Patent Application
20220373184
The present application claims the benefit of priority of United Kingdom (Great Britain) Patent Application No. 2107141.0 filed on May 19, 2021, the entire disclosure of which is expressly incorporated by reference herein.
This present invention relates to apparatus for controlling an air inlet valve for a solid fuel burner.
Many people across the world rely on solid fuel for heating and cooking, particularly biomass such as wood, agricultural waste, charcoal and animal dung. The use of wood as a fuel is not restricted to developing countries. Many people in developed countries see wood burning as an ecological, carbon neutral option. If the wood is sourced sustainably the burning of wood for heating and cooking can approach carbon neutrality.
Unfortunately, the burning of wood has potentially harmful consequences in the form of the emission of harmful pollution in the form of small carbon particles. These are referred to as PM2.5 (particulate matter less than 2.5 microns in diameter). The emissions of PM2.5 from woodburning stoves in Europe is already limited and legislation due in 2022 will reduce the limits to 40 mg/m3.
The emission of PM2.5 particles can be reduced in a solid fuel burner by ensuring that the flue gas is above a specific temperature, allowing the particles to be fully combusted before they leave the burner. For example, the instructions issued with a wood-burning stove should provide guidance on how to adjust the damper or dampers on the stove to ensure that the correct temperature is maintained in the stove. Unfortunately, the users often do not understand the damper controls or appreciate the importance of maintaining a high temperature.
Automatically controlling the temperature of the flue gas can reduce the emission of PM2.5 particles, ensuring that the level of pollution is reduced and the stove meets the requirements of legislation both now and into the future.
Most existing solutions involve a mechanism for automatically closing an air inlet valve as a specified temperature is reached. The method of sensing the temperature can take a number of forms, each of which has some disadvantage.
The temperature can effectively be sensed using an electrical sensor such as a thermocouple or thermistor. A simple electronic circuit can sense when a specified temperature has been reached and cause a solenoid or motor to be energized to close the valve. One problem is that many stove installations do not have an electrical power supply and the user does not want the complication of batteries in what is perceived to be essentially a simple low technology appliance.
Some devices us the capillary thermostat principle. This uses a hollow metal bulb connected to a diaphragm with a small diameter tube. The bulb, tube and diaphragm are filled with a liquid or gas with a relatively high coefficient of thermal expansion. As the bulb is heated the fluid expands causing the diaphragm to move. This movement is used to close an air inlet valve. The problem with this system is that if the stove reaches excessively high temperatures the fluid expands to such an extent that the bulb, tube or diaphragm rupture.
A third type of sensor is a bimetal strip. This works on the principle of bonding two strips of metal with different coefficients of thermal expansion together. The resultant strip will bend when subjected to a change in temperature. The movement can be used to close an air inlet valve. The problem with thermal bimetals is that the maximum temperature they can withstand is 550° C. These high temperature bimetals are made using two different grades of stainless steel: an austenitic stainless steel with an expansion coefficient of typically 17.2×10−6/K and a ferritic stainless steel with an expansion coefficient of typically 10.5×10−6/K. At 550° C. the stress at the interface between the two types of stainless steel is sufficient to plastically deform the material. The result is the relationship between the temperature and the shape of the strip and hence the relationship between the temperature and the opening or closing of the valve will change if the stove reaches an excessively high temperature. One solution to this is to locate the bimetal outside the stove and conduct the heat to the bimetal using a metal with a high coefficient of thermal conduction, such as copper or brass. The effect of this is to increase the response time of the device and also to add an unknown variable since the temperature difference between the inside of the stove and the outside can be influenced by the ambient conditions surrounding the stove installation.
In some embodiments, apparatus for controlling the flow of air through an air inlet in a solid fuel burner comprises: a mechanical temperature sensor for sensing the temperature within the solid fuel burner, the sensor comprising first and second elongate parts having different coefficients of linear thermal expansion and arranged such that a first end of the first elongate part moves linearly in the elongate direction relative to a first end of the second elongate part in response to a change in the sensed temperature; a movable valve member for controlling the flow of air through the air inlet; and a mechanism for coupling the first end of the first elongate part to the movable valve member so as to close or restrict the air inlet as the sensed temperature increases.
