Abstract:
Nebulization systems to generate a mist of microdroplets of a liquid for the purpose of refreshing the atmosphere. Such nebulization systems are sized for mounting on a sales display for humidifying and refreshing fresh products displayed for sale.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a National Stage Application of PCT International Application No. PCT/FR2015/051877 (filed on Jul. 7, 2015), under 35 U.S.C. §371, which claims priority to French Patent Application No. 1456863 (filed on Jul. 17, 2014), which are each hereby incorporated by reference in their respective entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to nebulization systems able to generate a mist of microdroplets of a liquid, for example water, for the purpose of refreshing the atmosphere, and more particularly small-sized nebulization systems that can be mounted on a sales display for humidifying and refreshing fresh products displayed for sale. 
       BACKGROUND 
       [0003]    Such systems are known per se. The patent EP 0 691 162 describes a nebulization system with a concentration nozzle in which a piezoelectric element immersed in water generates a mist of water droplets at the outlet of a nozzle that concentrates the ultrasound generated by said piezoelectric element at its outlet point; the mist is next taken away by a stream of air generated by a fan. This nozzle is as a general rule disposed vertically, with the focusing outlet pointing upwards; the nozzle may also be inclined, for example at 45°. 
       SUMMARY 
       [0004]    The aim of the invention is to present an improved nebulization device that can be used to humidify and/or refresh goods, in particular fresh products, displayed for sale on a stall, or for humidifying and/or refreshing and/or perfuming the atmosphere for other purposes, for example in order to cool and/or refresh and/or perfume (and/or disinfect) a volume of air, such as a room in a dwelling or a car passenger compartment, and which is compact, robust, reliable, inexpensive and simple to use, is easy to maintain, and the energy consumption of which is as low as possible. 
         [0005]    The requirement for compactness results from the need for an overall small size of the device, and in particular a limited height, which is particularly important when the device is to be integrated in a vehicle passenger compartment or under a table or stall. The requirement for robustness results from the need for resistance of the device to disturbed conditions, and its reliable functioning under disturbed conditions, such as mechanical movements (acceleration, braking, vibrations, shocks, variation in slope). It also results from the desire to avoid frequent maintenance of the nebulizer. The requirement for simplicity in use results in particular from its being impossible in practice to require the user to ensure the regular filling of the nebulizer with water. The requirement for lightness results from the general requirement to limit the weight that is added to a vehicle (and in particular to an aircraft, by the addition of additional options and functions; lightness in all cases facilitates maintenance when it is a case of manipulating the device, and also tends to reduce the environmental impact of the device. The requirement for low energy consumption results from the desire that is becoming widespread to reduce the environmental impact of the products, devices and machines in their lifecycle. The cost requirement militates in favor of a device of simple design. 
         [0006]    The problem is solved by a device of novel design that has three parts (modules): 
         [0007]    (i) A body comprising a water tank; this body constitutes a bottom part of the appliance. 
         [0008]    (ii) A top part comprising a tube for collecting the water jet and an elongate end piece through which the mist generated by the system escapes; this top part is placed on the body. 
         [0009]    (iii) An electronic unit fixed to the body, detachably or not, and which can be opened and/or retracted to facilitate maintenance. 
         [0010]    The appliance according to the invention is narrow and compact and can be fixed to runners below the table or stall (it can also be fixed above the table), and can be easily brought out and disconnected in order to place it on a table or stall for maintenance work. 
         [0011]    The water tank is removable and can be replaced easily. 
         [0012]    Each of the three bodies is of substantially elongate form, with the longitudinal axis substantially parallel to the direction of the elongate end piece through which the mist escapes and/or to the direction of the tube for collecting the water jet. 
         [0013]    The device comprises a collection tube suitable and arranged for collecting the liquid jet emerging from the outlet orifice and draining into said collection reservoir. This tube may be inclined with respect to the vertical. This collection tube can have an air flow pass through it, which takes away said mist of droplets to its outlet. This further reduces the height of the device and simplifies its design. 
         [0014]    Advantageously the collection tube is extended in the elongate end piece through which the mist generated by the system escapes. 
         [0015]    The device according to the invention advantageously comprises ventilation means for creating an air flow that takes away said mist of droplets to the outside of said device. This ventilation device can be located in said top part. In an advantageous embodiment, the air enters the system through said ventilation means, passes through the collection tube or a tube in which the collection tube is inserted, and leaves the system through said elongate end piece. 
         [0016]    By virtue of its nozzle with focusing of acoustic waves generated by a piezoelectric element, the nebulization device according to the invention is able to create and distribute a mist formed from droplets with a typical mean diameter of between 0.5 μm and 25 μm, preferably between 1 μm and 10 μm, even more preferentially between 1 μm and 5 μm. 
         [0017]    Said liquid to be nebulized is preferably water, which may comprise additives such as perfumes and/or disinfectant products (for example: H2O2, peracetic acid, citric acid). 
