Combustion system with high turn down ratio

Abstract

A low NOx combustion system for reducing the production of nitrogen oxides in its emissions while allowing unusually high turndown ratios in operation is shown. The system includes an improved burner which has a firing head with an outlet end which can be mounted on a sidewall of a heat exchanger such as a boiler. The burner includes a fan, a windbox, a burner drawer assembly and a gas manifold. The windbox has an internal scroll-shaped passageway which is contoured to improve air flow from the fan to the firing head. An improved air straightener and diffuser head in the burner drawer assembly cooperate with improvements in the air supply system to provide improved combustion characteristics and a higher than usual turndown ratio for the burner assembly.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a combustion system, such as a fire tube boiler or furnace combustion system. More specifically, the invention relates to such a combustion system that reduces the NO_x emissions while allowing an unusually high turn down ratio during operation.

2. Description of the Prior Art

In the operation of heat exchangers, such as furnaces or boilers, various gases are produced as by-products, including the so called nitrogen oxides (NO_x). Such oxides of nitrogen, when produced in combination with hydrocarbons present in the atmosphere, constitute a major source of pollution in the environment. Depending on the type of fuel being burned, there are generally two types of nitrogen oxides which can be formed. Fuel bound NO_x is formed as a result of nitrogen being present in the fuel itself, i.e., in fuel oils. During combustion, the nitrogen is released and quickly reacts with the oxygen in the combustion air to form NO_x. Thermal NO_x is formed, on the other hand, when high combustion temperatures break down the nitrogen gas in the combustion air, resulting in the formation of atomic nitrogen. When this occurs, the atomic nitrogen will very quickly react with oxygen to form thermal NO_x. If natural gas is employed as the furnace or boiler fuel, only thermal NO_x should be formed, because clean natural gas generally does not contain any significant nitrogen containing compounds. On the other hand, both thermal and fuel bound NO_x are formed when burning fuel oils.

Since the production of NO_x by the burning of fuels in the operation of boilers and furnaces is potentially damaging to the environment, various environmental emissions standards have been imposed by various governmental authorities and agencies to regulate and to suppress the formation of nitrogen oxides during operation of boilers and furnaces. Various techniques have been utilized in the design and operation of boilers and furnaces to meet those regulations.

For example, it is known that burning a hydrocarbon fuel in less than a stoichiometric concentration of oxygen will increase CO production. This concept is utilized in a staged air type low NO_x burner where the fuel is first burned in a deficiency of air in one zone to produce an environment that suppresses NO_x formation, and then the remaining portion of the air is added in a subsequent zone. The use of staged fuel has also has been suggested for suppressing the NO_x formation. In staged fuel, the air and some of the fuel is burned in the first zone and then the remaining fuel is added in the second zone. The subsequent lowering of the combustion temperature in the first zone is thought to suppress NO_x formation. Another widely used technique to reduce NO_x emissions is to recirculate flue gas to one or more of the combustion zones to lower the flame temperature and reduce NO_x formation.

Despite the success resulting from the use of such techniques as flue gas recirculation, certain of these prior art processes have exhibited deficiencies and associated problems which have led to limited commercial acceptance. For example, flame stability can be a critical factor when operating a burner at significantly sub-stoichiometric conditions. Moreover, many of the prior processes and systems have been complicated and expensive to build, install, use and maintain and require extensive modifications of standard furnaces, boiler and fuel burners.

In the case of a boiler burner or other industrial heat exchanger burner, the purpose of the burner is to mix molecules of fuel with molecules of air. A boiler will run only as well as the burner performs. For this reason, a poorly designed boiler with an efficient burner may perform better than a well designed boiler with a poor burner. Burners are designed to maximize combustion efficiency while minimizing the generation of emissions. Thus, a particularly efficient burner design may reduce or eliminate many of the emissions problems associated with NO_x and other undesirable by-products of the combustion process.

