Patent Description:
A sheath heater generally has a heating wire held inside a metal tube shaped sheath, and an insulating material having high thermal conductivity is filled in a gap between the metal sheath and the heating wire. Since the surface of a heating element is electrically insulated, it is possible for the sheath heater to directly heat a gas, liquid or metal and the like. In addition, it is possible for the sheath heater to have any shaped layout. Because of these conveniences it is used for various purposes. As a result, there is increasing demand for the sheath heaters having smaller diameter which can have more complex shaped layouts so as to meet various needs. On the other hand, since the sheath heater heats the heating wire by supplying electricity, it is necessary to come up with a means for improve the durability of the heating wire.

For example, a sheath heater arranged with a plurality of heating wires inside a single metal sheath is disclosed in the Patent Document <NUM>. Usually, heating is performed using one of a plurality of heating wires, and when this heating wire is disconnected, the power supply circuit is switched to another heating wire to recover easily and quickly.

<CIT> discloses a sheath heater according to the preamble of claim <NUM>.

However, the sheath heater described in the Patent Document <NUM> is arranged for disconnection of a heating wire, and no consideration is provided for suppressing the disconnection of the heating wire. In addition, there is no mention with regards to a reduction in the diameter of a sheath heater.

One of the objects of an embodiment of the present invention is to provide a small diameter sheath heater with improved reliability. Solution to Problem.

According to one embodiment of the present invention, a sheath heater is provided including a metal sheath;.

In addition, in another embodiment, an insulating material may be an inorganic insulating powder.

In addition, in another embodiment, the metal sheath may be aluminum, the heating wire may be a nickel-chromium alloy, and the insulating material may be magnesium oxide.

Hereinafter, each embodiment of the invention disclosed in the present application is explained below while referring to the drawings. However, the present invention can be implemented in various forms and should not be construed as being limited to the description of the embodiments exemplified below.

In addition, although the drawings may be schematically represented with respect to the width, thickness and shape or the like of each part as compared with the actual embodiment in order to make the explanation clearer, they are merely examples and do not limit an interpretation of the invention. In addition, in the present specification and each drawing, elements which have the same functions as those described with reference to previous drawings may be denoted by the same reference numerals, and overlapping explanations may be omitted.

The structure of the sheath heater according to the first embodiment of the present invention is explained using <FIG>, and <FIG>. The sheath heater according to the first embodiment of the present invention includes a heating mechanism. In addition, the sheath heater according to the first embodiment can be used to directly heat gas, liquid or a metal and the like. However, the sheath heater according to the first embodiment is not limited to being used heating the objects described above.

<FIG> are cross-sectional structural diagrams showing a sheath heater according to one embodiment of the present invention. As is shown in <FIG>, the sheath heater according to the first embodiment includes a band shaped heating wire <NUM>, an insulating material <NUM>, a metal sheath <NUM> and connection terminals <NUM>.

Referring to <FIG>, the heating wire <NUM> is arranged with a gap within the cylindrical metal sheath <NUM>. The heating wire <NUM> and the metal sheath <NUM> are insulated by the insulating material <NUM> which is arranged in the gap. Although the metal sheath <NUM> is shown as having a shape in which one end is closed in <FIG>, the shape is not limited to this and both ends may be open. The heating wire <NUM> is arranged so as to reciprocate in a cylindrical axis direction within the metal sheath <NUM>, and both ends of the heating wire <NUM> are arranged at one end of the metal sheath <NUM>. In other words, one heating wire <NUM> is arranged so as to be biaxial in most of the metal sheath <NUM> in a cylindrical axis direction. Each heating wire <NUM> which is arranged in the metal sheath <NUM> is arranged with a gap and is insulated by an insulating material <NUM> arranged in the gap.

<FIG> is a cross sectional diagram along the line C-C' in <FIG>. Referring to <FIG>, a width d1 of the band shaped heating wire <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. A thickness d2 of the band shaped heating wire <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. An inner diameter d3 of the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. A thickness d4 of the metal sheath <NUM> is to be in a range of <NUM> or more and <NUM> or less. An outer diameter d5 of the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. Since the sheath heater <NUM> according to the present embodiment has the structure described above, it is possible to reduce the diameter while maintaining reliability. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shape.

