U.S. patent number 3,651,304 [Application Number 05/129,893] was granted by the patent office on 1972-03-21 for electric resistance heating element.
This patent grant is currently assigned to Gould Inc.. Invention is credited to Robert J. Fedor.
United States Patent |
3,651,304 |
Fedor |
March 21, 1972 |
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.
Inventors: |
Fedor; Robert J. (Westlake,
OH) |
Assignee: |
Gould Inc. (Chicago,
IL)
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Family
ID: |
22442087 |
Appl.
No.: |
05/129,893 |
Filed: |
March 31, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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96322 |
Dec 9, 1970 |
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Current U.S.
Class: |
219/200; 219/538;
219/552; 392/347 |
Current CPC
Class: |
F24H
3/0405 (20130101); H05B 3/32 (20130101) |
Current International
Class: |
F24H
3/04 (20060101); H05B 3/22 (20060101); H05B
3/32 (20060101); H05b 001/00 () |
Field of
Search: |
;219/200,353-355,359,374-377,538,540,552,553
;338/208-210,280,281,283,284,287,291,315,316,333,334 ;13/25 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
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.
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.
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.
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