At least some embodiments of the invention comprise a mechanical device which relies on the different thermal expansion coefficients of two materials. However instead of configuring the materials as a bimetallic strip, the difference in the change of length of two components made from the two materials is used to sense the temperature. For example, if a rod made from a low expansion material is mounted inside a tube of relatively high expansion material and one end of each is fixed firmly together, a change in temperature will result in a relative movement of the free end of the rod, in the direction of the length of the rod, with respect to the free end of the tube. If the temperature is increased the free end of the rod will move towards its fixed end. A decrease in temperature will result in a movement away from the fixed end.
An advantage of this arrangement is that the resultant movement is dependent on the average temperature change along the length of the two components. Another is that unlike in a bimetal driven system, the force available to be applied by the relative movement is very high and only limited by the buckling force of the rod or tube.
The assembly of rod and tube can be made of any suitable materials with differing coefficients of thermal expansion. An alloy typically used in this type of temperature sensor is an iron/nickel alloy commonly referred to as Invar. The most common composition for this alloy is 36% nickel, with iron making up the balance. This alloy has a coefficient of thermal expansion of virtually zero between −100° C. to 200° C. At 150° C. the coefficient is 2×10−6/K, at 250° C. the coefficient is 4×10−6/K, and at 400° C. the coefficient is 8×10−6/K.
The problem of the increased coefficient of Invar type alloys at high temperatures can be overcome by using a suitable ceramic material. One such material is cordierite, for which the coefficient of expansion is less than 2×10−6/K across a wide temperature range. Another suitable material is quartz, for which the coefficient of expansion is 5.5×10−7/K between 20° C. and 300° C. Another suitable material is borosilicate glass.
Brass or copper have very high coefficients of expansion, so are suitable for the material with a high coefficient. Copper has a coefficient of 17.7×10−6/K (average 20° C.-300° C.), and brass has a coefficient of 21×10−6/K(average 20° C.-300° C.)
However the combustion gases present in a woodburning stove can be corrosive to copper and brass. A protective coating can be applied to overcome these issues. One suitable coating is nickel which can be applied electrochemically or using an electroless nickel process.
Another suitable material with a high coefficient of expansion is stainless steel, especially the austenitic or face centered cubic types. A suitable grade of stainless steel is grade 321 (SS321) or 1.4341, which has titanium added making it corrosion resistant at high temperatures. SS321 has a coefficient of 17×10−6/K (average 20° C.-300° C.).
An assembly of a SS321 tube and a quartz rod within the tube provides a suitable differential expansion. An assembly with a 300 mm tube and rod would result in a relative motion of approximately 0.005 mm per degree Kelvin and a motion of approximately 2 mm for a temperature change of 400K. This degree of motion is not enough to open or close a valve, so a suitable mechanical system is required to amplify the motion. This can be achieved with a simple lever mechanism.
An alternative arrangement is to replace the tube with one or preferably a plurality of rods. The rods can be joined together at their fixed ends. This allows the material with the higher coefficient of expansion to be in the center of the assembly whilst still exposing it directly to the hot flue gasses. This system can be arranged so that the mechanism is not damaged if the stove reaches excessive temperatures. If the tube is made from the material with the greater coefficient of expansion the end of the rod will move away from the lever if an excessive temperature is reached. If the rod is made from the material with the higher coefficient a similar system can be employed by creating a head or step on the end of the rod.
The rod(s) or tube typically have a length in the range 200-500 mm, so as to give sufficient relative motion for the temperature range encountered in a solid fuel burner.
There now follows, by way of example only, a detailed description of embodiments of the present invention, with reference to the figures identified below.
In the following description, functionally similar parts are indicated using the same reference numerals. References to directions such as clockwise or anticlockwise are with reference to the figures as shown. Parts may be omitted in some of the figures, for example to show other parts more clearly.
The free end of the actuating rod 2 bears on the lever 5 which pivots about the pivot 4 on the bracket 3. When the tube 1 and actuating rod 2 cool, the length of the tube 1 decreases more than the length of the actuating rod 2, so that the length of the actuating rod 2 protruding out of the tube 1 increases. When the temperature increases, the length of the tube 1 increases more than length of the actuating rod 2, so that the length of the actuating rod 2 protruding out of the tube 1 decreases. The change in the length of the actuating rod 2 protruding out of the tube 1 allows the lever 5 to pivot. The lever 5 is biased against the end of the actuating rod 2, for example by means of a spring (not shown) and/or by gravity. A moving valve part 6 is mounted at the end of the lever 5.