A power burner mechanically mixes fuel and combustion air and injects the mixture into the combustion chamber. All power burners provide essentially complete combustion while maintaining flame stabilization over a range of firing rates. Different burners, however, require different amounts of excess air and have different “turndown ratios.” The turndown ratio can be defined as the maximum inlet fuel or firing rate divided by the minimum firing rate. Turndown ratio can also be used to compare the maximum to minimum heat output. For example, a turndown ratio of 25:1 means that the burner can modulate from 4% to 100% of full fire. On the other hand, a turndown ratio of 2.5:1 would limit the heat output from 40% to 100% of full fire.

Most gas burners of the type under consideration in the marketplace exhibit turndown ratios in the range from about 5:1 to about 8:1, with little or no loss in combustion efficiency. A higher turndown ratio reduces burner starts, provides better load control, saves wear and tear on the burner, reduces refractory wear, reduces purge-air requirements, and provides fuel savings. It would thus be advantageous to provide a burner of the type under consideration with a higher turndown ratio without increasing NO_x emissions.

It is an object of the present invention to provide a particularly efficient combustion system for burning oil or gas, which combustion system utilizes a burner with an exceptionally high turndown ratio.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the invention is to provide a low NO_x combustion system that reduces the amount of NO_x formed during combustion.

It is a further object of this invention to provide a burner system which will provide low NO_x burning for a wide range of fuel burning rates and corresponding air or oxidant supply rates.

Another object of the invention is to provide a low NO_x combustion system which provides turndown ratios much higher than those conventionally achieved by commercially available equipment.

Another object of the invention is to provide a low NO_x combustion system which is relatively inexpensive to manufacture, install, use, and maintain, and requires no significant heat exchanger modification.

The foregoing objects are basically obtained by providing a low NO_x combustion system comprising a heat exchanger having sidewalls defining a closed interior containing a medium to be heated and a burner having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the heat exchanger. A fuel supply means is fluidly coupled to the burner for conveying a combustible fuel to the burner. An air supply means is fluidly coupled to the burner for conveying combustion supporting air to the burner. An igniting means is positioned adjacent the outlet end of the burner for igniting the combustible fuel to thereby heat the medium contained in the heat exchanger. A windbox forms a part of the air supply means which connects with the firing head, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway and inlet and outlet openings. Air from the air supply means travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the air supply means to the burner. The scroll-shaped passageway of the windbox is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner.

The firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the heat exchanger. The burner drawer assembly includes a diffuser, a pilot, a scanner and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate. The preferred air straightener comprises a single plate having a length and a width and opposing side edges which define opposing planar surfaces. The plate is mounted on the longitudinal support tube along a selected opposing side edge thereof. Preferably, the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate.

The firing head diffuser provides directional control of combustion air for mixing and combustion stability. The diffuser includes an manifold plate with a first and second annular wall regions. The first annular region forms a collar-like region surrounding a central opening in the manifold plate. The second annular wall region is made up of a series of overlapping fins which are separated by slits. The slits provide a swirling action to combustion air passing through the diffuser. A plurality of gas orifices are located about the outer periphery of the second annular wall region and are arranged in a circular array for conveying and communicating natural gas outward and into a combustion region of the burner.

The air supply means also includes a combustion air fan which attaches to the windbox, whereby the windbox routes combustion air from a fan inlet to the firing head. The combustion air fan has an associated air damper which communicates with an inlet to the combustion air fan. The air damper comprises a box-like enclosure which preferably houses a single damper blade. The damper blade has upper and lower longitudinal sealing edges and opposing side edges. The upper and lower longitudinal sealing edges and side edges are provided with resilient sealing strips which provide ease in adjustment for leakage.

In the preferred embodiment of the invention, the head extension connects to a cylindrical gas manifold which, in turn, connects to the heat exchanger sidewall. The gas manifold has radial gas ports used to direct gas fuel to the burner outlet. The radial gas ports may have gas spuds installed therein to improve the distribution of the gas. The gas manifold holds the outlet end of the burner assembly. The gas manifold also has an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate. Preferably, a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat.

