Low-noise heating element and formed container bottom

Abstract

What is claimed is a heating element which is for the bottom of a container for heating a liquid and comprises a metallic bottom of variable thickness having a linear electric heater which is attached to the metallic bottom from below or built into the body of the bottom. The special feature of the claimed heating element is that the bottom is made thicker at the heater penetration location and around the heater penetration location than the rest of the bottom surface. The bottom may radially include several concentric zones having different characteristics. The central zone has the smallest bottom thickness. The heating zone is located above the annular heater where the bottom is thickest. The thickness raises it upward. The annular heater may be designed as an incomplete ring with gaps. Also claimed is a container bottom device comprising a variable thickness bottom for heating a liquid.

Description

Technical Field

[0001] The present invention relates to a heating element for heating water or other liquids in household or industrial heating appliances. Background Art

[0002] Currently, there are various heating elements similar to the claimed one.

[0003] For example, CN101237797A discloses the device design of the most typical modern household kettle. The disadvantage of this device is the increased energy consumption and considerable noise during boiling due to the inefficiently organized boiling process.

[0004] Most devices for similar uses employ the same or similar designs of household appliances.

[0005] For example, GB2588888A discloses a device design for cooking solid or consistent (non - liquid) food. However, the design of this device is similar to the claimed design with bottom electric heating.

[0006] Its distinguishing features are the local heating of the bottom by a small part (less than 50%) of the heating element surface and an additional ventilation system in the area under the container, because the temperature for cooking solid or consistent (non - liquid) food in oil is higher than that of boiling water.

[0007] CN200948065 claims a device. This document discloses a heating element device for a household kettle. The heating element has a complex shape, is stamped from a metal plate, and an annular electric heater is welded inside the cavity.

[0008] The disadvantage of this heating element is that the bottom surface area is very narrow and is strongly heated by a tubular electric heater (TEH). Therefore, the heat exchange rate in this area increases, resulting in the formation of active cavitation bubbles and an increased noise level.

[0009] The device disclosed in CN200948065 was selected as a prototype.

[0010] The technical effect of the claimed invention is to create an efficient enclosed heating element with enhanced convection intensity and a reduced noise level.

[0011] The known devices cannot achieve the required technical effects.

[0012] The problems solved by the claimed device use the existing TEH. Summary of the Invention

[0013] A heating element is claimed, which is mainly used at the bottom of a container for heating a liquid and includes a metallic bottom with variable thickness having a linear heater. The linear heater is mainly an electric heater and is attached to the metallic bottom from below or built into the body of the bottom.

[0014] The special feature of the claimed heating element is that the bottom is made thicker at the heater penetration position and around this heater penetration position than the rest of the bottom surface.

[0015] There is a claim for a heating element which is mainly used at the bottom of a container for heating a liquid. The heating element is made in the form of a metallic bottom with variable thickness and has a circular linear heater. The heater is mainly a heater and is attached to the metallic bottom from below or built into the body of the bottom.

[0016] The features of the claimed heating element are as follows.

[0017] The bottom radially includes several concentric regions with different characteristics.

[0018] The central region has the minimum bottom thickness. This is a region for downward convection.

[0019] The heating region above and around the annular heater has the thickest bottom. The thickness causes it to rise up so that most of the heat radiation surface is in the liquid. This is a region for upward convection.

[0020] The annular heater is usually designed as an incomplete ring with gaps. The heating coil leads are at the ends of the ring. When such an annular heater heats the bottom of, for example, a kettle, this part is not heated.

[0021] Therefore, in order to improve convection, it is proposed to make the region above the annular heater have variable thickness. The places in the heater ring with gaps are made with smaller thickness. The opposite sides in the middle of the heating element are made with greater thickness.

[0022] The region between the central part and the region above the annular heater has variable bottom thickness, from the minimum thickness adjacent to the central part to the maximum thickness adjacent to the region above the annular heater.

[0023] The peripheral region from the annular heater to the region (if any) adjacent to the container wall has a bottom thickness close to the minimum.

[0024] The central region has an upwardly curved spherical or spherical to conical shape.

[0025] The region between the central region and the region above the heater can have a downwardly curved shape.

[0026] A variable - thickness container bottom unit mainly for heating liquids is claimed.

[0027] The bottom radially includes several concentric regions with different characteristics.

[0028] The central region has the minimum bottom thickness. This is a region of downward convection.

