High turn-down modulating burner

A high turn-down burner adapted to receive a fuel flow for combustion. The burner includes a housing having a side wall with an interior surface forming an inner periphery, a bottom wall adjoining the side wall, a top wall adjoining the side wall and a plurality of apertures disposed on the side wall; a supply tube adapted through the top wall of the housing, the supply tube including a side wall having an outer surface forming an outer periphery, a top end, a bottom end, wherein the supply tube is adapted to receive the fuel flow at the top end of the supply tube; and a disk having an opening adapted to accommodate the supply tube, wherein the disk is configured to slide along a length of the supply tube within the space delineated by the inner periphery of the housing and the outer periphery of the supply tube.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention is directed generally to a high turn-down modulating burner. More specifically, the present invention is directed to a high turn-down modulating burner of the burner tube type where the burner is disposed such that its central axis is disposed vertically while in use.

2. Background Art

Prior art burners include fixed surface areas at which combustion occurs, fixed volume conductors directing fuel to fixed surface areas at which combustion occurs and are either rectangular or cylindrical in shape. The fixed surface areas may include punched holes of various diameters, slots or interwoven metallic fibers/cross-hatched sintered metal fiber. The sizes of orifices or openings through which gas mixture is supplied to the surface areas are fixed due to the fixed punched hole/slot sizes or mat density or density of fiber weaving. Therefore, given fixed surfaces areas at which combustion occurs, prior art burners are incapable of supporting combustion at a very low combustion rate. For example, when modulated to a low flow of gas, the supply of gas is insufficient to be spread across now relatively large combustion surface areas to support combustion. Therefore, prior art burners may only support a minimum heat output setting that is still quite large, even when a heating demand does not justify this setting.

In order to achieve the effect of a high turn-down at low heat output regions, burners may also be shut off periodically. Upon shut off, the amount of materials heated can drop rapidly, potentially causing discomfort to users of such materials. Cycling frequency of the burner can be also be quite high, leading to energy losses and inefficiencies caused in the need to purge during both the shut-down and start-up phases. In addition, typical start-up times for burners can be quite long, leading to an inability to respond to sudden load demands.

Thus, there arises a need for a burner capable of a high turn-down ratio and one in which the effective combustion area is adjustable to accommodate heating demands without the need to shut off burners.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed toward a high turn-down burner adapted to receive a fuel flow for combustion. The burner includes:

(a) a housing including a side wall having a top edge, a bottom edge, an interior surface forming an inner periphery, a bottom wall adjoining the side wall at the bottom edge, a top wall adjoining the side wall at the top edge and a plurality of apertures disposed on the side wall;

(b) a supply tube adapted through the top wall of the housing, the supply tube comprising a top end, a bottom end and a side wall having an outer surface forming an outer periphery, wherein the supply tube is adapted to receive the fuel flow at the top end of the supply tube; and

(c) a disk having a weight and an opening adapted to accommodate the supply tube, wherein the disk is configured to slide along a length of the supply tube within the space delineated by the inner periphery of the housing and the outer periphery of the supply tube,

wherein the fuel flow is configured to exert a force equivalent to the weight of the disk thereby sustaining an optimal flowrate of the fuel flow through a plurality of apertures below the disk.

In one embodiment, the present burner further includes a travel limiter disposed on the bottom end of the supply tube for limiting the travel of the disk along the length of the supply tube.

In a second embodiment, the present invention is directed toward a burner adapted to receive a fuel flow for combustion. The burner includes:

(a) an outer housing comprising a central axis, a side wall having a top edge and a bottom edge, a plurality of apertures disposed on the side wall, a top wall adjoining the side wall at the top edge and a bottom wall adjoining the side wall at the bottom edge;

(b) an inner housing comprising a central axis, a side wall, a plurality of apertures disposed on the side wall, wherein the inner housing is configured to be coaxially inserted in the outer housing such that the inner housing is coaxially rotatable with respect to the outer housing; and

(c) an actuator adapted to harness and convert the power exerted by the fuel flow to a movement of the inner housing with respect to the outer housing, wherein the alignment of the plurality of apertures of the inner housing and the plurality of apertures of the outer housing are configured such that the movement is adapted to modify an effective combustion area of the burner which is defined by the amount of overlap between the plurality of apertures of the inner housing and the plurality of apertures of the outer housing.

