Dual fuel burner

Abstract

Airstream type burner capable of burning either gas or oil fuel or both, and having very high turn down ratio and good low rate burning characteristics on either fuel. Burner is used either in a make-up air duct or as a combustion head in a heat exchange unit. Burner has conical burner cone having a plurality of circumferential rows of air apertures along its length for receiving combustion air from the moving environmental airstream; an ultrasonic type oil fuel nozzle mounted centrally of a flame retention head at the narrow, or head end of the burner cone; an exterior, annular-shaped gas manifold which feeds raw gas through a circumferential row of gas ports adjacent to the head end of the cone and against the exterior of a cylindrical gas inlet baffle ring attached within, and aligned with the axis of the burner cone; a coneshaped diffuser mounted centrally of the forward or downstream end of the burner cone; a cylindrical air curtain baffle ring attached within the downstream end of the burner cone in concentrically surrounding spaced relation with the diffuser for establishing a cylindrical air curtain emanating from the last circumferential row of air apertures in the burner cone and projecting downstream from the forward end of the cone; and a cylindrical housing concentrically surrounding the burner cone and attached thereto by an annular flange at the exterior of the downstream end of the latter.

Description

This invention relates to air heating burners for space heating or make-up air applications, and more particularly to burners which burn either natural or other gas or fuel oil.

The invention was made while attempting to provide a dual fuel burner suitable for use as a direct heating airstream burner for space heating purposes, but was discovered to be also usable as a combustion head in an indirect type of primary space heater such as described in U.S. Pat. No. 3,695,250 (Rohrs et al.). Accordingly, and although it may have other uses, the invention will be described in connection with such uses.

Direct-heating airstream burners are placed in air supply ducts to heat the air being furnished as make-up air to factory buildings and similar places where industrial processes and concentrations of personnel absorb much of the air within the building. Such heaters must be capable of close temperature control over a wide range of summer and winter temperatures to avoid discomfort to workers, and must not contaminate the air being heated. Regarding the latter, the quality of combustion which the burner must constantly produce is such as will provide make-up air free from unacceptable levels of carbon monoxide, nitrogen dioxide, sulfur dioxide, aldehydes and particulates. Very effective airstream burners which burn either raw or premixed natural gas are known. See, for example, U.S. Pats. Nos. Re.25,626 (Yeo et al.) and 3,405,921 (Rohrs).

However, it is desirable that the same burner be capable of burning fuel oil as well as natural and other gas so that, as often happens in commercial and industrial establishments, during a required curtailment of natural or other gas use the burner may be switched over to an oil fuel supply and thus continue its normal operation. The difficulty of burning oil in an airstream burner used for space heating purposes is primarily that of obtaining complete combustion of the oil fuel so that the heated air, downwstream of the burner, is pure enough for humans to breathe with complete safety. A secondary difficulty is the obtaining of the very high turndown ratios necessary in such burners to afford a wide range of consistently efficient operation between conditions of low heat and high demand when burning either gas or oil.

Dual fuel airstream burners are known (for example, that shown in U.S. Pat. No. 4,682,451 (Vosper), but it is not known whether their combustion products are pure enough such that the heated airstream can be used for space heating purposes. However, it is intended by the present invention to provide an airstream type burner wherein combustion is virtually complete, whether gas or oil or a combination of both is used as a fuel, such that the directly heated air can be safely discharged into a room or the like for breathing by humans.

It is further intended to provide in such an airstream burner an extremely high turndown ratio between its maximum firing rate and a very low firing rate, whether the burner be fired using oil or gas or both. In this regard, the dual fuel burner of the present invention has been found to have a turndown ratio of about 40 to 1 using natural gas, and about 30 to 1 using fuel oil so that, for a preferred embodiment as will be described and which has a maximum firing rate of 2.5 million Btus per hour the low firing rates are 62,000 and 83,000 Btus per hour, respectively.

In addition to these characteristics, the burner must not be unduly complex so that it is relatively inexpensive to make, and dependable and maintenance-free in operation.

The same possibilities exist for curtailment of natural and other gas supplies used in other types of heat exchangers, such as in space heater units of the indirect type. Accordingly, it is desirable to have such units fired by burners which can operate efficiently on either gas or oil, or a mixture of both, and many such combustion heads are known. However, such combustion heads are normally considered as being of a different nature from burners of the airstream type, and it has not been generally believed that the same burner can be used either as a direct heater in an airstream or to fire a unit heater, with comparable efficiencies, especially in instances where the burner is capable of burning either gas or oil. In this regard, it has been discovered that the burner provided by the present invention can be used as a combustion head in indirect space heater units, and probably will find similar application in boilers, furnaces, industrial ovens, incinerators and other units whenever a blower of suitable capacity can be included in the arrangement to produce the required flow of combustion air through the burner.

