Fuel/Air Mixture and Combustion Apparatus and Associated Methods for Use in a Fuel-Fired Heating Apparatus

Abstract

A fuel-fired furnace incorporates specially designed fuel/air mixing and combustion structures. The fuel/air mixing structure is of a mixing sound-attenuating design and includes a venturi having a perforated sidewall portion and being surrounded by a noise-damping housing chamber communicating with the interior of the venturi via its sidewall perforations. During use of the mixing structure, air is flowed through the venturi in a swirling pattern while fuel is transversely injected internally against the swirling air. The combustion structure includes a burner box housing into which the fuel/air mixture is flowed, combusted, and then discharged as hot combustion gas into and through the heat exchanger tubes. The fuel/air mixture entering the burner box housing initially passes through a non-uniformly perforated diffuser plate functioning to substantially alter in a predetermined manner the relative combustion gas flow rates through the heat exchanger tubes.

Description

BACKGROUND

The present invention relates generally to fuel-fired heating apparatus, such as fuel-fired air heating furnaces, and more particularly relates to specially designed fuel/air mixing and combustion sections of such fuel-fired heating apparatus.

In fuel-fired heating appliances such as, for example, furnaces, a known firing method is to flow a fuel/air mixture into a burner box structure in which a suitable ignition device is disposed to combust the fuel/air mixture and thereby create hot combustion gases used to heat air (or another fluid as the case may be) for delivery to a location served by the heating appliance. The hot combustion gases are flowed through a series of heat exchanger tubes, externally across which the fluid to be heated is flowed, and then discharged from the heating appliance into a suitable flue structure. Due to various configurational characteristics of the heating appliance, during firing of the appliance undesirable uneven heating of the combustion product-receiving heat exchanger tubes may occur such that an undesirable non-uniform temperature distribution is present in the overall heat exchanger tube array.

In addition to this potential heat exchange unevenness problem, other problems that may arise in the design of fuel-fired heating appliances include an undesirable noise level generated in the creation of the fuel/air mixture delivered to the burner box, an undesirably low level of mixing of the fuel and air, and an undesirably high level of NOx generated in the fuel/air mixture combustion process.

As can be seen, a need exists for alleviating the above-noted problems associated with conventional fuel-fired heating appliances of various types. It is to this need that the present invention is primarily directed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, foreshortened depiction of a fuel-fired heating apparatus embodying principles of the present invention.

FIG. 2 is a schematic cut-away perspective view of a sound-attenuating primary fuel/air mixing structure portion of the heating apparatus.

FIG. 2A is an exploded, perspective view of the sound attenuating primary fuel/air mixing structure portion shown in FIG. 2.

FIG. 3 is an enlarged scale cross-sectional view taken through a burner box portion of the fuel-fired heating apparatus taken along line 3-3 of FIG. 1.

FIG. 4 is an enlarged scale cross-sectional view taken through a heat exchanger tube portion of the fuel-fired heating apparatus taken along line 4-4 of FIG. 1.

DETAILED DESCRIPTION

A specially designed combustion system 10 of a fuel-fired heating appliance, representatively an air heating furnace 12, is schematically depicted in FIG. 1 and includes, from left to right as viewed in FIG. 1, a primary fuel/air mixing structure 14, a secondary fuel/air mixing structure 16, and a fuel/air mixture combustion structure 18 to which a plurality of heat exchanger tubes 20 (representatively five in number) are operatively connected as later described herein.

Referring to FIGS. 1-2A, the primary fuel/air mixing structure 14 disposed at the left end of the combustion system 10 embodies principles of the present invention and comprises a rectangular housing structure 22 having an outer portion 22a and an inner portion 22b telescoped into the outer portion 22a as may be seen in FIGS. 2 and 2A. Outer housing portion 22a has an inlet end wall 24 and an open outlet end 26. A central circular opening 28 is formed in the inlet end wall 24 and is circumscribed by an annular end wall opening 30 radially across which an circumferentially spaced array of swirl-inducing vanes 32 radially extends. Inner housing portion 22b has open inlet and outlet ends 34, 36 and laterally circumscribes a venturi structure 38 having enlarged open inlet and outlet end portions 40 and 42.

