COMPOSITE HEAT EXCHANGER HAVING STRENGTHENED JOINTS

Abstract: A heat exchanger core (50) includes a stack of fins (54), plates (52) and closure bars (56). A first plurality of stiff fibers (60a) extend into contacting surfaces of the fins (54) and plates (52), a second plurality of stiff fibers (60b) extend into contacting surfaces of the closure bars (56) and plates (52), and a third plurality of stiff fibers (60c) extend into contacting surfaces of the closure bars (56) and manifolds (58). The stiff fibers can strengthen the heat exchanger assembly and they can also increase thermal conductivity in their direction of orientation.

Description: COMPOSITE HEAT EXCHANGER HAVING STRENGTHENED JOINTS BACKGROUND OF THE INVENTION The present invention relates to heat exchangers. More specifically, the invention relates to a method of bonding together members of a composite heat exchanger. An aircraft environmental control system typically includes an air cycle machine and a heat exchanger upstream the air cycle machine. The heat exchanger is used to transfer heat from bleed air to a coolant (e.g., ambient air). Heat exchangers for aircraft environmental control systems should be lightweight and strong. During operation, the heat exchanger is subjected to large thermal stress and high pressure. The heat exchanger should have sufficient strength to withstand the stress and pressure. Moreover, the walls of the heat exchanger should be made as thin as possible in order to reduce weight. Reducing the weight of the heat exchanger reduces fuel consumption of the aircraft. Reducing fuel consumption, in turn, reduces aircraft operating costs. The assignee of the present invention has been making aircraft heat exchangers out of carbon/carbon composites. Carbon/carbon composites offer high strength and light weight. A method of manufacturing a carbon/carbon composite plate-fin heat exchanger core is disclosed in Anderson et al. U.S. Serial No. 08/601 ,754 filed April 12, 1996, entitled "CARBON/CARBON HEAT EXCHANGER MANUFACTURING METHOD" and assigned to the assignee of the present invention. A first plurality of woven sheets of low modulus Pitch carbon fibers are impregnated with resin and shaped into fins. A second plurality of sheets of carbon fibers are impregnated with resin and shaped into plates. The resin-impregnated fins are stacked between the resin-impregnated plates to form a stacked assembly, the stacked assembly is then cured, pyro zed, densified and annealed to form a fully graphitized plate-fin heat exchanger core Joints between plate and fin tends to be the weakest part of the composite heat exchanger core It is most likely to separate, especially if subjected to large thermal stress and high pressure The plate-fin joints also tend to have the lowest thermal conductivity and, therefore, do not maximize the transfer of heat from the bleed air to the coolant It would be desirable to increase joint strength It would also be desirable to increase joint thermal conductivity SUMMARY OF THE INVENTION According to one aspect of the present invention, a heat exchanger comprises a first composite member having a first contacting surface, and a second composite member having a second contacting surface The second contacting surface is in contact with the first contacting surface The heat exchanger further comprises a plurality of stiff fibers extending into both contacting surfaces and being embedded in both members The fibers can strengthen joints between plates and extended heat transfer surfaces, closure bars and plates, and manifolds and closure bars The fibers can also increase thermal conductivity in their direction of orientation BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of an environmental control system including two heat exchangers according to the present invention, Figure 2 is an illustration of an exemplary heat exchanger core, Figure 3 is an illustration of a plate-fin joint of the heat exchanger core Figure 4 is a flowchart of a method of fabricating a carbon/carbon composite heat exchanger according to the present invention, Figure 5 is a flowchart of a method of fabricating a different composite heat exchanger according to the present invention, Figure 6 is an illustration of a first method of orienting stiff fibers during fabrication of a heat exchanger, and Figure 7 is an illustration of a second method of orienting stiff fibers during fabrication of a heat exchanger DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows an aircraft environmental control system 10 for supplying