GB2059155A

GB2059155A - PTC heater assembly

Info

Publication number: GB2059155A
Application number: GB8026655A
Authority: GB
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1979-08-17
Filing date: 1980-08-15
Publication date: 1981-04-15
Also published as: JPS5659493A; DE3030799A1; FR2464006A1; GB2059155B; FR2464006B1

Abstract

PTC heater assembly comprises an elongate strip 3 of a conductive polymer composition exhibiting PTC behaviour, two elongate parallel electrodes 1, 2, an electrically insulating jacket 4 surrounding the strip and the electrodes, and an envelope 5, 6 of a material having a thermal conductivity of at least 0.1 cal/cm 0c sec. The heater assembly has a relatively higher passive power dissipation, and as a result higher active power output. The envelope is typically of aluminium, and aluminium sheets 5, 6 are secured to the jacket by means of an epoxy adhesive. <IMAGE>

Description

SPECIFICATION PTC heater assembly This invention relates to self-regulating heaters comprising an elongate strip of a PTC conductive polymer, two (or more) parallel electrodes in electrical (and optionally also physical) contact with the strip, and an electrically insulating jacket. Such heaters are known and are described for exampie in U.S. Patents Nos.

3,793,716,3,823,217, 3,861,029,4,177,376 and 4,188,276 and the Thermal Design Guide published by the Chemelex Division of Raychem Corporation (H 50190 505 B5 1/78).

In defining such heaters, reference is often made to their "passive power output" and their "active power output". The "passive power output" of a heater is the vaiue of the term V2/R,, where R0 is the resistance of aunit length (usually 1 foot or 1 metre) of the heater at 21" C and V is the voltage of the source of electrical power connected to the heater, usually 120 or 240 volts AC.The "active power output" of a heater is the measured power output of a unit length of the heater (usually 1 foot or 1 metre) when connected to the source of electrical power, with one major surface of the heater in contact with a metal substrate which is maintained at some temperature related to the intended use of the heater, e.g. 7 to 65"C, such as 10 C for heaters designed to prevent pipes from freezing.Conventional strip heaters comprise two or more electrodes in the form of wires embedded in a PTC conductive polymer element and have passive power outputs of substantially less than 165 watts per metre (50 watts per foot) at their intended operating voltage; the use of such heaters having higher passive power outputs has been avoided because they give little or no useful increase in active power output and have substantial disadvantages, in particular shorter life.

We have now discovered that if a PTC conductive polymer heater is surrounded by an envelope of metal or other material of good thermal conductivity, it is possible to make use of the heater at voltages at which its passive power output is greater than 165 watts per metre without the disadvantages previously associated with such use, and that a very valuable increase in active power output can thereby be obtained. For example the addition of such an envelope surprisingly makes it possible to use, at at voltage of 240 volts, a PTC heater which has a passive power output at 120 volts of less than 165 watts/metre and at 240 volts of more than 165 watts/metre, even though such a heater, in the absence of the envelope, has an unacceptably short service life at 240 volts.

In one aspect, the invention provides a method of heating which comprises passing current through a self-regulating heater assembly comprising (a) a PTC heater which comprises (i) an elongate strip of a conductive polymer composition exhibiting PTC behavior, (ii) two elongate parallel electrodes in electrical contact with said strip, and (iii) an electrically insulating jacket which surrounds said strip and said electrodes; and (b) an envelope which surrounds said PTC heater and which is composed of a material having a thermal conductivity of at least 0.1 Cal/cm "C sec. preferably at least 0.3 Cal/cm "C sec.: the current being supplied by a source of electrical power having a voltage V and the heater having a passive power output at said voltage V of at least 165 watts/metre (50 watts/ft), preferably at least 230 watts/metre (70 watts/ft).

The heater assembly can be used to heat a solid substrate, in which case the envelope is preferably shaped to conform to the substrate; or it can be used as a space heater, in which case the assembly is preferably in the form of a coil, with space between the coils for air or other liquid to circulate.

In another aspect the invention provides a self-regulating heater assembly comprising a PTC heater and an envelope as defined above, the heater having a passive power output at 120 volts to at least 165 watts/metre.

The passive power output of a heater is of course dependent on the resistivity of the PTC composition and the size and shape of the electrodes and the PTC element. For a conventional strip heater, e.g. as described in the Example below, the passive power output is preferably 165 to 655 watts/metre (50 to 200 watts/ft), preferably 230 to 655 watts, metre (70 to 200 watts/ft). For the broad range of heaters contemplated by this invention, the preferred ranges of passive power outputs are better expressed by a term which includes the width of the PTC element over which heat is produced. Thus the passive power output of the heaters used in this invention is preferably 217 d to 862 d, especially 302 d to 862 d, watts/metre, where d is the largest cross-sectional dimension (in centimetres) of the PTC strip which lies between the electrodes.

The active power output of the PTC heater is substantially increased by the presence of the envelope, and preferred heater assemblies have an active power output at 10C which is at least 1.5 times the active power output at 1 00C of the PTC heater without the envelope.

The envelope is preferably formed by a pair of elongate sheets with the heater sandwiched between them, the sheets contacting each other either side of the heater. The sheets are preferably 0.025 to 1.3 mm, e.g.

0.075 to 0.2 mm, thick and composed of a material having a thermal conductivity of at least 0.3, e.g.

aluminium. They may be secured together and/or to the heater by means of an adhesive, e.g. an epoxy adhesive.

