Abstract:
A gas-fired heat gun is ignited using an electrode tip located downstream of a flame holder such that the electrode tip is in the path of the flowing gas. The electrode is connected to a voltage source that causes a spark to jump from the electrode tip to another part of the heat gun, such as the flame holder or a casing of the heat gun, when a trigger is pulled, thereby igniting the flowing gas. The electrode is typically a thin wire extending through a portion of a diffuser defining a portion of the gas flow path upstream of the flame holder. The electrode may be continuous from the diffuser to a terminal end downstream of the flame holder.

Description:
RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/US2009/064807, which designated the United States and was filed on Nov. 17, 2009, published in English, which claims the benefit of U.S. Provisional Application No. 61/199,541, filed on Nov. 18, 2008. The entire teachings of the above application are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to portable liquid propane gas (LPG) burners, which are used in a great variety of applications. One application is as a heat gun, wherein pressurized gaseous fuel aspirates air for combustion in a jet pump and the combustion products entrain additional air to create a blast of hot air. Such heat guns are commonly used to heat plastic to moderate temperatures to soften it for bending or to shrink it for packaging. To shrink wrap large objects, such as boats, it may be advantageous to lengthen the reach of the heat gun by mounting an extension between the jet pump and the burner. Thus, the ignition system for such heat guns has to be able to adapt to ignite the burner with and without an extension. Present gas-fired heat guns use a spark plug mounted at a side of a burner chamber, which may be difficult to light and also requires electrical wiring outside of the heat gun to power the spark plug. The wiring must be long enough or include couplings and extension wires to accommodate an extension. The spark plug and associated wiring add weight to the heat gun, making it harder to handle and manipulate. Also, the couplings and extension wires increase the likelihood of a short-circuit or of a failure of an electrical connection. 
     SUMMARY OF THE INVENTION 
     The present invention is an improvement for a heat gun, such as the heat gun described in U.S. Pat. No. 3,917,442 or U.S. Pat. No. 6,227,846, the contents of which are incorporated herein by reference. Rather than using a spark plug as used by prior art heat guns, embodiments of the present invention place an ignition electrode in a flame holder, which is in the flow path of the gas mixture burned by the heat gun. A voltage applied to the electrode cause a spark to jump from an electrode tip at the flame holder to a portion of the body of the heat gun. For example, the spark may jump to the surface of the burner chamber of the flame holder. The electrode may run through interior portions of the heat gun, such as a diffuser chamber to be electrically coupled to a voltage source. The electrode is electrically isolated from other portions of the heat gun by insulators. In one embodiment, a ceramic insulator having a bore therethrough is fixed to the flame holder. As the flame holder is mounted to a main housing, the igniter electrode passes through the bore with the tip of the electrode exposed beyond the insulator. The voltage source may be a piezo-electric element, a battery, or an external power source. Typically, the voltage source is actuated by a trigger mechanism. In another embodiment, an intermediate member with an electrode is placed on the electrode of the heat gun, and the electrode of the intermediate member passes through the bore of the ceramic insulator of the flame holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is a cross-sectional side view of an embodiment of a heat gun; 
         FIG. 2  is a cross-sectional exploded perspective view of an embodiment of a heat gun; 
         FIG. 3  is a cross-sectional exploded side view of an embodiment of a heat gun; 
         FIG. 4  is a cross-sectional exploded side view of an embodiment of a heat gun with an extension tube; and 
         FIG. 5  is a cross-sectional side view of an embodiment of a heat gun with an extension tube; and 
         FIG. 6  is a cross-sectional side view of an embodiment of a heat gun with an intermediate member between a short lead and a short insulator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of example embodiments of the invention follows. 
     The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
       FIG. 1  shows a cross-sectional view of a heat gun  100  employing an ignition system according to an embodiment of the invention. The heat gun  100  includes a handle  21 , which houses a valve  22 , an igniter  23  and a trigger  24 . A fuel line  25  leads from the valve  22  to the jet pump nozzle  26 . The jet pump nozzle  26  is located by a nozzle holder  27 , which is supported by a pair of struts  28  inside the pump inlet  29 . Internally, the pump contains a bell-shaped inlet  30 , a cylindrical mixing section  31  and a diffuser  32 . The term “diffuser,” as used herein, refers to either an expanding diffuser, such as expanding diffuser  32 , or a constant cross-sectional area passageway, such as passageway  34  in  FIG. 1  or extension tube  50  in  FIG. 4 . A burner chamber  38  with a flame holder  37  is attached to the pump outlet hub  33 . The burner chamber  38  is held in place by fastener  35  and the joint is sealed by O-ring seal  36  and can be disassembled from the pump easily by removing the fastener  35 . 
     An electrode  40  mounts inside the pump along its center axis by an insulating lead-through  41  and connects to an igniter  23  by means of the igniter lead  42 . The igniter  23  may be a piezo-electric igniter that does not require an external power source. Alternatively, battery-powered igniters or igniters powered by external electrical sources may also be used. The electrode  40  fits through a bore  44  of an insulator  39  that is mounted centrally on the flame holder  37 . The insulator may be an insert made of electrically insulating material, such as ceramic, as shown in  FIG. 1 . Alternatively, the flame holder  37  may be made of an electrically insulating material, such as ceramic, and the bore is integral to the flame holder  37 . The tip  80  of electrode  40  passes through the insulator  39  and extends into the burner chamber  38  downstream of the flame holder  37 . 
