EP0401583B1 - Method and device for exhaust fumes - Google Patents

Abstract

In a fume extractor hood (5) for cooking areas or other sources of dust or fumes, a blower nozzle arrangement (26) is provided on the front edge (27) of the hood which directs blown air (22) in the form of a wall jet approximately horizontally over the underside of the hood (28) against a filter (17) arranged in the rear hood region, so that the wall jet aspirates and entrains the flow of fumes. In a specific embodiment of the fume extractor hood, a downward-directed swirl nozzle (38) is provided on the underside of the front wall of the hood and generates a turbulent flow (37) which enters the flow of blown air. <IMAGE>

Description

The invention relates to an extractor hood according to the preamble of claim 1 and a method for extracting rising vapor according to the first part of claim 9.

To remove vapors and vapors, extractor hoods are arranged above the vapors, which suck the vapors and vapors through a filter surface using a fan and either return the filtered air to the work area (recirculation hood) or convey it to the outside via an exhaust air duct (exhaust hood). All conventional extractor hoods have in common that the steam or haze is drawn to the filter surface via a suction flow. Since the cooking steam is strongly accelerated upwards by the thermal buoyancy forces acting on it and thereby develops a strong dynamic of its own, it is very difficult to completely capture the steam or haze via a suction flow, since either a hood with a cross-sectional area that is essential is larger than the cooking surface, or a fan with extremely high suction power would be required.

The following considerations apply to illustrate the speeds and conditions prevailing on such hoods:

Hot water vapor has a density ρ _D at 100 ° C, which is about half the density ρ _{L of} the surrounding air. A thermal upward force acts on the steam, which tries to drive it upwards with an acceleration b ≈ 9.81 m / s². This acceleration is counteracted by the friction with the surrounding air at the edge of the vapor flow, which delays the edge areas and thus causes a mushroom-shaped vapor flow.

The freely rising water vapor reaches a theoretical upward speed of v _D = 3.1 m / s, for example after a distance of 0.5 m. The observation of ascending cooking steam confirms this and shows that the steam vapors develop their own dynamic with fluctuating ascending direction.

The suction flow of conventional extractor hoods is too weak to counteract this dynamic and to lead the vapor completely to the filter surface. The following consideration shows that there is a haze in front of the filter surface of normal hoods. It is assumed that the filter area F = 0.2 m² and the throughput V of the blower = 300 m³ / h. The entry velocity v of the air into the filter is thus:

The boiling water vapor has an upward speed of 3.1 m / s after a distance of 0.5 m. It must therefore be stowed in front of the filter area, since it can only flow through this filter area at the above-determined speed of 0.42 m / s. With this jam, part of the cooking steam escapes into the kitchen via the edges of the extractor hood without having enough storage space. In order to prevent this, various fresh air curtains have been proposed which are directed downwards from the edge of the hood. Corresponding suggestions can be found, for example, in the following DE-OSes 22 59 670, 19 63 456, 19 24 345, and 16 04 293.

DBGM 85 34 453 discloses an extractor hood with an approximately rectangular cross section with a separate feed line and suction line. The air coming from the supply line into the feed chamber formed by an inclined partition wall and the vertical side wall is discharged as a free jet via a rounded flow channel and an outlet opening. that is, directed as a jet without lateral limitation, against the filter arranged obliquely on the opposite side of the hood and arranged inside the hood. The flat free steel emerging from the outlet opening is cone-shaped diagonally upwards in the direction of the filter and is guided by one edge, and can be deflected by environmental influences, so that a clear guidance of the jet is not achieved, edge currents are not detected by the suction fan and that Effectiveness of the hood is reduced.

An extractor hood according to US-A-4 043 319 has a blowing nozzle arrangement on the front lower edge of the hood. Blown air is not directed as a wall jet and not nearly horizontally against the filter in the rear hood area. Air from the nozzle 70 is guided along the bottom side 62 as a wall jet for a short distance, but at the transition point 62/63 the wall jet is torn off and vortices V1 are formed along the surface 63, which cause the rising steam V is pushed somewhat back into the filter 50. When the steam flow V and the jet jet meet, there is no longer a horizontal wall jet, but the vortices, as extensions of the wall jet, rotate and are directed upwards. The underside of the hood is only approximately horizontal over an initial part, but in the second, decisive part it slopes steeply upwards.

