U.S. patent application number 11/356329 was filed with the patent office on 2007-08-16 for heater assembly for deicing and/or anti-icing a component.
This patent application is currently assigned to United Technologies Corporation. Invention is credited to George Alan Salisbury, John Henry SR. Vontell, Charles R. JR. Watson.
Application Number | 20070187381 11/356329 |
Document ID | / |
Family ID | 37997731 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070187381 |
Kind Code |
A1 |
Vontell; John Henry SR. ; et
al. |
August 16, 2007 |
Heater assembly for deicing and/or anti-icing a component
Abstract
A heater assembly for deicing and/or anti-icing a component
includes a metallic heating element adjacent to a densely woven
fabric layer impregnated with a resin that is capable of
withstanding temperatures of up to 550.degree. F. (288.degree.
C.).
Inventors: |
Vontell; John Henry SR.;
(Manchester, CT) ; Salisbury; George Alan; (East
Hampton, CT) ; Watson; Charles R. JR.; (Windsor,
CT) |
Correspondence
Address: |
KINNEY & LANGE, P.A.
THE KINNEY & LANGE BUILDING
312 SOUTH THIRD STREET
MINNEAPOLIS
MN
55415-1002
US
|
Assignee: |
United Technologies
Corporation
Hartford
CT
|
Family ID: |
37997731 |
Appl. No.: |
11/356329 |
Filed: |
February 16, 2006 |
Current U.S.
Class: |
219/202 |
Current CPC
Class: |
F04D 29/324 20130101;
F05D 2300/603 20130101; F04D 29/5853 20130101; F04D 29/023
20130101; F04D 29/584 20130101; Y02T 50/60 20130101; F05D 2300/44
20130101; Y02T 50/676 20130101; F01D 25/02 20130101; F05D 2300/614
20130101; F05D 2300/601 20130101; Y02T 50/672 20130101; Y02T
50/6765 20180501; F02C 7/047 20130101; F01D 5/18 20130101 |
Class at
Publication: |
219/202 |
International
Class: |
B60L 1/02 20060101
B60L001/02 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0001] This invention was made with Government support under
contract number N00019-02-C-3003, awarded by the U.S. Navy. The
U.S. Government has certain rights in this invention.
Claims
1. An electrothermal heater assembly suitable for anti-icing and
deicing a gas turbine engine component, the electrothermal heater
assembly comprising: a metallic heating element; and a densely
woven fabric layer adjacent to the metallic heating element and
impregnated with a resin capable of withstanding a heater assembly
operating temperature of up to 550.degree. F., wherein the fabric
layer includes a fabric material selected from a group consisting
of a fiberglass fabric, a ceramic fiber fabric, polymer film, and
combinations thereof; and wherein the fabric material accounts for
about 45 to about 70 percent by volume of the fabric layer.
2. The heater assembly of claim 1, wherein the resin is selected
from a group consisting of: bismaleimide, phthalonitrile, cyanate
ester, polyimide adhesive, and polyimide resin.
3. (canceled)
4. The heater assembly of claim 1, wherein the fabric layer is less
than about 0.005 inches thick.
5. The heater assembly of claim 1, wherein the metallic heating
element is a titanium heating element.
6. The heater assembly of claim 1, wherein the heating element has
a watt density in a range of about 1 to about 50
watts/in.sup.2.
7. The heater assembly of claim 1, wherein the metallic heating
element is adhered to the fabric layer with an adhesive selected
from a group consisting of: the resin and a thermoset adhesive.
8. The heater assembly of claim 1, wherein the metallic heating
element is about 0.5 to about 40 mils thick.
9. The heater assembly of claim 1, wherein the fabric layer
exhibits a thermal conductivity value of about 10.1
BTU-in/hr-ft.sup.2-.degree. F.