[0029] Then there is a region with variable bottom thickness, from the minimum thickness adjacent to the central part to the maximum thickness.

[0030] Farther from the center, there is a heating zone with the thickest bottom. This is a region of upward convection.

[0031] This region is considered the heating region, and the heating is considered to come from a linear heat source that is an incomplete un - enclosed ring.

[0032] Therefore, the heating region is thicker and has a variable thickness - smaller on one side and larger on the opposite side.

[0033] The peripheral region up to the region adjacent to the container wall has a thickness close to the minimum.

[0034] The minimum bottom thickness is selected and implemented due to structural and technical reasons or based on specific operating conditions, material properties, and many other factors. If using a sheet, the minimum thickness is equal to the thickness of the sheet used. If it is cast, the minimum thickness is determined by the casting and strength properties of the material.

[0035] The central region has an upward - curved spherical or spherical - to - conical shape.

[0036] The heating region with the maximum bottom thickness rises due to its thickness, so that the bottom surface remains flush with the adjacent region, while the protruding part and most of the heat - radiating surface are in the liquid.

[0037] The region between the central region and the thickest region of the bottom has a downward - curved shape.

[0038] Depending on the specific device implementation, the peripheral region up to the region adjacent to the container wall has a nearly flat shape, but can have a conical, spherical, cylindrical, or a combination thereof. Description of the Drawings

[0039] Figure 1 Shows the heating container bottom design according to patent GB2588888A.

[0040] Figure 2 Shows the heating container bottom design of the prototype device according to patent CN200948065.

[0041] Figure 3Shows the claimed bottom design

[0042] a) Isometric view,

[0043] b) Plan view,

[0044] c) Plan view showing regions 1.1, 1.2, 1.3 and 1.4.

[0045] Figure 4 Shows a schematic diagram of the liquid movement in the known design.

[0046] Figure 5 Shows a schematic diagram of the liquid movement in the claimed design.

[0047] Figure 6 - a), b), c) and d) show possible embodiments of the claimed design.

[0048] Figure 7 Shows the claimed design in cross-section.

[0049] Figure 8 Shows a temperature recording graph of the longitudinal section of a heating element according to Patent CN101237797A.

[0050] Figure 9 Shows a temperature recording graph of the longitudinal section of a heating element with a strong heat exchange region according to Patent CN200948065.

[0051] Figure 10 Shows a temperature recording graph of the longitudinal section of the claimed heating element with a uniform heat exchange distribution.

[0052] Figure 11 Shows a drawing of an experimental model. Detailed description of the invention

[0053] When water (aqueous solution or mixture) is heated, due to low pressure and high temperature, cavitation bubbles containing steam but no air appear at the nucleation sites in the strong heat exchange region. The cavitation bubbles break away from the surface and lose the heat energy in the form of the heating element, falling into a denser and cooler liquid layer where they violently collapse and emit shock waves, generating resonance in the heating element. This process produces boiling noise. The smaller the area of the strong heat exchange and the higher the temperature therein, the denser and faster the bubbles form, and the louder the noise. Thereafter, when a strong convection that heats the liquid more evenly is formed, the bubble separation rate decreases, the bubble size increases, and the pressure difference between inside the bubbles and the upper layer of the liquid becomes unimportant. The separated bubbles reach the surface without breaking. The noise decreases. The boiling process begins.

[0054] Therefore, by selecting the parameters of the body and shape of the bottom of the heating container with a built-in heating element, the required technical effect is achieved, which increases the area of the intensive heat exchange zone and accelerates the formation of convection, thus making the liquid temperature uniform.

[0055] The required technical effect is achieved by a heating element that is the bottom of the container for primarily heating the liquid. The heating element includes a metal bottom with variable thickness that has a linear heater. The linear heater is mainly an electric heater, attached from below to the metal bottom or built into the body of the bottom.

[0056] The special feature of the claimed heating element is that the bottom is made thicker at the heater penetration position and around this heater penetration position than the rest of the bottom surface.

[0057] In this case, the linear heating element can be made in any shape - circular ring, oval ring, rectangular ring or square or triangle, serpentine or any other closed or unclosed shape. The only important thing is that the bottom is thicker at the heater penetration position and around this heater penetration position (including the heater installation location) compared to the rest of the bottom surface.

[0058] To further increase the effect, the shape of the bottom surface can follow the expected trajectory of the convective movement of the liquid during heating and boiling.