In the extractor hood according to EP-A-0 163 763, the blowing jet is not designed as a wall jet, since there is a free space between the upper limit of the fan-shaped jet and the associated wall 21; the wall 21 is inclined upwards and backwards, and the blowing jet is directed slightly upwards. Furthermore, the wall 21 is an inner wall of the hood, not the underside of the hood. The filter 15 is also arranged in the interior of the hood at a distance from the rear of the hood housing.

US-A-41 53 044 shows an extractor hood with a throttle plate 78 and a rectilinear part 76a of the deflection device which together define a slot which forms a defined air flow 79 directed downwards against the filter 80. The air flow 79 is not a wall jet, but a free jet, as is evident from the jet-free area between the upper boundary of the jet cone 79 and the underside of the wall 74. Such a free jet is deflected upwards very easily by the upward-directed vapor flow or other flow influences outside the hood, so that the blowing jet would have to be generated with very high power in order to achieve a sufficient effect. However, this would have the consequence that the rising vapor flow would be prevented from flowing properly into and through the filter, especially since the filter is inclined upwards to the rear so that the free jet can strike vertically.

In order to prevent outbursts of haze in the kitchen, the fan power can be increased, which, however, usually results in a very high noise level, or fold-out screens can be installed at the front edge of the hood, but their effectiveness is relatively limited, which considerably impair the view of the cooking surface, and that contaminate relatively quickly due to water vapor and fat condensation.

The object of the invention is to develop an extractor hood of the generic type in such a way that the vapor stream rising from the vapor source is completely detected and escape into the kitchen or the surrounding space is prevented; At the same time, the performance of the suction fan and thus the noise level and the overall height of the hood should be as low as possible, and the effectiveness of the hood should be kept as large as possible.

This object is achieved according to the invention with the features of the characterizing part of claim 1. Further embodiments of the invention are the subject of the dependent claims.

The present invention thus proposes to blow a horizontal air jet from the front hood edge to the filter surface located at the rear hood end. This air jet is taken from the blower as blown air, deflected at the front edge of the hood and shaped through a slot in the front edge of the hood along the underside of the hood led by a wall jet. This wall jet detects the vertically rising vapor flow and leads it to the filter surface. The wall jet acts like an aerodynamic conveyor belt; the speed of the wall jet is higher than the speed of the rising cooking vapor, which is not delayed when it is detected on the underside of the hood. By using such a wall jet, the air flow to the filter is directed through the underside of the hood; the wall jet only draws in air on the free side, namely on the air surrounding the underside of the air flow facing the vapor source. The wall jet is guided safely through the wall, it adheres firmly to the wall and can even follow larger deflections on the wall. Furthermore, the wall jet is much less sensitive to influences from the environment than a free jet. Finally, the wall jet is necessary to generate the vortex due to the interaction with the air jet from the vortex nozzle.

The delivery rate of the suction fan arranged behind the filter area is dimensioned such that it delivers at least the amount of air from the wall jet at the filter entry area. Due to the suction effect of the jet, the initially blown air flow V _o rises continuously up to the filter surface. The blown air volume V _o can either be taken from the suction blower on the pressure side or sucked in via a small additional blower through the filter, or can be fed to the filter as an additional blower at the front edge of the hood in the form of a pressure blast jet. If the wall jet is generated via a slot nozzle, the most important flow variables along the jet can be roughly calculated according to the free jet theory, whereby the following applies:

The volume flow in the jet increases as follows:

The jet widens with increasing barrel length and its maximum speed decreases continuously.

The decrease in this maximum speed V in the beam is $$\text{v (x) / v (x = 0) ≈ 2.44}\frac{\text{1}}{\sqrt{\text{x / h}}}$$

The beam width is b _s = x · tg ϑ, where ϑ ≈ 14 °.

The beam spread can be estimated by the angle ϑ, which roughly defines the beam edge at which the speed is only 10% of the maximum value (ϑ ≈ 14 °).

The width of the filter surface must be adapted to the beam width so that the entire wall jet can be detected. Compared to a normal suction flow through a filter, in which the suction effect is distributed uniformly over the filter surface, the wall jet offers the advantage that the suction effect is greatest at the blowing nozzle located on the front of the hood.

