Abstract:
The invention pertains to ignition systems and more particularly to spark igniters for burners and burner pilots. A spark igniter is provide, which is configured so that the spark gap is on the outer side surface of the spark igniter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to ignition systems and more particularly to spark igniters for burners and burner pilots. 
     2. Description of the Related Art 
     A gas burner pilot is a device used to create a stable pilot flame by combustion of a low flow rate (relative to the main burner) gaseous fuel-air mixture. The pilot flame is used to light a larger main burner, or a difficult to light fuel. Gas pilot designs normally include an ignition system. One common type of ignition systems used in gas burner pilots, as well as other systems such as flare systems, is a high-energy ignition (HEI). 
     An HEI system typically utilizes a capacitive discharge exciter to pass large current pulses to a spark rod. The large current pulses are often greater than 1 kA. The spark igniter (also known as spark plug, spark rod or igniter probe) for an HEI system is generally constructed using a center electrode surrounded by an insulator and an outer conducting shell over the insulator such that, at the axially-facing ignition end of the spark rod, an air gap is formed between the center electrode and the outer conduction shell, i.e., a gap between the center electrode and the outer electrode shell or conducting shell. At this air gap, also called a spark gap, a high-energy spark can pass between the center electrode and outer conducting shell. Often a semiconductor material is applied to the insulating material at this gap to facilitate sparking. HEI systems have the ability to maintain powerful high energy sparks in adverse conditions such as cold temperatures, heavy fuels (heavy gases or oils), contamination of the igniter plug with coking or other debris and moisture presence due to steam purging or rain. 
     Past HEI spark igniter designs produced sparks on an axial-facing surface (referred to herein after as “axial-directed spark igniter”). One variable that affects spark energy is the size of the air gap on the axial-facing surface of the igniter. As the air gap increases, the amount of energy released during the spark event also increases. Air gaps generally range in size from 1 mm to 2 mm. 
     The center electrode, the electrode shell and semiconductor material erode away as sparking occurs over the course of an igniter&#39;s life. An igniter generally reaches the end of its life when either the semiconductor has worn away or when the air gap has become too large due to electrode erosion. Thus, while there is a desire to have relatively large air gaps because fuel ignition is more likely with higher energy release, problems are encountered with increasing the air gap size. Increased air gap size means either a shorter igniter life due to less material used in the center electrode and/or electrode shell or a larger higher-cost igniter due to an increased outer shell diameter and, hence, increased material. It would be desirable to have an igniter allowing for an increased gap size without significantly increasing the size or amount of material used and without adversely affecting igniter life. 
     In addition to the above considerations, the igniter life can be shortened by the exposure of the semiconductor material to flame radiation. In some burner pilot configurations, the flame may root in a position in which the igniter&#39;s semiconductor material is exposed to flame radiation. Flame radiation damages the semiconductor material, which generally reduces the life of an igniter. Accordingly, it would be desirable to avoid this problem in a burner pilot design. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the current invention there is provided a spark igniter comprising a plurality of electrodes and an insulator, which are configured to form an elongated body having a first end, a second end and an outer surface extending between the first end and the second end. The spark igniter is configured so as to produce a radially-directed spark. 
     In accordance with another embodiment of the current invention there is provided a burner pilot comprising a source of electrical energy, a spark igniter, and a housing. The spark igniter has a first end, a second end, an outer surface, a center electrode, an electrode shell and an insulator. The outer surface comprises an end surface at the first end and a side surface extending from the second end toward the first end. The center electrode extends from the second end toward the first end. The electrode shell surrounds the center electrode and forms at least part of the side surface. The insulator is between the center electrode and outer electrode shell. The center electrode, the electrode shell and the insulator are configured to form a spark gap on the side surface, which produces a radially-directed spark, and the center electrode and electrode shell are connected to the source of electrical energy at the second end. The housing has a fuel flow passage which contains the first end of the spark igniter such that the spark gap is within the fuel flow passage. 