The bracket 3 may be mounted on the outer surface of a side wall of a solid fuel burning stove, shown in dashed outline, with the tube 1 and actuating rod 2 projecting through an aperture in the side wall into the stove and the lever 5 extending vertically downwards from the bracket 3. The moving valve part 6 may be a flap which moves into contact with an air inlet on the side wall of the stove so as to block or restrict the air inlet when the valve assembly is in the closed state, as shown in
If the temperature increases excessively above the temperature at which the valve assembly moves to the closed state, the protrusion of the end of the actuating rod 2 past the end of the tube 1 continues to decrease, causing the actuating rod 2 to lose contact with or decouple from the lever 5, thereby preventing further movement of the moving valve part 6 and ensuring that no damage is sustained due to the excessive temperature.
To ensure that the lever 5 moves in the required direction in response to a change in temperature, the pivot 4 is located between the end of the lever 5 and the moving valve part 6 i.e. the lever 5 is a first order lever rather than the third order lever of the first embodiment.
Instead of tube 1 in the first embodiment, the bracket 3 is connected to first ends of a pair of fixed rods 1 of low coefficient of thermal expansion, with second ends of the fixed rods 1 being connected to a fixed end of the actuating rod 2 of high coefficient of thermal expansion, by means of a connector 10.
Alternatively, the fixed rods 1 could be omitted and the fixed end of the actuating rod 2 could be supported by a structural part within the interior of the burner, for example an opposite inner side wall. In another alternative, a pair of brackets 3 may be installed on opposite side walls of the burner, with the actuating rod passing through apertures in the opposite side walls and actuating corresponding levers on the opposite side walls. Hence, at least where the actuating rod has a high coefficient of linear thermal expansion, all that is needed is for the actuating rod 2 to be supported in some way so as to be able to actuate a mechanism for closing the air inlet when the temperature increases above a threshold.
The free end of the actuating rod 2 acts on the first lever 5a, arranged as a third order lever, causing it to rotate counter-clockwise around its pivot 4a as the temperature of the actuating rod 2 increases. The free end of the first lever 5a acts on a first end of the second lever 5b, arranged as a first order lever, causing the second lever 5b to rotate clockwise about its pivot 4b so as to move the moving valve part 6, attached to a second end of the second lever 5b, into its closed position. The free end of the first lever 5a and the first end of the second lever 5b are biased towards the bracket 3 by a spring 7.
The first lever 5a amplifies the movement of the free end of the actuating rod according to the ratio of the distances of the free end of first lever 5a, and that of the point of contact of the free end of the actuating rod 2, to the first pivot 4a. The second lever 5b further amplifies this movement by the ratio of the distances of the second end and the first end of the second lever 5b to the second pivot 4b, so that the total amplification is the multiple of these two ratios.
In the above embodiments, the moving valve part 6 is positioned some distance away from the temperature sensing parts, such as the fixed rods/tube 1, 2. This may be suitable where it is desirable to sense the temperature in an upper part of the stove or burner, for example just below a flue, but where the air inlet needs to be provided at a lower part of the stove or burner to allow combustion of particles. One such arrangement is shown in
Alternatively, in some stoves it may be desirable to place the air inlet part of the valve close to the temperature sensor 1,2. In this case the valve arrangement in a fifth embodiment as shown in
The free end of the actuating rod 2 actuates a pivoting lever 5 which acts to open and close a pair of moving valve parts 6 or flaps by means of a cam mechanism formed by a slot in the lever 5 and a pin connected to the moving valve members 6, which are biased into a closed position, for example by a spring.
In some embodiments of the invention, the moving valve member(s) 6 may be manually moved to an open or closed position, overriding the actuation by the temperature sensor. In particular, it may be desirable to allow the valve member(s) 6 to be manually moved to an open position, but not to a closed position. This may be achieved for example by a manually operable latch that latches the lever 5 and/or the valve member 6 in an open position.
Although the above embodiments have been described with reference to stoves such as wood or coal burning stoves, embodiments of the invention may also be applied to solid fuel burners of other types such as ovens and ranges.
In some embodiments, individual features as described above may be combined or omitted. On reading the above description, the skilled person may contemplate alternative embodiments which nevertheless fall within the scope of the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2107141.0 | May 2021 | GB | national |