       FIG. 2  shows a burner chamber  38  of an embodiment of a heat gun  200  partially disassembled from the pump  202 . By unscrewing the fastener  35 , the burner chamber  38  can be readily removed from o-ring seal  36 . As the burner chamber  38  is removed from the pump  202  and o-ring seal  36 , the electrode  40  slides out of the insulator  39 . Note that  FIG. 2  shows the burner chamber  38  separating from the pump  202  before the electrode  40  fully separates from the insulator  39 . This feature may ease assembly because it permits inserting the electrode  40  into the bore  44  before the burner chamber  38  slips over the pump outlet  33 . The insertion of the electrode  40  into the insulator bore  44  is further facilitated by the tapered insulator counter bore  45 . In an alternative embodiment (not shown), the insulator  39  may be configured in the burner chamber  38  such that the burner chamber  38  interfaces with the pump  202  and o-ring seal  36  before the electrode  40  interfaces with the insulator  39 . 
       FIG. 3  shows an embodiment of a heat gun  300  with a burner chamber  38  fully disassembled from the pump  302 .  FIG. 3  shows electrode  40  with free tip end  304  extending from the end of the pump  302 . When the burner chamber  38  is assembled onto the pump  302 , the free tip end  304  of the electrode  40  is inserted through the insulator bore  44  (and counter bore  45 ) before the burner chamber  38  interfaces with the pump  302  at o-ring seal  36 . Alternatively, the rear portion  34  of the burner chamber  38  may be lengthened such that the rear portion  34  interfaces with the o-ring seal  36  before the free tip end  304  of the electrode  40  interfaces with the insulator bore  44 . 
       FIG. 4  shows an extension tube  50  for an embodiment of a heat gun according to an embodiment of the invention before its assembly. At the entry of the extension tube  50 , an insulator  52  is mounted coaxially by means of struts  51 . Insulator  52  is fashioned similar to insulator  39  with a central bore  57  and a tapered counter bore  58 . An electrode  53  is mounted inside the bore  57  and leads to the outlet hub  54  end of the extension tube  50 , where it is held in place centrally by the insulated strut  55 . Electrode  53  extends beyond the outlet hub  54  end of the extension tube  50  by the same amount as the electrode  40  extends beyond the pump outlet  33 . 
     The outlet hub  54  is similar to the pump outlet  33  with an O-Ring seal  56  and a quick-connect fastener  67  so that it can mate with the burner inlet  34  of the burner chamber  38 . When burner chamber  38  is mated to the extension tube  50  via the burner inlet  34 , the electrode  53  in the extension tube  50  fits through the bore  44  of the insulator  39  that is mounted centrally on the flame holder  37 . The tip  70  of electrode  53  passes through the insulator  39  and extends into the burner chamber  38  downstream of the flame holder  37 . 
       FIG. 5  shows a heat gun according to an embodiment of the present invention with a pump  33 , an extension tube  50 , and a burner chamber  38  fully assembled. The extension tube  50  extends the length of the pump outlet hub  33  to the burner chamber  38 . In the fully-assembled state, the free tip end (not shown in  FIG. 5 , but see  304  in  FIG. 4 ) of electrode  40  presses against a tip end (not shown) of electrode  53  in the bore  57  of insulator  52 . In an embodiment, electrode  40  and electrode  53  interfere when the extension tube  50  is in the fully-assembled state. Electrode  40  is configured to flex slightly in the direction of the bell shaped inlet  30  in response to this interference. This flexing has a spring force effect, which maintains the tip ends (not shown) of electrodes  40  and  53  in contact when the extension tube  50  is in the fully-assembled state. The added pump length resulting from the extension tube  50  compared to a heat gun without an extension tube may cause the air/fuel mixture to take a longer amount of time to reach the burner chamber  38 . As a result, the igniter may have to create a spark for a longer period of time or be timed differently with respect to opening of the fuel valve as a result of a trigger pull. 
       FIG. 6  shows a heat gun according to an embodiment of the present invention, before its assembly, wherein the lead  140  in the pump outlet hub  33  is shorter than in other embodiments. Also, the insulator  142  in the burner chamber  38  of the embodiment in  FIG. 6  is shorter than in other embodiments. The embodiment of  FIG. 6  includes an intermediate member  141  that includes an intermediate electrode  143  and a tubular portion  144 . The tubular portion  144  includes a bore  146  and counter bore  145 . 
     When a heat gun according to the embodiment of  FIG. 6  is assembled, the bore  146  and counter bore  145  are installed over the end of lead  140  such that an end of electrode  143  makes electrical contact with the lead  140 . The opposite end of electrode  143  passes through a bore  150  and counter bore  151  in insulator  142  when the burner chamber is installed on the pump outlet hub  33 . 
     In operation, depressing the trigger  24  opens the valve  22  which sends pressurized gas to the nozzle  26 . The gas jet emanating from the nozzle  26  draws ambient air through the air inlet  29 . The gas and air mix in the mixing section  31  and the mixture is then pressurized in the diffuser  32 . As the trigger  24  is depressed further, it activates the piezo-electric igniter  23 , which sends a high voltage spike of electricity up through the lead  42  to the electrode  40  and creates a spark S at the tip of the electrode  40  that protrudes into the burner chamber  38 , igniting the combustible mixture. 
     To install the extension tube  50 , the burner chamber  38  has first to be removed as illustrated in  FIGS. 2 and 3 . The extension tube  53  is then mounted to the pump by first inserting the electrode  40  into the counter bore  59  of the insulator  52 , then sliding the extension tube  53  over the pump outlet hub  33  and securing the joint with fastener  35 . Then the burner chamber  38  is mounted to the outlet hub  54  of the extension tube  50 . Again, the connection between the tip of the electrode  53  and the counter bore  44  of the insulator  39  is established first before the burner inlet  34  is inserted. 
     Disassembly proceeds in the reverse order. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.