Additional objects, features and advantages will be apparent in the written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the combustion system of the invention, showing the burner assembly mounted in place on the sidewall of a boiler, the remainder of the boiler being broken away.

FIG. 2A is a side view of the windbox of the burner of FIG. 1, showing the profile of the improved internal scroll in dotted lines.

FIG. 2B is a view similar to that of FIG. 2, but showing the prior art scroll profile.

FIG. 3A is a side view of the burner drawer assembly of the burner of FIG. 1, showing the air straightener of the invention.

FIG. 3B is a view similar to FIG. 3A, but showing the prior art air straightener.

FIG. 3C is an isolated view of the prior art air straightener.

FIG. 3D is an isolated view of the improved air straightener of the invention.

FIG. 4A is a side view illustrating the gas manifold of the burner of the invention and showing the diffuser manifold plate housed therein.

FIG. 4B is an isolated view of the diffuser manifold plate used with the improved burner of the invention.

FIG. 5 is an isolated view of an improved air damper used with the combustion air fan of the burner of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 1 of the drawings, there is shown a combustion system of the invention which includes a burner assembly, designated generally as 11, mounted on the sidewall of a heat exchanger, such as a boiler 13. The boiler 13 can be, for example, a typical boiler intended for commercial and industrial use such as a Scotch Marine Firetube boiler which can fire gas or oil. The boiler is used to heat a fluid medium, such as water, either directly or indirectly. Heat exchanger 13 can also be, for example, a conventional furnace in which a gas such as air is heated directly or indirectly. It will be understood, that the sidewalls of the heat exchanger 13 form an outer housing. The housing being equipped with a flue gas stack (not shown) rigidly coupled thereto and forming a combustion chamber contained therein, in conventional fashion.

The invention described herein is an improved burner assembly 11 which offers improved emission performance and higher turndown ratios than burners of the same general type available at the present time in the industry. It features a new burner firing head and improvements to other components to support higher turndown and lower emissions. In addition to the novel features to be described, the burner assemblies of the invention are configured from a common group of components that vary in size and style depending on the capacity, NO_x level, fuels and end application. A typical installation will be described in the discussion which follows, along with the improved features of the invention.

The burner assembly 11 of the invention includes an air supply means, fluidly coupled to the burner, for conveying combustion supporting air to the burner. The air supply means includes at least a combustion air fan 15, a windbox 17 and an air damper 19. Typically, a backward curved fan (15 in FIG. 1) is used to supply the combustion air to burn the fuel. If the burner is equipped with flue gas recirculation for lowered NO_x emissions, the fan 15 will also provide the recirculated flue gas. The fan diameter and width will vary to match the required combusiton air flow rate and flue gas recirculation rate, along with altitude and vessel backpressure. Fans of the type under consideration are commercially available and will be familiar to those skilled in the art. An inlet cone (not shown) is used with the fan 15 to provide a smooth air flow transition to the fan. Each fan 15 has a matching inlet cone. In some cases, the inlet cone bolts directly to the housing and in other cases, it bolts to an adapter that bolts to the housing. The inlet cone extends into the fan inlet about ¼ inch. The combustion air fan and associated motor are assembled together on a mounting plate that attaches to the windbox 17. The fan and motor can easily be removed for inspection or maintenance by removing this assembly. The fan has a hub that is machined to match the motor shaft diameter and key. Setscrews are used to lock the fan hub to the motor shaft. The fan can be adjusted on the shaft to provide the correct overlap between the fan and inlet cone. Several different motor styles can be used, depending upon the application. An open-drip-proof style is most common and is used in a typical enclosed, clean environment. The fan motors in the instant application are typically face mounted, while larger units are typically base mounted.

