D E S C RIP T IO N

The present invention relates to a new concept cooking fumes suction system for cooktops.

The suction of fumes and vapours generated during a cooking process on a cooktop is a fundamental function in a kitchen but it has not yet been resolved satisfactorily.

As known, the hoods used in the kitchen are of two fundamental types: the filtering hoods and the suction hoods. The first ones filter and purify the extracted fumes to then introduce them again in the kitchen room from an expulsion outlet placed in a position not annoying for the user; the second ones expel the fumes to the outside. As to their position relative to the cooktop, it makes no difference whether they are filtering or suction hoods.

The most conventional position of the hoods is above the cooktop with the suction surfaces exposed to the cooktop. In the most common cases, the suction surfaces are flat and horizontal (such hood is hereinafter referred to as "horizontal hood"). Considering a common wall installation, with the same cooking configuration (type, geometry and use of the cooktop), it is known that the fume collection efficiency of a hood essentially depends on: volumetric flow of the fan, installation height, position and shape of the suction surfaces. At equal volumetric flow, and suction geometries, the efficiency increases as the installation height decreases, but for the latter there is a lower regulatory limit in the case of coupling to gas cooktops (h>65 cm). At the same time, the width of the hood is limited to two or three "sizes" (60, 90, 120[cm]) due to the standardization of the kitchen wall units and the user only chooses a width identical to that of the related cooktop; greater widths are felt as an unnecessary increase in price, while lower widths would compromise the performance and aesthetics of the cooking zone. As regards the depth of the hood, this is limited by the ergonomics: an excessive projection from the wall (that would improve the efficiency) does not allow easy inspection of the gas cookers and constitutes a risk to the head of the user.

Therefore, the operation of a horizontal hood is the result of a compromise of technical and regulatory requirements.

This compromise is not satisfactory: at equal number of active gas cookers on the cooktop, a more powerful fan is necessary and therefore noisier than for other known solutions commented below. Despite such suction power, the collection efficiency is limited by the fact that not all the cooking fumes can be intercepted at least if the active gas cookers are many and the outermost with consequent diffusion of odours and greasy vapours in the kitchen.

Of course, with a few active gas cookers, the fan may be operated at a lower speed in a substantially silent manner.

The inadequacy of the traditional horizontal hood is proven by the multiple alternative solutions proposed.

Document DEI 9843436999 shows an improvement in the horizontal hoods consisting in that the hood has mechanisms to descend vertically from a first position in which the inspection of the vessels is privileged to a second position in which the extraction effectiveness is privileged. The limits of such solution are in the costs of the mechanism, in the need to manoeuvre it during the cooking operations (operation that in the practice the user has no time to perform while he is focused on the cooking operations), in the dispersion, however, of odours and vapours when the hood needs to be risen to the first position.

Document F 2698022, it also related to horizontal hoods, provides a movable screen that descends vertically from a first position at the top, in which it does not interfere with the cooktop and is not operational, to a second operating position at the bottom in which it prevents the dispersion of the fumes but substantially prevents the user access to the cooktop. In a variant, the screen has means for blowing air towards the fumes greatly hindering the dispersion thereof in the environment.

As an alternative to the horizontal hoods many solutions with vertical hoods have been proposed, that is, with vertical or substantially vertical suction front. A widespread technical solution provides such vertical hoods fixed to the wall behind the cooktop. An advantage of such solutions is the lack of impediments to the inspection of the vessels. Even such solution is not free from drawbacks: the suction is very effective on the nearest gas cookers while it is less on the furthest and the hood constitutes an encumbrance that reduces the space allocable to the cooktop.

The so-called "downdraft" hoods are then known, e.g., see documents US 5.231.972, US 2.674.991 , EP 2.105.674 the suction inlet whereof in operating conditions emerges from the cooktop, usually among the gas cookers, while it is retracted in the cabinet below the cooktop in non-operating conditions. An advantage of such solution is that the suction capacity is high and similar for all the gas cookers. Drawbacks are: the possibility of spilling liquids on them at least when they are closed; the obstacle to the inspection of the vessels behind; the complication of the movement mechanisms; the great difficulty of arranging the fume venting duct, whether the hood is suction or filtering; finally, the fact that it occupies, with mechanisms and pipes, the cabinet below the cooktop that, for its ease of access, is much more useful to be used fully as a storage compartment or as a compartment to accommodate the oven.