In one embodiment, the actuator includes:

(a) a supply tube adapted through the top wall of the outer housing, the supply tube comprising a side wall, a top end and a bottom end, wherein the supply tube is adapted to receive the fuel flow at the top end of the supply tube;

(b) a flap having a shaft fixedly attached to the flap, wherein the shaft is pivotably mounted within the lumen of the supply tube about a rotational axis, the shaft is disposed substantially perpendicularly to the fuel flow within the supply tube, the shaft extending from the flap and terminating in a pinion configured for rotational engagement with a rack mounted to a portion of an inner surface of the inner housing;

(c) a return spring secured at one end to a portion of an inner surface of the supply tube and at another end to a portion of the flap,

wherein the flap is adapted to rotate about the rotational axis at a magnitude commensurate with the magnitude of the fuel flow to cause a relative rotation of the pinion with respect with the rack and the return spring is configured to return the flap to its neutral position when the fuel flow ceases.

In one embodiment, the burner further includes a fibrous burner surface disposed along an outer surface of the housing for aiding in distributing the fuel flow over the outer surface of the housing.

In one embodiment, each burner further includes an external housing disposed along an outer surface of the housing, the external housing having a side wall and a plurality of apertures disposed on the side wall of the external housing, wherein the plurality of apertures are configured for aiding in distributing the fuel flow over the outer surface of the housing or the outer housing.

In one embodiment, the fuel flow is a premixed fuel flow that is air-propane flow.

In another embodiment, the fuel flow is a premixed fuel flow that is air-natural gas flow.

Accordingly, it is a primary object of the present invention to provide a burner capable of a high turn-down ratio, thereby capable of maintaining efficient combustion in a wide range of fuel flowrates.

It is another object of the present invention to provide a burner capable of a high turn-down ratio and the high turn-down ratio is achieved through a means not requiring external power, i.e., power made available from outside of the burner.

It is a further object of the present invention to provide a burner that provides for automatic area compensation with respect to firing rate and allows for both an increased back pressure (as seen by the blower-gas valve train) and appropriate flame lift off (from burner surface) so as to not over heat the burner body including the housing, burner surface, external housing, etc.

Whereas there may be many embodiments of the present invention, each embodiment may meet one or more of the foregoing recited objects in any combination. It is not intended that each embodiment will necessarily meet each objective. Thus, having broadly outlined the more important features of the present invention in order that the detailed description thereof may be better understood, and that the present contribution to the art may be better appreciated, there are, of course, additional features of the present invention that will be described herein and will form a part of the subject matter of this specification.

PARTS LIST

2—burner4—housing5—inner periphery of housing6—supply tube7—outer periphery of supply tube8—travel limiter10—disk12—fuel14—fuel supplied space16—fuel starved space18—burner surface20—top wall or flange22—flame24—direction of hot flue gas26—coil tube28—igniter30—thermal insulator32—input port of top casting34—exit port of top casting36—top casting38—heat exchanger housing40—heat exchanger42—aperture of housing44—inner housing46—groove48—lip50—actuator52—pinion54—rack56—point to which spring is attached at flap58—return spring60—point to which spring is attached at anchor of supply tube62—flap64—angular offset66—aperture of inner housing68—shaft70—central axis of outer housing or inner housing72—flue gas exit port

PARTICULAR ADVANTAGES OF THE INVENTION

A high turn-down ratio is achieved by regulating the pressure of a fuel flow being fed to the burner. In one embodiment, the pressure regulation of the fuel flow is achieved by providing a burner capable of adjusting the effective burner area based on whether the fuel flow has access to the apertures in the burner surface. A prominent fuel flow causes more apertures to be exposed, increasing the effective area through which the fuel flow can be supplied to the combustion surface of the burner. In another embodiment, the pressure regulation of the fuel flow is achieved by providing a burner capable of adjusting the size of the apertures through which the fuel flow can be supplied to the combustion surface of the burner.

The present burner is capable of a high turn-down ratio without requiring complex powered moving parts and any moving parts required are contained within the burner itself, thereby eliminating any leaks which may be caused by having a power supply and actuator interface to the environment outside of the burner. In both embodiments disclosed, the prominence of a fuel flow itself is used to modulate the size of the effective area on which combustion takes place, making for sustained combustion at the combustion surface of the burner especially at low flowrates of a fuel flow and efficient heating as the desired firing rate is also the actual firing rate. In contrast to low turn-down burners, the present burner can be modulated to a low firing rate as heating demand decreases. In control applications where precise temperature adherence is important, a present burner aids in preventing overshoot of target temperatures of a medium heated.

