FIELD OF THE INVENTION

The present invention relates to a seal for use in an electrically heated vessel comprising a container space for containing liquid and a heating element for supplying heat to the container space, which seal is intended to be used at a position inside the vessel for protecting a portion of the vessel which is present at a side of the heating element facing away from the container space from penetration of liquid from the container space, and which seal has an annular shape, and comprises at least one resilient circumferential projection extending in an outward direction.

The present invention also relates to an electrically heated vessel, comprising a container space for containing liquid, a heating element for supplying heat to the container space, and a seal as mentioned, which seal is positioned inside the vessel for protecting a portion of the vessel which is present at a side of the heating element facing away from the container space from penetration of liquid from the container space.

BACKGROUND OF THE INVENTION

An electrically heated vessel is a well-known and widely used article, and is suitable to be applied in situations in which there is a need for heating a liquid such as water or milk. Practical examples of an electrically heated vessel include kettles, hot water jugs, steamers and milk frothers. In general, an electrically heated vessel comprises a container space for the purpose of containing liquid to be heated, and a heating element for supplying heat to the container space. In many cases, the type of heating element is a so-called planar heating element, i.e. a heating element comprising a plate and heat generating means arranged at a side of the plate which faces away from the container space, wherein the heat generating means may comprise a wire wound resistance heater, or a resistance heating track, for example. The heating element is normally arranged at a side of the electrically heated vessel which is a bottom side of the vessel in a normal orientation of the vessel, i.e. an orientation associated with the heating process. In such side of the plate of the heating element which faces the container space actually constitutes a bottom of the container space. For the purpose of avoiding leakage of liquid from the container space to the other side of the heating element, a seal is applied. A practical type of seal is known from EP 1 314 385. This seal is a resilient seal which is provided around the heating element, and which seals the periphery thereof with respect to a wall of the vessel. Particularly, the seal has one or more resilient circumferential projections, which are denoted as fingers or fins in the specification of EP 1 314 385, which extend outwardly, and which contact the vessel wall.

The known seal is especially suitable to be used with planar heating elements. In a situation in which the size of the plate of a heating element is chosen such as to substantially match the size of the vessel at its mounting position, the seal surrounds a circumferential portion of the plate extending more or less in a downward direction. When the size of the plate is chosen such as to be smaller, an annular mounting flange may be used at a position between the plate and the seal. Hence, in such a situation, the seal surrounds a circumferential portion of the flange extending in a downward direction, wherein it is possible for the seal to be integral with the flange if so desired.

The seal used with the heating element or the combination of the heating element and a mounting flange as mentioned has one or more relatively thin resilient radially- extending circumferential projections, as noted earlier, which deform to take the shape of the vessel wall against which they are fitted, and which absorb a wide range of size and shape variations of the vessel wall, which is normally made of plastic. Also, there is a safety aspect to the application of the seal, which resides in the fact that there is no need for the heating element to contact the vessel wall or other objects associated with the vessel wall than the seal, so that the risk of heat damage of the vessel wall is reduced. Heat damage as mentioned may otherwise occur in a situation of dry boil, for example.

The seal may comprise an elastomeric thermally insulating material such as silicone rubber. Furthermore, the seal may include a locating groove or other formation for reception and positive accommodation of a circumferential part of the heating element, although this is not necessary for realizing the sealing effect as desired. It appears in practice that even though the seal is a resilient seal and contacts the heating element at an inner side and the vessel wall at an outer side, it is still possible for leakage to occur, wherein liquid passes the seal and reaches the side of the heating element where the actual heat generating means are arranged. This is a situation which is not acceptable, as this may result in rust of metal components which are present outside of the container space, and/or may be dangerous as soon as the liquid reaches electrical components.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a seal like the seal known from EP 1 314 385, which is capable of yielding better results as far as avoiding leakage is concerned. This object is achieved by means of seal which has an annular shape, and comprises at least one resilient circumferential projection extending in an outward direction, wherein at least an outer portion of a top surface of the projection, which is a surface of the projection having a top position in a normal operational orientation of the seal, slants upward in the outer direction.

The present invention is based on the insight that in a situation in which the seal known from EP 1 314 385 is applied, leakage may occur as a result of a practical out of roundness of the vessel wall, i.e. as a result of a practical radial deviation of the shape of the vessel wall from ideal roundness. It is found that out of roundness of the vessel wall causes high pressures to be exerted on the seal, to such an extent that permanent deformation of the seal may take place, wherein it may be possible for lime scale to penetrate between the seal and the vessel wall.