In conclusion, the hood is noisy and nevertheless not very efficient in the capture of cooking fumes but the known alternative solutions solve this problem by creating other drawbacks, that the horizontal hood has not, as regards at least: easy inspection of the vessels; plan dimensions; complication and cost of mechanisms; volumes of cabinets occupied; difficulty in placing the cooking fumes venting conduit.

One object of the present invention is to improve the collection efficiency of horizontal hoods even conventional and known by using exclusively means integrated in the underlying cooktop.

A further object of at least some variants of the present invention, is to obtain such result, in particular cooktops, also using means already present in such particular cooktops.

A further object of at least some variants of the present invention, is to obtain such a result, in other particular cooktops, by using exclusively means already present in such other particular cooktops.

- Fig. La shows, in a plan view and schematically, the main elements of a per se innovative cooktop and which also incorporates a possible embodiment of the invention;

- Fig. Lb shows a detail of Fig. La.

- Fig. l .c shows, in a plan view and schematically, different possible arrangements of cooking vessels on the cooktop of Fig. La;

- Fig. 2 shows graphic symbols used in other figures;

- Fig. 3. a shows, in a plan view and schematically, the main elements of a further innovative cooktop, not only aesthetically but also functionally different from the cooktop of Fig. La, and which also incorporates a further possible embodiment of the invention;

- Figs. 3.b and 3.c show, in the particular conformation for the cooktop of Fig.

3. a, two possible states of a burner usable by the invention;

- Figs. 4. a and 4.b show two possible states of a feeding valve of a burner usable by the invention;

- Fig. 5 shows a further feeding valve of a burner usable by the invention;

- Fig. 6. a schematically shows, in a plan view, a cooktop incorporating two possible embodiments of the invention;

- Fig. 6.b shows, in a section view and schematically, one of the two possible embodiments of the invention of Fig. 6. a; - Fig. 7 shows, in a section view and schematically, the other of the two possible embodiments of the invention of Fig. 6. a;

- Fig. 8 shows, in a section view and schematically, both of the possible embodiments of the invention of Fig. 6. a jointly used;

- Fig. 9. a shows, in a plan view and schematically, a cooktop according to a further possible embodiment of the invention;

- Fig. 10. a shows, in a section view and schematically, the same cooktop of Fig. 9;

- Fig. lO.b shows a detail of Fig. 10. a.

Given the many variants of the invention, the figures represent only some particularly exemplifying thereof and not necessarily all the means represented in every single figure are essential for most of the variants.

Unless otherwise specified, any possible absolute spatial reference in this report such as the terms vertical/horizontal or lower/upper refers to the position in which the elements are arranged in operating conditions while any relative spatial reference such as the terms previous/subsequent, upstream/downstream should be understood with reference to the direction of circulation of the flows of airforms.

The invention provides a cooktop provided with:

- at least a fan seated so as to draw air from the environment and which must be made operational at least when the overlying hood is in operation;

- suitable air ducts leading at least part of said air drawn from environment, from the outlet of said at least one fan towards the outside of the same cooktop through at least some of a range of ventilation openings.

These ventilation openings have the following characteristics:

- are shaped so as to direct upwards, in the direction of the overlying hood, the air jets exiting therefrom;

- in some variants, they can be made operational, that is open to the passage of air, in a different number instant by instant according to a configuration that depends on the configuration of gas cookers switched on at that instant; - are sufficiently close to each other to ensure that the said air jets that those made operational produce form substantially a continuous curtain of ascending air with flow which, in a plan view, circumscribes, at least partially, at least all the burners operational at the moment, such curtain forming an air blade sufficient to constitute an obstacle to the lateral deviation of fumes and vapours produced by the operating gas cookers and a guide thereof, instead, towards the hood,

- where, in particular, said curtain may be composed of multiple portions also discontinuous and each surrounding a single one of more burners; - are suitably dimensioned and calibrated based on the default head of the fan so that the air jets have a speed comprised between a value that ensures a curtain of sufficient stability and persistence against perturbative air currents and a maximum value compatible with the formation of the said flow (preferably laminar).

Before proceeding with the description of the invention it is necessary to mention important innovations on cooktops and relative gas cookers introduced by the same applicant of this patent and whose teachings and means are in part used in some variants of the present invention.