[0059] The required technical effect is achieved by a heating element device that is the bottom of the container for primarily heating the liquid. The device includes a metal bottom with variable thickness that has a linear heater. The heater is mainly an electric heater, in a closed-loop shape, attached from below to the bottom or built into the body of the bottom.

[0060] The features of the device are as follows.

[0061] The bottom 1 radially includes several concentric regions with different characteristics.

[0062] The bottom thickness of the central region 1.1 is the smallest and is circular in shape.

[0063] Then there is the region 1.2 with variable bottom thickness, from the minimum thickness adjacent to the central part to the maximum thickness adjacent to the heating region 1.3 - the attachment of the linear ring heater.

[0064] Region 1.3 is the thickest region at the attachment point of the linear ring heater. The thickness causes it to rise, so most of the heat transfer surface is in the liquid. The region 1.3 above the heater can be 20 times or more thicker than the central region.

[0065] The shape of the bottom surface can follow the expected trajectory of the convective movement of the liquid during heating and boiling.

[0066] Linear heaters are typically designed as incomplete rings with gaps. The electrical leads of the heating coil leads are made at the ends of the ring. When such a ring heater heats the bottom of, for example, a kettle, the bottom part between the leads is not heated.

[0067] Therefore, in order to improve convection, it is recommended that the attachment area of the ring heater and the area around it have a variable thickness. Where there is a gap in the heater ring, it is made of a smaller thickness. The opposite sides in the middle of the heating element are made of a greater thickness.

[0068] Therefore, the central area (with a smaller bottom thickness) is slightly offset from the geometric center of the circular bottom towards the gap of the circular heater.

[0069] Depending on the heater power, the width and shape of the heat transfer surface, and other characteristics, the required (necessary) thickening value for areas 1.3 can vary. Therefore, the offset of the central area from the geometric center of the circular bottom towards the gap of the circular heater may also vary.

[0070] Since the central area is offset from the center towards the edge, the central area with a circular shape can, in some cases, change from almost circular to oval or elliptical. If there is a significant offset, the area with a larger bottom thickness may become open, i.e., have a gap. This gap is located at the position of the gap in the ring of the linear electric heater.

[0071] The transition from the area above the heater with a small thickness to the area with a large thickness can be smooth.

[0072] The peripheral area 1.4 from the ring heater all the way to the area adjacent to the container wall (if any) has a bottom thickness close to the minimum - approximately or exactly the same as the central area 1.1.

[0073] The central area 1.1 can have an upwardly curved spherical or spherical to conical shape to follow the direction of the convective liquid movement.

[0074] The area 1.2 between the central area and the heater attachment area has a downwardly curved shape.

[0075] In the current experiment, the thickness of the central area is 1.2 - 2.5 mm

[0076] The area 1.3 along the heater attachment can be 20 times thicker than the central area.

[0077] In the current experiment, the thickness of the bottom of the peripheral area is 1.2 - 2.5 mm

[0078] The transition between multiple areas can be smooth.

[0079] Radially oriented ribs can be added on the surface of the area above the annular heater. In the current experiment, the height of the ribs is 1.2 - 2.5 mm.

[0080] To prevent surface oxidation and fouling (oxidation and fouling can also reduce the number of nucleation sites and thus affect the technical effect), a waterproof, non-stick and / or any other coating can be additionally applied on the surface.

[0081] The shape of the bottom surface can follow the expected trajectory of the liquid convection movement during heating and boiling.

[0082] If there is an additional peripheral area further away (starting from the center) of the annular heater up to the area adjacent to the container wall, this area can have an approximately flat or conical or spherical shape.

[0083] In the current experiment, the thickness of the bottom of the peripheral area is 1.2 - 2.5 mm.

[0084] For the bottom device of the container mainly used for heating liquids, the required technical effect is achieved. The characteristics of the variable-thickness metal bottom device are as follows.

[0085] The bottom radially includes several concentric regions with different characteristics.

[0086] The bottom thickness of the central region is the smallest. This is a downward flow region.

[0087] Then there is a region with variable bottom thickness, from the smallest thickness near the central part to the largest thickness.

[0088] The heater (including a linear annular heater or any other heat source, possibly an electric heater or operating on any other principle) is placed, attached or built into the body of the heating region with the largest bottom thickness.

[0089] This region is assumed to be the heating region, and the heating is assumed to come from any, especially a linear annular heat source, including a heat source forming a complete ring or an unclosed ring.

