Electric Resistance Heating Element

Abstract

An electric resistance heating element for a moving air system in which the element is composed of a thin strip of apertured, foil-like material formed with a series of continuous and generally parallel extending corrugations which establish a wavy pattern. The foil is supported by a central, longitudinally extending, electrically insulated rod.

Parent Case Text

This application is a continuation in part application of U.S. Application, Ser. No. 96,322, filed Dec. 9, 1970 and now abandoned.

Description

The present invention relates generally to an electric resistance heating element and, more particularly, to an element which is composed of a thin strip of foil-like material and is adapted to be employed in moving air systems.

In the prior art electric resistance heating elements are commonly constructed by either utilizing a self-supporting expanded metal strip which is secured between two supporting members, or by using a coiled wire arrangement which is supported at intermediate locations by means of a ceramic bearing mount which surrounds the wire.

The conventional coiled wire construction can be characterized as having a low surface area and a high mass, or a low surface area to mass ratio. The high mass leads to a relatively high raw material cost, while the low surface area leads to inefficient heating and cooling. As a consequence of the high raw material cost, a constant effort is made to minimize the amount of material used by deliberately operating the element at as high a temperature as possible, usually well into the red heat range. The operation of the element at the high temperature level has at least two detrimental consequences. The areas of low air flow (inside the ceramic bushings) become overheated and are prone to failure; secondly, the efficiency of convective air heating is decreased as a significant amount of the energy is spent in radiant heating the solids surrounding the element. The low surface area to mass ratio also causes a slow heat-up and cooling rate.

In the past, considerable experimentation and effort has been expended to produce an element with a high surface area to mass ratio. However, to date, no economical method has been found to provide a system which would have the desired low weight and high surface area without the associated problem of excessive sagging.

It is the primary object of the present invention to provide an electric heating element which overcomes the disadvantages common in the prior art.

It is a further main object of the present invention to provide an electric resistance heating element which is very light weight, yet is supported in a manner that will prevent sagging.

It is a further object of the present invention to provide a heating element having an expanded or otherwise apertured metal foil grid which provides at least the same electrical characteristics as prior art devices; however, with a considerable saving in raw material.

It is another object of the present invention to provide a heating element which lends itself to assembly by automated means.

It is another object of the present invention to provide a heating element in which the support for the heating element is internal rather than external, as is the common practice today.

It is another object of the present invention to provide a heating element which is composed of corrugations establishing a wavy pattern which has the advantage of placing a long length of material in a relatively small area to obtain a relatively high electric resistance.

It is another object of the present invention to provide a heating element in which the heating element establishes a corrugated and wavy pattern which is supported at very short intervals to prevent, or a least to substantially reduce, creeping or sagging.

An aspect of the present invention resides in the provision of an electric resistance heating element for a moving, or forced air system which includes a thin strip of apertured foil-like, electric resistant material which is formed with a series of continuous and generally parallel extending corrugations which establish a wavy pattern. A supporting rod having an electrically non-conductive surface extends through most of the corrugations and is effective to structurally support the strip at very short intervals. A structural frame is provided to connect to the support rod for mounting the assembly of the rod and the strip.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawing:

FIG. 1 is a perspective view of a heating element in accordance with this invention;

FIG. 2 is a plan view of the heating element shown in FIG. 1; and

FIG. 3 is an elevational side view of the heating element.

Referring now to the drawing there is shown a foil-like strip 10 of commercially available material which has electric resistance characteristics. As the term "foil" is used herein, it is to denote materials having a thickness of between 4 to 16 mils (inch). While the invention is, theoretically, utilizable for strips having a thickness greater than 16 mils (inch), it should be noted, however, that as the thickness of the foil-like strip increases, the weight increases correspondingly and thus becomes, at a given point, uneconomical. Similarly, as the foil thickness decreases below 4 mils, the cost of the raw material increases significantly to the point where the use of the element as an electric resistance member becomes uneconomical. Conventional heating elements, by comparison, usually are composed of wire or sheet material having a thickness of 40 to 60 mils.

The heating element 10, as shown in the drawing, has a diamond shape expanded metal configuration. The expansion of the metal is accomplished by first splitting the solid foil strip intermittently so that the entire sheet has a series of closely spaced parallel cuts, to permit expanding it laterally to form the open screen. However, the invention is not limited to expanded metal elements. For example, the strip or element 10 can be composed of a solid foil which is mechanically perforated by impact, or wherein the apertured foil is electrochemically formed.

While the diamond shape configuration, as shown at 12 in the drawing, is usually made by an expanded metal process, such configuration is not necessarily the most desirable. For instance, it has been found that a rectangular or square-like grid pattern has a more direct current path and thus is preferred from an electrical performance point of view; however, such configuration does present cost and formability problems in the manufacture thereof.

The strip 10 is formed with a series of continuous and generally parallel extending corrugations 14 which establish in their totality a wavy pattern. While there are shown two rows of such heating elements, it will be appreciated that numerous longitudinally extending rows, which are electrically connected in series, can be constructed; all of such rows forming together a continuous loop with one end of the loop being connected to a current terminal 16 and the other end being physically and electrically connected to the other electric terminal 18. The terminals 16 and 18 are suitably mounted to a common plastic insulating mount 20.