In general, the seal according to the present invention is of the same design as the seal known from EP 1 314 385. However, a notable difference resides in the fact that according to the present invention, at least an outer portion of a top surface of a projection of the seal slants upward in the outward direction. In this respect, it is noted that the outward direction is a direction towards an outer end of the projection, i.e. a radial direction of the annular shape of the seal, outward from a center of the annular shape. Furthermore, it is noted that the top surface of the projection is a surface having a top position in a normal operational orientation of the seal, which is an orientation of the seal in which the seal is suitable to be applied in an actual electrically heated vessel, assuming a normal operational orientation of such a vessel, which is an orientation of the vessel in which the container space is present above the heating element, i.e. an orientation of the vessel in which the vessel is capable of containing water, milk, or another liquid as intended. Normally, the seal does not have a plane of symmetry perpendicular to a central axis of its annular appearance, and it is very well possible to identify which side of the seal is intended to be up and which side of the seal is intended to be down, taking into account the application of the seal in a vessel. Naturally, if the seal does have such a plane of symmetry, it is arbitrary which side is regarded as a top side and which side is regarded as a bottom side.

Due to the upwardly inclined appearance of at least an outer portion of the top surface of the projection of the seal, a situation in a vessel in which the seal is applied of scale penetrating between the seal and the vessel wall is avoided, as the scale is caught in a lowest section of the top surface of the projection as it were, and is kept there under the influence of gravity. This lowest section may be present at the position where the projection is connected to a main ring of the seal, which is particularly the case when the entire top surface of the projection slants upwards in the outward direction.

By having the upwardly inclined orientation of the top surface of the projection, leakage problems of vessels in which the seal is applied are prevented. Hence, by choosing a clever design of the seal, particularly a projection of the seal, the sealing function of the seal in a vessel can be significantly improved without needing to make additional costs and without needing to change the design of the vessel. With the seal according to the present invention, it is ensured that if liquid from the container space reaches the projection, the liquid will move to the lowest section of the top surface of the projection, which will result in scale depositing only at that lowest section. For sake of completeness, it is noted that in known seals comprising at least one projection, especially the seal known from EP 1 314 385, the projection has a top surface which slants down towards an outer end of the projection.

Therefore, it appears that in the case of known seals, problems of scale penetrating between the seal and the vessel wall may occur, which may eventually lead to failure of vessels in which the seals are applied.

In a preferred embodiment, the seal further comprises at least one additional resilient circumferential projection extending in the outward direction. In particular, the additional projection may of a type having at least one discontinuity in its surface. In the following, for sake of clarity, the projection as discussed in the foregoing, i.e. the projection having a top surface which is at least partially upwardly inclined, will be referred to as first projection, and the additional projection will be referred to as second projection.

In the known seal comprising at least one projection, the surface of the projection has a smooth, continuous appearance, but this is not the case with the second projection as mentioned, assuming that this projection is of the type as mentioned, i.e. the type having at least one discontinuity in its surface. By having a discontinuity such as a notch or a bulge, it is possible for the second projection to have a function in hindering a migration of scale in the area where the seal is present and avoiding penetration of scale between the seal and the vessel wall. It will be understood that normal, practical phenomena like surface roughness are not to be understood as being included in the term "discontinuity" as used here, but that the term is intended to cover possibilities which are different from the prior art in such a way that at least one deviation from a generally smooth surface can actually be seen in the surface of the second projection. By having both at least one first projection and at least one second projection, a double function of avoiding problems with lime scale reaching a location where the outer ends of the respective projections contact the vessel wall is realized, so that leakage is most unlikely to occur.

