Pellet stove with basket adjustment

Abstract

A pellet stove includes a hopper for storing pellets, a basket for receiving the pellets by gravity, a duct for conveying combustion air to the basket, and a conduit for conducting heated air away from the basket. The position of the basket is adjustable, such as by raising and lowering it, or by rotating it, so as to control the number of pellets in the area of most intense combustion. One or more burn plates, positioned below the basket, provide platforms on which pellets falling through the basket can burn down to a smaller size. The pellet stove requires no external source of power. The stove so constructed enables quiet, efficient and clean burning of the pellets.

Claims

1. A pellet stove, comprising: a) a hopper for storing a plurality of pellets comprising fuel for the stove, b) a basket defining a plurality of openings through which pellets of a predetermined size can fall, c) a slide having an inclined surface, the slide being positioned to receive pellets, by gravity, from the hopper and to deflect said pellets into the basket, d) means for adjusting a position of the basket, the adjusting means comprising means for varying an amount of pellets allowed in the basket and for causing contents of the basket to be moved into, or out of, a region of the most intense heat, e) a secondary burn plate, the secondary burn plate being positioned to receive pellets which fall through the basket, the secondary burn plate comprising a plurality of spaced-apart members which allow pellets to fall through when the pellets have burned down to a predetermined size, f) means for conveying air to a vicinity of the basket, to support combustion of pellets in the basket, and g) means for conducting heated air away from the vicinity of the basket.

2. The pellet stove of claim 1, wherein the secondary burn plate includes an angled portion defining an inclined surface for deflecting pellets which fall through the basket.

3. The pellet stove of claim 2, further comprising an ash tray positioned to receive residue of pellets which fall through the secondary burn plate.

4. The pellet stove of claim 1, wherein the stove includes a support stand, and wherein the adjusting means comprises means for moving the basket towards or away from the support stand.

5. The pellet stove of claim 1, wherein the adjusting means comprises means for rotating the basket.

6. The pellet stove of claim 3, further comprising a tertiary burn plate, the tertiary burn plate being positioned to receive pellets which fall from the secondary burn plate, the tertiary burn plate comprising a plurality of spaced-apart members which allow pellets to fall through when the pellets have burned down to a predetermined size.

7. The pellet stove of claim 6, wherein the spaced-apart members of the tertiary burn plate are spaced more closely to each other than the spaced-apart members of the secondary burn plate.

8. A method of operating a pellet stove, comprising the steps of: a) allowing a plurality of pellets to fall, by gravity, from a hopper onto a basket, the basket having a position which is adjustable, b) igniting the pellets in the basket, and c) adjusting the position of the basket so as to control an amount of heat produced by the stove, wherein the adjusting step comprises varying an amount of pellets allowed in the basket and causing contents of the basket to be moved into, or out of, a region of the most intense heat.

9. The method of claim 8, wherein the adjusting step comprises moving the basket towards or away from the hopper.

10. The method of claim 8, wherein the adjusting step comprises rotating the basket.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of one embodiment of the pellet stove of the present invention.

(2) FIG. 2 is a cross-sectional side view of the pellet stove of FIG. 1, taken along the lines 2-2 of FIG. 1.

(3) FIG. 3 is a perspective view of the essential components of the burn chamber, according to the embodiment of FIGS. 1 and 2.

(4) FIG. 3A provides a detailed end view of a portion of the structure of FIG. 3, showing the differences in bar spacing between the secondary burn plate and the tertiary burn plate.

(5) FIG. 4 provides an exploded perspective view of an alternative embodiment of the present invention.

(6) FIGS. 5 and 6 provide cross-sectional views of the burn tube of the embodiment of FIG. 4, showing the movement of the burn basket.

(7) FIG. 7 provides a perspective view of the embodiment of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

(8) In one aspect, the present invention comprises a gravity-fed pellet stove which comprises a primary burn basket. The position of the primary burn basket is adjustable, so as to vary the quantity of fuel pellets located within the primary burn basket, which are exposed to an air flow through the burn chamber.

(9) In another aspect, the present invention comprises a gravity-fed pellet stove which comprises a primary burn basket, and a secondary burn plate disposed beneath the primary burn basket. The spacing between the primary burn basket and the secondary burn plate may be variable or fixed.