Briefly describing the invention in its preferred embodiment, the dual fuel burner has a perforated burner cone or flame basket by which primary air is introduced for burning either raw natural or other gas or fuel oil emitted within the cone at its small end. Gas is furnished through an annular series of gas supply ports from an exterior gas manifold either adjacent to or surrounding the small end of the cone, and is received within a narrow annular V-shaped region between the interior surface of the burner cone and the exterior surface of a cylindrical gas inlet baffle ring which is attached within, and projects only a few inches forward of the headplate at the small end of the cone. Fuel oil is introduced via an oil nozzle, preferably of the sonic type, mounted on the headplate centrally of the gas inlet baffle ring. Although not essential, in order to reduce the length of the burner flame when burning oil, the headplate has radially disposed, relatively flat louvers about the oil nozzle, thus to serve as a flame retention head or turbulator affording a tight vortex patten of the air admitted therethrough.

Primary combustion air is received through the aforementioned perforations of the cone, these apertures being arranged in a number of circular rows of apertures of increasing size extending from the small end to the large end of the burner cone. In the preferred embodiment, some thirty-two air apertures are arranged circumferentially in each row, and the total of the areas of the apertures in each row is such as to admit, at that location along the length of the cone, the requisite amount of primary combustion air for that stage of the combustion. In addition, a small quantity of combustion air for low rate operation is received through a central aperture of the headplate within which the fuel oil nozzle is mounted. The air is received from the moving airstream within which the burner is situated, and the burner is provided with a surrounding cylindrical housing extending the length of the burner cone to provide uniform distribution of the air passing through the cone apertures.

The last row of apertures at the large end of the burner cone is employed to provide a cylindrical-shaped air curtain extending downstream of the burner, there being a forwardly projecting air curtain baffle ring attached within the cone on the upstream side of this last row of air apertures for the purpose. Thus, the emanating flame is not permitted to be dispersed laterally, but is contained within the projected forward area of the burner.

Combustion efficiency of the burner is further increased by a conical-shaped diffuser at the downstream end of the burner cone to assist in establishing a conical pattern of the air and fuel mixture, urging it outward towards the interior surface of the burner cone, as it courses the length of the latter.

Because this conical diffuser tends to spread the flame pattern in a manner tending to overcome the immediate effects of the aforementioned cylindrical-shaped air curtain as the flame emerges from the burner cone, the previously described air curtain baffle ring is made longer, extending some two inches forward of the large end of the burner cone, to redirect the flame inwardly. Beyond the downstream end of this air curtain baffle ring, the cylindrical curtain of air has its desired effect.

These and other objects, features and advantages of the invention will become apparent from the following detailed description of the invention, when taken together with the accompanying drawings in which:

FIG. 1 is a fragmentary side sectional showing of an airstream duct in which air to be heated for space heating purposes is flowing, and having a burner in accordance with the invention mounted centrally thereof;

FIG. 2 is a sectional side view of a dual fuel burner in accordance with the invention;

FIG. 3 is a view of the burner as seen from its upstream end at lines 3--3 of FIG. 2;

FIG. 4 is a view of the burner from its downstream end at lines 4--4 of FIG. 2;

FIG. 5 is a sectional side elevation of a space heater unit incorporating a dual fuel burner in accordance with the invention;

FIG. 6 is a transverse sectional view of the space heater as seen from lines 6--6 of FIG. 5; and

FIG. 7 is a fragmentary plan view of the space heater as seen from lines 7--7 of FIG. 5.

Referring to FIG. 1, a typical dual fuel burner in accordance with the invention, capable of delivering 2.5 million Btu per hour, is generally indicated by reference numeral 10. It is shown mounted centrally of a 48 inch square air duct 11 for use as a direct-heating burner for heating a flowing airstream indicated by the arrows 12, to heat the interior of a building or similar space. The forward or downstream end 10a of the burner is transversely aligned with a conventional profile plate 13 which is attached to, and extends peripherally about the interior of the air duct 11 and provides a 42 inch square opening. FIG. 1 also illustrates the "tight", heart-shaped flame pattern, indicated by reference numeral 14, which is emitted at the downstream end of the burner 10 when operating at full capacity on either gas or oil, and the distinct, upstanding flame pattens 14a which appear within the flame basket or cone 15 of the burner at low firing rates.

