US 3210059 A
Description (OCR text may contain errors)
1965 J. D. NESBITT ETAL 3,210,059
AIR HEATING BURNER 5 Sheets-Sheet l Filed June 20, 1955 INVENTORS. JUHNDNEsBITT. BY. I/IZILLABEAJAMBB.
DJZWMCX Oct. 5, 1965 J, NESBITT ETAL 3,210,059
AIR HEATING BURNER Filed June 20, 1955 3 Sheets-Sheet 2 I I a [I I], i I
I l 37 I 37 3/ 7.2:); l
JuHN E. NESBITT, Y WALLAEE AJAMEs.
Oct. 5, 1965 J, D. NESBITT ETAL 3,210,059
AIR HEATING BURNER 5 Sheets-Sheet 5 Filed June 20, 1965 INVENTORS J mm D. NLE'BIT T, By WALLACE A. JAME s.
m 'ATT Y5 FEW- United States Patent 3,210,059 AIR HEATING BURNER John D. Neshitt and Wallace A. James, Toledo, Ohio, assignors to Midland-Ross Corporation, Toledo, Ohio, a corporation of Ohio Filed June 20, 1963, Ser. No. 289,324 Claims. (Cl. 263-19) This invention relates to a burner for heating a stream of air. More particularly, the invention relates to a burner adapted to be positioned in a duct through which the stream of air to be heated is circulated.
Among the applications for air heating burners of the present invention is a class of equipment commonly called make-up air units. Equipment of this type is used to supply replacement or make-up air to commercial or industrial buildings to replace the air exhausted through one or more exhaust systems, as for the removal of fumes, to maintain pressure in the building at the desired value. A make-up air unit normally comprises a duct, having an inlet adapted to receive fresh air from outside the building and an outlet adapted to discharge the air at an appropriate point in the building; blower means adapted to circulate air from the inlet to the outlet; and burner means adapted to heat the air stream as it flows through the duct.
As is explained more fully in Canadian Patent 660,757, the type of air heating burner which was most widely used in prior art make-up air units, at least until a few years ago, is the type known as a pre-mix line burner. Because of the inherent drawbacks in pre-mix line burners for air heating, which are extensively described in the aforesaid Canadian patent, there has been much effort expended in recent years to develop improved air heating apparatus for make-up and other air heating applications. Much of this recent activity has been directed at the development of a nozzle-mixing type of burner capable of drawing all, or as much as possible, of its air for combustion from the relatively low pressure stream of makeup air, to eliminate the expense of a separate combustion air supply blower, but without impairing the turndown characteristics of the burner and without resorting to long flame combustion characteristic of prior art nozzle mixing combustion apparatus when operated on low pressure air.
For a further understanding of the invention, attention is directed to the following portion of the specification, the drawing, and the appended claims.
In the drawing:
FIG. 1 is a plan sectional view of a make-up air supply unit incorporating a preferred embodiment of the invention;
FIG. 2 is a sectional view taken on line 22 of FIG. 1;
FIG. 3 is a plan view of the burner of FIGS. 1 and 2;
FIG. 4 is a rear elevational view of the burner of FIGS. 1 and 2;
FIG. 5 is a sectional view taken on line 55 of FIG. 3;
FIG. 6 is a sectional view taken on line 66 of FIG. 5; and
FIG. 7 is a sectional view taken on line 77 of FIG. 5.
In accordance with the present invention there is provided novel burner apparatus 11 for heating a stream of air (or other fluid comprising a substantial fraction of air such as recirculated oven gases) which is caused to flow through a duct 12 by blower means 13. Duct 12, which is defined by wall means 14 and which has an inlet end 15 and an outlet end 16, may be constructed in a rectangular fashion as shown or in any other desired fashion, such as circular, to suit the needs of the installation. Inlet end 15 of duct 12 is adapted to be exposed to a source of air to be heated, as by locating it exteriorly of the building in which the remainder of duct 12 is located, and may be provided with filter means and/ or damper means 3,21%,959 Patented 0st. 5, 1965 (not shown) in accordance with the preference of the designer.
