed. Thus, a large
quantity of fuel can be charged to the firebox, providing an
,,'~
`:
optimal combustion process over a long period of time.
Simultaneously herewith there is obtained the highest
possible degree of safety, as the temperature prevailing in the
firebox will not exceed the prescribed maximum temperature. When
the damper is closed, the temperature in the firebox will gradually
fall, this fall in temperature taking place with a certain delay
in time as a result of the residual heat. Thereafter the damper is
reopened whereupon a further, highly efficient combustion process
commences. When carrying out tests it was found that even when the
l~ regulating periods were of relatively long duration (10-60 minutes)
with the damper closed, the temperature of the fuel did not fall
lower than a level at which it was rapidly reignited when the
damper was reopened. When the damper was reopened, the described
combustion process again took place with a high efficiency. If
the interval over which the damper remains closed using conventional
room-thermostats is too long, separate mechanical or electrical
time relays can be provided to ensure that the damper opens to
permit the combustion process to start.
In order to ensure that the fuel will re-ignite subsequent
. C3D to opening the damper, the temperature of the fuel can be maintained
during those periods when the damper is closed by limiting the
transfer of heat from the firebox to the medium to be heated during
these periods. This can be effected, for example, by restricting
- the circulation of the air flow around the firebox. This restric-
tion of air circulation is best effected by transmitting a signal
to the circulation fan to stop the same at the same time as the
signal is sent to the damper to close said damper.
In practice it is preferred that at least one thermostat
_ _ _ _ . _ _
- 6 -
is arranged at a given value exceeding a pre-set temperature,
to generate a mechanical movement which will rapidly cause the
damper to close substantially completely, or to guide such a
mechanical movement via electrical drive means.
In this way the system is safeguarded against over-
heating, even in those cases when the thermostat does not function
normally, e.g. in the case of a break in the current supply. The
method can also be used to advantage for heating rooms and other
spaces which are not connected to the mains supply.
In order to ensure the best function, it is preferred
that a mechanical or electrical drive means is arranged to open
and close the damper over an interval of time of a duration which
is sufficient for the fire in the firebox to generate a self-
maintaining heat.
The mechanical device may, for example, be a time clock.
The electrical device can, for example, be a delay circuit of
special type, comprising capacitors and resistors. The devices
may contribute to ensuring that the combustion process continues
without interruption under all prevailing conditions in the
aforedescribed manner.
Particular advantages can be gained when, in accordance
with a preferred embodiment, a thermo-electrical generator is
arranged to co-operate with one or more electrical thermostats in
a manner such the generator will not give out~ roduce energy when
the fireplace is cold and the damper will be held fully open
whilst when the temperature of the fireplace, and therewith the
output of the generator reaches a certain desired value the
generator closes the damper via setting devices provided that at
~rL~ ~
- 7 -
.- ~
lease one thermostat does not call for heat.
A thermogenerator of this type can be arranged between a
~- warm or heat transfer surface defining the firebox and, for
example, a surface in a passage for combustion air. When
using such a thermogenerator it is possible, under all con-
ditions, to ensure automatic control of the discontinuous
combustion process, which is of great advantage in many
different fields of application.
Further advantages can be gained when, in accordance
with a preferred embodiment, a thermostat is placed either
~- in the firebox, or in the path of the heated circulating air
or water, or in the flue tube of the firebox, this thermostat
being adapted in a manner such that the damper is automatic-
ally opened when the temperature falls beneath a given value,
the fire being reignited only momentarily, provided that no
other thermostat calls for heat.
For the purpose of adapting the combustion sequence to
different types of fuels it is convenient to provide, in
addition to the automatic control of movement of the damper,
some other type of manual control by means of which said
regulation for different fuels can be effected manually.
Of particular interest in this connection is the provision
of means whereby the amount of surplus air required for
different fuels which may be used can be set.
The invention also relates to a stove for burning solid
fuel in the afore-described manner, the main characterizing
, '~,
- 8 ~
'
features of the stove being disclosed in the accompanying claims.