cooled, conditioned air to an aircraft cabin 8 The environmental control system 10 includes a pre-cooler 12, a primary heat exchanger 14 and an air conditioning system 16 Hot, compressed air (e g , bleed air) from a source such as an auxiliary power unit or a compressor stage of an aircraft engine is supplied to the pre-cooler 12, which removes the heat of compression Temperature of the bleed air entering the pre-cooler 12 may be between 900°F and 1200°F, and temperature of the bleed air leaving the pre- cooler 12 may be between 350°F and 500°F The bleed air may be supplied at a pressure of about 100 psig The air leaving the pre-cooler 12 is further cooled in the primary heat exchanger 14 Air pressure inside the primary heat exchanger 14 may also be about 100 psig Air leaving the primary heat exchanger 14 is cooled and dehumidified in the air conditioning system 16 The air conditioning system 16 may include an air cycle machine for expanding the air and a water extractor for dehumidifying the air Cooled, conditioned air leaving the air conditioning system 16 may then be supplied to a mix manifold 18 Inside the mix manifold 18, the cooled conditioned air is mixed with recirculated cabin air The recirculated cabin air may be drawn from the cabin 8 to the mix manifold 18 by a recirculation fan 20 Mixed air leaving the mix manifold 18 is supplied to the aircraft cabin 8 In the alternative, the cooled, conditioned air leaving the air conditioning system 16 may be supplied directly to the cabin 8 Each heat exchanger 12 and 14 includes a core and manifolds The core includes a plurality of parting sheets (e g , plates) and a plurality of extended heat transfer surfaces (e g , fins) located between the parting sheets and bonded to the parting sheets The parting sheets and extended heat transfer surfaces define hot side and cold side passageways During operation of the heat exchangers 12 and 14, a hot fluid flows through the hot side passageways and a coolant flows through the cold side passageways of the heat exchanger 12 Heat is transferred from the hot fluid to the coolant The manifolds direct fluids to and from the hot side and cold side passageways The core and manifolds of the heat exchangers 12 and 14 may be made of a composite material The primary heat exchanger 14 may be made of a carbon fiber-based composite material such as carbon/carbon composite or epoxy graphite In the alternative, the primary heat exchanger 14 may be made of a composite material including fibers embedded in a cured thermoplastic The preferred material for the pre-cooler 12 is a carbon/carbon composite (coated with a material that protects against oxidation), which can withstand the higher operating temperatures and pressures The heat exchangers 12 and 14 are not limited to any particular design or configuration The heat exchangers 12 and 14 may have a cross-flow configuration, a counter-flow configuration, etc The primary heat exchanger 14, for example, may have the plate-rib design disclosed in Dewar U S Patent No 5,628,363, which is assigned to the assignee of the present invention, it may have the plate-fin design shown in Figure 2, etc Figure 2 shows a core 50 of an exemplary cross-flow heat exchanger The core 50 is fabricated from a plurality of spaced apart plates 52 and a plurality of continuous plain fins 54 The fins 54 are interposed between the plates 52 Contacting surfaces of the fins 54 are bonded to contacting surfaces of the plates 52 The core 50 also includes a plurality of closure bars 56 interposed between plates 52 A pair of closure bars 56 is shown between ends of the two bottom-most plates 52 Contacting surfaces of the closure bars 56 are bonded to contacting surfaces of the plates 52 Although only two closure bars 56 are shown, the core 50 includes additional pairs of closure bars between ends of the plates 52 Reference is now made to Figure 3 The bonds between contacting surfaces of the plates 52 and extended heat transfer surfaces 54 are strengthened by a first plurality of stiff fibers 60a Similarly, the bonds between contacting surfaces of the plates 52 and closure bars 56 are strengthened by a second plurality of stiff fibers 60b, and the bonds between contacting surfaces of the closure bars 56 and manifolds 58 are strengthened by a third plurality of stiff fibers 60c Thus, the bond between two members 52, 54 or 52, 56 or 56, 58 is strengthened by a plurality of stiff fibers 60a or 60b or 60c Each stiff fiber 60a, 60b and 60c extends into the contacting surfaces of both contacting members and is embedded