Fuel burning stove

A stove to burn solid fuels, said stove comprising a combustion chamber with a front (13) with a fuel charging aperture (14) therein and a door (3) with a transparent viewing panel (33) therein mounted over the aperture (14), a baffle (15) dividing the interior of the combustion chamber into a primary combustion zone (A) and a secondary combustion zone (B), a closeable port (16) in the baffle (15) connecting the primary and secondary combustion zones, a combustion gas discharge port (17) to atmosphere from the secondary combustion zone, a gas screen (24) in the gas path between the primary and secondary combustion zones, the gas screen being constructed from materials that will glow at a temperature below the ignition temperature of combustion gases, primary and secondary combustion air inlets (22 and 30) discharging into the combustion zone adjacent a top edge of the door panel (33) and the gas screen (24) respectively, manually resettable temperature actuated flow control means (34) regulating the amount of primary combustion air entering the combustion chamber.

BACKGROUND OF THE INVENTION 
This invention relates to solid fuel burning stove and in particular to 
stoves of the above type that burn solid fuels which develop smoke with a 
high soot content as a result of incomplete combustion. An example of the 
foregoing is a wood burning stove. 
Stoves of the above type have as desirable features complete combustion of 
fuel even at low temperatures, after-burning of the primary combustion 
gases and means to provide a soot free transparent viewing window (if a 
window is fitted) so that the owners of the stove can watch the fuel 
burning process. Attempts have been made to provide stoves having all of 
the above features, however in the area of maintaining substantially 
complete combustion for a range of combustion air intakes difficulties 
have arisen, largely because of the controls that have been used for the 
combustion cycle. 
In general, stoves for burning solid fuels comprise a combustion chamber 
with a draft generating flue provided with a damper to regulate the 
drawing ability of the flue and an air inlet with a flow control operable 
between a fully open condition (designed to provide sufficient combustion 
air to allow the stove to perform up to the maximum designed level) and a 
fully closed condition. In most stoves reliance is put upon leakage of air 
into the combustion chamber to maintain a slow combustion rate when the 
air inlet is closed off. This makes the operation of the stove easy for 
the user because if slow combustion is required then it is simply a matter 
of closing off the air inlet completely and relying on the air leakage 
into the combustion chamber to sustain combustion. The problem is that the 
amount of air leaked into the combustion chamber depends on many factors 
and can vary substantially from stove to stove of the same make and 
therefore efficient combustion in the slow combustion mode of operation 
cannot be guaranteed. If incomplete combustion occurs where wood is the 
fuel smoke and creosote can be generated. If this continues there will be 
a build up of soot in the flue from the combustion chamber and on the 
inner surface of the combustion chamber, including the viewing window if 
one is fitted. It is known that if a stream of air is directed across the 
inner face of a viewing window of a combustion chamber the window will be 
maintained substantially clear of soot irrespective of the rate of 
combustion occuring in the combustion chamber. 
One aspect of the present invention is the provision of a permanent supply 
of combustion air to the combustion chamber which is sufficient to 
maintain substantially complete, but slow, combustion when the combustion 
air control means is in the appropriate position. The invention also 
provides an air supply which is directed so as to inhibit the deposition 
of any soot which results from the combustion process on the inner surface 
of the viewing panel provided in the stove. A further feature of the 
invention is a catalytic means to ensure ignition of the products of 
primary combustion in a secondary combustion zone of the combustion 
chamber. 
SUMMARY 
Broadly stated the invention can be said to comprise a stove to burn solid 
fuel, said stove having a combustion chamber defined by a casing including 
at least a top, a back, two sides and a front, a door including a 
transparent viewing panel, said door is hingedly connected to the casing 
to cover a fuel charging aperture in the casing front, a baffle within the 
combustion zone, said baffle extends from side to side of the casing and 
forwardly from the back of the casing and terminates in a leading edge 
adjacent the casing front so as to provide a lower primary combustion zone 
and an upper but smaller secondary combustion zone, a combustion gas 
discharge opening through the casing connecting the secondary combustion 
chamber to atmosphere, a closable port in the baffle adjacent the back of 
the casing and connecting the primary and secondary combustion zones, a 
combustion gas screen at the leading end of the baffle separating the 
primary and secondary combustion chambers, a secondary combustion air 
intake passage open to the atmosphere and having a discharge end adjacent 
the leading end of the baffle, primary combustion air intake passage means 
open to atmosphere and having a discharge to direct primary combustion air 
over the inner surface of the viewing panel, control means including a 
manual primary adjustment and a temperature controlled secondary 
adjustment to regulate the quantity of primary combustion entering said 
passage means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The stove comprises an outer decorative casing 1 including louvres 2. The 
casing comprises sides, a top and a back and a partial front. The casing 
is mounted on and around a firebox which stands on a plinth 8. There is a 
door 3 mounted on the firebox so as to be accessible through the partial 
front of the casing 1. An ash spill tray 4 is located below the door 3. 
The casing top includes a lift-up door 5 to permit cooking by use of 
cookware placed on top of the firebox. There are louvres 6 in the top of 
the casing to facilitate convection circulation of air over the firebox 
and there is a flue 7 extending through the casing top whereby combustion 
gases are discharged from the firebox. 