With reference to FIGS. 1, 2A and 2B, the windbox 17 is an enclosure that routes the combustion air from the fan 15 to the firing head, and provides the primary mechanical structure for all of the components of the burner assembly. As shown in FIG. 1, the combustion air fan 15 and inlet cone are contained within the windbox 17. The firing head is connected by a head extension (21 in FIG. 1). The head extension 21 allows different combinations of firing heads to be used with different fan and windbox sizes. Some burners use longer head extensions to provide clearance for smoke boxes and other components. The head extension is typically provided with an access door to simplify inspection and maintenance. The flue gas recirculation adapter (where present) and air damper 19 are also connected to the windbox, opposite the combustion air motor. The windbox 17 serves as the building block of the burner. It requires good structural support to the boiler and floor to handle the weight and movement of rotating components.

As shown in FIG. 1, the windbox 17 has an exterior comprised of opposing sidewalls 23,25 connected by a mid wall 27. As best seen in FIGS. 2A and 2B, the windbox 17 also has an interior 29 including a scroll-shaped interior passageway 31 and inlet and an outlet openings 33, 35, respectively. The combustion air from the fan 15 travels at a right angle to the windbox opposing sidewalls 23, 25 as it enters the inlet opening 33 and as it passes out the outlet opening 35 in passing from the air supply means to the burner, The scroll-shaped passageway 31 is defined by lateral edges (as at 37 in FIG. 2A) which fit at right angles to the windbox opposing sidewalls 23, 25. As shown in FIG. 2A, the lateral edges 37 are flush with the outlet opening 35 without forming a lip region with respect to the opposing sidewalls 23, 25, thereby providing more uniform air flow though the windbox to the burner.

This feature can be further explained with respect to FIG. 2B in which the lateral edges 37A of the scroll-shaped passageway of the prior art actually form a lip region indicated by the dimensional radius “r” in FIG. 2B. This lip region in the prior art design tended to introduce a certain amount of disruption in the air flow which detracted from the efficiency of the overall combustion process.

The air damper (19 in FIG. 1) regulates the flow of air to the burner. As best seen in FIG. 5, the preferred embodiment comprises a box-like enclosure having upper, lower and opposing sidewalls, 39, 41, 43, 45, respectively, which define an interior space 48. The interior space 48 houses a single blade damper 47 which is mounted on a damper shaft 49. The shaft 49 is supported by ball bearings (51 shown in FIG. 5) for smooth operation. On a single point positioning system (linkage), the damper shaft is connected by a suitable linkage 53 to a jackshaft (to be described further). On a parallel positioning system (linkageless), the shaft is directly coupled to the actuator for the air damper. FIG. 5 illustrates the damper blade in the closed position in phantom lines. The single blade damper 47 has special edge strips along the upper and lower longitudinal sealing edges thereof which can contact stops 55, 57. These special edge strips are typically formed of a resilient material, such as a suitable rubber, plastic or other resilient synthetic material. The perpendicular side edges of the single blade damper are also provided with resilient sealing strips (illustrated in phantom lines as 58 in FIG. 5). The edge strips assist in providing ease of adjustment for leakage.

The air damper 19 operates on the fan inlet. On low NO_x burners, the air damper 19 is connected to a flue gas recirculation adapter plate, so that the flue gas can enter down stream of the damper where there is a negative pressure. On other burners, the air damper is bolted to the windbox housing. FIG. 1 shows a damper 19 equipped with an inlet grate 59 in place. If an optional silencer is used, it is mounted to the inlet of the air damper.

The combustion system of the invention also comprises a burner firing head with an outlet end mounted on a selected sidewall of the boiler. As shown in FIG. 3A, the preferred firing head includes a burner drawer assembly, designated generally as 61, which is located partly within the head extension (21 in FIG. 1) which connects the windbox to the sidewall of the boiler. The burner drawer assembly 61 includes a diffuser 63, a pilot 65, a scanner tube 66 and an air straightener 67 all carried on a longitudinal support tube 69 which extends perpendicular to a circular back plate 71. The longitudinal support tube 69 can conveniently comprise an oil gun for the burner assembly where fuel oil is to be burned either alone or with natural gas. The burner drawer assembly 61 is removed as a complete unit for easy adjustment and inspection. The burner drawer slides through the windbox, head extension 21 and into a gas manifold (73 in FIG. 1). It is attached to the burner by bolting the back plate to the windbox. The pilot, scanner, oil gun and diffuser position can be adjusted by sliding the tube 69 in an out through the back plate 71. Setscrews can be used to lock the support tube 69 into position. The oil gun can also be removed for inspection or extended gas firing without removing the burner drawer.