A shortest distance g1 between the metal sheath <NUM> and each heating wire <NUM> which is arranged in the metal sheath <NUM> in a cross section orthogonal to the cylindrical axis is preferred to be in a range of <NUM> or more and <NUM> or less. The shortest distance g1 between the metal sheath <NUM> and the heating wire <NUM> is more preferably in a range of <NUM> or more and <NUM> or less. By setting the distance g1 between the metal sheath <NUM> and the heating wire <NUM> to <NUM> or more, insulation between the metal sheath <NUM> and the heating wire <NUM> can be ensured. By setting the distance g1 between the metal sheath <NUM> and the heating wire <NUM> to <NUM> or less, the diameter of the sheath heater <NUM> can be reduced. The diameter of sheath heater <NUM> according to the present embodiment can be reduced while maintaining reliability by using the band shaped heating wire <NUM>. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shape.

A distance g2 of each heating wire <NUM> arranged in the metal sheath <NUM> in a cross section orthogonal to the cylindrical axis is preferred to be in a range of <NUM> or more and <NUM> or less. The shortest distance g2 of each heating wire <NUM> arranged in the metal sheath <NUM> is more preferably in a range of <NUM> or more and <NUM> or less. By setting the distance g2 of the biaxial heating wire <NUM> to <NUM> or more, the insulation of the heating wire <NUM> can be ensured. By setting the distance g2 of the biaxial heating wire <NUM> to <NUM> or less, the diameter of the sheath heater <NUM> can be reduced. The diameter of sheath heater <NUM> according to the present embodiment can be reduced while maintaining reliability by using the band shaped heating wire <NUM>. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shape.

Both ends of the heating wire <NUM> are arranged with a connection terminal 50a and a connection terminal 50b which are electrically connected respectively. Here, when the connection terminal 50a and the connection terminal 50b are not particularly distinguished, they are referred to as connection terminals <NUM>. The sheath heater <NUM> of the present embodiment has a biaxial single-terminal structure in which two connection terminals <NUM> are arranged at one end of the sheath heater <NUM>. One end of the sheath heater <NUM> including the connection terminals <NUM> is connected to an external device (heater controller, power source and the like). The sheath heater <NUM> is heated by electric power which is supplied from the external device which controls the temperature of the sheath heater <NUM>.

The band shaped heating wire <NUM> is arranged so as to rotate with respect to the cylindrical axis direction of the metal sheath <NUM> in a region where the heating wire <NUM> is biaxial within the metal sheath <NUM>. The band shaped heating wire <NUM> extends in the cylindrical axis direction in a state in which the long axis of the heating wire <NUM> rotates in a direction perpendicular to the cylindrical axis direction of the metal sheath <NUM>. That is, each heating wire <NUM> is in a spiral shaped coiled state. The rotation axes of the biaxial heating wires <NUM> are arranged substantially parallel to the cylindrical axis direction of the metal sheath <NUM> respectively. By arranging the heating wire <NUM> in a coiled state, the length of the heating wire <NUM> arranged in the metal sheath <NUM> is increased and the resistance value of the sheath heater <NUM> can be increased. Furthermore, since the heating wire <NUM> has a spring property by being arranged in a coiled state, disconnection during thermal expansion is suppressed. As a result, for example, even if the difference in thermal expansion coefficient between the metal sheath <NUM> and the heating wire <NUM> is large, it is possible to provide the sheath heater <NUM> with improved reliability.

A rotation pitch L1 which is the length in the cylindrical length axis direction of the metal sheath <NUM> in which the heating wire <NUM> arranged in the metal sheath <NUM> rotates once in a spiral, is preferably <NUM> or less. The rotation pitch L1 of the heating wire <NUM> arranged in the metal sheath <NUM> is more preferably <NUM> or less, and more preferably <NUM> or less. By setting the rotation pitch L1 of the heating wire <NUM> arranged in the metal sheath <NUM> to <NUM> or less, it is possible to provide the sheath heater <NUM> with improved reliability by suppressing disconnection during thermal expansion.