A stove according to the invention has a very high
efficiency, good control ability and is safe against overheating.
The stove is primarily intende'd to be fired with solid fuel or
- combustible, e.g. wood, peat, cole, coke and the like, but may
also be fired with liquid f~el, such as oil or paraffin, gaseous
fuels, s~h as town gas, naturai gas or gasol without rendering
the necessary auxiliary devices for the purp~se very expensive.
The stove also enables auxiliary devices for both direct and
accumulating electrical heating to be readily connected. The
stove can therefore be used both as a reserve-heat source and
oven in the case of a breakdown in the supply of electric current
and/or as the only source of heat in cottages, detached and terrace
houses. The stove can also function as a source of additional
heat to heat pumps and solar heating systems utilizing some form
of heat accumulating device.
In comparison with a stove according to the invention,
an open fire is uneconomical, primarily because it emits the major
part of its heat in the form radiation heat, besides which it burns
with a very high surplus of air, which impairs its efficiency.
Attempts to reduce this air surplus by means of dampers in the
chimney breast or flue tube often causes the fire to smoke, and
in the worst cases, to carbon monoxide poisoning. In distinction
hereto, an important characteristic of the stove according to the
invention is the closable flap or shutter which provides for
accurate delimitation of the firebox from the surroundings. In
accordance with a preferred embodiment of the invention there is
used a front flap or shutter of hardened glass whose dimensions
:
can be quite considerable, the glass flap retaining some of
the comforts of the open fire, primarily because the fire can
be seen through the flap and because some heat is radiated
therefrom.
In accordance with the invention the heat output of the
; stove is regulated by controlling the supply of combustion air
- by means of the damper located in the air-supply passage. In
this way there is obtained considerably increased efficiency and
an increased security against carbon monoxide poisoning, in
/ 0 comparison with conventional control by means of a damper in the
chimney breast or flue tube.
In.order to increase the efficiency still further there
is applied in accordance with a preferred embodiment of the
invention a so-called balanced draft, the temperature of the
combustion air being increased by heat exchange with the
combustion gases. The efficiency limit obtainable is determined
by the temperature of the fumes or smoke. In order for a chimney
effect to be obtained, the temperature of the smoke must exceed
the temperature of the combustion air by approximately 50-80C,
O if it is desired to avoid the use of smoke fans.
In order to utilize the capabilities of the stove,
particularly with low outside temperatures, without needing to
risk overheating at high outside temperatures, there is used in
accordance with one embodiment a thermostat-controlled damper
which at temperatures, for instance, of 50C causes room air at
a temperature of about 20C to be mixed with the combustion air.
If the building is insulated, pre-heated fresh air will flow in
as a replacement, thereby retaining the high efficiency.
ii
-- 10 --
By means of simple mechanical dampers or manually con-
trolled dampers, one embodiment of the stove according to the
invention can either rapidly heat circulating room air or
accumulate heat energy in a collection of stones connected to
its air passage - so-called ground-stored heat. For the purpose
of storing heat in such accumulators and removing heat therefrom
there is required a fan. The stove, however, will heat the room
air quickly and directly even without a fan, although the effect
will be lower and the efficiency poorer. Thus, in this way, the
/0 stove can be used as a reserve source of energy in the case of,
for example, a breakdown in mains supply.
The possibility of accumulating heat greatly facilitates
the use and increases the comfort value. For example, it is
possible during the evening to light several fires one after the
other, without the temperature of the room becoming too high,
whereafter the heat supplied to the accumulator can be used the
following evening in a known manner.
The aforedescribed intermittent, thermostat-controlled
firing sequence according to the invention enables the stove to
~0 be provided with a large fuel grate which can be filled completely,
so that the fire will last from 8-10 hours. Firing with wood is
not therefore particularly laborious and as a result of the high
efficiency of the stove, the quantity of wood consumed is
considerably lower in comparison with the amount consumed in known
conventional firing arrangements.
of~
An embodiment of a stove for circulation/air shall be
` capable of being used for cooking food in -the event of a power
supply failure and to this end it is provided with a horizontal
:
part forming a hot plate or cooking and grilling stand. This
stand may be incorporated in part of the stove to form an oven.