     In accordance with yet another embodiment of the invention, there is provided a method of igniting a fuel gas comprising: introducing the fuel gas into a flow passage having an ignition end wherein the flow passage defines an aperture at the ignition end and wherein the flow passage contains a spark igniter having an elongated igniter body terminating at a first end in a spark tip having a side surface and an end surface and wherein the spark tip is located adjacent to the ignition end of the flow passage; and producing a radially-directed spark to thus ignite the fuel and produce a flame at the ignition end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view with partial cutaway of a prior art spark igniter. 
         FIG. 2  is a perspective view with phantom walls of a burner pilot in accordance with an embodiment of the current invention. 
         FIG. 3  is a perspective view with partial cutaway of a spark igniter in accordance with an embodiment of the current invention. 
         FIG. 4  is a sectional view of an igniter tip in accordance with the embodiment illustrated in  FIG. 3 . 
         FIG. 5  is an elevation view of an igniter tip in accordance with another embodiment of the current invention. 
         FIG. 6  is an elevation view of an igniter tip in accordance with yet another embodiment of the current invention. 
         FIG. 7  is an elevation view of an igniter tip in accordance with still another embodiment of the current invention and illustrating flame radiation shielding. 
         FIGS. 8  A-C are elevation views from different angles of an igniter tip in accordance with still another embodiment of the current invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The description below and the figures illustrate a spark igniter and burner pilot of the type used in a furnace having a main burner that supplies a fuel and air mixture to the furnace and a burner pilot adjacent to the main burner for igniting the fuel and air mixture. While the invention is described in the context of a burner pilot for such a furnace, it will be appreciated that the inventive spark igniter is more broadly applicable as an ignition system for fuels and can be applied to other systems such as flare systems. 
     Referring now to  FIG. 1  a prior art axially-directed spark igniter  100  is illustrated. Spark igniter  100  has a center electrode  102  surrounded by an insulator (not shown) and an outer conducting shell or electrode shell  106  over the insulator such that, at the ignition end  108  of the spark igniter, an air gap  110  is formed between the center electrode  102  and the outer electrode shell  106 , i.e., a gap between the center electrode and the outer electrode shell. Often a semiconductor material is applied to the insulating material at this gap to facilitate sparking. At this air gap  110 , also called a spark gap, a high-energy spark can pass between the center electrode  102  and outer conducting shell  106 . 
     Housing  114  surrounds spark igniter  100 . Housing  114  forms a fuel channel  115 , which surrounds spark igniter  100 . The end  116  of housing  114  forms an opening. Fuel flows through fuel channel  115  and towards the opening in a generally axial direction parallel with the longitudinal axis of spark igniter  100 . 
     As can be seen from  FIG. 1 , air gap  110  is located on the end surface or axial-facing surface  112  of the ignition end  108 . Accordingly, spark igniter  100  produces an axially-directed spark, i.e., a spark directed along the longitudinal axis of the spark igniter at and away from the end surface  112 . Fuel is ignited by the spark downstream from axial-facing surface  112 , producing a flame adjacent to axial-facing surface  112 . This location of the flame and spark gap  110  means that the spark gap and any semiconducting material are exposed to flame radiation and damage therefrom. 
     Turning now to  FIGS. 2-4  a burner pilot  200  in accordance with one embodiment of the current invention is illustrated. Burner pilot  200  has a housing  202 . Housing  202  is comprised of a main pipe or tube portion  204 , electronics enclosure  216  and fuel introduction pipe  218 . Tube portion  204  has a wall  206  having a first end  208  and a second end  210  and a longitudinal fuel flow passage or fuel channel  212  defined by wall  206 . Second end  210  is connected to electronics enclosure  216  and the wall  206  defines an opening  214  at first end  208 . At or near second end  210  will be a sealing device  220  which seals fuel channel  212  so that it is not in fluid flow communication with electronics enclosure  216  and, hence, so that fuel cannot enter electronics enclosure  216 . 
     Fuel introduction pipe  218  is in fluid flow communication with a fuel source (not shown) and longitudinal fuel flow passage  212  of tube portion  204 . Generally, a fuel-air mixture will be introduced into passage  212  through pipe  218  such that the fuel-air mixture will flow in a generally longitudinal direction towards first end  208  and out opening  214 . 