<FIG> are cross-sectional structural diagrams showing a sheath heater according to one embodiment of the present invention. <FIG> are cross-sectional diagrams of the sheath heater <NUM> which is shifted by a quarter pitch (L1/<NUM>) in the cylindrical axis direction of the metal sheath <NUM>. The arrangement of the heating wire <NUM> in the present embodiment is explained in detail using <FIG>. The dotted line in <FIG> shows the trajectory of the heating wire <NUM> when the heating wire <NUM> is rotated once in a spiral. Referring to <FIG>, when moved by a quarter pitch (L1/<NUM>) in the cylindrical axis direction, each heating wire <NUM> rotates <NUM> degrees around the rotation axes. The rotation axes of each heating wire <NUM> are parallel to the cylindrical axis direction and are separated by the distance g2 of the biaxial heating wire <NUM>.

A surface direction formed by the width d1 of the heating wire <NUM> is substantially perpendicular to a normal line of the rotation surface. That is, the surface of the band shaped heating wire <NUM> is a tangential plane of the rotation surface. Furthermore, the surface directions of the biaxial heating wire <NUM> are substantially parallel. The direction in which the central axis of each heating wire <NUM> rotates spirally in the direction of the cylindrical axis of the metal sheath <NUM> is substantially the same. The rotation pitch L1 is also the same. When the rotation direction and the rotation pitch L1 of each heating wire <NUM> are the same, the distance g2 between the biaxial heating wires <NUM> can be constantly maintained, and the reliability of the sheath heater <NUM> can be maintained. The sheath heater <NUM> according to the present embodiment is designed so that it is possible to maintain the reliability even if the rotation of the heating wire <NUM> is considered by meeting the conditions described above.

The cross-sectional shape of the sheath heater <NUM> according to the present embodiment is circular. Since the cross-sectional shape of the sheath heater <NUM> is circular, the sheath heater <NUM> can be easily bent into a desired shape. However, the cross-sectional shape of the sheath heater <NUM> is not limited to this, and can have any shape and can be transformed into any shape as long as the conditions described above are met.

A conductor which generates Joule heat when conducting can be used for the band shaped heating wire <NUM>. Specifically, it is possible to include a metal selected from tungsten, tantalum, molybdenum, platinum, nickel, chromium and cobalt. The metal may be an alloy including these metals, for example, an alloy of nickel and chromium, or an alloy including nickel, chromium, and cobalt. In the present embodiment, a nickel-chromium alloy is used as the material of the heating wire <NUM>.

The insulating material <NUM> is arranged to suppress the heating wire <NUM> from being electrically connected to other members. That is, a material that sufficiently insulates the heating wire <NUM> from other members can be used. Furthermore, the thermal conductivity of the material which is used for the insulating material <NUM> is preferred to be 10W/mK or more. When the material used for the insulating material <NUM> has a thermal conductivity of 10W/mK or more, the heat energy which is generated by the heating wire <NUM> can be efficiently transmitted to the metal sheath <NUM>. As the insulating material <NUM>, magnesium oxide, aluminum oxide, boron nitride, aluminum nitride or the like can be used. In the present embodiment, magnesium oxide (MgO) powder is used as the insulating material <NUM>. The thermal conductivity of a compact powder of magnesium oxide (MgO) is about 10W/mK.

The thermal conductivity of the material which is used for the metal sheath <NUM> is preferred to be 200W/mK or more. When the thermal conductivity of the material used for the metal sheath <NUM> is 200W/mK or more, the thermal energy generated by the heating wire <NUM> can be efficiently transmitted to the object to be heated.

Furthermore, the coefficient of thermal expansion of the material which is used for the metal sheath <NUM> is preferred to be <NUM>×<NUM>-<NUM>/K or less. In the present embodiment, aluminum is used as the material of the metal sheath <NUM>. However, the material of the metal sheath <NUM> is not limited to aluminum and materials such as aluminum (Al), titanium (Ti) and stainless steel (SUS) can also be used. Since the thermal expansion coefficient of the material used for the metal sheath <NUM> is <NUM>×<NUM>-<NUM>/K or less, disconnection of the heating wire <NUM> due to the thermal expansion of the metal sheath <NUM> can be suppressed, and a sheath heater <NUM> with highly reliability can be provided.