Blower means 13 is illustrated as comprising two centrifugal blowers whose impellers are mounted on a common shaft 17 which is adapted to be rotated by appropriate drive means, not shown. It is to be understood, however, that a greater or lesser number of centrifugal blowers may be employed, depending mainly on the width of the duct, or an axial flow blower may be used, as in the case of a circular duct. Likewise, blower means 13 may be situated upstream of burner apparatus 11 instead of downstream as shown.
Burner apparatus 11, per se, is of the nozzle mixing type and comprises combustion stabilizing means 21, primary mixing means 22 disposed downstream of stabilizing means 21, and secondary mixing means 23 disposed downstream of primary mixing means 22. Combustion stabilizing means 21, which receives fuel from conduit 46, serves to mix a portion of the fuel stream with air, preferably drawn from the stream in duct 12, and to ignite the mixture to establish a stable flame front. Primary mixing means 22 serves to mix an additional portion of the fuel stream with air from the stream in duct 12 in such a manner that the additional mixture will be ignited by, and serve to augment, the flame from means 21. Secondary mixing means 23 serves to mix the remainder of the fuel stream with air from the stream in duct 12 to complete the combustion reaction. Because the flame from stabilizing means 21 is derived from the combustion of only a small portion of the fuel supply it will be relatively weak and readily extinguishable. Accordingly, it is important that the mixing action of primary mixing means 22 be relatively slow to avoid flame quenching and its associated problems. The flame which ultimately issues from primary mixing means 22, on the other hand, will be considerably stronger and less apt to be extinguished than the flame from stabilizing means 21, due to the fact that it is derived from the combustion of a larger portion of the fuel stream. Accordingly, it is important that the mixing action of secondary mixing means 23 be as rapid as possible to minimize the length of the flame extending therefrom.
In regard to the construction of flame stabilizing means 21, it would be feasible to employ the principle of stabilizing by means of premix piloting, a principle familiar to skilled artisans. For simplicity of construction and installation, however, it is preferable to employ the principle of primary air spin self stabilization described in Canadian Patent 651,907. Accordingly, stabilizing means 21 is provided with a chamber 25, defined by backplate 26 and circumferential wall 27, into which fuel is introduced in an axial direction from fuel nozzle means 28, in an annular pattern for reasons to be discussed subsequently. A small portion of the air required for stoichiometric combustion of the fuel introduced from nozzle means 28 when flowing at the maximum rate, preferably 410% thereof, is introduced tangentially to chamber 25 through one or more inlet conduits 29 whose axis is disposed perpendicularly to a plane through the axis of chamber 25.
The air which is added to chamber 25 through inlet conduits 29 may, of course, be provided by a separate air blower. However, from an economic standpoint, it is preferred that this air be drawn from the stream flowing in duct 12. To cause suflicient air from duct 12 to flow into chamber 25 through inlets 29 it is necessary that the air flow in duct 12 be restricted in the vicinity of inlets 29 to provide a modest drop in static pressure from the entrance to inlet 29 to the point where the fluid passing thereinto is ultimately mixed, in reacted or partially reacted state, with additional air from the stream of air from duct 12. This pressure drop ought to be of the order of 0.3 to 0.7 inch water column. Such a pressure drop can be obtained by restricting the flow of air in duct 12 to a velocity of the order of 1400 to 2600 feet per minute at a point immediately downstream of the entrance to inlets 29. In the illustrated embodiment the needed degree of restriction of the flow of air in duct 12 is obtained by means of burner apparatus 11 itself in combination with baffles 31 disposed in a plane extending generally transversely of duct 12 and located slightly downstream of the entrances to inlets 29. It is to be understood, however, that baflies 31 may be omitted where the area occupied by burner apparatus is sufficiently large in relationship to the area of duct 12 to provide an adequate degree of flow restriction. It is also to be noted that the admission of air to inlets 29 can be somewhat further enhanced by the relatively low degree of velocity pressure of the fluid in duct 12 by providing a scoop portion 29:: at the outer extremity of each inlet 29. Scoop portion 29a is provided with an inlet opening 32 disposed transversely of duct 12 and is exposed to the flow of air passing therethrough.