     Extending longitudinally inside and along longitudinal passage  212  is a spark igniter  300 . Generally, spark igniter  300  is held in place by sealing device  220  and structural supports  222 . Structural supports  222  can be perforated to limit obstruction of the flow of the fuel and air mixture and can be shaped into swirling or diffusion elements to induce premixing of fuel and air within longitudinal passage  212  and prior to reaching the first end  302  of spark igniter  300 . 
     Spark igniter  300  has a first end or igniter tip  302  located inside tube portion  204  but near the first end  208  of tube portion  204  and a second end  304  extending into electronics enclosure  216 . As can best be seen from  FIGS. 3 and 4 , spark igniter  300  is comprised of a center electrode  306 , an insulating sleeve or tube  308  and an outer electrode shell or electrode tube  310 . Center electrode  306 , insulating sleeve  308  and electrode shell  310  generally extend from the igniter tip  302  to the second end  304  of spark igniter  300  with center electrode  306  extending though the center of electrode shell  310  and insulating sleeve  308  being positioned between center electrode  306  and electrode shell  310  in order to prevent electrical conductivity between the two electrodes. 
     As illustrated, spark igniter  300  is a high-energy igniter (HEI) probe. Accordingly, spark igniter  300  should be suitable to pass large current pulses (often greater than 1 kA) from an energy source to the spark gap and, thereby, generate a spark at the spark gap. The purpose of an HEI probe is to provide high ignition power. In applications with low temperatures, heavy fuels (heavy gases or oils), contamination of the igniter plug with coking or other debris, or moisture presence due to steam purging or rain, the main fuel may be difficult to light but an HEI system has the ability to maintain powerful high energy sparks in these adverse conditions. 
     An HEI system generally has a spark igniter and capacitive discharge system to provide a high-energy pulse to the spark igniter. As illustrated in  FIG. 2 , electronics enclosure  216  has located therein an exciter  224 . Exciter  224  is connected to a power supply (not shown but generally located outside of electronics enclosure  216 ), which provides electrical power to exciter  224 . Exciter  224  can be any high-energy exciter known in the art and suitable to provide a rapid electrical pulse to spark igniter  300  and, thus, cause a spark at spark gap of the spark igniter, as discussed below. For example, exciter  224  can be a capacitive discharge device. 
     Spark igniter  300  is connected at its second end  304  to exciter  224  so that center electrode  306  is connected to a first terminal of exciter  224 , generally the high voltage terminal, and electrode shell  310  is connected to a second terminal of exciter  224 , generally the low voltage terminal, which can be electrically grounded. 
     Turning now on  FIGS. 3 and 4 , spark igniter  300  and igniter tip  302  will be further described. As described above, the spark igniter  300  is constructed using a center electrode  306 , an insulation system (typically comprising insulation sleeve or tube  308 ) and electrode shell  310 . Electrode shell  310  can be about 0.25 to 0.75 inches in diameter. It should be understood that while spark igniter  300  is illustrated as having a center electrode covered by a concentric insulating sleeve and a concentric electrode tube, it could have any other suitable design consistent with the spark tip  302  as hereinafter described. Generally, spark igniter  300  will have a first electrode and a second electrode that are electrically isolated from each other but with ends that are adapted to produce a radially-directed spark upon application of an electrical charge on the opposite ends of the electrodes. Thus, for example, the first and second electrodes could form two halves of a cylindrical spark igniter rod with an insulator sandwiched between them, as long as they culminated in a spark tip suitable for producing a radially directed spark such as, but not limited to, the embodiments described below. 
     The igniter tip  302  comprises an outer surface  314  comprised of a side surface  316  and end surface  318 . The side surface  316  typically extends between the end surface  318  of the igniter tip  302  and the second end  304  of the spark igniter  300 . The igniter tip  302  terminates in end surface  318 , which is an axially-facing surface, as can be seen from the figures. Generally, the igniter tip in accordance with the invention is configured so that a spark gap  312  is on a radially-facing surface such as side surface  316  of spark igniter  300 . In  FIGS. 3 and 4  an embodiment is illustrated where electrode shell  310  forms a portion of the side surface extending from second end  304  of the spark igniter  300  and terminating at edge  320 , which defines an open end  322  of the electrode shell. Electrode  306  extends from second end  304 , though the inside of the electrode shell  310  and out the open end  322  of the electrode shell. The portion of electrode  306  extending out the open end  322  forms a cap  324  forming the end surface  318  and the cap side surface  326 , which is the portion of the side surface  316  adjacent to the end surface  318 . The spark gap  312  is located between the side surface  326  of the cap  324  and the open end  322  of the electrode shell  310  or, more specifically, between edge  328  of the cap and edge  320  of the electrode shell  310 . 