         [0018]    Said nozzle is preferably disposed in line with the collection tube, and therefore preferably slightly inclined with respect to the horizontal. This helps to reduce the height of the device. 
         [0019]    The device according to the invention is in particular suitable for tables or stalls with a surface area of up to approximately 5 m2 or more, but it is possible to produce smaller models suitable for tables or stalls with a surface area of less than approximately 3 m2. 
         [0020]    In general, said circulation pump may be of any suitable type; a screw-type centrifugal pump is well suited. 
         [0021]    The device according to the invention may also comprise at least one means for detecting a lack of liquid associated with a feedback loop for cutting or reducing the intensity of the acoustic waves emitted by the piezoelectric element in the event of a lack of water. This detection means may be a sensor (for example a sensor for the water level in the primary reservoir, and/or a sensor for the pressure in the pressurized chamber), or a plurality of sensors, and/or may comprise a measurement of an electrical parameter of the circulation pump. 
         [0022]    In an embodiment that can be combined with all the others, the longitudinal axis of said nozzle forms an angle of inclination a with respect to the horizontal which is between 0° and 45°, preferably between 0° and 30° and even more preferentially between 5° and 20°. This allows a particularly compact design of the device. 
         [0023]    In an embodiment that can be combined with the previous ones, the collection reservoir and said nozzle form a single unit. 
         [0024]    Advantageously, the top part of the body is made from molded plastics material (preferably reinforced glass fibers, for example to the extent of at least 10% by mass, or at least 10% by mass, or approximately 30% by mass); this affords a lightweight and nevertheless robust construction, in the nature of an ecodesign. The lightness is advantageous for onboard apparatus. It also facilitates maintenance if, as is possible with the device according to the invention, the apparatus is removed entirely from its place of installation (for example under the stall or table that it equips). 
         [0025]    The plastics material is preferably a plastic suitable for contact with food, in order not to contaminate the water and mist with traces of undesirable products coming from the plastics materials. Advantageously a recyclable plastic is chosen. 
         [0026]    The device may comprise a heating means able to evaporate the residual liquid in said device, and in particular in the tank, after it has stopped. The same heating means may be used to heat the water contained in the device to a sufficient temperature to reduce its pathogenic-germ content. 
         [0027]    Another subject matter of the invention is a method for starting a device according to the invention, in which 
         [0028]    (a) liquid is put in the tank; 
         [0029]    (b) when the level of said liquid rises in said tank to a preset point that is detected by a detector for the liquid level in the tank, the circulation pump is started; 
         [0030]    (c) the circulation pump creates a pressure of liquid sufficient for the liquid to be able to enter the nozzle, optionally after having entered the pressurization chamber, and to form a stable jet of liquid that emerges from the outlet orifice, knowing that, during at least part of this time, additional liquid is put into the tank; 
         [0031]    (d) when the level of said liquid in said tank has reached a preset point that is detected by a level detector, the electrical supply to the piezoelectric element is activated in order to create liquid droplets. 
         [0032]    In step (d) said preset point and/or said level detector may be the same as at step (b). 
     
    
     
       DRAWINGS 
         [0033]    The device according to the invention is illustrated schematically by  FIGS. 1 to 10 , which show various views and/or various embodiments thereof. 
           [0034]      FIG. 1  shows an exploded view of the device showing its top part, its bottom frame and its electronic unit closed by a cap. 
           [0035]      FIG. 2 a    shows a lateral view,  FIG. 2 b    a front view of the same device. 
           [0036]      FIG. 3  shows an exploded view of the electronic unit with its cap that protects the electronic card. 
           [0037]      FIG. 4 a    shows the lateral view of a vertical longitudinal section of the device,  FIG. 4 b    shows the same cross section of the same device with the liquid levels in normal functioning. 
           [0038]      FIG. 5  illustrates the installation of the device under a table or under a stall. 
           [0039]      FIG. 6  illustrates the positioning ( FIGS. 6 d  and  e   ) and the removal 
           [0040]    ( FIGS. 6 a, b  and  c   ) of the device under a stall;  FIG. 6 d    shows the lateral perspective view of a vertical longitudinal section of the device,  FIG. 6 e    its lateral perspective view. 
           [0041]      FIG. 7  shows the lateral view of a longitudinal section of the device according to another embodiment of the invention. 
           [0042]      FIG. 8  shows an exploded perspective view of the top part of the device according to a particular embodiment. 
           [0043]      FIGS. 9 a  and 9 b    show a perspective view of a particular embodiment of the fixing of the piezoelectric element. 
           [0044]      FIG. 10  shows a perspective view of a particular embodiment of the device according to the invention in which the cover ( FIG. 10 a   : closed,  FIG. 10 b   : open) is fixed by lateral hinges. 
           [0045]      FIG. 11  shows an extract of the lateral view of a longitudinal section of another embodiment of the top part of a device according to the invention. 