Referring to FIGS. 2-4 in which the construction of the burner itself is shown, the several parts of the burner 10 are made from 18 gauge, type 309 stainless steel, except as indicated. It has a flame basket or burner cone 15 which is perforated by a number of rows of air apertures 16 which admit primary air for combustion from the passing airstream. In the embodiment being described, the cone 15 is 17 inches in diameter at its larger, downstream end 15a and about 5 inches in diameter at its smaller, head end 15b. Its length is 18 inches.

An annular gas manifold 17a is formed around the head end 15b of the burner cone by a gas manifold ring 17 of 12 gauge stainless steel which is attached, as by welding, to the exterior of the cone as shown, and by a head plate 18 which also generally closes the small end of the burner cone. Raw natural gas, or other gas such as propane, butane, propane air, etc. is supplied through a main gas line 19 to the manifold 17a, from whence it is admitted to the interior of the burner cone 15 through a single row of 65 annularly spaced aprat, one-eighth inch diameter gas port apertures 20 formed in the cone as shown. The gas is admitted within the narrow region exterior of, and adjacent to the apex of the V-shaped annular region formed by the line of welded attachment of a gas inlet baffle ring 21. The baffle ring 21 is about 2 inches long.

A pilot gas line 22 extends through the gas manifold directly to the same triangular or V-shaped region exterior of the baffle ring 21, and emits pilot ignition gas to the interior of the baffle ring 21 via a single, three-sixteenth inch diameter pilot gas aperture 23 through the latter. The pilot gas aperture 23 is aligned with the gas emitting end of the pilot gas line 22, and the pilot gas entering the baffle ring 21 is ingited by a spark plug 24 in a conventional manner. The main gas supply emanating from the gas ports 20 is thus ignited at the forward end of the baffle ring 21, and will burn only at this location under the lowest firing rate conditions.

An oil fuel nozzle 25 of the ultrasonic type is mounted centrally of the headplate 18, within a headplate opening 18a, by three annularly spaced apart, exteriorly located nozzle mounting struts 26 best seen in FIG. 3. The oil inlet line to the nozzle 25 is indicated by reference numeral 27, and the resonator air line for the nozzle by reference numeral 28. The sonic nozzle 25 is of the type made by Sonic Development Corporation of America, of Yonkers, New York, and is generally described in its U.S. Pat. Nos. 3,240,253 (Hughes), 3,240,254 (Hughes) and 3,371,869 (Hughes). This type of nozzle depends upon generated ultrasonic force to "atomize," or "shear" the emanating oil stream into micron-sized particles, e.g. 1 to 25 microns. However, it is possible that any other oil fuel nozzle having similar atomizing characteristics might be used.

A plain cylindrical housing 29 having a diameter of 19 inches concentrically surrounds the cone 15 and is joined to the cone by an annular flange 30 at the forward or downstream end 10a of the burner. The length of the housing 29 is the same as that of the cone 15. The main gas line 19 and pilot gas line 22 pass through the housing 29, as shown.

The cylindrical housing 29 and flange 30 provide uniform distribution of a portion of the moving airstream 12 to the burner cone apertures 16 to be used as primary combustion air when either gas or oil is burned. In the embodiment being described, fourteen longitudinally spaced rows of combustion air apertures 16 are provided. A fifteenth row of apertures 16a at the downstream end 15a of the cone provides a cylindrical air curtain extending forwardly of the burner as will be further described. Each row, including the last row 16a, has 32 circumferentially spaced apart, equally-sized air apertures, and the sizes of the apertures become progressively larger in succeeding rows in the direction of the downstream end. The first row of apertures 16 is located about three-eighths inch forward of the gas inlet baffle ring 21, and the total of the areas of the apertures in each row is appropriate to meter the desired quantity of combustion air at that combustion location along the length of the cone 15. Thus, the apertures 16 in the first row are each one-eighth inch in diameter, and in each of the succeeding thirteen rows are sized as follows:

Row Aperture diameter ______________________________________ 2 .136" 3 .147" 4 .152" 5 .152" 6 .187" 7 .187" 8 .250" 9 .312" 10 .375" 11 .437" 12 .500" 13 .625" 14 .625" ______________________________________ Preferably, the apertures 16 in each row are circumferentially offset, or "staggered" with respect to those in each adjacent row.

