Electric heating – Heating devices – With power supply and voltage or current regulation or...
Patent
1986-03-20
1988-01-26
Wong, Peter S.
Electric heating
Heating devices
With power supply and voltage or current regulation or...
219505, 338 22R, H05B 300, H01C 700
Patent
active
047218487
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an electrical heater.
Electrical heaters are used for a variety of purposes, for example to maintain the temperature of pipes in industrial plants at required levels. The heaters are usually in the form of tapes which are wrapped around the pipes to be heated and connected to an appropriate electrical supply.
Most known heating tapes comprise two or more conductors or busbars extending longitudinally of the tape and a resistance heating medium electrically connected between the busbars. Generally the tapes can be considered as falling into one of two categories, the first category being that in which the resistance heating medium is simply connected between the busbars across the width of the tape, and the second category being that in which the resistance heating medium is in discrete sections each extending a relatively short distance along the length of the tape between points at which it is connected to the busbars. There are many examples of the first category, the heating medium generally being in the form of an extruded mass in which the busbars are embedded. Such a heating tape is described in British Patent Specification No. 1521460. The extruded mass may be for example carbon-loaded polymer. An example of the second category is described in British Patent Specification No. 1523129.
Some of the heating tapes have the characteristic that as their temperature rises the electrical resistance also rises, thereby avoiding overheating in the event of a fault in the system of which the heating tape is a part. Tapes with such characteristics are known as self limiting heating tapes. Most self limiting heating tapes belong to the first category of tapes and comprise a heating medium in the form of an extruded mass of polymeric material containing a component which results in a temperature dependent resistance. The material has a positive temperature coefficient, and such materials are referred to hereinafter as PTC materials. A resistance heater incorporating such a PTC heating medium is described in British Pat. No. 1449261.
The structure of the known self limiting tapes of the first category is such that the electrical path from one busbar to the other is generally between six and twelve millimeters. This makes the dimensional accuracy and chemical composition critical to heat output. The situation is further complicated by the known phenomena that carbon-filled polymer extrudates are far less homogeneous across the direction rather than along the direction of extrusion. Consequently it is found that large variations in output can occur from point to point along the tape due to a combination of factors. This can result in either dangerous overheating and/or the waste of significant quantities of energy. An extreme example of the tolerances and degree of homogeneity required can be illustrated from one embodiment of U.S. Pat. No. 4,117,312. This heating tape has a coating of a PTC material applied to one of the busbars. The resistance of this coating should increase very sharply at a certain temperature and thus act as a thermal switch. However to be effective this switching should occur over a narrow temperature range and thus the PTC material should be almost perfectly homogeneous and the thickness of the coating should be the same along the entire length of the busbar. It would be extremely unlikely that this coating could be manufactured and applied within such strict tolerances throughout the whole tape.
The afore-mentioned problems associated with self limiting tapes of the first category have been recognized and alternative tape structures have been proposed. For example, British Patent Specification No. 2120909 describes the use of an elongate PTC strip which is spiraled around a pair of power supply conductors and electrically connected to the power conductors at spaced apart locations. Thus the structure described in British Patent Specification No. 2120909 is of the second category but the simple resistance material normally used as the heating element has been
REFERENCES:
patent: 4117312 (1978-09-01), Johnson et al.
Boshell Paul M.
Malone Neil S.
Heat Trace Limited
Todd Voeltz Emanuel
Wong Peter S.
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