LIQUID WARMER

Description

[0001] The invention relates to a liquid heater according to the preamble of claim 1 and to a method for heating a foamable liquid in a liquid heater according to claim 15.

[0002] Liquid heaters which also stir a liquid to be heated, for example milk, during the heating process are known from the prior art.

[0003] US Patent 9,107,533 B2 discloses an automatic milk frother comprising a container for receiving the milk, a base into which the container is inserted for heating and frothing the milk, a rotor housed within the container for stirring the milk, and a heating device for warming the milk. The rotor is driven by a motor located in the base via a magnetic coupling. The container is free of electrical components and a continuous drive shaft for the stirring device.

[0004] US Patent 2013 / 081545 A1 relates to a milk frother with a base and a stainless steel container that can be inserted into it. The container is heated by an induction coil in a side wall of the base. The base has a centrally located, hollow projection in which a magnetic head rotates, driven by an electric motor located in the base. The bottom of the stainless steel container has a central, hollow hub that accommodates the projection.

[0005] A disadvantage of the milk frothers according to US 9,107,533 B2 and US 2013 / 081545 A1 is that heating the milk contained in the container is only possible if the milk is frothed at the same time.

[0006] When warming liquids for subsequent consumption by babies, the formation of foam and air bubbles should be avoided. Preventing air pockets can help alleviate common feeding problems such as colic and gas. Furthermore, the liquid, especially milk, should be warmed gently, avoiding so-called hotspots—areas with temperatures that can damage the nutrients, enzymes, and antibodies in the liquid being warmed, particularly in breast milk. According to scientific studies, breast milk should not exceed a maximum temperature of 40°C when warmed (see, for example, Bransburg-Zabary S, Virozub A, Mimouni FB, Human Milk Warming Temperatures Using a Simulation of Currently Available Storage and Warming Methods. PLoS ONE 10(6), published on June 10, 2015).

[0007] US patent 2018 / 126339 A1 discloses a machine for homogenizing a foodstuff, such as for frothing milk or a milk-containing substance, optionally with inductive heating of the foodstuff. The machine has an electrically passive, cylindrical container, e.g., made of stainless steel and with a flat bottom, for holding the foodstuff, which can be inserted into a housing, as well as a magnetically driven impeller.

[0008] The object of the invention is to provide a liquid heater and a method for heating a foamable liquid as described above, which avoids or at least reduces the disadvantages of the prior art. The liquid heater and the method should enable the most uniform possible heating of the liquid contained therein, without the formation of overheated areas within the liquid volume and without foaming of the liquid. Furthermore, the heating of the liquid should be as efficient as possible. The liquid heater should also be inexpensive to manufacture and easy to clean.

[0009] This problem is solved by a liquid heater according to claim 1 and a method according to claim 15. Advantageous embodiments and further developments are specified in the dependent claims.

[0010] The liquid heater according to the invention is characterized in that the rotor is designed to rotate the liquid in the container in an operating state and to cause it to rise on the outer surface, and the rotational speed of the rotor is in a speed range in which a defined minimum quantity of the liquid in the container exposed to the rotating rotor has a height at least equal to the heating head, and in which speed range the heated liquid has a surface free of a continuous foam surface, wherein, in the case of a density of the liquid in the range of 1.018-1.048 g / cm³, a Newton number associated with the stirring device is between 0.1 and 0.3.

[0011] The method according to the invention is characterized in that, in an operating state, the rotor sets the liquid in the container into rotation and causes it to rise on the outer surface, wherein the rotational speed of the rotor is in a speed range in which, during its rotation, the rotor causes a defined minimum amount of the liquid in the container to rise to a height at least equal to the heating height, and with which speed range, after the rotation of the rotor, a heated liquid is obtained in the container which is furthermore free of a continuous foam surface on its surface, wherein, if a liquid with a density in the range of 1.018-1.048 g / cm³ is taken into the container, a Newton number associated with the stirring device is between 0.1 and 0.3.