There is provided a burner which allows automatic (e.g., in this case, passive) combustion surface area compensation with respect to firing rate and allows for both an increased back pressure (as seen by the blower-gas valve train) and appropriate flame lift off (from burner surface) so as to not over heat the burner body including the housing, burner surface, external housing, etc.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The term “about” is used herein to mean approximately, roughly, around, or in the region of. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 20 percent up or down (higher or lower). The term “turn-down” is used herein to mean the width of the operational range of a burner. The magnitude of “turn-down” is herein expressed as a ratio, or a maximum heat output divided by a minimum heat output.

In order to show the environment in which a present burner can be used,FIG. 1is provided.FIG. 1is a cross-sectional view of a heat exchanger40where a burner2of the burner tube type is used. In this example, the burner2is attached to a top casting36at the exit port34of the top casting36. In use, a fuel flow is drawn via the input port32of the top casting36, through the top casting and into the burner2. Combustion at the burner2causes thermal transfer from the burner2to water flowing through the coil tube26of the heat exchanger40. The fuel flow then exits the burner2to a burner surface18on the exterior periphery of the burner which is subsequently consumed to generate heat. In one embodiment, the burner surface includes a fibrous material having orifices defined by the density of the fibrous material. The denser the fibrous material, the finer the orifices will be.

In another embodiment, this burner surface includes orifices defined by punched apertures disposed on an external housing. In yet another embodiment, the burner surface18is defined by the outermost housing of a burner as an additional housing (fibrous or punch-holed) surrounding such outermost housing is not used. Any flue gas generated by such combustion is contained within the heat exchanger housing38and channeled to a flue gas exit port.

A typical heat exchanger further includes a thermal insulator30for reducing thermal loss via the top casting36and an igniter28for starting a flame22. The fuel used includes, but not limited to, a premixed air-propane or premixed air-natural gas. During operation, the flue gas generated due to combustion flows outwardly from the burner in direction24to the flue gas exit port72. A blower disposed upstream of the burner forces a fuel flow through the burner and the flue gas generated at the burner to continue to the exit port72.

FIG. 2is a perspective view of a burner2of the burner tube type.FIG. 3is a top orthogonal view of the burner2ofFIG. 2.FIG. 4is a partial orthogonal cross-sectional view of the burner2ofFIG. 3taken along line AA ofFIG. 3. In one embodiment, the outer structure of the burner2is essentially, but not limited to, a cylindrical or tubular structure encased on its side wall by a burner surface18. The tubular structure includes a side wall that is closed on one end with a bottom wall and fixedly attached to a top wall20having a centrally disposed opening. A supply tube6is connected to the opening such that supply tube6becomes the only means for the burner2to receive a fuel flow from the top casting36to the cavity of the burner2. During installation of a burner to the top casting36, the top wall20is aligned with exit port34of the top casting and secured to the top casting36such that no leaks can occur through the space between the top casting36and the top wall20. The supply tube6is essentially a tube used for channeling a fuel flow into the cavity of the burner2where a bottom flange8is disposed at its bottom end to serve as the bottom travel limit of the disk10configured to slide along the side wall of the supply tube6and the top wall20serves as the top travel limit of the disk10. The disk10is preferably constructed from an excellent thermal insulator, e.g., ceramic, light-weight aluminum, stainless steel, and the like, to avoid any effects of overheating of the disk and to maximize heat transfer to the materials to be heated, e.g., materials carried in the coil tube26, as shown inFIG. 1. It is conceivable that the housing, supply tube and disk be constructed in another shape, e.g. with a square or rectangular cross-section. However, the Applicants discovered that a cylindrical supply tube that is coaxially disposed with the housing works best as the number of potential pinch points are greatly reduced with a cylindrical housing and supply tube.