10-20. (canceled)
21. An electrothermal heater assembly suitable for anti-icing and
deicing a gas turbine engine component, the electrothermal heater
assembly comprising: a metallic heating element wherein the heating
element has a watt density in a range of about 1 to about 50
watts/in.sup.2, and wherein the metallic heating element is about
0.5 to about 40 mils thick; and a densely woven fabric layer
adjacent to the metallic heating element and impregnated with a
resin capable of withstanding a heater assembly 0operating
temperature of up to 550.degree. F., wherein the fabric layer
includes a fabric material selected from a group consisting of a
fiberglass fabric, a ceramic fiber fabric polymer film, and
combinations thereof; and wherein the fabric material accounts for
about 45 to about 70 percent by volume of the fabric layer wherein
the fabric layer is less than about 0.005 inches thick, and wherein
the fabric layer exhibits a thermal conductivity value of about
10.1 BTU-in/hr-ft.sup.2-.degree. F.
22. The heater assembly of claim 21, wherein the resin is selected
from a group consisting of bismaleimide, phthalonitrile, cyanate
ester, polyimide adhesive, and polyimide resin.
23. The heater assembly of claim 21, wherein the metallic heating
element is a titanium heating element.
24. The heater assembly of claim 21, wherein the fabric material
accounts for about 45 to about 70 percent by volume of the fabric
layer.
25. An electrothermal heater assembly suitable for anti-icing and
deicing a gas turbine engine component, the electrothermal heater
assembly comprising: a metallic heating element having first and
second surfaces, a watt density in a range of about 1 to about 50
watts/in.sup.2, and a thickness of about 0.5 to about 40 mils; and
a first densely woven fabric layer adhered to the first surface the
metallic heating element and impregnated with a resin capable of
withstanding a heater assembly operating temperature of up to
550.degree. F., wherein the fabric layer includes a fabric material
selected from a group consisting of a fiberglass fabric, a ceramic
fiber fabric, polymer film, and combinations thereof, the first
fabric layer being positioned to make direct contact with a portion
of the gas turbine engine component.
26. The heater assembly of claim 25, wherein the fabric layer is
less than about 0.005 inches thick.
27. The heater assembly of claim 25, wherein the metallic heating
element is a titanium heating element.
28. The heater assembly of claim 25, wherein the fabric layer
exhibits a thermal conductivity value of about 10.1
BTU-in/hr-ft.sup.2-.degree. F.
29. The heater assembly of claim 25, wherein the fabric material
accounts for about 45 to about 70 percent by volume of the fabric
layer.
30. The heater assembly of claim 25, and further comprising: a
second densely woven fabric layer positioned adhered to the second
surface of the metallic heating element and impregnated with a
resin capable of withstanding a heater assembly operating
temperature of up to 550.degree. F., wherein the fabric layer
includes of a fabric material selected from a group consisting of a
fiberglass fabric, a ceramic fiber fabric, a polymer film, and
combinations thereof.
Description
BACKGROUND
[0002] The present invention relates to a heater assembly. More
particularly, the present invention relates to an electrothermal
heater assembly that is suitable for removing and/or preventing ice
accumulation on a gas turbine engine component.
[0003] It is desirable to minimize or prevent the formation of ice
on certain components of a gas turbine engine in order to avoid
problems attributable to ice accumulation. For example, if ice
forms on air intake components, the flow of air into the gas
turbine engine compressor may become obstructed, which then
adversely affects engine operation and efficiency. Furthermore,
chunks of ice that break loose from a gas turbine engine component
during operation can damage other parts of the engine.
[0004] There are many existing methods of removing or preventing
the formation of ice on gas turbine engine components. Among these
methods is the incorporation (or embedding) of an electrothermal
heating element into a gas turbine engine component that is
susceptible to ice formation. The heating element may also be
applied to a surface of the component. The heating element heats
the susceptible areas of the component in order to help prevent ice
from forming. The heating element may be a metallic heating element
(e.g., a foil element) formed of stainless steel, copper, wire
cloth, etc., which typically converts electrical energy into heat
energy.
[0005] The metallic heating element is typically a part of a heater
assembly that also includes a thermally conductive fabric layer
attached to and supporting the heating element. For example, the
heater assembly may be formed of a metallic heating element
embedded into an epoxy fiber reinforced composite structure. In
some cases, the fabric layer also electrically insulates an
electrically conductive component from the heating element.
Typically, multiple plies of fabric are required for sufficient
electrical isolation of the metallic heater element.
[0006] When the heater assembly is embedded in a composite
structure of some gas turbine engine components, the heater
assembly replaces some structural elements of the composite in
order to maintain the dimensions of the component. In those cases,
the heating element accounts for a percentage of the composite
structure that forms the component. This may affect the strength
and the structural characteristics, such as the transfer of
structural loads, of the component. The larger the percentage the
heater assembly constitutes, the larger the reduction in composite
strength of the gas turbine engine component.