The temperature of the oven can be controlled within wide
limits by means of simple devices arranged to operate in
combination with dampers and thermostats.
For use, for example, in cottages or like dwellings
not connected to mains supply, one embodiment of the stove
according to the invention can be provided with means for heat-
ing water, e.g. a volume of 50-100 liters. Overheating is
/~ prevented by thermostats arranged to operate in combination
with the aforedescribed damper-control system. The hot-water
preparing system also tends to protect against overheating
:
through its cooling effect on the smoke gases. Accumulation of
heat in accordance with the principle of a tile-oven can also
readily be obtained by filling the space between the heat
exchanger and the outer casing with a heat-accumulating material,
such as sand.
The important components of the stove, such as the
firebox, heat-exchanger and damper arrangements can be sold as
a unit for incorporation into, for example, an open brick fire-
place. In this connection a balanced draft can be arranged by
mounting in the flue tube of the fireplace an optionally circular
pipe, preferably of non-rusting material, whilst the combustion
air is guided down through the space around the pipe. Instead
of a conventional, externally bricked chimney, it is possible to
use the same chimney and a heat exchanger which is utilized in
a combination stove according to the invention, thereby avoiding
corrosion problem at low flue gas temperatures. It has been
12 ~ ~ r . ~ ~.
found when carrying out tests that good heat distribution can
be obtained even in the case of a large building, owing to the
fact that all heat is released at ceiling level. The heat can
be dispensed to adjacent rooms by providing sound-insulated
openings at ceiling and floor level. The combination stove can
be provided with auxiliary means for electrical heating, in
which case the upper part of the stove is provided with electrical
heating elements having protective thermostats. The circulation
fan and filter are preferably arranged in a lower part of the
/0 stove. In this way risk of fire in the filter is considerably
reduced, this risk being comparatively high in conventional
arrangements.
Some embodiments of the invention will now be described
in more detail with reference to the accompanying schematic
drawing.
Figure 1 is a partly cut away front view of the stove
- shown in Figure 1.
Figure 2 is a sectional side view of the stove shown
- in Figure 1.
o~ 0 Figure 3 is a graph showing the different heat require-
ments during the process of combustion of the fire, Figure 3a
sh,owing a small heat requirement and Figure 3b a large heat
requirement.
Figure 4 is a partly cut away schematic side view of a
modified embodiment of the stove used together with a heat
accumulator in the form of a bed of stones.
Figures 5 and 6 are sectional views taken on the line
5-5 and 6-6 in Figure 4.
- 13 -
A stove 1 for intermittently burning solid fuel
comprises a firebox 2. The upper part of the firebox 2 is the
interior of a heat exchanger 5 which merges with a chimney 6.
The firebox 2 together with the upper part 5 is surrounded by
said heat exchanger through which circulation air 7 is passed
by means of a fan 10 driven by a motor 10a and arranged beneath
the firebox. The upper portion of the flue tube located in the
chimney 6 is also encircled by a heat exchanger 12 for combustion
air, the inlet of which opens out approximately in the region of
the outlet of the chimney 6, to provide a so-called balanced
draft. The air-supply passage of the stove is referenced 13.
On the front side of the stove the firebox 2 is delimited
by a displaceable shutter 15 of hardened glass. As will be seen
from the drawing, the shutter 15 is relatively large and is meant
to retain the comforting effect of a conventional fire, since the
fire can be seen through said shutter and radiation heat felt
therethrough.
The shutter 15 is arranged to effectively seal the Eire-
box from the room in which the stove is placed, wherewith the
process of combustion in the stove is independent of variations
in pressure both within the building and outside the building,
for example due to wind or the opening of a window or the like.
