Patent GB-900515-A (GB)
Title: Electric surface heating devices
IPC: F24D13/02
Filed: 1957-07-08
Abstract: 900,515. Heating resistors. EISLER, P. July 8, 1958 [July 8, 1957; July 22, 1957], Nos. 21496/57 and 23110/57. Class 39 (3). [Also in Groups XV and XXXVII] A heating resistor comprises a thin metal sheet 11 having some metal removed in a repeat fashion separated by a thin insulating layer 26 from a thin metal sheet 27 which covers at least the metal pattern of the resistor 11 and serves as an earth on breakdown of the insulator 13, and means for interrupting the supply to the resistor in the case of an external action, e.g. a penetrating nail, causing a small local connection between the resistor and sheet 27, the earthing sheet 27 being of such material, e.g. copper or aluminium, thickness and structure, that it is not easily melted and the connection will persist for the period necessary for the interrupting means to operate. In Fig. 5 the heater 11 is separated by insulating sheets 26 from two continuous metal sheets 27, e.g. of steel or aluminium, joined at their edges 28 to enclose the heater and form an earth. In Fig. 7 the heater 11 is separated by an insulating layer 26 from a metal earthing sheet 31 which is joined at its edges to an insulating sheet of waterproofed or varnished paper or of porous plastic on the other side of the heater 11 covered in turn by a corrugated sheet 29 of paper or metal. The corrugations of the sheet 29 are covered by an extensible material, e.g. crepe paper 32, having its folds parallel with the corrugations. The resistor 11 may also be corrugated so that the whole panel may be rolled up, e.g. when used on a wall or as a blind. The heater may be used under carpets or on a wall or ceiling and may be covered by wall paper or may form a heating mantle or an immersion heater. Part of the sheet resistor 11 may form a fuse, which may, however, comprise a metal sheet patterned to a replica of the resistor 11 and superposed thereon with an intervening insulating sheet, e.g. of plastic. The lines of the fusible pattern are so narrow that two such lines, connected in series or parallel, may lie under a single line of the heater pattern 11. Thus in Fig. 4 the heater patterns 11 are connected to bus-bars 15 of a mains supply and the fuse patterns 14 are supplied with low-voltage from a transformer 20 having output leads wound in opposition at 21, 22 on an iron core of a circuit-breaker 23. The fused patterns 14 are centrally tapped and connected to an earthed bar 18. Should any half of any line 14 be broken, the balance of the windings 21, 22 is upset and the iron core actuates the circuitbreaker 23. The fusible pattern 14 is of tin, lead or an alloy. The insulating layers between the metal conductors may be a thermoplastic film softening at a critical temperature, or it may be an insulating film or may be a semi-conductive film with a temperature coefficient or may be pigmented with carbon or graphite. Normally only a slight current passes from the resistor 11 to the earthing layer 13 but on softening of the plastic, fuses 24 on the mains blow, or a circuit-breaker coil 25 is actuated, and an alarm is given. The front surface of the metal envelope 27 may be a good heat emitter and the back surface a bad heat emitter, e.g. by colour treatment. The heater may be stuck on a wall or spaced from the wall by laths to form an air gap, or may be spaced from a ceiling by attaching it to a rim of a reflector of a tubular lamp adjacent the ceiling. The lamps may be recessed behind the heater to give concealed lighting. The heating pattern may form part of the inductance or capacitance required for fluorescent lamps. The repeat pattern may be formed by removing the metal in a pattern of ovals 42 elongated longitudinally of the strip and staggered to present meandering parallel paths to current supplied to the edges of the strip. The pattern may be further slotted at 45 to form a zig-zag sequence. Wide conductive lines 47 placed on the insulating sheet parallel with the edges of the heating strip are connected to supply current thereto by spaced metal tabs 48 which may be fuses. When the strip is wide, a further conductive line may run down its centre. One outer line may form an earth. The heating pattern may be coils giving capacitance and inductive effects. When an insulating layer carrying two lines of S coils is folded and enclosed in a steel envelope, the latter is heated by induction. The heaters may be placed on doors or chairs or be used as warm-plates and shelves in food trolleys. The pattern of metal on an insulating sheet is formed by removing some of the metal by brushing, polishing or abrasive action over a stencil, which may be an endless metal belt with rims or sharp edges on the patterns. The heater may be a paper strip with a lead foil bonded thereto or made by frictioncallendering an aqueous suspension of lead or till on to the paper. The coated paper passes over a roller and the endless stencil is pressed thereon, the excess metal being removed by a revolving brush which may have a heated axis. The brush may be almost entirely surrounded by a heating mantle, or a flame may play in front of the brush. If the stencil contains unperforated bridge pieces, the pattern is made in two stages, using two stencils in succession and register on the same strip. The metal may, however, be removed by a jet of abrasive particles directed on to the stencil openings. Specification 900,518 is referred to.