Patent Publication Number: US-6710310-B2

Title: Heating element with integrated heat safety device

Description:
The present invention concerns an electric heating element of the enclosed type and more particularly to the integration of a heat safety device into this element. 
     BACKGROUND OF THE INVENTION 
     In the field of heating elements of the enclosed type, i.e. having a tubular envelope at the interior of which is placed a heating element embedded in an insulator, there is known, from the document DE 34 33 688, a tubular heating body containing a spiral resistive heating wire enclosed in a powder of insulating material such as magnesia. A heat protection element is connected in series with the heating spiral at the interior of the tube. This protection element is a fusible bead that, in the case of overheating, frees a compression spring permitting definitive interruption of the circuit for supplying the heating spiral. 
     This heating element thus corresponds to a fuse placed in series with the heating element in the tubular envelope, requiring an intermediate connection. Furthermore, confinement of the fuse within the heating element is a source of possible re-ionization of gas around the fuse when this latter is vaporized by overheating of the device, being able to lead, within the insulating material, to a reclosing of the electric supply circuit that has just been opened. This possibility of dysfunctioning of the safety device must be avoided as much as possible because its consequences can be serious. 
     The document JP 6267641 provides an element of response to the problems mentioned above. The heating element presented in this document is shown in FIG.  1 . Thus, a heating element  70  has a spiral resistance  72  contained in a first tube  71  within an insulating powder  73 . The spiral resistance  72  is held by two connection pins  78   a  and  78   b  through two sealing beads  74   a  and  74   b  arranged in the extremities of this first tube  71 . A heat protection fuse  77  is housed between bead  74   b  and a third end bead  76   b  arranged in a second tube  75  containing the first tube. This fuse is connected to pin  78   b  as well as a pin  78   c  connected to an electric supply circuit. At its opposite extremity, a fourth bead  76   a,  traversed by the pin  78   a  assures sealing of the second tube. 
     In this device, the fuse is certainly not housed within the heating element, but is nonetheless confined between two sealing beads, which does not eliminate completely the risks of re-ionization of the surrounding gas after triggering, bead  76   b  constituting particularly a thermal brake. In addition, the insertion of the fuse requires a specific connection. Overall, four beads and two tubes are thus necessary to obtain a heating element having a heat fuse at one of its extremities, which increases in a notable manner the selling price of the assembly. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to overcome the drawbacks of the prior art by presenting an electric heating device constituted by a tubular envelope, at the interior of which is housed a heating element enclosed by an insulator, said heating element being connected to an electric supply circuit by connection rods and pins, said tubular envelope being provided with two cylindrical sealing beads arranged at its extremities, at least one of these beads, called a specific bead, being in direct contact with the insulator enclosing the heating element, characterized in that the specific bead is axially traversed by an opening into which is inserted a heat fuse connected electrically in series with the heating element. 
     By this arrangement, the number of constituent elements is reduced since one bead assures at the same time a tight seal of the insulator enclosing the heating element, but also maintenance and protection of the housing of the fuse. Thus, without a supplemental element other than the tubular envelope and the two sealing beads, it is permitted to constitute, not only the heating assembly, but equally the addition of a heat fuse. This arrangement avoids, moreover, wiring between the heating element and the fuse within the device, just like the specific means for maintenance and protection of the fuse in the product. 
     Advantageously, the heating element is in spiral form and mounted under tension in the tubular envelope. 
     This configuration permits notably the sealing beads, connected to the ends of the heating element, to be maintained by being supported on the extremities of the tubular envelope without a specific part. 
     Advantageously, the opening has a narrowing at the level of the extremity connected with the interior of the tube, against which the fuse is positioned. 
     Together with the traction effect exerted by the spiral element, the narrowing formed in the opening permits correct and systematic positioning of the fuse in the opening, without any particular mechanical part. This equally permits assurance of the sealing of the tube with respect to the insulator enclosing the heating element. 
     According to a preferred embodiment of the invention, the heat fuse has a housing within which is housed a compression spring exerting a pressure between, on the one hand, a heat block situated at the outer extremity of the housing and in contact with an external connection pin and on the other hand a first central sliding disc pushed while resting against an internal connection rod, and this against a restoring force exerted by an opening spring coaxial with the connecting rod. 
     This association permits, when the heat block is vaporized, opening of the electric supply circuit of the heating element, with the aid of the two springs, compression and opening, releasing while carrying away the first contact disc, into the space left free by the block, thus out of contact with the connecting rod. 
     Advantageously, the connecting rod is surrounded, within the heat fuse, by an insulator, equally permitting centering and/or maintaining the connecting rod at a distance from the housing of the fuse. 
     Advantageously, the heat block is situated at the extremity of the fuse housing situated outside of the tubular envelope. 
     This permits a better controlled reactivity of the protection constituted by the heat fuse, by slightly spacing it from the heating element. 
     Moreover, direct contact between the block and one of the external connection pins avoids re-ionization of the gas produced during melting of the block, notably by permitting a rapid flow of the heat generated. 
     In addition, this thermal escape, moreover relative, equally guarantees a better aging of the fuse, preventing its untimely triggering at a temperature lower than that for which it has been calibrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood with the aid of the description that will follow, in reference to the attached figures, among which. 
