Abstract:
An apparatus in which an atomizer is provided with two arrays of discharge holes adjacent and offset from each other.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to apparatus for dispersing fluids and more specifically to a method and apparatus for atomizing liquids. In particular, the invention relates to an atomizer or front plate for delivery of fuels to burners. 
     BACKGROUND OF THE INVENTION 
     Nozzles have long been used to atomize fluids, such as liquids, gases and liquid-solid slurries, to provide fine sprays. One particular application of atomizers that has received considerable attention is in fuel combustion. It is now well known that atomizers that deliver a fine spray of discrete minute droplets improve combustion efficiency. 
     Thus, considerable work has been done to develop atomizer nozzles that enhance fuel burning efficiency in power plant boilers. It is generally accepted that the finer the fuel spray pattern, the more complete and effective the combustion will be. However, other limitations such as nozzle vibration, tube temperatures, opacity and nitrogen oxide (NO x ) emissions must be considered when designing fuel atomizers. 
     In about 1989 Todd Combustion, Shelton, Conn. developed an atomizer nozzle that improved the combustion efficiency and to some extent reduced NO x  emission. The prior art Todd Combustion atomizer was developed for application in a now conventional furnace windbox. 
     The Todd atomizer is characterized as a Multi-Jet Single Fluid Atomizer. Structurally the atomizer has a cup-shaped internal whirling chamber into which fuel under pressure is delivered through an array of passages or slots that are arranged tangentially to the whirling chamber. An array of perimeter holes each of which are the same radial distance from the center of the whirling chamber provide passage of the fuel from the whirling chamber to the furnace combustion chamber. 
     In addition, prior art atomizers with a plurality of perimeter holes such as disclosed in U.S. Pat. No. 5,143,297 also exist. 
     Although the prior art designs reduced NO x  emissions over previous designs, the level of NO x  emission was still not satisfactory and opacity was higher than desirable. As a result considerable research was conducted to obtain an aerator nozzle capable of reducing opacity to minimal levels. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved atomizer. 
     It is a further object of the invention to provide an atomizer that will improve boiler combustion efficiency and opacity. 
     The invention is an atomizer having a centrally disposed whirling chamber into which fuel under pressure is delivered and an outer and inner circular array of holes to provide openings from the centrally disposed whirling chamber to the furnace fire box. The outer array of perimeter holes is arranged at an angle to the centerline of the centrally disposed whirling chamber of about 40°. The inner array of circular holes is arranged at an angle of about 35° to the center line of the centrally disposed whirling chamber. The inner and outer array of perimeter holes are arranged in relationship wherein each individual perimeter hole of the outer array is adjacent to an individual perimeter hole of the inner array to in effect form a pair of holes that have centerlines offset from each other. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood when the description of the preferred embodiment is considered with the following drawings wherein: 
     FIG. 1 is a partial sectional elevational view of a furnace windbox in which the atomizer of the present invention is installed; 
     FIG. 2 is a front elevational view of the atomizer of the present invention taken through line 2--2 of FIG. 1; 
     FIG. 3 is a back elevational view of the atomizer of the present invention taken through line 3--3 of FIG. 1; 
     FIG. 4 is a sectional elevational view of the atomizer of the present invention taken through line 4--4 of FIG. 2; 
     FIG. 5 is a sectional elevational view of the atomizer of the present invention taken through line 5--5 of FIG. 2; 
     FIG. 6 is a partial sectional view taken through line 6--6 of FIG. 2; 
     FIG. 7 is a front elevational view of a second embodiment of the atomizer of the present invention; 
     FIG. 8 is a graph illustrating the improved opacity performance of the atomizer of the subject invention; and 
     FIG. 9 is a graph illustrating the NO x  performance of the atomizer of the subject invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The atomizer of this present invention, although having more general application, will be described in the environment of a power plant boiler. 
     As seen in FIG. 1, the atomizer 2 of the present invention is shown centrally disposed in a power plant furnace windbox 4. The windbox 4 include a conventional swirler 6 and an array of poker tubes 3 arranged around the swirler 6. 
     The atomizer 2, as best seen in FIGS. 2, 3, 4 and 5 is provided with a large diameter circular first array of perimeter holes 8, a small diameter circular second array of perimeter holes 38, an external circular flange 10, an internal centrally disposed whirling chamber 12 and a plurality of slots 14 terminating tangentially at the inside upstream opening of the centrally disposed chamber 12. The front surface of the atomizer 2 is a frusto-conical surface 15 terminating in a central circular flat surface 16. 
     As best seen in FIGS. 2-4, the array of perimeter holes 8 comprise five holes equidistant from the centerline 17 of the centrally disposed whirling chamber 12 and from each other. The perimeter holes 8 are formed at an angle α in the range of 22.5° to 60° and preferably 40° to the centerline 17 of the whirling chamber 12, as best can seen in FIG. 4. As seen in FIG. 5, the perimeter holes 38 are formed at an angle δ in the range of 22.5° to 60° and preferably 35° to the centerline 17 of the whirling chamber 12. The whirling chamber 12 is formed in a cup-like configuration. The downstream section 11 of the whirling chamber 12 is hemi-spherical and the upstream section 13 is cylindrical. As seen in FIGS. 4 and 6 each of the five perimeter holes 8 has an inner upstream opening 7, an outer downstream opening 9 and a divergent passage 5. As seen in FIGS. 5 and 6 each of the perimeter holes 38 has an inner upstream opening 37, an outer downstream opening 39 and a divergent passage 35. 
