Background of the Invention
Field of the Invention
This invention relates to devices that are intended to transfer heat
from one fluid to another, particularly such devices that are suitable for use
in hot gas engines.
Description of the Prior Art
Heat is frequently needed to be added to, or extracted from a stream
of moving fluid. This is commonly done by heat exchangers which transfer heat
between two streams of moving fluid. A common example is in Stirling or hot-gas
engines which require three heat exchangers called a heater, cooler and regenerator.
The regenerator is a special type of heat exchanger with heat storage capability,
and will not be discussed further. Heaters and coolers have design requirements
which apply to other types of heat exchangers and include: High efficiency of
transfer; controlled resistance to flow of fluid; capability to resist high temperatures
and pressures; and, low cost of construction.
Several types of heater exchangers have been developed to accomplish
the above design criteria. The methods can be characterized as cylinder heating,
finned heat exchangers, tubular heat exchangers, baffle arrangements, and combinations
of the above. Additionally, heat pipes have been used with each of the above structures.
Construction of a suitable heater has, nonetheless, been one of the primary obstacles
to widespread adoption of Stirling or hot gas engines.
The simplest method used is aiming a burner or other heat source
at the cylinder of the engine. This method is used with small engines for demonstration
purposes. This method has the advantage of low cost and simplicity, both the efficiency
of heat transfer is low and the method cannot be scaled up for use in larger engines.
A variation of the above method is accomplished by adding fins to
the cylinder to make a finned heat exchanger. This method can also be accomplished
by adding a separate finned heat exchanger. The fins add to the efficiency of
heat transfer by increasing the area exposed to heat. On the negative side, the
addition of fins adds to the cost of construction as they must be welded or brazed
to the cylinder or heat exchanger to operate at high temperature. The fins do not
affect the pressure handling capability of the device where there is still a trade-off
between thin walls for efficient transfer and thick walls for pressure resistance.
As a result of the above factors, fins are generally only used in small engines.
The next most common approach has been the addition of a tubular
heat exchanger. A tubular heat exchanger usually consists of a series of bent tubes
running between the cylinder and regenerator. The tubes are arranged to be exposed
to a burner or other heat source. To achieve high thermal efficiency, the tubes
should have thin walls. Thin walled tubing, however, is limited in its pressure
handling capability. Additionally, the numerous small tubes must be assembled and
joined to the cylinder increasing the cost of construction. Nonetheless, tubular
heat exchangers are the type most commonly used for larger engines.
Combinations of the above methods have also been used, such as finned
tubes. Generally the additional cost of construction outweighs the advantages.
One addition frequently used is a heat pipe to convey heat from a small burner
to a large heat exchanger structure. A heat pipe is a cavity filled with a vaporizable
liquid, such as sodium metal and an inert gas. One portion of the cavity is heated,
and the liquid vaporized. This is called the evaporation area. The load is located
at another portion of the cavity called the condensation area. At the condensation
area, the liquid condenses, transferring its heat of evaporation to the load. The
result is a conveying of heat with low loss. The use of heat pipes, however, cannot
increase the pressure or temperature capabilities.
Finally, baffle arrangements have been tried alone, or in combination
with the other methods. The baffles can increase thermal efficiency, but at an
increase in construction cost. In summary, no method has been found that provides
high thermal efficiency at low cost with the ability to be exposed to constant
high temperatures and pressures at controlled flow resistance.
Summary of the Invention
An improved heat exchanger is provided by the present invention.
The device has the ability to transfer heat to or from a stream of fluid with high
efficiency and controlled flow resistance. Ability to withstand continuous high
temperatures and pressure is achieved at minimal construction cost. Finally, the
device is adaptable to mass production methods.
The device is comprised of at least one annular fluid passage that
is surrounded on one or both sides by heating walls. The heating walls are constructed
of large diameter thinwall tubing to minimize cost and maximize heat transfer.
The thin walls are separated by spacers which allow passage of a second fluid which
may be a coolant or heat transfer medium, while providing support to the walls.