They are herein referred to as:

- "primary air", the combustion air mixed with a fuel gas upstream of the point where the combustion takes place;

- "secondary air", the combustion air optionally to be flown to the flame in order to complete the combustion;

- "stoichiometric air", the amount of air required to obtain a stoichiometric mixture, that is, where fuel gas and combustion air are in the correct theoretical ratio for a complete combustion without residual oxygen; because in practice the complete combustion requires at least a small excess of air, that is, a mixture at least slightly lean in fuel, such mixture at least slightly lean in fuel is still referred to herein as "stoichiometric mixture" or "STC mixture"; - "atmospheric burner" a burner that requires both primary air, sucked by the fuel gas by Venturi effect, and secondary air recalled on the flames by natural draft;

- "blown burner", a burner where the primary air is fed through a fan;

- "premix burner" a particular blown burner where the primary air supplied is at least sufficient to produce a STC mixture; the premix burners do not need secondary air that in fact is deleterious because it cools the flames.

With reference at least to Figs. 1, 3 and 6, it is possible to feed a plurality of blown burners BRN (premix in particular) of a cooktop PC by introducing in them the primary air through a single fan FAN. Each burner may provide the possibility of modulation of its power at least in steps from a minimum to a maximum power while keeping in regular combustion conditions (that is, with STC mixture); in the single fan FAN the flow rate can be regulated so that it feed each burner BRN, if premix, with the stoichiometric air flow necessary to the power Pp set at the moment using devices described below.

Fig. 2 shows a double triangle graphic symbol that indicates the joint presence of an ignition device IGN and a flame detector FD. All the burners BRN of the cooktops PC shown in the plan view carry such symbol to indicate that all the burners BRN may be independently switched on and off, however such condition is not necessary for the invention.

With ref. to Figs. 3, 4 and 5, for each burner BRN, fuel gas and combustion primary air flow into a corresponding mixing chamber 2 from which, mixed, are guided to the flame spreading cap 6 of the same burner BRN where the flame is ignited by the ignition device IGN and a flame detector FD detects the existence of the flame. Primary air and gas reach the mixing chamber 2 from corresponding air 401 and gas 402 ducts through groups-orifices 403-404 comprising one or more air orifices 403 and gas orifices 404. Each group- orifices 403-404 has the corresponding air 403 and gas 404 orifices suitably calibrated so that the air and gas flow necessary at a predetermined power Pp considering the known or predetermined air and gas feeding pressures globally pass through them.

The gas pressure is known from the mains pressure.

As to the primary air pressure, it is guaranteed constant to the air orifices 403 at a predetermined value by acting on the rotation speed of the fan FAN; more precisely, the fan FAN speed is made to vary as the air flows required vary so as to keep its delivery pressure constant. This ensures the primary air flows provided by the calibrations of the air orifices 403 and may be obtained at least by continuously monitoring the delivery pressure and/or by other means and methods described below.

The air 401 and gas 402 ducts are preferably provided with sections wide enough to make the load losses of the airforms that run through them irrelevant so that the calibrations of the various air 403 and gas 404 orifices are independent of the length of the sections of air 401 and gas 402 ducts that serve them. In particular (see. Figs. 3 and 6 to 10) the air duct 401 may consist of a plenum 401 consisting of at least a part of the casing of the cooktop made sealed for the purpose.

Appropriate shutters 3 (see Figs. 3, 4 and 5) electromagnetically operated, normally closed, can open/close the air 403 and gas 404 orifices of each group- orifices 403-404 thus switching the ON/OFF state thereof.

The regulation of power to different steps is obtained by imposing different combinations of ON or OFF state to the various groups-orifices 403-404 (in particular, to the relative gas orifices 404) thus determining more ON states (one for each power set) or the OFF state of the corresponding burner BRN.

The general state of the cooktop and individual burners BRN is known and managed by an electronic command unit CMD which has functional links with a control dashboard CC, with the fan FAN and with the shutters 3 (the command unit CMD, the functional links and the control dashboard CC are not shown). Exchanges of signals and/or inputs among said components of the cooktop PC take place through such functional links. The user, by acting on the control dashboard CC, can control the change of state of the groups-orifices 403-404 through the command unit CMD.

Because it is the command unit CMD that determines the state of all the air 403 and gas 404 orifices that therefore knows slightly in advance, can also instantly impose on the fan FAN, at each change of state, the speed necessary to maintain the predetermined delivery pressure without having to wait for the signalling of an undesired variation of the delivery pressure to correct it.

Most part of the above is described in an exhaustive form in document AN2015A000041.