As described above, the diameter of the sheath heater <NUM> according to the present embodiment can be reduced by including the band shaped heating wire <NUM>. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> with a fine pattern shaped layout can be provided. By arranging the band shaped heating wire <NUM> within the sheath heater <NUM> in a spiral rotated state, disconnection of the heating wire <NUM> during thermal expansion can be suppressed. For example, the sheath heater <NUM> with improved reliability can be provided even when the difference in coefficient of thermal expansion between the metal sheath <NUM> and the heating wire <NUM> is large.

The structure of the sheath heater according to the second embodiment of the present invention is explained using <FIG>, and <FIG>. <FIG> are cross-sectional structural diagrams showing a sheath heater according to one embodiment of the present invention. As is shown in <FIG>, the sheath heater according to the second embodiment includes a band shaped heating wire <NUM>, an insulating material <NUM>, a metal sheath <NUM> and connection terminals <NUM> the same as in the first embodiment. Since the sheath heater <NUM> according to the second embodiment is the same in the first embodiment except for the arrangement of the heating wire <NUM> in the metal sheath <NUM>, an explanation of the overlapping structure and composition is omitted and mainly the differences are explained.

Referring to <FIG>, the heating wire <NUM> is arranged with a gap within the cylindrical metal sheath <NUM>. The heating wire <NUM> and the metal sheath <NUM> are insulated by the insulating material <NUM> which is arranged in the gap. Although the metal sheath <NUM> is shown in <FIG> in a shape in which one end is closed, the present embodiment is not limited to this, and the metal sheath <NUM> may be in a shape in which both ends are open. The heating wire <NUM> is arranged so as to reciprocate in the cylindrical axis direction within the metal sheath <NUM>, and both ends of the heating wire <NUM> are arranged at one end of the metal sheath <NUM>. That is, one heating wire <NUM> is arranged so as to be biaxial in most of the metal sheath <NUM> in the cylindrical axis direction. Each heating wire <NUM> which is arranged in the metal sheath <NUM> is arranged with a gap and is insulated by the insulating material <NUM> which is arranged in the gap.

<FIG> is a cross-sectional diagram along the line C-C' in <FIG>. Referring to <FIG>, the width d1 of the band shaped heating wire <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. The thickness d2 of the band shaped heating wire <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. The inner diameter d3 of the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. The thickness d4 of the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. The outer diameter d5 of the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less. By providing the sheath heater <NUM> according to the present embodiment with the structure described above, it is possible to reduce the diameter while maintaining reliability. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shape.

The shortest distance g1 between the metal sheath <NUM> and each heating wire <NUM> which is arranged in the metal sheath <NUM> in a cross section orthogonal to the cylindrical axis is preferred to be in a range of <NUM> or more and <NUM> or less. The shortest distance g1 between the metal sheath <NUM> and the heating wire <NUM> is more preferably in a range of <NUM> or more and <NUM> or less. By setting the distance g1 between the metal sheath <NUM> and the heating wire <NUM> to <NUM> or more, insulation between the metal sheath <NUM> and the heating wire <NUM> can be ensured. By setting the distance g1 between the metal sheath <NUM> and the heating wire <NUM> to <NUM> or less, the diameter of the sheath heater <NUM> can be reduced. By using the band shaped heating wire <NUM>, the diameter of the sheath heater <NUM> according to the present embodiment can be reduced while maintaining reliability. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shaped layout.

The distance g2 between each heating wire <NUM> arranged in the metal sheath <NUM> is preferred to be in a range of <NUM> or more and <NUM> or less in a cross section which is orthogonal to the cylindrical axis. The shortest distance g2 between each heating wire <NUM> arranged in the metal sheath <NUM> is more preferably in a range of <NUM> or more and <NUM> less. By setting the distance g2 between the biaxial heating wires <NUM> to <NUM> or more, the insulation of the heating wire <NUM> can be ensured. By setting the distance g2 of the biaxial heating wires <NUM> to <NUM> or less, the diameter of the sheath heater <NUM> can be reduced. By using the band shaped heating wire <NUM>, the diameter of the sheath heater <NUM> according to the present embodiment can be reduced while maintaining reliability. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> can be laid out in a fine pattern shape.