[0012] A liquid warmer is used to heat liquids, particularly milk, such as expressed breast milk, powdered milk, or animal milk. The liquid warmer comprises a container for holding the liquid to be heated, a base on which the container can be placed and from which it can be removed, a heating element for heating the liquid in the container, and a stirring device for stirring the liquid. The container has a base and an outer surface extending from it and can, for example, be designed as a fillable and emptyable container with a lid that can be opened and closed again and a handle for the user. The outer surface can, for example, be cylindrical.Preferably, the container is designed to hold a maximum intended fill quantity of 300 ml of the liquid to be heated, more preferably 200 ml, and particularly preferably 120 ml, whereby the actual capacity may be even higher. In use, particularly for heating the liquid, the container is positioned on the base, which includes the electrical components required for operating the liquid heater. For this purpose, the base may have a power cable for connection to a wall socket or batteries. To remove the heated liquid and to clean the container, it can be detached from the base by the user of the liquid heater.

[0013] The heating device serves to heat the liquid in the container and may include a control device for the user to set a desired target temperature of the liquid to be heated. The outer surface of the container, up to a predetermined heating height from the bottom, is part of the heating device and maintains a heating temperature up to this height for heating the liquid, i.e., during operation of the liquid heater. Thus, the liquid in the container is gently heated via the outer surface. Since the outer surface generally has a larger surface area than the bottom of the container, the heating of the liquid can be particularly efficient. Furthermore, due to the comparatively large surface area of ​​the outer surface, particularly high heating temperatures are not required. For example, the heating temperature is only up to 30 degrees, preferably up to 20 degrees, above the target temperature of the liquid to be heated.

[0014] The stirring device comprises a rotor, which is housed in the container and has magnetic field-generating or magnetizable first bodies, and a drive unit for the rotor, which is housed in the base and also has magnetic field-generating or magnetizable second bodies. The magnetic field-generating first and second bodies can be permanent magnets or coils capable of carrying an electric current, while the magnetizable first and second bodies can be metallic, in particular ferromagnetic. The rotor is housed in the container and is set in rotation by the drive unit located in the base during operation of the liquid heater, i.e., during the heating of the liquid in the container. For this purpose, the first bodies are magnetically coupled to the second bodies.Thus, the rotor is magnetically coupled to the drive unit via the first and second bodies, and the drive unit is designed to rotate the rotor at at least one speed. The drive unit includes a motor, in particular an electric motor. The rotation of the rotor during the heating of the liquid in the container ensures the most even distribution possible of the heat introduced by the heating device within the liquid, without creating overheated areas within the liquid volume, so-called hotspots. Avoiding such overheated areas or hotspots has a beneficial effect on the liquid's components and also prevents discomfort or injury when drinking the liquid if it is not thoroughly stirred beforehand.

[0015] To efficiently heat the liquid in the container while minimizing foam formation, the rotor speed is set within a range that ensures a defined minimum amount of liquid rises to at least the heating level during its rotation, and within a speed range that, after the rotor has finished rotating, ensures the heated liquid's surface is free of any continuous foam. Thus, the drive unit is designed to rotate the rotor at a speed that allows the liquid, set in motion by the rotor, to rise to at least the heating level, while preventing the formation of a continuous foam surface on the liquid's surface, such as that of (breast) milk, both at the end of the rotor's rotation and after it has finished rotating.In this way, even a defined minimum amount of liquid in the container is efficiently heated, as the liquid comes into contact with the entire surface area of ​​the container that is at the heating temperature. For this to work, the rotor speed must be at least as high as a lower limit of the speed range. Furthermore, by ensuring the rotor speed does not exceed an upper limit of the speed range, even with foamable liquids like milk, a continuous layer of foam is avoided. This allows the heated liquid to be drunk immediately after being removed from the container without swallowing air in foam that forms on the surface. Avoiding foam formation is particularly advantageous when a small child drinks the heated liquid. However, individual air bubbles or areas of the heated liquid's surface covered in foam may still appear after the rotor has finished turning.

[0016] Any rotational speed within the speed range is suitable to raise the liquid level in the container at least to the heating element's height and prevent a continuous foaming surface after the rotor has finished turning. The rotational speed or speed range depends on numerous design parameters of the liquid heater. The drive unit can also be designed to adjust the speed and / or speed range to adapt it to the properties of the liquid in the container. For example, a table or a pre-programmed setting aid in the liquid heater can be provided to indicate suitable speeds or speed ranges for different liquids. Within the speed range, the rotor speed can remain constant.

