Abstract:
An improved combustion assembly including a venturi mixer and a combustion tip attached to the discharge end of the venturi divergent section. The improvement comprises eliminating abrupt contractions and sudden expansions of the interior flow passage at the venturi mixer and combustion tip interface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to venturi mixers used for delivering fuel and air mixtures to burners and pilots. More particularly, but not by way of limitation, the present invention relates to combustion assemblies comprising venturi mixers having burner tips, pilot tips, or other types of combustion tips attached thereto. 
     BACKGROUND OF THE INVENTION 
     Premix burners and pilots have long been employed for producing and burning gaseous fuel and air mixtures in applications ranging from residential furnaces to large process heaters. Premix burners typically use venturi mixers to combine and mix the gaseous fuel and air components and to deliver the resulting fuel/air mixture to a combustion tip (e.g., a burner tip or a pilot tip). As used herein and in the claims, the term “venturi mixer” refers to and includes any venturi, eductor, or similar device wherein the ejection and/or flow of a gaseous fuel is used to draw air into the device and which includes a divergent discharge section from which the resultant mixture of gaseous fuel and air is delivered. Commonly, as fuel under pressure enters the venturi mixer, a low-pressure zone is created that acts to pull air into the mixer. For each cubic foot of fuel ejected into the venturi, between three and eight cubic feet of air may be educted into the venturi depending upon the efficiency of its design. 
     In a particularly preferred type of venturi mixer, fuel gas under pressure is ejected from an orifice toward the center of a convergent (typically bell-shaped) opening at the rearward (upstream) end of the venturi. A straight section or throat follows (i.e., is positioned forwardly/downstream of) the convergent inlet section. The throat connects, in turn, to a discharge section of the venturi having a divergent interior wall. A combustion tip is then typically welded or threadedly connected to the venturi at the exit end of the divergent discharge section. Fuel gas and air are mixed within the throat and the divergent section of the venturi to provide a uniform combustion mixture which is then delivered through the combustion tip. As used herein and in the claims, the term “combustion tip” refers to a burner tip, a pilot tip, or any other type of combustion tip structure employed for delivering a fuel/air combustion mixture from a venturi mixer into a combustion area preferably downstream of (i.e., outside of) the tip. 
     Venturi mixers operate in accordance with Bernoulli&#39;s Theorem. Because of the high velocity of the fuel gas jet delivered from the fuel gas ejection orifice, a negative pressure is created at the entrance of the venturi. This negative pressure zone continues into the straight section or throat of the venturi and draws air into the throat where the air begins to mix with the fuel gas. In the divergent section of the venturi located downstream of the throat, the gradual enlargement of the flow passage operates to convert the velocity pressure of the fuel gas and air mixture into static pressure. The resultant static pressure provides the motive pressure needed to expel the fuel/air mixture from the flow port(s) of the combustion tip. 
     Heretofore, premix burners have used methods of attaching the combustion tip to the venturi which have had little regard for minimizing pressure drop at the end of the divergent section. The attachment of the combustion tip often imposes a significant pressure drop due to the existence of a sudden contraction or sudden enlargement at the venturi/combustion tip interface. This pressure drop can significantly and undesirably reduce the degree of static pressure regain attained in the apparatus. Thus, it is an object of the present invention to significantly reduce or eliminate such pressure drop at the exit of the venturi divergent section as it transitions to the burner or pilot tip. 
     Depicted in FIG. 1 is a prior art combustion assembly comprising: a venturi mixer  10 ; a combustion tip  20 ; and a fuel gas supply pipe  12  containing a gaseous fuel  14  under pressure. Gas supply pipe  12  terminates in a fuel metering orifice  16 . Gaseous fuel  14  is emitted from orifice  16  in a jet  18 . Combustion air  20  is educted into the bell-shaped inlet  22  of mixer  10 . The gaseous fuel jet  18  and combustion air  20  are mixed in the venturi throat  24  and in the venturi divergent section  26 . As mentioned above, the purpose of the venturi divergent section  26  is to convert the velocity pressure of the fuel/air mixture exiting the throat  24  into static pressure. Burner or pilot tip  28  is secured at the exit end of the venturi divergent section  26  and includes outlet flow ports  30 . The fuel/air mixture exits combustion tip ports  30  and is combusted in furnace space  32 . 
     In prior art devices of the type depicted in FIG. 1, the combustion tip  28  is received in the exit end of the venturi divergent section  26  and is typically threadedly connected or welded thereto. In this prior art assembly, an abrupt contraction  34  of the type shown in FIG. 1 is commonly imposed upon fluid stream as it flows into combustion tip  28 . The flow obstruction  34  creates a significant pressure drop at the divergent zone exit. 
