Note: Descriptions are shown in the official language in which they were submitted.
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TUBULAR HEATING ELEMENT
WITH INSULATING CORE
Technical Field
The invention is related to the field of
electric heating elements and in particular to a tubular
electric heating element with an insulating core.
Background Art
Heating elements consisting of a resistive
heating wire enclosed is a metal sheath are known in the
art. The sheathed resistance heater taught by Naruo et
al in U.S. Patent 4,506,251 is typical of such a heating
element. This sheathed resistance heater consists of a
heating wire coaxially supported in a metal sheath by an
electrically insulating powder. Similar heating ele-
ments are taught by Neemanns et al in U.S. Patent
4,080,726, Neidhardt et al in U.S. Patent 3,621,204 and
Read in U.S. Patent 1,127,281.
The problem with these electric heater ele-
ments is that the resistivities of the metal alloys
which are resistant to oxidation and corrosion at
elevated temperatures are relatively low. Therefore, to
achieve the desired resistance, either the diameter of
the heater wire must be relatively small or considerable
lengths are required. Reducing the diameter of the
heater wire makes it relatively sensitive to oxidation
or corrosion and therefore subj ect to failure . Reducing
the diameter of the wire heater also increases the
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surface loading required to achieve the desired radiated heat
energy.
Alternatively, increasing the length of the wire to
obtain the desired electrical resistance increases the total
quantity of the heater wire required which, in turn, increases
the cost of the elE~ctric heater element. Increasing the length
of the wire often results in excessive bulk which may cause
packaging problems.
What is needed is a heater element in which the
resistivity of the metal heater is effectively increased,
permitting shorter lengths and lower surface loading.
Summary of the Invention
The invention is a heater element consisting of a
metal tube filled with an insulating ceramic material. The
diameter of the metal tube can be selected to produce the
desired surface loading while the cross-sectional area of the
metal tube and its length can be selected to produce a desired
resistance and a desired surface load. In a second embodiment,
a wire element may be coaxially supported in the metal by the
ceramic powder to increase the structural rigidity of the heater
element.
The invention may be summarized according to one broad
aspect as a tubular heating element comprising: a preformed
metal tube having a predetermined temperature coefficient of
resistance for generating heat when an electrical current passes
through the preformed metal tube; a ceramic material disposed in
and filling said preformed tube; and a single, continuous linear
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metal wire coaxial:Ly supported in said metal tube along the
entire length of s<~id metal tube by said ceramic material.
Accordin<~ to another broad aspect the invention
provides a tubular heating element comprising: a performed metal
tube having a predcstermined temperature coefficient of
resistance for gencsrating heat when an electrical current passes
through the preforrned metal tube; a single, continuous linear
wire coaxially disposed in said preformed metal tube along the
entire length of the tube; and a ceramic material filling said
preformed metal tube and mechanically supporting said wire
coaxially within said preformed metal tube.
According to yet another aspect the invention provides
a tubular heating element comprising: a preformed inner metal
tube; an outer metal layer overlaying said inner metal tube
along its entire length, at least one of said inner metal tube
and said outer metal layer having a predetermined temperature
coefficient of resistance; and a ceramic material filling said
inner metal tube.
One advantage of the tubular heating element is that
its diameter can be selected to produce a desired surface
loading.
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Another advantage of the tubular heater
' element is that the cross-sectional area of the metal
tube may be selected to produce a desired linear resis-
' tivity.
Still another advantage of the tubular heater
element is that a coaxial wire may be used to improve
the rigidity of the tubular heater element.
Further, another advantage of a coiled tubular
heater element is that it is structurally more rigid
than a coiled solid wire.
Yet another advantage is that a hardenable
wire may be coaxially supported within the metal heater
tube to stiffen the heater element after forming by heat
treatment.
These and other advantages of the tubular
heater element will become more apparent from a reading
of the specification in conjunction with the drawings.