[0017] Furthermore, in the case of a liquid density in the range of 1.018–1.048 g / cm³, the Newton value assigned to the stirring device is between 0.1 and 0.3. The density of the liquid in the specified range is essentially that of milk. Preferably, the Newton value assigned to the stirring device is also between 0.1 and 0.3, more preferably between 0.1 and 0.2, if the surface tension and / or the viscosity of the liquid is essentially that of milk. In this case, the surface tension can be in the range of 30 to 43.5 mN / m and the viscosity in the range of 1.06386 to 3.27726 mm² / s. The specified values ​​for density, surface tension, and viscosity depend on the temperature and the type of liquid, particularly milk.It is obvious that differently designed rotors may require different rotational speeds to raise the defined minimum volume of liquid in the container at least to the heating head. However, the different rotors can also have different Newton values, since the Newton value is characteristic of the power transferred to the liquid via the rotor. Thus, the rotor design is sufficiently defined by specifying the Newton value. Therefore, the rotor installed in the container can have one of several different shapes. In particular, the Newton value indicates what proportion of the rotor's power P is actually available as hydraulic power. The following applies: . Ne = P / ρ * n 3 * d 5 with: Newton number, P rotor power, W; ρ density of the liquid, kg / m³ < n rotational speed, s⁻¹ < d rotor diameter, m

[0018] The rotor power P is calculated from the resistance torque of the rotor: P = M * 2 * Π * n with: PRotor power, W MRotor torque, Nm nSpeed, s -1<

[0019] If the description refers to locations or directions such as above, above, below or below, these are to be understood in relation to a position of use of the liquid heater in which liquid in the container is heated.

[0020] According to a preferred embodiment of the invention, in the case of a liquid density in the range of 1.018-1.048 g / cm³, the Newton number associated with the stirring device is between 0.1 and 0.2.

[0021] It is particularly advantageous if, with a rotor power output in the range of 0.05 to 0.1 W, a heating height to rotor height ratio of 3 to 5, and a ratio of the vessel's shell diameter to the rotor's shell diameter in the range of 1.20 to 1.4, the rotor speed is in the range of 370 to 450 rpm. The rotor height is defined in the axial direction, and the shell diameter of the vessel or rotor is defined as the smallest diameter within which, when viewed axially from the operating liquid heater, the inner surface of the vessel or rotor is completely enclosed.

[0022] To enable the container to be designed without electrical components in the heating device, the heating device can be an induction heating device. Thus, the container, and in particular the outer surface up to the heating height, can be heated by induction from an external energy source. For this purpose, the outer surface, at least in the area up to the heating height, is made of a material that can be heated by induction.

[0023] For heating the liquid in the container, it is particularly advantageous if the base has a bottom and a side surface projecting from it, which is arranged next to the outer surface of the container positioned on the base and preferably includes a magnetic field-generating element of the induction heating device extending to the heating height. It is particularly advantageous if the side surface of the base is curved, at least in sections, around the outer surface of the container. In particular, the side surface can completely surround the outer surface, for example, in an annular or cylindrical shape. Thus, the container can be inserted into the base for heating the liquid. If the magnetic field-generating element of the induction heating device is arranged over as large an area as possible on the side surface of the base, the heating of the outer surface of the container, and consequently the liquid in the container, can be particularly efficient.For example, the magnetic field-generating device can span more than half the surface area, in particular more than three-quarters of the surface area of ​​the side face. The magnetic field-generating device can, in particular, be at least one coil that generates an alternating magnetic field when an alternating current flows through it.

[0024] If the base also incorporates a magnetic field-generating element for the induction heating device, this can also heat the bottom of the container, making the heating of the liquid inside even more efficient. In this case, the bottom of the container is also made of an electrically conductive material that can be heated by induction.

[0025] To make cleaning as easy as possible and to keep the container free of any electrical contacts that might detract from its appearance, the container is preferably electrically insulated from the base. The container therefore has no electrical connection to the base and can be immersed in water for cleaning without risk of damaging the contacts. Furthermore, the bottom and outer surface of the container can be made of thin walls, as these are free of electrical components.

[0026] For a durable container construction, efficient heating of the liquid, and the most effective possible prevention of foaming of the liquid within the container, the outer surface of the container, at least up to the heating element, can be made of metal, particularly stainless steel, and preferably cylindrical. The outer surface can nevertheless include a metal-free section, e.g., a transparent viewing window, preferably at least in the area of ​​the heating element, to allow observation of the stirring process or the level of the liquid introduced. Stainless steel also offers the advantage of corrosion resistance.

[0027] If at least one underside of the container base is flat, the container can be easily designed, cost-effectively manufactured, and reliably placed on a surface. The underside of the container base is understood to be the side facing away from the interior of the container.