       
    
    
     DESCRIPTION 
       [0046]    The mist diffused by the system (device)  1  according to the invention is generated by the nozzle module  9 , which comprises a piezoelectric element  46  stimulating the formation of mist from a liquid that circulates in the system and more precisely in its bottom frame  4 . Said nozzle module  9  comprises an acoustic concentrator (nozzle)  49  open in the direction of the mist outlet tube  23  of the top part of the system  1 . 
         [0047]    The nozzle module  9  comprises a concentration nozzle  49  (also referred to as acoustic concentrator), of a known type, able to contain a liquid to be atomized (typically water) and having an outlet orifice  50 , the cross section of the inside of said concentration nozzle  49  having a gradual narrowing in the direction of said outlet orifice  50 . Said nozzle  49  further has, on the side opposite to its outlet orifice  50 , a (ceramic) piezoelectric element  46  able to emit acoustic waves in the liquid. The inside wall of said nozzle  49  is made from a hard material able to reflect the acoustic waves generated by said piezoelectric element  46 . Advantageously, the convergent form of the inside walls of the nozzle  49  is determined so as to focus the ultrasonic acoustic waves at a point close to the central part of the outlet orifice  50 ; thus a mist of microdroplets of the liquid to be atomized is generated when the nozzle  49  is filled with liquid and the piezoelectric ceramic  46  emits acoustic waves of suitable frequency and intensity. Said convergent form of the inside walls of the nozzle  49  advantageously has radial symmetry. This convergent form is preferably parabolic, which improves the efficiency of the concentration nozzle  49 . 
         [0048]    According to the invention, and as illustrated in  FIG. 4 a   , the longitudinal axis of the nozzle  49  is inclined with respect to the vertical. This inclination, expressed by the angle α with respect to the horizontal, is aimed at reducing the total height of the system  1  and in particular its top part  2 . The angle α may be less than 45°, preferably less than 30° and even more preferentially less than 20°. In the context of the present invention, an angle α between 0° and 30° is preferred, and preferably between 5° and 20°. 
         [0049]    In addition an almost horizontal functioning of the nozzle  49  (angle 15° for example) assists an increase in flow rate. In this case, the hydraulic pressure on the surface of the piezoelectric element  46  is less and the acoustic waves can therefore propagate more easily in comparison with a vertical functioning of the nozzle. The almost horizontal functioning of the nozzle requires the presence of a circulation pump  42 . This pump  42  is optional for vertical or even inclined functioning (for example at 45°), provided however that the whole of the surface of the piezoelectric element  46  is covered with liquid, but the presence of the pump  42  is preferred since it makes the system less sensitive against the effect of variations in flow rate. Moreover, it allows the use of a pressurization chamber  52  as explained below. 
         [0050]    The outlet orifice  50  of the nozzle  49  preferably has a circular form. In one embodiment, its diameter is between 3 and 8 mm, and advantageously between 4 and 6 mm; the inside length of the nozzle is advantageously between 25 mm and 42 mm, knowing that this distance corresponds to the near field of the ultrasound generated by the piezoelectric ceramic  46 . By way of example, it is possible to use a nozzle  49  with a height of 38 mm, with an outlet orifice with a diameter of 6 mm. The inlet cross section of the nozzle  49  (i.e. the sum of the surfaces of the inlet orifices  51 ) must be greater than the cross section of the outlet orifice  50  (preferably at least three times greater) in order to avoid the phenomenon of cavitation in the nozzle  49  (as well as a lack of water). This condition is fulfilled for example with four inlet orifices  51  with a diameter of 5 mm for an outlet orifice  50  with a diameter of 6 mm. These inlet orifices  51  ensure good filling of the nozzle  49 , reducing the pressure drops in the system and reducing the pressure on the piezoelectric element  46 . It is also found that the presence of a plurality of inlet orifices  51  distributed in the bottom part of the nozzle reduces the risk of the formation of bubbles by cavitation during the functioning of the piezoelectric ceramic  46 . 
         [0051]    Liquid coming from the tank  7  is admitted into the nozzle  49  through at least one inlet orifice  51 . Preferably, a plurality of inlet orifices  51  are provided around the longitudinal axis of the nozzle  49  in a zone close to the piezoelectric ceramic element  46 . This filling of the nozzle  49  with liquid has two functions. Firstly, knowing that in operation some of the liquid contained in the nozzle  49  starts in the form of a mist, it is necessary to resupply the nozzle  49  with liquid. Secondly, continuous filling of the nozzle  49  associated with the recirculation of the liquid stabilizes the operating conditions of the system  1  even in the presence of mechanical disturbance to the system  1 , for example when there is a mechanical impact against the stall on which said system  1  is mounted. 
         [0052]    On the rear face of said acoustic concentrator  49  there is said piezoelectric element  46 . The latter has an active surface directed towards the outlet  50  of the nozzle  49 ; this active surface may in particular be planar or concave. During the functioning (electrical excitation) of the piezoelectric element  46 , this active surface is immersed in said liquid in order to transmit to it the acoustic waves that it generates. Said piezoelectric element  46  is preferably cylindrical in shape, typically a circular-shaped plate. By way of example, said piezoelectric element  46  may have a diameter of 10 mm or 405 mm, or any diameter lying between these two values. The ultrasonic frequency is advantageously between 0.3 MHz and 3 MHz, preferably between 1.3 MHz and 2.3 MHz. It may for example be 1.68 MHz. 