in both contacting members The stiff fibers may be oriented roughly orthogonal to the contacting surfaces into which they extend At least one-quarter the length of each stiff fiber is inserted into a contacting member Each stiff fiber 60a, 60b and 60c may have a length between 1/16" and 1/32", a diameter between seven and ten microns, and a fiber-like geometry The stiff fibers 60a, 60b and 60c have a high modulus of elasticity, between 30 Msi and 120 Msi Additionally, the stiff fibers 60a, 60b and 60c are distributed with a density of between about 50% and 80% of the contacting surface. The stiff fibers 60a, 60b and 60c may be Pitch carbon fibers or graphite fibers. In addition to strengthening the joints, such fibers 60a, 60b and 60c can increase thermal conductivity in the direction of orientation, thereby increasing heat transfer efficiency. Although not preferred, the fibers could be made of a PAN precursor. Figure 4 shows a method of fabricating a carbon/carbon composite heat exchanger assembly having a piate-fin design. A first plurality of woven fabric sheets of carbon fibers are impregnated with a thermoset-type resin and shaped into fins, a second plurality of woven sheets of carbon fibers are impregnated with the thermoset-type resin and shaped into plates (block 102), and a third plurality of woven sheets of carbon fibers are impregnated with the thermoset-type resin and shaped into closure bars. Resulting are "pre-preg" members. Stiff carbon fibers are interposed between contacting surfaces of the prep-preg members (block 104) and the members are stacked to form a stacked assembly (block 106). Some stiff carbon fibers will become embedded into plates and fins of the stacked assembly, and other stiff fibers will become embedded in plates and closure bars. Still other stiff fibers will extend from contacting surfaces of the closure bars and will, eventually, be embedded in manifolds. The stiff carbon fibers are oriented in a particular direction, such as orthogonal to the contacting surfaces. Any gaps between plates and fins are filled with additional thermoset- type resin (block 108). The resulting stacked assembly may then be cured, whereby the plates, extended heat transfer surfaces, closure bars and stiff carbon fibers are bonded together (block 110). Pre-preg manifolds may then pressed onto the cured assembly (block 112). Additional resin is added to fill any gaps between the manifolds and the core assembly, and the resulting assembly is cured (to bond the manifolds to the core), pyrolized, densified and annealed (block 114) Resulting is a fully graphitized plate-fin heat exchanger assembly Figure 5 shows a method of fabricating a different plate-fin heat exchanger assembly Sheets of high conductivity fibers (e g , graphite) are formed into plates, fins and closure bars and embedded in a thermoplastic- type resin such as "PEEK" resin or "NYLON" resin (block 202) Each member is also cured (block 204) The closure bars and fins are heated to soften the thermoplastic-type resin (block 206), and individual stiff fibers are inserted into contacting surfaces and into the bars and fins (block 208) The bars and fins are cooled to harden the thermoplastic-type resin (block 210) The plates are heated to soften the thermoplastic-type resin (block 212), and the plates, fins and closure bars are stacked and loaded to form a core assembly (block 214) As the plates are stacked onto the fins and closure bars, the stiff fibers are inserted into the softened thermoplastic of the plates The plates are then cooled to harden the plate thermoplastic (block 216) Gaps between the members may be filled with additional thermoplastic- type resin and cured (218) Resulting is a core assembly Manifolds may also be added (block 220) The manifolds, which may also be made of fibers embedded in a cured thermoplastic-type resin, are heated (to soften the thermoplastic-type resin) and pressed onto the core assembly As the manifolds are pressed onto the core assembly, fibers extending from the closure bars become embedded in the manifolds After the manifolds have been pressed onto the core assembly, the manifolds are cooled Resulting is a heat exchanger assembly Figure 6 shows one way in which the stiff fibers 312 may be oriented relative to the contacting surfaces of the members The stiff fibers 312 extend through a thin mesh 314 in a preferred orientation The fiber-penetrated mesh is interposed between contacting surfaces of the members If for example, the members are made of Pitch carbon fibers 312 that are impregnated with