[0007] In order to increase the strength of the component that
includes the heater assembly, it is desirable to reduce the amount
of space the heater assembly takes up in the component. One way of
achieving the reduction in space is by reducing the thickness of
the heater assembly.
BRIEF SUMMARY
[0008] The present invention is a heater assembly suitable for
deicing and/or anti-icing a gas turbine engine component. The
heater assembly includes a metallic heating element and a densely
woven fabric layer impregnated with a high-temperature resin
capable of withstanding temperatures of up to 550.degree. F.
(288.degree. C.).
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1A is a schematic cross-sectional view of a heater
assembly in accordance with a first embodiment of the present
invention.
[0010] FIG. 1B is a schematic cross-sectional view of a heater
assembly in accordance with a second embodiment of the present
invention.
[0011] FIG. 1C is a schematic cross-sectional view of a heater
assembly in accordance with a third embodiment of the present
invention.
[0012] FIG. 2 is a perspective view of an airfoil, which is
cut-away to show a heater assembly that is embedded along a leading
edge of the airfoil.
[0013] It should be understood that the figures are not drawn to
scale.
DETAILED DESCRIPTION
[0014] The present invention is an electrothermal heater assembly
that includes a metallic heating element and a densely woven fabric
layer impregnated with a high temperature resin that is capable of
withstanding operating temperatures of up to 550.degree. F.
(288.degree. C.). The resin reinforces the fabric layer. The
metallic heating element is attached to the fabric layer using a
thermoset adhesive and is electrically connected to a source of
electrical power using any suitable conductor, such as a wire or
flexible circuit. In one embodiment, the resin that is introduced
into the fabric layer is also the thermoset adhesive that adheres
the metallic heating element to the fabric layer.
[0015] The heater assembly of the present invention is suitable for
incorporating (or embedding) into a composite structure of a
component (i.e., an internal application), including a gas turbine
engine component, or for attaching to a surface of a component
(i.e., an external application) in order to deice the component
and/or prevent ice from forming thereon. The heater assembly may
also be used in a hybrid configuration, which includes both
internal and external applications. The component may be any
component that is susceptible to ice formation. For example, the
component may be an aircraft component or a gas turbine engine
component such as, but not limited to, a vane, an airfoil leading
edge, a front bearing of the engine, a structural strut that
supports the front bearing, and a duct. The component may be formed
of materials such as, but not limited to, fiberglass, metal, or
carbon composite.
[0016] FIG. 1A is a schematic cross-sectional view of
electrothermal heater assembly 10 in accordance with the present
invention. Heater assembly 10 includes densely woven fabric layer
12 and metallic heating elements 14, which are attached to densely
woven fabric layer 12 with thermoset adhesive 16. Fabric layer 12
is impregnated with a high-temperature resin that is capable of
withstanding temperatures of up to 550.degree. F. (288.degree. C.).
Examples of suitable high-temperature resins that may be used in
accordance with the present invention include, but are not limited
to, bismaleimide, phthalonitrile, cyanate ester, polyimide
adhesive, and polyimide resin.
[0017] Fabric layer 12 includes about 45 to about 70 percent by
volume of a fabric and about 30 to about 55 percent by volume of
the high-temperature resin. In one embodiment, fabric layer 12
includes about 55 to about 60 percent by volume of the fabric and
about 40 to about 45 percent by volume of the high-temperature
resin. Suitable fabrics for including in fabric layer 12 include
densely woven materials that have a continuous fiber. Preferably,
the fabric is not easily distorted and maintains its weave pattern
prior to and during the introduction of resin into the fabric
during manufacture of heater assembly 10. Examples of suitable
densely woven materials that may be used include a fiberglass
fabric, such as Style 106, which is made commercially available by
Clark Schwebel Tech-Fab Company of Anderson, S.C., and a polymer
film, such as Kapton, which is made commercially available by
DuPont High Performance Materials of Circleville, Ohio.