Venturi structure 38 has perforations 44 formed in its sidewall. Representatively, the perforations 44 are formed only in the inlet end portion 40 of the venturi structure 38, but could be located on additional or other portions of the venturi structure sidewall if desired. As shown in FIGS. 1 and 2A, a longitudinal axis 46 extends centrally through the interior of the venturi structure 38. With the inner housing portion 22b telescoped into the outer housing portion 22a, the axis 46 extends centrally through the central housing wall opening 28, and the outlet ends 26, 36 of the housing portions 22a, 22b combinatively define an open outlet end 48 of the overall primary fuel/air mixing structure 14. The inner housing portion 22b defines a sound-attenuating chamber 50 that laterally circumscribes the venturi structure 38 and communicates with its interior via the venturi sidewall perforations 44. In the assembled overall housing 22, a radial fuel injector 52 is operatively received in the central housing wall opening 28, and projects axially into the open inlet end portion 40 of the venturi structure 38 for purposes later described herein.

Turning now to FIG. 1, the secondary fuel/air mixing structure 16 comprises a secondary mixing housing 54 having an open inlet end 56 coupled to the open inlet end 48 of the housing 22, and an open outlet end 58 coupled to the open inlet end 60 of a burner box housing portion 62 of the fuel/air mixture combustion structure 18. Positioned at the juncture between the housings 54 and 62 is a specially designed perforated diffuser plate 64 embodying principles of the present invention and uniquely functioning in a manner later described herein. The housing 62 has a closed right end wall 66 spaced apart from and facing the perforated diffuser plate 64. Positioned between the diffuser plate 64 and the end wall 66 is an igniter 68 operative to ignite a fuel/air mixture entering the housing 62 as later described herein.

The previously mentioned heat exchanger tubes 20 form with the fuel/air mixture combustion structure 18 a heat transfer structure portion of the furnace 12 and have, as viewed in FIG. 1, left inlet end portions coupled to the housing 62 end wall 66 and communicating with the interior of the housing 62. As viewed in FIG. 1, right outlet ends of the heat exchanger tubes 20 are communicated with the interior of a collector box structure 70 within which a draft inducer fan 72 is operatively disposed.

Still referring to FIG. 1, during firing of the furnace 12 the draft inducer fan 72 draws combustion air 74 into the open inlet end portion 40 of the venturi structure 38, across the vanes 32, and then rightwardly through the interior of the venturi structure 38. Vanes 32 cause the combustion air 74 to internally traverse the venturi structure 38 in a swirling pattern 74 a generally centered about the venturi structure longitudinal axis 46. At the same time, the fuel injector 52 receives gaseous fuel via a fuel supply line 76 and responsively discharges gaseous fuel jets 78 radially outwardly into the swirling combustion air 74a. The gaseous fuel in the jets 78 mixes with the swirling combustion air 74a to form therewith a fuel/air mixture 80 that enters the secondary mixing housing 54 and is further mixed therein.

The fuel/air mixture 80 within the secondary mixing housing 54 is then drawn through the perforated diffuser plate 64 into the interior of the burner box housing portion 62 wherein the igniter 68 combusts the fuel/air mixture 80 to form therefrom hot combustion gas 82 that is flowed rightwardly through the heat exchanger tubes 20.

Simultaneously with the flow of hot combustion gas 82 through the heat exchanger tubes 20, a supply air fan portion of the furnace 12 (not shown) flows air 84 to be heated externally across the heat exchanger tubes 20 to receive combustion heat therefrom and create a flow of heated air 84a for delivery to a conditioned space served by the furnace 12. Combustion heat transfer from the heat exchanger tubes 20 to the air 84 causes the tube-entering hot combustion gas 82 to rightwardly exit the heat exchanger tubes 20 as cooled combustion gas 82a that enters the collector box 70 and is expelled therefrom, by the draft inducer fan 72, to a suitable flue structure (not shown).

Compared to conventional fuel/air mixing structures, the venturi-based primary fuel/air mixing structure 14 provides several advantages. For example, due to the cross-flow injection technique utilizing the combustion air 74a swirling through the venturi interior in combination with the radially directed interior fuel jets 78, an improved degree of fuel/air mixing is achieved within the venturi structure 38. This enhanced degree of fuel/air mixing is further increased by the use of the secondary fuel/air mixing structure 16 which serves to further mix the fuel and air by providing further “residence” time for the fuel/air mixture created in the venturi structure 38 before it enters the fuel/air mixture burner box housing 62 for combustion therein.

Additionally, the construction of the primary fuel/air mixing structure 14 substantially reduces the fuel/air mixing noise during both start-up and steady state operation of the furnace 12. In the primary fuel/air mixing structure 14 the perforations 44 in the sidewall of the venturi structure 38 permit the fuel/air mixture traversing it to enter and fill the chamber 50 circumscribing the venturi structure 38. This creates within the chamber 50 a fluid damping volume that absorbs and damps noise-creating fluid pressure oscillations in the venturi interior, thereby desirably lessening the operational sound level of the primary fuel/air mixing structure 14. Moreover, the enhanced mixing of the fuel/air mixture to be combusted desirably reduces the level of NOx emissions created by the furnace 12 during firing thereof.