[0028] If the upper surface of the container base is flat and, preferably, the rotor is free of a circular or ring-shaped recess for a rotating shaft, the upper surface of the container base facing the interior can be cleaned particularly easily and reliably. For this purpose, the upper surface of the container base does not have, for example, cylindrical or pin-shaped protrusions that would serve as a rotating axis for the rotor. Furthermore, the rotor can be easily inserted into the container before commissioning the liquid heater without having to be positioned precisely over or on a rotating axis. This is particularly advantageous if the container has a small diameter compared to its height, making it difficult for a user to insert their hand deep enough into the container to position the rotor precisely on a rotating axis. Ideally, the rotor is also free of a circular or ring-shaped recess.A recess, in particular a cylindrical receptacle or recess, for a rotating shaft. This allows the rotor to be cleaned easily and thoroughly, as it has no receptacle or recess for a rotating shaft where residues of the heated liquid could accumulate.

[0029] To enable the container to be constructed independently of the drive unit, it is advantageous for the second bodies of the drive unit to be arranged below the bottom of the container positioned on the base. In contrast, the rotor is arranged above or on the bottom of the container. Since the magnetic coupling between the second bodies of the drive unit and the first bodies of the rotor, and thus the power transmission from the drive unit to the rotor, occurs through the bottom of the container, the container bottom is advantageously designed with thin walls for efficient power transmission. For example, the thickness of the container bottom is at most 3 mm, preferably at most 2 mm.

[0030] It is particularly advantageous if the rotor, in a state coupled to the drive unit, is magnetically centered by the second bodies of the drive unit. This eliminates the need for a mechanical axis of rotation for the rotor. The centering of the rotor, i.e., the position of the rotor that is favorable or necessary for stirring the liquid in the container, is achieved by the magnetic attraction between the first and second bodies. It is particularly advantageous for the distance between the second bodies of the drive unit and the distance between the first bodies of the rotor to be as large as possible, for example, at least half, preferably at least two-thirds, of the rotor's circumscribed circle diameter, in order to transmit the highest possible torque from the drive unit to the rotor via the magnetic coupling.Furthermore, as a result of the large distance, the rotor can be reliably centered even if the rotor was inserted into the container by a user outside the center of the container bottom, e.g., touching the outer surface.

[0031] To achieve reliable magnetic coupling between the rotor and the drive unit, the extent of at least one of the first and second bodies in a plane parallel to the bottom of the container positioned on the base is preferably greater than the extent of at least one of the first and second bodies in a plane perpendicular to the bottom of the container positioned on the base. Preferably, at least one of the first and second bodies extends in the plane parallel to the bottom of the container positioned on the base by at least 10%, more preferably at least 20%, and most preferably at least 30% of the rotor's circumscribed circle diameter.

[0032] For ease of use of the liquid heater, the rotor can be removed from the container without tools and, in particular, can move freely in the radial direction. Thus, for example, for cleaning the rotor and the container, the rotor can fall out of the container by turning the container, once removed from its base, so that the container bottom is facing upwards. Removal of the rotor is further facilitated by the fact that, according to this embodiment, the rotor of the container, once removed from its base, is free to move in the radial direction, i.e., without any mechanical guide.

[0033] According to a further embodiment, the rotor can have at least two, preferably three, arms extending in a radial direction, on which the first bodies are arranged. The arms projecting from a center point or center of gravity of the rotor serve to stir the liquid. Furthermore, the arrangement of the first bodies on the arms, particularly in a radially outer half or a radially outer third of the arms, enables efficient transmission of torque from the drive unit to the rotor. For example, the arms of the rotor inserted into the container extend at least partially in a plane parallel to the bottom of the container. It is also advantageous if the longitudinal extent of the arms in the radial direction of the rotor is greater than the vertical extent of the arms in the axial direction of the rotor.

[0034] For efficient stirring of the liquid and the most loss-free possible transmission of torque from the drive unit to the rotor, it is advantageous if the rotor has three arms curved in a rotor plane, preferably in the direction of rotation, and preferably an axially projecting ridge on the side facing the bottom of the container, serving as a bearing surface. The rotor plane is perpendicular to a virtual axis of rotation of the rotor, i.e., perpendicular to the axial direction of the rotor. The curvature of the arms in the direction of rotation promotes stirring of the liquid. The axially projecting ridge, which serves as a bearing surface, advantageously reduces the frictional resistance of the rotor against the bottom of the container. For example, the axially projecting ridge of the rotor can be a bulge or a pin pointing towards the bottom of the container.