[0018] Fabric layer 12 acts as a backing material to support
heating elements 14, and in some embodiments, also acts as a
structural element of a component (if heater assembly 10 is
embedded in the component). Because fabric layer 12 supports
heating elements 14, it may also be referred to as a "structural
layer." Some heating elements 14 require a backing material because
they are thin and fragile and, as a result, cannot be easily
handled during the manufacturing process. For example, heater
assembly 10 may be etched into a shape prior to application in or
on a component. The shape typically depends upon the type of
component and the area of the component that requires deicing
and/or anti-icing. Some of these fragile heating elements 14 tend
to break apart during the etching process without a backing
material (i.e., fabric layer 12). Fabric layer 12 contributes to
the mechanical integrity of heating elements 14.
[0019] In comparison to many fabric layers in existing heating
assemblies, fabric layer 12 of the present invention is load
bearing and more stiff, due to the type of the fabric that is
selected for including in fabric layer 12. As a result, if heater
assembly lo is embedded into a component, fabric layer 12
contributes to the structural integrity of the component and can
act as a structural element of the component, rather than merely
taking up space in the component that could be occupied by a
structural element. The fabric material included in fabric layer 12
may also constitute all or substantially all of the structural
material in a composite component, such as a vane.
[0020] In some embodiments, fabric layer 12 is also electrically
insulating and configured to electrically insulate an electrically
conductive component, such as one formed of a carbon composite or a
metal alloy, from metallic heating elements 14, while at the same
time, thermally conduct heat generated by heating elements 14. In
situations where fabric layer 12 also electrically insulates
heating elements 14, it is desirable for the fabric material
forming fabric layer 12 to be woven tightly enough to be
electrically insulating. Electrically insulating materials that may
be used to form fabric layer 12 include fiberglass, Nextel or
another suitable ceramic fiber fabric.
[0021] A thickness of heater assembly 10 is minimized because
fabric layer 12 is thinner than many existing heating assemblies,
which include structural layers that are about 0.020 inches (0.0508
centimeters) thick. In contrast, heater assembly 10 of the present
invention includes fabric layer 12 that is less than about 0.005
inches (0.0127 centimeters) thick. In one embodiment, fabric layer
12 is about 0.003 inches (0.00762 centimeters) to about 0.005
inches (0.0127 centimeters) thick. Given the increased structural
integrity of fabric layer 12, and in some embodiments, its ability
to electrically insulate heating elements 14, it has been found
that only one layer of material is typically required to form
fabric layer 12. Of course, fabric layer 12 may also be formed of
multiple layers of material.
[0022] If fabric layer 12 is also an electrically insulating layer,
the thickness of fabric layer 12 varies depending on the voltage
feeding heating elements 14. When heater assembly 10 is embedded in
a component, the amount of structural material of the component
that is displaced by heater assembly 10 is reduced because the
thickness of heater assembly 10 is reduced. Furthermore, as
previously discussed, fabric layer 12 is also load bearing, and in
some embodiments, is a suitable structural substitution for
structural elements of the component. As a result, the component is
more structurally sound than a similar component that incorporates
an existing heater assembly. Reducing a thickness of fabric layer
12 of heater assembly 10 helps reduce the weight of heater assembly
10, which may be desirable in the case of gas turbine engine
components, which are used in aircrafts.
[0023] Those skilled in the art recognize that it is important for
heater assembly 10 to distribute heat substantially evenly. Thermal
conductivity of fabric layer 12 contributes to the even
distribution of heat that is generated by heating elements 14. In
the embodiment of FIG. 1A, fabric layer 12 exhibits a thermal
conductivity value of 10.1BTU-in/hr-ft.sup.2-.degree.F. (1.45
W/m-K).
[0024] In many existing heating assemblies that include structural
layers having a thickness of about 0.020 inches (0.0508
centimeters) or greater, the heating assemblies include thick
heating elements in order to achieve the necessary operating
temperatures through the thick fabric layers. It has been found
that is difficult to incorporate these heating assemblies into a
component having a small radius (e.g., less than about 0.50 inches
(1.27 centimeters)) because the thick heating elements fracture
upon bending around a radius of less than about 0.50 inches (1.27
centimeters). Heater assembly 10 in accordance with the present
invention, however, is more flexible than many of these existing
heating assemblies due to the thinner fabric layer 12. In
combination with suitably thin metallic heating elements 14 (e.g.,
about 0.5 mils (.00127 centimeters) to about 40 mils (0.1016
centimeters)), heater assembly 10 can be used with components
having a small radius while retaining the full capability of heater
assembly 10.