(10) In another aspect, the present invention comprises a gravity-fed pellet stove which comprises a primary burn basket, and a secondary burn plate disposed beneath the primary burn basket. The secondary burn plate may comprise a plurality of parallel bars. The spacing of the bars may be adjustable, for example by replacing a first secondary burn plate having a particular bar spacing with a second secondary burn plate having a different bar spacing.

(11) In another aspect, the present invention comprises a gravity-fed pellet stove which comprises a primary burn basket, a secondary burn plate beneath the primary burn basket, and a tertiary burn plate beneath the secondary burn plate.

(12) The above description sets forth, rather broadly, a summary of one embodiment of the present invention so that the detailed description that follows may be better understood and contributions of the present invention to the art may be better appreciated. Some of the embodiments of the present invention may not include all of the features or characteristics listed in the above summary. There are, of course, additional features of the invention that will be described below and which will form the subject matter of claims. In this respect, before explaining at least one preferred embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of the construction and to the arrangement of the components set forth in the following description or as illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

(13) In FIG. 1, there is shown a natural draft pellet stove in accordance with one embodiment of the present invention. The stove uses a unique configurable burn chamber having an adjustable primary burn basket, an adjustable secondary burn plate, and an adjustable tertiary burn plate. This configurable burn chamber is used in conjunction with a gravity-fed and natural draft system as applied to the operation of a pellet stove for use in home, office and general space heating and related utility use.

(14) The stove of FIG. 1 includes a pellet fuel hopper 1 which stores a supply of burnable pellets. The hopper 1 is shaped to direct pellets, by gravity, to a feed tube 8 which houses a primary burn basket 7, the basket being visible in FIGS. 2 and 3. Control of the flow of pellets from the hopper 1 to the feed tube 8 is provided by a pellet fuel shutoff gate 6. At the base of the feed tube 8 is a burn chamber 9 which can be accessed via a burn chamber door 10. A smaller porthole and porthole door 11 is provided in the burn chamber door 10.

(15) The burn chamber 9 connects with an air duct system 12 which includes an inlet below the burn chamber that connects to the burn chamber 9. Extending from the burn chamber 9 is a heat tube 5 that extends upwardly and parallel to the side of the hopper 1, from the burn chamber 9, through a number of angled sections, to a chimney connector 3. The chimney connector can be connected to any suitable flue or chimney arrangement for venting the burn chamber exhaust to an exhaust area, such as an area outside of the premises in which the stove is installed. The chimney arrangement is not considered a component of the stove, being largely dependent on the particular premises, and thus no further description of the chimney arrangement is considered necessary herein.

(16) A heat shield 2 protects the hopper 1 from the heat radiating from the heat tube 5. One or more air deflectors 4 may be provided above the sections of the heat tube 5 to deflect heated air from around the heat tube towards a heated space of the premises.

(17) The stove is supported by a support stand 13.

(18) FIG. 2 shows an internal view of some of the components of the stove, in particular the components of the primary burn basket 7, a secondary burn plate 14 below the primary burn basket, a tertiary burn plate 15 below the secondary burn plate 14 and an ash pan or tray 16 below the tertiary burn plate 15. These components are shown in more detail in FIG. 3.

(19) Prior art stoves using gravity feed and natural draft use a fixed primary burn basket. This fixed primary burn basket contains the same amount of pellet fuel on a high heat burn as it does on a low heat burn. These stoves try to control the amount of heat the stove is generating by slowing the natural draft, causing less air to pass over the primary burn basket. This method slows the burn and reduces the heat output, but also allows the pellet fuel to burn at a lower temperature, creating a burn that is not as clean as a higher-temperature burn using less pellet fuel.

(20) The primary burn basket 7, of the embodiment of the present invention represented in FIGS. 1-3, extends upwardly into the feed tube 8 and is adjustable. It can be lowered, exposing more pellets to the flame and air flow for a high temperature burn, or it can be raised, to allow fewer pellets to be exposed to the flame and air flow for a low temperature burn. The height of the primary burn basket may be set by turning handle 17, in the direction of the arrow at the top of FIG. 3, which in turn rotates a threaded stem 18 on which the primary burn basket 7 is supported. The threaded stem may engage a thread in a lid or cover of the feed tube 8. Alternatively, a lockable sliding stem may set the height of the primary burn basket.

