U.S. patent application number 10/689813 was filed with the patent office on 2005-04-21 for laminar air flow, low temperature air heaters using thick or thin film resistors.
Invention is credited to Cooper, Richard, Fich-Pedersen, Thomas.
Application Number | 20050084254 10/689813 |
Document ID | / |
Family ID | 34314185 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050084254 |
Kind Code |
A1 |
Cooper, Richard ; et
al. |
April 21, 2005 |
LAMINAR AIR FLOW, LOW TEMPERATURE AIR HEATERS USING THICK OR THIN
FILM RESISTORS
Abstract
An elongate heating element that includes an insulative
substrate and a resistive coating that heats to a predetermined
temperature when an electrical current is passed through the
coating. The insulative material forming the substrate is
preferably a ceramic such as cordierite, but the invention is not
limited to any particular insulative material. An air moving device
directs an air stream over the resistive coating in order to heat
the air stream.
Inventors: |
Cooper, Richard; (Bend,
OR) ; Fich-Pedersen, Thomas; (Copenhagen S,
DK) |
Correspondence
Address: |
GLENN C. BROWN, PC
777 NW WALL STREET, SUITE 308
BEND
OR
97701
US
|
Family ID: |
34314185 |
Appl. No.: |
10/689813 |
Filed: |
October 20, 2003 |
Current U.S.
Class: |
392/379 ;
219/543; 392/485 |
Current CPC
Class: |
F24H 3/0405
20130101 |
Class at
Publication: |
392/379 ;
219/543; 392/485 |
International
Class: |
F24H 003/02 |
Claims
What is claimed is:
1. A heater comprising: an insulative substrate, the insulative
substrate being at least one hollow tube; a resistive film on the
substrate having first and second spaced apart portions; first and
second connectors in communication with the respective first and
second spaced apart portions of the resistive film, and adapted for
directing an electrical current from an electrical source through
the resistive film; and, an air moving device for directing an air
stream over the resistive films, wherein the at least one hollow
tube includes at least one hollow, conical tube disposed within a
cylindrical hollow tube, each of the conical and cylindrical tubes
having the resistive film formed thereon.
2. A heater according to claim 1 further comprising the insulative
substrate including an insulative film formed on a substrate.
3. A heater according to claim 1 further comprising the insulative
substrate being at least one hollow tube.
4. A heater according to claim 1 further comprising the insulative
substrate being a plurality of hollow tubes.
5. A heater according to claim 3 further comprising a housing and
the at least one hollow tube mounted in the housing.
6. A heater according to claim 4 further comprising a housing and
the plurality of hollow tubes mounted in the housing.
7. A heater according to claim 3 further comprising the housing
having at least one mounting bracket protruding from an inner
surface of the housing and the at least one hollow tube mounted in
the at least one mounting bracket.
8. A heater according to claim 3 further comprising the housing
having a plurality of mounting brackets protruding from an inner
surface of the housing and the plurality of hollow tubes mounted in
the mounting brackets.
9. A heater according to claim 7 further comprising the at least
one mounting bracket in communication with an electrical source and
the first and second resistive film connectors.
10. A heater according to claim 8 further comprising the mounting
brackets in communication with an electrical source and the first
and second resistive film connectors of the plurality of hollow
tubes.
11. A heater according to claim 5 further comprising the at least
one hollow tube having inner and outer heat transfer surfaces, the
outer heat transfer surface comprising the resistive film.
12. A heater according to claim 6 further comprising the plurality
of hollow tubes each having inner and outer heat transfer surfaces,
each outer heat transfer surface comprising the resistive film.
13. A heater according to claim 1 further comprising the hollow
tube having a longitudinal axis and being disposed in the housing
with the longitudinal axis parallel to a direction of airflow
through the housing.
14. A heater according to claim 1 further comprising the hollow
tube having a longitudinal axis and being disposed in the housing
with the longitudinal axis perpendicular to a direction of airflow
through the housing.
15. A heater according to claim 1 further comprising the insulative
substrate including at least one plate.
16. A heater according to claim 1 further comprising the insulative
substrate being a plurality of plates.
17. A heater according to claim 15 further comprising a housing and
the at least one plate mounted in the housing.
18. A heater according to claim 16 further comprising a housing and
the plurality of plates mounted in the housing.