Brief Description Of 1'he Drawings
FIGURE 1 is a perspective view of a first
embodiment of the tubular heating element;
FIGURE 2 is a perspective view of a second
embodiment of the tubular heating element;
FIGURE 3 is a perspective view of a third
embodiment of the tubular heating element;
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FIGURE 4 shows a first circuit arrangement of
the tubular heating element of Figure 1 with a source of
electrical power;
FIGURE 5 shows a second circuit arrangement of
the tubular heating element of Figures 2 or 3 with a
source of electrical power;
FIGURE 6 shows a tubular heating element wound
around a ceramic tube; and
FIGURE 7 shows atubular heating element in a
self standing coiled configuration.
Detailed Description of the Invention
The details of a first embodiment of the
tubular heating element are shown in Figure 1. The
tubular heating element 10 has a metal tube 12 filled
with an insulating mineral powder 14 such as magnesium
oxide. Alternatively, the metal tube 12 may be filled
with a PTC (positive temperature coefficient) conductive
ceramic powder in place of the insulating mineral
powder. The metal tube 12 is made from a metal alloy
such as Chrome!-C or any other alloy having a near zero
or a slightly positive temperature coefficient of resis-
tance. Constantan and various nickel-chromium alloys
meet these criteria. The outside diameter (OD) and
inside diameter (ID) of the metal tube 12 are selected
to produce desired linear electrical resistivity. The
heating element may be made using any conventional
method used to fabricate metal tubes. In the preferred
embodiment, the metal tube 12 is a ceramic filled welded
seam tube fabricated using known welded seam tube
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manufacturing techniques such as taught by Lewis in U. S .
Patent 4,269,639. After fabrication, the heating
element is drawn to the desired outside diameter. The
insulating material 14 is used to prevent the metal tube
12 from collapsing while it is being drawn down to the
desired diameter.
The advantages of the tubular heating element
may best be described by way of an example of a
commercial application such as a heating element for an
10 electric clothes dryer. Conventionally, this heating
element consists of a solid 16-gauge (.0508 inch dia-
meter) Chromel-C solid wire having a 10 ohm cold resis-
tance and an 11 ohm resistance at its operating tempera-
ture. The cold linear resistance of the Chromel-C wire
is approximately 0.26 ohms per ft. , therefore a Chromel-
C wire approximately 38.5 feet long is required to
produce the desired resistance. This solid wire heating
element has a weight of approximately 0.28 pounds. In
operation, approximately 240 volts are applied across
the solid wire heating element, producing 5,200 watts of
. heat energy. The surface loading, i.e., heat radiated
per square inch of surface area, of this solid wire
heating element is 70.5 watts/inch2.
This same surface loading may be achieved with
a tubular heating element 10 having a Chromel-C metal
tube whose cross-sectional area is approximately 220 of
the total cross-sectional area of metal tube 12. The
metal tube 12 for example may have 0.084 inch O.D. and
a 0.074 inch I.D. The linear resistivity of this metal
tube is approximately 0.427 ohms per foot; therefore
only 23.5 feet are required to produce the desired 10
ohm total resistance. The weight of this tubular
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heating element a.s approximately 0.1 pounds which
represents a 64 o reduction in the amount of the metal
required to make the tubular heating element.
In a second example, the cross-sectional area
of the metal tube sheath is 40% of the total cross-
sectional area of the tubular heating element. For
example, the metal tube 12 may have a 0.067 inch OD and
a 0.052 inch ID. The linear resistance of this metal
tube is approximately 0.378 ohms and requires a length
of 27 feet to produce the desired 10 ohm resistance.
The weight of this tubular heating element is approxi-
mately 0.14 pounds, which is approximately one-half
(1/2) the weight of the equivalent heating element made
from the 16-gauge solid wire.
Both embodiments of the tubular heating
element described above would operate at the same
temperature as the solid wire_ heating element, but
because of their larger diameter would last longer.
In an alternate example of the advantages of
the tubular heating element 10, consider a tubular
heating element 10 whose metal tube 12 has the same
linear resistance as the solid wire heating element
described above, has the same length, and has a cross-
sectional area which is 22 0 of the total cross-sectional
area of the tubular heating element 10. In this embodi-
ment, the metal tube 12 would have a 0.109 inch OD and
a 0.096 inch ID. The weight of the metal tube is the
same as the weight of the solid Chromel-C wire, however,
its surface load would be reduced to approximately 33
watts/inch2. This lower surface load would significantly
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reduce the surface temperature of the tubular heating
element 10 and substantially increase its life.
