US3861858A

US3861858A - Throat mix burner

Info

Publication number: US3861858A
Application number: US467928A
Authority: US
Inventors: Klaus H Hemsath; Arvind C Thekdi
Original assignee: Midland Ross Corp
Current assignee: Surface Combustion Corp; Grimes Aerospace Co
Priority date: 1972-12-11
Filing date: 1974-05-08
Publication date: 1975-01-21
Anticipated expiration: 1992-01-21

Abstract

A throat mix burner providing efficient air and fuel mixing, flame stability, high turndown ratio and safety. These results are achieved through selected size relationships among the various dimensions of the burner combustion chambers.

Description

United States Patent Hemsath et al.

[4 1 Jan. 21, 1975 THROAT MIX BURNER inventors: Klaus H. Hemsath, Sylvania; Arvind C. Thekdi, Toledo, both of Ohio Assignee: Midland Ross Corporation, Cleveland, Ohio Filed: May 8, 1974 Appl. No.: 467,928

Related US. Application Data Continuation of Ser. No. 313,689, Dec. 11, 1972, abandoned.

US. Cl 43l/353, 431/174, 431/263,

239/425 Int. Cl. F23d 15/02 Field of Search 431/174, 177, 181, 187,

[56] References Cited UNITED STATES PATENTS 1,975,033 9/1934 Wolff 239/4165 2,368,370 1/1945 Maxon. 431/187 3,154,134 10/1964 Bloom 239/4165 3,676,048 7/1972 Sellors et al. 431/187 Primary Examiner-Carrol1 B. Dority, Jr. Attorney, Agent, or Firm-Peter Vrahotes; Frank J. Nawalanic [57] ABSTRACT A throat mix burner providing efficient air and fuel mixing, flame stability, high turndown ratio and safety. These results are achieved through selected size relationships among the various dimensions of the burner combustion chambers.

1 Claim, 4 Drawing Figures SHEET 2 BF 2 PATENTEBJANZI ms FIG.3

THROAT MIX BURNER This is a continuation, of application Ser. No. 313,689, filed Dec. 11, 1972, now abandoned.

BACKGROUND OF THE INVENTION Modern industrial heating processes require precise and accurate control of a number of variables such as furnace temperature and atmosphere composition. In most cases, the operating characteristics of the burner used in the furnace, or the process reactor, have a significant effect on these variables. It is advantageous to use a burner which can operate under a variety of conditions such as firing rate, air-fuel ratio, type of fuel and the like. In addition to this, the burner must have a reliable ignition and flame supervision system to avoid mishaps. When all of these requirements are considered, a good burner should possess the following characteristics:

1. Efficient mixing of air and fuel, and complete combustion of the air-fuel mixture within the combustion chamber of the burner.

2. Flame stability without excess and erratic noise.

3. Wide range of turndown while maintaining a preset air-fuel ratio.

4. The designed flame shape should be maintained within a specified range of the burner operation.

5. The burner parts should not overheat, deform, or be damaged during an extended period of burner operation.

6. Minimum amount of pollutants such as CO, NO, and other noxious gases, which originate from the combustion process.

7. Dependable and safe ignition system.

8. Foolproof and positive flame supervision system.

The above and other characteristics, advantages and objects are achieved through the present invention as will be readily apparent from the following description, reference being made to the accompanying drawing wherein:

FIG. 1 is a longitudinal sectional view through a throat mix burner embodying the principles of this invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is an end view of the apparatus shown in FIG. 1; and

FIG. 4 is a detailed view showing the means for igniting the burner in FIG. 1.

SUMMARY OF THE INVENTION The burner of this invention is of the balanced pressure type which uses a ratio control device, such as a zero regulator, to maintain the desired air-fuel ratio. Both the air and the gas travel axially into a first chamber portion of the throat of the burner with the air forming an annulus about the gas supply means. The gas is directed radially into this air annulus after which the gases travel down the throat into a second chamber throat portion having diverging walls. From the second chamber the gases go into a third portion having a right cylindrical shape. The geometrical proportions of the three chambers relative to one another are critical and upon selection of the proper proportions the objects of this invention are achievable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, a burner which incorporates the principles of this invention is shown generally at 10. The burner has a generally annular block 12 made of a refractory such as high alumina castable. Extending through the burner is a preconfigured bore or throat having a first right cylindrical chamber 14 at the upstream end thereof. At the downstream end of first chamber 14 is a shoulder or step 16 which is part of a conically shaped chamber 18. Downstream from the conically shaped chamber 18 is a shallow second right cylindrical chamber 20.

The block 12 is received within a metallic holder 24, which holder has axially extending flanges 26, that are generally annular, and a backplate 28. The backplate 28 extends radially beyond the annular flanges 26 and has a central opening'30 therein, which opening is coaxially aligned with first chamber 14. The backplate also has a plurality of outlying openings 31 which provide means for securing the burner 10 to a furnace wall (not shown). The block 12 is held securely within the holder 24 as by cement 32.