In a practical embodiment of the seal comprising the second projection besides the first projection, the second projection may have at least one bulged area. In such a case, it is possible for the bulged area to be annular, thereby forming a circumferential rib on the second projection. In that way, the bulged area can be effective along the entire circumference of the seal. The number of bulged areas may be at least two, wherein the bulged areas can be arranged such as to form concentric circumferential ribs on the second projection in case they are chosen to be annular. As far as the number of second projections is concerned, it is noted that this is at least one, like the number of first projections, but that it is an advantageous possibility to have at least two second projections for sufficient and reliable sealing capacity. As far as the positioning of the second projection is concerned, it is noted that it is advantageous for the second projection to be at a higher level than the first projection in the normal operational orientation of the seal. As a consequence, in a vessel in which the seal is applied, a first barrier encoutered by scale-containing liquid is the second projection. As explained in the foregoing, the second projection hinders a migration of lime scale to a portion of the vessel which is present at a side of the heating element facing away from the container space on the basis of the presence of discontinuities in the way of the lime scale. Still, even if it happens that lime scale passes between the second projection and the vessel wall, the lime scale will definitely be caught by the first projection due to the upwardly inclined orientation of at least an outer portion of the top surface of the first projection, as will be any scale-containing liquid leaking to the first projection.

Assuming that a second projection has a top position with respect to the at least one other projection of the seal, it is most practical for the second projection if a top surface of the second projection slants downward in the outward direction. During assembly of a vessel, the seal is arranged around the heating element first. Subsequently, a generally tube- shaped component including the vessel wall is put in place with respect to the assembly of the heating element and the seal by pushing the component downward. In the process, an inner surface of the tube-shaped component is made to contact the second projection first. Among other things, as a consequence, the inner surface of the tube-shaped component is made to deform a little bit, wherein the inner surface perfectly assumes a shape as intended. For example, when the inner surface of the vessel is supposed to have a circular inner

circumference, it is not necessary for the inner surface of the tube-shaped component to be perfectly round before being fitted onto the assembly of the heating element and the seal, as the inner surface assumes the ideal shape automatically on the basis of contact to the second projection of the seal.

It will be understood that the assembling process as described can be performed in a most smooth manner when the top surface of the second projection slants downward in the outward direction, as in that way, it is achieved that the second projection is oriented according to the direction of the assembling movement of the tube-shaped component with respect to the assembly of the heating element and the seal. As a result of the fact that the inner surface of the vessel wall obtains a shape as desired as the tube-shaped component is pressed downward and the second projection is passed, it is possible for the inner surface of the vessel wall to pass the first projection as well without exerting downward local pressure on the first projection to such an extent that the first projection is made to flip from an upward inclination to a downward inclination. Thus, in practice, having a second projection with a downward inclination contributes to the possibility of actually having a projection with an opposite inclination, i.e. an upward inclination, wherein both projections may be in sealing contact to the inner surface of the vessel wall.

In the case of the seal known from EP 1 314 385, it also appears to be possible for scale to penetrate between the seal and the heating element, so that leakage may also occur at that position. In order to avoid this problem, it is proposed to equip the seal with a tapering area at its inside surface, and to let the seal comprise resilient material at the position of the tapering area. When the seal is put in place around the heating element, the tapering area is gradually compressed, and additional pressure is built up between the seal and the heating element. Hence, the connection of the seal to the heating element can be tighter than in a situation without the tapering area. For sake of completeness, in respect of the material of the seal, it is noted that the seal may be made entirely of resilient material, and that most effective functioning of the seal can be achieved when this is the case.

The present invention also relates to an electrically heated vessel, comprising a container space for containing liquid, a heating element for supplying heat to the container space, and an annular seal which is provided with at least one projection having a capability of preventing migration of lime scale on the basis of applying a surface or at least a portion of a surface which is oriented such as to slant upward in an outward direction, possibly also at least one projection which may have a capability of preventing migration of lime scale on the basis of applying a surface which is provided with at least one discontinuity, and possibly also a tapering area at its inside surface, as described in the foregoing, and which is positioned inside the vessel for protecting a portion of the vessel which is present at a side of the heating element facing away from the container space from penetration of liquid from the container space. The seal is provided around the heating element, and seals an outer periphery of the heating element with respect to an inner surface of the vessel by contacting the inner surface through at least one projection.

Apart from the above-described particular aspects, aspects of the seal and the electrically heated vessel equipped with the seal as known from EP 1 314 385 are also applicable in the case of the present invention. These aspects include the fact that the seal may be made entirely of a suitable resilient material which is preferably also a thermally insulating material, the fact that the seal may include an annular mounting flange for completely bridging a distance between the inner surface of the vessel and the heating element in situations in which the heating element has a diameter which is considerably smaller than a diameter of the vessel at the mounting position of the heating element in the vessel, and the fact that the seal may comprise a locating groove or other formation for reception and accommodation of a circumferential part of the heating element, as mentioned earlier.

