U.S. patent application number 09/754467 was filed with the patent office on 2002-07-04 for fluid heat exchanger.
This patent application is currently assigned to Tamin Enterprises. Invention is credited to Isaac, Donald JR..
Application Number | 20020084065 09/754467 |
Document ID | / |
Family ID | 25034913 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020084065 |
Kind Code |
A1 |
Isaac, Donald JR. |
July 4, 2002 |
Fluid heat exchanger
Abstract
An annular heat exchanger suitable for a Stirling engine is
provided. The heat exchanger has helical fins, and an outer
reinforcing sleeve about the fins. The sleeve improves the pressure
resisting ability of a thin separating wall between a pressurized
fluid and an outside working environment, resulting in a
high-pressure heat exchanger with high heat transfer efficiency. In
addition, the sleeve and helical fins together define fluid
passages for the flow of heating fluid. The heat exchanger
according to the invention has the ability to resist high pressures
at high temperatures without distortion, has improved heat transfer
capability, better reliability, and lower production cost than
prior art heat exchangers.
Inventors: |
Isaac, Donald JR.; (Half
Moon Bay, CA) |
Correspondence
Address: |
David P. Gordon, Esq.
65 Woods End Road
Stamford
CT
06905
US
|
Assignee: |
Tamin Enterprises
|
Family ID: |
25034913 |
Appl. No.: |
09/754467 |
Filed: |
January 4, 2001 |
Current U.S.
Class: |
165/156 ;
165/155; 60/912 |
Current CPC
Class: |
F28F 1/36 20130101; F28D
7/12 20130101; F02G 1/053 20130101 |
Class at
Publication: |
165/156 ;
165/155; 60/912 |
International
Class: |
F28D 007/12 |
Claims
1. A heat exchanger for exchanging heat with a working fluid,
comprising: a) a tubular wall having an interior surface and an
exterior surface, said interior surface in contact with the working
fluid; b) a plurality of external fins about said exterior surface
of said wall; c) a tubular sleeve having an interior surface about
and in contact with said external fins; and d) a heat exchange
fluid in contact with said tubular wall, said external fins, and
said interior surface of said sleeve, said heat exchange fluid
adapted to exchange heat with the working fluid.
2. A heat exchanger according to claim 1, wherein: said external
fins are helical about said exterior surface of said tubular
wall.
3. A heat exchanger according to claim 1, wherein: said external
fins are one of annular and axial relative to said wall.
4. A heat exchanger according to claim 1, wherein: said external
fins taper in thickness from said tubular wall to said sleeve.
5. A heat exchanger according to claim 1, wherein: said wall, said
external fins, and said sleeve together define a plurality of fluid
passages for said heat exchange fluid.
6. A heat exchanger according to claim 1, wherein: said wall and
said external fins are a unitary construct.
7. A heat exchanger according to claim 1, wherein: said external
fins and said sleeve are a unitary construct.
8. A heat exchanger according to claim 1, wherein: said external
fins and said sleeve are made from stainless steel.
9. A heat exchanger according to claim 1, further comprising: e)
internal fins in contact with said interior surface of said tubular
wall and the working fluid.
10. A heat exchanger according to claim 9, wherein: said internal
fins are axial relative to said wall.
11. A heat exchanger according to claim 9, wherein: said internal
fins and said wall are a unitary construct.
12. A heat exchanger according to claim 9, wherein: said external
fins and said internal fins are in an orthogonal grillage
relationship.
13. A Stirling engine, comprising: a) a cylinder containing a
working fluid; b) a first heat exchanger means for heating said
working fluid; c) a second heat exchanger means for cooling said
working fluid; d) a regenerator for storing heat energy released by
the working fluid; and e) a means for converting a pressure wave in
said working fluid into mechanical energy, wherein at least one of
said first and second heat exchangers includes (i) a tubular wall
having an exterior surface, and (ii) a plurality of helical fins
about said exterior surface of said wall.
14. A Stirling engine according to claim 13, wherein: said at least
one of said first and second heat exchangers further includes,
(iii) a tubular sleeve about and in contact with said helical fins,
wherein said wall, said fins and said sleeve together define a
plurality of helical fluid passages.
15. A Stirling engine according to claim 14, wherein: said fins and
said sleeve are a unitary construct.
16. A Stirling engine according to claim 13, wherein: said at least
one of said first and second heat exchangers further includes,
(iii) a plurality of internal fins, wherein said wall and said
internal fins are a unitary construct.
