Tubular electric heater

Abstract

A tubular electric heater (1), in particular a sheathed resistor, comprising - a metal casing (2); - a first pin (3), or inner pin, arranged inside the metal casing (2); - a heating wire (5), adapted to generate heat when it is electrically powered, wound about an axis (X); - a thermal fuse (6) electrically connected to the first pin (3); wherein the heating wire (5) comprises an end stretch (51), formed by one or more coils having a first diameter (D1) that are welded to the first pin (3); a central stretch (54), formed by coils having a second diameter (D4) greater than the first diameter (D1); a transition stretch (53) extending between the end stretch (51) and the central stretch (54), wherein the coils of said transition stretch (53) have an increasing diameter, in particular towards the central stretch (54), wherein said transition stretch (53) has a length (L3) along said axis (X) from 1 to 30 mm; wherein said first diameter (D1) is from 1 to 5 mm; and wherein said second diameter (D4) is from 3 to 7 mm.

Description

Field of the invention

[0001] The present invention relates to the field of tubular electric heaters, in particular to the field of sheathed resistors.

[0002] In particular, the invention regards a tubular electric heater adapted to generate a high specific thermal power (in particular per area unit), preferably adapted to operate in water.Background art

[0003] Tubular electric heaters conventionally comprise a metal casing or metal sheath inside which a resistive element is arranged, in particular a metal wire, adapted to generate heat by Joule effect.

[0004] Some tubular electric heaters are designed to generate great specific powers and operate in water.

[0005] For safety reasons, tubular electric heaters can be provided with at least one thermal fuse.

[0006] The thermal fuse intervenes, for example, in dry operating conditions or in other abnormal working conditions, in which there is an undesired overheating.

[0007] The thermal fuse can be electrically connected to a pin to which the metal wire is welded.

[0008] In particular for this type of tubular electric heaters, which are adapted to generate great specific powers and operate in water, it is not trivial to manage to optimize both the features which allow optimizing normal working conditions and the intervention of the thermal fuse in abnormal working conditions.

[0009] CN102821496B, EP0876081A2 and US4697069A represent background art.Summary of the invention

[0010] It is an object of the present invention to provide a tubular electric heater, in particular adapted to generate great specific thermal power and to operate in water, which allows the optimization of both the features which allow optimizing normal working conditions and also the intervention of the thermal fuse in abnormal conditions.

[0011] In particular, it is an object of the invention to provide a tubular electric heater which allows an effective and efficient heating in normal working conditions, and which also allows a rapid intervention of the thermal fuse in the event of abnormal working conditions.

[0012] It is a particular object of the invention to provide a tubular electric heater in which the temperature of the area where the thermal fuse is arranged, in normal working conditions, is as low as possible; and in which the thermal fuse may intervene quickly to cut off the electrical supply under abnormal working conditions of overheating, for example in the absence of water for a tubular electric heater designed to operate in water.

[0013] The present invention achieves at least one of such objects, and other objects which will become apparent in the light of the present description, by means of a tubular electric heater, in particular a sheathed resistor, comprising a metal casing; a first pin, or inner pin, arranged inside the metal casing; a heating wire, adapted to generate heat when it is electrically powered, wound about an axis; a thermal fuse electrically connected to the first pin; wherein the heating wire comprises an end stretch, formed by one or more coils having a first diameter, the one or more coils being welded to the first pin; a central stretch, formed by coils having a second diameter greater than the first diameter; a transition stretch extending between the end stretch and the central stretch, in which the coils of said transition stretch have an increasing diameter, in particular towards the central stretch, wherein said transition stretch has a length along said axis from 1 to 30 mm; wherein said first diameter is from 1 to 5 mm; and wherein said second diameter is from 3 to 7 mm.

[0014] A tubular electric heater according to the invention, in particular wherein the heating wire has the features mentioned in claim 1, advantageously allows to optimize the heating of the central stretch during normal operation, and to optimize the speed of intervention of the thermal fuse in the event of anomalous operation, i.e., in the event of overheating of the central stretch such as, for example, in the case of dry operation.

