ELECTRIC TUBULAR HEATING ELEMENT AND TEMPERATURE CONTROLLER THEREFOR

Abstract: An electric tubular heating element comprises a tubular metal casing provided with electric terminals at either one of both ends for a coil of resistance wire surrounded by an insulating material. The coil extends through the metal casing between the terminals. A temperature controller is, via an inner terminal (7), connected to a first of the terminals of the tubular heating element and has an outer terminal (8) thus forming a terminal of the tubular heating element. The temperature controller comprises an insulating sleeve (13), the inner terminal (7) being provided at the inner end of the sleeve and the outer terminal (8) being provided at the outer end of the sleeve; a bimetal switch (18, 21) which, when in closed position, interconnects the inner and the outer terminal; and a resistor element (19) connected in parallel with the bimetal switch and having such a resistance that the heat generated by the resistor element when the bimetal switch is open, is sufficient to maintain the switch in the open position.

Description: ELECTRIC TUBULAR HEATING ELEMENT AND TEMPERATURE CONTROLLER THEREFOR The present invention relates to an electric tubular heating element comprising a tubular metal casing which contains a coil of resistance wire surrounded by an insu¬ lating material, and a temperature controller connected in series with the resistance-wire coil and arranged at one end of the metal casing and comprising an insulating sleeve which contains a bimetal switch and a resistor ele¬ ment, which is supplied with current when the temperature controller is tripped, for heating a bimetal spring in the bimetal switch, the resistor element having such a resis¬ tance that the heat generated by said element when the bimetal switch is open, is sufficient to maintain the bimetal switch in open position. The invention also concerns a temperature controller for an electric tubular heating element, comprising a body in the form of an insulating sleeve which contains a bimetal switch and a resistor element, which is supplied with current when the temperature controller is tripped, for heating a bimetal spring in the bimetal switch, the resistor element having such a resistance that the heat generated by said element when the bimetal spring is in open position, is sufficient to maintain the bimetal spring in this position. The prior art encompasses electric tubular heating elements of this type. The temperature controller used in the prior-art tubular heating elements is of single-use type, such as a fuse, or of reclosing type, such as a bimetal switch. Single-use temperature controllers have the disadvan¬ tage that the entire tubular heating element has to be replaced when the controller has blown. It is true that the temperature controller can be made replaceable, but this involves safety problems. Such temperature controllers as bimetal switches, which always return automatically when the temperature falls to the normal value, are not usually sanctioned by the authorities for safety reasons. The object of the present invention is, therefore, to provide a tubular heating element with a temperature con¬ troller not impaired by the drawbacks of the prior art. Such a temperature controller must meet the following requirements: a) It should be of recloseable, not single-use, type, . but should not reclose automatically when the tempe- . rature drops. b) The heat conduction from the resistance-wire coil to the temperature-sensing part of the temperature con- troller should be satisfactory. c) After opening, it should be insensitive to the ambient temperature as to maintaining the open posi¬ tion. d) It should be mountable in the tubular heating ele- ent after this has been completed. e) It should be mountable in a tubular heating element of comparatively small diameter, e.g. 8 mm. or even 5.4 mm. f) It should cause a distinct opening at a given excess temperature. According to the invention, these requirements are met by an electric tubular heating element which is of the type stated in the introduction to this specification and which is characterised in that the bimetal spring is dis- posed at the centre of the insulating sleeve and is con¬ nected between an outer terminal forming a terminal of the tubular heating element, and an inner terminal which later, when the temperature controller is mounted in the tubular heating element, contacts a terminal connected to the resistance-wire coil, such that the bimetal spring is live when in closed position, and that the resistor ele¬ ment is connected in parallel with the bimetal spring and is disposed on that side of said spring which is opposite to the opening direction thereof. According to the invention, these requirements are also met by a temperature controller which is of the type stated in the introduction to this specification and which is characterised in that the bimetal spring is disposed at the centre of the insulating sleeve and is connected between two electric terminals which project in opposite directions from said body and of which one is adapted to contact a terminal of the tubular heating element, and of which the other is adapted to form a terminal of the tubu¬ lar heating element, such that the bimetal spring when in closed position interconnects the terminals, and that the resistor element is connected in parallel with the bimetal spring and ' is disposed on that side of the bimetal spring which is opposite to the opening direction thereof. In a preferred embodiment, the inner and outer termi¬ nals have tongues facing one another, the bimetal switch and the resistor element being fixed one on each side of said tongues. To this end, rivets may be used, one of which may also constitute the fixed contact of the bimetal switch. According to the invention, the resistor element advantageously is a PTC-resistor, i.e. a resistor element whose resistance has a positive temperature coefficient. The temperature controller according to the invention meets the above requirements a) - f) in that 1 ) it employs a bimetal switch with a parallel-connected resistor for maintaining the open position (require- ment a), 2) it comprises a casing having such an inner connection that the temperature controller can be mounted last, i.e. after the tubular heating element otherwise is complete, e.g. stress-relieved, which would not be possible if the temperature controller had been mounted (requirement d) , 3) the bimetal spring proper is live and disposed at the centre of the temperature controller, which is faci¬ litated by the fact that the resistor element is arranged on that side of the bimetal spring which is opposite to the opening direction thereof. This arrangement permits maximum width of the bimetal spring, which is important in preventing the bimetal spring from being overheated by the current through- flow, and in enabling such a design of the bimetal spring that it