A flexible electric heater element in the form of a band, comprising
heating wires disposed in an electrically insulating base enveloped
in a sealed sheathing, the current-carrying wires extending outside
the sealed sheathing and disposed along at least one of the narrower
sides of the heater element.
What is claimed is:
1. A flexible electric heater element in the form of a band, comprising:
a plurality of heating wires;
an electrically insulating base in which said heating wires are
a sealed envelope enclosing said electrically insulating base,
said envelope having opposite pairs of wide and narrow sides,
at least one current-carrying wire extending outside said sealed
envelope along a corresponding narrow side of said sealed envelope;
means securing said current-carrying wire to said sealed envelope.
2. A heater element as claimed in claim 1 wherein said means securing
said current-carrying wire to said sealed envelope comprises a jumper
supporting said current-carrying wire and interlock means on said
sealed envelope and said jumper extending along said narrow sides.
3. A heater element as claimed in claim 1 wherein said means securing
said current-carrying wire to said sealed envelope comprises a lug
extending along the narrow side of the envelope at which the current-carrying
wire is disposed, said lug having a groove in which said current-carrying
wire is received.
4. A heater element as claimed in claim 3, wherein said lug is
shaped as a split cylinder and is made of a material having a residual
plastic deformation, said cylinder having a horizontal axis of symmetry
coinciding with the neutral axis of said heater element.
5. A heater element as claimed in claim 4 wherein said lug is integral
with said sealed envelope.
6. A heater element as claimed in claim 1 wherein said heating
wires are adjacent one another in a row extending parallel to the
wide sides of the envelope.
7. A heater element as claimed in claim 6 wherein said current-carrying
wire is aligned with said row of heating wires.
The present invention relates to electrothermics and, more particularly,
to flexible, band-like electric heater elements comprising resistance
Due to their flexibility, electric heater elements of this type
are wound around an object to be heated without any structural modifications
in the latter and ensure adequate heat transfer and uniform distribution
of the heat flux over considerable surfaces. Flexible electric heater
elements have found extensive application in heating vessels, containers
and chemical reactors in order to provide adequate conditions for
processes to be carried out at elevated temperatures; such heater
elements are also used for heating or freeze-protection of pipelines
and equipment employed in the transfer of hightly viscous or easily
The latter include food and technical oils, lubricants, bitumens,
paraffins, low-melting metals, synthetic resins, mastics and pastes,
fuel oils, etc.
Flexible electric heater elements are utilized in the chemical,
oil refining and food industries, as well as in oil extraction,
transport, agriculture, construction, metallurgy and municipal services.
There are known and extensively used flexible band electric heater
elements having heating and current-carrying wires disposed in an
electrically insulating base which locks said wires in a predetermined
position with respect to each other. Such elements are also provided
with a sealed sheathing.
Electric heater elements with heating and current-carrying wires
are advantageous over those which only have heating wires, as they
make it possible to have a greater number of internal wire connections,
which, in turn, makes it possible to provide heater elements of
different length and heating capacity constructed on the basis of
one type of electric heater element. In addition, the presence of
two current-carrying wires makes it possible to connect a number
of electric heater elements to one another and thus heat pipelines
of a considerable length. This helps to reduce the amount of supply
cable used, as power is supplied only to one point, which, in turn,
reduces the amount of construction, assembly and excavation work,
including cable laying, ditch digging and installation of junction
One of the known electric heater elements comprises a band of a
flexible, heat-resistant, electrically insulating material, for
example, glass fabric, which performs the function of the dielectric
base. Two current-carrying copper wires are secured along the edges
of the band. Arranged in between said wires is a zigzag heating
wire which is connected to the current-carrying wires at evenly
spaced points. The electrically insulating base with the wires mounted
thereon is additionally insulated with two layers of glass fabric
and is provided with a sealed sheathing to make it waterproof.
Another known flexible electric heater element comprises a band
of an elastic, electrically insulating material, which band performs
the function of an electrically insulating base. Pressed into said
base are two current-carrying wires insulated from each other and
from a heating wire by the material of the base. The heating wire
is wound in coils around the outer surface of the base and is connected
at evenly spaced points to the current-carrying wires. This electric
heater element is provided with a sealed sheathing to make it waterproof.
In addition to the latter function, the sealed sheathing also serves
to provide electric insulation for the heating wire.
There is known still another flexible electric heater element which
is a flat band of an electrically insulating, plastic and moisture-resistant
material, which band serves both as a sealed sheathing and as an
electrically insulating base. Pressed into the base are two current-carrying
wires and heating wires insulated by the material of the base.