The inflow speed in the filter is evenly distributed over the entire filter surface, while in the wall jet the inflow speed v _{z increases} corresponding to v _z ≈ 1 / √x towards the blowing nozzle. The following applies in particular: $${\text{v}}_{\text{e.g.}}{\text{(x) = 0.58v}}_{\text{O}}\text{·}\frac{\text{1}}{\text{2nd}}\frac{\text{1}}{\sqrt{\text{x / h}}}\text{; (x / h> 1).}$$

Since the blow jet is guided on one side on a wall on the underside of the hood, the free jet formulas can only be used as a first approximation. In principle, it is a wall jet with only one free inflow surface for the vapor flow. With the formulas given above, only a rough first interpretation of the blowing and suction surface geometry is possible. The conditions can be fine-tuned experimentally using a defined hood.

With conventional extractor hoods, vapor often escapes from the front hood area. Mechanical shields, so-called vapor shields, have already been used to prevent this type of vapor escape.

In contrast to this, the wall jet principle according to the present invention results in an aerodynamic vapor screen in the form of a vortex which arises at the front hood edge from the interaction of the wall jet with an additional weaker free jet or vortex jet. This weaker free jet is generated with the help of a small slot, which is called the swirl nozzle, and which is approximately perpendicular to the blow slot. The interaction of the two rays, namely the wall jet and the vortex jet, creates a vortex that lies below the front hood edge and whose direction of rotation points downwards and inwards from the front hood edge. This vortex acts beyond the edge of the hood, thus capturing the haze that blows past the edge of the hood and guides it to the blowing jet, which transports it to the filter surface.

The slot width b _{w of} the swirl nozzle is approximately one third to one quarter of the blow slot width. The air from the swirl nozzle is taken from the supply duct in front of the blowing nozzle, which is formed between the fan and the front hood limitation. Blowing nozzle and swirl nozzle thus work at the same total pressure.

The position of the vortex core can be controlled via the width of the vortex nozzle. If the width b ^{w of} the nozzle is small compared to the width of the blowing nozzle, the core is very close to the blowing jet. With a width of the blow jet nozzle of 6 mm and a width of the vortex nozzle of 2 mm, the primary vortex core is approximately 2 cm below the hood. Flow observations with smoke or strong cooking vapor show that the vortex is generated according to the described method and that it also detects the cooking vapor outside the hood. The core of the cooking vapor vortex is somewhat lower than the core of the primary air vortex.

In a special embodiment of the invention, the hood is equipped with a suction fan, of which about 25% of the air on the pressure side is used for the wall jet. The remaining 75% are discharged outside via the exhaust air duct. The blowing air is guided in a hollow chamber or a hollow channel to the front edge of the hood, where it exits via the blowing nozzle and the vortex nozzle. The rising mist is captured by the vortex and the wall jet and transported very quickly to the rear filter, where it is captured by the fan with the additionally drawn in air.

For a perfect effect of the extractor hood according to the invention, the flow continuity at the filter inlet is important, i. that is, the air flow of the wall jet at the filter inlet must not be greater than the conveying capacity of the fan, taking into account the flow resistance of the filter. If the blown air flow is too large, part of the air flow sweeps past the filter and down the rear wall.

With a hood edge vortex according to the invention, the escape of the vapor outside the hood area is prevented and cooking vapor, which rises from the stove or the source of the vapor from outside the hood, is detected and fed to the extractor hood.

With the help of the wall jet, due to the high speed of this jet, the entire rising haze is detected and fed to the filter. With a suitable design, the haze does not penetrate through the wall jet, but is taken along by it, so that no precipitation of water vapor or grease can be found on the wall side of the jet, ie the underside of the extractor hood, even with strong haze development at the haze source. In contrast to normal hoods, the underside and front of the hood remain dry and clean. The formation of water vapor, which condenses on the vapor screen of a normal hood in the event of strong vapor development, is caused by the hood vortex completely avoided. The underside of the hood is always exposed to the cleaned air of the jet and therefore contaminates much more slowly than with normal hoods with pure suction operation.

Fig. 1

das Prinzip einer bekannten Dunstabzugshaube über einer Dunstquelle,

Fig. 2

das Prinzip der erfindungsgemäßen Dunstabzugshaube mit Blasströmung, die als Wandstrahl geführt wird, über einer Dunstquelle,

Fig. 3

eine detaillierte Darstellung der Strömung des Wandstrahles nach der Erfindung an der Dunstabzugshaube,

Fig. 4

eine schematische Darstellung der Verteilung der Zuströmgeschwindigkeit am Absaugfilter und bei der Blasströmung des Wandstrahles,

Fig. 5

eine weitere Ausgestaltung der Erfindung nach Fig. 3 mit Haubenrandwirbel, und

Fig. 6

eine spezielle Ausführungsform der erfindungsgemäßen Dunstabzugshaube.