(21) How much heat is generated by the stove is controlled by how many pellets are exposed to the flame and air flow. Using fuel burn rate instead of restricting air flow to the burn area, to set the temperature of the stove, is a more stable and repeatable way of controlling the heat range while maintaining fuel efficiency and reducing emissions. With the presently described arrangement, the pellets receive all the air they can consume, thus maximizing the efficiency of the stove.

(22) The ability to adjust the amount of pellets exposed to the burn also allows the spacing of the bars defining the primary burn basket 7 to be reduced, thereby keeping the pellets in the primary burn basket 7 longer, so they complete more of their total burn while they are still in the basket 7. If more heat is needed, more pellets can be exposed to the flame and air flow by lowering the primary burn basket 7.

(23) Keeping the pellets in the primary burn basket 7 longer, where they complete more of their total burn, allows the pellets to be smaller when they drop to the secondary burn plate 14. Being farther along in their total burn process when they hit the secondary burn plate 14 means that the pellets spend less time on the secondary burn plate 14 before dropping down to the tertiary burn plate 15, thereby reducing pellet buildup on the secondary 14 and tertiary 15 burn plates.

(24) The secondary 14 and tertiary 15 burn plates are located directly below the primary burn basket 7. In the embodiment shown in FIGS. 1-3, the secondary 14 and tertiary 15 burn plates are constructed of round rods arranged parallel to each other. Both burn plates resemble an oven rack used in a kitchen oven. The parallel rods are spaced a specific distance apart. The burn plates could be constructed of other kinds of members, and are not limited to rods.

(25) The roundness of the rods makes it more difficult for the burning pellets to sit in a specific location without falling through the spaces between the rods to the next level as they burn down and become smaller. The length of time the burning pellets sit on the secondary 14 and tertiary 15 burn plates is determined by the spacing of the rods defining these burn plates. The wider the spacing between rods, the less time the burning pellets stay on the secondary 14 and tertiary 15 burn plates before burning to a size small enough to fall through to the next level. The smaller the spacing between the rods, the more time the burning pellets stay on the secondary 14 and tertiary 15 burn plates before burning to a size small enough to fall through to the next level.

(26) After burning to a size small enough to allow the pellets to pass through the secondary burn plate 14, the burning pellets drop through to a tertiary burn plate 15 located directly below the secondary burn plate 14.

(27) The tertiary burn plate 15 is very similar to the secondary burn plate 14, as it is made of round rods which are generally parallel to each other. The spacing of the tertiary burn plate 15 rods is smaller than the spacing on the secondary burn plate 14 rods so the pellets have additional time to complete the burn process before dropping into the ash pan 16. This difference in spacing is illustrated by the detailed view of FIG. 3A, which shows that the spaces between the bars of the tertiary burn plate 15 are smaller than the spaces between the bars of the secondary burn plate 14.

(28) Using secondary 14 and tertiary 15 burn plates allows time for the burning pellets to complete their burn before dropping into the ash pan 16. Spending time on each burn plate keeps the secondary burn plate 14 from backing up with burning pellets which reduces the draft and/or plugs up the stove causing the pellets to smoke, smolder, and become extinguished.

(29) The secondary 14 and tertiary 15 burn plates are located close to the primary burn basket 7, allowing the secondary 14 and tertiary 15 burn plates to be exposed to the higher heat coming from the primary burn basket 7. Being close to the primary burn basket 7 also allows the burning pellets on the secondary 14 and tertiary 15 burn plates to be exposed to more air movement caused by the natural draft. Both a hotter location and more air movement help the burning pellets on the secondary 14 and tertiary 15 burn plate to burn faster, more completely, and with fewer emissions.

(30) In one embodiment, the secondary 14 and tertiary 15 burn plates can be set in adjustable locations either closer or farther from the primary burn basket 7. For example, the secondary and tertiary burn plates may be slidable in and out of a support rack with multiple racks at various heights. The secondary 14 and tertiary burn plates may be accessed and removed through the burn chamber door 10. Having secondary 14 and tertiary 15 burn plates that are removable by sliding in and out allows for easy servicing, as well as for exchanging the secondary 14 and tertiary 15 burn plates with an alternate set of secondary 14 and tertiary 15 burn plates with a different spacing between the rods. Secondary 14 and tertiary 15 burn plates having different rod spacing can be substituted as needed if hardwood pellets are used or if a lesser than premium grade of pellets are used, and the time the burning pellets need to spend on the secondary 14 and tertiary 15 burn plates changes.