19. A heater according to claim 15 further comprising the housing
having at least one mounting bracket protruding from an inner
surface of the housing and the at least one plate mounted in the at
least one mounting bracket.
20. A heater according to claim 15 further comprising the housing
having a plurality of mounting brackets protruding from an inner
surface of the housing and the plurality of plates mounted in the
mounting brackets.
21. A heater according to claim 19 further comprising the at least
one mounting bracket in communication with an electrical source and
the first and second resistive film connectors.
22. A heater according to claim 20 further comprising the mounting
brackets in communication with an electrical source and the first
and second resistive film connectors of the plurality of
plates.
23. A heater according to claim 15 further comprising the at least
one plate having first and second heat transfer surfaces, the first
heat transfer surface comprising a resistive film.
24. A heater according to claim 16 further comprising the plurality
of plates each having first and second heat transfer surfaces, each
first heat transfer surface comprising the resistive film.
25. A heater according to claim 15 further comprising the plate
disposed in the housing with the first parallel to a direction of
airflow through the housing.
26. A heater according to claim 15 further comprising the plate
being disposed in the housing with the first surface perpendicular
to a direction of airflow through the housing.
27. A heater according to claim 1 wherein the resistive film
comprises a thin film.
28. A heater according to claim 27 wherein the thin film is formed
by a process selected from the group consisting of chemical vapor
deposition and physical vapor deposition methods.
29. A heater according to claim 1 wherein the resistive film
comprises a thick film.
30. A heater according to claim 29 wherein the thick film is formed
by a process selected from the group consisting of screening and
spraying.
31. A heater according to claim 3 wherein the at least one hollow
tube includes at least two concentric tubes, each tube having a
resistive film formed thereon.
32. A heater according to claim 3 wherein the at least one hollow
tube includes at least one hollow, conical tube.
33. A heater according to claim 32 wherein the at least one hollow
tube includes at least one hollow, conical tube disposed within a
cylindrical hollow tube, each of the conical and cylindrical tubes
having a resistive film formed thereon.
34. A heater according to claim 15 wherein the resistive film
comprises a trapezoidal resistive film.
Description
BACKGROUND OF THE INVENTION
[0001] This invention is related to heaters, and in particular to
heaters used to heat streams of air. Air heaters are used to create
hot airflows in appliances such as dryers, room heaters, and other
heating devices. Most air heaters heat airstreams by directing a
flow of air over coiled resistive wires that are electrically
heated to a relatively high temperature; in many cases the heated
coils are red hot. This configuration provides a very high
temperature difference between the wire and the air, and provides
the desired rate of heat transfer into the air stream despite the
relatively small surface area of the wire. In dryers the high
temperatures of the heating elements can cause fire hazards, and in
general it leads to relatively large inefficiencies. As energy
costs continue to rise, the efficiency losses in air heaters
represent a greater and greater disadvantage. A need remains for an
improved technology for heating air in a variety of heating devices
for home and industrial use.
SUMMARY OF THE INVENTION
[0002] This invention serves to eliminate some of the
inefficiencies inherent in current designs by providing air heaters
that can be configured in many different designs and sizes, and in
which the temperature of the heated surface is relatively low
compared to known heating devices. The lower temperature heating
surface is relatively large to provide the necessary heat transfer
to the air. The heating surface is formed of a resistive thick or
thin film deposited over a relatively large area compared to the
wire in the traditional heaters, but which can at the same time be
packaged in a relatively small enclosure. The heating elements of
the present invention can be formed as tubes, plates, and in other
configurations as described in greater detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] FIG. 1 is a schematic depiction of a preferred embodiment of
the invention.
[0004] FIG. 2A is an end view of a second preferred embodiment of
the invention that includes multiple heating tubes of the type
depicted in FIG. 1.
[0005] FIG. 2B is a cross-sectional view of a heater depicted in
FIG. 2B.
[0006] FIG. 3A is an end view of another preferred embodiment of
the invention that includes multiple transverse heating tubes of
the type depicted in FIG. 1.
[0007] FIG. 3B is an end view of another preferred embodiment of
the invention that includes multiple transverse heating tubes of
the type depicted in FIG. 1.
[0008] FIG. 3C is a cross-sectional view of a heater depicted in
FIG. 3B.