The invention is explained below in connection with the drawing using exemplary embodiments. It shows:

Fig. 1

the principle of a known extractor hood over a vapor source,

Fig. 2

the principle of the extractor hood according to the invention with blowing flow, which is guided as a wall jet, over a vapor source,

Fig. 3

a detailed representation of the flow of the wall jet according to the invention on the extractor hood,

Fig. 4

1 shows a schematic representation of the distribution of the inflow speed at the suction filter and with the blowing flow of the wall jet,

Fig. 5

a further embodiment of the invention according to FIG. 3 with hood edge vortex, and

Fig. 6

a special embodiment of the extractor hood according to the invention.

Known extractor hoods capture the haze by suction flow. In Fig. 1, the stove 1 has a hotplate 2 with saucepan 3 as a source of haze. The vapor flow 4 runs upward to the filter 6 provided in the extractor hood 5, above which a fan 7 is arranged. 8 shows the suction chamber, 9 the pressure chamber inside the extractor hood 5. The filtered air passes from the blower 7 through the exhaust line 10 either into the open air or as purified recirculated air back into the room. The speed of the rising vapor flow is indicated by arrow 11 with 12 the suction flow and with arrow 13 the exhaust air. 14 denotes the underside of the extractor hood 5, 15 an intermediate floor that separates the filter 6 from the fan 7. The speed of the vapor flow 11 (V _D ) is many times greater than the speed of the suction flow 12 (V _F ).

2, cooker 1, hotplate 2 and vapor source 3 corresponding to FIG. 1 are shown. The extractor hood 16 has a filter 17 which is arranged in the rear area of the extractor hood and positioned obliquely between the rear area of the hood underside and the hood rear, forming part of the hood underside. The fan 18 is arranged inclined to the filter 17. With 19 the suction chamber, with 20 the pressure chamber and with 21 the exhaust air duct. Furthermore, 11 denotes the vapor flow, 22 the wall jet on the underside of the hood, 23 the suction flow, 24 the exhaust air and 25 the blown air in the hood. A blowing nozzle 26 is formed at the front, lower hood end 27. 28 shows the underside of the hood, which is the wall delimiting the flow of the wall jet 22, and 29 the intermediate floor.

The darkness detection using the flow of the wall jet is shown in more detail in FIG. 3. Here, the intermediate floor 28 is formed with a rounded front end 30 within the hood 16; the extractor hood 16 has at the front end an approximately vertically extending end face 31 and an adjoining, approximately horizontally extending lower limit 32. The elements 30, 31 and 32 form the deflection channel for the blown air flow 25 within the hood and the blower nozzle 26, from which the wall jet 22 emerges, the lower limit of which is designated by 33. With 34 partial flows of the suction flow 11 are shown by arrows. Furthermore, the velocity distribution of the wall jet 22 is given in FIG. 3 in the form of the volume flow V̇.

Fig. 4 shows the distribution of the inflow speed in the filter, namely a) with suction flow and b) with blowing flow. In the case of the pure suction flow in known extractor hoods, the suction effect is distributed uniformly over the entire filter surface. Curve 35 is a straight line with the value v (x) / V̇ / L = 1. In the extractor hood working with blowing flow, the suction effect at the blow-out point on the front of the hood is greatest and decreases according to curve 36 with an increasing aspect ratio x / L from curve 36. The mean inflow V̇ / L is the same in both cases.

According to the invention, a hood edge vortex, which is designated by 37, is formed in the front hood region in order to support the detection of darkness with blowing flow. For this purpose, the underside 32 of the blowing nozzle 26 has a slot 38 through which a part 39 of the blowing air 25 emerges from the hood 16 approximately vertically downwards, while the larger part in the form of the wall jet 22 is directed against the filter 17. The jet 39 emerging from the vortex nozzle 32 forms, together with the wall jet 22, a vortex whose center 40 is formed just below the hood 16; A vortex flow 37 is formed around this vortex core 40, which is directed clockwise in the indicated direction of view and introduces into the wall jet 22 and is then transported together with this to the filter 17. This vortex also detects the rising haze 4 outside the hood and feeds it to the blowing jet. The slot width of the swirl nozzle 38 is a fraction of the slot width of the blow nozzle, about a third to a quarter. The position of the vortex core 40 can be controlled via the width of the vortex nozzle. The nozzle opening of the swirl nozzle 38 can preferably be designed to be adjustable, so that the ratio of the slot width from the swirl nozzle to the blowing nozzle is variable.