(31) In one embodiment, and as represented in FIGS. 1-3, one or both of the secondary 14 and tertiary 15 burn plates may be mechanically attached to the primary burn basket 7, such that the burn plate(s) move upwardly and downwardly with the primary burn basket 7. FIGS. 2 and 3 show that the secondary and tertiary burn plates are attached to the basket by rods 19. Mechanically tying the secondary 14 and tertiary 15 burn plates to the primary burn basket 7 keeps the distance between the primary burn basket 7 and the secondary 14 and tertiary 15 burn plates the same during hot temperature burns and low temperature burns as the primary burn basket 7 is raised up and down. The secondary 14 and tertiary 15 burn plates may be attached to the primary burn basket 7 via racks with multiple levels that allow the spacings between the primary burn basket 7 and secondary 14 and tertiary 15 burn plates to be adjusted.

(32) The stove uses solid fuel pellets for fuel, a gravity-fed pellet delivery system to move the pellet fuel from the storage area in the hopper through the feed tube and into the primary burn basket, and a natural draft created by the energy of the burning pellet fuel to move air past the burning pellets. No electronics, combustion fans, convection fans, or auger motors are required.

(33) To light the stove, the adjusting handle 17 for the primary burn basket 7 is turned until the basket has been moved down to the high heat position (i.e. with most pellets exposed) with the pellet fuel shut off gate 6 in the closed position. A heat gun or propane torch (not shown) may be inserted through the porthole 11 to preheat the exhaust tube for several minutes.

(34) After several minutes of preheating the exhaust/heat tube 5, the pellet fuel shut off gate 6 is opened, causing the pellet fuel to fall, by gravity, into the feed tube and fill the primary burn basket 7.

(35) One continues to apply heat with a heat gun or propane torch until the pellet fuel ignites and the natural draft is established. When the pellet fuel ignites and the natural draft is established, it may be possible to hear the sound of the heated air moving up the exhaust/heat tube 5.

(36) After the pellet fuel ignites and the natural draft is established, one continues to heat the pellet fuel in the primary burn basket 7 with the heat gun or propane torch for approximately 10 seconds, and then removes the heat gun or propane torch. After a few minutes, one closes the porthole door 11.

(37) The pellet fuel will continue to burn in the primary burn basket 7 until the pellet size burns down to a diameter small enough to allow the burning pellets to drop through the spacing between the primary burn basket bars 7. The smaller burning pellets drop through the spaces between the primary burn basket bars down to the secondary burn plate 14 where the pellets continue to burn.

(38) The pellets will continue to burn on the secondary burn plate 14 until they burn down to a size small enough to allow them to drop though the spaces between the bars of the secondary burn plate 14. The smaller burning pellets drop through the spaces between the bars of the secondary burn plate 14, to the tertiary burn plate 15, where the pellets continue to burn. When the burning pellets reach the tertiary burn plate 15, they complete their burn, with the ash falling into the ash pan 16.

(39) After lighting the stove, one continues to run the stove on high heat for a minimum of 30 minutes to burn out any creosote and to ensure the burning pellets in the primary burn basket 7 have stabilized into a complete burn.

(40) After operating the stove at high heat for 30 minutes, the heat output may be adjusted to the desired heat range by moving the adjusting handle 17 for the primary burn basket until the basket is in the medium high, medium, medium low, or low position.

(41) Moving the adjusting handle 17 such that the primary burn basket moves up reduces the number of pellets exposed to the burn in the primary burn basket 7, decreasing the heat output of the stove. Moving the adjusting handle such that the primary burn basket moves down increases the number of pellets exposed to the burn in the primary burn basket 7, thereby increasing the heat output of the stove.

(42) In the embodiment of FIGS. 1-3, the secondary 14 and tertiary 15 burn plates are mechanically attached to the primary burn basket 7. When the primary burn basket 7 is adjusted up or down, the secondary 14 and tertiary 15 burn plates move with the primary burn basket 7, keeping the distance constant between the primary burn basket 7, the secondary burn plate 14, and the tertiary burn plate 15.