[0035] The invention is further explained below with reference to preferred embodiments, to which it is not limited. The drawings show: Fig. 1 a liquid heater according to the invention, with a lid, in a perspective view; Fig. 2 the liquid heater Fig. 1 without the lid; Fig. 3 a top view of and partially inside the liquid heater Fig. 2 ; Fig. 4 a cross-sectional view of the liquid heater made of Fig. 1 ; Fig. 5 another sectional view of the liquid heater Fig. 1 , corresponding to a comparison with Fig. 4 Cut rotated by 90°; Figs. 6a to 6d Views of a rotor of the liquid heater from Fig. 1 ; Fig. 7 a diagram of rotor power as a function of time; and Fig. 8 a graph of the Newton number as a function of time.

[0036] Fig. 1shows a liquid heater 1 according to the invention, with a Fig. 1 The liquid heater 1 consists of a container 2 (not shown) for receiving a liquid to be heated (not shown), a base 3 on or in which the container 2 is positioned according to a usage position, a removable or otherwise openable lid 4 for covering or closing the container 2, and a handle 5 for easy gripping of the liquid heater 1 by a user (not shown). A control element 6 in the form of an electrical on / off switch 6a is also visible. The base 3 has a bottom 7 and a side surface 8 projecting upwards towards the lid 4, which surrounds a large part of the container 2. The bottom 7 of the base 3 may well have a cavity containing components of the liquid heater 1, cf. e.g. Fig. 4 . In the Fig. 1In the illustrated example, the container 2 is completely enclosed by the base 3 and the lid 4. In another embodiment not shown, the container 2 may only be partially surrounded or covered by the base 3 and the lid 4.

[0037] Fig. 2 shows the liquid heater 1 from Fig. 1 without the lid 4, whereby the container 2, in particular the outer surface 9 of the container 2, is visible. In the illustrated example, the container 2 also has the handle 5 and a spout 10 and is at least partially cylindrical. Preferably, the outer surface 9 of the container 2 is at least partially cylindrical.

[0038] Fig. 3 shows the liquid heater 1 from Fig. 2in a top view, looking into container 2. Clearly visible are the container 2, which is placed on or inserted into the base 3, and a rotor 11 of a stirring device 12, which is housed in container 2 and used to stir a liquid in container 2. The rotor 11 can be made of plastic, in particular polypropylene.

[0039] Fig. 4 shows a vertical sectional view of the liquid heater made of Fig. 1Clearly visible are the container 2, the base 3, a heating device 13 for heating the liquid in the container 2, and the stirring device 12. The container 2 has a bottom 14 and a shell surface 9 extending from the bottom 14. The shell surface 9, up to a predetermined heating height H of the shell surface 9 extending from the bottom 14 of the container, is part of the heating device 13 and has a heating temperature TH for heating the liquid up to this heating height H. The stirring device 12 has the rotor 11 housed in the container 2 and a drive unit 15 for the rotor 11 housed in the base 3, wherein the rotor 11 has first bodies 16 that generate or magnetize magnetic fields, and the drive unit 15 has second bodies 17 that generate or magnetize magnetic fields.The drive unit 15 is designed to rotate the rotor 11 at at least one speed, for which purpose the first bodies 16 are magnetically coupled to the second bodies 17, and the drive unit 15 comprises a drive 18, in particular a motor 18a, which can be operated at a defined speed, and most preferably an electric motor 18b. The speed of the rotor 11 lies within a speed range in which the rotor 11, during its rotation, causes a defined minimum quantity M of the liquid in the container 2 to rise at least to the heating height H, and within which speed range, after the rotation of the rotor 11, the heated liquid is free of a continuous foam surface on its surface O. Fig. 4 The minimum amount M of liquid in container 2 is symbolically represented by the surface area O of the liquid. In the illustrated embodiment, the heating device 13 is an induction heating device 13a. Fig. 4It is also apparent that the side surface 8 of the base 3, projecting upwards from the bottom 7 of the base 3, is arranged next to the outer surface 9 of the container 2 positioned on the base 3 and, preferably up to the heating height H, has a magnetic field-generating device 13b of the induction heating device 13a. The side surface 8 of the base 3 can also be understood as a side wall with a wall thickness that may vary, in order to accommodate the magnetic field-generating device 13b of the induction heating device 13a. The magnetic field-generating device 13b of the induction heating device 13a can, for example, be at least one coil 13c, which is connected to an alternating current source (not shown) and generates an alternating magnetic field during operation of the liquid heater 1. This alternating magnetic field heats the outer surface 9 of the container 2 and thus the liquid in the container 2.For this purpose, the outer surface 9 of the container 2 is advantageously made of metal, in particular stainless steel, at least up to the heating height H. Furthermore, the base 7 of the base 3 can have a device 13b of the induction heating device 13a that generates a further magnetic field. This is shown in . Fig. 4 This is symbolically represented by the extension of the magnetic field-generating device 13b to below the rotor 11. Naturally, the device 13b, which generates the further magnetic field, can also extend below the bottom of the container 14.

