Introduction Typically two-fluid, parallel plate heat exchangers are constructed of metal parts, including a plurality of plates spaced apart and stacked in a parallel configuration to form a series of passageways therebetween. Within alternating passageways flow hot and cold fluids, the passageways through which hot fluids flow being referred to as hot passageways and the passageways through which cold fluids flow being referred to as cold passageways. Metal fins are provided between adjacent plates to assist the transfer of heat from the fluid in the hot passageways through the adjacent plates to the cold fluid by ensuring turbulent fluid flow. These fins are bonded to and between the plates to provide an extended heat transfer area and sufficient structural support to provide pressure containment of the fluids. In order to minimize flow blockage, the fins are disposed in parallel with the fluid flow and define a flow path with minimum additional flow resistance.

Additionally, the thickness and number of fins are selected to maximize the heat transfer area in contact with the fluid. Generally, thinner fins satisfy these requirements. However, many different geometric configurations may be used to satisfy the specific requirements of a given design situation.

Until now, integral passages have not been used in compact parallel plate heat exchangers because it is impossible to achieve a sealed metal

tube-like structure within a parallel plate array with an acceptable shape to be effective in transferring heat between the two fluids.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a parallel plate heat exchanger with integral passages or loop columns constructed of pluralities of individual loops. Each loop is comprised of specially constructed and processed materials having a higher thermal conductivity than that of available metals. The individual loops of successive rows are aligned to form loop columns and are interconnected in fluid communication with one another, thereby forming loop passageways through which a fluid may flow to facilitate the transfer of heat to or from the fluid flowing between adjacent plates. The heat exchanger can be visualized as comprising a stacked array of alternating levels of metal plate fins and rows of separate and spaced apart loops. A further object of this invention is to employ the potential anisotropic heat transfer properties of carbon/carbon composite materials to improve the transfer of heat within the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features, and advantages will become more apparent from the following detailed description of the invention, shown in the accompanying drawing wherein: Figure 1 is an illustration of the counter flow heat exchanger of the present invention.

Figure 2 is an illustration of a composite loop that can be used in the practice of the heat exchanger of the present invention.

Figure 3 is an illustration of an embodiment of the present invention in which the plates are coextensive with the loops.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figure 1 of the drawings, the heat exchanger 10 comprises a plurality of plates 1 and several rows of separate and

substantially uniform individual loops 5. The plates 1a-1d may have various planar shapes, preferably such that when configured in the heat exchanger 10, they are substantially parallel to one another. The loops 5aa-5df are interposed between the plates 1a-1d, and aligned preferably forming loop columns 7. The adjacent individual loops in each column are in fluid communication with each other. Fluid communication can be established by extending the loops 5aa-5df beyond the edge of the plates 1 a-1 d, to create an overhanging portion, as shown in Figure 1, with the bottom plate 1a extending beyond the overhanging portion to seal the bottom of the loop columns 7. The overhanging portions of the loops in each loop column are sealed together to prevent leakage. The assemble heat exchanger 10 can thus be seen to include a stack array of alternating plates 1 a-1 d and loop rows 5a-5d. The area between the plates 1a-1d and the loops 5aa-5df comprise a first passage. The area internal to the loops sealed together comprises a second passage.

Still referring to Figure 1, fluid flow through the heat exchanger involves a first fluid 12 that flows between the plates 1 a-1 d and the loop columns 7a- 7f, and a second fluid 14 that flows within the loop columns 7. The passageways formed between the plates will be identified as the plate passageways 18 and the passageways formed inside the loop columns 7 will be identified as the loop passageways 20. The entrance 22 to the loop passageways 20 and the exit 24 from the loop passageways 20 may be formed at varying positions on loop columns 7 in order to optimize the flow of fluid 14 through the loop passageways 20. One such construction would involve providing the entrance 22 to the loop passageways 20 at a diagonally opposing position to the exit 24 from the loop passageways 20 to help ensure minimal pressure effects on the flows in different loop passageways 20.

Different geometric constructions may also be employed for different layers of individual loops 5 to improve fluid flows in the loop passageways 20.

The individual loops 5 used to form loop columns 7 can exhibit varying regular or irregular geometric shapes to enhance the transfer of heat from the fluids to the heat exchanger material. Substantially serpentine or other

irregular-shaped loop columns may be used to enhance turbulence within the fluids, and prevent the formation of boundary layers, to improve overall heat transfer within the heat exchanger 10. Additionally, other surface enhancements, such as increased surface roughness, may be added to either the loops 5 or the plates 1 to facilitate improved heat transfer.

In accordance with the present invention, both the plates 1 and the individual loops 5 may be formed of carbon/carbon composite materials to take advantage of its relatively high thermal conductivity to facilitate the heat transfer between the two fluids. Additionally, the possible anisotropic thermal conductivity of some carbon/carbon composite materials may be used to further enhance heat transfer. Carbon/carbon materials also have a relatively low density, which permits the construction of a relatively light-weight heat exchanger.

In operation, the first fluid 12 and the second fluid 14, flowing in the plate passageways 18 and the loop passageways 20 respectively, are at different temperatures to facilitate the heat transfer between the fluids. For instance, the first fluid 12 can be hotter than the second fluid 14. When this hotter fluid 12 flows in the plate passageways 18, heat is transferred from the fluid to the plates 1 and to the loop walls 5aa-5df. Heat is then transferred from the plates 1 to the fluid passageways 20 formed by the individual loops 5 of the loop columns 7, and to the cooler second fluid 14. The second fluid 14 then exits and flows from the heat exchanger 10, carrying the exchanged heat away from the heat exchanger 10 allowing the continuous flow of the hot fluid to be continuously cooled by the continuous flow of the cool fluid.