     As shown, insulator  308  extends concentrically around electrode  306  within electrode shell  310  so that the two electrodes do not make electrical contact. Further, spark gap  312  is between the electrode  306  and the electrode shell  310  and extends down to insulator  308  so that electrode  306  and electrode shell  310  do not make electrical contact. Additionally, there can be a semiconductor material  330  deposited on the insulator at the bottom of the spark gap  312 . Semiconductor material  330  forms a conductive path between the electrode  306  and the electrode shell  310 . This semiconductor can be a film applied to the insulator itself. This semiconductor assists the spark igniter  300  with spark initiation by allowing a low level of current to pass in the semiconductor when the energy source applies an ignition pulse to the electrode  306 . This low level current flowing through the semiconductor creates a small ionized air zone above the path of current in the spark gap  312 . This small ionized air path is a low impedance pathway for current flow. Once the pathway is established, the electrical energy is able to flow unresisted except for circuit impedance, thereby creating a very high current and energy spark at spark gap  312 . 
     As illustrated in  FIGS. 3 and 4 , the electrode  306 , insulator  308  and electrode shell  310  are cylindrical and spark gap  312  extends circumferentially completely around the cylindrical side surface  316 . It should be understood that other shapes are within the scope of the invention as is a spark gap that extends only partially around the circumference by either extending the insulator into the spark gap or limiting the semiconductor material to a portion of the spark gap. For example, side surface  316  can have a square or rectangular cross section with the spark gap extending only across one, two or three of the sides of the square or rectangle. 
     The spark generated at spark gap  312  is projected perpendicular to the longitudinal axis of spark igniter  300  and outward into the fuel-air mixture flowing through tube portion  204  and will, thus, be projected perpendicular to the flow of the fuel-air mixture, as shown by arrow  313 . In the embodiment illustrated, the spark igniter is cylindrical and thus the spark is projected radially outward; however, a similar spark projection, perpendicular to the longitudinal axis, will apply to other configurations, such as a spark igniter having a square, rectangular, triangular or oblong cross-section, and will generally be herein referred to as a “radially-directed spark.” The spark will ignite the fuel-air mixture forming a flame that will be located downstream from end surface  318 ; that is, the flame will be located on the other side of end surface  318  from spark gap  312 . Accordingly, cap  324  will act to shield the spark gap  312  and semiconductor material  330  from flame radiation generated from the flame. Turning now to  FIG. 7 , flame  332  is illustrated downstream from end surface  318 . As can be seen, flame radiation, indicated by arrows  334 , is blocked by end surface  318 . In this embodiment, the outer diameter  336  of end surface  318  (also called herein “end diameter”) is larger than the outer diameter  338  of the electrode shell  310 . Thus, the shielding effect is increased by the larger end diameter  336 . As illustrated, the end cap can have a partial conical side surface  326  so that its diameter is reduced towards the cap gap edge  328  to having an edge diameter  340 , which is smaller than end diameter  336  but can be equal to or larger than outer diameter  338  of the electrode shell  310 . 
     Turning now to  FIGS. 5 and 6 , alternative embodiments of a spark tip  400  in accordance with the invention can be seen. In  FIGS. 5 and 6 , as well as the other embodiments described below, like parts have been given like reference numerals.  FIGS. 5 and 6  illustrate an electrode shell  310  forming a portion of the outer surface  314 , including end surface  318  and a first portion of side surface  316 . Electrode shell  310  has a first gap edge  402  defining an aperture  404  in the side surface  316 . Generally, electrode shell  310  can form all of the outer surface  314 , except for within aperture  404 . The electrode  306  extends concentrically through electrode shell  310  with insulating sleeve  308  being concentric with and between electrode shell  310  and electrode  306  so that the two electrodes do not make electrical contact. Electrode  306  forms a second portion of the side surface  316  by extending up through insulating sleeve  308  within aperture  404 ; thus, electrode  306  forms a second gap edge  406  within aperture  404  such that first gap edge  402  