ELONGATE ELECTRICAL HEATING DEVICE AND A SYSTEM COMPRISING SUCH DEVICES

Abstract: Elongate electrical devices, comprising two conductors with electrical elements connected in parallel between them, have improved performance if the power supply is connected to one conductor at the near end and to the other conuuctor at the far end. Particularly useful devices are heaters, e.g. PTC conductive polymer heaters. The power supply is connected to the far end of the device through a connection means whose electrical properties can be correlated with those of the device in order to obtain a wide range of useful results. For example the connection means can have PTC, NTC or ZTC character and can be a simple conductor or another elongate device. The power supply can be DC or single-phase, two-phase or three-­ phase AC. A circuit of the invention is shown in Figure 3.

Description: This invention relates to elongate electrical heating devices, and to circuits containing them. Many elongate electrical heaters, e.g. for heating pipes, tanks and other apparatus in the chemical process industry, comprise two (or more) relatively low resistance conductors which are connected at one end to the power source and run the length of the heater, with a plurality of heating elements connected in parallel with each other between the conductors. An advantage of such heaters is that they can, if necessary, be cut to length. In one class of such heaters, the heating elements are in the form of a continuous or segmented strip of conductive polymer which lies between the conductors. In a second class, the heating elements are in the form of one or more resistive heating wires which progress down the length of the heater and are connected at intervals to alternate conductors; such heaters are usually referred to as zone heaters. Zone heaters, when cut to length, have a cold spot at the cut end, the length of the cold spot depending on where the cut is made. For many uses, elongate heaters are preferably self-regulating. This can be achieved, for example, in the first class given above, by using a continuous strip of conductive polymer at least a part of which exhibits PTC behavior, and in the second class, by connecting the heating wire(s) to one or both of the conductors through a connecting element composed of a PTC material. Although the conductors in such elongate heaters are of relatively low resistance, there is still a finite loss of potential between them as the distance from the power source increases, and this limits the length of heater which can be employed, since the power generated by the heating elements depends in part upon the potential difference between the conductors. The maximum length of such a heater can be increased by increasing the size of the conductors, but this is expensive and results in a heater which is heavier and has reduced flexibility. Another limitation of self-regulating heaters is that their resistance, when cold, is often much less than their resistance at steady state operation; consequently they draw a much larger current then they are first switched on, and therefore suffer from the problem of current inrush. Another limitation of many heaters is that they can only be powered by supply voltages within a particular range. Elongate heaters of various kinds, and conductive polymers for use in such heaters, are disclosed in U. S. Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,255,698, 4,272,471, 4,314,230, 4,315,237, 4,318,881, 4,330,704, 4,334,351, and 4,361,799, J Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978(, Narkis et al; and DE-A-2,634,999; DE-A-2,755,077; DE-A-2,746,602; DE-A-2,755,076; and DE-A-2,821,799; and published European Patent Applications Nos. 0038713, 0038714, 0038715, 0038718, 0063440 and 0067679. In the Standard Handbook for Mechanical Engineers (Mc Graw-Hill Book Co, 7th Edition, 1967, pages 15-86 and 15-87) there is disclosed an electrical loop circuit in which a plurality of receivers (electrical loads) connected in parallel with each other are supplied with power such that the electrical distance from one generator terminal to the other through any receiver is the same as that through any other receiver, and so that the voltage at the receivers is maintained more nearly equal. A self-limiting heater strip sold by Raychem Corporation under its tradename AUTO-TRACE may be monitored by the Raychem AUTOSENSE system, whereby an insulated conductor electrically insulated from the heater strip but incorporated within a common outer jacket is used to monitor voltage to determine the continuity of the complete cable. Our invention is based on the finding that a self-regulating electrical heating system of substantially greater length can be produced through the use of a three phase power source and three substantially identical composite heating devices of a particular kind. The composite heating devices are arranged sequentially end to end to provide a heating system whose length is equal to the sum of the lengths of the individual devices. The invention includes the composite heating devices, which are novel per se , and the heating systems made from them. Thus in one aspect, the present invention provides a composite electrical heating device comprising: (i) an elongate self-regulating electrical heater which has a near end and a far end, the near end and the far end being at opposite ends of the heater, and the near end, in use, being closer than the far end to a power source for the heater, and which comprises (a) a first elongate electrical connection means which has a near end and a far end, the near end being closer to the power source than the far end, (b) a second elongate electrical connection means which is adjacent the first connection means and which has a near end and a far end, the near end being closer to the power source than the far end, and (c) a plurality of electrical elements which are connected in parallel with each other between the first and second electrical connection means; and (ii) a third elongate electrical connection means which comprises a metal conductor and which has a near end and a far end, the near end being closer to the power source than the far end, and which is adjacent to the heater but electrically insulated therefrom, between the near and far ends thereof: wherein the heater and the third connection means are enclosed within an electrically insulating jacket; the device being characterised by a fourth elongate electrical connection means (112) which has a near end and far end, which is adjacent the heater and the third connection means but is insulated therefrom along the length of the