The spacers are designed to be placed under compression by either multiple annular
passages, or annular passages and insulation passages, so that no thick walls
of high temperature materials are needed. The device is designed so that all components
are cylinders of various sizes with the exception of the ends and manifold block.
The device is shown as attached to a Stirling cycle engine as a heater, but it
is realized that the invention is equally useable in other applications, or as
a cooler. A second embodiment is illustrated which includes a single heating wall
for lower cost of construction.
Brief Description of the Drawings
Description of the Preferred Embodiment
- Figure 1 is a sectional elevation view of the device in a working environment.
- Figure 2 is a sectional plan view of the Figure 1 device through line A-A.
- Figure 3 is a sectional plan view of the Figure 1 device through line B-B.
- Figure 4 is a perspective view of the manifold Block of the Figure 1 device.
- Figure 5 is a sectional elevation view of a second embodiment of the invention.
- Figure 6 is a sectional plan view of the Figure 5 device through line C-C.
Figure 1 is a sectional elevation view of the device in use as a
heater on a Stirling or hot gas engine. This view is a partial sectional view of
concentric cylinders and only one side of the assembly is shown for clarity, the
other side being identical. As the device is shown attached to an engine, several
components of the engine are shown to illustrate the use of the device that are
not essential to use of the invention in other heat exchanger applications.
The entire apparatus is enclosed in a pressure shell 1. Pressure
shell 1 is of substantial thickness to contain the high pressure fluids within
the device and engine. Since pressure shell 1 is not subjected to high temperatures,
it may be constructed of any high strength material, such as steel, which has the
added advantage of low cost. Within pressure shell 1 is the cylinder 2 of the
engine, a displacer 3 oscillates within cylinder 2 of the engine. One end of cyliner
2 of the engine is constantly exposed to high temperatures and must, therefore,
be constructed of a super alloy. Such materials are expensive, but only a thinwall
tube is required in this embodiment. The top of cylinder 2 is closed by a cylinder
head 6, which, along with cylinder 2 and the top of displacer 3, defines the expansion
space 4 of the engine. The only exit from expansion space 4 of the engine is a
port 5 which forms the entrance to the heater. Port 5 is shown as an annular opening
in this embodiment, but it is realized that equivalent construction, such as holes
or slots could be substituted. The exit of the heater is a Regenerator 7. Regenerator
7 is housed between cylinder 2 and an outer wall 8, which is also made of heat
resistant material. In operation, gas shuttles between regenerator 7 and expansion
space 4, and through the heater under the influence of displacer 3. The purpose
of the invention is to transfer heat from an outside source 9 (not shown) to the
shuttling gas stream. The device will also work in reverse if the engine is powered
by a motor to act as a refrigerator, in which case, heat is transferred from the
gas stream to the outside 9.
If the device is used as a heater, outside source 9 is a burner or
equivalent heat source, such as radioactive material or solar concentrator. While
the preferred embodiments are herein functionally described with reference to
heat delivery by means of a heat pipe, it is readily apparent that a pumped heat
transfer material, such as a liquid metal, can be substituted for the heat pipe
10 and adjacent structure with minimal modification. Heat source 9 is connected
to a heat pipe 10. Heat pipe 10 is a space containing gas and a vaporizable material,
such as sodium or potassium metal. Heat source 9 vaporizes the material which then
fills heat pipe 10 and areas connected to heat pipe 10. The vaporized material
condenses on the walls of such areas, releasing its heat of vaporization to such
walls. The above process results in an efficient transfer of heat from source
9 to the walls. Heat pipe 10 enters the device via a hole or annular hole in top
plate 11. The junction may be brazed or welded. Top plate 11 forms the top housing
of the device and is ring shaped with an inner housing cylinder 12 joined to its
inner surface, and a outer housing cylinder 13 joined to its outer surface. The
housing cylinders 12, 13 may be joined to top plate 11 by brazing or welding, and
are constructed of a high temperature alloy. In this embodiment, inner housing
cylinder 12 is joined at its other end to cylinder head 6 by brazing or welding.