A second embodiment of the tubular heating
element 10 is shown in Figure 2. In this embodiment, a
wire 16 is coaxially supported within the metal tube 12
and is electrically insulated therefrom by the ceramic
powder 14. The wire 16 may be a single solid wire, a
plurality of wires, or a braided wire. This wire 16 may
perform a variety of functions as discussed below. The
wire 16 may be used to provide rigidity to the tubular
heating element 10 during the forming process. The wire
16 may provide rigidity to the tubular heating element
10 at room and elevated temperatures. The wire 16 also
may be made from an hardenable metal and used to stiffen
the finished tubular heating element 10, after being
formed, by heat treating. Also, the wire 16 may be made
from a metal such as copper or any other metal or alloy
having a low electrical resistivity or be made from a
material used to provide a temperature control to avoid
catastrophic overheating as shall be explained relative
to Figure 5.
As shown in Figure 3, the metal tube 12 may be
overlayed with one or more layers of different metals or
alloys to provide a performance superior to the perfor
mance attainable from any single alloy alone. In the
embodiment shown in Figure 3, the metal tube 12 consists
of an inner tube 18 having an outer layer 20 disposed
thereon. It is recognized that additional layers may be
used as desired.
In a first example, an alloy having good hot
strength may be selected for the inner tube 18 and an
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alloy having good oxidation or corrosion resistance may
be selected for the outer layer 20. The outer layer 20
will protect the inner tube 18 from oxidation and
corrosion. Alternately, the outer layer 20 may be made
from a premium heat resistant alloy and the inner tube
may be made from a less expensive alloy.
The tubular heating element shown in Figure 3
may have a solid coaxial wire 16 as shown and described
relative to Figure 2 or the solid coaxial wire may be
omitted, as shown in Figure 1.
Referring now to Figure 4, a source of elec-
trical power, illustrated as battery 22, may be electri-
cally connected to the opposite ends of the metal tube
12 of the tubular heating element 10. It is recognized
that although the source of electrical power 22 is
illustrated as a battery, the source of electrical power
may be an alternating current generator or electrical
power received from conventional commercial or household
alternating current electrical power outlets.
In Figure 5, one output terminal of an alter-
nating source of electrical power 24 is electrically
connected to one end of the metal tube 12 and the other
terminal of the source of electrical power 24 is con-
nected to one end of the coaxial wire 16. The opposite
end of the coaxial wire 16 is connected to the opposite
end of the metal tube 12 completing the circuit between
the terminals of the source of electrical power 24.
With this arrangement, the coaxial wire 16 may be made
from an alloy which will melt when the current through
the metal tube approaches a value preselected to prevent
a catastrophic failure of the tubular metal heater 10.
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Alternatively, the wire 15 may be made from an alloy
having a positive temperature coefficient of resistance
such that when the current through the metal tube 12
exceeds a predetermined value, the resistivity of the
wire 16 rapidly increases, thereby maintaining the
current flow through the metal tube 12 at a value less
than a current sufficient to produce catastrophic
failure of the tubular heating element 10.
The tubular heating element 10 may be used in
the same manner as a conventional solid wire heating
element. As shown in Figure 6, the tubular heating
element may be wound around a ceramic cylinder 26, or
may be coiled or may be spiral wound as shown in Figure
7 to form a free standing heater.
The disclosed tubular heating element has many
operational and economic advantages over solid wire
heating elements:
~ The tubular heating element can have a higher
linear resistance than comparable solid wire
heating elements having approximately the same
diameter.
~ The tubular heating element can reduce the quantity
of a premium resistive alloy required to make a
heating element having the desired resistance and
surface load.
~ The tubular heating element can reduce surface
loading on the heater element having the desired
resistance.
~ The tubular heating element can operate at lower
surface temperatures, thereby increasing its life.
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It is recognized that while various embodi-
ments of the invention have been shown in the drawings '
and discussed in the specification, those skilled in the
art may make changes and/or improvements to the tubular
heating element as set forth in the appended claims.