[0015] More in details, the geometric features of the heating wire allow to modulate the electric power so that the central stretch heats adequately while the temperature of the area close to the thermal fuse remains sufficiently low, during usual operation; and so that the thermal fuse may intervene quickly in the event of anomalous operation.

[0016] The geometric features of the heating wire are selected so that the balance between the operating temperatures of the heating wire during normal operation and the speed of intervention of the thermal fuse in the event of an anomalous low heat exchange is always ensured.

[0017] In particular, the aforesaid values, in particular the length of the aforesaid transition stretch, in particular in combination with the aforesaid first diameter and second diameter, allow optimizing the distance between the central stretch (or heating stretch) and the thermal fuse (in particular so that they are close enough to each other), so as to allow having a zone having a high specific power load (i.e., the heating stretch or central stretch), which remains cool enough during normal working (because the heating wire, close to the sheath exchanges well with the liquid to be heated), and so as to quickly heat the thermal fuse in abnormal conditions. Advantageously, a thermal fuse configured (or set) to intervene at a relatively low temperature may also be used.

[0018] The geometric features of the heating wire may advantageously be selected according to the needs.

[0019] The Applicant has experimentally found that designing the heating wire with geometric features different from the dimensional features according to the invention does not ensure an optimal heating of the central stretch and a quick intervention of the thermal fuse.

[0020] Further features and advantages of the invention will become more apparent in the light of the detailed description of exemplary but not exclusive embodiments.

[0021] The dependent claims describe particular embodiments of the invention.Brief description of the Figures

[0022] The description of the invention refers to the accompanying drawings, which are provided by way of non-limiting example, in which: Figure 1 diagrammatically shows a top plan view of the electric heater according to the invention; Figure 2 diagrammatically shows a view, in particular, of internal components of the electric heater in Figure 1; Figure 3 diagrammatically shows some components of an electric heater according to the invention.

[0023] The same elements or components have the same reference numerals.Description of exemplary embodiments of the invention

[0024] With reference to the Figures, exemplary embodiments of a tubular electric heater 1, in particular a sheathed resistor, are described. The tubular electric heater 1 is preferably configured to function in water, in particular it is preferably configured to be immersed at least partially in water.

[0025] In all the embodiments, the tubular electric heater 1 comprises a metal casing 2, or metal sheath; a first pin 3, or inner pin, arranged inside the metal casing 2; a heating wire 5, adapted to generate heat when it is electrically powered, i.e., adapted to generate heat due to the Joule effect, wound about an axis X; a thermal fuse 6 electrically connected to the first pin 3; wherein the heating wire 5 comprises an end stretch 51, formed by one or more coils having a diameter D1, or first diameter for descriptive purposes, the one or more coils being welded to the first pin 3; a central stretch 54, formed by coils having a diameter D4, or second diameter for descriptive purposes, greater than the first diameter D1; a stretch 53, also called transition stretch 53, extending between the end stretch 51 and the central stretch 54, wherein the coils of said transition stretch 53 have an increasing diameter, in particular towards the central stretch 54, wherein said transition stretch 53 has a length L3 along the axis X from 1 to 30 mm; wherein said first diameter D1 is from 1 to 5 mm; and wherein said second diameter D4 is from 3 to 7 mm.

[0026] Preferably, but not exclusively, the heating wire 5 further comprises a stretch 52, called first stretch 52 for descriptive purposes, which extends between the end stretch 51 and the transition stretch 53, said first stretch 52 being formed by coils having said first diameter D1. Alternatively, for example, the transition stretch 53 may be consecutive to the end stretch 51, i.e., the first stretch 52 may not be provided.

[0027] Said first stretch 52 (when provided) has, preferably, a length L2 along the axis X from 0.3 to 30 mm. Said values are selected so as to optimize the distance between the central stretch 54 and the thermal fuse 6.

[0028] In particular, the end stretch 51, the first stretch 52, the transition stretch 53 and the central stretch 54 are axial stretches. In other words, each of said end stretch 51, first stretch 52, transition stretch 53 and central stretch 54 is formed by a plurality of coils of heating wire 5, wound about a respective stretch of the axis X.