The apertures 16a in the last, or air curtain row of apertures are each three-fourths inch diameter. The air entering the interior region of the cone 15 through these apertures 16a is not used for combustion, but is directed against a cylindrical air curtain baffle ring 31 attached within the cone adjacent to its downstream end 15a as shown. The baffle ring 31 is arranged concentraically within the burner cone opening, and is 4 inches long. Along its inner end the ring 31 is attached, as by welding, to the interior surface of the cone 15 immediately behind, or upstream of the row of air curtain apertures 16a and, being some 15 inches in diameter, its outer end projects about 2 inches forward of the downstream end 15a of the burner cone, as shown. Thus, air curtain air is introduced within the annular V-shaped region between the interior surface of the cone and the exterior of the ring 31.

Mounted centrally of the air curtain baffle ring 31 is a cone-shaped diffuser 32, its apex pointing in the upstream direction and its base lying within the plane of the downstream end 15a of the burner cone. The base diameter of the diffuser 32 is 11 inches, and the height of the cone-shape is 31/2 inches. Fitted within the 15 inches diameter air curtain baffle ring 31, these dimensions afford a free area for emission of combustion products between the diffuser and the ring which is from about 38 square inches to about 45 square inches per 1,000,000 Btu/hr. input, and through which the combustion products pass at a desired velocity. The diffuser tends to spread and urge the flame pattern outwardly towards the interior surface of the burner cone 15, after which the air curtain baffle ring 31 tends to contain and redirect the flame inwardly.

As perhaps best understood from FIG. 4, the diffuser 32 is mounted by three diffuser support struts 33a, 33b. These are made of three-eighths inch diameter stainless steel rod. The lowermost strut 33a is welded both to the diffuser 32 and to the baffle ring 31, whereas each of the upper struts 33b is welded both to the diffuser 32 and to the baffle ring 31, whereas each of the upper struts 33b is welded only to the diffuser 32 and is permitted to "float" at its outer end within appropriate apertures 34 formed in the baffle ring 31. This slidable mounting of two of the three diffuser support struts permits relative expansion and contraction movement of the parts as the burner is heated and cooled.

Referring again to the head end of the burner 10, the headplate 18 may optionally be provided with a flame retention head or turbulator formed by a series of radially extending louvers 18b to permit entry of a quantity of combustion air in a swirling vortex pattern of "tight" of narrow pitch. The louvers 18b are preferably substantially flat, being simply displaced a short distance (e.g. one-sixteenth inch) out of, and remaining parallel to the plane of the headplate 18. That is, they are offset towards the interior of the burner, and almost no pitch.

Although because of the faster flame propogation rate the turbulator has little effect when firing gas, the tightly swirling air retains the combustible fuel oil and air mixture within the length of the cone 15 for a longer period than would otherwise be the case. Thus, substantially complete combustion of the oil fuel occurs within the length of the burner cone 15 at all but the highest fuel rates, and at the highest fuel rates the length of flame downstream of the burner is shorter, as is desirable. Conversely, and as will be apparent, it may be said that the addition of the flame retention head or louvers 18b permits the burner to have shorter length than otherwise for optimum oil firing characteristics.

However, when operating on either oil or gas, the burner 10 has good flame retention at low firing rates in the absence of the flame retention louvers 18b, and inclusion of the latter does not significantly affect the turndown ratio of the burner in either case. The flame retention head or turbulator is therefore considered optional, and may be eliminated, especially in lower rated units. If the flame retention head is omitted, sufficient air for even very low rate combustion is admitted through the central aperture 18a of the headplate, which surrounds the oil fuel nozzle 25. The air admitted through this aperture 18a also keeps the nozzle 25 cool.

Regarding the rated capacities of burners 10 in accordance with the invention, it should be noted that burners rated at 1.25 million Btu/hr. and 5.0 million Btu/hr., as well as the preferred embodiment being described, are presently contemplated.

When the burner 10 is mounted in a duct 11 as shown in FIG. 1, its gaslines 19 and 22 are connected to a raw natural gas source; its oil fuel line 27 is connected to a source of common, No. 2 fuel oil to deliver from 0.5 to 17.0 gallons per hour, and its oil nozzle resonator line 28 is connected to air under pressure from 10 to 40 p.s.i.g. Of course, all of these connections are made through modulating valves which may be automatic to respond to temperature changes, and through appropriate pressure regulators and shutoff valves, none of which are shown. Airstream velocity at the profile plate 13 is anywhere from about 2700 feet per minute to about 4300 feet per minute.