device, and which is a metal conductor; and a fifth elongate electrical connection means which has a near end and a far end, which is adjacent the heater and the third and fourth connection means but is insulated therefrom along the length of the device, and which is a metal conductor; in that the fourth and fifth connection means are enclosed within the said insulating jacket; and in that the heater and third, forth and fifth connection means are suitable for being assembled with two further devices of the same kind and with a source of three phase electrical power so as to form an elongate electrical heating system in which the three devices are physically located end-to-end so as to extend sequentially away from the power source and are interconnected with the power source and each other by means of the third, fourth and fifth connection means. In another aspect, the present invention provides an electrical heating system which comprises (1) a source of three phase electrical power; and (2) first, second and third composite electrical heating devices as defined above, each of the heating devices being at least 52 metres, preferably at least 100 metres long; the near end of the first connection means of the first heating device being connected to the first phase of the power source: the near end of the first connection means of the second heating device being connected to the second phase of the power source through the fourth connection means of the first heating device: the near end of the first connection means of the third heating device being connected to the third phase of the power source through the fifth connection means of the first heating device and the fourth connection means of the second heating device; the far end of the second connection means of the first heating device being connected to a junction point at the far end of the third heating device through the fifth connection means of the second heating device and the fourth connection means of the third heating device; the far end of the second connection means of the second heating device being connected to said junction point through the fifth connection means of the third heating device; the far end of the second connection means of the third heating device being connected to said junction point; and said junction point being connected to the neutral of the power source through the third connection means of the first, second and third heating devices. For ease and clarity in describing and claiming the invention, the terms "near end" and "far end" are used in this specification to identify the ends of the elongate connection means and the devices containing them. It is to be understood, however, that the invention includes not only systems in which each of the heating devices is located so that one end of the device is both physically and electrically nearer to the power supply than the other, but also systems in which the devices are arranged, e.g. in a loop, so that the "far end" electrically speaking, is physically closer to the power supply than the "near end" or so that the near and far ends are equidistant from the power supply. The first, second, third, fourth and fifth electrical connection means will often be simple conductors, which can be composed of the same or different materials, e.g. round metal wires (which may be solid or stranded) or flat metal strips, and are sometimes simply referred to herein as conductors. Generally it is desirable that in each elongate heater, the first and second conductors (a) are substantially the same as each other; (b) have substantially the same cross-section throughout the length of the heater; (c) have a resistance which is as low as is consistent with other factors such as weight, flexibility and cost; and (d) are at a constant distance from each other (they may be for example, straight or spiralled). The electrical elements, which are connected in parallel with each other between the first and second connection means, will usually be the same as each other, but this is not necessary. In one preferred embodiment of the invention, at least some of the elements comprise a PTC element, which can be composed of a conductive polymer or a ceramic. The PTC element can itself be the sole heating element; alternatively it can have a ZTC resistive heating element in series with it. The elements can be in the form of at least one element composed of a conductive polymer, for example a continuous strip or web of conductive polymer or a plurality of segments of conductive polymer. The composition of the conductive polymer element maybe the same throughout, or can vary; thus the conductive polymer element can comprise two or more longitudinally extending components which have different electrical characteristics. Suitable conductive polymer elements include (a) elements which consist essentially of a conductive polymer which exhibits ZTC behavior; and (b) elements which comprise a PTC conductive polymer element, e.g. an element which consists essentially of a PTC conductive polymer or an element which comprises a ZTC component element and at least one PTC component element, for example at least one PTC component element which surrounds one of the elongate conductors. The invention is illustrated by the accompanying drawing, in which Figs. 1 and 2 are cross-section views of two embodiments of an elongate self-regulating electrical heater included in a composite electrical heating device according to the invention, and Fig. 3 is a diagrammatic view of an electrical heating system according to the invention including three interconnected heating devices. Referring now to the drawings, Figures 1 and 2 are cross-sections of heating devices in accordance with the invention. In each of Figures 1 and 2, a tube 114 of insulating polymeric material encloses (i) a PTC conductive polymer heater (101) which is composed of first and second conductors (1,2), a strip of PTC conductive polymer (11) in which the conductors are embedded, and an insulating jacket (12) which surrounds the conductive polymer strip; (ii) a third conductor (111) surrounded by an insulating jacket (13); (iii) a fourth conductor (112) surrounded by an insulating jacket (14); and (iv) a fifth conductor (113) surrounded by an insulating jacket (15). Figure 3 shows how three heating devices of the invention, as shown for example in Figure 1 or 2, are connected to form a heating system of the invention, the dashed line being the connection to the neutral (N) from the junction point at the far end of the third heating device, through the third connection means of the first, second and third heating devices.