17. A Stirling engine, comprising: a) a cylinder containing a
working fluid; b) a first heat exchanger means for heating said
working fluid; c) a second heat exchanger means for cooling said
working fluid; d) a regenerator for storing heat energy released by
the working fluid; and e) a means for converting a pressure wave in
said working fluid into mechanical energy, wherein at least one of
said first and second heat exchangers includes (i) a tubular wall
having an exterior surface, (ii) a plurality of external fins about
said exterior surface of said wall, and (iii) a tubular sleeve
about and in contact with said fins.
18. A Stirling engine according to claim 17, wherein: said external
fins are one of annular, axial, and helical relative to said
wall.
19. A Stirling engine according to claim 17, wherein: said wall,
said external fins, and said sleeve together define a plurality of
fluid passages.
20. A heat exchanger according to claim 17, wherein: said tubular
wall has an interior surface in contact with a plurality of
internal fins.
21. A heat exchanger according to claim 10, wherein: said wall,
said external fins, said interior fins, and said sleeve are of a
unitary construct.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates broadly to heat exchange devices.
More particularly, the invention relates to devices that exchange
heat energy from one fluid to another where one or both fluids may
be pressurized (above or below atmospheric pressure) and do not
mix, such as in a Stirling engine.
[0003] 2. State of the Art
[0004] Frequently heat energy must be exchanged between two or more
fluids which do not mix and which may be flowing or stagnant. The
heat energy is transferred from the hotter fluid to a separating
wall by convection and/or radiation. Heat energy is conducted
through the wall from the hot side to the cold side. Heat energy is
then transferred from the separating wall to the cooler fluid by
convection and/or radiation. The purpose of the heat exchanger may
be to raise the temperature of a relatively cool fluid (as a
heater) or to lower the temperature of a relatively hot fluid (as a
cooler).
[0005] Except for radiative only heat exchangers, all heat
exchangers have large surfaces where heat energy is absorbed or
given off by the surface contacted by the fluids. There are
basically three types of fluid heat exchangers for Stirling engines
defined by the fluid interfacing configurations. Heat exchangers
for Stirling engines may be annular, finned, or tubular, or various
combinations of these. Annular heat exchangers consist of
concentric tubes with the fluids contained in or between them. The
tubes may be cylindrical or of other closed cross sections. One
tube separates the fluids and provides the surface area and
conductive path required for heat exchange. Finned heat exchangers
increase the surface area exposed to one or both fluids by
providing finned structures on one or both sides of the wall, which
effectively increase the surface area of the wall thus improving
heat transfer. Tubular heat exchangers contain one fluid within
relatively small diameter tubes that are surrounded by the other
fluid. Heat is conducted through the tube wall. Various
combinations of these three types may also be used in a heat
exchanger. For example, fins may be added to the tubes of an
annular heat exchanger to increase the contacted surface area.
[0006] Annular (with and without fins) and tubular heat exchangers
have been used for Stirling engines. Tubular heat exchangers (with
and without fins) have been traditionally used for engines with
power outputs greater than 1 kW mechanical. Many small diameter
tubes provide large surface area and the small diameters have lower
stress at high pressures. Tubular heat exchangers are the most
expensive to produce and are susceptible to burnout due to uneven
heating and high stresses at the attachment points due to thermal
expansion deformation of long tubes.
[0007] Often one or more of the fluids may be pressurized to a
relatively high level. In such case, the separating wall must
structurally resist the difference in pressure between the fluids.
For high heat exchanger efficiency, large fluid contacted surfaces
and low thermal resistance through the separating wall are desired.
Low thermal resistance is achieved by using a thin separating wall,
large contact area, and a material with high thermal conductivity.
On the other hand, high structural strength to resist deformation
by pressure is achieved by using thick walls, small surface areas,
and high strength materials. In general materials with high thermal
conductivity do not have high strength and high strength materials
have low thermal conductivity. Thus, the desired characteristics of
heat exchanger designs assuring high thermal efficiency and high
strength conflict.
[0008] With particular reference to Stirling engines, such engines
are typically provided with four heat exchangers: a heater, a
regenerator, a cooler, and an exhaust/inlet air preheater. A more
detailed explanation of the respective functions of the heat
exchangers of Stirling engines can be found in G. Walker in
"Stirling Engines", Clarendon Press, 1980, pp. 124-126, 133-144,
and 156-159, which is hereby incorporated by reference herein in
its entirety. The above described annular, tubular, and finned heat
exchangers, as well as combinations thereof, have all been used in
various Stirling engines for heaters and coolers. For example, U.S.
Pat. No. 4,671,064, which is hereby incorporated by reference
herein in its entirety, describes an annular heat exchanger for a
Stirling engine. C. M. Hargreaves in "The Philips Stirling Engine",
Elsevier, 1991, pp. 185-187, describes finned heat exchangers
(referred to as "concertina" and "partition" heaters) in Stirling
engines.