When gas-fired operation of the burner 10 is desired, the ignition (not shown) is actuated to activate the spark plug 24. Pilot gas is introduced through the pilot gas line 22 and pilot gas port 23 to the vicinity of the spark plug within the gas inlet baffle ring 21 where it ignites. The main gas supply is then admitted through the line 19 to the gas manifold 17a and thence through the gas ports 20 to the exterior of the gas inlet baffle ring 21 where it is ignited by the pilot gas, and burns. Air for combustion is supplied through the combustion apertures 16 in the burner cone 15 and through the opening 18a in the headplate 18.

At minimum fuel input, flame is confined to the annular area exterior of the baffle ring 21, which is the area of outlet of the gas. As fuel rate is increased, the flame burns along a progressively longer interior length of the cone 15 from the baffle ring 21, and it will be noted that radially inward projecting upstanding flame patterns 14a (FIG. 1) progressively appear at each of the rows of air apertures 16. The larger sized apertures 16 towards the downstream end 15a of the cone supply the larger incremental air volumes needed at increased gas input rates. Although at full fuel input most combustion takes place within the cone 15, some flame does extend downstream of the forward end 10a of the burner. However, it is contained in a short or "tight," heart-shaped pattern 14 (FIG. 1) by the diffuser 32, the air curtain baffle ring 31, and the cylindrical air curtain which surrounds the ring 31 and is produced by the air curtain apertures 16a.

As previously mentioned, turndown ratios of better than 40 to 1 are obtained using raw natural gas fuel.

When it is desired to fire the burner 10 using oil fuel, it may be directly ignited by the spark plug 24 as it is emitted from the oil fuel nozzle 25, or it may be ignited by pilot gas admitted and ignited as previously described in connection with gas operation. In any event, the oil fuel is atomized by the sonic generator type nozzle 25, and is mixed with combustion air introduced through the headplate opening 18a. The combustible mixture is ignited and burns in a cone-shaped pattern just forward of the nozzle 25 and, if flame retention louvers 18a are employed, the blue flame will burn in a pinwheel pattern.

As the fuel rate is increased, the flame burns progressively farther out within the burner cone 15, and combustion air is supplied in the correct proportions for progressively increased firing rates by the sequential rows of air apertures 16. The flame color becomes predominantly white at increased rates.

As in the case of gas firing, at full oil fuel input rates the flame is mostly contained within the cone 15, but also appears in a short, heart-shaped pattern, as at numeral 14 in FIG. 1, forward of the downstream end 10a of the burner. At lower rates the flames appear in distinct flame jets 14a (FIG. 1) along a portion of the length of the burner or, if flame retention louvres 18b are included, in a swirling vortex pattern within the cone.

Turndown ratios of better than 30 to 1 are obtained using oil fuel.

Referring briefly to FIGS. 5-7, it will be understood that the same dual fuel burner 10 may also be utilized as a combustion head within a space heater unit of the rotary regenerative type as described in U.S. Pat. No. 3,695,250 (Rohrs et al.) and generally indicated by reference numeral 40. The space heater 40 has a slowly rotating, metallic regenerative heat exchange wheel 41 which is heated as it passes through the combustion chamber 42 in which the burner 10 is mounted, and which transfers its heat to a flowing airstream, generally indicated by reference numeral 43, emitted from a blower 44 as the rotating wheel 41 passes through the fresh air supply duct 45.

Although the burner 10 might alternatively extend through the backwall 40a, it is mounted on and extends through one of the sidewalls 40b of the combustion chamber 42, as shown. However, rather than being mounted centrally of an air inlet aperture similar to provide plate aperture to receive air from a passing stream, the burner housing 29 fits closely within appropriate circular openings of the exterior wall 40b and interior double wall 40c and a ventilating cover box 36 is attached across the headplate end 10b of the burner. The cover box 36 has a backplate 37 and apertured topwall, bottomwall and sidewall panels 38 whose numerous apertures 38a impart a screenlike appearance. Thus, the induced draft fan 46 located on the downstream side of the wheel 41 draws not only the burner combustion products in the direction of arrow 47 through the axially aligned corrugation openings of the wheel 41, but also combustion air into and through the burner 10 itself. In addition, air is drawn into the combustion chamber 42 via the some 54 peripherally disposed air openings 48a in the interior double wall end panel 48 of the heater 40, this air being secondary or excess air used not only to ensure complelte combustion but also to actually cool the burning gases and combustion products before they impinge on the wheel 41. This secondary air enters the double wall through grillwork, such as grillwork 49, in one or more exterior walls of the heater. This instruction and the operation of the space heater 40 is more fully described in the aforesaid U.S. Pat. No. 3,695,250 (Rohrs et al.), the specification of which is hereby incorporated by reference.