The above-described and other aspects of the present invention will be apparent from and elucidated with reference to the following detailed description of a seal for use in an electrically heated vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in greater detail with reference to the figures, in which equal or similar parts are indicated by the same reference signs, and in which:

Figure 1 diagrammatically shows a sectional view of an electrically heated vessel comprising a container space for containing liquid, a heating element for supplying heat to the container space, and a seal for protecting a bottom portion of the vessel from penetration of liquid from the container space, wherein most other components of the vessel are omitted for sake of clarity;

Figure 2 shows a detail A of figure 1; and

Figure 3 shows a sectional view of the seal in an unmounted condition.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 diagrammatically shows a sectional view of an electrically heated vessel 1 comprising a container space 10 for containing liquid, a heating element 20 for supplying heat to the container space 10, and a seal 30 for protecting a portion 2 of the vessel 1 which is present at a side of the heating element 20 facing away from the container space 10 from penetration of liquid from the container space 10. In figure 1, the vessel 1 is shown in a normal, upright position, in which an open side of the vessel 1 for receiving liquid from a source such as a tap is up. Furthermore, in figure 1, only the components of the electrically heated vessel 1 which are relevant in the context of the present invention are shown, which is justified in view of the fact that such a vessel 1 is a well-known and widely used article. Among other things, it is known for the vessel 1 to be part of a cordless electrical water heater which furthermore comprises a base for supporting the vessel 1 and supplying the heating element 20 with power when the vessel 1 is put in place on the base. Also, it is known for the vessel 1 to be provided with a lid or other suitable cover at a top side, and to comprise a handle or the like for allowing a user to easily take hold of the vessel 1 if so desired. In a bottom portion 2 of the vessel 1 , among other things, electrical connectors are present for cooperating with connectors of the base as mentioned earlier. Nevertheless, the base and the other components as mentioned are not shown in figure 1. It will be understood that within the scope of the present invention, it is not essential for the vessel 1 to be part of a cordless water heater, or to be shaped in a generally known way with a cover or a handle.

Within the scope of the present invention, it is possible to apply a circular clip or the like for securing the heating element 20 and the seal 30 to the vessel 1. The

functionality of such a clip may easily be realized on the basis of the fact that an inner surface 12 of the vessel wall 11 can be provided with suitable slots for accommodating the clip, for example. Nevertheless, a clip or other means for supporting the heating element 20 and the seal 30 inside the vessel 1 may be omitted when a choice is made to only rely on a self- positioning effect of the seal 30 as can be obtained on the basis of resilient proporties of the seal 30 and friction phenomena.

Normally, the vessel 1 has a tapering shape, wherein a diameter of the vessel 1 is larger at a bottom portion than at a top portion thereof. Also, it is common use for the vessel 1 to have an annular cross-section with a circular outer circumference. The heating element 20 as shown in figure 1 is a planar heating element 20 which comprises a heating plate 21 and a tube-shaped resistance heater 22 arranged at a bottom side of the heating plate 21. The heating plate 21 has a circular appearance, and is dimensioned such as to fit inside the vessel 1 with only small circumferential play. A suitable material of the heating plate 21 is aluminum. It is noted that other suitable means may be used instead of the heater 22 as shown. For example, it is possible to have a resistance heating track.

The heating plate 21 as shown has a stepped appearance. At a bottom side, a portion 23 extending more or less parallel to the vessel wall 1 1 is present, as can be clearly seen in the detail of figure 1 as shown in figure 2, which has an outer surface 24 and an outwardly bent bottom rim 25. Basically, the vessel 1 according to the present invention can be the same as the vessel known from EP 1 314 385, including the same components with the exception of the seal 30, which is of another, improved design as will be explained in the following with reference to figures 2 and 3 in which the seal 30 according to the present invention is shown in detail. Furthermore, the exact way in which the seal 30 is mounted at a position between the outer surface 24 of the bottom portion 23 of the heating plate 21 and the inner surface 12 of the vessel wall 11 will become apparent with reference to figure 2.