Both ends of the heating wire <NUM> are arranged with a connection terminal 50a and a connection terminal 50b which are electrically connected respectively. Here, when the connection terminal 50a and the connection terminal 50b are not particularly distinguished, they are referred to as connection terminals <NUM>. The sheath heater <NUM> of the present embodiment has a biaxial single-terminal structure in which the two connection terminals <NUM> are arranged at one end of the sheath heater <NUM>. One end of the sheath heater <NUM> including the connection terminals <NUM> is connected to an external device (heater controller, power source and the like). The sheath heater <NUM> is heated by electric power which is supplied from the external device which controls the temperature of the sheath heater <NUM>.

The band shaped heating wire <NUM> is arranged so as to rotate with respect to the cylindrical axis direction of the metal sheath <NUM> in a region where the heating wire <NUM> is biaxial within the metal sheath <NUM>. The band shaped heating wire <NUM> extends in the cylindrical axis direction in a state where the long axis of the heating wire <NUM> rotates in a direction perpendicular to the cylindrical axis direction of the metal sheath <NUM>. Furthermore, the rotation axes of each heating wire <NUM> are arranged in a state where they substantially match. That is, the biaxial heating wire <NUM> is coiled in a double helix shape. The rotation axis of the biaxial heating wire <NUM> is arranged substantially parallel to the cylindrical axis direction of the metal sheath <NUM>. By arranging the heating wire <NUM> in a coiled state, the length of the heating wire <NUM> arranged within the metal sheath <NUM> is increased, and the resistance value of the sheath heater <NUM> can be increased. Furthermore, since the heating wire <NUM> provided with spring properties by being arranged in a coiled state, disconnection during thermal expansion is suppressed. As a result, for example, it is possible to provide the sheath heater <NUM> with improved reliability even if the difference in the coefficient of thermal expansion between the metal sheath <NUM> and the heating wire <NUM> is large.

A rotation pitch L2, which is the length in the cylindrical length axis direction of the metal sheath <NUM> in which the heating wire <NUM> arranged in the metal sheath <NUM> rotates once in a spiral, is preferred to be <NUM> or less. The rotation pitch L2 of the heating wire <NUM> which is arranged in the metal sheath <NUM> is more preferably <NUM> or less, and even more preferable <NUM> or less. By setting the rotation pitch L2 of the heating wire <NUM> which is arranged in the metal sheath <NUM> to <NUM> or less, it is possible to provide the sheath heater <NUM> with improved reliability by suppressing disconnection during thermal expansion. Furthermore, it is preferred that the shortest distance L3 in the rotation axis direction of each heating wire <NUM> is <NUM> or more in the region where the heating wire <NUM> is biaxial in the metal sheath <NUM>. By setting the distance L3 of the biaxial heating wires <NUM> to <NUM> or more, insulation of the heating wire <NUM> can be ensured.

<FIG> are cross-sectional structural diagrams showing a sheath heater according to one embodiment of the present invention. <FIG> are cross-sectional diagrams of the sheath heater <NUM> which is shifted by a quarter pitch (L2/<NUM>) in the cylindrical axis direction of the metal sheath <NUM>. The arrangement of the heating wire <NUM> in the present embodiment is explained in detail using <FIG>. The dotted line in <FIG> shows the trajectory of the heating wire <NUM> when the heating wire <NUM> rotates spirally once. Referring to <FIG>, when moving by a quarter pitch (L2/<NUM>) in the cylinder axis direction, each heating wire <NUM> rotates <NUM> degrees around the same rotation axis. The rotation axis of the heating wire <NUM> is parallel to the cylindrical axis direction.

A surface direction formed by the width d1 of the heating wire <NUM> is substantially perpendicular to a normal line of the rotation surface. That is, the surface of the band shaped heating wire <NUM> is a tangential plane of the rotation surface. Furthermore, the surface directions of the biaxial heating wire <NUM> are substantially parallel. The direction in which the central axis of each heating wire <NUM> rotates in a double helix spiral in the direction of the cylindrical axis of the metal sheath <NUM> is misaligned by <NUM> degrees. The rotation pitch L2 is substantially the same. That is, the rotation of each heating wire <NUM> is misaligned by one half pitch. When the rotation pitch L2 of each heating wire <NUM> are the same, the distance g2 between the biaxial heating wires <NUM> can be constantly maintained, and the reliability of the sheath heater <NUM> can be maintained. However, the present invention is not limited to this, and the misalignment of the rotation direction of each heating wire does not have to be <NUM> degrees. The sheath heater <NUM> according to the present embodiment is designed so that it is possible to maintain reliability even if the rotation of the heating wire <NUM> is considered as long as the condition that the shortest distance L3 of the biaxial heating wire <NUM> in the cylindrical axis direction of the metal sheath <NUM> is g2 or more is met.