A burner casting 34 is rigidly secured to the backplate 28 as by bolt assemblies 35. The burner casting 34 has an axial opening 36 that receives a gas line 38. A zero regulator 39 is connected to the gas line 38. Additionally, the casting 34 has a radial opening 40 that receives an air line 42. Air is supplied to the air line 42 by an air header 41, there being a back load line 43 providing communication between the zero regulator 39 and the air header 41. In essence, the combination of the block 12, the backplate 28 and the casting 34 may be considered a housing for the burner to support other components.

Received within the burner casting 34 and extending into first chamber 14 is a tubular nozzle 44 which defines, in combination with the block 12 interior walls, an annulus 45. The end of the nozzle 44 receives a solid disc 46 and the nozzle has a plurality of peripherally spaced openings 48 located near its downstream end. A generally star-shaped disc 49 is received within the opening 30 to properly position the nozzle 44.

In order to provide ignition means for the mixture of gas and air in the throat, the block 12 and backplate 28 are provided with an angular opening 50 which receives a spark plug 52 whose electrodes extend to first chamber 14. It will be understood that although the ignition means is shown extending into first chamber 14, it may also be located at the step 16 or conical chamber 18. Additionally, a sight glass 54 is included to provide visual examination and supervision of the flame. As can be seen from the drawing, the location of the sight glass 54 is such that light from the ignition means does not reach the view angle of the sight glass 54. The flame may be controlled automatically by providing a control means 56 such as an ultraviolet detector at the sight glass or by use of a flame rod.

In operation, being a nozzle mix burner 10, the combustion air and fuel are supplied separately to the burner. The fuel is admitted into the burner 10 from the nozzle 44 which may have radial holes 48 or an axial hole in the plate 46, although radial holes, as shown, are preferable. The size and number of radial holes 48 depend on the type of fuel, available fuel pressure and the size of the burner 10. The radial holes 48 promote better mixing of air and fuel, and give a short, intense, nonluminous flame. In cases where a relatively long flame is required, the gas nozzle 44 may have an axial hole, rather than the radial holes. The size of such an axial hole, again, would depend on the factors mentioned above.

Combustion air is introduced through the annulus area 45 between the gas nozzle 44 and wall of first chamber 14. The area of the air passage depends on the available air pressure drop, the rated burner capacity and the air density. The combustion air creates a jet pump effect.

The air and fuel then travels into and through first chamber 14. The ratio of first chamber length (Lt) to throat diameter (Dt) should be from 0.5 to 10.0, in order to obtain satisfactory burner performance. Preferably the ratio of Lt to Dt should be from 6.5 to 8.5. In practice, the ratio is determined by the required burner block length and the position of the burner ignition means 52. The distance between the end of the fuel nozzle 44 and the position of the ignition device 52 can vary from at least one throat diameter (Dt) to that permitted by first chamber length (Lt).

First chamber 14 is followed by the step 16 in the burner block 12. The step 16 is used to stabilize the flame by providing recirculation of the air-fuel mixture and the products of combustion. The fresh air-fuel mixture mix during the recirculation, thereby increasing the air-fuel mixture temperature until the mixture is ignited. The ratio of the step diameter (Ds) to first chamber diameter (Dt) should be within the range of 1.15 to 1.5. After the step 16, the burner block 12 is shaped in the form of a conical chamber 18, which is followed by the shallow short right cylinder chamber 20. The half angle a of the conical chamber 18 should vary from zero to about l degrees. The conical chamber 18 shape affects the stability and the aerodynamic noise of the burner 10. A proper conical chamber 18 angle provides an unobstructed passage for the combustion products. The ratio of conical chamber 18 length (Lc) to the step 16 diameter (Ds) should be in the range of 1.5 to 3.0, depending on the allowable block length. The shallow cylindrical chamber 20 diameter (Dc) should be equal to the outer diameter of the conical chamber 18. The cylindrical length (Lc) to the cylindrical diameter (Dc should vary from 0.2 to 0.75, again depending on the allowable block length. The block 12 length depends on the burner size and application.

The throat mix burner 10 of this invention is designed to operate primarily on natural gas, but it can be adapted for use with fuels such as propane, manufactured gas, or No. 2 fuel oil.

The burner 10 is a balanced pressure type which uses a ratio control device, such as a zero regulator 39, to maintain the desired air-fuel ratio. With the direct spark plug 52 ignition available on this burner 10, normally it does not need any premix pilot connection.

The burner 10 can be operated over a wide range of air-fuel ratios. The limit on air-fuel ratio varies slightly for different burner sizes, but with lean operation an air-fuel mixture of up to 70 percent excess air is achievable, and with rich operation, up to 90 percent excess fuel is normal. This range of operation covers most major fields of applications, especially where the fuel efficiency and controlled atmosphere conditions are to be satisfied simultaneously. In the range of air-fuel ratios previously given, the burner 10 has a turndown ratio of 10 to l. The flame remains stable and gives a strong ultraviolet signal to the detection unit 56 at all firing rates. This wide range of turndown allows considerable flexibility of furnace operation. It allows the use of fewer high capacity burners, resulting in low initial costs on burners, piping and wiring. Using radial openings 48 in the nozzle 44, the flame length is short and provides a high degree of circulation in a furnace. This circulation in a furnace helps to maintain uniform furnace atmosphere temperature, and improves convection heat transfer to the work.