Fig. 6 shows a practical embodiment of an extractor hood with wall jet and hood edge swivel. The hood consists of a rear wall 41, a top wall 42 with a recess 43 and a flange 44 for the exhaust air duct, approximately one vertically arranged partial front wall 45, an inclined wall 46 extending from wall 45 to the front, an approximately vertical wall 47 representing the lower front boundary of the hood, a bottom wall 48 and an inclined wall between bottom wall 48 and rear wall 41 receiving filter 17 Wall and side walls, not shown. In the interior of the extractor hood, the fan 49 is separated from the filter area by an intermediate wall 50, so that a suction chamber 51 is formed between the intermediate wall 50 and the filter and a pressure chamber 52 is formed above the intermediate wall. The intermediate wall 50 is designed as a spiral housing surrounding the fan 49, which has an opening 53 to the front of the hood, through which the blowing air 25 can reach the blowing nozzle and the vortex nozzle. The blown air flows through an air duct formed by the hood housing, the shape of which is designed so that the blown flow reaches the blower nozzle or the vortex nozzle in a defined manner.

The embodiment shown can be easily converted into an air hood. For this purpose, the exhaust air (as usual with other switchable hoods) is led into the kitchen with the help of a switch flap, not shown.

Claims (9)

1. Exhaust hood (5) for installation over a cooking place (2) or a corresponding source of dust or fumes, comprising a suction fan (18), a filter system (17) and a blast nozzle arrangement (26) at the lower front edge which directs blast air (22) discharged from the flow passage (20, 21, 27) formed by the casing of the hood as a wall jet substantially horizontal under said hood against a filter (17) positioned in the rear hood area, whereby the wall jet (22) is restricted upwardly by the substantially horizontal lower side (28) of the hood,
   characterised in that

   a) the wall jet (22) passing from the blast nozzle (26) to said filter (17) is slightly downwardly directed, said filter (17) forms part of the lower side (28) of the hood within the rear hood area, and extends in an inclined manner from the underside (28) of the hood downwardly to the rear wall of the hood, and that

   b) on the underside (32) of the front hood wall a downwardly directed swirl nozzle (38) is provided which in view of the interaction between the wall jet (22) and the jet of the swirl nozzle (38) generates a vortex flow acting as an aerodynamic vapor screen.
2. Exhaust hood according to claim 1, characterised in that blast air (22) is removed from fan (18) on its pressure side (52) in the form of a bleeding flow (25), is conducted in a flow duct formed by upper side (46) of the hood and the inner side of bottom (48) of the hood, is diverted in the area of blast nozzle (26) and is conducted as wall jet (22) out of the blast nozzle (26).
3. Exhaust hood according to claim 1, characterised in that on the front hood edge (27) a separate fan is provided which produces the wall jet (22) and directs it against the filter (17).
4. Exhaust hood according to claim 1, characterised in that in the area of filter (17) a separate fan is installed with which the blast air of the wall jet is generated.
5. Exhaust hood according to claim 1, characterised in that the blast nozzle arrangement (26) is a slot formed within the housing of the hood.
6. Exhaust hood according to one of claims 1 - 5, characterised in that the velocity V_B of the wall jet (22) is higher than the velocity (V_D) of the rising fume.
7. Exhaust hood according to one of claims 1 - 6, characterised in that the blower output of the suction fan at least corresponds to the quantity of air of the wall jet at the filter intake surface.
8. Exhaust hood according to claim 2, characterised in that the swirl nozzle (38) is designed so that the vortex core (40) is formed somewhat underneath the free edge of the underside (32) and at a small distance from the underside, and that the eddy flow (37) passes around the core (40) downward from the front edge of the hood, and joins the airflow (22) of the wall jet towards the filter (17).
9. Method for drawing off fumes or steam rising above sources of fumes of steam by a suction fan and a filter which is connected upstream, purifying the suction air drawn off, whereby the suction flow is removed through the filter and the fan in the exhaust air or circulating air operation,
   characterised in that at the bottom of the exhaust hood a wall jet is generated which is conducted in a slight downward direction immediately underneath the bottom of the hood and is passed to a filter arranged in the path of the blast air flow so that the approx. vertical flow of fumes is carried along by the approx. horizontal blast air flow, and that in addition a vortex is generated, which turns around a vortex core in the front area of the hood, is directed downward and beyond the front edge of the hood so that this vortex catches the fumes arising at the front area and feeds them to the flow of the wall jet.

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