In order to support the strip 10, an particularly the individual corrugations 14 thereof, there is provided a longitudinally extending rod 22 which protrudes through the individual corrugations in a skewer-like manner. The rod 22 extends through the corrugations 14 about a central axis to provide maximum support for the corrugations 14 which are also generally symmetrically arranged along and parallel to the longitudinal axis of the rod. The rod 22 has a dielectric strength of at least 21/2 thousand volts. This is typically accomplished by providing a steel rod coated with ceramic material, although a solid glass or quartz rod may be utilized. Alternatively, a steel rod having a ceramic sleeve has also been found to be satisfactory.

The assembly of the rod 22 and the corrugated strip structure 10, 14 is supported by means of a simple frame which comprises a network of rods 24 connecting at opposite ends to the insulated rods 22 and to a mounting plate 26.

The present invention will be better understood when the operating characteristics are compared with those of the conventional devices. For example, a typical 5,600 watt laundry heating element of standard coiled wire construction has the following characteristics:

Surface Area 69 in..sup.2 Weight 0.265 .music-sharp. Surface Area to Mass Ratio 260 in..sup.2 /.music-sharp. Room Temperature Resistance 9.3 ohms Operating Temperature at 236 volts nominal 1600.degree.-1800.degree. F. Alloy Type C Wire Gage 16 =(.0508") Time to dry 10.music-sharp. load of towels 70 minutes Length of Wire 35.6 ft.

The Type C resistance alloy, with a nominal composition of 60Ni-24Fe-16Cr, is commonly used for this application because it has sufficient oxidation resistance at the operating temperatures with a minimum utilization of expensive nickel. On the other hand, a Type D alloy, with a composition of 35Ni-45Fe-20Cr, would be less expensive because of the lesser amount of nickel; however, its elevated temperature properties are not conducive to a long operating life. Typically, when failure of the wire does occur, the break is in an area where air blockage (such as inside a ceramic bushing) causes the temperature of wire to exceed the temperature capability of the wire.

As a comparative specific example of the operating characteristics of the high surface area to mass ratio heating element in accordance with the invention, the performance data of a 5,600 watt laundry type heating element can be described as:

Surface Area 96 in..sup.2 Weight 0.100 .music-sharp. Surface Area to Mass Ratio 960 in..sup.2 /.music-sharp. Room Temperature Resistance 9.3 ohms Operating Temperature Black to 1550.degree. F. (236 volt nominal) Alloy Type C Thickness 0.012" Pattern Diamond shaped apertures .50 in. in length .20 in. in width .018 in. in strand width Length of strip 11 ft. Time to dry 10.music-sharp. load of towels 54 min.

The low temperature of operation is mostly due to the efficient heat transfer afforded by the high surface area to mass ratio of the element. The increased efficiency has a significant effect upon the drying time and, consequently, the operating expenses. It will be noted that there is a significant difference between the "surface area to mass ratio" of a typical prior art heating element v. the surface to mass ratio of an element in accordance with the present invention. Experimentation has shown that the desirable ratio range is approximately 900 to 2,500 in..sup.2 /.music-sharp.. When the upper end of the ratio range is appreciably exceeded, economic considerations such as excessive cost of material and fabricating expenses nullify or depreciate the advantages of a thin foil heating element. Similarly, when the lower end of the ratio range is exceeded, the desired performance characteristics are significantly lost.

In addition, the skewered thin foil construction does not have areas of air blockage analagous to the bushing areas of the coiled wire design. And the operating temperature can be maintained considerably below the upper limit of performance of the Type C alloy, thus permitting the use of less expensive alloys such as, for example, Type D (35Ni-45Fe-20Cr).

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. An electric resistance heating element particularly for moving air systems, comprising:

a thin strip of apertured foil-like, electric resistance, material formed as a grid with a series of continuous and generally parallel extending corrugations establishing a wavy pattern;

a support rod with an electrically nonconductive surface extending through most of said corrugations, effective to structurally support said strip at short intervals; and

frame means connecting to said support rod for mounting the assembly of said rod and strip.

2. An electric resistance heating element according to claim 1, wherein said rod extends through said corrugations about a central axis.

3. An electric resistance heating element according to claim 1, wherein said corrugations are generally symmetrical and extend along a longitudinal axis.

4. An electric resistance heating element according to claim 1, wherein said corrugations form a continuous loop of said strip of foil-like material.

5. An electric resistance heating element according to claim 1, wherein the apertures of the foil-like material are symmetrically arrayed.

6. An electric resistance heating element according to claim 1, wherein said material is electrochemically formed.

7. An electric resistance heating element according to claim 1, wherein said material is an expanded metal foil grid.

8. An electric resistance heating element according to claim 1, wherein said corrugations are supported on said rod in a skewer-like fashion.

9. An electric resistance heating element according to claim 1, wherein said foil has a thickness approximately in the range of 4 to 16 mils inch.