[0090] Therefore, based on this, the heating region is made of a constant or variable thickness - with the smallest thickness on one side and the largest thickness on the opposite side.

[0091] The transition from the small thickness to the large thickness of the heating region can be carried out linearly, exponentially, power-law, or in any other way considered appropriate by the R & D personnel or production engineers.

[0092] Regardless of whether the track of the heating element is a closed loop or an open loop, the central region with the minimum bottom thickness can be slightly deviated from the geometric center of the circular bottom towards the heating line gap. At certain parameter values, the deviation of the central region may become significant, and the heating region may become an open loop with a gap.

[0093] The peripheral region from the area with the maximum bottom thickness to the area adjacent to the container wall has a bottom thickness close to the minimum.

[0094] The central region can have an upwardly curved spherical shape.

[0095] The region above the heat source has the maximum bottom thickness. The thickness causes it to rise up so that most of the heating surface is in the liquid.

[0096] The region between the central region and the region above the heater can have a downwardly curved shape.

[0097] The peripheral region from the area with the maximum bottom thickness (starting from the heat source) to the area adjacent to the container wall (starting from the center) can have a spherical, cylindrical, conical or nearly flat shape.

[0098] In the current experiment, the thickness of the bottom of the central region is 1.2 - 2.5 mm.

[0099] The thickest region of the bottom can be 20 times thicker than the central region. In the experimental study, the thickness of the bottom of the peripheral region is 1.2 - 2.5 mm.

[0100] The transition between regions can be smooth.

[0101] Radially oriented ribs can be formed on the surface of the region above the annular heater. In the conducted research, the height of the ribs is 1.2 - 2.5 mm.

[0102] To prevent surface oxidation and fouling, the surface can be coated with a waterproof, non - stick and / or any other coating.

[0103] During heating and boiling, the surface shape can follow the expected trajectory of the liquid convection movement.

[0104] If there is an additional peripheral region further away (from the center) from the thickest region until the region adjacent to the vessel wall, it can have a nearly flat shape.

[0105] The bottom thickness of such a peripheral region can be the minimum, 1.2 - 2.5 mm in the conducted research.

[0106] The device operates as follows.

[0107] When water (aqueous solution or mixture) is heated, due to low pressure and high temperature, cavitation bubbles containing steam but no air appear at the nucleation sites in the strong heat exchange zone. The cavitation bubbles break away from the surface and lose the heat energy source in the form of a heating element, fall into a denser and cooler liquid layer, and violently collapse there. This process generates boiling noise. The smaller the area of the strong heat exchange and the higher the temperature therein, the denser and faster the bubbles form, and the louder the noise. Thereafter, when strong convection that more evenly heats the liquid is formed, the bubble separation rate decreases, the bubble size increases, and the pressure difference between inside the bubble and the upper layer of the liquid becomes unimportant. The separated bubbles reach the surface without breaking. The noise decreases. The boiling process begins. Therefore, the required technical effect is achieved by selecting the parameters of the body with a built-in heating element and the shape of the bottom of the heating container, which increases the area of the dense heat exchange zone and accelerates the formation of convection, thereby making the liquid temperature uniform.

[0108] Therefore, the required technical effect is achieved by selecting the parameters of the body and the shape of the bottom of the heating container with a built-in heating element, which increases the area of the dense heat exchange zone and accelerates the formation of convection, making the liquid temperature uniform, and arranging the necessary surface properties, for example, by applying a waterproof, non-stick or any other coating, in which case the coating acts as a protective layer to prevent surface oxidation and the closing of contaminants and thus reduces the number of steam generation sources.

[0109] The claimed heating element is the bottom 1 of the container for primarily heating the liquid, and its operation is as follows.

[0110] The device includes a metal bottom 1 of variable thickness, which has a linear ring heater. The heater is mainly an electric heater, attached, installed or built into the metal bottom.

[0111] To enhance convection, the bottom is shaped as described and includes a heating area that forms an upward flow and an area other than the heating area that forms a downward flow.

[0112] When heating 2 is turned on in the area 1.3 with the maximum bottom thickness, the temperature rises and local liquid heating occurs. Above this area, the liquid starts to move upward.

[0113] The central area 1.1 has the minimum bottom thickness and is far from the heater, which causes the liquid to move downward above this area.

[0114] Figure 4 The liquid trajectory is shown.

