Stoves and furnaces – Stove hoods
Reexamination Certificate
1999-12-02
2002-01-08
Yeung, James C. (Department: 3743)
Stoves and furnaces
Stove hoods
C126S29900R, C055SDIG003
Reexamination Certificate
active
06336451
ABSTRACT:
This invention refers to a process and a device for confining, retaining and drawing off vapors, fumes, dust or similar fluid materials, resulting from kitchen ovens, cooking places and industrial working stations. However, the invention also can be used for retaining and drawing off other fluid materials, such as solutions, dispersions or suspensions. Especially, the invention refers to exhaust hoods to be used in the kitchen field and in the field of clean-air rooms.
Vapors, dust, fumes and the like are pollutants to be removed from air by drawing off the pollutants through a filter, for example a vapor hood. These materials often are included in very fast and turbulent streams of air. A mere suction flow is not suitable for retaining such streams because it's intensity nor it's stability is sufficient to divert and draw off a turbulent flow. For this reason the volume of the drawing off stream is chosen to be considerably larger than the volume stream of the pollutants, or a larger suction screen with high suction power is used.
DE 39 18 870 C2 discloses a method for improving the suction flow field of an exhaust hood. A downwardly directed free jet and a wall jet directed towards the suction surface cooperate with each other and generate a frontal vortex. The flow field produces an aerodynamic wall around the exhaust hood.
DE 42 03 916 C1 discloses a process for forming a blast flow according to DE 39 18 870 in such a manner that it results in a higher inherent stability and is formed as a helix, and passes the frontal vortex along the lateral sides of the exhaust hood. The drawback of both described processes is the expensive structure of a twin slot nozzle for generating the frontal vortex and the wall jet, as well as the problem in diverting the frontal vortex at the edges of exhaust hoods.
DE 33 04 262 C2 discloses a recirculation hood forming an air curtain around the lateral walls of an exhaust hood. As can be seen from the Schlieren photographs of this type of air curtain, no distinguished front is generated. Especially, this type of recirculation hood is to be further developed and improved by the subject invention.
In the field of meteorology, the expression ‘front’ is the boundary between different air masses. A front is a strongly convergent flow area on which extreme gradients, for example temperature or moisture gradients, preferably adjacent to boundary surfaces, such as the ground or a wall can occur. According to the invention, this type of front also is generated as an area of flow between the vapor section and the blow out area of the exhaust hood.
It is an object of this invention to improve the suction flow field of an exhaust hood for vapors, fumes and dust in such a manner that vapors, fumes, and/or dust are separated from ambient air, and a front is formed.
SUMMARY OF THE INVENTION
A blast jet exiting around the front edge of the hood is diverted into a movement extending towards the suction surface, and is transformed into a vortex or a curved shearing flow or shearing layer. Ideally, a vortex comprises a fixedly rotating nucleus surrounded by a shearing flow or a shearing layer. For generating a front it is vital that the shearing flow is able to build a convergent stream field generating a front if the stream hits a wall or a counterstream. With the invention front vortexes, as well as vortex or shearing streams are generated, and devices are proposed which build a front on the underside of the exhaust hood in a more stable and more effective manner, and also generate helical suction flows.
Diverting a jet for obtaining a curved vortex or shearing flow is obtained in different ways:
1. A direct suction effect acts upon a jet. The jet is blown out at the front edge of the hood into the area below the hood and, by means of gap suction formed within the deeper inner edge area of the hood, is diverted towards the bottom surface of the hood. Optimum orientation of the jet depends upon the intensity and the distance of the edge suctioning from the blast slot. Preferably, the jet is orientated in an angle of +/−30° to the vertical in order to obtain a positive generation of a front vortex and a front layer. The aperture of the suction slot is provided towards the center of the hood. The exit aperture and suction aperture in their most simple embodiments are separated from each other by a straight surface, the distance depending on the radius of curvature. The suction rate is in the order of the exhaust rate and is approximately 3-5 m/sec. In front of the suction slot, a trough can be arranged to act as a receiving trough and as a means for diverting the suctioned free jet and the vapor elements pulled along with the free jet.
2. The jet diversion is caused by the action of the Coanda effect on a wall jet over a curved surface or by blowing the jet in an inclined direction over a plane surface. The suction effect causing a curvature and acting upon a free jet also can be generated by a free jet by blowing the jet over a curved surface. Such jet adheres to the curved surface and is diverted up to 240°. This effect is known as the so-called Coanda effect and generates a vortex flow or a shearing flow. The curved surface takes the function of a vortex nucleus, either partly or totally. If a break edge is provided within the curvature, a vortex can be generated at this break edge. A jet is directed outwardly over a circular profile or a partial circular profile in a horizontal direction and generates a flow which at the bottom side of the hood is directed against the interior of the hood. For improving the adherence of a wall jet on a curved surface boundary layer, suctioning can be provided within the area where the flow separates from the surface.
A further possibility to provide a jet with a curved direction is to blow the jet at an angle a in view of the exit direction towards an inclined plate, a correspondingly inclined profile or a curvature if the jet adheres to the plate at an angle of 0<&agr;<50°. This is possible with a plate which is approached with the indicated range of angles. The jet adheres in a distance of 5-30% of the thickness of the blow out slot behind the slot at an angle of 25°<&agr;<30°.
Another way of diverting the jet is to blow it onto a straight surface in a tangential direction, which means that &agr;=0°, and the jet is a wall jet. This plane surface is joined by a curvature or a profile in order to generate a corresponding flow. If a half-circular, a circular segment type, a profiled or a similarly curved element is inserted between the vertical blast jet and the horizontal wall jet of a nozzle according to DE 39 18 870 C2, the effect of the process according to the invention will improve because the nucleus of the generated front vortex does not need to be built up completely or partly. Therefore, a larger proportion of the jet can be transformed into a vortex flow for generating a front.
3. Another possibility of diverting the jet is to direct a free jet leaving the front edge of the hood against a profile in such a manner that the jet is diverted in the direction towards the bottom side of the hood and towards the interior of the hood so that a curved vortex or shearing flow is generated. Such diversion of the jet towards the bottom side of the hood is equivalent to the effect of a flat of an airplane which, at high angles of incidence, passes the approach flow towards the airfoil profile.
4. A fourth possibility of diverting the jet is to combine the generation of a front vortex or a front vortex type flow according to the above possibilities mentioned in items 2 and 3 with edge suctioning according to item
1
, whereby surface suctioning can be dispensed with.
If several fans are used within an exhaust hood, either all fans can be operated in the suction mode, and part of the suction air can be diverted, or alternatively, separated fans for suctioning off or fans for blowing out vapors are used. The former process lends itself to an option if a single fan is used. When operating in t
Koppenwallner Georg
Koppenwallner Georg Emanuel
Röhl-Hager Hannelore
Hoffman Wasson & Gitler
Yeung James C.
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