[0029] Length of a stretch of heating wire 5 along the axis X means, in particular, the length of the stretch of the axis X about which the coils are wrapped, in particular all the coils, of the stretch of heating wire.

[0030] The end stretch 51 also has a length L1 along the axis X, and the central stretch 54 also has a length L4 thereof (not shown in the Figures) along the axis X.

[0031] In particular, the length L4 of the central stretch 54 is greater than the length L1 of the end stretch 51, is greater than the length L2 of the first stretch 52, and is greater than the length L3 of the transition stretch 53.

[0032] In particular, the length L4 of the central stretch 54 is greater than the sum of the length L1 of the end stretch 51, the length L2 of the first stretch 52, and the length L3 of the transition stretch 53 (i.e., L4 is greater than L1+L2+L3).

[0033] Preferably, the central stretch 54 of the heating wire 5 has a length L4 along the axis X of 50 to 500mm.

[0034] Preferably, the end stretch 51 of the heating wire 5 has a length L1 along the axis X of 1 to 5 mm.

[0035] The aforesaid axis X can be a rectilinear axis, a curved axis or an axis comprising one or more rectilinear stretches and one or more curved stretches.

[0036] In the non-limiting example shown, the axis X comprises two rectilinear stretches joined to each other by a curved stretch, in particular so as to form a substantially "U" shape.

[0037] In particular, the term "coils" is understood to mean coils of the winding formed by the heating wire 5.

[0038] Preferably, the central stretch 54 is configured to generate a specific thermal power of at least 20 W / cm 2< , preferably from 20 W / cm 2< to 60 W / cm 2< .

[0039] As said, the coils of said transition stretch 53 have an increasing diameter, i.e., which increases from one coil to the next. Preferably, the diameter of any coil of the transition stretch 53 is smaller with respect to the diameter of the coil of the transition stretch 53 next thereto, where next is understood to mean closer to the central stretch 54.

[0040] Preferably, a surface S3, or envelope, tangent to the coils of the transition stretch 53 is frustoconical or substantially frustoconical.

[0041] Preferably, the coil density of said first stretch 52 is smaller than the coil density of said end stretch 51.

[0042] "Coil density" of a stretch of heating wire 5 means the number of coils per unit of length, in particular along the axis X. In particular, it means the linear density.

[0043] In particular, a greater coil density implies a greater number of coils per unit of length.

[0044] Preferably, the pitch between the coils of said end stretch 51 is equal to the diameter of the heating wire 5, in particular the coils of said end stretch are in contact with each other; and / or the pitch between the coils of said first stretch 52 is greater than the diameter of the heating wire 5.

[0045] In particular, the coils of the first stretch 52 are spaced apart from the first pin 3, in particular they are not welded to the first pin 3.

[0046] Preferably, said first stretch 52 comprises a first sub-stretch 52' and a second sub-stretch 52", in particular consecutive to each other; the first sub-stretch 52' and the end stretch 51 are consecutive to each other; the second sub-stretch 52" and the transition stretch 53 are consecutive to each other; the coil density of the first sub-stretch 52' is different from the coil density of the second sub-stretch 52'; preferably, the coil density of the first sub-stretch 52' is greater than the coil density of the second sub-stretch 52" (as in the example shown).

[0047] The length L2' along said axis X of the first sub-stretch 52' may be equal to or different from the length L2" along the axis X of the second sub-stretch 52".

[0048] Preferably, the length L2' along the axis X of the first sub-stretch 52' is less than the length L2" along the axis X of the second sub-stretch 52".

[0049] Preferably, the coils of said first stretch 52 are separated, in particular radially separated, from the first pin 3.

[0050] The first pin 3, in particular, comprises a part 32 about which some of the coils of the heating wire 5 extend.

[0051] Preferably, said part 32 of the pin 3 has a length Lp, along the axis X, which is less than or equal to L1+L2 (if provided)+L3+20 mm.