     Another typical prior art combustion assembly is shown in FIG.  2 . The prior art assembly of FIG. 2 is very similar to the prior art assembly of FIG. 1 except that, in the embodiment of FIG. 2, the discharge end of the divergent section of a venturi mixer  40  is received in a combustion tip  44 . The burner tip or pilot tip  44  is commonly attached by threaded means or by welding. The combustion tip  44  includes one or more flow ports  46  through which a gaseous fuel/air mixture  50  exits the tip  44  to be consumed in the furnace space  48 . A significant pressure drop occurs in this apparatus because of an abrupt expansion  52  at the interface of the divergent section  42  and the combustion tip  44 . 
     SUMMARY OF THE INVENTION 
     The present invention provides a venturi mixer and a combustion assembly which alleviate the problems and satisfy the needs discussed above. In the inventive design, the divergent section of the venturi mixer extends into or otherwise interfaces with the interior of the combustion tip in a manner such that abrupt expansion or sudden contraction is either eliminated or at least greatly reduced. 
     In one aspect, the present invention provides an improvement for a combustion assembly. The combustion assembly includes a venturi mixer and a combustion tip attached to a discharge end section of the venturi mixer. The discharge end section of the venturi mixer has a divergent interior wall. The improvement comprises an extension of the discharge end section which projects the divergent interior wall into the combustion tip. The divergent interior wall preferably terminates at a sharp distal end of the extension which is positioned inside and adjacent to the interior wall of the combustion tip. In this improved assembly, the sharp distal end of the extension preferably touches the interior wall of the combustion tip. 
     In another aspect, the present invention provides an improvement for a combustion assembly including a venturi mixer and a combustion tip attached to a discharge end section of the venturi mixer wherein the discharge end section has a divergent interior wall and the improvement comprises: the combustion tip having an interior shoulder, the discharge end section of the venturi mixer having a forward end substantially abutting the interior shoulder; the divergent interior wall having an interior diameter at the forward end of the discharge end section of the venturi mixer; and the combustion tip having an interior diameter at the interior shoulder which is substantially equal to the interior diameter of the divergent interior wall at the forward end of the discharge end section of the venturi mixer. 
     The benefits derived from the present invention will be apparent to those skilled in the art. Because of the reduction in pressure drop at the venturi exit, the device is able to induce more combustion air. Thus, the premix gas burner tip or pilot tip is capable of operating over a wider air to fuel ratio range. Further, the flame propagation speed of the inventive assembly desirably increases as a result of the induction of more combustion air. In addition, with the increase in flame propagation speed comes a desirable increase in flame temperature as well as increased flame stability. 
     Another benefit of the present invention is the ability of the burner or pilot tip to operate at a higher internal pressure. The port(s) of the combustion tip can therefore be made smaller because of increased discharge pressure. Moreover, a higher internal pressure in the tip desirably provides greater ability to increase the tip exit velocity when needed to match the high flame propagation speeds encountered with certain fuel compositions. 
     Matching the tip exit port velocity to the flame propagation speed is of particular benefit for burners and pilots used in refinery applications. In an upset condition in a petroleum refinery, the fuel supplied to the burners in the process heaters typically contains a higher percentage of hydrogen. Hydrogen has a very high flame propagation speed. Thus, a low exit velocity at the burner tips or pilot tips can result in flame instability and flashback. However, the burner and pilot assemblies produced in accordance with the present invention can be designed to provide exit velocities exceeding the worst-case flame propagation speeds, thus resulting in greater stability and resistance to flashback. 
     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a prior art venturi mixer/burner tip combination. 
     FIG. 2 is a sectional view of a second prior art venturi mixer/burner tip combination. 
     FIG. 3 is a sectional view of an embodiment 55 of the inventive combustion assembly. 
     FIG. 4 is a sectional view of an alternative embodiment 90 of the inventive combustion assembly. 
     FIG. 5 is a sectional view of another alternative embodiment 110 of the inventive combustion assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment 55 of the inventive combustion assembly is depicted in FIG.  3 . Combustion assembly  55  comprises a venturi mixer  60  having a burner tip or other combustion tip  78  attached to the discharge end thereof. Assembly  55  further comprises a gas supply pipe  62  which terminates in a fuel metering orifice  64 . Gaseous fuel  66 , under pressure, is emitted from orifice  64  in a fuel jet stream  68  and causes combustion air  70  to be educted into the bell mouth opening  72  of venturi  60 . The fuel jet stream  68  and combustion air  70  mix in venturi throat  74  and in the venturi divergent section  76 . Combustion tip  78  is secured on and is in communication with the venturi divergent section  76 . The combustion tip  78  includes one or more flow port(s)  80  through which the gaseous fuel/air mixture exits the burner tip  78  to be consumed in the furnace space  82 . 