If the burners BRN of such cooktop PC are of the blown but not premix type, they must be sufficiently spaced from each other (arrangement not shown in the figures) to allow, from around them, the inflow to the flame spreading cap 6 of the secondary air necessary for the completion of the combustion. Moreover, on such flame spreading caps 6 only one peripheral crown of holes may be provided, and thus a corresponding crown of flames so that the secondary air can reach also the innermost part of the same flames. Preferably, for the purposes of the invention, the holes of the flame spreading cap 6 have vertical axis.

On the contrary, if the burners BRN of such cooktop PC are of the premix type, they can have their flame spreading caps 6 with the vertical holes arranged on the whole surface so as to form a bed of flames. The flame spreading caps 6 may also be closely adjacent (as shown in the figures) to form a continuous bed of flames composed of a matrix of an undefined number of burners BRN adjacent to one another. It is then provided that each burner BRN is served by a mix valve 1 thereof, an ignition device IGN thereof and a flame detector FD thereof so that each burner BRN can, totally independently of the others, be switched on, modulated if served by a modulable mix valve 1, or excluded from the gas feeding in case of accidental switching off of the flame.

As a consequence, the cooktop PC may have a cooking zone 5 composed of a number of contiguous flame zones Z equal to the number of burners BRN present, each flame zone Z corresponding to the bed of flames of each flame spreading cap 6. In this way the cooking zone 5 can simultaneously accommodate more vessels RCP variously arranged and subject to independent cooking processes by switching on only the appropriate burners BRN.

For merely aesthetic reasons also, the cooktop PC may have more cooking zones 5 separate from each other.

For the purpose of the ventilation of the cooktop provided in the present invention, it is appropriate to note the following regarding the means and methods for regulating the power of a premix burner BRN through said normally closed shutters 3 and the possibility of having a continuous ventilation through the their flame spreading caps.

Let's assume that Ppl, Pp2 and Pp3 are three increasing powers suppliable by a premix burner BRN by introducing in its mixing chamber 2 corresponding pairs of gas and air flows, respectively, Gl/Al, G2/A2, and G3/A3.

A premix burner BRN provided to supply a single power Ppl has a single group- orifices 403-404 (as shown in Figs. 3 and 4) and, of this, it is sufficient to open/close the only gas orifices 404 in order to enable the gas flow Gl ; in fact, even with the premix burner BRN turned off, the primary air to flow rate Al can flow through the flame spreading cap 6 without producing drawbacks, in fact with an advantage that shall be highlighted hereinafter. Such air flow through the flame spreading cap 6, that does not require any enabling command except for the operation of the fan FAN, is herein referred to as "passive ventilation".

For a premix burner BRN intended to supply two powers Ppl and Pp2 two groups-orifices 403-404 are necessary (see Fig. 5) and, for the power Ppl , a first group-orifices 403-404 with calibrated orifices for the corresponding flow rates Al/Gl opens. As to the second greater power Pp2 it is possible to opt for two alternatives.

In the first alternative the first group-orifices 403-404 is kept open and also the second herein referred to as "complementary" is opened the air 403 and gas 404 orifices whereof are calibrated for complementary flow rates A2' and G2' such that A2' = A2 - Al and G2' = G2 - Gl . This way the overall flow rates G2 and A2 necessary for the power Pp2 are obtained and it is seen that actually it has never been necessary to intercept the primary air to flow rate Al allowed by the first group-orifices 403-404; therefore, a passive ventilation is still possible. In the second alternative the first group-orifices 403-404 closes and the second one the air 403 and gas 404 orifices whereof are calibrated for total flow rates A2 and G2 required opens. In this case the primary air to flow rate Al that would be permitted by the first group-orifices 403-404 should be intercepted when operating to power Pp2 otherwise an STC mixture would not be obtained having an air flow Al + A2; therefore the passive ventilation should be prevented.

In fact, however, a premix burner BRN provided with two groups-orifices 403- 404 may be regulated to three power steps of Ppl, Pp2, and Pp3 by combining appropriately and in a known manner, the ON/OFF state of the two groups- orifices 403-404 in order to have the powers Ppl, Pp2 and P3 = Ppl + Pp2 but the primary air to flow rate Al allowed by the first group-orifices 403-404 must be intercepted when the contribution of the power Ppl is not required otherwise a too lean mixture is obtained. Even in such a case, therefore, a passive ventilation is impossible.

All this to affirm that in said cooktops PC that provide one or more continuous cooking zones 5 composed of premix burners BRN with contiguous flame spreading caps 6 it is possible to have a passive ventilation with, in particular a flow rate Av equal to the air flow rate Al corresponding to the flow rate necessary for the minimum flow rate Ppl, whether such premix burners BRN provide a single power step Ppl or they provide two of them PP1 and PP2 > PP1, respectively.