Claims: A composite electrical heating device comprising: (i) an elongate self-regulating electrical heater (101) which has a near end and a far end, the near end and the far end being at opposite ends of the heater, and the near end, in use, being closer than the far end to a power source for the heater, and which comprises (a) a first elongate electrical connection means (1) which has a near end and a far end, the near end being closer to the power source than the far end, (b) a second elongate electrical connection means (2) which is adjacent the first connection means (1) and which has a near end and a far end, the near end being closer to the power source than the far end, and (c) a plurality of electrical elements (11) which are connected in parallel with each other between the first (1) and second (2) electrical connection means; and (ii) a third elongate electrical connection means (111) which comprises a metal conductor and which has a near end and a far end, the near end being closer to the power source than the far end, and which is adjacent to the heater but electrically insulated therefrom between the near and far ends thereof; wherein the heater and the third connection means are enclosed within an electrically insulating jacket (114); the device being characterised by a fourth elongate electrical connection means (112) which has a near end and far end, which is adjacent the heater and the third connection means (111) but is insulated therefrom along the length of the device, and which is a metal conductor; and a fifth elongate electrical connection means (113) which has a near end and a far end, which is adjacent the heater and the third (111) and fourth (112) connection means but is insulated therefrom along the length of the device, and which is a metal conductor; in that the fourth (112) and fifth (113) connection means are enclosed within the said insulating jacket (114); and in that the heater and third (111), fourth (112) and fifth (113) connection means are suitable for being assembled with two further devices of the same kind and with a source of three phase electrical power so as to form an elongate electrical heating system in which the three devices are physically located end-to-end so as to extend sequentially away from the power source and are interconnected with the power source and each other by means of the third, fourth and fifth connection means. A device according to claim 1, characterised in that it is at least 52 metres, and preferably at least 100 metres long. A device according to claim 1 or 2, characterised in that the plurality of electrical elements comprises a continuous strip (11) of a PTC conductive polymer. A device according to claim 3 wherein the first (1) and second (2) electrical connection means are metal wires that are embedded in the conductive polymer strip. An electrical heating system comprising (1) a source of three phase electrical power (Ø1,Ø2,Ø3); and (2) first, second, and third composite electrical heating devices, each of which is in accordance with any one of claims 1 to 4, and each of which is at least 52 metres, and preferably at least 100 metres long; the first, second, and third heating devices being physically located end-to-end so as to extend sequentially away from the power source (8); the near end of the first connection means of the first heating device being connected to the first phase (Ø1) of the power source; the near end of the first connection means of the second heating device being connected to the second phase (Ø2) of the power source through the fourth connection means of the first heating device; the near end of the first connection means of the third heating device being connected to the third phase (Ø3) of the power source through the fifth connection means of the first heating device and the fourth connection means of the second heating device; the far end of the second connection means of the first heating device being connected to a junction point at the far end of the third heating device through the fifth connection means of the second heating device and the fourth connection means of the third heating device; the far end of the second connection means of the second heating device being connected to said junction point through the fifth connection means of the third heating device; the far end of the second connection means of the third heating device being connected to said junction point; and said junction point being connected to the neutral of the power source through the third connection means of the first, second and third heating devices.

Inventors: HULETT, RICHARD HENRY, MIDGLEY, JOHN ARTHUR
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1983
Main classification: H05B3
IPC codes: H05B3, H05B6