Self-regulating heater

Details Self-regulating heater Self-regulating heater

2001-08-15

2002-02-26

Walberg, Teresa (Department: 3742)

Electric heating

Heating devices

With power supply and voltage or current regulation or...

Reexamination Certificate

active

06350969

ABSTRACT:

2.0 FIELD OF THE INVENTION
The present invention relates to electrical heating devices that use positive temperature coefficient ceramic elements in self-regulating heaters.
3.0 BACKGROUND
Introduction
Production of heating tape is a substantial industry in the United States, with a large existing body of background art. Before developing my prototype for use in the heating system development in connection with the above cited contracts, some of the commercially available tape was purchased. I attempted to use it around areas on composite surfaces, to thermally isolate the areas from ambient so that they could be heated to high and precisely regulated temperature for the purpose of composite repair. Results with the available tape were not acceptable for a variety of reasons:
The most common use for heat tape is to prevent the cooling of an already warm, insulated object. The heat transfer rates that available heating tapes can provide are generally quite limited. The maximum power density found was 72 watts/foot, for an unregulated tape.
An “unregulated” tape is one which delivers heat at a substantially constant rate, based on the applied voltage and its own electrical resistance, regardless of its own temperature. In contrast, a “self-regulated” tape is one which automatically reduces its rate of heating at elevated temperature.
If the tape is truly “self-regulating”, the rate of heat delivery decreases very dramatically at and above a specific threshold temperature. Alternatively, some heating tapes deliver a gradual and continuous reduction in heating rate as temperature increases. Such tapes are better described as “self-limiting”; they cannot truly regulate to a specific temperature, but they prevent the tape from heating to a potentially destructively high temperature.
As indicated above, the maximum power density found in a commercially available tape was 72 watts/foot. This was not only too low for our purpose (since our objective was to effectively heat to very high temperature in the presence of heat losses), but the lack of regulation made the tape unsuitable due to potential for destructive overheating.
The heating tapes are invariably electrically insulated. Electrical insulations generally have low thermal conductivity, e.g. on the order of 0.1 Btu/hr-ft-°F., as compared with about 200 Btu/hr-ft-°F. for copper. Therefore, the temperature of the surface being heated will be very much lower than the temperature of the heating element or wire, by an unpredictable amount, unless steps are taken specifically to minimize heat transfer resistance.
If a PTC (positive temperature coefficient) heating element is used, the material is generally either a specially prepared polymer or barium titanate ceramic. Thermal conductivity is low in either case, in the range of about 0.1 to 2.0 Btu/hr-ft-°F. Therefore heat developed throughout the body of the element is partially trapped, and the element interior is much hotter than the element surface.
As a result of thermal resistance and gradients within the element, power density of available self-regulating or self-limiting tape is even lower than for the unregulated components. The maximum power density found in an available self-regulating component was 50 watts/foot. Most commercial tapes are limited to less than 20 watts/foot and, as a practical matter, even 20 watts/foot is only attainable when the surface being heated is very much colder than the tape.
If the component is made literally in the form of a tape, being typically wide but thin, it typically has very little flexibility except in “vertical” bending (in the plane perpendicular to its width). It cannot be bent laterally, and it cannot readily be stretched or compressed.
If it is a PTC “tape”, which typically is quite thick, it also is fairly rigid to torsion.
As a result, available PTC tapes can be applied to a flat surface or follow the simple contour of a cylinder (a pipe or tank, for example), but cannot follow arbitrary or complex contours to heat a complex structural surface, e.g. for the purpose of composite repair.
Other PTC components, which are designed to address the flexibility issue, do not have the relatively extreme width of “tapes”. Width and thickness are essentially identical in such components, and they are called heating “cables” or “ropes” rather than tapes. As a result, they can follow complex contours, but available contact area for heat transfer is severely reduced. Power density and temperature regulation are inherently further compromised.
These problems were the impetus for the development of a new heat tape. The prototype showed much better power density, heat transfer and temperature regulation than the commercially available tape that had first been tried. However the prototype was handmade and was fragile, with limited flexibility. The tape according to the present invention includes:
1. Available, actually attainable power density up to or even exceeding 120 watts/foot.
2. Thin-film (e.g.≈0.010″ (0.25 mm) or less) electrical insulation, for improved heat transfer.
3. Wide, flat tape geometry.
4. Easy flexibility in stretching and compression, torsion, and both lateral and vertical bending.
Purpose and Background Art
The purpose of the invention is to heat a surface to a prescribed temperature, with a compact and inexpensive heater that does not require any external regulator, i.e. is self-regulating.
For example, the invention may be used:
a. to provide a warm ambient temperature boundary around a region of controlled temperature, to facilitate accurate control and temperature uniformity within the region.
b. to heat a region to an elevated temperature with moderate precision, with no other means of regulation required.
c. to maintain elevated temperature, as of a pipe or vessel, without risk of overheating.
In addition, of course, the invention can also be used in the manner of background art self-regulating heaters, to prevent a surface from becoming cool with no risk of overheating in varying conditions.
Existing devices directed to the same objectives include heater tapes, heating ropes, and a variety of special purpose heaters, especially those constructed with self-regulating (positive-temperature-coefficient, or PTC) heating elements.
In general, a PTC resistive heating element is one whose electrical resistance rises significantly with an increase in its temperature, thus limiting its potential power dissipation at constant voltage.
Some examples of relevant background art include U.S. Pat. Nos. 5,937,435; 5,922,233; 4,341,949; 4,324,974; 4,223,208; 4,117,312; 4,072,848; 4,673,801; 4,574,187; 4,425,497; 4,395,623; 4,330,703; 4,177,376; 3,914,727; 3,861,029; 3,749,879; and 3,748,439.
The following performance related problems are associated with the background art:
The regulated temperature is that of the heater, not of the surface being heated, so good heat transfer (low and predictable thermal resistance) between the heater and surface is essential for regulation. However good heat transfer is never achieved in the background art and the tolerance on “regulated” surface temperature is extremely wide.
The heating element must almost always be electrically insulated for safety, and electrical insulators are invariably thermal insulators as well. As a result, there will always be a large and uncertain thermal resistance and &Dgr;T (temperature drop) within the insulating layer between the heating element and the surface.(Indeed, since PTC elements are generally either polymers or ceramics, not metals, there is often a high differential temperature between the element interior and its own surface.) Adjacent surface temperature will be substantially below heater temperature.
Background art devices offer very low power density. The “high power” variant or model of a typical commercially available self-limiting heater tape provides a maximum of about 20 watts/foot at normal room temperature, and power density decreases further with increasing temperature.
The devices are bulky and complex structures, relative to the simpli

Affiliated with

Also associated with

Rating

Self-regulating heater does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Self-regulating heater, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Self-regulating heater will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2969480