a thermoset-type resin, the fiber-penetrated mesh 314 is interposed between contacting surfaces of the members, the members are pressed together, additional resin is added, and the members are cured If, for example, the members are made fibers 312 that are impregnated with a thermoplastic-type resin that has been cured, the members are softened, the fiber-penetrated mesh 314 is interposed between contacting surfaces of the members, the members are pressed together, and the members are cooled The mesh 314 may be made of a carbon or graphite veil having a thickness between one-half and two mils Prior to inserting the stiff fibers 312 into the mesh 314 those mesh areas not corresponding to contacting surfaces may be masked The stiff fibers 312 may then be inserted through the unmasked portions of the mesh 314 The stiff fibers 312 may be secured to the mesh 314 by a spray adhesive Figure 7 shows another way in which the stiff fibers 412 may be oriented relative to the contacting surfaces of a member 414 The stiff fibers 412 are sprinkled onto a contacting surface of the member 414, and a magnetic field (from a magnet 416) is applied so that the stiff fibers 412 line up vertically in the desired orientation Resulting is a dense pile of stiff fibers 412 Ends of the polarized fibers 412 may then be pressed into the member by such means as a plate 418 The depictions of the stiff fibers shown in Figures 3, 6 and 7 are not intended to portray accurate size, geometry, density, etc They are merely provided to help explain the invention The invention is not limited to the specific embodiments described above The method of Figure 4 may be applied to an epoxy graphite heat exchanger, except that graphite fibers would be used as the starting material (instead of Pitch carbon fibers), a different thermoset-type resin would be used, and the steps of pyrolizing, densifying and annealing would not be performed. Extended heat transfer surfaces other than continuous plain fins may be used. For example, the extended heat transfer surfaces may be ribs as disclosed in assignee's U.S. Patent No. 5,628,363. The heat exchanger core is not limited to a plate-fin configuration. Nor is the heat exchanger core limited to Pitch carbon or graphite fibers. The invention is not limited to an environmental control system. It may be used in any high temperature, high pressure environment. Therefore, the invention is not limited to the specific embodiments described above. Instead, the invention is construed according to the claims that follow.

Claims: WHAT IS CLAIMED IS 1 A heat exchanger (12, 14) comprising a first composite member (52 or 58) having a first contacting surface, a second composite member (54 or 56) having a second contacting surface, the second contacting surface being in contact with the first contacting surface, and a plurality of stiff fibers (60a, 60b or 60c), each fiber extending into both contacting surfaces and being embedded in both members (52, 54 or 56) 2 The heat exchanger of claim 1 , wherein the first member is a plate (52) and the second member is an extended heat transfer surface (54) 3 The heat exchanger of claim 1 , wherein the first member is a plate (52) and the second member is a closure bar (56) 4 The heat exchanger of claim 1 , wherein the first member is a manifold (58) and the second member is a closure bar (56) 5 The heat exchanger of claim 1 , wherein the fibers (60a, 60b, 60c) are roughly orthogonal to the contacting surfaces 6 The heat exchanger of claim 1 , wherein the first and second members (52, 54, 56, 58) are made of carbon/carbon composite 7 The heat exchanger of claim 1 , wherein the first and second members (52, 54, 56, 58) are made of fabric embedded in a cured thermoplastic-type resin. 8. The heat exchanger of claim 1 , wherein between at least one- quarter the length of each fiber (60a, 60b, 60c) is embedded in the first member (52 or 58) and at least one-quarter the length of each fiber (60a, 60b, 60c) is embedded in the second member (54 or 56). 9. The heat exchanger of claim 1 , wherein the fibers (60a, 60b, 60c) have a length between 1/16" and 1/32"; a diameter between 7 and 10 microns; and a stiffness between 30 Msi and 120 Msi. 10. The heat exchanger of claim 1 , wherein the fibers (60a, 60b, 60c) have a density of about 50% to 85% of the contacting surface area.

Inventors: ANDERSON ALEXANDER F, ANDERSON, ALEXANDER, F.
Applicants: HONEYWELL INT INC, HONEYWELL INTERNATIONAL INC.
Application year: 2001
Main classification: F28F21
IPC codes: B29C65, C04B35, F28D9, F28F3, F28F21