SHELL AND TUBE HEAT EXCHANGER

Abstract

A heat exchanger is provided that includes a shell defining a first fluid space and one or more tubes within the first fluid space having interiors fluidly isolated therefrom. The tubes define a second fluid space and are configured to permit thermal energy transfer between the first fluid space and the second fluid space. One or more heat pipes are disposed within one of the first fluid space and the second fluid space and are configured to transfer thermal energy within the respective fluid space.

Description

BACKGROUND OF THE INVENTION

The embodiments herein generally relate to heat exchangers and more particularly to shell and tube heat exchangers.

Numerous heat exchangers have been devised for transferring heat stored in a first medium or fluid to a second medium or fluid. One example of a heat exchanger for high temperature/high pressure applications is a shell and tube heat exchanger. Several features are essential for efficient heat transfer in shell and tube type heat exchangers.

A large tube surface area is necessary for effective heat transfer, wherein the surface area increases with tube length and tube diameter. However, the advantage gained from a larger tube diameter is offset by a decreased thermal energy exchange which results from the medium inside of the large tubes tending to flow through the middle area of the tube where thermal energy transfer is lowest rather than adjacent the peripheral tube wall where thermal energy exchange is greatest. Further, a long tube length poses a problem with longitudinal expansion. When a high temperature shell fluid is employed, the tube temperature increases resulting in thermal expansion of the tubes, which can lead to damage and/or leaks between the mediums. Thus, there are size constraints that impact the efficiency of tube and shell heat exchangers, resulting in smaller heat exchangers.

Another factor affecting the thermal energy transfer between mediums is the flow of the fluids in relation to each other. Optimum thermal energy transfer is achieved when the shell fluid and tube fluid are in a contraflow, or counter-flow, configuration allowing for small heat exchangers that are efficient. However, in extreme temperature conditions, a counter-flow configuration may not be sufficient to warm a cold fluid at the point where the cold fluid enters the heat exchanger. If the cold fluid is not warmed sufficiently, icing or other impacts on fluid flow may occur.

BRIEF DESCRIPTION OF THE INVENTION

According to one embodiment, a heat exchanger is provided that includes a shell defining a first fluid space and one or more tubes within the first fluid space having interiors fluidly isolated therefrom. The tubes define a second fluid space and are configured to permit thermal energy transfer between the first fluid space and the second fluid space. One or more heat pipes are disposed within one of the first fluid space and the second fluid space and are configured to transfer thermal energy within the respective fluid space.

According to another embodiment, a method of transferring thermal energy between two mediums is provided. The method includes providing a heat exchanger defining a first fluid space and a second fluid space that is fluidly isolated from the first fluid space, the heat exchanger configured to allow thermal energy transfer between the first fluid space and the second fluid space, and providing one or more heat pipes within one of the first fluid space and the second fluid space, the heat pipes configured to transfer thermal energy within the respective first fluid space or second fluid space.

Technical effects of embodiments of the invention include providing an improved heat exchanger that enables efficient thermal energy transfer between mediums, or fluids, in a shell and tube heat exchanger that is configured for high pressure applications. Further, thermal energy transfer for a given heat exchanger size can be optimized in accordance with embodiments disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional illustration of an exemplary shell and tube heat exchanger;

FIG. 2A is a schematic view of a heat exchanger showing a parallel-flow configuration;

FIG. 2B is a relative temperature plot of the temperatures of the mediums within the parallel-flow heat exchanger of FIG. 2A as they flow therethrough;

FIG. 3A is a schematic view of a heat exchanger showing a counter-flow configuration;

FIG. 3B is a relative temperature plot of the temperatures of the fluids within the counter-flow heat exchanger of FIG. 3A as they flow therethrough;

FIG. 4 is a cross-sectional illustration of a heat exchanger in accordance with an exemplary embodiment of the invention;

FIG. 5 is a relative temperature plot of the temperatures of the fluids within the heat exchanger of FIG. 4 as they flow therethrough.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a cross-sectional illustration of an exemplary shell and tube heat exchanger 100 is shown. The heat exchanger 100 includes a shell 102 and one or more tubes 104 located within the shell 102. Shell 102 defines a domed pressure vessel having a cylindrical body 106, a domed first end 108, and a domed second end 110. Of course, the first and second domed ends 108, 110 could take on other shapes and/or geometries.

The cylindrical body 106 defines a first fluid space, labeled as interior shell space 112, located in the center of the shell 102 and bounded at a first end by a first tube sheet 114 and at a second end by a second tube sheet 116. The first end tube sheet 114 and the second end tube sheet 116 fluidly isolate the shell space 112 from a first end cavity 128 and a second end cavity 130. The first end cavity 128 and the second end cavity 130 are fluidly connected by the interior(s) of the one or more tubes 104. A second fluid space may be defined as the volume within the tubes 104, and may further include the first and second end cavities 128, 130. It shall be understood that in order for the first and second end cavities 128, 130 to fluid connect to the tubes 104, at least one tube 104 may pass completely through each tube sheet 114, 116.

A first medium 101, such as a fluid, flows through the shell space 112 by entering the shell space 112 at a point 103 through first port 118 and exiting the shell space 112 at a point 105 through second port 120. The first medium in the shell space 112 is in contact with the exterior surfaces of the tubes 104. This allows for thermal energy transfer between a medium within the shell space 112 (first medium 101) and a medium within the tubes 104 (second medium 107), without mixing of the two mediums. The flow path of the first fluid within the shell space 112 can be controlled or directed by the inclusion of one or more baffles 122, 124. As shown in FIG. 1, the first medium enters the first port 118 and flows downward, around the first baffle 122, upward and around the second baffle 124, and then downward and out the second port 120, as indicated by the arrows within the shell space 112. The first medium generally flows from left to right in FIG. 1, and defines a first fluid path.

A second medium 107 flows through the heat exchanger 100 along a second fluid path. The second medium 107 enters the heat exchanger 100 at point 109 through a third port 126 and enters the first end cavity 128. The second medium 107 then flows through the tubes 104 and into the second end cavity 130. The second medium 107 will then exit the heat exchanger 100 at point 111 by way of a fourth port 132. Similar to the first medium 101, the second medium 107 also flows generally from left to right through heat exchanger 100 in FIG. 1.

As noted, the first tube sheet 114, the second tube sheet 116, and the tubes 104 fluidly isolate the first medium 101 and the second medium 107 from each other to prevent mixing. This allows for the first medium 101 and the second medium 107 to be of different compositions and, more importantly, of different temperatures. The tubes 104 are formed from thermally conductive material(s) in order to transfer thermal energy from the first medium 101 to the second medium 107, or vice versa. For example, thermal energy from a relatively warm or hot medium can be transferred to a relatively cool or cold medium when passing through the heat exchanger 100.

In order to facilitate heating of a cold medium (or cooling of a hot medium), the cold medium is passed through the heat exchanger 100 in one of the shell space 112 and the tubes 104, such as shown in FIG. 1. At the same time a hot medium is passed through the heat exchanger 100 in the other of the shell space 112 and the tubes 104. For example, the cold medium may be a fuel for an aircraft and the hot medium may be oil of an aircraft. Due to the low temperatures and other conditions of flight, the fuel may chill to temperatures that are sufficient to cause icing. The icing results from water that is in the fuel freezing and forming ice crystals that may clog lines through which the fuel flows and either reduces the fuel flow or, in extreme cases, may prevent fuel flow entirely. To heat the cold fuel and prevent icing, the cold fuel is passed through the tubes 104 and the hot medium, e.g., hot oil, is passed through the shell space 112. The hot medium surrounds the tubes 104 and transfers heat through the surfaces of the tubes 104, thus heating the fuel.

As shown in FIG. 1, the first fluid path and the second fluid path flow generally in the same direction, i.e., generally from left to right. This fluid flow configuration is a parallel-flow configuration (see FIG. 2A). As an example, in parallel-flow heat exchangers, the two mediums may enter the heat exchanger 100 generally at the same end (118, 126) and flow in the same general direction, relatively parallel to one another (arrows of FIG. 1), to the other end (120, 132) of the heat exchanger 100. An advantage of a parallel-flow configuration is that the hottest point of the hot medium is adjacent to the coldest point of the cold medium. Accordingly, the two mediums start at the highest temperature difference and approach the same temperature when they exit the heat exchanger. Advantageously, in the case of aircraft fuel, a parallel-flow configuration can prevent icing at the point that the fuel is at it coldest by locating the hottest temperature oil in proximity to the coldest fuel.

In an alternative configuration, one of the mediums flows from right to left in FIG. 1, i.e., the fluids flow opposite to each other. This is an example of a counter-flow, or contraflow, configuration (see FIG. 3A). In counter-flow heat exchangers the mediums enter the heat exchanger from opposite ends, for example, and flow in opposite directions. This results in the temperature at the outlet/exit of each medium approaching the temperature at the inlet/entry of the other medium. An advantage of counter-flow heat exchangers is that they can optimize the thermal energy transfer efficiency between the mediums for given heat exchanger sizes. Thus, a counter-flow configuration is preferred when size is a constraint or factor.

FIGS. 2A, 2B, 3A, and 3B illustrate the differences between parallel-flow and counter-flow configurations.

Turning to FIG. 2A, a parallel-flow heat exchanger 200 is shown. Although schematically shown, elements of heat exchanger 200 are substantially similar to heat exchanger 100 of FIG. 1; thus like features are preceded with a “2” rather than a “1.” In the parallel-flow heat exchanger 200, a first medium 201 is a relatively hot fluid that enters on the left side of FIG. 2A at point 203, cools off as it transfers thermal energy to the second medium 207 while passing through the shell space 212, and exits the heat exchanger 200 on the right side at point 205. The medium fluid 207 is a relatively cold fluid that enters on the left side of FIG. 2A at point 209, warms up as thermal energy is transferred to it from the relatively hot first medium 201 while passing through tubes 204, and exits the heat exchanger 200 on the right side at point 211. This configuration enables the hottest point of the hot fluid to be in thermal contact with the coldest point of the cold fluid. As the mediums 201, 207 pass through the heat exchanger 200, they will approach the same temperature, as shown in FIG. 2B.

A relative temperature gradient representative of the first and second mediums 201, 207 passing through the parallel-flow heat exchanger 200 is shown in FIG. 2B. The solid line represents a relative temperature of the first medium 201 as it passes through the heat exchanger 200, from point 203 (inlet/entry) to point 205 (outlet/exit). The dashed line represents the temperature of the second medium 207 as it passes from point 209 (inlet/entry) to point 211 (outlet/exit). The arrows indicate relative direction of flow of the two mediums 201, 207 through heat exchanger 200. As shown, the first medium 201 starts at a relatively high temperature at point 203 and then decreases in temperature to point 205 as thermal energy is transferred away from the first medium 201. In contrast, as thermal energy is transferred to the second medium 207, the temperature of the second medium 207 increases from point 209 to point 211. The parallel fluid flow enables a high transfer rate of energy from the hot medium to the cold medium quickly, and thus prevents icing, e.g., the hot medium is provided at the coldest location in the heat exchanger to prevent icing in the cold medium. Specifically, when both mediums enter the heat exchanger, the hottest temperature of the first medium 201 at point 203 is adjacent to the coldest temperature of the second medium 207 at point 209. This presents the highest temperature gradient between the two mediums, and thus the best solution to counter icing.

Turning now to FIG. 3A, a counter-flow heat exchanger 300 is shown. Although schematically shown, elements of heat exchanger 300 are substantially similar to heat exchanger 100 of FIG. 1; thus like features are preceded with a “3” rather than a “1.” In the counter-flow heat exchanger 300, a first medium 301 is a relatively hot fluid that enters on the left side of FIG. 3A at point 303, cools off as it transfers thermal energy to the second medium 307 while passing through the shell space 312, and exits the heat exchanger 300 on the right side at point 305. The second medium 307 is a relatively cold fluid that enters on the right side of FIG. 3A at point 309, warms up as thermal energy is transferred to it from the relatively hot first medium 301 while passing through tubes 304, and exits the heat exchanger 300 on the left side at point 311. This configuration enables the mediums to maintain a relatively constant temperature gradient as they pass through the heat exchanger 300, as shown in FIG. 3B.

A relative temperature gradient representative of the first and second mediums passing through the counter-flow heat exchanger 300 is shown in FIG. 3B. The solid line represents a relative temperature of the first medium 301 as it passes through the heat exchanger 300, from point 303 (inlet/entry) to point 305 (outlet/exit). The dashed line represents the temperature of the second medium 307 as it passes from point 309 (inlet/entry) to point 311 (outlet/exit). The arrows indicate relative direction of flow of the two mediums 301, 307 through heat exchanger 300. As shown, the first medium 301 starts at a relatively high temperature at point 303 and then decreases in temperature to point 305 as thermal energy is transferred away from the first medium 301. In contrast, the second medium 307 flows in the opposite direction, as indicated by the arrows, and is at the coldest temperature at point 309 and the warmest temperature at point 311. The counter fluid flow enables a consistent thermal energy transfer that is efficient and enables the heat exchanger 300 to be optimized for sizing.

Regardless of the type of heat exchanger, the principle of operation is to have two mediums of different temperatures brought into close contact but prevent the mediums from mixing. This allows for cold mediums to be warmed and warm mediums to be cooled without energy being added or removed from the system; it is merely an exchange of thermal energy between the mediums. Further, there is also a change in pressure in the mediums, as the temperature changes, which transfers energy, e.g., a pressure drop occurs as each fluid moves from the entrance of the heat exchanger to the exit of the heat exchanger, transferring energy. In the example of heat exchangers employed in aircraft, size and weight constraints apply, in additional to the requirement of providing a vessel for high pressure mediums. Due to the size and weight constraints, a counter-flow shell and tube heat exchanger provides the best advantage, but due to icing problems during flight, parallel flow may be preferred.

Turning now to FIG. 4, a heat exchanger 400 in accordance with an exemplary embodiment of the invention is shown. Heat exchanger 400 includes similar features as heat exchanger 100 of FIG. 1; thus like features are preceded with a “4” rather than a “1.” Similar to heat exchanger 100 of FIG. 1, heat exchanger 400 includes a shell and tube assembly, with similar components as described above and is arranged as a parallel-flow configuration. The primary difference between heat exchanger 400 and the embodiments described above is the inclusion of heat pipes 450, 452, which may be dimpled heat pipes. Heat pipes as used herein refer to thermal-transfer devices that combine the principles of both thermal conductivity and phase transition to efficiently manage the transfer of thermal energy between two solid interfaces. At a hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid—releasing the latent thermal energy. The liquid then returns to the hot interface through capillary action, centrifugal force, gravity, or other process, and the cycle repeats.

The addition of heat pipes 450, 452 allows for a parallel-flow heat exchanger to include the benefits of a counter-flow heat exchanger, i.e., optimization of thermal energy transfer efficiency, and thus the size of the heat exchanger can be optimized with the benefits/advantages of both parallel-flow and counter-flow heat exchanger configurations. The materials and mediums of the heat pipes are configured such that the mediums of the heat exchanger will cause a phase transition of the heat pipe medium, thus enabling efficient intra-medium thermal transfer.

As shown in FIG. 4, heat pipes 450 are included within the tubes 404 of the heat exchanger 400. The heat pipes 450 allow for thermal energy transfer within the fluid that passes through the tubes 404. Similarly, heat pipes 452 are included within the shell space 412 and allow for thermal energy transfer within the fluid that passes through the shell space 412. Accordingly, in heat exchanger 400, there are two types of thermal energy transfer. First, thermal energy transfer occurs between the first and second mediums through the tubes 404 without mixing of the first and second mediums, similar to that described above (inter-medium thermal transfer). Second, thermal energy transfer occurs within the first medium and within the second medium because of the heat pipes 450, 452 (intra-medium thermal transfer).

In operation, in the parallel-flow heat exchanger 400 of FIG. 4, the temperature extremes of the two mediums occur at the entry point to the heat exchanger 400, which are adjacent. The first medium enters at the first port 418 at a high temperature (hot fluid), and the second medium enters at the third port 426 at a low temperature (cold fluid). Thus, the hottest temperature of the first medium is adjacent to the coldest temperature of the second medium, which prevents icing, as discussed above with respect to a parallel-flow configuration. With the addition of the heat pipes 452, located in shell space 412, the high temperature of the first medium within the shell space 412 is transferred toward the portions of the shell space 412 where the first medium is cooler. Similarly, in the tubes 404, the heat pipes 450 allow for the warm thermal conditions of the second medium located toward the second cavity 430 to be carried back toward the first cavity 428, thus providing additional heat to the cold second medium.

As shown in FIG. 5, a relative temperature plot representative of the temperatures of the first and second mediums 401, 407 as they flow through heat exchanger 400 is shown. The entry points of first port 418 and third port 426 are shown on the left side of the plot and indicate the largest temperature difference between the two mediums. However, because the heat pipes 450 and 452 are included, the temperature difference between the first medium 401 and the second medium 407 equalizes very quickly, and provides a relatively constant temperature gradient between the first and second mediums 401, 407 throughout heat exchanger 400. This enables an optimized thermal energy transfer similar to a counter-flow configuration, but also includes the inlet temperature advantages of a parallel-flow configuration.

Advantageously, embodiments of the invention provide maximum thermal energy transfer and maximum absolute pressure capability for a given volume. Furthermore, advantageously, icing within a fuel line, such as on an aircraft, can be efficiently prevented. Moreover, heat pipes added to a shell and tube heat exchanger provide a uniform temperature gradient and thermal energy transfer throughout the heat exchanger while maintaining the benefit of icing prevention and optimizing the heat exchanger size.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions, combination, sub-combination, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments.

For example, although described herein as a particular shell and tube heat exchanger in each of the embodiments, other types of shell and tube heat exchangers may employ heat pipes without departing from the scope of the invention. One such alternative configuration is a U-shaped shell and tube heat exchanger, with heat pipes located within the U-shaped tubes and within the shell space of the heat exchanger. Furthermore, variations of shell and tube heat exchangers may include any number of tubes, shapes, sizes, and/or configurations without departing from the scope of the invention. Moreover, although described above in FIG. 4 with heat pipes located within both the tube space and the shell space, alternative embodiments may include heat pipes in only one of the tube space and the shell space. Further, although shown as having a heat pipe in each tube, this is merely an example, and any number of heat pipes may be used in each of the tube space and the shell space of the heat exchanger. The mediums discussed above are also not limiting, and other mediums beside fuels and oils may be employed, either as the hot medium or as the cold medium, and the type or composition of the medium is not intended to be limiting. Moreover, different types of heat exchangers that are not tube and shell may employ similar heat pipes or heat transfer devices without departing from the scope of the invention.

Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A heat exchanger comprising:

a shell defining a first fluid space;

one or more tubes within the first fluid space having interiors fluidly isolated therefrom, the tubes defining a second fluid space and configured to permit thermal energy transfer between the first fluid space and the second fluid space; and

one or more heat pipes disposed within one of the first fluid space and the second fluid space and configured to transfer thermal energy within the respective fluid space.

2. The heat exchanger of claim 1, further comprising a first medium configured to flow through the first fluid space, and a second medium configured to flow through the second fluid space.

3. The heat exchanger of claim 2, wherein the first medium is a relatively hot oil and the second medium is a relatively cold fuel.

4. The heat exchanger of claim 2, wherein the first medium and the second medium flow in generally parallel directions through the respective fluid spaces.

5. The heat exchanger of claim 2, wherein the first medium and the second medium flow in generally opposite directions through the respective fluid spaces.

6. The heat exchanger of claim 1, wherein the one or more heat pipes define at least one first heat pipe disposed within the first fluid space, the heat exchanger further comprising at least one second heat pipe disposed within the second fluid space.

7. The heat exchanger of claim 1, configured to be installed on an aircraft.

8. A method of transferring thermal energy between two mediums, the method comprising:

providing a heat exchanger defining a first fluid space and a second fluid space that is fluidly isolated from the first fluid space, the heat exchanger configured to allow thermal energy transfer between the first fluid space and the second fluid space; and

providing one or more heat pipes within one of the first fluid space and the second fluid space, the heat pipes configured to transfer thermal energy within the respective first fluid space or second fluid space.

9. The method of claim 8, further comprising providing one or more additional heat pipes within the other of the first fluid space and the second fluid space, the one or more additional heat pipes configured to transfer thermal energy within the respective first fluid space or second fluid space.

10. The method of claim 8, wherein the first fluid space is defined by a shell and the second fluid space is defined by one or more tubes that pass through the shell.

11. The method of claim 8, further comprising providing a first medium within the first fluid space and a second medium within the second fluid space.

12. The method of claim 11, wherein the first medium is a relatively hot oil and the second medium is a relatively cold fuel.

13. The method of claim 11, wherein the first fluid and the second fluid flow in generally parallel directions through the respective fluid spaces.

14. The method of claim 11, wherein the first fluid and the second fluid flow in generally opposite directions through the respective fluid spaces.