The seal 30 has an annular appearance, and is made of a resilient material such as rubber. For sake of completeness, it is noted that the term "annular" should be understood such as to mean that the seal 30 is shaped like a ring, wherein the inner circumference and the outer circumference of the ring may have any suitable shape, and do not necessarily need to be circular, and wherein rims which are at the bottom and the top as seen in an axial direction of the ring do not necessarily need to be in planes intersecting a longitudinal axis of the ring in a perpendicular fashion, but may be in planes having a slanting orientation and/or may have a corrugated appearance, for example. In the case of the seal 30, among other things, it depends on the shape of the vessel wall 11 which shape is most appropriate, as long as the seal 30 is capable of performing the sealing function as desired.

Like the seal known from EP 1 314 385, the seal 30 according to the present invention comprises at least one resilient circumferential projection extending outwardly. In the shown example, two projections 40, 50 are provided, which both serve for contacting the inner surface 12 of the vessel wall 11 in a mounted condition of the seal 30. Furthermore, an inside surface 31 of the seal 30 serves for contacting the outer surface 24 of the bottom portion 23 of the heating plate 21 in the mounted condition. In this way, as a result of the fact that the seal 30 has resilient properties and is a continuous component which contacts both the vessel wall 11 and the heating plate 21 when mounted inside the vessel 1 , it is achieved that leakage of liquid from the container space 10 to a space 2 underneath the heating plate 21 cannot occur, as the seal 30 totally blocks liquid from passing through a space between the bottom portion 23 of the heating plate 21 and the vessel wall 11 which would be present when the seal 30 would not be applied.

The two projections 40, 50 are mutually different, wherein a projection 40 according to a first type, which will hereinafter be referred to as first projection 40, is located at a lower level than a projection 50 according to a second type, which will hereinafter be referred to as second projection 50. A particular feature of the first projection 40 resides in the fact that a top surface 41 of the first projection 40 slants upward in an outward direction, i.e. in a direction towards an outer end 42 of the first projection 40. In the shown example, the first projection 40 has an overall upwardly inclined appearance, as a bottom surface 43 of the first projection 40 slants upward in an outward direction as well.

A particular feature of the second projection 0 resides in the fact that the second projection 50 is provided with bulged areas 1. Advantageously, the bulged areas 51 are annular, thereby forming circumferential ribs on the projection 50, wherein a concentric arrangement of the ribs may be realized. In the shown example, the number of bulged areas 51 is three, but it is also possible to have another number of bulged areas 1. In general, there is a discontinuity 51 in a surface 52, 53 of the second projection 50, which may be a bulged area 51 as shown, but which may also be a notch, for example. It is preferred for the discontinuity 1 as mentioned to be oriented such as to face the container space 10, as is the case in the shown example.

In the shown example, contrary to the first projection 40, the second projection

50 has an overall downwardly inclined appearance, caused by the fact that both a top surface 52 and a bottom surface 53 of the second projection 50 slant downward in an outward direction, i.e. in a direction towards an outer end 54 of the second projection 50. As a consequence, a configuration is obtained in which the top surface 41 of the first projection 40 and the bottom surface 53 of the second projection 50 converge towards each other in the outward direction.

Both the upwardly inclined orientation of the top surface 41 of the first projection 40 and the bulged areas 51 of the second projection 50 as mentioned in the foregoing have a function in avoiding penetration of lime scale between the seal 30 and the vessel wall 11. It is a well-known fact that scale is formed over time when an electrically heated vessel 1 is used. According to an insight underlying the present invention, leakage of liquid from the container space 11 in a downward direction across the seal 30 can occur when scale particles end up at a position between the seal 30 and the vessel wall 11, and also when scale particles end up at a position between the seal 30 and the heating element 20. The bulged areas 51 of the second projection stop scale particles from actually reaching a position in which they are stuck between the seal 30 and the vessel wall 11, and in which they provide passages where liquid may penetrate. Even when scale-containing liquid leaks to the first projection 40, it is guaranteed that the liquid and scale particles cannot pass the seal 30, due to the fact that the top surface 41 of the first projection 40 slants upward in the outward direction. Under the influence of gravity, the liquid flows towards an area where the first projection 40 is connected to a main ring 60 of the seal 30, i.e. an area where the upwardly inclined top surface 41 of the first projection 40 is delimited by another surface of the seal 30.