[0115] To improve the convection efficiency, the area 1.3 with the maximum bottom thickness bulges upward so that most of the heat radiation surface is in the liquid.

[0116] The area above the heater wire gap, Region 1.3, has the minimum thickness. The thickness gradually increases along the heating wire in Region 1.3. The part of Region 1.3 opposite to the area above the linear heater gap is made of the maximum thickness. The thickness gradually decreases along Region 1.2 until it reaches its minimum value above the linear heater gap.

[0117] When the heater is connected from below, the heat from the electric heater 2 is transferred through the lower surface to the thickened part of the bottom 1.3. If the heater is built into the body of the thickened region 1.3, the heat is transferred directly.

[0118] If there is a peripheral region 1.4 from the thickened part of the bottom 1.3 to the region adjacent to the container wall, a downward flow that also increases convection is formed there due to the small bottom thickness.

[0119] To improve heat transfer, the central region 1.1 can have an upwardly curved spherical or spherical-to-conical shape.

[0120] The transition between regions can be smooth.

[0121] To improve heat transfer, radially oriented ribs 3 can be added on the surface of the region above the heated annular heater.

[0122] To prevent surface oxidation and fouling, the heating element 1 can be additionally coated with a composition having non-stick, anti-friction, and hydrophobic properties. This coating effectively prevents the oxidation of the heating element body and ensures a significant increase and uniform distribution of nucleation sites.

[0123] The shape of the bottom surface can follow the expected trajectory of the convective movement of the liquid during heating and boiling.

[0124] For the bottom device of a container mainly used for heating liquids, the required technical effects can also be achieved. In addition, a specific feature of the device using a metal bottom with variable thickness is that heating should only occur in the region with the maximum bottom thickness.

[0125] Example of a comparative test between a conventional heating element built into the container bottom and the claimed heating element.

[0126] Input parameters

[0127] Bottom diameter - 138 mm (see Figure 11 )

[0128] The thickness of Region 1.1 is 1.3 - 1.5 mm

[0129] The thickness of Region 1.3 is 10 - 15 mm

[0130] The thickness of Region 1.4 is 1.3 - 1.5 mm

[0131] The height of rib 3 is 1.5 - 3.5 mm

[0132] The TEH power is -1800 - 2200 W

[0133] The water temperature is 60 degrees

[0134] The air temperature is 30 degrees

[0135] The TEH power is 2000 W

[0136] The lower part of the heating element is in contact with air

[0137] The upper part is in contact with water

[0138] The air convection coefficient is -5, and the water convection coefficient is -3600 W / (m2*S) (equivalent to the water coefficient at 60 degrees Celsius).

[0139] Cavitation occurs at approximately 60 degrees, increases at 70 degrees, and disappears at 80°C

[0140] Results

[0141] Assuming the heater power is the same and the temperature difference is the same, the energy transferred per unit time is also the same. The energy obtained by water per unit time is the same. Only in the first case, the energy will be transferred to the water through a small area equal to the contact area of the heating element with the bottom. In the second case, the energy is distributed over a larger surface of the thickened part of the body. Obviously, the same energy is transferred over a smaller area, and more heat is generated

[0142] When the temperature changes strongly, thermal cavitation occurs. Therefore, the smaller the change, the milder the cavitation

[0143] It seems that in this case, a wavy, thick-walled, and preferably aluminum heating element would be preferred

[0144] It is also worth reminding that the claimed device additionally solves the problem of using existing TEHs

[0145] The device has industrial applicability because each part can be mass-produced in large quantities on industrial equipment

[0146] Supplementary explanation

[0147] When water (aqueous solution or mixture) is heated, due to low pressure and high temperature, cavitation bubbles containing steam but no air appear at the nucleation sites in the strong heat exchange zone. The cavitation bubbles detach from the surface and lose the heat energy source in the form of the heating element, falling into a denser and cooler liquid layer where they violently rupture and emit a hydraulic shock wave. This process generates boiling noise. At the same time, the greater the power of the heating element and the smaller the area of strong heat exchange, the greater the density and speed of the cavitation bubbles, and the louder the noise. Thereafter, when a strong convection that heats the liquid more evenly forms, the bubble separation rate decreases, the bubble size increases, and the pressure difference inside the bubble and in the upper layer of the liquid becomes unimportant. The separated bubbles reach the surface without rupturing. The noise decreases. The process of boiling begins.