         [0053]    The nozzle module  9  has a flow of liquid to be nebulized passing through it. This flow is generated by a circulation pump  42  that is advantageously situated in the electronic unit  8 . The liquid to be nebulized is usually water. The liquid leaves said pump  42  and enters the bottom part of the pressurization chamber  52   b  of the nozzle module  9 , and then the nozzle  49  itself through the at least one inlet orifice  51  in the nozzle. The liquid emerging through the outlet orifice  50  of the nozzle  49 , preferably in the form of a small jet, is projected into the collection tube, which enters into the tank  7 ; the circulation pump  42  draws on the liquid in the tank  7  and makes it enter the nozzle module  9 . 
         [0054]    When the tank  7  is filled, the liquid enters first of all the bottom part  52   b  of the pressurization chamber, and then enters (through the inlet orifices in the nozzle  51 ) the nozzle  49  (and covers the internal face of the piezoelectric element  46 ) as soon as the level of liquid in the nozzle module  9  is sufficient, the liquid also enters the top part of the pressurization chamber  52   a  of the nozzle module  9 . A small jet of liquid leaves the outlet orifice  50  of the nozzle  49  and, when the level of liquid rises again, it also enters the top part  52   a  of the pressurization chamber. The circulation pump  42  keeps the liquid in circulation, preferably at a level just sufficient to ensure perfect filling of the nozzle vessel. When the piezoelectric element  46  is operated, the jet of liquid  53  elongates (acoustic pump effect, this is to assist this phenomenon when the pump must preferably be centrifugal in order not to constrict the acoustic pump) and the nozzle  49  produces a mist of fine droplets. 
         [0055]    The jet of liquid  53  generated at the outlet  50  of the nozzle  49  under the effect of the excitation by the piezoelectric element  46  empties into a collection tube  23 , the longitudinal axis of which is preferably inclined with respect to the vertical. Preferably, the axis of the collection tube  23  is parallel to the axis of the acoustic concentrator  49 , and even more preferentially these two axes coincide. 
         [0056]    The collection tube  23  may have passing through it an air flow generated by a ventilation means  24 , which is preferably adjustable for flow rate and is situated upstream of, downstream of or inside the collection tube  23 . Said air flow enters the nebulization system  1  through an air inlet  21  and (by thrust or aspiration) carries away the microdroplets of liquid generated by the nozzle  49  around the jet of liquid  53 . Thus a mist of fine droplets forms, which leaves the collection tube  23  through its outlet  22  and enters its destination environment, for example the passenger compartment of a vehicle or the air space above a stall  8 , possibly being conducted through a nebulizer  10  having at least one orifice  16  advantageously provided so as to confer on the mist a required diffusion direction and/or height and/or width. 
         [0057]    The jet of liquid  53  is projected against the internal wall of the collection tube  23 , and the liquid thus collected flows into the tank  7 . Thus the collection tube  23  also serves as a guide tube for the diffusion of the mist. 
         [0058]    Advantageously, the guide tube  23  has optimized aeraulic characteristics in order to increase the flow rate of droplets. In particular, the surface around the jet is homogeneous and smooth in order to collect the maximum amount of droplets; the path of the pipe follows the shape of the jet in order not to disturb the hydraulics. This is illustrated in  FIG. 4 b   . This form assists the flow of liquid, and prevents any retention of water and any vortex that could disturb the flow of the liquid, in order not to have stagnation zones that would present a risk for the hygiene of the device. This form also reduces the noise related to the flow of liquid and the flow of air: the device according to the invention is particularly silent. 
         [0059]    Several other embodiments may be adopted in order to improve the aeraulic characteristics of the system. It is possible to produce the guide tube so as to cause a venturi effect, as illustrated in  FIG. 7 . It is possible to provide for the air pressurization to be done from the top of the tank, in particular by positioning the fan so that its blades rotate about a substantially vertical axis ( FIG. 11 ). In general terms it is possible to use an axial or centrifugal fan. 
         [0060]    In the embodiment of the invention illustrated in  FIG. 1 , the collection reservoir is represented by the output tube for the mist  23  and forms a single-piece element with the nozzle  49 . This simplifies its design; moreover, such a single-piece element is more robust. 
         [0061]    Alternatively it is possible to produce the body of the top part  2  of the device  1  in two parts  54   a,    54   b,  preferably symmetrical, generated by a longitudinal section, as illustrated in  FIG. 8 . The two parts  54   a,    54   b  can be assembled by any suitable technique, such as screwing, adhesive bonding or snapping on. It is advantageous to insert at least one ring  55  (preferably held in a groove  56  and/or between two rims  57   a,    57   b ) in the direction perpendicular to the axis in order to consolidate the body  54  and provide the sealing and connection to the accommodating compartment  13 . 