As is explained in the aforementioned Canadian Patent 651,907, there is formed in chamber 25, immediately downstream of backplate 26, a readily ignitable air-fuel mixture at the interface of the streams of spinning air and axially flowing fuel. When once ignited, as by a sparkplug (not shown) extending through backplate 26, this mixture will continue to burn with great stability thereby obviating the need for premix pilot burners frequently employed to stabilize prior art nozzle mixing burners.
Primary mixing means 22 comprises a generally circumferential wall 33 defining a chamber 30 open at both ends, and axially aligned with stabilization means 21. Wall 33 serves as a continuation of wall 27 of chamber 25, is provided with a number of ports 34, and defines a chamber 30 which increases in area as it proceeds downstream from stabilizing means 21 to allow for the admission of air from the stream in duct 12 to the volume encompassed by wall 33 by virtue of the inherent small static pressure drop across ports 34 and by virtue of the small velocity pressure of the stream. Ports 34 are arranged in a plurality of circumferential stages. It has been found advantageous to locate the first stage of ports at a distance from backplate 26 of chamber 25 equal to a multiple of 2 /23 of the width of inlet conduit 29. Wall 33 is constructed in divergent fashion to allow for some scooping of air from the passing stream and to provide a layer of air between wall 33 and unreacted fuel to minimize contact between said fuel and wall 33 which could lead to the formation of undesirable carbon deposits on wall 33.
While the amount of air that will enter chamber 30 through ports 34 will be quite substantial, the rate of mixing between this air and the unmixed fuel from nozzle means 28 will be relatively slow because of the relatively high ratio of free stream velocity pressure to static pressure gradient across ports 34. Because the free stream velocity is the major force causing air to enter ports 34 and because the forces tending to direct the stream normally to wall 33 are small, the velocity vector of the portion of the air stream which enters chamber 30 remains substantially unchanged except for a slight decrease .in magnitude due to frictional losses. Consequently, the direction of the air jets entering cham ber 30 through ports 34 is substantially that of the external free air stream and is more nearly parallel to the axis of chamber 30 than normal to wall 33. Because the angle of inclination between the forwardly moving stream of reacting air and fuel and the air stream entering chamber 30 through ports 34 is quite small, the rate of mixing of these streams will be quite slow. Hence, the rate at which combustion is advanced in mixing means 22 by virtue of addition of fresh combustible mixture to the flame from stabilizing means 21 will, quite desirably, be relatively slow. The length of primary mixing means 22 need only be suflicient to maintain continuous ignition of a combustible mixture of air and a sufiicient portion of the fuel from nozzle 28 that the flame temperature Will be sufliciently high to resist quenching in subsequent more rapid mixing means 23. In practice, it is sufficient it about 30-50% of the fuel from nozzle 28 has mixed with a combustion-supporting degree of air by the time it emerges from mixing means 22.
The mixing action imported by secondary mixing means 23 between the unmixed portion of the fuel and air from the duct must be very rapid in spite of the low energy of the relatively slow-moving air in order to result in sufliciently short flame combustion to avoid the need for locating burner apparatus excessively far upstream of outlet end 16 of duct 12. It has been found that satisfactorily rapid mixing can be obtained when secondary mixing means 23 comprises a mixing bafiie 35 disposed downstream of primary mixing means 22 extending generally transversely of duct 12 and against which the fluids (comprising products of combustion, burning mixture, fuel, and air) passing from mixing means 22 are impinged. The turbulence imparted to the fluids from mixing means 22 as a result of the rapid change in direction caused by baflie 35 has been found to result in much. shorter flame combustion than is obtainable with any known prior art concentrated input nozzle mix air stream heating burner. In order to ob tain uniform fluid flow off mixing baflie 35 it is important that it be of a regular geometric configuration (e.g., circular, square, rectangular, etc.) and that it be axially aligned with mixing means 22. Preferably, the configuration of mixing baflie is similar to that of the opening in duct 12 defined by restricting bafiles 31. In the illustrated embodiment the opening in duct 12 defined by bafi'les 31 is rectangular and mixing baffle 35 is likewise rectangularly shaped. It has also been found advantageous in obtaining uniform fluid flow off mixing baffle 35 to construct mixing bafiie in the configuration of a V with the apex 35a pointing upstream and being axially aligned with mixing means 22. For instance, it has been found that a V-shaped mixing batfle with an included angle of between the legs, when so positioned, is somewhat superior to a flat mixing bafiie.
