Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/10—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor
  • H05B3/12—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material

Definitions

• Thermally regulated heating elements of a wide variety of types have existed for some time. Most often these elements have utilized some form of feedback control system in which the temperature produced is sensed and the source of electrical energization to the heating element is controlled either in a continuous, proportional or step-wise switching fashion to achieve more-or-less constant temperature. Utilizing a wide variety of thermal sensors and various control systems, these approaches continue to be successfully used in many applications.
• a second situation in which the prior art feedback control systems are not adequate is where the heating element itself is so small as to make adequate monitoring of its temperature by a separate sensing means impractical.
• a thermally dependent parameter of the heating element as a means of sensing its own temperature.
• the resultant apparatus is complex and relatively expensive.
• the principal object of the present invention is to provide a resistive heating element which is intrinsically self-regulating at a substantially constant temperature despite large changes in thermal load.
• a second object of the present invention is to provide such a resistive heating element which is selfregulating at a temperature determined by a physical parameter of the materials used to make the heating element.
• a third object of the present invention is to provide a resistive heating element which utilizes the skin effect, whereby alternating currents are most heavily concentrated near the surface of a conductor, as a means to achieve intrinsic temperature regulation.
• a fourth object of the present invention is to provide a resistive heating element in which localized variations in thermal load over the surface extent of the heating element are locally compensated to achieve a high degree of temperature constancy uniformly over the extent of the heating element.
• a fifth object of the present invention is to provide a resistive heating element in which a high degree of temperature stability despite significant fluctuations in thermal load is achieved without resort to complex feedback systems to control electrical energization.
• a sixth object of the present invention is to provide a resistive heating element in which a high degree of temperature control can be achieved merely by energization with a constant-current alternating source operating typically in the frequency range from 8-20 MHz.
• an electrically resistive heating element comprises: a substrate member of a non-magnetic material having high thermal and electrical conductivity, and a surface layer of a ferromagnetic material having a Curie temperature in the region about which temperature control is desired, the surface layer extending substantially the full length of the heating element.
• the declining magnetic permeability of the ferromagnetic surface layer markedly reduces the skin effect causing a migration or spreading of the current into the non- magnetic member of the heating element.
• the resistance of the heating element declines sharply near the Curie temperature such that at constant current, the power dissipated by the heating element likewise declines.
• any localized variations in thermal load on the heating element are automatically compensated, since the resistance of any axial portion of the heating element, however short, is a function of its temperature.
• the high thermal conductivity of the non-magnetic member is a further aid in equalizing temperature over the extent of the heating element.
• the heating element according to the present invention can provide accurate temperature regulation despite extremely small physical size.
• the constant current R.F. source can be significantly cheaper than the complex feedback-controlled power supplies of the prior art.
• FIG. 1 is a partially schematic representation showing a heating element according to the present invention
• FIG. 2 is a schematic representation of a cylindrical heating element and its current density profile
• FIG. 3 is a graph of power versus temperature illustrating the operational advantages of the present invention
• FIG. 4 is a cross-sectional view of a fluid conduit employing the heating element of the present invention.
• FIG. 5 is a view partly in section and partly in elevation of a soldering iron tip employing the teachings of the present invention.
• FIG. 1 there is shown a simplified cylindrical heating element 1 connected in series circuit relationship with an R.F. source 3 and an on-off switch 5.
• R.F. source 3 might provide high frequency alternating current power typically in the range from 8-20 MHz, for example, and might desirably include constant current regulation for reasons that will appear from what follows.