The diffuser 63 provides the directional control of the combustion air, for mixing and combustion stability. As will be explained in greater detail, a combination of outer swirl air and inner straight air is used. The diffuser must fit properly in the gas manifold 73 without large gaps between the diffuser 63 and gas manifold 73. The back surface of the diffuser may be flared out to provide a tight fit and to conform to any irregularities in the housing. As best seen in FIGS. 3A and 4B, the diffuser terminates in a diffuser manifold plate 75. The plate 75 has an outer circumference 77 and an inner circumference 79 which defines a central opening 80. A first annular wall region 83 forms a collar-like region which surrounds the central opening 80.

A second annular wall region 85 is located between the first annular wall region 83 and the outer circumference 77. The second annular wall region 85 is made up of a series of overlapping fins, such as fin 87 having terminal edges 89. A plurality of gas orifices are located about the outer periphery of the second annular wall region 85 and are arranged in a circular array for conveying and communicating natural gas outward and into a combustion region of the burner.

The terminal edges 89 of the fins 87 are bent inwardly as view in FIG. 4 and are thus spaced apart slightly from the planar surface of the next adjacent fin to provide a slit for the passage of air. The overlapping fins provide a swirling action to combustion air passing through the diffuser. The swirling action improves mixing and flame stability of the burner head. This action allows the burner to operate with higher flue gas recirculation rates and lower NO_x emissions. FIG. 4A shows the high swirl diffuser manifold plate 75 installed within the gas manifold 73 prior to installing the gas manifold onto the boiler.

Returning to the description of the burner drawer assembly (61 in FIG. 3A), a scanner is mounted to longitudinally oriented tube 66 that is routed past the diffuser manifold plate. This mounting insures that the scanner only sees the flame produced at the burner head, and not other light sources. A variety of different commercially available scanners can be utilized. However, an IR scanner is typically recommended due to the fact that IR scanners tend to work better with flue gas recirculation and the longer burner drawer assemblies used in the instant burner assembly.

With reference again to FIG. 3A, the pilot 65 is formed from a machined casting having an internal venturi shaped passageway which is used to pull air and mix natural gas with the air prior to burning. In the particular pilot shown in FIG. 3A, a perforated screen (68 in FIG. 3A) is used on the outlet of the venturi to shield the flame base from high velocity air. A raw gas tube 93 is used to provide additional gas to the pilot and generate the proper pilot flame size. An ignition electrode 97 operates in conventional fashion to provide a spark within the perforated screen 68 to ignite the pilot. The pilot 65 is positioned on the backside of the diffuser manifold plate 75, and the pilot flame actually passes through the diffuser manifold plate to ignite the main flame. It is located close to the scanner tube, and in the upstream direction to cause the flame to pass in front of the scanner tube. The pilot is connected to a gas pipe 95 that extends through the back plate 71 in the burner drawer, and can be adjusted by moving the tube in the back plate. The electrode 97 is mounted to the venturi casting.

As shown in FIG. 3A, the preferred burner drawer assembly of the invention features an air straightener in the form of a single plate 67. The air straightener is used to straighten the air before it reaches the diffuser. It is located in the windbox 17 in a location where it is clear of other components and can be securely locked into position. The air straightener helps to control the air flow pattern, and can be used to adjust the pattern by moving the straightener backwards and forwards. FIG. 3D shows the air straightener moved backwards in the direction of the back plate 71. The air straightener plate 67 can also be rotated 360° with respect to the back plate 71. As shown in FIG. 3A, the air straightener plate 67 is generally rectangular shaped, having a length and a width and opposing side edges 99, 101 which run along the length of the plate and which, along with the width of the plate define opposing planar surfaces (surface 103 shown in FIG. 3A). The air straightener plate 67 is mounted on the longitudinal support tube (oil gun) along a selected opposing side edge 101 thereof, as by tack welding.