produces a snap action when opening (requirements e and f) 4) the centrally disposed, live bimetal spring, which, constitutes the temperature-sensing element, is, when in closed position, connected in excellent thermally conductive manner to the resistance-wire coil by the inner terminal and the pin (requirement b), and 5) the employed casing is insulating (both as to elec¬ tricity and as to hea ), such that the required elec¬ tric strength and the desired independence of the ambient temperature can be achieved, especially in combination with the centrally disposed and live bimetal spring (requirement c). The invention will be described in more detail below, reference being had to the accompanying drawings, in which Figs 1 and 2 are longitudinal sections of two embodi¬ ments of a tubular heating element with one and two, respectively, temperature controllers according to the invention, Fig. 3 is a longitudinal section of a preferred embo- diment of a temperature controller according to the inven¬ tion, Fig. 4 is a plan view of a bimetal spring in the tem¬ perature controller of Fig. 3, and Fig. 5 is an end view of a resistor element in the temperature controller of Fig. 3. The inventive electric tubular heating element shown in Fig. 1 consists of a tubular metal casing 1 which has a resistance-wire coil 2 extended between two terminals in the form of pins 3 and 4. The resistance-wire coil is sur- rounded by, and insulated from the casing 1 by means of, a pulverulent insulating material 5, such as magnesia pow¬ der. The two ends of the wire coil are in conventional manner fixed to the inner ends of the pins 3, 4, e.g. by welding. A temperature controller 6 is according to the invention mounted in the left end of the tubular heating element, as seen in Fig. 1. The temperature controller 6 . has an inner terminal in the form of a contact socket 7 (Fig. 3) which encloses and electrically contacts the ter¬ minal pin 3 of the resistance-wire coil 2. The oppositely directed terminal 8 of the temperature controller 6 pro¬ jects from the end of the tubular heating element in the form of a pin and forms the second electric terminal thereof. The metal casing 1 extends a certain distance along the terminal 8, i.e. it contains substantially the entire temperature controller 6, and an end bushing 9 of steatite or polymers forms a seal between the metal casing 1 and the terminal 8 and also serves to maintain the latter in place. The terminal 4 may be either directly connected to the resistance-wire coil 2 or connected thereto via a tem¬ perature controller corresponding to the controller 6. Fig. 2 illustrates an alternative tubular heating ele¬ ment, in which the terminals 3', 4' of the resistance-wire coil 2' are provided at the same end of the metal casing 1'. The other end of the metal casing 1' is closed, and the resistance-wire coil 2 ' extends from one end of the casing 1 ' to the other and back embedded in an insulating material 5'. In this embodiment, two temperature controllers 6', 6" according to the invention are each connected to a terminal 3', 4'. The temperature controllers 6' and 6" in the embo¬ diment of Fig. 2, unlike that of Fig. 1, are not substan¬ tially contained in the metal casing 1 ' but in an extension piece thereof which consists of a bushing 10 and a tubular member 11. The tubular member 11 is mounted in the bushing 10 which in turn is fixedly mounted on the metal casing 1' . Further, the bushing 10 has a threaded portion for fixing the tubular heating element. The outer terminals 8', 8" of the temperature controllers 6', 6" are electrically con¬ nected to two conductors in an electric cable 12 through which current is supplied to the resistance-wire coil 2 ' . The preferred embodiment of an inventive temperature controller shown in Fig. 3 comprises a sleeve 13 of an insulating material, such as plastic, preferably of a cir¬ cular cross-section corresponding to e.g. the cross-sec¬ tion of the metal casing 1 in Fig. 1. The terminals 7 and 8 are mounted each in one end of the sleeve 13, the ter- minal 7 consisting of a cylinder 14 which is made in one piece with a contact socket 15 slotted in the longitudinal direction, and a tongue 16 facing away from the socket and disposed in a plane through a diameter of the cylinder 14. The terminal 8 consists of a pin which is fixedly mounted in an end wall of the sleeve 13 and has a tongue 17 extending into the free space inside the sleeve 13. The tongue 17 is located in the same plane as the tongue 16. One end of a resistor element 19 is fixed to the tongue 17 by a rivet 18. The other end of the resistor element 19 is fixed to the tongue 16 by a rivet 20. One end of a bimetal spring 21 is fixed to the tongue 16 by the rivet 20 on that side of the tongue 16 which is oppo¬ site to the resistor element 19. The bimetal spring 21 extends in over the tongue 17 and, when in closed posi- tion, contacts the rivet 18 which thus forms the fixed contact of the bimetal switch formed by the rivet and the bimetal spring 21. Thus, the bimetal spring 21 is located substantially at the centre of the casing 13 and has the shape illustrated in Fig. 4. In cross-section, the resis- tor element 19 has the shape shown in Fig. 5. For fixing the resistor element 19 and the bimetal spring 21 to the tongues 16 and 17, respectively, holes have been formed in the tongues 16 and 17 and in the resistor element 19 and one end of the bimetal spring 21, as shown in Figs 4 and 5. When mounting the temperature controller of Fig. 3 in e.g. the tubular heating element of Fig. 1, the contact socket 15 will contact the pin 3, and the terminal 8 will form one terminal of the tubular heating element. It should here be observed that the sleeve 13, after the details housed therein have been mounted, has been given a decreasing diameter at its free end, just beyond the con¬ tact socket 15, thereby ensuring the electric strength between the contact socket 15 and the metal casing 1. The resistor element 19 can be made of any suitable material of sufficiently high resistivity, but a material whose resistance has a positive temperature coefficient is preferred, since it ensures the desired limitation of the current through the tubular heating element after the bimetal switch has been tripped. If a tubular heating element according to the inven- tion is overheated when in operation, the bimetal spring 21 will, at a given temperature, rise from the fixed con¬ tact 18, thereby breaking the normal path of the current through the resistance-wire coil 2. A slight current, depending on the resistance of the resistor element 19, will continue to flow through the resistance-wire coil 2 via the resistor element 19. Thus, this element is heated and the generation of heat is so dimensioned that the bimetal spring 21 cannot bend back and contact the fixed contact 18. To reconnect the tubular heating element, thus also the slight current through the resistor element 19 must be interrupeted.