In another environment a separate flange would replace cylinder head 6. Similarly,
outer housing cylinder 13 is attached to the outer regenerator housing 8. Between
housing wall 13 and pressure shell 1 is a plurality of layers of multifoil insulation
14. Multifoil insulation 14 is comprised of several layers of heat reflective material,
such as nickel separated by layers of an insulating fluid, such as Xenon or carbon
dioxide gases. Wall 13 is at a high temperature so insulation is necessary if
pressure shell 1 is made of material that cannot be exposed to constant high temperatures.
Additionally, if the hot engine gas is operating at high pressure, the counter
balancing force of the pressure of insulating gas allows walls 12 and 13 to be
constructed of thin wall tubing as there is little pressure differential. This
completes the outer housing of the device.
Continuing with Figure 1 is the internal construction of the device.
In this embodiment outer regenerator wall 8 continues into the housing to constitute
the outer most heating wall 17 of the device. The annular space 18, between walls
17 and 13, forms the outer annular condenser section of a heat pipe. In this area,
during operation, some vapor present in heat pipe 10 condenses, transferring its
heat to wall 17. Wall 17 is constructed of thinwall tubing to increase the heat
transfer. In a similar manner, an inner condenser section 19 is formed between
inner housing wall 12 and an inner heating wall 21. One end of inner heating wall
21 is attached to cylinder head 6 in this embodiment. Finally, a central condenser
section 22 is formed between outer central heating wall 24 and inner central heating
wall 23. In this embodiment, wall 23 is an extension of cylinder 2 and is joined
to wall 24 by a central flange 26. The outer ends of heating walls 23 and 24 are
attached to manifold block 27. Walls 21 and 17 are also attached to manifold block
27. Manifold block 27 is sized so that an inner heating area 28 is formed between
heating walls 21 and 23, and an outer heating area 29 is formed between heating
walls 17 and 24. Communication between the inner heating space 28 and the outer
heating space 29 is possible through a passage 35 in manifold block 27. Manifold
block 27 also provides several passages 30 for material in heat pipe 10 to enter
the central condensing region 22. In summary, it is apparent that all heating
walls 17, 21, 23 and 24 are adjacent to condenser regions of the heat pipe 10,
therefore, when heat pipe 10 is heated by heat source 9, walls 17, 21, 23 and
24 will rapidly rise to high temperature. All joints are assumed to be connected
by welding or brazing, unless otherwise described.
In operation, the fluid pressure in areas 4, 28, 29 and 7 is higher
than that in the condenser sections 18, 19 and 22. This pressure differential results
in a force tending to collapse the condenser sections. This force is countered
by placing spiders 31, 32 and 33 in condenser sections 18, 22 and 19, respectively,
and by pressurizing areas 14, 34 and 36 with insulating gas. Spiders 31, 32 and
33 are thus put under compressive stress. Spiders 31 and 33 have passages on the
side facing the heating walls 17 and 21 for passage of fluid from heat pipe 10.
Inner spider 32 has such passages on both sides. Spiders 31, 32 and 33 can be constructed
of any material that is resistant to high temperatures and is strong under compression.
Ceramics and glasses are particularly suitable as they can be cheaply molded, but
corrugated metal can also be used. In summary, the use of spiders 31, 32 and 33,
as integrated into the described heat exchanger, allow low cost construction and
high thermal efficiency. These advantages are achieved primarily as a result of
allowing thin annular heating walls 17, 21, 23 and 24 by carrying the pressure
loads in heating spaces 28 and 29 with the spiders 31, 32 and 33 used in conjunction
with pressurized regions 14, 34, and 36.