It has previously been mentioned that it is important to maintain a close degree of axial alignment between mixing bafiie 35 and mixing means 22. It has also been found that there will be an optimum spacing therebetween for each particular burner apparatus. Accordingly, it has been found that closer control over the alignment and spacing of mixing bafiie 35 and mixing means 22 can be had by providing attachment means for the direct attachment of mixing baffle 35 to mixing means 22 rather than, for instance, attaching mixing baffle directly to wall means 14 of duct 12. Accordingly, there is provided attachment means comprising a plurality (preferably 4) of elongate members 36 attached at their downstream ends to mixing bafile 35 and attached at an upstream point, by means of inwardly extending ribs 37 to an outwardly extending rim 38 at the outlet of circumferential wall 33 of mixing means 22. The upstream ends of elongate members 36 are attached to a structural frame defined by spaced apart structural members 39. In the preferred embodiment, baflies 31 are continued from the sides of duct 12 all the way out to structural members 39 and the spacing between members 39 defines the profile opening in duct 12 which, in turn, establishes the desired 1400 to 2600 feet per minute velocity. By virtue of this arrangement, alignment of all critical members, including mixing means 22 and 23 and baffle means 31, is established by elongate members 36. It has also been found that the operating temperatures of elongate members 36 can be kept suitably low to permit them to be constructed of relatively inexpensive material (e.g., aluminized steel) if they are of hollow construction and open to the flow of cool air from duct 12. Annular flange 38 serves an additional function in that it provides for additional mixing of those peripheral portions of the fluids emerging from mixing means 22 which are apt to by-pass mixing baflle 35 of mixing means 23.
It has previously been mentioned that the fuel which is delivered to chamber 25 from muel nozzle means 28 flows in an annular stream. The purpose of this is to provide for the admission of air to the interior of primary mixing means 22 by way of center air inlet tube 41 which is open at both its upstream and downstream ends. The downstream end of inlet tube 41 is located within chamber 30 sufliciently far downstream from back wall 26 of stabilizing means 21 soas to prevent the air admitted thereby from interfering with the flame stabilizing of means 21. The admission of air to the interior of means 22 in this manner is advantageous in a number of respects: it provides a layer of cool air immediately adjacent the surface of mixing baflle 35, thereby minimizing its operating temperature and allowing it to be constructed of less expensive materials; and it increases the area of the fuel stream in contact with air thereby to increase the rate of mixing, both in secondary mixing means 23 and also in primary means 22, without increasing the risk of flame quenching in mixing means 22. The layer of cool air immediately adjacent mixing baffle 35 also keeps unreacted fuel from contact with the baflle and keeps carbon deposits fro-m forming thereon.
For simplicity of construction, the upstream end of center air inlet tube 41 forms a portion of fuel nozzle means 28 which also comprises outer circumferential wall 42 and axially spaced, radially extending annular walls 43 and 44 forming an annular chamber 45 with wall 42 and the upstream end of inlet tube 42. Fuel is delivered to annular chamber 45 by suitable piping means 46 and passes from chamber 45 into chamber 25 of stabilizing means 21 through ports 47 arranged in a circumferential pattern. It is to be noted that while all of the fuel for combustion is shown as being admitted to stabilizing means 21 by means of nozzle 28, it is also conceivable that some of the fuel could be added to burner 11 at points downstream of stabilizing means 21.
As an additional feature of the invention, it has been found beneficial in minimizing the number of spin air inlets 29 needed for satisfactory stabilization to employ a frusto conical baffle 48 within chamber 25 having its inner periphery disposed upstream attached to center air inlet tube 41, preferably closely adjacent backplate 44.
As will be evident to those skilled in the art, the invention herein can be employed in other embodiments and various modifications can be made to the illustrated embodiment without departing from the spirit or scope of the following claims.