The burner I0 is direct spark ignited at low firing rate, which is normally one-tenth of the rated capacity. If necessary, a premix pilot, or even manual ignition, can be used with this burner 10. An ultraviolet flame supervision system 56 can be used to monitor the flame at all firing rates within the operational range of the burner. Due to the unique flame stabilization associated with the burner 10, the flame signal strength, as represented by the flame detector output, remains almost constant at all firing rates. The location of the supervision system insures that there is no interference between two or more burners in a multi-burner arrangement in a furnace.

The burner operations were tested under both positive and negative back pressures in a furnace. It was found that a positive back pressure up to 8 osi and a negative pressure down to 8 osi do not affect the flame stability or other characteristics of a burner 10 having a rated capacity of 0.25 MM BTU/hr. It is obvi ous that a proper correction for the air and gas supply pressures should be allowed when the burner 10 is used under off-atmospheric back pressure. The same precaution applies for the burner 10 operations at elevated pressures.

Extensive testing was carried out to check the noise level and N0 CO and hydrocarbon emissions from the burner 10. The noise level of the burner is considerably lower than that from similar commercial burners. The N0 CO and hydrocarbon levels depend on the furnace temperature. Laboratory test results at furnace temperatures varying from 1,000F. to 2,950F. show that the concentration of N0 CO and hydrocarbon is much lower than the existing allowable concentrations.

The throat mix burner 10 can be used for most of the general purpose heating applications. Its operation characteristics, as discussed earlier, makes it attractive to furnaces used in ferrous and nonferrous metal treatment, ovens, dryers, incinerators, steam generators, etc. It can be used on batch type or continuous furnaces. In either case, the high turndown capacity allows the use of fewer high capacity burners, contrary to the use of a large number of small capacity burners. The possible savings in initial cost of burners, piping, flame supervision system, and the maintenance cost during the life of the burner are substantial.

What is claimed is:

l. A burner operable by a source of air and fuel comprising:

a block of refractory material having a preconflgured bore extending therethrough, said bore defined by a first cylindrical chamber at one end of the block, a second larger cylindrical chamber at the opposite end of said block and an intermediate connecting frusto-conical chamber therebetween, said frustoconical chamber having its large diameter portion equal to and adjacent said second chamber and its small diameter portion adjacent to and greater than said first chamber to define a step at the juncture between said first and second chambers;

a hollow tubular member having a closed end section within said first chamber, said end section being coaxially positioned within said first chamber to define an annulus between said member and said first chamber, said tubular member having a plurality of radially directed openings extending theret'hrough in said end section;

ignition means in fluid communication with said bore and operable to ignite said burner;

gas means operable to supply said gaseous fuel within said tubular member and through said opening into said annulus;

air means operable to supply combustion air under pressure through said annulus effective as a jet pump to draw said gaseous fuel along with said air through said first chamber, to recirculate at said step said fuel and air into a mixture sufficient to support combustion from said ignition means and to stabilize the products of combustion from said recirculated mixture at said step; the ratio of the diameter of said step to the diameter of said first chamber is in the range of [.5 to 3.0 and the ratio of the first chambers axial length to its diameter is 0.5 to 10.0; and the ratio of the second chambers axial length to its diameter is 0.2 to 0.75.

Claims

1. A burner operable by a source of air and fuel comprising: a block of refractory material having a preconfigured bore extending therethrough, said bore defined by a first cylindrical chamber at one end of the block, a second larger cylindrical chamber at the opposite end of said block and an intermediate connecting frusto-conical chamber therebetween, said frusto-conical chamber having its large diameter portion equal to and adjacent said second chamber and its small diameter portion adjacent to and greater than said first chamber to define a step at the juncture between said first and second chambers; a hollow tubular member having a closed end section within said first chamber, said end section being coaxially positioned within said first chamber to define an annulus between said member and said first chamber, said tubular member having a plurality of radially directed openings extending therethrough in said end section; ignition means in fluid communication with said bore and operable to ignite said burner; gas means operable to supply said gaseous fuel within said tubular member and through said opening into said annulus; air means operable to supply combustion air under pressure through said annulus effective as a jet pump to draw said gaseous fuel along with said air through said first chamber, to recirculate at said step said fuel and air into a mixture sufficient to support combustion from said ignition means and to stabilize the products of combustion from said recirculated mixture at said step; the ratio of the diameter of said step to the diameter of said first chamber is in the range of 1.5 to 3.0 and the ratio of the first chamber''s axial length to its diameter is 0.5 to 10.0; and the ratio of the second chamber''s axial length to its diameter is 0.2 to 0.75.