         [0062]    Said piezoelectric element  46  can absorb a large amount of electrical power, for example 30 to 60 W for a diameter of 20 mm. Approximately 40% of this power is rendered in the form of acoustic energy transmitted to the liquid, the rest being dissipated in heat form. According to one embodiment of the invention, the piezoelectric element  46  is mounted on a support  33  that has in its rear part an orifice  34  that emerges on the rear part of the piezoelectric element  46 ; this orifice  34  contributes to the natural cooling of the piezoelectric element  46 . This support  33  can be mounted directly on the rear part  38  of the pressurization chamber  52  in which the nozzle  49  is inserted. It is advantageous to use a gasket  36  to provide the seal between the support  33  and the rear part  38  of the pressurization chamber  52 . The support can be fixed by means of preferably reversible fixing means  35 , in order to facilitate maintenance, such as a clamping screw. The support  33  can be produced from metal or plastics material, for example polyamide, such as PA66, advantageously containing glass fibers); it must be able to adapt to the thermal stresses between the inside (contact with the piezoelectric element  46 ) and the outside (air). 
         [0063]    In order to avoid damaging the piezoelectric element  46  during its functioning, it must be constantly cooled by the liquid in order to prevent damage thereto by overheating. The inventors realized that, when the piezoelectric element  46  is functioning dry, even for a very short period, it risks being damaged or even destroyed. 
         [0064]    To prevent this, the inventors provided for the nebulization system  1  to be able to comprise suitable means for preventing said piezoelectric element  46  functioning (i.e. not emitting acoustic waves or only acoustic waves of very low power) when the piezoelectric element  46  is not immersed in the liquid to be atomized. These means may take various forms, and in general comprise at least one means for detecting the lack of liquid and/or a means for detecting heating of the piezoelectric element  46 , and a means of feedback on the electrical supply of said piezoelectric element  46 . 
         [0065]    Said means for detecting the lack of liquid may be a level sensor or a presence sensor that cuts off or regulates the functioning of the piezoelectric element  46 . This sensor may be an optical sensor or a capacitive sensor or an inductive sensor but, among these three, an optical sensor that is preferred, which has better reliability and in particular very low hysteresis (±1 mm). This sensor may be situated at various points, in particular in the collection receptacle, or inside the nozzle  49 , or in the pressurization chamber  52  of the nozzle  49 . In one embodiment a sensor situated in the tank  7  is used. 
         [0066]    Various types of pump may be used for the circulation pump  42 . The pump is advantageously of the adjustable rate type; a pump adjustable between 0.1 and 2.8 liters/minute is suitable for a nozzle  49  that has the dimensions indicated above. In one embodiment the circulation pump  42  is a helical pump. Advantageously, this pump absorbs a direct current and the voltage is adjusted so as to vary the rotation speed and therefore the discharge rate at the outlet of the nozzle  49 , which does not make it possible to modify the length of the jet. 
         [0067]    The device  1  according to the invention, by virtue of its nozzle  49  for focusing acoustic waves generated by a piezoelectric element  46 , is able to create and give out a mist formed from droplets with a typical mean diameter of between 0.5 m and 30 m, and more particularly between 0.5 m and 10 m, and preferably between 1 μm and 5 μm. This particle size depends in particular on the frequency of the acoustic waves produced by said piezoelectric element  46 . 
         [0068]      FIG. 1  shows the three parts of the device  1  according to the invention, namely its top part  2 , its bottom frame  4  and its electronic unit  8  closed by a cap  3 . Said electronic unit  8  is advantageously disposed behind the bottom frame  4 . These three parts are substantially elongate in form. The top part  2  comprises the tube  23  for discharging the mist, which is projected to the front. The bottom frame  4  comprises a tank module  5  comprising a tank  7  and the nozzle module  9 . The tank  7  may be separate from the tank module  5  so as to be replaced if necessary. The tube  23  discharging the mist may be curved towards the horizontal in its part close to its outlet  22 , and the axis of its outlet is preferably horizontal. 
         [0069]      FIG. 2 a    shows the roughly elongate form of the device  1 , with the tube  23  discharging the mist projecting to the front. 
         [0070]      FIG. 2 b    shows that the device  1  is narrow, which assists manipulation thereof for installation, dismantling and maintenance operations. 
         [0071]    As shown in  FIG. 3 , the electronic unit  8  is covered with a cowl  3 . The electronic unit  8  also comprises at least one electronic card  30  provided with electronic components  31 ; it may be fixed under the cowl  3 . The electronic unit  8  advantageously comprises a radiator  6 , which is advantageously in thermal contact with the electronic card  30 ; said radiator may be integrated in the cowl  3 . 
         [0072]    Said cowl  3  may be produced from metal, which assists the dissipation of heat; in some cases this may make the presence of a radiator superfluous. 