1. In combination with a duct, burner apparatus for heating a stream of air flowing in the duct from an upstream end thereof to a downstream end thereof, said burner apparatus being operably positionable in the duct intermediate the upstream and the downstream ends thereof and comprising, in combination; fuel supply means for delivering a stream of fuel to the burner apparatus; annular fuel nozzle means for receiving a portion of the fuel stream; flame stabilizing means comprising a generally circular outer wall defining a stabilizing chamber and an air inlet conduit for delivering air tangentially to said stabilizing chamber and in a plane substantially perpendicular to an axial plane of the stabilizing chamber, said stabilizing chamber being located downstream of the fuel nozzle means for receiving fuel therefrom and for initiating and stabilizing the combustible reaction of at least a portion of the fuel stream; primary mixing means comprising an elongate perforate wall defining a primary mixing chamber having an open upstream end in communication with said stabilizing chamber and an open downstream end, said primary mixing means sustaining a combustible reaction by mixing another portion of the fuel stream with air from the stream flowing in the duct at a relatively slow rate; center air inlet means comprising a tube extending through the stabilizing chamber to prevent the air flowing in the tube from passing into said stabilizing chamber, and for admitting air from the stream in the duct to the interior of the primary mixing chamber to flow generally coaxially therethrough, said air in the tube being parallel to the axis of said stabilizing chamber and said annular fuel nozzle means circumposing said center air inlet tube and delivering fuel in an annular pattern; and secondary mixing means downstream of the primary mixing means for further sustaining said combustible reaction by mixing yet another portion of the fuel stream with air from the stream flowing from the duct at a relatively rapid rate.
2. Apparatus according to claim 1 comprising a plurality of said air inlet conduits circumferentially spaced with respect to the flame stabilizing chamber and further comprising an air inlet scoop extending from an upstream portion of each air inlet conduit and having an entrance opening adapted to be disposed generally transversely of the direction of flow of the stream of air in the duct.
3. In combination with a duct, burner apparatus for heating a stream of air flowing in the duct from an upstream end thereof to a downstream end thereof, said burner apparatus being operably positioned in the duct intermediate the upstream and downstream ends thereof and comprising, in combination: fuel supply means for delivering a stream of fuel to the burner; annular fuel nozzle means for receiving at least a portion of the fuel stream; flame stabilizing means downstream of the fuel nozzle means for receiving an annular stream therefrom and for initiating and stabilizing the combustible reaction of at least a portion of the annular fuel stream; perforate wall means defining a primary mixing chamber having an open upstream end in communication with the stabilizing means and an open downstream end; bafile means in the duct disposed transversely of the burner for directing the air flowing in the duct past the burner and for increasing the velocity thereof to 1400 to 2600 feet per minute; mixing baflle means disposed downstream of the open downstream end of the primary mixing chamber and transversely thereof and constituting secondary mixing means; and means for interposing a layer of cool air adjacent the upstream side of the mixing baflle intermediate the baflle and fuel flowing from the primary mixing chamber.
4. Apparatus according to claim 3 and further comprising an annular rim extending generally radially outwardly from the perforate wall means at a locus adjacent the open downstream end of the mixing chamber defined by said perforate wall means.
5. Apparatus according to claim 3 comprising spaced apart structural members, wherein the baflle means in the duct extend from the duct all the way to the spaced apart member, and further comprising attachment means to which the perforate wall means and the mixing baffle means are attached and which, in turn, are attached to the spaced apart structural members.
6. Apparatus according to claim 5 wherein the attachment means comprise a plurality of hollow tubes.
7. Apparatus according to claim 3 wherein said means for interposing a layer of cool air comprises a center air inlet tube disposed axially of the primary mixing chamber, having an upstream end open to the flow of air in the duct, passing through the interior of the annular fuel nozzle means, and terminating downstream of the flame stabilizing means.
8. Apparatus according to claim 7 wherein the flame stabilizing means comprises a flame stabilizing chamber, wherein the annular fuel nozzle means is coaxial with the flame stabilizing chamber, and further comprising air inlet conduit means for introducing air tangentially to the flame stabilizing chamber and perpendicular to an axial plane of the flame stabilizing chamber.
9. Apparatus according to claims and further comprising a frusto-conical baffle disposed within the flame stabilizing chamber and having its inner periphery disposed upstream and attached to the center air inlet tube.
10. Apparatus according to claim 3 wherein the perforate Wall means diverges toward the downstream end. 10
References Cited by the Examiner UNITED STATES PATENTS 9/62 Lindsay 26319 8/63 Varvel 263--l9