[0009] For maximum efficiency, the Stirling engine working fluid
temperature should be as high (as close to the heating fluid
temperature) as possible at the heater and as low (as close to the
cooling fluid temperature) at the cooler as possible. For maximum
power production, the working fluid pressure should be as high as
possible. This requires high thermal conductivity of the wall
separating the fluids and high strength at the operating
temperature. Heating fluid temperature should be as high as the
heat exchanger construction material can withstand at the working
fluid pressure.
[0010] One manner of increasing the pressure-resisting strength of
a pressure vessel is to use "orthogonal grillage" about a
separating wall; i.e., providing straight internal fins parallel to
the cylinder axis combined with disk-like external fins
perpendicular to the axis and integral to the separating wall. The
straight and disk-like fins cross each other at right angles.
"Orthogonal grillage" is described in more detail in J. F. Harvey
in "Theory and Design of Modern Pressure Vessels", 2.sup.nd Ed.,
Van Norstrand Reinhold, 1974, pp. 120-122, which is hereby
incorporated by reference herein in its entirety. However,
orthogonal grillage has the disadvantage in that it is complicated
and difficult to move a heating fluid around the pressure vessel to
permit the heat exchange.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the invention to provide a heat
exchanger for heating a fluid in a high pressure vessel.
[0012] It is another object of the invention to provide a heat
exchanger which has a relatively high structural integrity.
[0013] It is a further object of the invention to provide a heat
exchanger through which it is relatively easy to circulate heating
fluid.
[0014] It is an additional object of the invention to provide a
heat exchanger which has a high heat transfer efficiency.
[0015] It is also an object of the invention to provide a heat
exchanger which is relatively light weight.
[0016] It is still another object of the invention to provide a
heat exchanger which is relatively inexpensive to manufacture.
[0017] It is yet another object of the invention to provide a heat
exchanger for a Stirling engine.
[0018] In accord with these objects, which will be discussed in
detail below, an annular heat exchanger having helical fins is
provided. According to preferred aspect of the invention, an outer
reinforcing sleeve is provided about the helical fins. The sleeve
improves the pressure resisting ability of a thin separating wall
(e.g., the heater wall of a Stirling engine) resulting in a
high-pressure heat exchanger with high heat transfer efficiency. In
addition, the sleeve and helical fins together define fluid
passages for the flow of a heating fluid.
[0019] The heat exchanger according to the invention has an ability
to resist high pressures at high temperatures without distortion,
has an improved heat transfer capability, better reliability, and
lower production cost than prior art heat exchangers.
[0020] Additional objects and advantages of the invention will
become apparent to those skilled in the art upon reference to the
detailed description taken in conjunction with the provided
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a partial cut-away side elevation view of a
Stirling engine according to the invention;
[0022] FIG. 2 enlarged partial cut-away side elevation view of a
hot end heat exchanger and heating fluid passages of a Stirling
engine according to the invention, revealing heating fluid
passages;
[0023] FIG. 3 is a section view across line 3-3 in FIG. 2;
[0024] FIG. 4 is a section view across line 4-4 in FIG. 2; and
[0025] FIG. 5 an enlarged section through a cylinder wall, and heat
wall fins and outer sleeve of the heat exchanger according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Referring now to FIG. 1, a Stirling engine 10 generally
includes a pressure vessel 12, a hot end heat exchanger (heater)
16, a cold end heat exchanger (cooler) 18, a regenerator 20, a
piston 22, a displacer 24, and a crank assembly 25. The pressure
vessel 12 defines a working space containing a pressurized working
fluid (not shown). The heater 16 (described in detail below) adds
heat to the working fluid in the pressure vessel (to increase total
working fluid pressure in the system). The cooler 18 removes heat
from the working fluid (and decreases total working fluid pressure
in the system). The regenerator 20 serves as a thermal storage
medium and increases the engine efficiency by reducing energy
losses as the working fluid is alternately transferred between the
hot and cold ends. The heater 16 is preferably integrated with the
regenerator 20, and both are preferably positioned on top of the
cooler 18.
[0027] The working space, mentioned above, is defined as all of the
space or internal volume occupied by the working fluid, and
includes the fixed internal volumes of the heater 16, regenerator
20, and cooler 18 as well as any connecting ducts or passageways.
The working space also includes a variable compression space 26 and
a variable expansion space 27. The compression space 26 is the
volume contained between the displacer 24 and the piston 22 that
varies as the displacer 24 and piston 22 move axially in a cylinder
29 (discussed below) relative to each other. The expansion space 27
is the volume contained between the displacer 24 and a closed hot
end of the pressure vessel (end cap 38, discussed below).