         [0073]    Said cowl  3  may be connected to said unit  8  by hinges (see  FIG. 10 ); this facilitates access to the components (pump, solenoid valves) during maintenance work. 
         [0074]      FIG. 4 a    explains in greater detail the functioning of the device according to the invention. The electronic unit  8  comprises in particular a circulation pump  42  that generates the jet of liquid  53  (water) in the nozzle  49 , as described above. The system  1  may be supplied with water from outside: the water enters through a water inlet  44  situated on the rear of the electronic unit, passes through a solenoid valve  41  in the open position and enters the nozzle module  9 . To cut off the water the solenoid valve  41  can be closed; this operation can be carried out automatically by the electronic card. 
         [0075]      FIG. 4 b    illustrates the functioning of the device  1  according to the invention: during the filling of the tank  7 , the liquid first of all enters the bottom part  52   b  of the pressurization chamber, and then enters (through the inlet orifices in the nozzle  51 ) the nozzle  49  (and covers the inside face of the piezoelectric element  46 ) and finally the top part  52   a  of the pressurization chamber; a small jet of liquid  53  emerges from the outlet orifice  50  of the nozzle  49 . The circulation pump  42  keeps the liquid in circulation. 
         [0076]      FIG. 7  shows the lateral view of a longitudinal section of the device  1  according to another embodiment of the invention, in which the air enters in parallel to the jet of liquid that emerges from the nozzle; thus the mist generated by the nozzle is aspirated by the air flow (venturi effect). 
         [0077]    As shown in  FIG. 5 , the device  1  according to the invention is able to be integrated in a table (stall)  60 , that is to say a surface on which objects are displayed, in particular for sale, and which is subjected to the effect of the mist of liquid generated by said device  1 . This effect may in particular be a humidifying and/or refreshing effect. Said objects may be food products, in particular fresh plant products (for example fruits or vegetables), seafood (for example fish, crustaceans or mussels), cheeses, meat, meat-based products (for example cooked meats) or non-food products (for example pot plants or cut flowers). 
         [0078]    It is also possible to use the device  1  according to the invention in other humidification or refreshing situations, for example in rooms in dwellings, in passenger compartments (in vehicles for example). 
         [0079]    In an embodiment illustrated in  FIGS. 5 and 6 , the device  1  is disposed under said table  60 , in an accommodating compartment  13  that emerges on one face of the table  60 ; said accommodating compartment  13  may be closed by a cowl (not shown in the figures). If the device is mounted under a table  60 , the outlet tube  23  for the mist is advantageously extended by a pipe  14  for mist; advantageously said outlet tube  23  for the mist may be inserted in said pipe  14  for mist. The mist is conveyed through said pipe  14  (under the influence of the air flow generated by the fan  24  that takes away said mist) to at least one diffusion orifice. Said diffusion orifice may be mounted directly on the surface of the table  60  or at a low height, or may be mounted on a diffusing mast  15  that projects above the surface of the table  60 , as shown in  FIG. 5 . 
         [0080]    In another embodiment said device  1  is fixed under the table  60  by means of runners  11  that may be situated in the accommodating compartment  13  ( FIG. 6 e   ) or which may replace the accommodating compartment. 
         [0081]    According to a particularly advantageous aspect of the invention, the system  1  according to the invention can be removed easily, for example it can be removed from its accommodating compartment  13  or can be removed by means of said runners. Three steps of this removal method are illustrated in  FIGS. 6 a , 6 b  and 6 c    (removal in the direction of the arrow). The removal method is greatly facilitated by the fact that the outlet tube  23  for the mist is simply inserted in said pipe  14 ;  FIGS. 6 d  and 6 e    show two variants of this embodiment. It is not necessary for the connection between the outlet tube  23  for mist and the pipe  14  to be fixed; the seal is provided by the ring  55  situated in the groove  56  of the outlet tube  23 . The removal can take place without a tool. The device  1 , optionally after having disconnected its electrical supply cable and/or its liquid supply pipe, can next be placed on a table or a stall close by, or can be taken away for a maintenance operation. The device according to the invention, through its improved design, which is in particular modular, and which facilitates its access and dismantling, can simply be exchanged against another device  1 , either new or in a good state of maintenance. This way of proceeding has the enormous advantage of not interrupting the functioning of the nebulization system except for a very brief period (around one minute or a few minutes) and avoiding carrying out maintenance on the device at its place of use, which is in general a place accepting the public. 
         [0082]    The system  1  according to the invention may be supplied by a mains supply cable. Being designed so as to consume very little electrical energy (and to function at extra low voltage ELV) it can also be supplied by a supply cable that leads to a low-voltage transformer. It may also be supplied by a battery and a water reservoir (solid or flexible). 
         [0083]    In this case, the battery and reservoir assembly is fixed to the table or to a mobile carriage. The user can move the carriage to a recharging zone, he connects the water inlet and the electrical inlet. This device is optimized in terms of use, the filling of the reservoir and the charging of the battery are automated. 