[0040] Fig. 4 This also shows that container 2 is electrically isolated from base 3, i.e., there is no electrical connection between container 2 and base 3. In particular, container 2 is free of any current-carrying components during the operation of liquid heater 1.

[0041] Furthermore, in the example according to Fig. 4It is evident that at least one underside 19 of the container base 14 is flat. Likewise, one upper surface 20 of the container base 14 may be flat. The flat underside 19 and / or upper surface 20 permits slight waviness, particularly due to the manufacturing process, but is free of protrusions or depressions whose dimensions in the axial direction A of the liquid heater 1 are greater than the thickness of the container base 14.

[0042] In the Fig. 4In the example shown, the second bodies 17 of the drive unit 12 are arranged below the base 14 of the container 2, which is positioned on the base 3. Thus, the container base 14 does not require any depressions or recesses to accommodate the second bodies 17. The rotor 11 is placed in the container 2 and, in a magnetically coupled state with the drive unit 12, is magnetically centered by the first bodies 16 of the rotor 11 and the second bodies 17 of the drive unit 12. In particular, the rotor 11 is, according to the illustration in Fig. 4 The example shown can be removed from container 2 without tools and is freely movable in the radial direction R.

[0043] Fig. 5 shows another cross-sectional view of the liquid heater made of Fig. 1 , generated by a compared to Fig. 4 Section plane rotated by 90°. In Fig. 5In addition, a shell circle diameter KB of the container 2 is shown, which is measured on the inside of the shell surface 9.

[0044] The Figs. 6a to 6d Figure 11 shows an exemplary rotor 11, which has at least two, or in the illustrated example three, arms 21 extending in a radial direction R of the rotor 11, on which the first bodies 16 are arranged. Fig. 6a a bottom side of the rotor 11, which in the operating state of the liquid heater 1 faces the bottom of the container 14. Fig. 6b shows rotor 11 in a side view, Fig. 6c shows an upper side of the rotor 11, which in the operating state of the liquid heater 1 is facing away from the bottom of the container 14, and Fig. 6d Figure 1 shows a cross-sectional view through rotor 11 along line CC. Rotor 11 has a height HR and a diameter (shell diameter KR).

[0045] In the Figs. 6a to 6cIn the illustrated example, it is also evident that the rotor 11 is free of a circular or ring-shaped recess or any other depression for a rotating shaft. Furthermore, it is evident that the rotor 11 has three arms 21 curved in a rotor plane E (in the illustrated example, in the direction of rotation DR) and an axially projecting elevation 22 as a bearing surface on the side facing the container bottom 14, particularly in the center of the rotor 11.

[0046] Fig. 7 shows a diagram of the rotor power P (in watts) as a function of time t (in seconds) for three different rotor speed combinations. Combination 1: Rotor height HR = 16.5 mm, rotor diameter KR = 70 mm, rotational speed = 380 rpm Combination 2: Rotor height HR = 16.5 mm, rotor diameter KR = 65 mm, rotational speed = 390 rpm Combination 3: Rotor height HR = 10.73 mm, rotor diameter KR = 70 mm, rotational speed = 420 rpm

[0047] Fig. 8shows a diagram of the Newton number Ne as a function of time t (in seconds) for the three rotor speed combinations from Fig. 7 .

[0048] Experiments were carried out with the described liquid heater 1 to investigate rotor dimensions and associated speed ranges with which the rotor 11, during its rotation, causes a defined minimum amount of liquid in the container 2 to rise at least to the heating height H, wherein after the rotation of the rotor 11 the heated liquid is free of a continuous foam surface on its surface.