All the foregoing types of flexible electric heater element have,
to a varying extent, one disadvantage in common: their current-carrying
wires are arranged in immediate proximity to the heating wires,
i.e. directly in the intensive heating zone.
All the foregoing types of flexible electric heater element employ
copper wire which is easily oxidized, especially at elevated working
The operating principle of the above-mentioned electric heater
elements is based upon the release of heat by a conductor produced
due to the effective resistance of this conductor as current passes
therethrough. The heat thus released is supplied both to an object
to be heated and to the electric heater element itself; some portion
of the heat is released through the thermally insulating coating
of the object being heated to the surrounding medium.
Heating of the current-carrying wires is undesirable because, due
to a positive temperature resistance coefficient of these wires,
it raises their resistance, which results in a release of heat in
the current-carrying wires. The latter, in turn, leads to overheating
the current-carrying wires which are then rapidly oxidized and rendered
In order to prevent destruction of current-carrying wires as a
result of oxidation, attempts have been made to reduce the working
temperatures of electric heater elements or make use of protective
plate coatings for the copper wire. For example, for electric heater
elements operating within a temperature range of 200.degree. to
250.degree.C, use is made of copper current-carrying wires plated
with silver or nickel.
Silver plating is expensive due to the cost of silver. On the other
hand, nickel plating is an extremely arduous and, consequently,
expensive process. At the same time, by using a plating it is hard
to ensure a uniform coating over the entire length of the wires;
as a result, pinpoint coating defects are possible, which may lead
There is known a flexible electric heater element, wherein the
foregoing disadvantages are partly eliminated.
This flexible electric heater element is a band woven from fibrous
materials, which band performs the function of an electrically insulating
base. In this case there are arranged in two parallel rows heating
and current-carrying wires separated by a thermally insulating layer.
To make it moisture-resistant, the electric heater element of this
type is provided with a sealed sheathing of silicone rubber. The
current-carrying wires are outside the zone of intensive heating,
being spaced from the heating wires at a distance equal to the thickness
of the thermally insulating layer. This reduces somewhat the working
temperature of the current-carrying wires.
This flexible electric heater element has proved to be convenient
and effective for heating pipelines of large diameters both with
the heater element being spiral-wound around the pipeline and being
arranged along the pipeline.
Although having a number of advantages over the conventional types
of electric heater elements, this latter type has some drawbacks
which limit the sphere of its application.
In heating small-diameter pipelines, heating wires sometimes are
forced out from the electrically insulating base at places where
the heater element bends around coupling flanges of the pipeline;
as a result, the heating wires are shorted against the current-carrying
wires or the pipeline itself, which renders the electric heater
element inoperable and may even lead to a fire.
This is accounted for by the fact that while being arranged in
two layers, one above the other, the heating and current-carrying
wires are displaced with respect to the neutral axis of the electric
The current-carrying wires are above the neutral axis, whereas
the heating wires are below that axis. AS the heater element is
bent, its layers that are above the neutral axis are stretched,
whereas the layers below the neutral axis are compressed. The greater
the distance of a layer from the neutral axis, the greater the stretching
or compression force acting upon it.
If a heater element is wound around a pipeline having a diameter
of upwards of 0.5 m, these forces are insignificant due to the great
radius of curvature and are compensated by the elasticity of the
woven electroinsulating material. If, however, the electric heater
element is wound around a small-radius pipeline or is bent at a
small curvature radius, these forces may exceed the strength of
the electroinsulating material. This reduces the longitudinal rigidity
of the heating wires and forces them out of the base and the sealed
It should also be pointed out that in view of the foregoing factors,
the thickness of the thermally insulating layer which separates
the heating and current-carrying wires must not be in excess of
2 to 2.5 mm, which only accounts for an insignificant drop in the
working temperature of the current-carrying wires.
These disadvantages reduce the reliability and confine the application
of the latter type of flexible heater element to heating large-diameter
Among the disadvantages of this type of flexible electric heater
element are the following: when several electric heater elements
are connected to make up one circuit, the heating wires of these
heater elements are connected in parallel with the common current-carrying
wires. If N electric heater elements are connected, there flows
through the current-carrying wires of the first electric heater
element the total current of all the electric heater elements; through
the current-carrying wires of the second element there flows current
N - 1; through the current-carrying wires of the third element there
flows current N - 2, etc.