[0025] Metallic heating elements 14 are resistive heating elements,
such as titanium, stainless steel, copper or wire cloth heating
elements, which convert electrically energy into thermal heat, as
is known in the art. Although FIG. 1A illustrates four heating
elements 14, heater assembly 10 may include any number of heating
elements 14, and those skilled in the art can modify the number of
heating elements depending upon the specific application of heater
assembly 10. Heating elements 14 are each electrically connected to
an electrical power source using any suitable conductor, such as a
wire or a flexible circuit. Various power arrangements may be used
to provide power to heating elements 14 of heater assembly 10. For
example, heating elements 14 may be electrically connected to one
another, or each of the heating elements 14 may be separately
electrically connected to the power source. The electrical energy
may be intermittently or continuously supplied to heating elements
14, depending upon whether a deicing or anti-icing function is
desired. Typically, in the case of a deicing function, power is
intermittently supplied to heating elements 14, whereas in an
anti-icing function, power is continuously supplied to heating
elements 14.
[0026] Metallic heating elements 14 each include a watt density in
a range of about 1 to about 50 watts/in.sup.2(7.75 watts/cm.sup.2).
The watt density, however, varies depending on the particular
application of heater assembly 10. Similarly, in some embodiments,
the watt density varies between heating elements 14. In some
situations, it may be desirable for more heat to be applied to one
area than to an adjacent area. This is often referred to as "zone"
heating. To achieve zone heating, heater assembly 10 includes at
least two heating elements 14 having different watt densities. The
zone heating may require a secondary power distribution system in
order to vary the power distribution between heating elements
14.
[0027] Metallic heating elements 14 are capable of operating at
temperatures of up to 550.degree. F. (288.degree. C.) due to the
high-temperature resin that impregnates fabric layer 12. Many
existing heating assemblies include a structural layer impregnated
with a resin, such as epoxy, that is unable to withstand
temperatures greater than 300.degree. F. (148.89.degree. C.). In
those cases, the integrity of the fabric layer is compromised and
the heating elements may become detached from the fabric layer if
the heater assembly operates at temperatures greater than
300.degree. F. (148.89.degree. C.) and if the heater assembly is
exposed to temperatures greater than 300.degree. F. (148.89 C.).
The high-temperature resin in heater assembly 10, however, is able
to withstand temperatures of up to 550.degree. F. (288.degree. C.),
thereby helping to maintain the integrity of heater assembly 10 at
temperatures greater than 550.degree. F. (288.degree. C.).
[0028] The ability of heater assembly 10 to withstand and operate
at higher temperatures increases the number of applications heater
assembly 10 may be used in because heater assembly 10 may be used
to deice/anti-ice gas turbine engine components that operate at
temperatures greater than 300.degree. F. (148.89.degree. C.). Many
existing heating assemblies that use resin that cannot withstand
temperatures greater than about 300.degree. F. (148.89.degree. C.)
will fail and be unable to deice and anti-ice components that
operate at higher operating temperatures (i.e., the temperatures
between 300.degree. F. (148.89.degree. C.) and up to 550.degree. F.
(288.degree. C.)).
[0029] In one method of forming heater assembly 10, a layer of
thermoset adhesive 16 is applied to a ply of high-density material
that forms fabric layer 12. Heating elements 14 are then positioned
on the layer of thermoset adhesive 16 and positioned with respect
to one another as desired. High-temperature resin is then injected
into the material, thereby impregnating the material with the resin
and forming fabric layer 12. The resin and thermoset adhesive are
then cured, and as a result, heating elements 14 are adhered to
fabric layer 12 after the curing step. The resulting heater
assembly 10 may then be etched into a shape suitable for the
specific application of heater assembly 10.
[0030] FIG. 1B is a schematic cross-sectional view of
electrothermal heater assembly 20 in accordance with a second
embodiment of the present invention. Heater assembly 20 includes
densely woven fabric layer 22, which is impregnated with a
high-temperature resin, and metallic heating elements 24, which are
adhered to fabric layer 22 with the high-temperature resin. Fabric
layer 22 is similar to fabric layer 12 of heater assembly 10 of
FIG. 1, and heating elements 24 are similar to heating elements 14
of heater assembly 10. Rather than using a separate layer of
adhesive (e.g., adhesive 16 of heater assembly 10 of FIG. 1A), the
high-temperature resin that impregnates fabric layer 22 also
adheres metallic heating elements 24 to fabric layer 22. Although
the high-temperature resin adheres metallic heating elements 34 and
fabric layer 22, the resin is not a distinct layer. In one method
of forming heater assembly 20, heating elements 24 may be
positioned next to a ply of material that forms fabric layer 22. A
high-temperature resin is then injected into the ply of material
and the resin is cured (thereby forming fabric layer 22). After
fabric layer 22 is injected with resin, resin is present along
surface 22A of fabric layer 22, on which heating elements 24 are
positioned. Therefore, after the resin on surface 22A is cured, the
cured resin along surface 22A acts as an adhesive to adhere heating
elements 24 to fabric layer 22.