If used as a heater for a Stirling engine, gas moves through regenerator
7 into outer heating area 29 where the gas is surrounded by heating walls 17 and
24, which heat the gas. The gas then proceeds through the passage 35 in manifold
block 27, and into inner heating area 28. In area 28 the gas is surrounded on both
sides by heating walls 21 and 23, and is heated further. The hot gas then exits
through port 5 into expansion space 4. When displacer 3 moves toward cylinder head
6, the gas returns through port 5 into space 28, 35, then 29, and returns to regenerator
7. From the above description, it is apparent that the device can work in either
direction. In addition, if the engine is used as a refrigerator, heat can be extracted
from the gas and transferred to a heat sink at 9. It should be noted that the device
is normally operated in a position inverted to that shown in Figure 1, so that
condensed fluid in heat pipe 10 returns to the vicinity of heat source by gravity.
However, heat pipe operation when oriented as shown, is possible by utilizing
metal wicking (not shown), such as wire mesh screens attached to the condensation
walls so that capillary action returns the condensate to the heat source 9 through
heat pipe 10. Finally, it should be noted that the device can also operate as a
cooler, if heat pipe 10 and areas 18, 19 and 22 are filled with a cooling fluid.
Figure 2 is a plan section view along line A-A of Figure 1. This
figure clearly shows the circular nature of the device, although only a segment
of each circle is shown, it being realized that the device is symmetrical. Pressure
shell 1 surrounds the device and is filled with high pressure insulation gas in
regions 14 and 36. Area 14 may also be filled with multifoil insulation as described
above. Next, is outer housing wall 13, which along with outer heating wall 17,
defines the outer condensing section of the heat pipe. Spider 31 fits between
walls 13 and 17, and provides a plurality of pillars for transmitting compressive
forces between walls 13 and 17. The area between pillars 41 forms channels which
constitute the condensing section 18 of the heat pipe. The outer heating passage
29 is seen to be an annular section between heating walls 17 and 24. The central
condenser section is similar to the outer condenser section except that condensing
section passages 22 are formed on both sides of spider 32. It should be noted
that spider 32 is designed to provide unbroken pillars between walls 24 and 23.
The inner condensing section is formed similar to, but in reverse of, the outer
condensing section, with inner heating wall 21, spider 33 and inner housing wall
12. The inner heating area 28 is thus formed between heated walls 21, 23 and is
also seen to be an annular space. Finally, the central area 36 of the device is
filled with a compressed insulating gas that supports inner housing wall 12 against
the compressive forces transmitted to it from inner heating area 28 through heating
wall 21 and spider 33.
Figure 3 is a sectional plan view of line B-B of Figure 1. Line B-B
passes through the passage 35 between annular heating areas 28 and 29. Parts 1,
14, 13, 31, 17, 29, 28, 21, 33, 12 and 36 are substantially identical to those
in Figure 2. It is apparent that the fluid flows from outer heating passage 29
through area 35 and into inner heating passage 28. Flow in the reverse direction
is also possible. It will be seen that passage 35 is formed by the spaces between
a series of tubes 42, the interiors of which form passages 30. A portion of the
central spider 32 can be seen through passages 30. As passages 30 area is also
filled with hot vaporized liquid, additional heat transfer can occur in this crossover
region. Tubes 42 must be sufficiently thick to contain the pressure differential
between areas 30 and 35, but this is the only high temperature portion of the
device that may require thicker walled tubing, although the relatively small diameter
of the tubes minimizes this potential problem.
Figure 4 is a detail of the manifold block of the invention. Only
half of manifold block 27 is shown to allow view of sections through passages 30
and 35. Manifold block 27 is comprised of a top flange 47, a bottom flange 43,
and a series of tubes 42. The interiors of tubes 42 form a passage 30, and the
area defined by the top of flange 43, the bottom of flange 47 and the areas between
tubes 42 form passage 35. In the construction shown in Figure 4, flanges 43 and
47 are provided with holes to accept tubes 42. This construction would be used
if manifold 27 is constructed by welding. Manifold block 27 could also be made
by casting or electroforming, in which case it would be one piece. Lower flange
43 provides an outer surface 44 to accept and join to outer central heating wall
24 and a surface 46 to accept and join to inner central heating wall 23 as shown
in Figure 1. Similarly, top flange 47 provides surfaces 48 and 49 to accept and
join to outer heating wall 17 and inner heating wall 21 respectively. The radial
distance between surfaces 44 and 48, and between 49 and 46, determine the size
of passages 28 and 29. The size of passages 28 and 29 determines the resistance
to flow of the device. Manifold block 27 provides a simple means to adjust the
flow resistance to a pre-determined value.