The circulation air is drawn in by the fan 10 through
a grid 17 or a gap at the lower part of the stove and passes
through a filter 18 before heat exchange takes place as the air
passes the firebox through a passage l9. The circulating air is
then discharged through a grid 20 in the upper part of the stove,
this grid being located, in the illustrated embodiment, in the
- 14 -
~region of the ceiling of the room in which the stove is
located. The grid 20 forms an outer limiting member of a
module 21 surrounding the upper part of the stove, said
module may also carry the electrical devices for the respec-
tive functions of the stove, as will be described more in
detail hereinafter. Arranged between the module 21 and the
ceiling 22 is a protective plate 23 adapted to reduce the
risk of overheating.
Arranged in the air-supply passage 13 is a da~per 29 by
means of which the rate of combustion can be controlled.
This damper is arranged to be guided between either a fully
open position or a fully closed position by means of a
number of control means (not shown) such as thermostats and
time relays. With the damper in its closed position, the
supply of air to the fireplace 2 is cut off. Combustion
will then practically cease. The control means, however,
ensure, at the same time, that the fuel maintains a tempera-
' ture high enough for the combustion process to recommence
when the damper 29 is reopened. The control means also
ensure automatically that the desired predetermined tempera-
ture is maintained in the heated room or in the hot-plate 8
located above the firebox 2. In addition, it is automatic-
ally ensured that the power output of the stove and the
temperature of the heated circulation air is restricted to
the desired maximum value or to the maximum value prescribed
by the authorities.
During those periods in which the damper 29 is closed,
the temperature of the fuel is maintained by limiting the
heat output of the firebox. This is primarily effected by
shutting off the fan 10 for circulation air. Alternatively,
or in addition hereto, circulation of air around the firebox
can be restricted
~- - ~ .
- 15 - ~$~
by means of a damper (not shown) placed in a suitable position
in the circulation passage, e.g. adjacent the grid 20.
One or more thermostats 50, 51 controls the damper 29.
From these thermostats one 50 can be a security thermostat
controlling the flue gas temperature and adapted to generate
a mechanical movement or to control a mechanical movement by
electrical drive means when it senses a temperature in excess
of a predetermined value. The damper 29 or another similar
damper will be quickly closed, substantially completely, as a
result of said mechanical movement.
Another thermostat denoted 51 is a room thermostat
which will close the damper 29 when the room temperature takes
a predetermined value.
In addition the same mechanical or electrical drive
means or additional mechanical or electrical drive means (not
shown) can be arranged to open and close the damper 29 respectively
for periods of time of sufficient duration for the fire in the
firebox to generate a self-maintaining heat.
The combustion sequence of the fire with different heat
requirements is illustrated in Figure 3. Figure 3a illustrates
a small heat requirement and Figure 3b a large heat requirement,
the width of the vertical peaks showing the duration of those
periods during which combustion took place with full power. The
spaces between these peaks illustrate time periods when combustion
in the stove practically ceased.
The thermostat 50 in the firebox or the flue tube or
a further thermostat (not shown) is adapted to open the damper
29 automatically when the flue gas temperature falls beneath a
given value even if the room thermostat 51 will keep the damper
- 16 -
' `
closed. The fire will then be reignited momentarily only in
a manner illustrated by the narrow peaks in Figure 3a.
The reference 32 indicates a thermo-electrical generator
one side of which is contacted by the hot flue gases in the
firebox 5, while the cold side is cooled by combustion air
coming from without. The generator is arranged to cooperate
with one or more electrical thermostats in a manner such
that when the thermo electro-generator does not emit energy,
when the fireplace is cold, the damper 29 is held fully
open, whilst when the temperature of the firebox, and
therewith the output of the generator 32, has reached a
certain maximum value, the generator causes the damper to
be closed through setting means (not shown). The thermo-
electrical generator 32 may also produce electrical energy
only, in which case the thermostat 51 is controlled in the
way described above.
A further damper (not shown) by means of which a quantity
of air suitable for different types of fuel may be set manu-
ally can be arranged in the air-supply passage. Alternativ-
ely, the damper 29 may be adapted to fulfil this function.