It is important that the envelope and the PTC heater should be in intimate thermal contact, and the envelope preferably contacts (either directly or through an adhesive) at least the areas of the insulating jacket adjacent those parts of the heater in which heat is generated. Preferably there are no voids between the envelope and the heater. It is preferred that the envelope should extend outwards from the PTC heater in the form of fins, so that the exposed surface area of the envelope is at least 1.5 times, e.g. at least 2 times, the surface area of the insulating jacket of the PTC heater, especially at least 2 times. The envelope may also serve to limit access of oxygen to the PTC composition as taught by U.K. Application Serial No. 2038549A.

The heater assemblies can comprise two or more PTC heaters. The heaters may be spaced apart from each other and connected by an envelope which surrounds each of them, e.g. a plurality of parallel strip heaters sandwiched between a pair of metal sheets.

The invention is illustrated in the accompanying drawings, in which Figure 1 is an isometric view, partly in cross-section, of a heater assembly; and Figure2 is a graph showing the relationship between the electrical current and the substrate temperature in the tests described in the Example.

Referring now to Figure 1, this shows PTC heater comprising electrodes 1 and 2 embedded in a strip 3 of PTC conductive polymer composition which is surrounded by an insulating jacket 4. The heater is sandwiched between a pair of aluminium sheets 5 and 6 which are bonded to each other and to the heater by means of an adhesive (not shown).

The invention is further illustrated by the following Example.

Example The PTC heater used was a 61 cm length of a PTC strip heater as shown in Figure 1. The electrodes were copper wires 1.3 mm in diameter, with a center-to-center spacing of 7.6 mm. The PTC element was 10 mm wide and 1.8 mm thick and was composed of a composition comprising a dispersion of carbon black in a mixture of polyethylene and an ethylene/ethyl acrylate copolymer. The insulating jacket was composed of a polyurethane and was about 0.25 mm thick. The heater had a passive power output of about 250-275 watts/metre. The active power output of this heater, alone or as part of a heater assembly, was measured by securing the heater or heater assembly to an aluminum plate 1.25 x 15.25 x 61 cm, connecting the heater to a 120 volt AC power supply and allowing the system to reach equilbrium while maintaining the plate at a desired temperature. In the tests, the heater alone (Sample A) or the heater sandwiched between two identical aluminum sheets 61 cm long and 0.003 cm thick, and having widths of 11.4, 6.3,3.8 and 1.9 cm (Samples B, C, D, and E) was used. Figure 2 shows the relationship between the temperature of the plate and the current passing through the heater. The Table below shows the calculated active power output (current x applied voltage) of the heater when the plate is at 50"F (10"C).

TABLE Width of Active Power Sample Metal Envelope (cm) at 10"C (watts/metre) A none 70 B 11.4 140 C 6.3 145 D 3.8 150 E 1.9 140

Claims

1. A method of heating which comprises passing current through a self-regulating heater assembly comprising a PTC heater which comprises (i) an elongate strip of a conductive polymer composition exhibiting PTC behavior, (ii) two elongate parallel electrodes in electrical contact with said strip, and (iii) an electrically insulating jacket which surrounds said strip and said electrodes; characterised in that the heater (a) is surrounded by an envelope which is composed of a material having a thermal conductivity of at least 0.1 Cal/cm. C sec, and (b) has a passive power output at the supply voltage of at least 165 watts/metre.

2. A method according to Claim 1 characterised in that said PTC heater has a passive power output of 217 d to 862 d watts/metre, where d is the largest cross-sectional dimension in centimetres, of the PTC strip between the electrodes.

3. A method according to Claim 1 or 2 characterised in that said PTC heater is sandwiched between a pair of elongate sheets which contact each other either side of the heater and which are composed of a material having a thermal conductivity of at least 0.3 Cal/cm "C sec.

4. A method according to Claim 3 characterised in that the exposed surface area of said sheets is at least 1.5 times the surface area of the insulating jacket of the PTC heater.

5. A method according to any one of the preceding claims characterised in that the heater has an active power output at 1 0"C which is at least 1.5 times the active power output at 1 0"C of the PTC heater without the envelope.

6. A method according to any one of the preceding claims characterised in that the supply voltage is 120 or 240 volts AC.

7. A self-regulating heater assembly comprising a PTC heater which comprises (i) an elongate strip of a conductive polymer composition exhibiting PTC behavior, (ii) two elongate parallel electrodes in electrical contact with said strip, and (iii) an electrically insulating jacket which surrounds said strip and said electrodes, characterised in that said PTC heater (a) has a passive power output at 120 volts at least 165 watts/metre, and (b) is surrounded by an envelope which is composed of a material having a thermal conductivity of at least Cal/cm "C sec.

8. A heater assembly according to Claim 7 characterised in that said PTC heater is sandwiched between a pair of elongate sheets which contact each other either side of the heater and which are composed of a material having a thermal conductivity of at least 0.3 Cal/cm. "C sec.

9. A heater assembly according to Claim 8 characterised in that the exposed surface area of said sheets is at least 1.5 times the surface area of the insulating jacket of the PTC heater.

10. A heater assembly according to Claim 7, 8 or 9 characterised in that the heater has an active power output at 1 0"C which is at least 1.5 times the active power output at 1 0"C of the PTC heater without the envelope.