(43) The secondary 14 and tertiary 15 burn plates are removable for servicing and replacement. The secondary 14 and tertiary 15 burn plates supplied with the stove are set with spacing between the bars on the secondary 14 and tertiary 15 burn plates for premium grade softwood pellets.

(44) If hardwood pellets or less than premium grade pellets are used, alternative secondary 14 and tertiary 15 burn plates with different spacing may be required.

(45) The stove is shut off by closing the pellet fuel shutoff gate 6. When the remaining pellets located in the feed tube 8 burn up, the stove will shut down.

(46) The pellet fuel only burns in the primary burn basket 7, the secondary burn plate 14, and the tertiary burn plate 15. No pellet fuel is burned in the feed tube 8 due to the pellet fuel being packed in the feed tube 8 and not allowing sufficient oxygen in to support the burning of the pellets.

(47) As the hot exhaust gases move up the exhaust/heat tube 5 and out the chimney connector 3, heat is transferred from the gases to the metal exhaust/heat tube 5. The convection heating process then moves the heat out into the room. The heat shield 2 and air deflectors 4 help to direct the hot air coming from the exhaust/heat tube 5 out into the room and away from the stove.

(48) As the pellet fuel burns in the main burn chamber 9, a natural draft is created moving air from outside the stove across the primary burn basket 7, secondary burn plate 14, tertiary burn plate 15, and out and up the exhaust/heat tube 5. Air flow to support the natural draft is supplied through the air duct system 12. Air duct ports are located at the bottom back and upper front sides of the burn chamber 9. The air duct system can be configured to receive air from inside the building or the air duct system can be attached to an outside air supply based on the local building codes established in the location where the stove is installed.

(49) The pellet stove as described herein provides a highly adjustable and configurable system that can take into account variations in fuel grade, material as well environmental factors. One particular factor is the primary burn basket may be adjusted up or down to compensate for burn conditions at different altitudes as well as adjusting the amount of heat generated by the stove. Another particular factor is the secondary and tertiary burn plate spacing can be changed to compensate for fuel grade and material ensuring that a strong secondary burn can occur without clogging the air flow.

(50) The tertiary burn plate should be considered optional, as it is also feasible to provide a pellet stove having only an adjustable basket and a secondary burn plate.

(51) As noted above, the pellet stove of the present invention does not use electricity, so the stove will continue to operate in conditions of power outages. There are no electronic components, or fans or motors, to be serviced or replaced. The operation of the stove is therefore very quiet.

(52) The stove of the present invention works similarly to a wood-burning stove, but the burn is much cleaner, so the stove of the present invention is more likely to comply with governmental restrictions on the use of wood stoves.

(53) FIGS. 4-7 illustrate another preferred embodiment of the present invention. The perspective view of FIG. 7 shows this embodiment from the outside, and FIGS. 4-6 better illustrate the internal components.

(54) As shown in FIG. 7, the pellet stove includes hopper 20, which is connected to transfer tube 21 having shutoff gate 22. The shutoff gate opens and closes a path, within the transfer tube, to the flow of pellets from the hopper. The transfer tube 21 is connected to burn chamber 23. The internal components of the burn chamber are shown in FIGS. 4-6 and will be described later.

(55) The burn chamber includes door 24 which can be opened and closed by moving handle 25. The stove is supported by stand 26.

(56) The pellet stove includes exhaust or heat tube 27, and air deflectors 28 positioned along segments of the heat tube. The heat tube terminates at chimney connector 29, which connects to a chimney (not shown) or other means for disposing of exhaust gas. Support leg 30 for the burn chamber includes an air inlet 31, similar to that of the first embodiment. Basket adjuster 32 controls the position of the basket, in a manner to be described later.

(57) In the embodiment of FIGS. 4-7, the top section 33 of the heat tube is horizontal, i.e. parallel to the floor, and not at an angle, as was the case in the first embodiment. This feature allows the operator to use this flat area to heat coffee, water, and food. A heat shield can be provided along the top section 33, and such heat shield could be hinged, allowing it to be in place, close to the top section 33, during normal use, and to be swung upward and out of the way when the top section is being used for heating food and the like.