and second gap edge  406  define the spark gap  312 . Generally, the first gap edge  402  and the second gap edge  406  can have the same shape and can be concentric. As illustrated in  FIG. 5 , the first gap edge  402  and the second gap edge  406  are circular and are concentric so as to form a circular spark gap  408 . As illustrated in  FIG. 6 , the first gap edge  402  and the second gap edge  406  are oblong so as to form oblong spark gap  410 . In the embodiments illustrated in  FIGS. 5 and 6 , there can be a single spark gap or a plurality of spark gaps on the side surface and these spark gaps can be distributed about the circumference of the side surface or can be confined to a portion of the side surface. For example, in  FIG. 5 , multiple circular spark gaps  408  are distributed evenly about the circumference of the side surface  316 . In  FIG. 6 , a single oblong spark gap  410  is confined to one-half of the circumference of the side surface  316  or less. 
     Turning now to  FIGS. 8A, 8B and 8C , a further embodiment of a spark tip  500  can be seen. The embodiment of  FIGS. 8A-8C  is similar to the embodiment of  FIGS. 3 and 4  with the electrode  306  forming cap  324 , except that cap  324  has a tab  502  extending therefrom. Thus, cap  324  includes a first side surface portion  504  and tab  502 . First side surface portion  504  is a portion of the side surface  316  adjacent to end surface  318  and extending longitudinally towards second end  303  (see  FIG. 2 ) of spark igniter  300  by a first length. Tab  502  is a second portion of the side surface adjacent to the end surface  318  and extending longitudinally towards second end  303  of spark igniter  300  by a second length. The first length of first side surface portion  504  is less than the second length of tab  502 . As can be seen from the figures, this means tab  502  has an edge having two longitudinal portions and a circumferential portion. Likewise, electrode shell  310  has a notch  506  having an edge having two longitudinal portions and a circumferential portion. Thus the spark gap is a longitudinal and circumferentially extending gap having a first circumferential portion  508 , a first longitudinal portion  510 , a second circumferential portion  512  and a second longitudinal portion  514 . 
     In the above embodiments, the semiconductor may be deposited on only a portion of the surface of the insulator in the spark gap to effectively reduce the spark direction. For example, in the embodiment illustrated in  FIGS. 8A-C , first circumferential portion  508  of the spark gap cannot have semiconductor material on the insulator surface while the first longitudinal portion  510 , second circumferential portion  512  and second longitudinal portion  514  can have semiconductor material on the insulator surface; thus, resulting in a spark production restricted to the gap around the tab. 
     In order to further illustrate the spark igniter of this invention, its operation and the methods of the invention, the following example is given. 
     EXAMPLE 
     Three igniter tips were life tested by repeatedly firing each igniter tip until it no longer fired. Control 1 and Control 2 were axially-directed spark tips and Example 1 was a radially-directed spark tip in accordance with the embodiment of the invention illustrated in  FIGS. 3 and 4 . The electrode material (center and shell) for each igniter tip were made from an austentic nickel-chromium-based superalloy sold under the trademark INCONEL 600 by the Special Materials Corporation group of companies. The igniter tips were connected to a 4 joules stored energy exciter producing a spark rate of 15 sparks per second and with a duty cycle of 30 seconds on, 30 seconds off. Further information on the igniter tips and the results of the life test are given in Table I. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Control 1 
                 Control 2 
                 Example 1 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Outer Diameter (inches) 
                 0.5 
                 0.625 
                 0.5 
               
               
                 Air Gap or Spark Gap 
                 1 
                 1.5 
                 1 
               
               
                 (mm) 
               
               
                 Total Number of Sparks 
                 2,193,434 
                 2,674,194 
                 4,790,000 
               
               
                 Until Failure 
               
               
                   
               
             
          
         
       
     
     As can be seen from Table I, the inventive radially-directed spark tip (Example 1) had a significantly longer spark life than either of the traditional axially-directed spark tips (Control 1 and Control 2). Example 1 had a 218% longer spark life than Control 1 and a 179% longer spark life than Control 2. 
     Other embodiments of the current invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Thus, the foregoing specification is considered merely exemplary of the current invention with the true scope thereof being defined by the following claims.