[0052] In particular, the coils of the end stretch 51 extend about said part 32 of the pin 3.

[0053] Preferably, the coils of said first stretch 52 extend about said part 32 of the first pin 3, as in the example shown.

[0054] Preferably, all (as in the example shown) or some of the coils of the transition stretch 53 extend about said second part 32 of the pin 3.

[0055] Preferably, some of the coils of the central stretch 54 extend about said second part 32 of the pin 3, as in the example shown.

[0056] Advantageously, the first pin 3 can act as a thermal bridge to bring the heat from the central stretch 54 towards the thermal fuse 6, in particular in dry working conditions.

[0057] To this end, it is particularly preferable that said part 32 of the pin 3 is conical or frustoconical, in particular tapered towards the central stretch 54 of the heating wire 5; or (as in the example shown) that said part 32 of the pin 3 comprises a conical or frustoconical portion 322, in particular tapered towards the central stretch 54 of the heating wire 5. In particular, in the latter case, the part 32 comprises a cylindrical part 321 from which the conical or frustoconical part 322 extends. In particular, the coils of the end stretch 51 are welded to said cylindrical part 321 of the first pin 3. Preferably some or all of the coils of said first stretch 52 extend about said conical or frustoconical part 322; preferably some or all of the coils of the transition stretch 53 extend about said conical or frustoconical part 322; optionally some of the coils of the central stretch 54 extend about said conical or frustoconical part 322.

[0058] The heating wire 5 is, in particular, a metal wire.

[0059] In particular, the heating wire 5 is incorporated into an insulating material, which is preferably magnesium oxide.

[0060] Preferably, as in the example shown, the tubular electric heater 1 comprises a metal body 7, or metal sleeve, in which the first pin 3 and the thermal fuse 6 are at least partially arranged; said metal body 7 fastens, in particular, it mechanically fastens, the first pin 3 and the thermal fuse 6 together, and electrically connects the first pin 3 and the thermal fuse 6 together. Alternatively, the first pin 3 and the thermal fuse 6 may be crimped together.

[0061] Said metal body 7 preferably fixes the first pin 3 and the thermal fuse 6 to each other exerting an elastic force thereon, in particular an elastic recall force; and / or said metal body 7 fastens the first pin 3 and the thermal fuse 6 to each other by interference.

[0062] Said metal body 7 preferably has elastic properties, in particular it is preferably adapted to elastically deform when the first pin 3 and the thermal fuse 6 are inserted into the metal body 7.

[0063] Said metal body 7 is preferably a metal cage, in particular an elastic metal cage.

[0064] In particular, the first pin 3 is adapted to lead an electric current to the heating wire 5.

[0065] The first pin 3 may for example be made of steel or copper or copper alloy.

[0066] In particular, the tubular electric heater 1 comprises a second pin 4, or external pin, for the electrical connection to a source of electricity. The second pin 4 is arranged at least partially outside the metal casing 2, for example, partially inside and partially outside the metal casing 2. The second pin 4 is in contact, in particular directly in contact with the thermal fuse 6. The second pin 4 is preferably crimped to the thermal fuse 6.

[0067] In particular, the tubular electric heater 1 comprises two end stretches 11, 12. The aforesaid first pin 3, thermal fuse 6, metal body 7, end stretch 51, first stretch 52 and transition stretch 53, in particular, are part of an end stretch 11 of the tubular electric heater 1.

[0068] The other end stretch 12 can optionally also be provided with a respective thermal fuse and be equal or substantially equal to the end stretch 11. Alternatively, the other end stretch 12 of the tubular electric heater 1 may not be provided with a thermal fuse. In this case, as shown in Figure 2, a pin 4a is provided, for example arranged partly inside and partly outside the metal casing 2. The pin 4a has an end to which the heating wire 5 is welded and another end for the electrical connection to a source of electricity.