[0009] FIG. 4 is a perspective end view of another preferred
embodiment of the invention that utilizes concentric tubular
heating elements.
[0010] FIG. 5A is a perspective end quarter view of another
preferred embodiment of the invention that utilizes a tubular
heating element and in which the ends of the tubular heating
element protrude from the housing.
[0011] FIG. 5B is a side elevational view of the embodiment shown
in FIG. 5A.
[0012] FIG. 6A is a perspective end view of another embodiment of
the invention that includes a conical tubular heater and a
concentric cylindrical tubular heater.
[0013] FIGS. 6B and 6C are partial cutaway views of the embodiment
shown in FIG. 6A.
[0014] FIGS. 7A and 7B are perspective end views of another
preferred embodiment of the invention that utilizes an array of
conical tubular heaters mounted in a housing.
[0015] FIG. 8A is an end perspective view of another embodiment of
the invention in which planar heating elements are mounted in a
housing.
[0016] FIG. 8B is a cutaway side view of the heater shown in FIG.
8A and which illustrates in greater detail a planar heater
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Referring now to FIG. 1, the present invention in its most
general form is embodied in an elongate heating element that
includes an insulative substrate and a resistive coating that heats
to a predetermined temperature when an electrical current is passed
through the coating. The insulative material forming the substrate
is preferably a ceramic such as cordierite, but the invention is
not limited to any particular insulative material. One preferred
embodiment in the form of a heating tube is illustrated in FIG. 1
at 10. This embodiment includes a tube 12 with a resistive layer 14
formed on the outer surface of the tube 12.
[0018] Resistive heating layer 14 is preferably a thick resistive
film such as a graphite based sol gel manufactured by Datec
Corporation of Milton, Ontario, Canada. The sol gel is preferably
screen printed or sprayed onto tube 12 as a liquid, and cured at
350.degree. C. or above. It is then stable in air up to a
temperature of over 350.degree. C. In other embodiments the
resistive film could also be a thin film such as SnO.sub.2:F
deposited by an evaporation process like PVD or CVD.
[0019] Electrical terminals 16 and 18 are formed at each end of the
resistive layer 14. Electrical terminals 16 and 1 8are preferably
formed of silver and are positioned along the left and right edges
of the resistive film before curing, and are bonded to the sol gel
during curing. Electrical terminals 16 and 18 are formed by
applying a curable silver-containing emulsion such as DuPont No.
7713. The buses could also be formed of other conductive metals
such as aluminum or copper applied in ways familiar to those of
skill in the art.
[0020] When a voltage is applied to the terminals 16 and 18 the
resulting electrical current heats the resistive layer 14 and tube
12.
[0021] Turning to FIG. 2, in preferred embodiments a number of
tubes 12 are placed inside a housing 20 through which the air is
caused to flow. In this manner the heat is transferred by
convection to the bypassing air. The heater tubes 12 can be placed
either with their axis parallel to the flow direction, as
illustrated in FIGS. 2Band 2B, or with the tube axis perpendicular
to the air flow as shown in FIGS. 3A-3C. Referring to FIGS. 2A and
3C, tubes 12 are mounted in housing 20 by in terminal clamps 22
with each tube 12 being engaged with two clamps 22, one each on
terminal 16 or 18 of each tube 12. Each terminal clamp 22 is
mounted to the inner surface 21 of housing 20 by a connector 26
which extends through the housing 20. Connectors 26 are connected
to an external electrical source 28 to provide an operating current
for the heater. In the embodiments shown in FIGS. 3A-3C, the tubes
12 are mounted in a similar manner in a transverse orientation to
the air flow through housing 20.
[0022] In another embodiment as shown in FIG. 4A, two concentric
tubular heating elements 10a and 10b are mounted in a housing 20 in
a manner similar to that described above using clamps 22 and
connectors 26. Each tubular heating element includes a resistive
layer as described above.
[0023] FIGS. 5A and 5B illustrate another embodiment in which
terminals 16 and 18 of heating element 10 extend beyond the ends of
housing 20, and clamps 22 are disposed outside housing 20.