The cross-sectional shape of the sheath heater <NUM> according to the present embodiment is circular. Since the cross-sectional shape of the sheath heater <NUM> is circular, the sheath heater <NUM> can be easily bent into a desired shape. However, the cross-sectional shape of the sheath heater <NUM> is not limited to this shape, and can have any shape, and can be deformed into any shape as long as the conditions described above are met.

As described above, the diameter of the sheath heater <NUM> according to the present embodiment can be reduced by including the band shaped heating wire <NUM>. By reducing the diameter of the sheath heater <NUM>, the sheath heater <NUM> with a fine pattern shaped layout can be provided. By arranging the band shaped heating wire <NUM> in the sheath heater <NUM> in a double helix shape, disconnection of the heating wire <NUM> during thermal expansion can be suppressed. For example, the sheath heater <NUM> with improved reliability can be provided even if there is a large difference in the coefficient of thermal expansion between the metal sheath <NUM> and the heating wire <NUM>.

Each embodiment described above as embodiments of the present invention can be implemented in combination as appropriate as long as they do not contradict each other. In addition, those skilled in the art could appropriately add, delete or change the design of the constituent elements based on the each embodiment, as long as it does not depart from the concept of the present invention and such changes are included within the scope of the present invention.

In addition, even if other actions and effects different from the actions and effects brought about by the aspects of each embodiment described above are obvious from the description of the present specification or those which could be easily predicted by those skilled in the art, such actions and effects are to be interpreted as being provided by the present invention.

Although the present invention is explained in more detail below based on examples and comparative examples, the present invention is not limited thereto.

<FIG> is a cross-sectional structural diagram showing the sheath heater according to Example <NUM> of the present invention. Example <NUM> has substantially the same structure as in the first embodiment described above, and each parameter is as follows.

Since the Comparative Example <NUM> has the same structure as Example <NUM> except that a round heating wire <NUM> is used, an explanation of the same structure is omitted.

The resistance values in the sheath heaters of Example <NUM> and Comparative Example <NUM> described above were measured. The resistance value in the sheath heater of Example <NUM> was <NUM> to 40Ω/m. On the other hand, the resistance value in the sheath heater of Comparative Example <NUM> was 170Ω/m or more. In the sheath heater obtained by coiling the band in Example <NUM>, output per unit length could be increased.

The sheath heaters in Example <NUM> and Comparative Example <NUM> described above were observed by a CT scan. <FIG> shows a CT scan image of the sheath heater according to the Example <NUM>. <FIG> shows a 3D image of the sheath heater according to the Example <NUM>. As is shown in <FIG>, in the sheath heater in Example <NUM>, an insulation distance between the coiled band shaped heating wire and the metal sheath, and the insulation distance between pairs of heating wires could be ensured of <NUM> or more. On the other hand, in the sheath heater of the Comparative Example <NUM>, sections were observed where an insulation distance between a coiled round heating wire and the metal sheath and the insulation distance between pairs of heating wires was <NUM> or less. In the band shaped coiled sheath heater in Example <NUM>, it was possible to perform coiling while ensuring insulation within a small diameter metal sheath.

Claim 1:
A sheath heater (<NUM>) comprising:
a metal sheath (<NUM>);
a heating wire (<NUM>) arranged in a space within the metal sheath so as to rotate with respect to an axis direction of the metal sheath, wherein the heating wire is arranged in a double helix structure in a biaxial region of the metal sheath;
an insulating material (<NUM>) arranged in the space; and
connection terminals arranged at one end of the metal sheath, the connection terminals electrically connected with both ends of the heating wire respectively;
characterised in that
the heating wire has a band shape,
wherein the surface directions formed by the width of the biaxial heating wire are substantially parallel,
wherein the rotation direction of each helix of the heating wire is substantially the same and the rotation pitch is substantially the same,
and
wherein a thickness (d4) of the metal sheath is in the range of <NUM> or more and <NUM> or less.