Patent Application
20160377350
The present invention generally relates to heat exchangers and, more particularly, apparatus and methods for minimizing strain in heat exchangers to achieve a longer useful life or enabling the heat exchanger to tolerate a more severe operating environment.
Plate-fin heat exchangers are brazed to form a core that is a monolithic structure where the each tube sheet layer is locked into close synchronicity with its two neighboring tube plate. Each of the internal tube sheets is connected to a neighboring tube sheet via the hot side fins and to the other neighboring tube sheet via the cold side fins. With this rigid, non-compliant construction, heat exchangers can exhibit a significant strain when the heat exchanger has a temperature difference placed on the unit, especially when the operationally temperature difference is large. This strain can lead to conditions where, with repeated exposure, the tube sheet cracks, thus negatively impacting the performance of the system and the useful life of the heat exchanger. A typical situation where this issue may occur is with bleed air pre-coolers that are found on most commercial (large and small) and military environmental control/bleed air systems. Any heat exchanger that is repeatedly exposed to rapid and large temperature differences may exhibit this issue.
As can be seen, there is a need for improved apparatus and methods for reducing strain in heat exchangers.
In one aspect of the present invention, a heat exchanger comprises a hot passage; a cold passage adjacent the hot passage; a pair of tube sheets, with one tube sheet on each opposing side of one of the hot passage and the cold passage; a fin in one of the hot passage and the cold passage; wherein the fin includes a first distal portion, a second distal portion, and an intermediate portion between the first and second distal portions; wherein the fin is configured to provide, at only the first distal portion and the second distal portion, three degrees of freedom of movement within one of the hot passage and the cold passage.
In another aspect of the present invention, a plate fin heat exchanger comprises a hot passage; a cold passage adjacent the hot passage; a pair of tube sheets on opposing sides of the cold passage; a fin in the cold passage; wherein the fin includes a first distal portion, a second distal portion, and an intermediate portion between the first and second distal portions; wherein the fin, at only the first distal portion and the second distal portion, is separated laterally into a first lateral portion and a second lateral portion.
In a further aspect of the present invention, a plate fin heat exchanger comprises a plurality of cold passages; a plurality of fins in the cold passages; a plurality of hot passages; a plurality of tube sheets; wherein the cold passages, hot passages and tube sheets are positioned in an recurring pattern of cold passage, tube sheet, hot passage, and tube sheet; wherein one of the plurality of fins has a distal portion with a divided configuration to provide three degrees of freedom of movement of the fin at the distal portion.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Various inventive features are described below that can each be used independently of one another or in combination with other features.
Broadly, embodiments of the present invention generally provides a heat exchanger that can minimize the locking of a tube sheet to its two neighboring tube sheets. In embodiments, the heat exchanger can include a hot circuit usually at high pressure and a cold circuit at relatively low pressure. This is the case for bleed air pre-coolers found on commercial and military aircraft. Tube sheets that surround a hot passage can be brazed to the hot side fin to provide for pressure containment of the high pressure fluid. However, the tube sheets that surround the cold side passage are not connected or otherwise affixed, in certain locations, to one another. This is possible due to the low pressure of the fluid in this passage.
A core of the heat exchanger 10 can include a plurality of cold passages 13 that can receive the cold flow 11 at the cold inlet face 17 of the heat exchanger 10. The cold passages 13 can include a plurality of main or primary cold passages 13a that may be generally located in a center area of the core. The cold passages 13 can further include outer or distal cold passages 13b at one side of the main cold passages 13a, in addition to outer or distal cold passages 13c at an opposite side of the main cold passages 13a. The outer passages 13b, 13c can be at the far ends/side areas of the core.