     FIG. 1 represents a state of the art previously described, 
     FIG. 2 is a cross-sectional view of the heating device at the level of the extremity of the tubular envelope having the heat fuse, 
     FIG. 3 is a detail view of the heat fuse in an untriggered operating state, 
     FIG. 3 b  is a detail view identical to the view  3   a,  but after triggering of rupture of said fuse. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     According to the invention, a heating device  1  is presented in FIG. 2 in the form of that which is currently called an enclosed resistance. This device comprises a tubular envelope  2  at the interior of which is housed a spiral heating element  4  enclosed by insulator  3  such as magnesia. Sealing beads for this insulator are arranged at each extremity of tubular envelope  2 . At least one of these beads, called a specific bead as will be described in detail below, is in direct contact with insulator  3  enclosing spiral heating element  4 , as is completely visible in FIG.  2 . Preferably, the two sealing beads are in direct contact with said insulator. 
     Extremities  6  of heating element  4  are arranged, for example by soldering, each around a cylindrical connection or a terminal terminal spade lug  8  serving to connect the heating element to the exterior electric supply circuit. This cylindrical connection is thus extended by an electrical connection rod  10 . At one of the extremities (not shown) of tubular envelope  2 , electrical connection rod  10  passes through one of the sealing beads in order to establish electrical contact with the exterior supply circuit. 
     At the other extremity of the tubular envelope, the second sealing bead, called specific bead  16  is cylindrical and has an opening  18  axially traversing it. This opening is intended to receive a heat fuse  20 . Preferably, the fuse will be mounted in the opening with a minimum of play. 
     Moreover, the opening has a narrowed zone at its internal part, i.e. situated at the side of the heating element, in the form for example of a cone  21 . This cone cooperates with heat fuse  20  equally having a conical part  22  in order to prevent this latter from sliding towards the heating element. 
     Moreover, the spiral heating element is mounted under tension in the tubular envelope, so that the fuse undergoes a pulling force that tends to maintain it against conical part  22  of the opening, assuring, at the same time a sealing of the heating element, but also maintenance of the fuse, and jointly maintenance of bead  16  on the tubular envelope. 
     Heat fuse  20  is mounted in series with heating element  4  by the intermediary of connection rod  10 . It is connected to the exterior electrical supply circuit by the intermediary of a pin  12  carrying a connector  14 . This fuse thus provides the connection between the exterior electrical circuit and the interior of the tubular envelope. 
     With respect to tubular envelope  2 , fuse  20  can be housed either partially at the interior of said envelope, or completely at the interior or completely at the exterior of said envelope, depending on the desired heat reaction characteristics for the protection device. 
     This fuse permits in effect cutting the electrical supply of the heating element in case of malfunctioning of the regulator element of the appliance, provoking an overheating of the resistance. 
     Calibration of the fuse, as well as its position in tubular envelope  2  are adapted to the temperature obtained at the level of the bead during an overheating, a lower temperature than within the heating element. 
     Heat fuse  20  is shown in detail in FIGS. 3 a  and  3   b  which illustrate its state respectively before and after triggering, i.e. respectively in the passing state of operation and in the cut state. 
     Thus, FIG. 3 a,  heat fuse  20  has a housing  23  at the interior of which is housed a heat block  24  having the particularity of vaporizing when it is subjected to a given temperature, corresponding, in our case, to an overheating of the heating element. 
     This block is arranged at the external extremity, i.e. furthest from the heating element, so as to not present an overly great reactivity which could lead it to vaporize while the heating element is not in an overheated state. 
     In order to interrupt the electrical supply circuit of the heating element, the fuse has a first compression spring  28  mounted between two discs  26 . One of these discs is in contact with block  24 , while the other assures the connection between this first spring  28  and a third disc  30  arranged against the restoring force of a second spring  32 . Disc  30  is in contact with connection rod  10  immobilized in housing  23  by an insulator  34 , with the aid particularly of recesses  38  on the connection rod. 
     In the untriggered state, spring  28  constrains internal disc  26  and contact disc  30  against the end of connection rod  10  assuring, by the heat block continuity of the electrical supply circuit. 
     By this fact, disc  30  is present in the form of a star contact in order to assure a good electrical contact. Other forms are equally possible without departing from the framework of the present invention. 
     Moreover, housing  23  has an opening  40  at the extremity where the heat block is housed, at the level of the connection with pin  12 , permitting a direct contact between block  24  and pin  12 . In addition, at the level of this connection the housing is exposed to the outside air by cavity  18 . 
     Thus, when the heat block vaporizes, the heat generated can primarily and rapidly be dissipated, at the same time by exchange with the ambient air, due to cavity  18 , but equally by conduction in pin  12 . The risks of ionization are then very low, which provides a high reliability for the device. 
     The volatilization of the block leads to the release of the two springs  28  and  32  arriving at a mechanical equilibrium where particularly disc  30  is no longer in contact with connection rod  10 . The electric supply circuit for the heating element is then interrupted. 
     Utilization of a conventional heat fuse in place of the usual connection terminal for the heating element, by integrating the thermal protection into the resistance permits in addition a direct temperature capture while eliminating a diffuser, while causing the thermal inertia to be more rapid and more reliable. The heating element is thus self-regulated. 
     In addition, by the present invention, the fuse is positioned in a single operation during attachment of the sealing bead. It is thus no longer a connected piece that requires additional steps for its integration within the heating device. The assembly time and the costs are thus reduced.