• FIGs. 1, 2 and 4 of this application are plainly circular cylinders, it is to be understood that the use of the term “cylinder” in this application is by no means limited to the special case of circular cylinders; it is intended that this term encompass cylinders of any cross-sectional shape except where otherwise indicated.
• the electrical circuit arrangements illustrated all employ direct or ohmmic connection to a source of alternating current electric power, it is to be understood that the invention is not so limited since the range of its application also includes those cases where the electric power source is electrically coupled to the heating element inductively or capacitively.
• Heating element 1 is traversed along its major axis or length by a high frequency alternating current from R.F. source 3. The effect of this current is to cause I ⁇ R heating or "Joule" heating. If, as suggested above, R.F. source 3 is provided with constant current regulation, then I 2 is a constant and the power absorbed by heating element 1 from R.F. source 3 is proportional to the resistance R of element 1 between the points of connection to the external circuit. As can also be seen in FIG. 1, heating element
• an inner core or substrate 7, which night be made of copper or other non-magnetic, electrically and thermally conductive, material is surrounded by or clad by a sheath or plating in the form of layer 9 which is made of a magnetic material such as a ferromagnetic alloy having a resistivity higher than the resistivity of the conductive material of core 7.
• FIG. 2 the current density profile across the cross-section of a conductor carrying high frequency current is illustrated. If the conductor is in the form of a circular cylindrical conductor of radius r, then the current density profile has the general form, under conditions of relatively high frequency excitation, illustrated by characteristic 11 in FIG. 2, showing a marked increase in current density in the surface regions of conductor 1'.
• characteristic 11 clearly illustrates the " skin ef fect" whereby alternating currents are concentrated more heavily in the surface regions of the conductor than in the interior volume thereof.
• the high concentration of current at the surface region of the conductor is more pronounced the higher the frequency is.
• the skin effect is dependent upon the magnetic permeability of the conductor:
• a "thick" conductor having a planar surface and a thickness T, energized by an alternating current source connected to produce a current parallel to the surface
• ⁇ is the permeability of the material of conductor
• cr is the electrical conductivity of the material of the conductor
• ⁇ is the radian frequency of the alternating- current source.
• FIG. 2 The lower part of FIG. 2 is a graph of current density j across the diameter of conductor 1'.
• current density profile 11 shows the expected high current density at the surface of conductor 1' tapering rapidly to a very low current in the interior of conductor 1'.
• Profile 13 illustrates the current density for a temperature in the region of the Curie temperature of the ⁇ ferromagnetic material of conductor 1': the characteristic shows a considerable lessening of the skin effect with only a moderate falling off of current away from the surfaces of conductor 1'.
• Characteristic 15 is for a uniform ferromagnetic conductor such as, for example, the conductor 1' shown in FIG. 2, carrying a constant current I 1 . As shown, characteristic 15 exhibits a sharp drop in power absorbed from an R.F. energizing source such as R.F. source 3 in FIG. 1, as the Curie temperature T c is approached. Following this sharp drop in power, characteristic 15 levels off at a level labeled P min in FIG. 3.
• Characteristic 16 in FIG. 3 shows a typical power versus temperature curve for a composite heating element such as element 1 in FIG. 1 in which a non-magnetic conductive core is surrounded by a ferromagnetic surface layer. Characteristic 16 also illustrates the very similar behavior of a hollow, cylindrical non-magnetic conductor which has been provided with a ferromagnetic layer on its inside surface, or indeed any composite conductor formed principally of a non-magnetic conductive member with a ferromagnetic surface layer according to the present invention. Although qualitatively the shape of characteristic 16 is similar to that for characteristic 15, it is to be noted that characteristic 16 descends more nearly vertically to a lower value of minimum power input.
• a third characteristic 17 illustrates the effect of increasing the current carried by the composite heating element to a new value I 2 which is greater than I 1 .
• characteristic 17 shows the effect of such a current increase where I2 has been selected so as to produce the same level of minimum power P min as was obtained in the case of the characteristic for a uniform ferromagnetic conductor 15 operating at current I 1 .
• Load lines 19 and 21 are graphs of total power lost through conduction, convection, and radiation, shown as a function of temperature. As will be apparent to those skilled in the art, load line 19 is for a condition of greater thermal lossiness than load line 21. For example, line 19 might represent the thermal load when a fluid coolant is brought into contact with the heating element.