     The venturi discharge section  76  of inventive assembly  55  is received in the combustion tip  78 . As will be understood by those skilled in the art, the combustion tip  78  can be secured on the venturi discharge section  76  by threaded attachment, welding, or any other suitable technique. 
     Inventive combustion assembly  55  is designed and adapted to overcome the adverse pressure drop effects encountered in the prior art devices. In accordance with the present invention, the venturi discharge section  76  of inventive assembly  55  includes an extension  65  which projects the divergent interior wall  75  of discharge section  76  into the combustion tip  78 . The extension  65  and the divergent interior wall  75  preferably terminates at a sharp forward end  86 . The sharp end  86  will preferably be circular in shape. To eliminate any sudden expansion in the interior of inventive assembly  55  at the divergent section/combustion tip transition point, the sharp forward end  86  of extension  65  is preferably positioned inside and adjacent to the interior wall  79  of combustion tip  78 . Most preferably, the sharp forward end  86  of extension  65  will touch the interior wall  79  of the combustion tip  78 . 
     In the particular embodiment of inventive combustion assembly  55  shown in FIG. 3, the combustion tip  78  is secured on venturi mixer  60  by a threaded connection between internal threads  85  provided in combustion tip  78  and matching exterior threads  84  provided around the venturi divergent section  76 . The sharp forward end  86  of venturi extension  65  is spaced forwardly (i.e., downstream of) threads  84  and  85 . The sharp forward edge  86  of extension  65  provides a smooth transition between the termination of the divergent section  76  and the combustion tip  78 . 
     An alternative embodiment 90 of the inventive combustion assembly is shown in FIG.  4 . The alternative embodiment 90 differs from embodiment 55 in that the combustion tip  95  of inventive assembly  90  is extended a greater distance downstream of the venturi divergent section  92 . In applications such as industrial pilots, the venturi mixer is often required to be remotely located from the pilot tip. Thus, in inventive assembly  90 , the pilot tip  95  includes an extended rearward section  94 . 
     As with embodiment 55, the venturi divergent section  92  of inventive assembly  90  includes a forward extension  93  which projects the divergent interior wall  97  of the venturi into the rearward section  94  of the combustion tip  95 . The venturi extension  93  terminates in a sharp forward edge  99  which most preferably touches the interior wall of the rearward section  94  of combustion tip  95 , thus providing a smooth transition from the divergent interior wall  97  of the venturi into the combustion tip  95 . 
     In inventive assembly  90 , the venturi is joined to the rearward section  94  of combustion tip  95  by a threaded coupling  96  having interior threads  100 . Matching exterior threads  102  and  104  are provided respectively around the venturi divergent section  92  and the rearward section  94  of the combustion tip. The distal end  99  of the venturi extension  93  is preferably spaced forwardly (downstream of) threads  102 . As also shown in FIG. 4, an initial portion  106  of the interior of rearward section  94  of combustion tip  95  can also be tapered to accommodate the venturi extension  93 . 
     Another embodiment 110 of the present invention is shown in FIG.  5 . Combustion assembly  110  is a close-coupled combination of a venturi  115  and a burner tip  120 . Burner tip  120  is attached on the end of venturi divergent section  122 . In the particular embodiment of the inventive combustion assembly  110  shown in FIG. 5, exterior threads  124  provided on the venturi divergent section  122  mate with interior threads  126  provided in burner tip  120 . However, it will be understood that a welded connection or other suitable attachment could alternatively be used. 
     In inventive combustion assembly  110 , combustion tip  120  preferably includes an interior shoulder  125  positioned forwardly of the combustion tip interior threads  126 . When combustion tip  120  is secured on venturi mixer  115 , the forward end  127  of the venturi divergent section  122  preferably abuts the combustion tip interior shoulder  125 . In addition, the inside diameter  128  of combustion tip  120  at interior shoulder  125  is preferably substantially equal to the inside diameter  130  of the venturi divergent section  122  at its forward end  127 . Thus, a smooth transition from the venturi divergent section  122  to the burner tip  120  is provided. 
     Although the inventive structures shown in FIGS. 3-5 are preferred, it will be understood that, in each of embodiments 55,90, and 110, the attachment between the venturi mixer and the combustion tip could be reversed such that the rearward end of the combustion tip, and/or an extension thereof, having a rearwardly convergent (i.e., forwardly divergent) interior wall is received in the forward (discharge) end of the venturi mixer. In embodiment 110, such reversal would require that the rearward end of the combustion tip abut against an interior shoulder formed within the venturi divergent section. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the appended claims.