[0069] Optionally, the pin 4a also comprises a conical or frustoconical portion, in particular similar to the frustoconical portion 322, and / or optionally the heating wire 5 comprises another end stretch 51a, welded to the pin 4a, analogous to the end stretch 51, another stretch (or another first stretch) analogous to the first stretch 52, and another transition stretch 53a, analogous to the transition stretch 53.

Claims

1. A tubular electric heater (1), in particular a sheathed resistor, comprising - a metal casing (2); - a first pin (3), or inner pin, arranged inside the metal casing (2); - a heating wire (5), adapted to generate heat when it is electrically powered, wound about an axis (X); wherein the heating wire (5) comprises an end stretch (51), formed by one or more coils having a first diameter (D1), the one or more coils being welded to the first pin (3); a central stretch (54), formed by coils having a second diameter (D4) greater than the first diameter (D1); a transition stretch (53) extending between the end stretch (51) and the central stretch (54), wherein the coils of said transition stretch (53) have an increasing diameter, in particular towards the central stretch (54); characterized in that the tubular electric heater (1) comprises a thermal fuse (6) electrically connected to the first pin (3); and in that said transition stretch (53) has a length (L3) along said axis (X) from 1 to 30 mm; said first diameter (D1) is from 1 to 5 mm; and said second diameter (D4) is from 3 to 7 mm.

2. A tubular electric heater (1) according to claim 1, wherein a surface (S3), or envelope, tangent to the coils of the transition stretch (53) is frustoconical or substantially frustoconical.

3. A tubular electric heater (1) according to claim 1 or 2, wherein said central stretch (54) has a length (L4) along said axis (X) from 50 to 500 mm.

4. A tubular electric heater (1) according to any one of the preceding claims, wherein the pitch between the coils of said end stretch (51) is equal to the diameter of the heating wire (5).

5. A tubular electric heater (1) according to any one of the preceding claims, wherein the heating wire (5) comprises a first stretch (52) extending between the end stretch (51) and the transition stretch (53), formed by coils having said first diameter (D1), preferably wherein said first stretch (52) has a length (L2) along said axis (X) from 0.3 to 30 mm.

6. A tubular electric heater (1) according to claim 5, wherein the coil density of said first stretch (52) is less than the coil density of said end stretch (51); and / or wherein the pitch between the coils of said first stretch (52) is greater than the diameter of the heating wire (5).

7. A tubular electric heater (1) according to claim 5 or 6, wherein said first stretch (52) comprises a first sub-stretch (52') and a second sub-stretch (52"); wherein the first sub-stretch (52') and the end stretch (51) are consecutive to each other; wherein the second sub-stretch (52") and the transition stretch (53) are consecutive to each other; wherein the coil density of the first sub-stretch (52") is different from the coil density of the second sub-stretch (52").

8. A tubular electric heater (1) according to claim 7, wherein the coil density of the first sub-stretch (52') is greater than the coil density of the second sub-stretch (52").

9. A tubular electric heater (1) according to claim 7 or 8, wherein the length (L2') along said axis (X) of the first sub-stretch (52') is equal to or different from the length (L2") along the axis (X) of the second sub-stretch (52"); preferably wherein the length (L2') along said axis (X) of the first sub-stretch (52') is less than the length (L2") along said axis (X) of the second sub-stretch(52").

10. A tubular electric heater (1) according to any one of claims 5 to 9, wherein the coils of said first stretch (52) are separated, in particular radially separated, from the first pin (3).

11. A tubular electric heater (1) according to any one of the preceding claims, wherein said first pin (3) comprises a conical or frustoconical part (322) about which some of the coils of the heating wire (5) extend; in particular wherein the coils of said end stretch (51) extend about said conical or frustoconical part (322); preferably wherein some or all of the coils of said first stretch (52), if provided, extend about said conical or frustoconical part (322); preferably, wherein some or all of the coils of the transition stretch (53) extend about said conical or frustoconical part (322); optionally, wherein some of the coils of the central stretch (54) extend about said conical or frustoconical part (322).

12. A tubular electric heater (1) according to any one of the preceding claims, wherein the central stretch (54) is configured to generate a specific thermal power of at least 20 W / cm2.

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