Claims: CLAIMS 1. Electric tubular heating element comprising a tubular metal casing (1; 1') which contains a coil of resistance wire (2; 2') surrounded by an insulating material (5; 5'), and a temperature controller (6; 6', 6") connected in series with the resistance-wire coil and arranged at one end of the metal casing and comprising an insulating sleeve (13) which contains a bimetal switch (18, 21) and a resistor element (19), which is supplied with current when the temperature controller is tripped, for heating a bimetal spring (21) in the bimetal switch, the resistor element having such a resistance that the heat generated by said element when the bimetal switch is open is sufficient to maintain the bimetal switch in open position, c h a r a c t e r i s e d in that the bimetal spring (21) is disposed at the centre of the insulating sleeve (13) and is connected between an outer terminal (8) forming a terminal of the tubular heating element, and an inner terminal (7) which later, when the tempera¬ ture controller (6; 6' , 6" ) is mounted in the tubular heating element, contacts a terminal (3) connected to the resistance-wire coil (2), such that the bimetal spring (21) is live when in closed position, and that the resistor element (19) is connected in parallel with the bimetal spring (21) and is disposed on that side of said spring which is opposite to the opening direction there¬ of. 2. Electric tubular heating element as claimed in claim 1, c h a r a c t e r i s e d in that the inner and outer terminals (7, 8) have tongues (16, 17) facing one another, the bimetal switch (18, 21) and the resistor ele¬ ment (19) being fixed one on each side of said tongues. 3. Electric tubular heating element as claimed in claim 2, c h a r a c t e r i s e d in that the bimetal switch (18, 21) and the resistor element (19) are fixed by means of rivets (18, 20), one (18) of which also consti¬ tuting the fixed contact of the bimetal switch. 4. Electric tubular heating element as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the resistor element (19) is a PTC-resistor. 5. Electric tubular heating element as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the temperature controller (6) is contained in the metal cas¬ ing (1) in its entirety. 6. Electric tubular heating element as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the temperature controller (6', 6") is arranged substantially outside the metal casing (1') in a sleeve (10, 11) which forms an extension of the metal casing (1' )• 7. Electric tubular heating element as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that the inner terminal (7) of the temperature controller has a contact socket (15) which in its entirety is contained in the insulating sleeve (13). 8. Temperature controller for an electric tubular heating element, comprising a body in the form of an insu¬ lating sleeve (13) which contains a bimetal switch (18, 21) and a resistor element (19), which is supplied with current when the temperature controller is tripped, for heating a bimetal spring (21) in the bimetal switch, the resistor element having such a resistance that the heat generated by said element when the bimetal spring is in open position is sufficient to maintain the bimetal spring in this position, c h a r a c t e r i s e d in that the bimetal spring (21) is disposed at the centre of the insu¬ lating sleeve (13) and is connected between two electric terminals (7, 8) which project in opposite directions from said body and of which one (7) is adapted to contact a terminal (3; 3 ! , 4') of the tubular heating element, and of which the other is adapted to form a terminal (8; 8', 8") of the tubular heating element, such that the bimetal spring when in closed position interconnects the terminals (7, 8), and that the resistor element (19) is connected in parallel with the bimetal spring (21) and is disposed on that side of the bimetal spring which is opposite to the opening direction thereof.

Inventors: ELOFSSON KJELL-AAKE, ELOFSSON, KJELL-AAKE
Applicants: BACKER ELEKTRO VAERME, BACKER ELEKTRO-VAERME AB
Application year: 1991
Main classification: H05B3
IPC codes: H05B1, H05B3