• characteristic line 17 in FIG. 3 which is representative of heating elements having a composite structure with a non-magnetic conductive core and a ferromagnetic surface layer, operating at the relatively higher current I2, most nearly approaches the ideal since both thermal load lines 19 and 21 intersect characteristic 17 defining equilibria which lie on the long, straight, nearly vertically falling portion of characteristic 17.
• the composite heating element according to the present invention only a relatively thin surface layer of the heating element is formed of ferromagnetic material, while the remainder consists of a substrate member made of non-magnetic material having high electrical conductivity. Consequently, the decline in resistance and power consumption which is experienced with a purely ferromagnetic heating element is greatly increased by the use of a non-magnetic, highly conductive core.
• the ratio ⁇ min/ ⁇ max is sufficiently close to 1 such that to a good approximation, Since ⁇ r maa has values which fall in the range from 100-600 for commercially available magnetic materials, and further since ⁇ r min (the value above T c ) is approximately equal to 1, the ratio R max /R min has a range of values for ferromagnetic materials from approximately to or approximately 10 to 25.
• this modest ratio of resistances can be vastly increased by selection of the relative cross-sectional areas and conductivities of the non-magnetic member and its ferromagnetic surface layer.
• the temperature at which regulation will take place is also variable.
• FIG. 4 there is shown a novel application of the present invention to form a heated conduit for the transmission of fluid such as, for example, crude oil over long distances while maintaining the fluid at a selected elevated temperature designed to minimize viscosity.
• the conduit 23 of FIG. 4 comprises a hollow cylindrical core 25 which may be made of copper or a less expensive non-magnetic material, for example.
• a ferromagnetic layer 27 Surrounding and immediately adjacent and in contact with the surface of core 25 is a ferromagnetic layer 27 which is in good thermal and electrical contact with core 25 substantially throughout its length.
• An insulative layer 29 which might be made of a plastic chosen to withstand the environment in which conduit 23 will be used surrounds core 25 and layer 27, electrically and thermally separating them from an outer sheath 31 which might be a woven mesh of fine copper wires, or any other suitable conductive sheath material.
• a source of R.F. current to energize conduit 23 would be connected between sheath 31 and core 25 and layer 27.
• sheath 31 would be operated at ground potential in order to avoid accidental short circuits.
• FIG. 5 is shown an additional application of the present invention to a soldering iron tip 33 of conical shape.
• Tip 33 is comprised of an outer nonmagnetic shell 35 which might be made of copper or molybdenum, for example, and which is in good thermal and electrical contact with an inner ferromagnetic shell 37, thus forming a composite self-regulating heating element in accordance with the present invention.
• An inner conductive, non-magnetic stem 39 extends axially into conical shells 35 and 37 and may be joined to inner shell 37 as by spot welding, for example.
• An R.F. source 41 is shown schematically interconnected between stem 39 and outer shell 35.
• Soldering iron tip 33 makes particularly good use of the advantages of the composite heating element structure of the present invention.
• the path of current flow through the structure of tip 33 is along stem 39 to its point of juncture with inner shell 37 and axially along the conical, inside surface of tip 33 in an expanding current flow path to return to R.F. source 41.
• a current flow path would inevitably produce excessive absorption of electric power at the apex portion of soldering iron tip 33, since the cross-section of the current flow path is smallest at this point and the resistance would in the usual case be higher therefore.
• the result would be that unless large amounts of copper were used in the formation of outer shell 35, the apex region of tip 33 would be overheated while portions near the broad base of the cone received inadequate heat.
• each portion of the current flow path will adjust its temperature to very nearly the desired regulated value despite significant changes in current-path cross-sectional area, or differential thermal loading.

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• General Induction Heating (AREA)
• Control Of Resistance Heating (AREA)
• Cookers (AREA)

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• 1981
  • 1981-03-02 AT AT81901612T patent/ATE53737T1/de not_active IP Right Cessation
  • 1981-03-02 DE DE8181901612T patent/DE3177193D1/de not_active Expired - Lifetime
  • 1981-03-02 WO PCT/US1981/000278 patent/WO1982003148A1/en active IP Right Grant
  • 1981-03-02 EP EP81901612A patent/EP0073190B1/de not_active Expired - Lifetime

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