The difference in the air straightener 67 of the invention and the prior art can best be understood with reference to FIGS. 3C-3D. The prior art air straightener consisted of a series of plates such as plates 107, 109 (FIG. 3C) mounted to the oil gun tube 69. The series of plates formed a radiating pattern about the longitudinal support tube. The single plate air straightener shown in FIG. 3D has been found to more effectively break up the rotating air flow and provide more uniform mixing.

The burner head of the invention would be utilized with a conventional gas train of the type familiar to those skilled in the relevant industry. Such a gas train typically contains the safety shut-off valves, manual shut-off valves, pressure switches, and other components that may be required for the specific installation, available gas pressure, insurance codes and local regulations. The details of the gas train can vary greatly from burner to burner. Gas trains tend to be designed for each application. A typical installation might utilize a gas pressure regulator upstream of two safety shutoff valves The gas metering valve would be downstream of these components. Another common style is to have the gas pressure regulation built into the second safety shutoff valve. The shutoff valves are usually motorized to open and spring return to close.

A gas control valve is used to modulate the flow of gas to the burner. On a single point positioning system (linkage), it is connected to the jackshaft, with a fuel cam used to make fine adjustments to fuel flow. With a parallel positioning system (linkageless), an actuator is connected to the gas control valve, and modulated by electronic control to the desired position. The gas control valve is located on the pipe that connects to the gas manifold.

The gas manifold 73 of the invention is shown in FIGS. 1 and 4A. The gas manifold 73 is a cylindrical chamber that has radial gas ports used to direct the gas fuel. Gas spuds (109 in FIG. 4A) are generally installed in the radial ports to improve the distribution of the gas to the air stream. These gas spuds are located around the circumference of the gas manifold. The gas spuds are typically stainless steel pipe nipples (⅛ or ¼ inch) that are screwed into the gas manifold. Some of the holes in the gas manifold may be plugged with pipe plugs in some configurations. The arrangement of the gas spuds can change by fuel type, input and NO_x control level. In some cases, field adjustment of these spuds is necessary to meet different furnace configurations and field conditions.

The gas manifold 73 also holds the diffuser end of the burner drawer assembly 61, which fits tightly into the gas manifold. This centers the diffuser manifold plate 75 in the gas manifold, which is required to obtain good mixing of the gas and air. The face of the manifold is protected from the high flame temperatures by a refractory front plate (illustrated as 111 in FIG. 1), which is formed from a material capable of withstanding the expected operating temperatures. In addition a ceramic blanket (112 in FIG. 4A) is used between the face of the manifold and the refractory to further prevent the transfer of heat. The ceramic blanket is generally about 1 inch to 2 inches wide.

In operation, the burner assembly of the invention may be equipped with a standard single point positioning system (linkage) for fuel-air-ratio control, as is conventional in the art. The burner may also be equipped with optional fuel cams, a multiple setting modulating motor or a parallel positioning system (linkageless). All of these systems provide the basic fuel-air-ratio control required for good combustion. The linkages are indicated generally at 113 in FIG. 1 but are not further described, as such linkage systems are conventional in the art.

The burner modulates to match the energy requirements of the load. It does this by using a sensor that measures the pressure or temperature of the system and a matching sensor in the modulating motor that moves to match the readings of the sensor. In some optional systems, a similar process is used with an external control that provides a signal to the motor to increase or decrease to a certain rate. These systems may include multiple burner sequencing, outside temperature compensation and numerous other control strategies.