FIGS. 5 and 6are partial orthogonal cross-sectional views of the burner2ofFIG. 4, depicting the disk10disposed in a position corresponding to a fuel flow at the lowest and highest flowrate setting, respectively. It shall be noted inFIG. 5that, at this flowrate, the fuel flow does not generate a sufficient uplifting force to sustain the disk10above the bottom flange8. The disk10therefore rests upon the bottom flange8, confining the fuel flow to the space14substantially below the bottom flange8and forcing the fuel flow to exit the apertures42accessible from this space14. The space16above the disk10is therefore fuel starved and incapable of sustaining combustion or generating heat. The fuel12flow at its minimum setting is preferably configured to a flowrate just below the rate capable of lifting the disk10. InFIG. 6, the space16above the disk10does not exist as the disk10is disposed at its upper limit along the supply tube6, making for the maximum setting for space14and maximum access to the apertures42. Compared to a constant volume burner, the present burner is capable of maintaining efficient combustion as the size of the effective burner surface is commensurate with the fuel flowrate. In a constant volume burner, if a fuel flowrate drops below a critical level, there will not be sufficient fuel flow to maintain an even distribution of fuel on the entire combustion surface. In contrast, the present burner is capable of maintaining an effective combustion surface area that is commensurate with the fuel flow, thereby allowing for a low firing rate when the fuel flowrate is low and a high firing rate when the fuel flowrate is high. The ratio of the high firing rate and low firing rate can then be higher than a conventional burner as the low firing rate of the present burner can be much lower than the low firing rate of a conventional or prior art burner. In one embodiment, the present burner2is capable of a heat rate ranging from about 30,000 BTU/hr. to about 250,000 BTU/hr. which is equivalent to the consumption of 30 Cubic Feet Per Hour (CFH) to 250 CFH of natural gas. In another embodiment, the present burner is adapted to provide a firing rate having a turn-down ratio of at least about 27:1 (e.g., in turning down from about 200,000 BTU/hr. to about 7,500 BTU/hr.). In one embodiment, the ratio of the number of blocked apertures42and the number of unblocked apertures42ranges from about 9/1 at low firing rate of 5% to about 0/10 at 60 to 100% firing rate. As the combustion area of the present burner can be adjusted, an increased back pressure (as experienced by the blower-gas valve train) can be effected. As a result, flames over the burner surface can be controlled to be lifted off from the burner surface so as to not over heat the burner body including the housing, burner surface, external housing, etc. Suitable disk weight and area upon which the fuel flow acts shall be configured to achieve the desired back pressure of the blower-gas valve train. It shall also be noted that the present burner is capable of reducing any potential backflow of flue gases through the burner in a multi-burner system having a common shared vent. In blower-equipped systems that lack measures to prevent backflow of flue gases from one or more burners to a non-functioning burner, flue gases can travel through the exit port of a heat exchanger, through a burner and into the blower area. As the effective combustion area of a present burner is proportional to the fuel flow magnitude, the availability of a through path for flue gas from another burner to flow through the present burner is greatly reduced as shown in the embodiment shown inFIGS. 1-9as the number of fluid accessible apertures42in space14is greatly reduced or eliminated in the embodiment shown inFIGS. 10-14as the amount of overlap of apertures42and66is eliminated.

The surface upon which the fuel flow acts shall be configured such that the fuel flow acts to center the disk10within the pathway in which the disk10slides. In one embodiment, the disk possesses substantially parallel top and bottom surfaces. In another embodiment, the bottom surface of the disk is concaved as shown inFIG. 7.FIG. 8depicts yet another embodiment of the disk10, where in this case, its bottom surface is configured in a convexed shape.FIG. 9depicts yet another embodiment of the disk10, where in this case, its bottom surface is configured in a frusto-conical shape. In these embodiments, the surface upon which the fuel flow acts upon exiting the supply tube6is not parallel to the horizontal plane so as to reduce opportunities for the disk from getting cock-eyed and getting stuck within its pathway during its ascent or descent. Notice the concave bottom surface74, convex bottom surface74or slanted bottom surface of the disk10inFIGS. 7-9. Suitable clearance between the inner periphery5of the housing and the disk10and between the outer periphery7of the supply tube6and the disk10shall be provided so as not to allow excessive leakage of fuel flow from space14to space16while allowing the disk to rise or drop substantially free of resistance.