(58) The exploded view of FIG. 4 shows the internal components of the burn chamber. All of the internal components are contained within burn chamber 23, which includes door 24, having handle 25, and basket adjuster 32. These components include burn tube 35, basket 36, slide 37, secondary burn plate 38, tertiary burn plate 39, and ash tray 40. When the components are installed, the basket 36, the slide 37, and the secondary and tertiary burn plates 38 and 39 are positioned within the burn tube 35, and the ash tray 40 sits under the burn tube but inside the burn chamber 23.

(59) The burn tube 35 includes fitting 41 for connection of the burn tube 35 to the transfer tube 21, to permit entry of the pellets into the burn tube.

(60) It can be seen, from FIG. 4, that when the components are assembled, the basket 36 does not sit directly below fitting 41, but instead will be laterally offset (to the left, in FIG. 4). Pellets falling through fitting 41 do not directly reach the basket, but instead hit slide 37, which comprises an angled surface 42 for deflecting the pellets. Pellets hitting the angled surface 42 are deflected into basket 36.

(61) When the pellets burn down so that they can fall through the bars defining the basket, the pellets fall onto the secondary burn plate 38. The secondary burn plate is similar to that of the first embodiment, except that secondary burn plate has an angled portion 43 which causes pellets falling onto the secondary burn plate to be deflected, and to be grouped together in one general location on the secondary burn plate.

(62) Pellets which then burn down to a smaller size may fall through the bars defining the secondary burn plate, onto the tertiary burn plate. The bars of the tertiary burn plate may have a spacing which is smaller than that of the secondary burn plate, as is true for the first embodiment. The pellets falling through the bars of the tertiary burn plate are collected in ash tray 40. Thus, the ash tray receives the residue of pellets, after they have burned down to a predetermined size.

(63) The secondary and tertiary burn plates are held within burn tube 35, by shelves 44 and 45. The burn tube also includes openings, such as openings 46, which comprise ducts for combustion air. The exact shape of the ducts can be varied considerably; what is desired is that air be conducted from the outside, into the interior of the burn tube, to support the combustion therein.

(64) As in the first embodiment, the tertiary burn plate is optional. If the tertiary burn plate is omitted, then pellets drop from the secondary burn plate directly into the ash tray.

(65) FIGS. 5 and 6 illustrate the control of the position of the basket in the second embodiment. Basket 36 is attached to control arm 47. The control arm is engaged by threaded portion 48 connected to basket adjuster 32. The adjuster 32 is turned as indicated by arrow 49 in FIG. 6. Turning the adjuster 32 thereby causes the basket to rotate, in either of the directions indicated by arrow 50. The positions shown in FIGS. 5 and 6 show the extreme positions of the basket. In the position shown in FIG. 5, the basket has been rotated such that it has been moved generally farther from the floor. When the basket is raised up in this position, fewer pellets are allowed in the basket for a lower temperature burn. In the position shown in FIG. 6, the contents of the basket are closer to the floor. When the basket is lowered in this position, more pellets are allowed in the basket for a higher temperature burn. Thus, the movements of the basket, shown in FIGS. 5 and 6, change the amount of pellets allowed in the basket and cause the contents of the basket to be moved into, or out of, the region of the most intense heat.

(66) Note also that FIGS. 5 and 6 further illustrate the offset position of the basket 36 relative to the fitting 41. It can be seen most clearly, in FIGS. 5 and 6, that pellets falling through fitting 41 will generally impact angled surface 42 of the slide before falling into the basket.

(67) In the embodiment of FIGS. 1-3, the basket is rigidly attached to the secondary and tertiary burn plates. In the embodiment of FIGS. 4-7, there is no comparable attachment, as the basket moves without affecting the position of the burn plates. Indeed, in the embodiment of FIGS. 4-7, the burn plates do not move at all. It is also possible to reconfigure the embodiment of FIGS. 1-3 to remove the connection between the burn plate(s) and the basket, so that the basket can move without moving the plate(s). The latter alternative should also be deemed within the scope of the present invention.

(68) The spacing between the bars of the burn plates can be changed by replacing one or more burn plates with another component having a different spacing. In the embodiment of FIGS. 4-7, such adjustment is made easier by the fact that the burn plates slide into, or away from, shelves formed in the burn tube.

(69) The present invention should not be deemed limited to the specific arrangements described above. The invention may be modified in various ways, as will be apparent to those skilled in the art. Such modifications should be considered to be within the spirit and scope of the following claims.