         [0084]    Being able to avoid a direct supply to the mains makes it possible to install the device according to the invention in wet areas, where perfect isolation of individuals close to the device (customers, salespersons in the shop, maintenance personnel) cannot be guaranteed in all circumstances (a shelf for seafood for example). This represents another advantage of the system  1  according the invention. 
         [0085]    The reduction in energy and in particular electrical consumption is obtained by virtue of the optimization of the output of the circulation pump  42 , the diameter of the outlet  50  of the nozzle  49 , the inclination of the nozzle  49 , the form and inclination of the outlet tube  23  for the mist, the geometry of the ventilation pipes and the efficacy of the refreshing of the piezoelectric element  46  by virtue of its support  33  provided with an orifice  34 . 
         [0086]    For example, for an angle of 15o with respect to the horizontal, a nozzle diameter of 6 mm and a pump output of 1 liter/minute, the nebulization rate is approximately 1.9 liters per hour, which represents an output of 32 watt-hours per liter. With conventional systems, the output is approximately 75 watt-hours per liter. With a system according to the invention, the gain is approximately 58% of electrical energy for the same nebulization output. 
         [0087]    According to another aspect of the invention, a certain number of the structure elements and functional elements of the device can be produced from plastics material (in particular polyamide, such as PA66), advantageously reinforced by glass fibers or other fibers. This makes it possible to lighten the device in order to facilitate manipulation thereof, and this can also facilitate the mass production of said elements by plastic molding methods. The following can be produced from suitable plastics materials in particular: the bottom frame  4 , the tank module  5 , and the top part  2  with the outlet tube  23  for the mist (with the exception of certain components of the motor of the fan  24 ). The cowl  3  of the electronic unit can also be produced from plastics material but, for reasons of electromagnetic shielding and heat dissipation, the use of a metal sheet may have advantages. 
         [0088]    According to another aspect of the invention, the bottom frame  4  may comprise one or more stabilization plates  48 , visible in  FIG. 11 , which, in the event of mechanical disturbance of the system  1 , prevent an excessively great fluctuation of the level of liquid in the tank  7  and in the nozzle  49 , said fluctuation in level being able to impair the continuous covering of the emitting surface of the piezoelectric element  46  with liquid. Said stabilization plates  48  may be disposed horizontally and/or vertically. They may comprise orifices. They may be disposed aslant. 
         [0089]      FIG. 11  illustrates another embodiment of the invention that relates to the relationships between various volumes. The volume of liquid in the tank  7  is designated V 1 , the volume of liquid in the circulation pump and the conduits connecting it to the tank  7  (not shown in the figure) are designated V 2 , the volume of liquid in the pressurization chamber  52  at the bottom part of the nozzle  49  (i.e. less than the height that defines the bottom plane of the volume V 5  (see below) is designated V 3  and the volume of liquid in the nozzle  49  is designated V 4 . The volume V 3  may be very small or even zero. V 5  designates the volume of liquid in the top part of the pressurization chamber  52  situated at a liquid level higher than the highest of the following three points: the water inlet orifice  51  of the nozzle situated highest, or the top edge of the outlet orifice  50  at the outlet of the nozzle  49 , the highest point of the piezoelectric ceramic  46 . Thus, whatever the inclination a of the nozzle  49 , any point of the piezoelectric ceramic  1  is situated at a level lower than the volume V 5 . 
         [0090]    In normal operation of the system  1 , the volumes V 1 , V 2 , V 3 , V 4  and V 5  are filled with liquid, the circulation pump  42  and the piezoelectric element  46  are functioning and generate a water jet  53  of approximately constant length, which illustrates the steady state of the system. 
         [0091]    According to the invention, the pressurization chamber  52  is sized so that it has a buffer volume (safety volume) V 5  sufficient with respect to the volume V 4  of the nozzle  49 , so that, in the case where the circulation pump  42  is no longer pumping any liquid (for example when the liquid level in the tank  7  is insufficient, or when the circulation pump  42  is drained), the volume V 5  provides, for a certain period of time ts, the supply of water to the volume V 4  of the nozzle  49 , so that the piezoelectric element  46  is still immersed during this period of time ts. This period of time ts may, in whole or in part, be taken advantage of to cut off the supply to the piezoelectric element  46 , and/or to wait to see if the level of liquid is re-established by itself (in particular in the case of mechanical disturbance or when the circulation pump  42  has simply taken in an air bubble). The time ts must be sufficiently long to allow total cutoff of the supply to the piezoelectric element  46  and stoppage of the functioning thereof; the applicant has in fact observed that the stoppage of functioning of the piezoelectric element  46  is not instantaneous when its electrical supply is cut off: the piezoelectric element  46  continues to vibrate while the circuits of its electrical supply are emptied. 
         [0092]    In general, it is preferred in the context of the present invention for the ratio of the volumes V 5 /V 4  to be at least 2 and preferably at least 6, and even more preferentially at least 12. 