The outer cold passages 13b can include a distal portion or area 13b-1 and a distal portion or area 13b-2 at opposed corners of the heat exchanger 10 where the cold passages 13 meet hot passages 14 as described below. Between the distal portions 13b-1, 13b-2 can be an intermediate portion 13b-5. Similarly, the outer cold passages 13c can include a distal portion or area 13c-1 and a distal portion or area 13c-2 at opposed corners of the heat exchanger 10 where the cold passages 13 meet the hot passages 14. Between the distal portions 13c-1, 13c-2 can be an intermediate portion 13c-5.
In embodiments of the distal portion 13b-2, the height 13b-4 can be measured in numbers of cold passages 13b. For example, the number of cold passages 13b in height 13b-4 can be from about 4 to about 8 passages specifically and from 2 to 10 as a general range, as an example. The number of passages that form the passages 13b can depend on the thermal stresses that are induced upon the core with the operational conditions.
In embodiments of the distal portion 13b-2, the ratio of the length 13b-3 to the height 13b-4 may be from about 0.5 to about 1.0 specifically and from 0.2 to 2.0 as a general range.
In embodiments, the length 13b-3 may be from about 0.5 to about 1.5 inches specifically and from 0.2 to 2 inches as a general range.
In embodiments, the ratio of the height 13b-4 to the overall height of the cold inlet face may be from about 5% to about 10% specifically and from 3% to 18% as a general range, though this number is dependent upon the cold inlet face width which can vary greatly for the heat exchanger.
The foregoing ratios may be the same or different among the other distal portions 13b-1, 13c-1, and 13c-2.
Referring back to
Further, the core of the heat exchanger 10 can include a plurality of tube sheets 15 that, in an embodiment, can form a recurring pattern of cold passage 13, tube sheet 15, hot passage 14, and tube sheet 15 (
In the core of the heat exchanger 10, a fin may be disposed in one or more cold passages 13. Differently configured fins may be disposed in different cold passages 13. In an embodiment, a fin 17 having a main or base configuration can be disposed in one or more of the main cold passages 13a. In an embodiment, a fin 16 having an outer or dual configuration can be disposed in one or more of the outer cold passages 13b, 13c.
For example, the fin 17 may have a wave or sinusoidal configuration over its entire cross section—from a first distal portion at a first end of the fin 17 and to a second distal portion at a second send of the fin 17 (
In contrast, and as an example, the fin 16 may have a wave or sinusoidal configuration 16c but only over its cross section in the intermediate portion (e.g., 13b-5). At one or both of its first and second distal portions, respectively located at its first and second distal ends, the fin 16 may have a divided wave or sinusoidal configuration. In other words, the fin may be separated laterally through its cross section at one or both of its distal portions or ends, but not in its intermediate portion. Thereby, the fin 16, at one or both of its distal portions/ends can have a first lateral portion 16a and a second lateral portion 16b (
In embodiments, the fin may be laterally separated at its cross-sectional mid-point (
As depicted in
In yet another embodiment, a splitter plate may be disposed laterally through a fin, keeping fin 16a′ and 16b′ separated. To relieve stress, portions of the fin at its distal ends may be unbrazed to the fins or otherwise disconnected from the plate. Or, the plate may be eliminated at the distal ends of the fin.
If the disconnection of the complete cold side fin causes other issues (such as pressure containment), the fins could be modified in select locations such as near the hot-hot, hot-cold and cold-hot corners, for −2 inches into the core and −2 inches away from the bar (
For embodiments, preliminary structural analysis and investigation of strain range reduction associated with this invention is 17% on a typical commercial transport pre-cooler for the worst transient maneuver if applied to the whole heat exchanger. The strain reduction is 8% if the cold side fin cut were applied only to the top and bottom six cold side passages with the passages in the center of the core unmodified. This is a significant strain decrease and would result in a much more increase in estimated pre-cooler useful life than the 17% due to the associated strain reduction. This analysis is preliminary in nature and was conducted without reflecting the reduction in the stackwise conduction due to the fin cut at the mid-plane. The structural analysis was conducted with a complete disconnection of the fins in the three directions at the mid-plane.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