[0148] Therefore, by selecting the parameters of the body with the built-in heating element and the shape of the bottom of the heating container, the required technical effect is achieved, which increases the area, reduces the temperature in the strong heat exchange zone, and accelerates the formation of convection, more strongly equalizing the liquid temperature.

[0149] The device includes a metal bottom 1 with variable thickness, and the metal bottom 1 has an annular heater 2, which is preferably an electric heater, attached to the metal bottom 1. To enhance convection, the shape of the bottom is as described above and has a downward convection flow region 1.1, a preheating region 1.2, and an upward convection flow region 1.3 with a top shape. Radially oriented ribs 3 can be added on the surface of the top 1.3 above the annular heater 2 to increase the heat transfer area. If there is a peripheral region 1.4 from the thickened part of the bottom 1.3 to the region adjacent to the container wall, due to the small bottom thickness, a downward flow that also increases convection is formed there.

[0150] When the heater 2 is turned on in the region 1.3 with the maximum bottom thickness, the temperature rises and a heat exchange zone appears, and the temperature in the preheating zone 1.2 rises partially, increasing the effective heat exchange area. The liquid begins to move upward above the upward convection flow region 1.3. Due to the thicker heating element body and the high heat capacity and high thermal conductivity of the body material, the heat is more evenly distributed over the regions 1.3 and 1.2, thereby reducing the risk of the strong heat exchange region and thus the risk of forming a region of dense cavitation bubbles.

[0151] The downward convection flow region 1.1 has the smallest bottom thickness and is not heated by the heater 2. Therefore, an effective downward liquid flow is formed in this region. Through the concave preheating zone 1.2, the liquid is heated and transported upward to the upward convection zone 1.3, where the liquid enters the strong heat exchange zone, is heated, and rises upward. This is how the closed convection is formed, resulting in rapid mixing and even heating of the liquid.

[0152] To prevent oxidation, reduce friction, and form a uniform surface, the upper heating element 1 in contact with the liquid can be further coated with a composition having non-stick, anti-friction, and hydrophobic properties. This coating effectively prevents the oxidation of the heating element body and ensures a significant increase and uniform distribution of nucleation sites.

[0153] The device includes a metal bottom 1 of variable thickness having an annular heater 2, preferably an electric heater, attached to the metal bottom 1. To enhance convection, the bottom has the shape as described above and has a downward convection flow region 1.1, a preheating region 1.2, and an upward convection flow region 1.3 of a top shape. Radially oriented ribs 3 can be added on the surface of the top 1.3 above the annular heater 2 to increase the heat transfer area. If there is a peripheral region 1.4 from the thickened part of the bottom 1.3 to the region adjacent to the container wall, a downward flow that also increases convection is formed there due to the small bottom thickness.

[0154] Reference numerals

[0155] 1 - Bottom of variable thickness

[0156] 1.1 - Central region

[0157] 1.2 - Transition region from the central region to the heating region

[0158] 1.3 - Heating region (region with the maximum bottom thickness)

[0159] 1.4 - Peripheral region

[0160] 2 - Heater

[0161] 3 - Ribs for increasing the heat transfer surface

Claims

1. A heating element for heating the bottom of a container for a liquid and comprising a metallic bottom with a variable thickness, said metallic bottom having a linear ring heater which is a ring-shaped electric heater forming an open ring and attached to the metallic bottom from below or built into the body of the metallic bottom, wherein - at the connection or passage of the linear ring heater and around the connection or passage of the linear ring heater, the thickness of the metallic bottom increases compared to the rest of the surface of the metallic bottom; - the metallic bottom radially comprises several regions with different characteristics: - the central region has the smallest thickness, is approximately circular in shape and is offset from the geometric centre towards the open part of the ring-shaped electric heater, - the region above the linear ring heater is the thickest and bulges upwards such that most of the radiating surface is in the liquid, - the region above the linear ring heater has a variable thickness, - the region between the central region and the region above the linear ring heater has a variable thickness.

2. The heating element according to claim 1, wherein there is a waterproof coating on the surface.

3. The heating element according to claim 1, wherein there is a non-stick coating on the surface.

4. The heating element according to claim 1, wherein the shape of the surface follows the trajectory of the convective movement of the liquid during heating and boiling.

5. The heating element according to claim 1, wherein the central region has a spherical shape curved upwards and the region between the central part and the region above the linear ring heater has a shape curved downwards.

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