As may best be seen in FIG. 4, the draft inducer fan 72 is representatively centered in a left-to-right direction within the collector box 70 and with respect to the five illustratively depicted heat exchanger tubes 20. Accordingly, the suction force of the fan 72 is similarly centered relative to the array of heat exchanger tubes 20. Without the incorporation in the furnace 12 of a subsequently described feature of the present invention, the result would be that the per-tube flow of hot combustion gas 82 is greater for the central tubes 20b than it is for the end tubes 22a. In turn, this would create an undesirable non-uniform temperature distribution across the heat exchanger tube array, with the central tubes 20b having higher operating temperatures than those of the end tubes 20a.

With reference now to FIGS. 1 and 3, the previously mentioned diffuser plate 64 installed at the juncture between the secondary fuel/air mixing housing 54 and the burner box housing 62 representatively has an elongated rectangular shape, and is substantially aligned with the open inlet ends of the heat exchanger tubes 20. Along substantially the entire length of the diffuser plate 64 are formed a series of relatively small perforations 86 (see FIG. 3), with relatively larger perforations 88 being additionally formed through the opposite end portions of the diffuser plate 64. This perforation pattern, as can be seen, provides opposite end portions of the diffuser plate 64 (which are generally aligned with the inlets of the end heat exchanger tubes 20 a) with greater fuel/air mixture through-flow areas than the diffuser plate fuel/air mixture through-flow areas aligned with the inlets of the central heat exchanger tubes 20b.

Accordingly, during firing of the furnace 12, the presence of the diffuser plate 64 lessens the flow of hot combustion gas 82 through the central heat exchanger tubes 20b and increases the flow of hot combustion gas 82 through the end heat exchanger tubes 20a, with the perforation pattern in the diffuser plate 64 functioning to substantially alleviate non-uniform temperature distribution across the heat exchanger tube array that might otherwise occur. As can readily be seen, principles of the present invention provide a simple and quite inexpensive solution to the potential problem of non-uniform temperature distribution across the heat exchanger tube array. Additionally, in developing the present invention it has been discovered that the use of the non-uniformly perforated diffuser plate 64 also provides for further mixing of the fuel/air mixture 80 entering the burner box housing 62, thereby providing an additional beneficial reduction in the NOx level of the discharged combustion gas 82a.

While a particular hole pattern in the diffuser plate has been representatively described herein, it will be readily appreciated by those of ordinary skill in this particular art that a variety of alternative hole patterns and sizes may be alternatively be utilized if desired. For example, while a combination of different size perforation has been representatively illustrated and described, the perforations could be of uniform size but with more perforations/area being disposed on the opposite ends of the diffuser plate 64 than in the longitudinally intermediate portion thereof. Further, the hole pattern could be a non-uniformly spaced pattern to suit the particular application. Additionally, if desired, the diffuser plate hole pattern could have a different overall configuration operative to alter in a predetermined, different manner the relative combustion gas flow rates through selected ones of the heat exchanger tubes 20.

While principles of the present invention have been representatively illustrated and described herein as being incorporated in a fuel-fired air heating furnace, a combustion system utilizing such invention principles could also be incorporated to advantage in the combustion systems of a wide variety of other types of fuel-fired heating apparatus using fire tube-type heat exchangers to heat either a gas or a liquid.

The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.

Claims

1. A method of mixing fuel and air in a fuel-fired heating apparatus, the method comprising: providing a venturi structure having a longitudinal axis extending through its interior, sidewall perforations, and first and second opposite end portions;forming around the venturi structure a chamber that communicates with the interior of the venturi structure through its sidewall perforations;creating a flow of air that flows through the interior of the venturi structure from its first end portion to its second end portion while swirling about its longitudinal axis;creating a flow of gaseous fuel that interiorly impacts and mixes with the swirling flow of air in a direction transverse to the longitudinal axis; andutilizing the chamber to damp pressure oscillations within the venturi structure in a manner attenuating fuel/air mixing noise generated within the venturi structure.

2. The method of claim 1, wherein the venturi structure comprises a sidewall that extends from the first end portion to the second end portion such that the sidewall tapers from the first end portion and the second end portion towards a substantially mid-portion of the venturi structure.

3. The method of claim 2, wherein the sidewall perforations are circumferentially disposed around the side wall from adjacent the venturi inlet to the substantially mid-portion of venturi structure.