FIGS. 10 and 11are partial orthogonal cross-sectional views of a second embodiment of the present burner, depicting the second embodiment of the present burner2with and without fuel flow, respectively.FIGS. 12, 13 and 14are partial top orthogonal views of the housing of the second embodiment of the present burner, depicting an inner housing disposed in a position corresponding to a fuel flow at a flowrate between the maximum and minimum settings, a fuel flow at a minimum setting and a fuel flow at a maximum setting, respectively. It shall be noted that the shape of the flap62is configured substantially according to the cross-section of the supply tube6with suitable clearance provided between the flap62and the inner surface of the supply tube6to avoid binding but yet allowing the flap62sufficient surface area to harness the forces of the fuel flow.FIGS. 12, 13 and 14are shown with the top wall removed to more readily reveal the actuator50. In this embodiment, in order to maintain efficient combustion at the burner surface, the size of the cavity into which fuel is supplied is not altered as in the embodiment shown inFIGS. 2-9. Instead, the effective size of the openings through which the fuel flow traverse is adjustable. Such adjustment is performed using the concept of aligning apertures of two housings to increase or decrease the openings. The amount of overlap between the plurality of apertures66of the inner housing44and the plurality of apertures42of the outer housing4determines such effective size of the openings.

In this embodiment, a second housing called the inner housing44is coaxially disposed within the housing or outer housing4such that the apertures66of the inner housing44can be aligned precisely or misaligned completely to cause openings ranging from about 0% to about 100%. As shown inFIG. 10, the inner housing44is essentially a hollow cylindrical tube supported at a groove46disposed at the top end of the housing by a lip48extending outwardly from the top end of the inner housing44. More preferably, the openings are configured to range from about 5% to about 100%. Referring toFIG. 12, the housing4is disposed about its central axis70at an angular offset64with respect to the inner housing44. The resulting openings are then between about 0% and about 100%. The openings can be altered by changing the magnitude of angular offset64to a configuration where the apertures42of the housing4are completely misaligned with the apertures66of the inner housing44as shown inFIG. 13and another configuration where the apertures42of the housing4are wholly aligned with the apertures66of the inner housing44as shown inFIG. 14. Apertures42,66of suitable sizes and shapes may be used to yield a substantially linear proportional correlation between the angle of rotation of one housing relative to another and the size of openings due to overlapping of the housings. The actuator50ofFIG. 10represents a mechanism capable of rotating the inner housing44with respect to the housing4. In this embodiment, the actuator50includes a flap62disposed within the lumen of the supply tube6for harnessing the power exerted by the fuel flow in causing such rotation. The flap62is essentially a disk that is pivotably connected to the supply tube6such that when no flow occurs, the flap62is disposed in an orientation substantially perpendicular to the fuel flow within the supply tube6(i.e., in a closed orientation as shown inFIG. 10) and when a flow occurs, the flap62rotates in a direction to an open orientation as shown inFIG. 10to allow flow through the supply tube6. The edges of the disk are rounded so that the flap transitions smoothly from one orientation to another. Referring toFIGS. 10-14, a shaft68fixedly connected to the flap62extends from the supply tube6to an end having a pinion52. A rack54fixedly mounted on a portion of the inner surface of the inner housing44is configured to cooperate with the pinion52. As the flap62rotates, the pinion52rotates with it, causing rotary motion of the inner housing44with respect to the outer housing4. The actuator52has been configured such that the degree of rotation of the flap62is directly proportional to the effective combustion area of burner. It shall be noted that in the second embodiment, the supply tube6of the second embodiment may be configured substantially shorter than the supply tube shown inFIGS. 2-9as the lumen of the inner housing is filled in its entirety regardless of the fuel flowrate. The difference between a low fuel flowrate and a high fuel flowrate lies in the effective apertures leading to the fibrous burner surface18. As a fuel flow ceases, a return spring58causes the flap62to return to its orientation as shown inFIG. 10. The return spring58is pivotably connected at one end to an anchor60disposed on a portion of inner wall of the supply tube6and pivotably connected at another end to a portion56of the flap62.

In yet another embodiment, the housings4,44may be configured such that relative linear axial movements of the housings4,44are used to determine the amount of overlaps of their corresponding apertures42,66. In this embodiment, the housings4,44may be configured in a different shape, e.g., rectangular or square.

The detailed description refers to the accompanying drawings that show, by way of illustration, specific aspects and embodiments in which the present disclosed embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice aspects of the present invention. Other embodiments may be utilized, and changes may be made without departing from the scope of the disclosed embodiments. The various embodiments can be combined with one or more other embodiments to form new embodiments. The detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, with the full scope of equivalents to which they may be entitled. It will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of embodiments of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description. The scope of the present disclosed embodiments includes any other applications in which embodiments of the above structures and fabrication methods are used. The scope of the embodiments should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.