[0028] The axial position of the displacer 24 in the cylinder 29 is
always ahead of the position of the piston 22 with respect to time.
Oscillating motion of the displacer 24 transfers or displaces
working fluid alternately between the compression space 26 and
expansion space 27. Working fluid flow to and from the compression
space 26 and expansion space 27 must flow through the heater 16,
regenerator 20 and cooler 18.
[0029] In general, the working fluid pressure in the total working
space is uniform at any instant in time. When working fluid flow is
from the regenerator 20, through the heater 16, and into the
expansion space 27, working fluid temperature and pressure increase
and the piston 22 is forced out by having a higher pressure on the
working fluid side than on the opposite side. When working fluid
flow is from the regenerator 20, through the cooler 18, and into
the compression space 26, working fluid temperature and pressure
decrease and the piston 22 returns. Thus, the oscillating motion of
the displacer 24 creates an oscillating pressure wave in the
working fluid that moves the piston 22 in and out. The piston,
acting on crank assembly 25, moves the displacer 24 to provide the
pressure wave and also produces mechanical energy at an output
shaft 28.
[0030] Before explaining the heater 16 of the invention, it is
helpful to more fully understand particular elements of the
pressure vessel 12 containing the working fluid. Referring to FIGS.
2 through 5, the pressure vessel 12 includes the cylinder 29, a
tubular wall 30 about the cylinder, preferably axial internal fins
32 between the cylinder 29 and the wall 30, axial flow fluid
passages 34 bounded by the cylinder 29, wall 30, and internal fins
32 between the cylinder and the wall, a transition cone 36, and an
end cap 38. At the location of the transition cone 36, the cylinder
29 includes radial ports 40 which open into the fluid passages 34,
thereby permitting the working fluid to move from the cylinder 29
to the axial flow fluid passages 34. The pressure vessel also
includes a flange 39 which mates with the cooler 18 and provides a
sealed annular opening at the bottom of the regenerator 20 for
passage of the working fluid between the regenerator and the
cooler.
[0031] The function of the heater 16 is to add heat to the
pressurized working fluid within the axial fluid passages 34. The
heater 16 is an annular heat exchanger which, according to a first
preferred aspect of the invention, has external helical fins 42
integral with the exterior of the wall 30. The helical fins 42
preferably taper away from wall 30. An exemplar size for the fins
includes a width of 0.125" at the root 44a of the fin (against the
wall 30), a width of 0.06" at the tip 42b, and a height 42c of 0.5"
(FIG. 5), though fins of other sizes may be used. It will be
appreciated that because in FIG. 5 the fins are sectioned at an
oblique angle, the exemplar preferred relative dimensions of the
fins are distorted. A preferred lay angle for the helical fins 42
is one revolution every 3.5 inches about a 3.5 inch diameter wall
30. The helical fins 42 increase heat transfer across the wall 30
by effectively increasing the surface area of the wall that can be
wetted (contacted) by the heating fluid. It will be appreciated
that helical fins 42 are longer than either of annular fins or
longitudinal fins, and therefore provide a relatively larger
surface over which heat transfer between the heating fluid and the
working fluid can occur. Longer fins 42 imply longer passages 48
and therefore more time for heat transfer with the heating fluid at
any given heating fluid velocity. Furthermore, the helical fins 42
add substantial structural integrity to the heat exchanger.
[0032] According to a second preferred aspect of the invention, an
outer tubular reinforcing sleeve 44 is attached to the outer edges
of the helical fins 42. The resulting unified construction of the
wall 30, axial fins 32, helical fins 42, and sleeve 44 provides a
composite pressure vessel wall with an effective thickness much
greater than the wall 30 alone; in effect, providing a wall with an
effective thickness approximating the combined material of the
sleeve 44, the helical fins 42, axial fins 32, and the wall 30,
without the weight of a solid wall of that thickness. As such, the
sleeve 44 greatly improves the pressure resisting ability of the
wall 30 resulting in a high-pressure heat exchanger with high heat
transfer efficiency.
[0033] The sleeve 44, transition cone 36, lower portion of end cap
38, and wall 30 define a plenum 46 (FIG. 2) which distributes
heating fluid to numerous inlets of the relatively long helical
fluid passages 48 defined between the sleeve 44, the helical fins
42, and the wall 30. The number of helical fins 42 and passages 48
are optimized according to a particular application, and is based
on factors such as fluid nature (liquid, gas, or a combination),
fluid velocity, temperature, viscosity, etc. The thermal and
structural properties of the wall 30, helical fins 42, axial fins
32, and sleeve 44 determine the optimum dimension of those
components. A preferred material for both of the helical fins and
sleeve is a high temperature metal or alloy, such as stainless
steel.
[0034] The sleeve 44 is preferably permanently bonded to the ends
of the helical fins 42 by welding, casting, brazing, or some other
permanent attachment process. The wall 30, axial fins 32, and
helical fins 42 are also preferably a unitary construction. The
cylinder 12 is optionally permanently bonded to the end of the
axial fins 32 by welding or brazing to increase the pressure
resisting strength of the vessel.
[0035] The heater 16 also includes an insulating barrier 54, an
exhaust cylinder 56, and an insulating wall 58. The insulating
barrier 54 deflects the heating fluid leaving the helical passages
48 at the bottom of the heater and protects the flange 39 and other
engine components from heat. The exhaust cylinder 56 forms an
exhaust passage 60 through which the heating fluid exhausts after
passing through the helical passages 48. The exhaust cylinder can
be insulated or non-insulated. Once heating fluid is exhausted, it
can be directed to another location for use in preheating incoming
fluid at 64 (FIG. 1) or other purposes needing heated fluid. The
insulating wall 58 surrounds the sleeve 44 and insulates the sleeve
from the relatively cooler heating fluid in the exhaust passage 60,
thus maintaining a relatively high temperature at the sleeve.
[0036] The heater 16 is less expensive to produce than the tubular
heat exchangers of the prior art, has increased surface area over
traditional annular heat exchangers of the prior art, and does not
have the thermal expansion and uneven heating problems associated
with tubular heat exchangers.
[0037] In operation, heated fluid is created (e.g., as combustion
gas) at 66 (FIG. 1). The heated fluid enters the Stirling engine
10, surrounds the cap 38 (thereby heating the cap), and enters the
plenum 46 of the heater 16. Because the net heat flow in the
structure composed of the sleeve 44, helical fins 42, and the wall
30 is from the sleeve 44 to the axial fins 32, there is a
temperature gradient where the temperature of the sleeve 44 is
higher than the temperature of the wall 30. As a result, there is
heat transfer from the sleeve 44 to the wall 30 to heat the working
fluid in the axial passages 34 defined by the axial fins 32.
[0038] The work output and efficiency of a Stirling engine are
directly related to the high working fluid pressure and the
temperature differential obtained. In view thereof, it will be
appreciated that the ability of the heat exchanger 16 to operate
under extremely high working fluid pressures (e.g., 150 psi-450 psi
or more) and large temperature differentials (e.g., 1000.degree.
F.) permit the realization of a high efficiency heat exchanger and
enable a relatively high output and particularly efficient engine.
The heat exchanger of the invention can be used anywhere a high
efficiency heat exchanger operating with high-pressure fluid is
needed.
[0039] There have been described and illustrated herein a Stirling
engine and particularly a heat exchanger suitable for a Stirling
engine. While particular embodiments of the invention have been
described, it is not intended that the invention be limited
thereto, as it is intended that the invention be as broad in scope
as the art will allow and that the specification be read likewise.
Thus, while a both helical fins and an outer reinforcing sleeve
have been disclosed on the heat exchanger, it is believed that each
component provides advantage over prior art heat exchanger, and
each component may be used alone without the other. As such, the
external fins may be radial or axial in shape with a reinforcing
sleeve thereabout. Regardless of which shape, it is preferable that
the angle between the internal and external fins should be
relatively large (e.g., 70.degree.-110.degree.) such that the
strengthening advantage of orthogonal grillage is maintained. In
addition, if desired, bumps, wall variations and/or inserts can be
added to the helical passages or axial passages to induce
turbulence in the fluid flows. Also, while a particular heating
fluid (combustion gas) has been disclosed, it will be appreciated
that other heating fluids, in gas and liquid form, may be used as
well. Furthermore, while the axial internal fins are described as
defining axial flow passages, it will be appreciated that such fins
may be radial or helical in shape other shaped fluid passages, as
this may be an advantage in lengthening the working fluid flow path
to give more time for heat exchange at higher fluid velocities. In
addition, the heating fluid direction may be reversed with flow
through the helical fluid passages in the opposite direction. Flow
may also be reversing or oscillating, if desired. Moreover, it will
appreciated that the heat exchanger can be configured as a Stirling
engine cooler. When used as a cooler, the sleeve and helical fins
are preferably made from aluminum. Also, while particular materials
have been disclosed, it will be appreciated that other suitable
materials may be used. It will therefore be appreciated by those
skilled in the art that yet other modifications could be made to
the provided invention without deviating from its spirit and scope
as claimed.
* * * * *