         [0093]    More precisely, the desirable reaction time for the system to cut off the electrical supply to the piezoelectric element  46  in the event of lack of water is taken into consideration. It is not necessarily desirable to cut off the supply at the least drop in level in the pressurization chamber  52 , which would risk leading to an excessively intermittent generation of mist. However, it must be certain that, when this drop is prolonged or is aggravated beyond a certain period, the electrical supply to the piezoelectric element  46  should be cut off or at least greatly reduced. Thus the inventors consider that, in a mechanically unstable environment (vehicle, stall surrounded by a crowd of persons), the nebulization system  1  according to the invention must allow functioning of the piezoelectric element  46  during a period ts of between 1 and 10 seconds without a supply of liquid by the circulation pump  42 , and preferably between 2 and 5 seconds. 
         [0094]    In this context, an important parameter is the flow rate of liquid generated by the piezoelectric element  46  at the outlet of the orifice  50  of the nozzle  49  in the absence of any pumping by the circulation pump  42 ; this rate (which is often manifested by the presence of a small jet of water referred to as an “acoustic fountain”) depends (for a given angle α of the positioning of the nozzle  49  and a given liquid) essentially on the power of the piezoelectric element  46 . 
         [0095]    Even more precisely, the rate of the acoustic fountain may be expressed by 
         [0000]      Qpiezo= K ×Pmax
 
         [0000]    where Pmax is the maximum electrical power consumed by the piezoelectric element and Qpiezo is the flow rate of the acoustic fountain at this power Qpiezo, and K is a proportionality factor. A period of safety functioning of ts seconds is desired, that is to say when the circulation pump  10  ceases to function (in particular because of draining), the system has a period of approximately ts seconds for cutting off the supply to the piezoelectric element  46 . Advantageously, the time ts is between 1 and 10 seconds, and a value of between 2 and 5 seconds is preferred. According to the invention, this objective can be achieved by providing a sufficient safety buffer volume V 5 , which corresponds to the volume of the pressurization chamber  52  situated at a level of liquid higher than the top edge of the outlet orifice  50  of the nozzle  49 . This volume must be greater than the volume V 4  of the nozzle  49 . 
         [0096]    It is therefore wished that V 5 &gt;V 4 =Qpiezo×ts. 
         [0097]    This relationship can be expressed by V 5 &gt;V 4 +K×Pmax×ts. 
         [0098]    In a typical example, a nozzle  49  with a volume V 4  of 0.0054 liters is used, and Qpiezo is 50 W for a supply voltage of 22 V with an acoustic efficiency of approximately 40%; the angle α is between 0o and 30o. Under these conditions Pmax is approximately 1.5 liters/min, and consequently K=0.0005 liters/W.s. If a value ts=5 seconds is aimed at, V 5  must be at least equal to 0.13 liters. The ratio V 5 /V 4  is therefore 24. As indicated above, the value is may be less than 5 seconds, which tends to decrease the ratio V 5 /V 4 . 
       LIST OF REFERENCE NUMERALS 
       [0099]      1  System 
         [0100]      2  Top part 
         [0101]      3  Cowl 
         [0102]      4  Bottom frame 
         [0103]      5  Tank module 
         [0104]      6  Radiator 
         [0105]      7  Tank 
         [0106]      8  Electronic unit 
         [0107]      9  Nozzle module 
         [0108]      10  Nebulizer (diffuser) 
         [0109]      11  Runner 
         [0110]      12  Quick connection of end piece 
         [0111]      13  Accommodating compartment 
         [0112]      14  Pipe for mist 
         [0113]      15  Diffuser mast 
         [0114]      16  Orifice of nebulizer (diffuser) 
         [0115]      21  Air inlet 
         [0116]      22  Mist outlet end piece 
         [0117]      23  Mist outlet tube 
         [0118]      24  Fan 
         [0119]      30  Electronic card 
         [0120]      31  Electronic components 
         [0121]      32  Electrical connector or cable 
         [0122]      33  Piezoelectric element support 
         [0123]      34  Orifice in support 
         [0124]      35  Fixing element 
         [0125]      36  Seal 
         [0126]      37  Orifice 
         [0127]      38  Rear wall of chamber 
         [0128]      39  Liquid-level detector 
         [0129]      41  Solenoid valve 
         [0130]      42  Circulation pump 
         [0131]      43  Level sensor 
         [0132]      44  Water inlet 
         [0133]      45  Heating element 
         [0134]      46  Piezoelectric element 
         [0135]      47  Drain 
         [0136]      48  Stabilization plate 
         [0137]      49  Acoustic concentrator (nozzle) 
         [0138]      50  Nozzle outlet 
         [0139]      51  Nozzle inlet orifice 
         [0140]      52  Pressurization chamber 
         [0141]      53  Jet of liquid 
         [0142]      54  Body of top part 
         [0143]      55  Seal ring 
         [0144]      56  Groove 
         [0145]      57  Rim 
         [0146]      58  Hinge 
         [0147]      60  Table (stall)