The peripheral disposition of the end panel air openings 48a with relation to the positioning of the burner 10 has the effect of drawing the burner flame 14b back towards the end panel, as further assists in preventing direct impingement of flame on the wheel 41. It will also be noted that the flame 14b is desirably longer than the heart-shaped flame 14 of FIG. 1, and such longer flame is produced by omitting any flame retention head in the burner.

When operating either in a duct as in FIG. 1 or in heat exchange unit as shown in FIGS. 5-7, the flame will be stable and combustion will be found to be virtually complete even where the airstream is turbulent.

Thus has been described a dual fuel burner which achieves all of the objects of the invention.

Claims

What is claimed is:

1. A dual fuel airstream type burner having a high turndown ratio characteristic when burning gas or oil, comprising an elongated conical burner cone having a plurality of longitudinally spaced apart, circumferential rows of air apertures disposed along substantially its length, said apertures being smaller in diameter in said rows thereof towards the small end of said burner cone and being larger in diameter in said rows thereof towards the large end of said burner cone whereby the sequential rows of said apertures introduce increasing amounts of combustion air in the correct proportions for progressively increasing firing rates of the burner, an oil fuel nozzle mounted centrally of the small end of said burner cone, said oil fuel nozzle having means for atomizing fuel oil fed therethrough to a particle size smaller than about 25 microns, gas port aperture means adjacent to said small end of said burner cone, and gas manifold means on said small end of said cone and communicating with said gas port aperture means.

2. A dual fuel airstream type burner according to claim 1 wherein said oil fuel nozzle is an ultrasonic nozzle.

3. A dual fuel airstream type burner according to claim 1, wherein said gas port aperture means provides a circumferential row of gas port apertures, and which further comprises means defining a gas inlet baffle ring disposed concentrically within said small end of said burner cone adjacent to said circumferential row of gas port apertures for diverting gas introduced into said cone through said gas port apertures towards the opposite end of said burner cone.

4. A dual fuel airstream type burner according to claim 3 wherein said circumferential row of gas port apertures is formed in said cone, the tip of said oil fuel nozzle being substantially within the plane of said circumferential row of gas port apertures, and said gas inlet baffle ring being attached within said cone having its longitudinal axis aligned with that of the cone and having length extending from a location on the upstream side of said row of gas port apertures to a location downstream of said oil fuel nozzle.

5. A dual fuel airstream type burner according to claim 4 which further comprises ignition means within said gas inlet baffle ring, means defining a pilot gas aperture through said gas inlet baffle ring, and a pilot gas line for introducing pilot gas within said cone to a location exterior of said gas inlet baffle ring adjacent to said pilot gas aperture thereof.

6. A dual fuel airstream type burner according to claim 3, wherein said plurality of circumferential rows of air apertures in said burner cone includes a circumferential row of apertures adjacent to the large end of said burner cone, and which further comprises a cylindrical air curtain baffle ring attached within said burner cone and having its longitudinal axis aligned with that of the cone, said air curtain baffle ring being attached at a location adjacent to and on the upstream side of said one row of air apertures and having length extending to a location downstream of said one row of air apertures.

7. A dual fuel airstream type burner according to claim 6 which further comprises a conical-shaped diffuser mounted centrally on said burner cone within, and in annularly spaced relation with respect to said air curtain baffle ring, the apex of said conical-shaped diffuser facing towards said small end of said burner cone.

8. A dual fuel airstream type burner according to claim 7 wherein the base of said conical-shaped diffuser lies substantially within the plane of said large end of said burner cone, and said air curtain baffle ring has length to project a distance in the downstream direction beyond said plane of the larger end of said burner cone.