     As best seen in FIG. 2, each perimeter hole 38 is adjacent to a perimeter hole 8 and is offset at an angle γ about the center of the atomizer 2. 
     As best seen in FIG. 6, a wall 50 separates the perimeter holes 8 from the perimeter holes 38. 
     In the preferred embodiment, the diameter of the cylindrical section 13 is 0.512 inches and the radius of the hemi-spherical section 11 is 0.256 inches. The diameter of a pitch circle 19 made through the center lines of the perimeter holes 8 at the inner upstream opening 7, best seen in FIG. 3, is in the range of 0.300&#34; to 0.350&#34; preferably 0.350&#34;, and the diameter of a pitch circle 21 made through the centerline of the outside downstream openings 9 of the perimeter holes 8, best seen in FIG. 2, is 0.680&#34;. The diameter of a pitch circle 49 made through the center lines of the perimeter holes 38 at the inner upstream opening 37, best seen in FIG. 3, is in the range of 0.200&#34; to 0.300&#34; preferably 0.271&#34; and the diameter of a pitch circle 51 made through the center lines of the perimeter holes 38 at the outer downstream opening 39 is 0.540&#34;. The divergence angle β of passage 5 of the perimeter holes 8 is 12° and the divergence angle ε of the passages 35 of the perimeter holes 38 is also 12°. Each of the holes 8 and 38 has an inlet opening 7 and 37 respectively formed with a partial hemi-spherical section which in the preferred embodiment is formed with a 3/16 inch ball mill that penetrates 0.025&#34; into the respective passages 5 and 35. The offset angle δ is in the range of 10° to 25°, preferably 20°. 
     The embodiment of the atomizer 2 of FIG. 7 is essentially the same as the embodiment of FIGS. 2-6 but includes a centrally disposed hole 28. 
     In operation oil under pressure up to 1200 psig, is directed by a backing plate (not shown) to the outer perimeter of the rear of the atomizer. The oil under pressure enters the atomizer 2 at the outer edge of the slots 14 cut in the rear of the atomizer 2. The oil is accelerated to high velocity in the slots 14, and jets into the whirling chamber 12 at an angle almost tangent to the outer diameter of the whirling chamber 12. This produces a high velocity rotating flow in the whirling chamber 12 that accelerates as the oil proceeds to the perimeter holes 8 and 38. Oil passes through the perimeter holes 8 and 38, where atomization occurs from a combination of centrifugal force and shearing of the oil by air as it jets into the air stream. 
     The embodiment of FIG. 7 functions similarly to the embodiment of FIGS. 2-6 but fluidized fuel also discharges from the centrally disposed hole 28 which has a diameter of 3/16 to 5/16 inch. At the exit of the center hole 28, the swirling oil forms a thin film around the perimeter of the hole, which atomizes the oil into small droplets. Centrifugal force from the swirling oil causes the oil to be discharged from the perimeter holes 8 and 38 in an enlarging fan pattern, which results in small droplets that ignite easily. 
     Extensive tests were conducted to develop the atomizer 2 of the present invention. FIG. 8 displays the improved opacity performance of the atomizer 2 of the present invention compared to the TODD prior art atomizer and the TODD Advanced Low NO x  atomizer described in Ser. No. 08/724,442, filed Oct. 1, 1996 filed coincidentally with this application. The prior art nozzle identified as the standard nozzle differed from the nozzle of the present invention in the location of the perimeter holes 8 and the absence of the second array of perimeter holes 38. In the standard nozzle the divergence angle through the centerline of each perimeter hole to the centerline of the whirling chamber is 421/2°; the interior (upstream) pitch circle has a diameter of 0.350 and the exterior (downstream) pitch circle has a diameter of 0.680. 
     A Todd DYNASWIRL-LN burner employing advanced fuel staging for Residual Fuel (RFO) was used in the test. A single conventional differential pressure atomized burner (RFO gun) is located along the burner centerline, inside the gas pipe. 
     Through the use of the database developed during the course of the tests, spray lab testing and mathematical modeling, the atomizer 2 of the present invention was developed and performed to result in opacity levels generally in the 5% range. Compared to TODD&#39;s &#34;standard and advanced low NO x  design, the atomizer of the present invention reduced opacity levels from the 7-12% range to the 4-10% range. The results are reported on FIGS. 8 and 9 wherein PTF #1 Standard refers to the prior art TODD atomizer and #1 Advanced Low NO x  refers to the TODD atomizer developed contemporaneously with the atomizer of the subject invention. #1 Advanced Low Opacity refers to the atomizer 2 of the subject invention. 
     Although the tests were performed using a differential pressure atomizer (return flow atomizer), the performance will be the same when using a simplex (non-return flow) atomizer.