An electric heating element 24 may be arranged in themodule 21, which encircles the upper part of the stove,
adjacent the grid 20. In this way it is possible also to
heat the circulation air electrically, i.e., the stove
becomes a combination
:
- 17 -
"':
stove using solid fuel and electrical energy.
The space 40 between the circulation-air passage 7 and
the outer casing of the stove may optionally be filled with a
- heavy material, such as sand, to enable heat to be accumulated
- for a short period of time.
Tlle hotplate 8 may be contained in a casing section la
associated with the stove and having a door lb, to form an oven
space whose bottom comprises the hotplate 8. ~niform temperatures
with regard to the hotplate and the oven,respectively,are
~O maintained by means of thermostats (not shown) arranged to control
the combustion process via the damper 29.
The fan 10 is arranged to co-operate with the filter 18
in a manner such that when the fan is working air is drawn in via
the filter, whilst when tlle fan is stationary the filter is
automaticajly moved to one side so that air can be supplied to
the circulation-air passage 19 without passing the filter.
Close to the module 21 there is a further damper 27
suitably a bimetal construction which, when heated combustion air
coming from without exceeds a certain magnitude, e.g. 50, is
~ ~ automatically opened so that room air having a temperature of
approximately 20 is drawn into the firebox instead of combustion
air. If the building is so sealed that a subpressure is obtained
thereby, the air drawn in from without is automatically replaced
by and heat exchanged with incoming, pre-heated outside air.
Figures 4-6 illustrate a modified embodiment in which
the stove 1 is used together with a heat accumulator which, in
the illustrated embodiment, has the form of a collection of stones
45. In this embodiment the air is caused, by means of suitable
- 18 -
damper arrangements~ 41, 42, 43, whilst charging the stones with
heat, to first pass the front side of the firebox 2 and thereafter
the upper part of said firebox as a result of the damper 42
being closed, along the rear side of the stove and to flow down
into the stone collection 45.
An advantage afforded by this embodiment is that the
heat exchange is more effective, because the exchange of heat
between the flue gases and the heated outside air takes place in
accordance with the counterflow principle, i.e. the airflow passes
the hottest part of the firebox immediately before its passage
to the accumulator 45. In this case any restrictions made by the
authorities on maximum temperature have no significance, since
the air is able to pass to the accumulator without coming into
contact with any part of the building.
Instead of the air - when the stove is used for direct
heating purposes - passing the firebox in parallel flows in
Airections downwardly and upwardly through the passages l9a and
l9b, as shown by the solid arrows in Figure 4, and thereafter out
into surrounding space at the upper side of the stove, the air
is forced, when the damper is adjusted for heat accumulation,
down on the rear side of the fireplace following a circulating
cause in which upward movement of the air takes place in the
passage l9a and downward movement in passage l9b. This course
taken by the air is indicated by hollow arrows in Figure 4. When
charging, the dampers 41a shown in Figure 5 are opened whilst the
dampers 41b are closed. As a result of this damper arrangement,
the stove 2 obtains two separate usages. Thus it can be used to
rapidly heat a space, e.g. a building which has been uninhabited
`
for some time, owing to the fact that all heat is immediately
supplied -to the building. Alternatively when the stove is fired,
the heat can, instead, be stored in an accumulator from which
heat is removed in a known manner through devices constructed
therefor. As will be understood, it is also possible for the
dampers to be adapted in a manner such that surrounding space is
- heated at the same time as heat is stored in the accumulator.
The electrical equipment of the stove may be positioned
close to the fan at the bottom of the stove instead of in the
O module 21.
Instead of the flue gas thermostat 50 there may be
another thermostat positioned outside the flue pipe and being
adapted to be activated by a temperature which depends on the
flue gas temperature.
The maximum temperature of the circulating heated air
will depend on local requirements. Typically the temperature of
the heated air must not exceed 200-210C. If the air takes this
temperature the damper 29 or another similar damper (not shown)
will immediately shut off the supply of combustion air to the
fireplace.
~.