[0024] In other preferred embodiments of the invention, one or more
of heating elements 10 is conical rather than tubular. FIGS. 6A-6C
schematically illustrate one embodiment of the invention that
incorporates concentric heating elements, an inner conical element
60 and an outer tubular heating element 62. Both include a
resistive layer as described above. Applicants have found that the
conical shape of heating element 60 provides enhanced heat transfer
from element 60 to the surrounding airflow, and at the same time
promotes increased heat transfer from outer tubular heating element
62 to the surrounding airflow. Each heating element is mounted to
housing 20 in the manner described in the embodiments described
above using clamps (not shown) and electrical connectors (not
shown) passing through the housing 20.
[0025] The advantages of a conical heating element configuration
can also be exploited by mounting multiple conical heaters within a
housing as shown in FIGS. 7A-7C. Each heating element 70 is
generally as described above with reference to FIG. 1, and includes
an insulative conical member having a resistive layer and terminals
formed on its outer surface. Each heating element 70 is mounted in
housing 72 by mounting brackets 74, which can include a clamp at
its inner end to receive the terminal of the heating element as
described above, and which passes through housing 72 and is
connected to an electrical source (not shown).
[0026] Further to the use of the conical or trapezoidal designs, a
cone, which is a trapezoid rolled up, changes the number of squares
in the heater which affects the watt density. At the inlet end of
the cone or planar substrates the width is narrower resulting in a
higher watt density and more energy per square unit. This results
in higher heat at the beginning of the structure and lower heat
(watt density) at the downstream part of the structure. The film
thus does not get overheated, resulting in inefficiency as the air
moves down the structure.
[0027] The number of squares can be calculated by dividing length
by width. Length is direction of current flow (bus to bus). The bus
to bus resistance is equal to the sheet resistance times the number
of squares. The sheet resistance is calculated by dividing the bus
to bus by the number of squares. The power in watts is equal to the
voltage squared times the width divided by the resistance (in ohms)
per square X length.
[0028] Referring to FIGS. 8A and 8B, another preferred embodiment
of the invention is illustrated. Multiple planar heating elements
80 are mounted in a housing 82. Each heating element includes an
insulative substrate 84, preferably formed of a mica material, and
a resistive heating layer 86 formed over the insulative substrate
84, on either one or both sides of heating element 80. Terminals 83
and 85 are formed at respective ends of each resistive layer 86 for
connection of the heater to an electrical source. Insulative
substrate 84 is not limited to mica, but could also be formed of
any suitable insulative material. Resistive layer 86 can be formed
of any suitable thick or thin film resistive material as described
above with reference to FIG. 1. In the embodiment shown, resistive
layer 86 is formed in a trapezoidal shape to optimize the heat
transfer from heating element 80 to the surrounding airflow, but
could also be square or rectangular.
[0029] The unexpected advantage demonstrated by a heater according
to this invention is a very high thermal efficiency compared to
conventional coiled wire heaters. The following examples
demonstrate the improved efficiency achieved in tests of heaters
according to the invention.
[0030] In on example, a heater similar to that shown in FIG. 2A-2B
was compared to a conventional resistive wire air heater. The
resistive wire air heater consisted of a tubular housing having a
diameter of 21/2" inches and 8 inches in length, and in which was
mounted a coiled resistive wire heating element. Ambient air at
71.degree. F. was introduced at a rate of 60 cfm. Air exited the
heater at 138.degree. F., and the heater demonstrated an efficiency
of 43%. The efficiency was calculated by dividing the energy
transferred to the air stream by the electrical energy provided to
the heater which was 870 Watts. A heater according to the present
invention, and generally configured as shown in FIG. 2A-B, was then
tested.
[0031] The heater was comprised of 7 tubes with a thick film
coating. The tubes were 1/2" O.D..times.3/8" I.D and 4 inches Long.
They were mounted in a pipe (duct) 21/2" I.D.times.8 inches long.
The heater according to the present invention heated a 60 CFM flow
of ambient air from 71.degree. F. to 138.degree. F. utilzing 515
Watts with an efficiency of 60%, an increase of approximately 40%
over the resistive wire heater. This increase in efficiency is
highly surprising, and represents a significant advance over
resistive wire heaters.
[0032] While the invention has been described by reference to the
embodiments described above, the description of the preferred
embodiments is intended to be illustrative and not limiting to the
following claims. Those of skill in the art will recognize that
numerous changes in detail and arrangement are possible without
departing from the scope of the claims.
* * * * *