An additional advantage of having the passive ventilation available consists in the total elimination of the risk of backfire from the burners. In fact, the voluntary switching off of a blown or premix burner BRN takes place by interrupting the gas flow while the air flow remains constant or only partly reduced making the flame propagation speed slower.

Of course, whatever the type of burner BRN, only blown or even premix and with multiple power steps, it is always possible to ventilate the flame spreading cap 6 thereof when the burner BRN is switched off and provided that a normally closed ventilation shutter 3v is provided (see Fig. 5) intended for the opening of the corresponding orifices that enable the passage of such air flow Av where preferably such flow rate Av coincides with one of those equal to Al , A2, A3, etc. required with a switched on burner BRN and such normally closed shutter 3v coincides with the normally closed shutter 3 responsible for the enabling of one of said flow rates.

Such ventilation imposed by opening and keeping deliberately open the said bistable shutter 3v for the passage of an air flow rate Av is herein referred to as "active ventilation".

Since, as said, the command unit CMD determines and recognises the state of all the air 403 and gas 404 orifices, it can enable the active ventilation of one or more of the switched off burners BRN when these are not of the type that allows a passive ventilation. The command unit CMD may command the active ventilation of all the flame spreading caps 6 of the switched off burners BRN or at least of the all switched off burners BRN that surround the switched on burners BRN.

For blown but not premix burners BRN, instead, where the primary air is insufficient for the complete combustion, the conditions of the exact combination between gas and primary air flow may be less stringent and it is possible to always provide the active ventilation at least to the flow rate Al correspondent to the minimum power Ppl without drawbacks even when the burner is switched on to powers higher than said minimum Ppl .

So far it has been shown that it is possible, in blown burners (premix in particular) that require a fan FAN for their operation, to obtain, through passive or active ventilation, the discharge of air jets of flow rate Av from flame spreading caps 6 when the corresponding burners BRN are switched off; in particular, the active ventilation can be obtained by choosing with predetermined criteria only some of the burner BRN. Therefore, a first group of variants of the invention provides that said continuous curtain of ascending air with preferably laminar flow that at least partially circumscribes at least all the gas cookers operational at the moment, is obtained, in cooktops as described above, through passive ventilation of all the switched off burners BRN or with the active ventilation of at least the switched off burners BRN necessary for the formation of the curtain.

In short, the first group of variants provides that the ventilation of the cooktop may be obtained through burners BRN that are of the blown type (premix in particular).

Figs. 3 and 7 exemplify the variant with passive ventilation showing a cooktop PC below a horizontal hood H. The grey arrows indicate cooking fumes coming from a blown burner BRN ON (i.e., switched on) and from the overlying vessel RCP; the white vertical arrows indicate the ascending laminar flow coming from the cooktop due to the passive or active ventilation of all the blown burners BRN OFF (i.e., switched off); the curved lines with a vertical pattern indicate the confinement of the cooking fumes; the large curved arrows indicate air recalled from the environment towards the edges of the horizontal hood H.

No figure exemplifies the active ventilation variant

A limit of such variant is that when the outermost burners BRN of a cooking zone 5 are operational, it is impossible to create a curtain that surrounds (all together or one by one individually) all the operating burners BRN but has the advantage that in cooktops PC employing blown burners BRN (premix in particular) it may use means already necessarily present for other purposes.

A second group of variants of the invention provides that said continuous curtain of ascending air with laminar flow is obtained through ventilation openings 7 obtained in appropriate zones of the air duct 401 so as to rise in a substantially vertical direction towards said horizontal hood H air jets coming directly from the same air duct 401 ; that is, without passing through the burners BRN.

In such way, the invention is applicable:

- both in cooktops PC that provide blown burners BRN (premix in particular) that require a fan FAN and a corresponding air duct 401

- and in cooktops PC that provide atmospheric burners BRN.

In the second case the fan FAN and the corresponding air duct 401 are not required for feeding the burners BRN and should be specially provided only for the purposes of the invention.

In this second group of variants, therefore, the ventilation openings 7 may remain always open without there being interference with the correct combustion and even here we can then talk about passive ventilation.