The firebox comprises a back 9, a top 10, two sides 11, a bottom or hearth 
12 and a front 13 having a fuel charging aperture 14 over which the door 3 
is hingedly mounted. Fixed within the firebox there is a baffle plate 15 
which extends from side to side of the firebox and extends forwardly from 
the back of the firebox terminating in a front edge spaced from the front 
of the firebox. The baffle plate 15 divides the firebox interior into a 
lower combustion zone A and a smaller upper combustion zone B. Preferably 
the baffle is inclined upwardly forwardly as shown in the drawings but 
this is not an essential. 
There is a hole 16 through the baffle plate 15 interconnecting the upper 
and lower combustion zones, the hole 16 is aligned with hole 17 through 
the top of the firebox with an associated flue coupling collar and the 
holes 16 and 17 are approximately the same size. The hole 17 can be 
located in the back 9 of the firebox if a rear discharge for combustion 
gases is required. In such a case the flue would not exit through the top 
of the casing and the louvred top of the casing would be suitably 
modified. 
There is a slidable cover 18 adapted to be placed over the hole 16 by 
operation of the handle/rod assembly 19 which exits from the front of the 
casing. The cover is fixed on a shaft 20 having ends mounted in bearing 
holes in the firebox top and the baffle plate 15 and the rod 19 is coupled 
to an arm 21 fixed to the shaft 20 above the top 10 of the firebox so that 
the cover movement is arcuate about the shaft 20. Located below the baffle 
plate 15 there is a U shaped secondary air inlet pipe 22 which takes air 
from the rear of the firebox, although the position at which the pipe 22 
exits from the firebox is optional. The base of the U is adjacent the 
front edge of the baffle plate 15 and is provided with discharge ports 23 
which are forwardly directed. 
A grid 24 is located in the mouth to the upper combustion zone, i.e. 
between the front edge of the baffle and the top of the firebox. The grid 
plays an important part in the efficient working of the stove. The grid 
comprises two plates 25 with a layer of fibrous material 26 sandwiched 
therebetween and there are holes 27 through the sandwiched assembly to 
provide the grid effect. The plates 25 are made of heat resistant 
non-corroding light gauge metal, for example stainless steel of 1.2 mm 
thick. The fibrous material is preferably that known by the trade name 
Kaowool and is made from refractory fibre compositions, as used it is in 
the form of a board. The function of the grid will be described later. 
The aperture 14 in the front of the firebox is rectangular and on the two 
upright sides and the top it is respectively bordered by the legs 28 and 
the base 29 of a U shaped tubular member 30. The open ends 31 of the legs 
28 extend through the bottom of the firebox so as to draw primary 
combustion air from outside the stove. The base 29 has slot openings 32 
therein which are adapted to direct the primary combustion air entering 
the firebox through the member 30 downwardly over the inner face of the 
glass panel 33 in the door 3. 
The flow of primary combustion air into the open ends 31 of the legs 28 has 
a predetermined minimum and maximum and is controlled by the position of 
flap members 34 fixed to a common shaft 38 pivotally mounted in bearing 
means on the bottom of the firebox with the flaps 34 aligned with the ends 
31 of the legs 28. To provide a minimum flow there is a stop screw 35 of 
adjustable length on each flap 34 to engage against the bottom of the 
firebox. 
The position of the flaps is determined by manual and temperature 
controlled means. The temperature controlled means comprises a bi-metal 
spring coil 36 with a free end connected through link means 37 (preferably 
a chain) to a lever arm 39 on the shaft 38. The inner end of the coil 36 
is mounted on a plate which can be moved about its connection to the 
firebox so that the coil 36 can be rotationally positioned in accordance 
with required burning characteristics for the fire. In a first position 
the plate is positioned so that a plate moving handle 40 points to the 
letter S to bring to stop screws 35 into close proximity to the bottom of 
the firebox. The combustion will now be slow because of the small quantity 
of air available. There will be heat generated which will cause the coil 
to expand the bring the screws 35 against the firebox bottom thereby 
limiting the amount to which the combustion air can be restricted. As the 
combustion slows due to the reduction of available air the coil will 
contract and the flaps will open again to the limit allowed by the 
contracted coil. Thus there will be a repeated opening and closing of the 
flaps over a low temperature range. 
At the other end of the operating scale the handle is set to F where the 
flaps are fully open allowing a superabundance of combustion air to enter 
the firebox. As the fire heat increases the coil will expand closing the 
flaps to a degree dependent upon the deflection of the coil. When the fire 
heat decreases as a result of partial flap closure the coil will contract 
again thereby opening the flaps. Thus there will be repeated opening and 
closing of the flaps for a predetermined high temperature range. 
When the handle is set to I the flap opening and closing cycle will also 
occur but for an intermediate temperature 
range. The burning cycle includes primary combustion and secondary 
combustion. Once the fire has achieved a required operating temperature 
the damper 18 is closed over and the products of primary combustion are 
forced to pass through the grid holes 27 in order to reach the flue. The 
temperature within the firebox is such as to cause the edges of the holes 
27 to glow and the non-combustible fibres of the Kaowool around the holes 
27 to glow. This source of ignition in the presence of combustible gases 
(from the primary combustion phase) in an atmosphere rich in combustion 
air (from the pipe 22) provides the ideal conditions for secondary 
combustion into the secondary combustion zone. The result is that the 
gases exhausted from the secondary combustion zone are substantially 
devoid of combustible material thereby ensuring the maximum heat energy 
extraction from the fuel consumed on the stove. 
If desired the case around the firebox can be fitted with heat radiant 
surfaces or members such as ceramic tiles.