4. The method of claim 1, wherein the chamber laterally extends around the venturi structure and communicates with an interior of the venturi structure through the sidewall perforations such that a fuel and air mixture traversing the sidewall perforations enters and fills the chamber to create a noise attenuating volume that damps the pressure oscillations within the venturi structure in a manner that attenuates the fuel/air mixing noise generated within the venturi structure.

5. The method of claim 1, wherein a vane structure disposed on a housing that houses the venturi structure causes the flow of air to swirl about the longitudinal axis of the venturi structure.

6. A method of sound-attenuation in a fuel-fired furnace, the method comprising: creating a flow of air through an interior of a venturi structure having (i) a longitudinal axis extending through its interior, (ii) sidewall perforations, and (iii) first and second opposite end portions, while swirling about its longitudinal axis;creating a flow of gaseous fuel that impacts and mixes with the swirling flow of air in a direction transverse to the longitudinal axis; anddampening pressure oscillations within the venturi structure, via a chamber structure disposed around the venturi structure, wherein the chamber is in fluid communication with the interior of the venturi structure through its sidewall perforations, in a manner effective to attenuate fuel/air mixing noise generated within the venturi structure.

7. The method of claim 6, wherein the venturi structure comprises a sidewall that extends from the first end portion to the second end portion such that the sidewall tapers from the first end portion and the second end portion towards a substantially mid-portion of the venturi structure.

8. The method of claim 7, wherein the sidewall perforations are circumferentially disposed around the side wall from adjacent the venturi inlet to the substantially mid-portion of venturi structure.

9. The method of claim 8, wherein the chamber laterally extends around the venturi structure and a fuel and air mixture traversing the sidewall perforations enters and fills the chamber to create a noise attenuating volume.

10. The method of claim 9, wherein the swirling of the flow through the interior of the venturi structure about its longitudinal axis is caused by a vane structure.

11. The method of claim 6, wherein the flow of fuel is provided by radially directed interior fuel jets disposed within the first end portion of the venturi structure.

12. The method of claim 11, wherein a fuel/air mixture exiting the venturi structure flows through a secondary fuel/air mixing structure before entering a fuel/air mixture burner box housing for combustion therein.

13. The method of claim 12, wherein the fuel/air mixture has enhanced mixing of the fuel/air mixture to be combusted desirably reduces a level of NOx emissions created by the furnace 12 during firing thereof.

14. A method for reducing a NOx level of discharged combustion gases in a fuel-fired heating apparatus, the method comprising: creating, via a vane structure, a flow of air through an interior of a venturi structure having (i) a longitudinal axis extending through its interior, (ii) sidewall perforations, and (iii) first and second opposite end portions, while swirling about its longitudinal axis;creating, via radially directed interior fuel jets, a flow of gaseous fuel that mixes with the swirling flow of air in a direction transverse to the longitudinal axis; anddampening pressure oscillations within the venturi structure, via a chamber structure disposed around the venturi structure, wherein the chamber is in fluid communication with the interior of the venturi structure through its sidewall perforations;flowing a fuel/air mixture exiting the venturi structure through a secondary fuel/air mixing structure; and thenflowing the fuel/air mixture into a burner box of the fuel-fired heating apparatus,wherein the secondary fuel/air mixing structure is effective to further mix the fuel and air by providing further residence time for the fuel/air mixture created in the venturi structure housing, which is effective to reduce the level of NOx emissions created by the fuel-fired heating apparatus during firing thereof.

15. The method of claim 14, wherein the secondary fuel/air mixing structure comprises a diffuser plate.

16. The method of claim 15, wherein the burner box is coupled to plurality of heat exchanger tubes comprising one set of heat exchanger tubes and another set of heat exchanger tubes.

17. The method of claim 16, wherein the diffuser plate comprises a first set of holes and a second set of holes, wherein the first set of holes are larger than the second set of holes, and wherein the diffuser plate is disposed such that the first set of holes face the one set of heat exchanger tubes and the second set of holes face the one set of heat exchanger tubes and the another set of heat exchanger tubes.

18. The method of claim 14, wherein the sidewall perforations are circumferentially disposed around the side wall from adjacent the venturi inlet to the substantially mid-portion of venturi structure.

19. The method of claim 18, wherein the chamber laterally extends around the venturi structure and a fuel and air mixture traversing the sidewall perforations enters and fills the chamber to create a noise attenuating volume.

20. The method of claim 16, wherein the heat exchanger tubes are arranged in a linear array and have inlets in fluid communication with the interior of the burner box through the outlet end of the burner box for receiving the hot combustion gas generated within the interior of the burner box, and wherein the outlets of the heat exchanger tubes are coupled to a collector box.