The single point positioning systems use a single modulating motor to vary the fuel input, air flow and other flow changes such as flue gas recirculation and atomizing air flow. Suitable linkages are used to connect these flow control elements together to provide a unified fuel-air-ratio control system. Other elements in the system would typically include the jackshaft, fuel cam and modulating motor.

The jackshaft is a shaft that is used to tie the fuel, air and flue gas recirculation valves together with linkage, to provide a uniform change in the flow as the burner modulates. The modulating motor is used drive the jackshaft, driven by the requirement for heat in the system and as allowed to operate by the flame safe guard. The jackshaft in the present case is a ¾ inch shaft that rotates and is mounted in bearing supports. This provides a common means of modulating all of the valves from a single drive mechanism. The length can vary to meet overall dimensions and individual drive arms are used to connect to each valve. A fuel cam may also be utilized and comprises a mechanical linkage that allows for small fuel rate changes without changing the linkage setting. It can simplify the fuel-air-ratio adjustments in the burner setup.

The modulating motor is driven in response to the boiler load. For example, a sensor that measures steam pressure or water temperature provides the feedback to the modulating motor to increase or decrease the firing rate. This sensor is adjusted to provide the desired pressure or temperature range to modulate from low to high fire and return. The standard modulating motor has two internal proving switches. One switch, the low fire start switch, proves the low fire position where the burner will lite. This is also the position the modulating motor will travel to when the burner shuts down. The second switch, the high fire purge switch, proves he high fire purge position during pre-purge operation of the combustion system. The combination of features previously described allows the present burner assembly to operate with unusually high turndown ratios as compared to burner assemblies of the same general type available in the marketplace.

An invention has been provided with several advantages. The improved scroll-shaped passageway within the windbox improves the air flow to the burner. The air straightener plate also immproves mixing and air flow at the burner drawer prior to the diffuser. The high swirl flame produced by the diffuser manifold plate offers excellent flame stability for very low NO_x performance. The improved firing head places a high spin on the combustion air that improves mixing and flame stability. This allows the burner to operate with higher flue gas recirculation rates while maintaining lower NO_x emissions. The firing head is also designed to keep the low fire flame off the burner head, and allows it to operate at low rates without heat damage to the firing head. The high turbulence which is produced by the diffuser allows the burner to be used in smaller furnace diameters. The improved air damper provides improved air turndown as well as easily adjustable opening speeds to match a variety of fuel valves. The combination of features described provides a burner assembly with turndown ratios which exceed about 12:1 and in some cases may be as high as 18:1.

While the invention has been shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.

Claims

1. A low NOx combustion system, the combination comprising: a heat exchanger having sidewalls defining a closed interior containing a medium to be heated; a burner assembly having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the heat exchanger; fuel supply means, fluidly coupled to the burner, for conveying a combustible fuel to the burner; air supply means, fluidly coupled to the burner, for conveying combustion supporting air to the burner; igniting means, positioned adjacent said outlet end of the burner, for igniting the combustible fuel to thereby heat the medium contained in the heat exchanger; and a windbox provided as a part of the air supply means connecting with the firing head, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway, an inlet and an outlet openings, and wherein air from the air supply means travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the air supply means to the burner, and wherein the scroll-shaped passageway is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner.

2. The combustion system of claim 1, wherein the fuel supply means utilizes natural gas as a primary combustible fuel.

3. The combustion system of claim 1, wherein the fuel supply means utilizes oil as a primary combustible fuel.

4. The combustion system of claim 1, wherein the heat exchanger is a fire tube boiler or furnace.

5. The combustion system of claim 1, wherein the firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the heat exchanger, the burner drawer assembly including a diffuser, a pilot and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate, and wherein the air straightener comprises a single plate having a length and a width, opposing side edges and opposing planar surfaces, the plate being mounted on the longitudinal support tube along a selected opposing side edge thereof.

6. The combustion system of claim 5, wherein the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate, the oil gun tube also being rotatable with respect to the opening provided in the back plate.