[0031] FIG. 1C is a schematic cross-sectional view of
electrothermal heater assembly 30 in accordance with a third
embodiment of the present invention. Heater assembly includes
fabric layers 32 and 36 and heating elements 34, which are adhered
to fabric layers 32 and 36 with a high-temperature resin. Fabric
layers 32 and 36 are similar to fabric layers 12 and 22 of FIGS. 1A
and 1B, respectively, and heating elements 34 are similar to
heating elements 14 and 24 of FIGS. 1A and 1B, respectively. Heater
assembly 30 is similar to heater assembly 20 of FIG. 1B, except
that second fabric layer 36 is adhered to heating elements 34. If
fabric layers 32 and 36 are electrically insulative and heater
assembly 30 is embedded into or attached to an external surface of
electrically conductive component, fabric layers 32 and 36
electrically insulate the electrically conductive component from
heating elements 34.
[0032] A method similar to the method discussed above in reference
to heater assembly 20 may be used to form heater assembly 30.
However, a second ply of material is positioned adjacent to heating
elements 34 so that heating elements 34 are "sandwiched" between
the plies of material. The high-temperature resin is then injected
into the plies of material and the resin is cured. The cured resin
adheres heating elements 24 to fabric layer 36.
[0033] The first embodiment of heater assembly 10 (FIG. 1A) may
also be modified to include a second fabric layer to electrically
insulate heating elements 14. However, because a separate thermoset
adhesive 16 is used in heater assembly 10, a second layer of
thermoset adhesive is used to adhere heating elements 14 to the
second fabric layer.
[0034] A heater assembly in accordance with the present invention
(e.g., heater assemblies 10,20 and 30) may be embedded in a
composite component. FIG. 2 illustrates an embodiment of a
composite component that includes a heater assembly. FIG. 2 is a
perspective view of airfoil 40, where a portion of body 41 of
airfoil 40 has been cutaway along leading edge 42 to expose heater
assembly 44. Body 41 of airfoil 40 is a composite structure, and
heater assembly 44 is embedded in body 41 as part of the composite.
Heater assembly 44 is similar to heater assembly 10 of FIG. 1A, and
includes fabric layer 46 embedded with a high-temperature resin and
heating elements 48 (in phantom), which are attached to fabric
layer 46 using a thermoset adhesive. As shown, fabric layer 46 is
positioned between the exterior surface of body 41 and heating
elements 48. However, other configurations are also
contemplated.
[0035] Airfoil 40 is a gas turbine engine component, and may be,
for example, an airfoil in a compressor. If the gas turbine engine
is used in an aircraft, moisture may accumulate on leading edge 42
of airfoil 40, and as the aircraft reaches higher elevations and
the atmospheric temperature decreases, the moisture may turn into
ice. In order to prevent the accumulation of ice: (i.e., anti-ice)
along leading edge 42 of airfoil 40, or remove the ice (i.e.,
deice) therefrom, heater assembly 44 is embedded in leading edge
42. As heating elements 48 receive electrical energy from an
external power source (not shown), heating elements 48 convert the
electrical power into thermal energy, thereby heating leading edge
42 of airfoil 40. Leading edge 42 of airfoil 40 is heated
sufficiently enough to melt any accumulated ice and/or prevent ice
from forming on. leading edge 42.
[0036] The terminology used herein is for the purpose of
description, not limitation. Specific structural and functional
details disclosed herein are not to be interpreted as limiting, but
merely as bases for teaching one skilled in the art to variously
employ the present invention. Although the present invention has
been described with reference to preferred embodiments, workers
skilled in the art will recognize that changes may be made in form
and detail without departing from the spirit and scope of the
invention.
* * * * *