With an equal section of the current-carrying wires, some part
of them is overloaded, whereas the other part is underloaded. This
accounts for overheating, rapid oxidation and failures of current-carrying
wires in the former case and wastage of copper in the latter case.
It is an object of the present invention to provide a flexible
electric heater element which makes it possible to substantially
reduce the working temperature of the current-carrying wires and
thus prolong the service life and ensure complete operational safety
of the electric heater element. The flexible electric heater element
of this invention must also reduce the optimum loads in the current-carrying
wires and thus make it possible to economize such a scarce material
The object of the present invention is attained by providing a
flexible electric heater element in the form of a band, comprising
heating wires disposed in an electrically insulating base enveloped
by a sealed sheathing, and current-carrying wires, wherein the current-carrying
wires are arranged, in accordance with the invention, along at least
one of the narrower sides of the heater element.
It is expedient that along the narrower sides the sealed sheathing
be provided with recesses or projections forming a lock joint with
respective recesses and projections of at least one jumper intended
for the attachment of the current-carrying wires.
It is also expedient that the sealed sheathing be provided with
at least one lug disposed along the narrower side of the element,
there being inside said lug a channel for the current-carrying wires.
It is desirable that the lug be shaped as a longitudinally cut
cylinder whose symmetry axis coincides with the neutral axis of
the heater element, the material of the lug having residual plastic
In an alternative embodiment of a sheathing made of a material
with residual plastic deformation, the lug has an open channel,
which makes it possible to lay current-carrying wires of a desired
section in said channel with subsequent squeezing of the lug walls
to close said channel.
The arrangement of the current-carrying wires in the lug channels
of the sealed sheathing or outside that sheathing, when the current-carrying
wires are secured with the aid of jumpers, reduces the heat flux
from the heating wires to the current-carrying wires and produces
a temperature difference in the order of 30.degree. to 35.degree.
with respect to the temperature of the heating wires.
A ten-degree reduction in the temperature results in a two-fold
reduction in the oxidation rate. This substantially prolongs the
service life of the current-carrying wires and, consequently, of
the heater element as a whole. The result is a substantial reduction
in the production and maintenance costs.
This is due to reduced copper consumption, an expanded sphere of
application and an increase in the operational safety of the flexible
electric heater element.
Other objects and advantages of the present invention will become
more apparent from the following detailed description of preferred
embodiments thereof taken in conjunction with the accompanying drawings,
FIG. 1 is a side elevational view of a flexible electric heater
element broken in length with one current-carrying wire secured
with the aid of jumpers, in accordance with the invention;
FIG. 2 is a similar view of a flexible electric heater element
with two current-carrying wires according to a second embodiment;
FIG. 3 is a transverse sectional view of jumper provided with projections
for laying two current-carrying wires;
FIG. 4 is a transverse sectional view of a jumper provided with
projections for laying one current-carrying wire;
FIG. 5 is a cross-sectional view taken on line V-V in FIG. 1 of
an electrically insulating base and a sealed sheathing with recesses
for locking the jumpers in place;
FIG. 6 is a cross-sectional view taken on line VI-VI in FIG. 2
of a flexible electric heater element with current-carrying wires
and a jummper whose projections form a lock joint with recesses
in the sealed sheathing;
FIG. 7 is an axonometrical view of a flexible elect-ring heater
FIG. 8 is a general plan view of an alternative embodiment of a
flexible electric heater element with two current-carrying wires
arranged in lug channels of a sealed sheathing;
FIG. 9 is a partially axonometrical view of a flexible electric
heater element with current-carrying wires disposed in channels
provided in a sealed sheathing;
FIG. 10 is a cross-sectional view taken on line X-X in FIG. 8 of
a flexible electric heater element with two current-carrying wires
disposed in closed channels provided in lugs of a sealed sheathing;
FIG. 11 is a similar sectional view of a flexible electric heater
element with two current-carrying wires disposed in open channels
provided in lugs of a sealed sheathing, one of said channels being
closed (after being rolled); and
FIG. 12 is a sectional view of an assembly coupling two flexible
electric heater elements by means of connection boxes.
Referring now to FIGS. 1 and 2 of the attached drawings, the flexible
band-type electric heater element of the present invention comprises
resistance-alloy-based heating wires 1 woven, sewn, plaited or pressed
into an electrically insulating base 2 of a heat-resistant, electrically
insulating material, for example, glass fabric.