7. The combustion system of claim 5, wherein the firing head diffuser provides directional control of combustion air for mixing and combustion stability, the diffuser comprising a manifold plate having a first annular wall region which forms a collar-like region about a central opening, the manifold plate also having a second annular wall region having a series of overlapping fins which are separated by slits which provide a swirling action to combustion air passing through the diffuser, and wherein a plurality of gas orifices are located about an outer periphery of the second annular wall region for conveying and communicating natural gas outward and into a combustion region of the combustion system.

8. The combustion system of claim 1, wherein the air supply means includes a combustion air fan which attaches to the windbox, whereby the windbox routes combustion air form the fan to the firing head, and wherein the air supply means also includes an air damper which communicates with an inlet to the combustion air fan, the air damper comprising a box-like enclosure which houses a single damper blade, the damper blade upper and lower longitudinal sealing edges and opposing side edges, the upper and lower longitudinal sealing edges and opposing side edges being provided with sealing strips which provide ease in adjustment for leakage.

9. The combustion system of claim 5, wherein the head extension connects to a cylindrical gas manifold which, in turn, connects to the heat exchanger sidewall, the gas manifold having radial gas ports used to direct gas fuel to the burner outlet, at least selected ones of the radial gas ports having gas spuds installed therein to improve the distribution of the gas, and wherein the gas manifold holds the outlet end of the burner assembly, the gas manifold having an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate.

10. The combustion system of claim 9, wherein a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat.

11. A low NOx combustion system for firing a Scotch Marine boiler, the combination comprising: a boiler having sidewalls defining a closed interior containing a fluid medium to be heated; a burner assembly having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the boiler; fuel supply means, fluidly coupled to the burner assembly, for conveying a combustible fuel to the burner assembly; a combustion air fan, fluidly coupled to the burner assembly, for conveying combustion supporting air to the burner assembly; a pilot, positioned adjacent said outlet end of the burner assembly, for igniting the combustible fuel to thereby heat the fluid medium contained in the boiler; and a windbox connecting the combustion air fan and the firing head of the burner assembly, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway, an inlet and an outlet openings, and wherein air from the combustion air fan travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the windbox to the burner assembly, and wherein the scroll-shaped passageway is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner assembly.

12. The combustion system of claim 11, wherein the firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the boiler, the burner drawer assembly including a diffuser, a pilot and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate, and wherein the air straightener comprises a single plate having a length and a width, opposing side edges and opposing planar surfaces, the plate being mounted on the longitudinal support tube along a selected opposing side edge thereof.

13. The combustion system of claim 12, wherein the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate, the oil gun tube also being rotatable with respect to the opening provided in the back plate.

14. The combustion system of claim 13, wherein the firing head diffuser provides directional control of combustion air for mixing and combustion stability, the diffuser comprising a manifold plate having a first annular wall region which forms a collar-like region about a central opening, the manifold plate also having a second annular wall region having a series of overlapping fins which are separated by slits which provide a swirling action to combustion air passing through the diffuser, and wherein a plurality of gas orifices are located about an outer periphery of the second annular wall region for conveying and communicating natural gas outward and into a combustion region of the combustion system.

15. The combustion system of claim 14, wherein an air damper communicates with an inlet to the combustion air fan, the air damper comprising a box-like enclosure which houses a single damper blade, the damper blade upper and lower longitudinal sealing edges and opposing side edges, the upper and lower longitudinal sealing edges and opposing side edges being provided with sealing strips which provide ease in adjustment for leakage.

16. The combustion system of claim 15, wherein the head extension connects to a cylindrical gas manifold which, in turn, connects to the boiler sidewall, the gas manifold having radial gas ports used to direct gas fuel to the burner outlet, at least selected ones of the radial gas ports having gas spuds installed therein to improve the distribution of the gas, and wherein the gas manifold holds the outlet end of the burner assembly, the gas manifold having an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate.

17. The combustion system of claim 16, wherein a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat.