Patent Publication Number: US-RE37134-E

Title: Heat exchanger containing a component capable of discontinuous movement

Description:
The Government has rights in this invention pursuant to contract Number De-AC21-89MC26051 awarded by the Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     This invention is related to regenerative heat exchangers. 
     BACKGROUND OF THE INVENTION 
     Heat exchangers are devices used to transfer heat between a hot fluid stream and a cold fluid stream. In conventional heat exchangers the heat is transferred from one stream to another through a wall and the heat transfer is limited by the conductivity of the material of which the wall is made. 
     Regenerative heat exchangers typically are capable of achieving higher heating temperatures. Regenerative heat exchangers expose a heat-absorbing mass or matrix alternately to a hot stream and to a cold stream. In general, therefore, regenerative heat exchangers have periodic flow. 
     Periodic-flow exchangers operate differently from conventional fixed-surface heat exchangers in that heat is transferred from the hot fluid to the cold fluid by alternatively heating and cooling a high surface area matrix material. This matrix or core is either rotated through or shuttled back and forth between the hot and cold fluid streams of the fluid streams are switched between or among two or more stationary matrices. One type of periodic flow regenerative heat exchanger is the rotary regenerative heat exchanger in which a heat-absorbing matrix is rotated relative to streams of hot and cold fluids. The matrix generally comprises a disk or drum-shaped body having a plurality of internal passageways oriented axially. The fluid streams flow through these passageways alternately heating the matrix body or extracting heat therefrom. Such rotary heat exchangers are particularly useful as air preheaters in boiler plants and in gas turbine engines. Seals are provided that either have rubbing contact or maintain a very small gap with the matrix and serve to separate the hot and cold streams thereby reducing leakage losses that occur between the hot and cold fluid streams. 
     Rotary regenerators have advantages that make them well suited for gas-turbine engines. One of these advantages is compactness. In laminar flow of the fluid streams, the volume needed for a given quantity of heat to be transferred is proportional to the square of the hydraulic diameter of the passage used (Wilson,  The Design of High Efficiency Turbomachinery and Gas Turbines,  MIT Press, Cambridge, Mass., 1984). The passages in rotary regenerators for gas-turbine applications can be made much smaller than those of conventional tubular or plate fin type heat exchangers. In tubular or plate fin type heat exchangers, problems can be encountered if the passages are small because deposits from the hot and cold fluids can accumulate and block the small passages. This problem is alleviated or reduced in rotary regenerators because the fluid streams alternate and reverse flow direction in each passage, thereby removing deposits and reducing blockage. In addition, because hot and cold-stream separation is controlled by the seals rather than by complex ducts that are required in recuperators, the cost of making many small passages is low. 
     Another desirable feature of rotary and other regenerators is low pressure drop. The pumping power required to force gas through a heat exchanger is directly proportional to the square of the Mach number and is rather independent of matrix geometry (Wilson, 1984, cited supra. Therefore, large face areas must be used to minimize fluid velocity. In the rotary and other regenerators, elaborate manifold schemes to interleave the fluids are unnecessary, so a large flow area is practical. In contrast, with fixed surface heat exchangers, achieving both compactness and large, interleaved flow areas simultaneously is more difficult. 
     A problem encountered with conventional rotary regenerators is leakage of fluid from the exchanger which decreases its efficiency. Leakage occurs either through the seals that separate the high and low-pressure chambers or through void-volume carryover. Void volume carryover occurs because hot high pressure fluid trapped in the matrix is carried through the seals during rotation of the matrix to the cool, low pressure side. This leakage, although relatively small, worsens as the speed of rotation of the matrix increases. 
     SUMMARY OF THE INVENTION 
     According to the invention the seal leakage rate of a heat exchanger is reduced by discontinuously moving either the matrix or ducting while releasably isolating portions of the matrix with sealing members. In one embodiment of the invention a regenerative heat exchanger for transferring heat between a hot fluid stream and a cold fluid stream has a matrix defining a first group of passageways for fluid to flow therethrough. A seal means is provided capable of releasably sealing the matrix so that said passageways are interconnected with one of said hot or said cold fluid stream. Means are provided for causing discontinuous movement of the matrix relative to said hot or cold fluid streams and relative to the seal means, and the seal means seals only when said matrix is stationary with respect to said seal means. 
     Preferably the matrix is a rotary disc. In some cases the means can be formed of two portions comprising modules which are each exposed to hot and cold fluid flows and then exchanged to transfer heat from one flow to the other. 
     According to a method of this invention heat is transferred between a hot fluid and a cold fluid by establishing a hot fluid stream and a cold fluid stream. A matrix having portions thereof carrying means for exchanging heat with the hot and cold fluid streams is positioned so as to have at least one of said streams exchange heat therewith. The matrix is discontinuously moved in preselected increments such that matrix portions alternately contact the hot fluid and cold fluid streams and carry heat between the hot fluid stream and the cold fluid stream. The matrix is contacted with a seal for at least one of said hot or cold fluid streams when the matrix is stationary with respect to the one stream thereby creating a substantially leakproof area. The seal is released from the matrix when the matrix is moved with respect to the one stream. 
     Generally, the invention provides an improvement in a method of exchanging heat between a hot fluid stream and a cold fluid stream in a regenerative heat exchanger having a matrix with first and second passageways for said hot and cold fluid streams respectively and said heat exchanger having sealing means to seal said hot and cold streams from each other. The improvement comprises sealing the hot and cold fluid streams from each other to prevent mixing of said fluids when the matrix is stationary with respect to the position of the fluid streams and releasing the seal and moving the matrix with respect to the position of said fluid streams to exchange heat through said matrix. 
     It is a feature of this invention that the sealing of the matrix during the stationary phase reduces the amount of fluid leaking from one side of the seal to the other which significantly reduces the seal leakage rate. 
     It is an object of this invention to provide a regenerative heat exchanger that shares the benefits of conventional rotary heat exchangers while significantly reducing the leakage rate of such conventional rotary heat exchangers. 
     It is another object of this invention to provide a rotary heat exchanger having a matrix capable of discontinuous movement. 
     It is another object of this invention to provide a modular heat exchanger having matrix modules capable of discontinuous movement. 
     It is another object of this invention to provide a rotary heat exchanger that shares the benefits of conventional rotary heat exchangers while significantly reducing the leakage rate thereby. 
     It is another object to provide heat exchangers useful in gas-turbine engines. 
     It is yet another object to provide heat exchangers that can be integrated within the ducting of systems presently using conventional-type rotary exchangers. 
     It is still another object of this invention to provide a method of exchanging heat between hot and cold fluid streams by sealing said streams when a heat exchange matrix is stationary with respect to the position of the fluid streams and releasing the seal formed when said matrix moves with respect to said fluid streams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic view of a rotary heat exchanger. 
     FIG. 1B is a schematic, side view cutaway depicting the rotary heat exchanger contained in a housing. 
     FIGS. 2A-2D schematically show the rotary heat exchanger in operation as the seals are contacting and released from the matrix. 
     FIG. 3 is a perspective view of a modular heat exchanger. 
     FIGS. 4A-4C show the modular heat exchanger in operation as the modules move through a cycle within the housing. 
     FIG. 5 is a schematic representation of a heat exchange system using modular heat exchangers. 
     FIG. 6 is a side elevation view of a rotary heat exchanger. 
     FIGS. 6A-D are schematic, side elevation views of honeycomb, corrugated, egg crate and grid matrices, respectively, taken along the lines  6 A,  6 B,  6 C and  6 D of FIG. 6. 
    
    
     DETAILED DESCRIPTION 
     This invention pertains to regenerative heat exchangers for transferring heat between a hot fluid and a cold fluid. The heat exchangers are capable of providing discontinuous movement of a matrix relative to the hot and cold fluid streams. Sealing members are provided to isolate portions of the matrix when the components are stationary and released upon movement. By providing a system that isolates matrix portions only when the matrix is stationary, losses and wear associated with dynamic, sliding seals of conventional, continuously moving regenerative heat exchangers are significantly reduced. Additionally, by providing isolation of matrix portions only when the components are stationary, leakage across the seals that isolate the matrix portions and power losses resulting therefrom can be significantly reduced. The discontinuous movement of the matrix relative to the fluid streams is intended to encompass either discontinuous movement of the matrix or discontinuous movement of the ducting carrying the fluid streams to the matrix. The discontinuous movement preferably is in preselected increments. 
     In one embodiment of the invention, the heat exchanger has sealing members capable of releasably contacting the matrix surface so that a seal is provided to isolate matrix portions when the matrix or ducting is stationary and released when the matrix is in motion. The sealing members preferably clamp against the matrix to provide a seal having very low leakage when the matrix is stationary. 
     The regenerative heat exchanger of this invention can be any regenerative heat exchanger that is susceptible to heat and pressure losses resulting from flow leakage through a seal between moving and stationary components. Examples of such regenerative heat exchangers include rotary heat exchangers and modular heat exchangers. 
     In a first preferred embodiment, a rotary heat exchanger  10  is used as the regenerative heat exchanger as shown in FIGS. 1A and 1B. The rotary heat exchanger  10  contains an annular matrix  12  provided with a plurality of fluid-flow passageways (not shown) and capable of rotating about a central axis α via a rotor  13 . The matrix  12  is contained within a housing  14  conforming to the shape of the matrix  12  in its mid section while having substantially conical portions  16 ,  18 ,  20 ,  22  extending outwardly in pairs on both sides of the housing  14 . The conical portions  16 ,  22  have inlets  24 ,  26  respectively and conical portions  18 ,  20  have outlets  28 ,  30  for connection to the hot fluid stream  32  and the cold fluid stream  34 . The rotary heat exchanger also has sealing members  36 ,  38 ,  40 ,  42  positioned on both sides of the matrix  12  for separating the hot fluid stream  32  and the cold fluid stream  34 . In this embodiment, the sealing members preferably are shaped to isolate a surface of the matrix being exposed to either the hot or cold fluid stream and are arranged orthogonal to the matrix. Each sealing member preferably comprises a generally annular sector having a pair of arcuate walls  44 ,  46  connected by a pair of radial walls  48 ,  52 . The surface of each sealing member which faces the fluid inlet  24 ,  26  or fluid outlet  28 ,  30  is sealed thereto, preferably using a flexible member  54 ,  56 ,  58 ,  60  to effectively prevent leakage between each sealing member and inlet or outlet. An actuating system  52  is provided for discontinuously moving the sealing members. In one embodiment, the actuating system  52  is connected to a series of rods  51  in communication with sealing members  36 ,  38 ,  40 ,  42 , the rods  41  serving via actuating system  52  to move the sealing members toward and away from the matrix  112  at the appropriate time. 
     In operation, a portion of the matrix  12  is heated as the hot fluid stream  32  flows therethrough. When the matrix is rotated to place the heated portion in contact with the cold fluid stream  34 , the heated portion carries heat to the cold fluid which stream  34  flowing through inlet  24 . The matrix  12  preferably contains a plurality of internal, axially aligned passageways, (not shown) e.g. such as a honeycomb configuration, through which the hot and cold fluids pass as they travel through the matrix. A portion of the matrix  12  is heated by rotating through the hot fluid and is cooled as it is subsequently rotated through the cold fluid. In so doing, heat is transferred from the hot fluid to the cold fluid to preheat the cool fluid and recycle thermal energy in the system. The matrix  12  is rotated discontinuously in preselected increments, preferably of approximately 20° to approximately 180°. 
     The sealing members  36 ,  38 ,  40 ,  42  contact the matrix  12  when the matrix is stationary and are released from the matrix when the matrix is moved or just prior to matrix movement. By providing a seal only when the matrix is stationary, a substantially leakproof seal can be achieved and losses and wear associated with sliding, dynamic seals can be substantially reduced or eliminated. 
     FIGS. 2A-2D depict the operation of a rotary heat exchanger in which the matrix sealing members isolate a matrix portion only when the matrix is stationary. For ease of understanding, only a single sealing member  36  is shown in FIGS. 2A-2D, however it is to be understood that in actual operation a plurality of sealing members as depicted in FIGS. 1A and 1B would be in operation. In FIG. 2A, the sealing member  36  is shown contacting the matrix  12  and reference point  62  is provided to demonstrate the rotation of the matrix  12  during operation. FIG. 2B shows the sealing member  36  released from the matrix  12  and FIG. 2C shows the rotation of the matrix  12  during operation as the reference point  62  moves outside of the matrix region isolated by sealing member  36 . It is noted that the sealing member  36  need not be entirely released during the rotation of the matrix  12 , but rather may remain in loose contact with the matrix to reduce leakage during the phase in which the matrix is rotated. FIG. 2D shows the sealing member  36  contacting the matrix  12  after the matrix has moved a preselected increment. 
     A second preferred embodiment of the regenerative heat exchanger of the present invention is a modular heat exchanger comprising a pair of modular heat exchange units  100 , one of which is depicted in FIG.  3 . Each modular heat exchange unit comprises a housing  102  for containing a plurality of heat exchange modules  104 . The housing is maintained across a fluid stream  106  which may be either a hot or a cold fluid. A central portion  108  of the heat exchange unit is not enclosed to enable the fluid stream  106  to pass therethrough. A plurality of heat exchange modules  104  are maintained within the central portion  108  and are subject to heating by the fluid stream  106  if the stream is a hot stream, or releasing heat into the fluid stream  106  if the stream is a cold stream. Sealing members  110 ,  112  can be engaged or disengaged to alternately isolate the modules  104  contained within the housing  102  or to allow rows of modules to be moved into the housing through module inlet  114  and out of the housing through module outlet  116 . 
     Movement of the modules is achieved using a system of linear actuators  118 ,  120  which serve to transport individual rows or modules through the housing. The activators can be controlled by any of a variety of methods well known in the art, including but not limited to hydraulic means or solenoids. 
     The housing  102  further includes a plurality of walls  122 ,  124 ,  126 ,  128  which define a path for flow of the fluid stream through and across the housing. 
     In operation, a plurality of modules are conveyed into the housing through module inlet  114 . Once the modules have been conveyed into the housing, the sealing members  110  and  112  are engaged, effectively sealing the module inlet  114  and module outlet  116 . A fluid stream  106  is flowed across the housing  102  in the path defined by walls  122 ,  124 ,  126 ,  128  and, in the case of a hot fluid stream, heats the modules contained in the central portion  108  of the housing. The modules contained in the central portion  108  define a heat exchange matrix. After a predetermined time, the sealing members  110 ,  112  are released and the linear actuator  120  pushes the top row of modules through the module outlet. These modules, heated from their previous residence within the central portion are conveyed to a second identical heat exchange unit through which is flowed a cold fluid stream. Upon contact with that stream, the modules release heat, thereby serving to heat the stream. 
     Returning to heat exchange unit  100 , linear actuator  120  is retracted and linear actuator  118  is advanced, thereby moving each of the module rows within the housing up one row. Actuator  118  is then retracted and a new row of modules is advanced into the housing through the module inlet  114 . By connecting the module outlet of a first exchange unit with the module inlet of a second exchange unit and the module inlet of the first unit with the module outlet of the second exchange unit, a module circuit is defined. 
     FIGS. 4A-4C depict the movement of the modules  104  during operation of the modular heat exchanger. In FIG. 4A, the housing  102  is full of modules and a linear actuator  120  is moving the top row of modules (labeled A) horizontally to the left providing space for upward movement of modules (contained in rows labelled B-H). In FIG. 4B, a linear actuator is moving the modules of rows B-H in a vertical direction providing space for an incoming row modules (labelled I) to enter the housing  102  as shown in FIG.  4 C. 
     When using the modular heat exchanger system, one modular heat exchange unit  100  would be used for the hot fluid stream and a second modular heat exchange unit  100  would be used for the cold fluid stream. As depicted schematically in FIG. 5, heat exchange modules are conveyed along paths  200 ,  202  between the separate heat exchanger units  204 ,  206  in the system, first contacting the hot fluid stream  208  before being conveyed to the cold fluid stream  210 . Thus, in the example described previously, the heated modules would be transferred from a first heat exchange unit to a second heat exchange unit, and a fluid would be passed through the modules to either heat the modules or to extract heat therefrom. Heated modules would be cycled to lie in the path of the cold fluid stream and cooled modules would be cycled to lie in the path of the hot fluid stream. 
     The matrix is designed to accommodate the particular type of regenerative heat exchanger chosen. For example, an annular matrix can be used in a rotary heat exchanger. Alternatively, if a modular heat-exchanger system is used, the matrix comprises a plurality of separate heat exchange modules. 
     The individual modules are of a shape that can arrange to fit into a housing. In a preferred embodiment, the modules are rectangular, allowing them to be arranged in the housing in a symmetrical manner. 
     The matrix is fabricated from a material capable both of allowing fluid flow therethrough and withstanding the temperature and pressure conditions of the hot fluid stream. Examples of such materials include metals, stainless steels, ceramics, heat resistant cast alloys, refactory materials, thoria-disbursed alloys, graphite and carbon-fiber-reinforced carbon-based materials. The term ceramic is intended to include glass ceramic, silicon carbide, silicon nitride and many others. Examples of metals for high temperature regeneration include nickel-chromium-cobalt beryllium alloy. 
     The configuration of the matrix is of a form that allows fluid to flow through the matrix. Preferred configurations include porous matrices with barriers to prevent transverse flow, or a matrices having internal passageways directed in the direction of fluid flow. Examples of such configurations include honeycombs (see FIG.  6 A), corrugated portions (see FIG.  6 B), “egg crate” portions (see FIG.  6 C), and wire grids (see FIG.  6 D). 
     The matrix can be moved discontinuously using mechanisms that are art-recognized. For the rotary heat exchanger, the matrix can be carried on a rotor and moved discontinuously using a mechanical mechanism such as a Geneva-drive. Alternatively, a stepping motor or electronic control device can be used to provide the discontinuing rotation. 
     In a modular system, the modules are moved discontinuously using linear actuators arranged to provide movement in both the vertical and horizontal directions. Linear actuators are art-recognized and can be comprised of hydraulic or air-piston actuators, mechanical actuators or electromagnetic devices such as solenoids. 
     The sealing members of this invention can be made of any material capable of sealing against fluid leakage and surviving the temperature and pressure conditions of the system. Examples of such materials include graphite, metals, ceramics and pressed carbon. It should be understood that some materials can be appropriate for the hot side of the matrix receiving the hot fluid, e.g., stainless steel or another high-temperature metal, and other materials can be appropriate for the side receiving the cold fluid, e.g., graphite. In a rotary heat exchanger, the sealing members can be arranged such that they contact the matrix leaving about one half to three fourths of the surface area of the matrix for contacting the hot fluid and about one-fourth to one half of the surface area of the matrix for contacting the cold fluid. 
     In one embodiment of operation, the sealing members are clamped to the matrix when it is stationary and released when the matrix is in motion as shown in FIGS. 2A-2C for rotary heat exchangers. The clamping action of the sealing members can be provided by a variety of art recognized mechanisms. One example of such a mechanism includes a differential-pressure system in which pressure is used to press the seal against the matrix. Another mechanism that can be used is that having a piston and bellows for pushing the seal against the matrix as described in Cox et al., Internal Combustion Turbines, pp 193-205; particularly see FIG. 12, the contents of which are hereby expressly incorporated by reference. 
     The clamping action of the sealing members is provided when the sealing members are either contacting or are very close to the matrix, e.g. within 0.001″ of the matrix surface. Seals for rotary heat exchangers have been movable or slidable in the prior art but such prior art seals were dynamic seals which followed the contour of the matrix surface as the matrix was rotated. Prior art seals have not been provided with the clamping action of the present invention. 
     The seal leakage rate is the amount of fluid that leaks through or across the seal. It is generally stated as a proportion or percentage of the higher-pressure flow. This rate can be determined directly by capturing the escaping fluid around the seal or by measuring and comparing the flow of fluid going into one side of the heat exchanger against the flow of fluid coming out the other side of the heat exchanger. Conventional rotary heat exchangers in small gas turbines have a seal leakage rate generally in the range of 2-10%. The rotary heat exchangers of the present invention can have seal-leakage rates as low as about 0.5%. 
     The housing and ducting of the heat exchanger can be made of a material capable of withstanding the heat and pressure requirements of the system. Examples of such materials for gas-turbine applications include stainless steel, ceramics and aluminum. 
     This invention also pertains to methods for transferring heat between a hot fluid and a cold fluid. The methods can be conducted using the heat exchangers discussed above. In the method, the matrix or ducting is discontinuously moved in preselected increments. Portions of the matrix are isolated by the sealing members when the matrix or ducting is stationary creating a substantially leakproof environment and the sealing members is released from the matrix when the matrix or ducting is moved or just prior to movement. The discontinuous movement and contacting steps are preferably repeated a plurality of times. 
     The movement in preselected increments can be either a rotary movement when the method is being conducted using a rotary heat exchanger or a linear increment when the method is being conducted using a modular heat exchanger. In a rotary system, the preselected increments are typically between 20° and 120° of rotation. In a modular system, the preselected increments are typically equal to the distance required to move a single row of modules into the heat exchanging section. 
     The fluids of this invention can include liquids and gases, but the invention is preferably directed at heat exchangers useful for exchanging heat between gas streams. 
     The benefit derived from using the method of the present invention is the creation of a regenerator having substantially reduced flow leakage. A substantially leakproof environment is an environment in which there is no leakage or the leakage is significantly reduced, e.g. to a value of approximately 5%-20% of that currently achieved using conventional regenerative heat exchangers. 
     EQUIVALENTS 
     Those skilled in the art will be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. 
     These and all other equivalents are intended to be encompassed by the following claims.