Figure 5 is a section elevation view of a second embodiment of the
invention. In this embodiment, the device is surrounded by a pressure wall 101
and insulation 114 in a manner similar to the Figure 1 embodiment. The device
is shown connected to a Stirling engine as in the Figure 1 embodiment. The regenerator
of the engine is shown at 107 and a piston 103 is near the top 106 of cylinder
102. Spaces 114, 134 and 136 are pressurized with insulating gas as in the Figure
1 embodiment. A heat pipe 110 is connected to a source of high temperature and
is filled with a conductive media such as sodium vapor. The conductive media is
contained between an outer annular containment wall 113 which also serves as an
outer wall for regenerator 107 and an inner annular containment wall 112 which
are connected to heat pipe 10 by a top cap 111. A bottom cap 126 seals to outer
containment wall 113 and an outer heater wall 117. An inner heater wall 121 is
connected to outer heater wall 117 by a heater top cap 147. In this embodiment,
inner heater wall 121 and inner containment wall 112 are sealed to cylinder head
106. It is thus apparent that the conductive media area between walls 113 and 117
and caps 126 and 111 is connected to the area between walls 121 and 112 and cap
111 and cylinder top 106. An interior non-heated wall 102 protrudes into the annular
space between heater walls 117 and 121 to divide this area into inner and outer
annular heat spaces 128 and 129, respectfully. Spaces 128 and 129 are connected
by a passage 135. The result is a long path for gases circulating between regenerator
107 and the space 104 between piston 103 and cylinder top 106. Ceramic or metal
spacers 131 and 133 between walls 113 and 117 and 121 and 112, respectively, allow
heater walls 117 and 121 to be thin for good heat transfer. Spacers 131 and 133
include passages 118 and 119, respectively, for passage of the conductive media.
In use, since the passages 118 and 119 are filled with condensing
conductive media, heater walls 117 and 121 are rapidly heated to a temperature
similar to that of heat pipe 110. Any gas flowing between regenerator 102 and
area 104 must thus pass along the entire length of heating walls 117 and 121 and
is thereby heated with high efficiency.
Figure 6 is a partial plan section view along line C-C of Figure
5. This figure shows the annular nature of the invention. An outer pressure shell
101 surrounds the device. Multifoil insulation 114 filled with pressurized insulating
gas is adjacent to shell 101. Outer containment wall 113 and inner containment
wall 112 enclose the hot portion of the device and are adjacent to and in contact
with ceramic spiders 131 and 133, respectively. Spiders 131 and 133 include passages
118 and 119 for the conductive media. Outer heating wall 117 and inner heating
wall 121 also contact spiders 131 and 133. A wall 102 divides the space between
heating walls 117 and 121 into two passages 129 and 128. In operation, area 136
and the vicinity of insulator 114 are filled with pressurized insulating gas. Passages
129 and 128 are filled with pressurized working fluid. Since the conductive media
in passages 118 and 119 is at a lower relative pressure, spacers 131 and 133 are
under constant compressive stress. Use of ceramic for spacers 131 and 133 allows
use of thin walls for heater walls 117 and 121 due to its high compressive strength.
Other materials having similar characteristics may be substituted for ceramics.
It will be understood that the invention may be embodied in other
specific forms without departing from the spirit of the central characteristics
thereof. The present examples and embodiments, therefore, are to be considered
in all respects as illustratiave and not restrictive, and the invention is not
to be limited to the details thereof, but may be modified within the scope of
the appended claims.
The embodiments shown are illustrative only, the invention being
defined solely by the claims attached.