Combustion – Mixer and flame holder
Reexamination Certificate
2003-06-02
2004-11-09
Price, Carl D. (Department: 3749)
Combustion
Mixer and flame holder
C239S548000
Reexamination Certificate
active
06814570
ABSTRACT:
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 a
Isaacs Rex K.
Little Cody L.
McDonald John
Zink Darton J.
Brown Dennis D.
Fellers Snider Blankenship Bailey & Tippens, P.C.
Price Carl D.
Zeeco Inc.
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