[0049] Tables 1 and 2 below contain some experimental results. Table 1 Rotor No. 1 2 3 4 5 6 Rotor height (mm) 16,5 16,5 16,5 16,5 16,5 8 Rotor diameter (mm) 70 70 40 50 60 70 Minimum speed for 60ml of liquid, heating height wetted (rpm) 380 380 >1100 870 600 505 Minimum speed for 100ml of liquid, heating 295 295 >1100 1100 470 300 height wetted (rpm) Max. speed for a foam-free surface (rpm) 390-420 380-410 510-650 470-560 420-460 320-380 Table 2 Rotor No. 7 8 9 10 11 Rotor height (mm) 32 16,5 14,03 12,38 10,73 Rotor diameter (mm) 70 65 70 70 70 Minimum speed for 60ml of liquid, heating height wetted (rpm) 325 390 370 380 420 Minimum speed for 100ml of liquid, heating height wetted (rpm) 200 n / a n / a n / a n / a Max. speed for a foam-free surface (rpm) 360-380 380-410 400-420 400-420 410-430

[0050] The tests show that rotors 11, numbered 1, 2, and 7, are particularly suitable for achieving the stated objectives. Rotors 11, numbered 8 to 11, are also at least conditionally suitable. In particular, with these rotors 11, at the specified minimum speed, the outer surface 9 can be wetted with liquid up to the heating height H when 60 ml or 100 ml, as a defined minimum quantity of liquid, are present in the container 2. Furthermore, up to the specified maximum speed, the heated liquid remains free of any continuous foam on its surface at the end of the stirring process. Milk at 7°C was used as the liquid to be heated.

[0051] The liquid heater 1 can, for example, be designed and operated with the following parameters: Inner diameter of the container: 84.7 mm; Heating height: approx. 50 mm; Rotor height: 10.725 mm; Ratio of heating height to rotor height: 4.662; Rotor diameter: 65 mm; Ratio of inner diameter of the container to rotor diameter: 1.303; Maximum speed without frothing (60 ml milk): approx. 450 rpm (depending on the rotor shape); Minimum rotor power: P = 0.05 W; Newton number: Ne = 0.1

[0052] A lower height HR or a lower diameter KR of the rotor 11 would require a higher rotational speed to allow the liquid to rise on the outer surface 9 through the rotating rotor 11 to the heating height H, but this rotational speed could cause foaming on the surface of the liquid.

Claims

1. Liquid heater (1) with a container (2) for receiving a liquid to be heated, a base (3) on which the container (2) is positioned, a heating device (13) for heating the liquid in the container (2) and a stirring device (12) for stirring the liquid in the container (2), the container (2) comprising a bottom (14) and a lateral surface (9) extending from the bottom (14), the lateral surface (9) being part of the heating device (13) up to a predetermined heating level (H) of the lateral surface (9), the heating level (H) starting from the bottom (14), and the lateral surface (9) comprising a heating temperature (TH) up to the heating level (H) for heating the liquid, and the stirring device (12) comprising a rotor (11) that is received in the container (2) and comprises magnetic field-generating or magnetisable first bodies (16), and a drive mechanism (15) for the rotor (11), the drive mechanism (15) being received in the base (3) and comprising magnetic field-generating or magnetisable second bodies (17), the drive mechanism (15) being designed to rotate the rotor (11) at least at one speed, for which the first bodies (16) are magnetically coupled to the second bodies (17), wherein the rotor (11) is designed to set the received liquid in the container (2) in rotation in an operating state and to allow it to rise up the lateral surface (9), characterised in that the rotational speed of the rotor (11) lies in a rotational speed range in which a defined minimum amount of the liquid in the container (2) exposed to the rotating rotor (11) comprises a height at least equal to the heating level (H) and with which rotational speed range the heated liquid comprises a surface free of a continuous foam surface, wherein in the case of a density of the liquid in the range of 1.018-1.048 g / cm3, a Newton number assigned to the stirring device (12) is between 0.1 and 0.3.

2. Liquid heater (1) according to claim 1, characterised in that, in the case of the density of the liquid in the range of 1.018-1.048 g / cm3, the Newton number assigned to the stirring device (12) is between 0.1 and 0.2.