9. A dual fuel airstream type burner according to claim 7 wherein said conical-shaped diffuser is mounted on a diffuser strut attached to and extending between said air curtain baffle ring and one side location on said diffuser, and at least one additional diffuser strut extending between said air curtain baffle ring and another side location on said diffuser, one end of said additional diffuser strut being attached to its associated element and the other end being slidably connected to its associated element.

10. A dual fuel airstream type burner according to claim 8 wherein said burner cone has a peripheral flange attached on, and within the plane of its said large end and projecting in radially outward direction, and said burner further comprises a cylindrical housing having its longitudinal axis aligned with that of said burner cone and being attached at its forward end to the outer periphery of said annular flange.

11. A dual fuel airstream type burner according to claim 10 wherein said cylindrical housing has length substantially equal to the length of said burner cone.

12. A dual fuel airstream type burner according to claim 2 which further comprises a headplate attached to said small end of said cone, said headplate having flame retention means formed therein.

13. A dual fuel airstream type burner according to claim 3 wherein said gas inlet baffle ring is disposed concentrically within said small end of said burner cone, said gas port apertures are formed through said burner cone, and said baffle ring extends across the plane of said circumferential row of gas port apertures.

14. A dual fuel airstream type burner according to claim 6 wherein said air curtain baffle ring extends downstream of said one row of air apertures at least to the plane of said large end of the burner cone.

15. A dual fuel airstream type burner according to claim 14 wherein said air curtain baffle ring extends downstream of said one row of air apertures to a location beyond said plane of the large end of said burner cone.

16. A dual fuel airstream type burner according to claim 1 wherein said plurality of circumferential rows of apertures adjacent to the large end of said burner cone, and which further comprises an elongated cylindrical air curtain baffle ring attached within said burner cone and having its longitudinal axis aligned with that of the cone, said air curtain baffle ring being attached at a location adjacent to and on the upstream side of said one row of air apertures and having length extending to a location downstream of said one row of air apertures.

17. A dual fuel airstream type burner according to claim 16 which further comprises a conical-shaped diffuser mounted centrally of said burner cone within, and in annularly spaced relation with respect to said air curtain baffle ring, the apex of said conical-shaped diffuser facing towards said small end of the burner cone.

18. A dual fuel airstream type burner according to claim 1 which further comprises a conical-shaped diffuser mounted within, and in annularly spaced relation with respect to the large end of said burner cone, the apex of said conical-shaped diffuser facing towards the small end of said burner cone.

19. An airstream type burner having a high turndown ratio characteristic, comprising an elongated conical burner cone having a plurality of longitudinally spaced apart, circumferential rows of air apertures disposed along substantially its length, said apertures being smaller in diameter in said rows thereof towards the small end of said burner cone and being larger in diameter in said rows thereof towards the large end of said burner cone whereby the sequential rows of said apertures introduce increasing amounts of combustion air in the correct proportions for progressively increasing firing rates of the burner, said plurality of circumferential rows of air apertures including a circumferential row of apertures adjacent to the large end of said burner cone, an elongated cylindrical air curtain baffle ring having its longitudinal axis aligned with that of the cone and being attached within said burner cone at a location adjacent to and on the upstream side of said one row of air apertures, said air curtain baffle ring having length extending to a location downstream of said one row of air apertures, and means for introducing fuel at the small end of said burner cone.

20. An airstream type burner according to claim 19 which further comprises a conical-shaped diffuser mounted centrally of said burner cone within, and in annularly spaced relation with respect to said air curtain baffle ring, the apex of said conical-shaped diffuser facing towards said small end of the burner cone, said air curtain baffle ring having length to project a distance in the downstream direction beyond the plane of the large end of said burner cone, and the plane of the base of said conical-shaped diffuser being substantially within the length of said air curtain baffle ring.

21. An airstream type burner having a high turndown ratio characteristic, comprising an elongated conical burner cone having a plurality of longitudinally spaced apart, circumferential rows of air apertures disposed along substantially its length, said apertures being smaller in diameter in said rows thereof towards the small end of said burner cone and being larger in diameter in said rows thereof towards the large end of said burner cone whereby the sequential rows of said apertures introduce increasing amounts of combustion air in the correct proportions for progressively increasing firing rates of the burner, and an oil fuel nozzle mounted centrally of the small end of said burner cone, said oil fuel nozzle having means for atomizing fuel oil fed therethrough to a particle size smaller than about 25 microns.

22. An airstream type burner according to claim 21 wherein said oil fuel nozzle is an ultrasonic nozzle.