Venturi mixer and combustion assembly

Information

  • Patent Grant
  • 6814570
  • Patent Number
    6,814,570
  • Date Filed
    Monday, June 2, 2003
    21 years ago
  • Date Issued
    Tuesday, November 9, 2004
    20 years ago
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'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.



Claims
  • 1. In a combustion assembly including a venturi mixer and a combustion tip attached to a discharge end section of said venturi mixer, said discharge end section having a divergent interior wall, the improvement comprising an extension of said discharge end section which projects said divergent interior wall into said combustion tip, said divergent interior wall terminating at a sharp end of said extension which is positioned inside and adjacent to an interior wall of said combustion tip wherein:said combustion tip is attached to said discharge end section of said venturi mixer by a threaded connection between exterior threads provided around said discharge end section of said venturi mixer and corresponding interior threads provided inside said combustion tip and said sharp end of said extension is spaced forwardly of said interior threads provided inside said combustion tip.
  • 2. The combustion assembly of claim 1 wherein said sharp end of said extension touches said interior wall of said combustion tip.
  • 3. In a combustion assembly including a venturi mixer and a combustion tip attached to a discharge end section of said venturi mixer, said discharge end section having a divergent interior wall, the improvement comprising:an extension of said discharge end section which projects said divergent interior wall into said combustion tip; said divergent interior wall terminating at a sharp end of said extension which is positioned inside and adjacent to an interior wall of said combustion tip; said combustion tip being attached to said discharge end section of said venturi mixer by a coupling; and said coupling is threadedly connected between first exterior threads provided around said discharge end section of said venturi mixer and second exterior threads provided around said combustion tip.
  • 4. The combustion assembly of claim 3 wherein said sharp of said extension is spaced forwardly of said first exterior threads.
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