The base 2 is provided with a sealed sheathing 3 of a moisture-resistant
material, for example, silicone rubber. Current-carrying copper
wires 4 provided with a sealed insulating coating 5 extend outside
the sealed sheathing 3 of the electrically insulating base 2 and
are arranged along the narrower sides of the electric heater element.
The heating wires 1, the current-carrying wires 4 and lead wires
6 are connected by means of connection boxes 7.
The current-carrying wires 4 are locked in place by means of jumpers
8 (FIGS. 3 and 4) having projections 9 which form with recesses
10 (FIG. 5) in the sealed sheathing 3 of the electrically insulating
base 2 a lock joint 11 (FIG. 6).
It should be pointed out that depending upon their purpose, flexible
electric heater elements may have one current-carrying wire 4, as
shown in FIG. 1, or current-carrying wires 4, as shown in FIG. 2.
This accounts for different modifications of the jumpers 8 (FIGS.
3 and 4), depending upon the number of the current-carrying wires
It should also be pointed out that the jumpers 8 and the sealed
sheathing 3 of the electrically insulating base 2 may have recesses
or grooves 10 and projections 9 of diverse configurations which,
of course, correspond to each other to produce reliable lock joint
11. It should be borne in mind that the jumpers 8 are only found
at the narrower upper and lateral sides of the sealed sheathing
3 of the electrically insulating base 2, which means that they do
not produce any additional thermal resistance to the heat flux from
the heating surface of the flexible electric heater element to an
object being heated.
An axonometrical view of the proposed flexible electric heater
element with two current-carrying wires is shown in FIG. 7. In alternative
embodiments, the jumper 8 may be continuous throughout the entire
length of the electric heater element (not shown) or broken, as
is shown in FIGS. 1, 2, 4, 6 and 7.
Another alternative embodiment of the present invention is shown
in FIG. 8, wherein the heating wires 1 in the electrically insulating
base 2 having the sealed sheathing 3, the current-carrying wires
4 in the coating 5, and the lead wires 6 are connected in suitable
fashion, depending upon the connection diagram, by means of the
connection boxes 7. The current-carrying wires 4 are locked in place
with the aid of lugs 12 of the sealed sheathing 3 of the electrically
insulating base 2 in closed channels 13 inside the lugs 12.
The lugs 12 extend along the narrower sides of the heater element,
throughot its length. An axonometric view of this structure of a
flexible electric heater element is shown in FIG. 9; its cross-sectional
view is shown in FIG. 10.
Other alternative embodiments of the invention include a flexible
electric heater element whose cross-sectional view is shown in FIG.
11. Unlike the previously discussed embodiments, the sealed sheathing
3 of the electrically insulating base 2 is made of a material having
a residual plastic deformation, for example, aluminum, whereas the
lugs 12 have open channels 13 which are rolled after the current-carrying
wires 4 have been arranged in the coating 5, which makes it possible
to produce a longitudinally cut cylindrical channel 13 with an open
side 14 (FIG. 11).
In all the foregoing embodiments the axis of symmetry of the lugs
12 must correspond to the neutral axis of the electrically insulating
base 2, i.e. of the whole element.
If it is necessary to warm up pipelines 300 to 350 m long, several
heater elements are joined to one another. An example of joining
electric heater elements and the design of a connection box are
shown in FIG. 12.
FIG. 12 shows a connection of two electric heater elements into
one circuit, wherein the heating wires 1 are connected with the
aid of a terminal 15 to the current-carrying wires 4 and to the
leads 6 with the aid of screws 16.
At the place where the lead 6 enters the connection box 7 there
is installed a gasket 17.
As a flexible electric heater element is mounted on a pipeline
to be warmed up, the beginning thereof is secured to the pipeline,
after which, depending upon the specific uses of the heater element,
it is wound at a preselected pitch around the pipeline to be heated
or is arranged in a straight line along the pipeline. If the heater
element is wound around a pipeline to be warmed up, its end is secured
to the pipeline. If the heater element extends along the length
of a pipeline to be heated, it is attached to the pipeline with
the aid of a glass fabric band, the attachment points being spaced
from one another at a distance of 0.4 to 0.6 m. If necessary, a
second element is attached to the end of the first element, etc.
In order to reduce heat losses due to release of heat into the surrounding
medium, the object being heated and the electric heater element
mounted thereon are thermally insulated. After this, protective
metal casings are mounted above the thermally insulating layer (not
shown). When the heater element is connected to a power source,
heat is released from the heating wires and is transferred to the
object to be warmed up.