In Fig. l .a such air duct 401 is shown as a hollow element that extends within the cooktop PC but externally to the burners BRN of the cooking zone 5 along some of its sides. In particular in the figure it is shown in the shape of a tubular element that feeds the air orifices 403 and is part of a possible ramp 4 which also includes a gas duct 402 that feeds the gas orifices 404. In this particular case the gas duct 402 is internal to the air duct 401. The ramp 4 has, in a plan view, a C- shape that extends at least along the sides and the front side of the cooktop PC. Each ventilation opening 7 (see. Fig. l .b) comprises a channel 701 that connects directly, that is, without passing by the burners BRN, the air duct 401 with the outer surface 702 of the cooktop PC.

In Figs. 6. a and 6.b such air duct 401 is shown as a plenum 401 that extends in a plan view externally to all the burners BRN present and has the upper surface 702, at least at edges 703 thereof, in contact with or coinciding with the outer surface 702 of the cooktop PC. Each ventilation opening 7 (see. Fig. 6.b) is made in proximity of such edges 703 in the shape of a vertical axis hole with suitably jointed edges that places the plenum 401 in communication with the outer surface 702 of the cooktop PC.

Fig. 8 shows a cooktop that makes simultaneously use of ventilation through flame spreading caps 6 of the burners BRN (which, in particular, is a passive ventilation) and through the ventilation openings 7 of the air duct 401 (which, in particular, is a plenum 401).

In Figs. 9, 10. a and lO.b such air duct 401 is shown as a plenum 401 that extends in a plan view up to at least some of the outer edges 704 of the cooktop PC and has at least the external vertical confinement walls 705 coinciding with the external vertical walls 705 of the cooktop PC.

The ventilation openings 7 are made on such external vertical walls 705 and each comprise a passage with guided inlet 706 shaped so as to invite the air to exit in a substantially descending direction and, externally, in front of such passage with guided inlet 706, a conveyor 708 which has the task of reversing the flow from descending to ascending, obtaining a vertical flow that laps the external vertical wall 705 of the cooktop PC. The edges 704, in turn, have an appropriate bend 709 whereon the outgoing vertical flow exiting the conveyor 708 adheres partly by the Coanda effect deflecting inwards of the cooktop PC. Preferably, the conveyor 708 is removable for cleaning and, when mounted leaves draining slots below between itself and the external vertical wall 705. Such variant therefore provides the following two additional results:

1) the single air jets of the curtain, adhering to the edges 704 provided with said appropriate bend 709 are deviated toward the centre of the cooking zone, greatly confining the fumes and increasing the conveying effect of the same towards the hood; in this way it would be possible to achieve the same collection efficiency by reducing the width and depth of the hood.

2) the shape of the ventilation openings 7 is such as to evacuate any liquid that overflow from the cooking perimeter during the same cooking or during the cleaning. This avoids dirt to get inside the plenum 401 as possible in the previous variants.

It is seen that the second group of variants of the invention still allows to create a curtain that confines all burners BRN present in one or more continuous cooking zones 5.

For such confinement it is sufficient to create an air curtain only at the sides of the cooktop PC that are not provided close to a vertical wall. Fig. 1 exemplifies a solution that forms a curtain on three sides because the fourth (the upper one in the figure) is provided against a wall. Figs. 6 and 8 show solutions suitable for cooktops PC installed as an "island", at the centre of a room, a position, moreover, where the interception of the cooking fumes, not sheltered from air currents, is particularly difficult with conventional cooktops.

For the purposes of the present invention the holes of the flame spreading caps of the blown burners BRN (premix in particular) are to be considered as ventilation openings 7 and must have their features.

It has been said that the ventilation openings 7 must have appropriate sizes and calibration to obtain the predetermined air flows rates to the predetermined head of the fan FAN adapted to form the preferred laminar flow. On the other hand, if the cooktop PC according to the invention makes use of blown burners BRN (premix in particular) and the fan FAN employed for the ventilation of the cooktop PC is the same that feeds the said blown burners BRN, the head of the fan FAN must be determined according to the composition of the fuel mix required that, in turn, depends on the type of gas in use (for example, natural gas or LPG). In conclusion the head of the fan FAN, dependent on the type of gas in use, involves a consequent variability of the speed of the ventilation air jets but this does not however constitute an obstacle for obtaining a preferably laminar ascending flow which, as already said, accepts at least a 1 to 2 ratio between the minimum and maximum speed necessary and which, therefore, allows a compromise sizing.

It is seen that the cooktop according to the invention may also be ventilated by jointly using more than one of the described variants, in particular one of the variants included in the first group of variants with one of the second group, as exemplified in Figs. 8 and 10.