For avoiding penetration of lime scale between the seal 30 and the heating element 20, the seal 30 according to the present invention may be provided with a tapering area 32 at its inside surface 31, as can be seen in figure 3. In the shown example, the tapering area 32 as mentioned is present at a top side of the seal 30. When the seal 30 is in the mounted condition, the tapering area 32 is compressed, and additional pressure is obtained at the position of the tapering area 32, so that a tighter fit of the seal 30 to the heating element 20 is realized in comparison with a situation without the tapering area 32.

On the basis of the upward inclination of the top surface 41 of the first projection 40 of the seal 30, the presence of the bulged areas 51 as discontinuities of the surface 52, 53 of the second projection 50 of the seal 30, and the presence of the tapering area 32 of the inside surface 31 of the seal 30, an improved and durable sealing function of the seal 30 is realized, which is not easily disturbed by penetration/deposition of lime scale as is the case with conventional, comparable seals, such as the seal known from EP 1 314 385. For sake of completeness, it is noted that the aspect of having an upward inclination of a top surface 41 of a projection 40, the aspect of having a discontinuity 51 in a surface 52, 53 of an additional projection 50, and the aspect of having a tapering area 32 at an inside surface 31 are solutions to the leakage problem which can be applied independently, although it is understood that a combination of these aspects yields favorable results, as such a combination has effect at both an outer circumference of the seal 30 and an inner circumference of the seal

30, wherein the effect at the outer circumference of the seal 30 is a double effect based on two different scale-blocking measures.

The seal 30 as described in the foregoing can be defined as a seal 30 for use in an electrically heated vessel 1 comprising a container space 10 for containing liquid and a heating element 20 for supplying heat to the container space 10, which seal 30 is intended to be used at a position inside the vessel 1 for protecting a portion 2 of the vessel 1 which is present at a side of the heating element 20 facing away from the container space 10 from penetration of liquid from the container space 10, and which seal 30 has an annular shape, and comprises two resilient circumferential projections 40, 50 of mutually different types, extending in an outward direction, wherein at least an outer portion of a top surface 41 of a projection 40 as present at a lower level in a normal operational orientation of the seal 30, which is a surface 41 of the projection 40 having a top position in the normal operational orientation of the seal 30, slants upward in the outward direction. The projection 50 as present at a higher level in the normal operational orientation of the seal 30 may be provided with at least one discontinuity 51 in its surface 52, 53.

It will be clear to a person skilled in the art that the scope of the present invention is not limited to the examples discussed in the foregoing, but that several amendments and modifications thereof are possible without deviating from the scope of the present invention as defined in the attached claims. While the present invention has been illustrated and described in detail in the figures and the description, such illustration and description are to be considered illustrative or exemplary only, and not restrictive. The present invention is not limited to the disclosed embodiments.

Variations to the disclosed embodiments can be understood and effected by a person skilled in the art in practicing the claimed invention, from a study of the figures, the description and the attached claims. In the claims, the word "comprising" does not exclude other steps or elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the present invention.

In general, it is advantageous if the design of the seal 30 is chosen such that effective functioning is obtained with a minimum of material, wherein effective functioning involves an excellent sealing function and a scale blocking function as described in the foregoing in order to guarantee the sealing function as time passes and a vessel in which the seal is used has been operated from time to time. In view of the fact that the seal 30 is a product which can be made in mass production, small savings of material per product can still lead to significant reduction of costs. The present invention can be summarized as follows. A seal 30 for use in an electrically heated vessel 1 comprising a container space 10 for containing liquid and a heating element 20 for supplying heat to the container space 10 is intended to be used at a position inside the vessel 1 for protecting a portion 2 of the vessel 1 which is present at a side of the heating element 20 facing away from the container space 10 from penetration of liquid from the container space 10. To this end, the seal 30 has an annular shape and comprises at least one resilient circumferential projection 40 extending in an outward direction. At least a portion of a top surface 41 of the projection 40 slants upward in the outward direction, whereby it is possible to prevent scale particles from reaching an outer end 42 of the projection 40 and penetrating between the seal 30 and an inner surface 12 of the vessel 1, which may otherwise occur and lead to leakage problems. Furthermore, it is preferred if the seal 30 further comprises at least one additional projection 50 having at least one discontinuity 51 in its surface 52, 53, so that an additional scale blocking function can be realized, and/or if an inside surface 31 of the seal 30 is provided with a tapering area 32 having resilient properties, so that a sufficiently tight fit of the seal 30 to the heating element 20 can be realized, which also leads to a reduction of the risk of leakage across the seal 30.