3. Liquid heater (1) according to claim 1 or 2, characterised in that, with a power of the rotor (11) in the range from 0.05 to 0.1 W, a ratio of the heating level (H) to a height of the rotor (HR) in the range from 3 to 5 and a ratio of an enveloping circle diameter (KB) of the container (2), the enveloping circle diameter (KB) being provided at the heating level (H), to an enveloping circle diameter (KR) of the rotor (11) in the range from 1.20 to 1.4, the rotational speed of the rotor (11) is in the range from 370 to 450 rpm.

4. Liquid heater (1) according to one of claims 1 to 3, characterised in that the heating device (13) is an induction heating device (13a).

5. Liquid heater (1) according to claim 4, characterised in that the base (3) comprises a bottom (7) and a side surface (8) projecting therefrom, which is arranged next to the lateral surface (9) of the container (2) positioned on the base (3) and, preferably up to the heating level (H), comprises a magnetic field-generating mechanism of the induction heating device (13a) and preferably also the bottom (7) of the base comprises a magnetic field-generating mechanism of the induction heating device (13a).

6. Liquid heater (1) according to one of claims 1 to 5, characterised in that the container (2) is electrically insulated from the base (3).

7. Liquid heater (1) according to one of the claims 1 to 6, characterised in that the lateral surface (9) of the container (2) is made of metal, in particular stainless steel, at least up to the heating level (H), and is preferably cylindrical.

8. Liquid heater (1) according to one of claims 1 to 7, characterised in that at least a bottom side (19) of the container bottom (14) is flat.

9. Liquid heater (1) according to one of the claims 1 to 8, characterised in that a top side (20) of the container bottom (14) is flat and preferably the rotor (11) is free of a circular or ring-shaped receptacle for a rotary shaft.

10. Liquid heater (1) according to one of claims 1 to 9, characterised in that the second bodies (17) of the drive mechanism (15) are arranged below the bottom (14) of the container (2) positioned on the base.

11. Liquid heater (1) according to one of claims 1 to 10, characterised in that the rotor (11) is magnetically centred in a state coupled to the drive mechanism (15) by the second bodies (17) of the drive mechanism (15).

12. Liquid heater (1) according to one of claims 1 to 11, characterised in that the rotor (11) can be removed from the container (2) without tools and, in particular, is movable in the radial direction (R).

13. Liquid heater (1) according to one of the claims 1 to 12, characterised in that the rotor (11) comprises at least two, preferably three, arms (21) extending in a radial direction (R) of the rotor (11), on which arms (21) the first bodies (16) are arranged.

14. Liquid heater (1) according to one of the claims 1 to 13, characterised in that the rotor (11) comprises three arms (21) curved in a rotor plane (E), preferably in the direction of rotation (DR), and preferably comprises an elevation (22) projecting in the axial direction (A) as a bearing surface on the side facing the container bottom (14).

15. Method for heating a foamable liquid in a liquid heater (1), wherein the liquid to be heated is received in a container (2) free from a continuous foam surface on its surface, the container (2) is positioned on a base (3), the liquid in the container (2) is heated with a heating device (13) and the liquid in the container (2) is stirred with a stirring device (12), the container (2) comprising a bottom (14) and a lateral surface (9) extending therefrom, which is part of the heating device (13) up to a predetermined heating level (H) of the lateral surface (9), the heating level (H) starting from the bottom (14), and the lateral surface (9) being heated to a heating temperature (TH) up to the heating level (H) in order to heat the liquid, and the stirring device (12) comprising a rotor (11) that is received in the container (2) and comprises magnetic field-generating or magnetisable first bodies (16), and a drive mechanism (15) for the rotor (11), the drive mechanism (15) is received in the base (3) and comprises magnetic field-generating or magnetisable second bodies (17), which drive mechanism (15) rotates the rotor (11) at least at one rotational speed, for which rotation the first bodies (16) are magnetically coupled to the second bodies (17), wherein the rotor (11) sets the received liquid in the container (2) in rotation in an operating state and allows it to rise up the lateral surface (9), characterised in that the rotational speed of the rotor (11) lying in a rotational speed range in which the rotor (11), during its rotation, causes a defined minimum amount of the liquid in the container (2) to rise to a height at least equal to the heating level (H) and with which rotational speed range, after the rotation of the rotor (11), a heated liquid is obtained in the container (2), the heated liquid still being free of a continuous foam surface at its surface, wherein, when a liquid with a density in the range of 1.018-1.048 g / cm3 is received in the container (2), a Newton number assigned to the stirring device (12) is between 0.1 and 0.3.