The first fluid 12 and the second fluid 14 may, in addition to being inherently unequal in temperature, be unequal in pressure. The plates 1 and the individual loops 5 must be of a thickness sufficient to provide for structural integrity between plate passageways 18 and loop passageways 20, but sufficiently thin to minimize weight and not interfere with heat transfer. Plate thickness and loop thickness must be gauged to account for the fluid pressure difference between plate passageways 18 and loop passageways 20, as this difference will tend to warp the shape of the individual loops 5 and loop

columns 7, thus affecting the overall performance of the heat exchanger 10 by affecting the fluid flow within both the plate passageways 18 and the loop passageways 20.

Carbon/carbon composite heat exchanger core manufacture is performed in several successive steps. First, a thin layer of carbon fiber material, which may consist of uni-directional fibers or a woven or mat material is impregnated with a high carbon char yield resin system. This impregnated material is then formed into the flat parallel plates 1. Both the loops 5 and the plates 1, as welE as interconnections therebetween, must be leakage-free to prohibit commingling of the first fluid 12 and the second fluid 14. The plates 1 and loops 5 are then stacked in alternating layers of sufficient quantity to make up a core stack 10 of the desired size, preferably having. 003-. 005 inch thick plates stacked at 12-28 plates per inch.

Additional resin with a high carbon char yield may then be used between each alternating layer to bond the plates 1 and individual loops 5. The individual loops are preferably stacked to form loop columns 7, which can then be sealed to prevent leakage between the loop passageways 20 and the plate passageways 18. The stacked core can then be processed into a carbon/carbon composite by several steps. These steps include, but are not limited to, carbonization to convert the resins to carbon, densification by chemical vapor deposition (CVD) into a porous structure, and heat treatment (1600° to 2800°C) to provide a carbon or graphite of the desired crystal structure. This resulting carbon/carbon composite structure will have the desired high thermal conductivity, low coefficient of thermal expansion, and low potential for corrosion. One or more of the above steps may be repeated to achieve the desired characteristics. Alternatively, the CVD process can be replace by repeated cycles of impregnation with a high char yield carbon based thermosetting resin such as phenolic followed by a curing and charring.

These steps are repeated until the desired properties are obtained.

A method of improving heat exchanger performance and extending life is to use the correct selection of carbon/carbon composite materials. Fibers, used in the construction of carbon/carbon composite materials, are presently

available which have a wide range of thermal conductivities. Additionally, carbon/carbon composite materials may be anisotropic or isotropic depending on how the fibers are oriented within the material. Isotropic materials conduct heat substantially uniformly along all three orthogonal axes X, Y, and Z, while anisotropic materials conduct heat predominantly along a first axis, such as the Z-axis, and to a lesser extent along the remaining two Y and X axes.

In the counter-flow heat exchanger of this invention, high conductivity of the plates 1 in the direction between the loops 5 and perpendicular to the flow of the first fluid 12 is desirable. High thermal conductivity of the loops 5 is also desirable, but is not as much as that of the plates. By using a highly thermally conductive anisotropic carbon/carbon composite material for the plates 1 with the conduction path in the direction between the loops 5, and a highly thermally conductive carbon/carbon composite material for the loops 5, performance is maximized. An additional, and very significant, benefit in the use of carbon/carbon composite materials is that the coefficient of thermal expansion is much lower than that of conventional heat exchanger metals, and thus greatly reduces thermal expansion and the resulting stress. Note, although carbon/carbon materials are discussed extensively as the preferred material for this invention, its construction is not limited exclusively to carbon/carbon materials, and may consist of many other materials including other conventional heat exchanger materials such as metals. In accordance with this invention, it is recognized that a number of different carbon fiber and polymeric resin precursor carbon/carbon composites, which may be either isotropic or anisotropic, can be selected for its fabrication such that the thermal flux exceeds the value which would be achieved with an identical heat exchanger fabricated from metal. Various other modifications may be contemplated by those skilled in the art without departing from the true spirit and scope of the present invention as here and after defined by the following claims. In addition to the loop geometry and flow configurations mentioned above, the heat exchangers could be formed in other than the illustrated rectangular shape. Accordingly, heat exchangers of cylindrical, circular, or conical configurations are also within the scope of the present invention, with

particular emphasis on the applicability as a non-segmented heat exchanger for use around the circumference of a turbofan jet engine fan duct.

Figures 2a and 2b show two different views of the loops 5.

Figure 3 shows an alternate embodiment of the present invention in which the ends of the loops 5, and thus the loop columns 7, do not extend beyond the plates 1. In this embodiment, the entrance 22 and exit 24 of the loop passageways 20 are formed by punching holes 30 in the plates 1. These holes may be of any geometric shape, but are preferably"D"shaped as shown in Figure 3. In order to control the degree of fluid flow between successive plate layers, the holes in the plates may vary from layer to layer.

The use of holes such as shown in Figure 3 allows the loops 5 to be easily sealed with the plates 1. Furthermore, because it is easy to vary the size of the holes 30 from plate to plate, the fluid flow within the loop passageways 20 is easily controllable to ensure efficient heat transfer within the heat exchanger.