CA1069883A - Compact primary surface heat exchanger - Google Patents

Abstract

COMPACT PRIMARY SURFACE HEAT EXCHANGER

A B S T R A C T
A compact primary surface heat exchanger is disclosed which includes a plurality of transversely corrugated sheets arranged in a stack in superimposed crest-to-crest askewed bridging relation to define first and second serpentine fluid flow paths alternately between them, and with each of the sheets having a plurality of repetitive transverse convolutions which are so constructed as to provide a plurality of sub-stantially vertically extending and substantially parallel walls with an integrally joining plurality of crest members to provide an effective heat exchanger with a decreased number of sheets.

Description

~8~3 Background of the Invention Stacked plate heat exchangers are continually being developed for use with internal combustion engines in order to improve the overall efficiency thereof. For example, a heat exchanger of the type shown in U.S. Patent No. 3,291,206 issued December 13, 1966 to T. P. Nicholson, and subsequently improved as disclosed in U.S. Patent 3,759,323 issued September 18, 1973 to H. J. Dawson et al and assigned to the assignee of the present invention~ has been found particularly effecti.ve in reducing the fuel comsump-tion of a gas turbine engine system. Such heat exchanger includes an alternating series of thin, equally folded corrugated sheets arranged in a stack to define alternating fluid passages for transferring heat from hot engine exhaust gas in a counterflowing manner to relatively cool air which is utilized in the combustion process. While the referenced primary surface heat exchanger is a considerable advancement in the art, it has a relatively large number of heat transferring sheets and the sizing of its passages for the two fluids flowing through it is not specifically tailored for maximum effectiveness, which results in a higher than desired pressure drop and an overall volume or package size larger than necessary for a given performance.
Further exemplifying the art is the gas-to-air heat exchanger disclosed in USAAVLABS Technical Report No. 65-37, published July, lg65 under contract with the U.S. Army Aviation Material Laboratories and entitled "Heat Regenerative System for T53 Shaft Turbine Engines". The heat exchanger disclosed in the referenced report is set forth as belng particularly suitable for use with an aircraft .. . . . . .
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1~16~3 gas turblne engine because of its corrugated stacked plate construction, which construction establishes a ratio of gas-to-air cross sectional flow area of 1.5:1. Undesirably, however, such heat exchan~er discloses sheets which are brazed or welded together at the internally matched crests thereof to form a rigidly aligned assembly and to avoid possible fretting at these locations. A further disadvantage is that the sheets have relatively flat corrugated or repeti-tively profiled patterns transverse to the direction of fluid flow which are arranged in facing relation to define straight fluid passages therethrough. This disadvantageously results in a high number of sheets, the expensive requirements of joinably bra~ing them, and a relatively structurally weak heat exchanger.
As is to be expected, it is extremely difficult to properly proportion the surfaces of the individual sheets for maximum effectiveness and economy. By way of example, reference is made to Stanford University Technical Report No. 23 by W. M. Kays and A. L. London dated November 15, 1954 and entitled "Compact Heat Exchangers--A Summary of Basic ~Ieat Transfer and Flow Friction Design Data", which discloses a conventional plate-fin type heat exchanger, rather than the primary surface heat exchangers noted immediately above, with a considerable number of alternate surface con-figurations for the individual sheets. The equally foldedcorrugated sheets of this report are alternately interleaved with flat plates to provide relatively elongated flow passages with performance somewhat analogous to that obtained with long tubes. Naturally, it will be appreclated it is not enough ~ust to remove the flat plates shown in the reference, in .. . . .

order to convert the plate-f;n heat exchanger to the primary surface type because of the la~ter's complex construction.
For example, directing ~low to two fluids to and from the primary surface heat exchanger is difficult when attempting to solve the pressure drop and volumetric requirements, and another major consideration is the stacking of the plates so that they will not nest and will remain in predetermined posltions .
~ligh performance heat exchanger sheet surfaces have been extremely difficult to make, and as far as is known, it ~as not until the development of the sheet material form-ing apparatus disclosed in United States Patent No, 3,892,119, issued July 1, 1975 to K. J. Miller, et al and assigned to the assignee of the present invention, that relatively thin sheet metal material in a range of from two to eight mils in thickness could be effectively formed into corrugated sheet with a large number of repetitive convolutions per inch trans-verse to the flow direction, a greatly extended height, and sinuous profiling in the direction of fluid flow. This improvement in the art allows a relatively large heat transfer surface area per unit volume, and attractively offers the potential of reducing the total number of profiled sheets needed, with further savings resulting from minimizing the brazing and sealing of the marginal edges thereof.
In accordance with the invention, a primary sur-face heat exchanger comprises a plurality of transversely cor-rugated sheets which are arranged in a stack with the corruga-tion crests of adjacen~ sheets in contact with one another and crossing over one another at their points of contact to provide first and second tortuous fluid passageways alternately ~33 between them, the corrugations of each sheet prcsenting d repetitivé
pattern with a plurality of substantially parallel wall portions extending substantially perpendicularly to the general plane of the sheet and integrally joined by a crest portion, the transverse corrugations of the sheets being so profiled that the first and second fluid passageways have different cross-sectional areas.
The sheets of this heat exchanger have an effective profile allowing the total number of the sheets to be significantly reduced with accompanying economical advantages.
The ratio of the cross-sectional areas may be so chosen to minimize the overall volume of the heat exchanger for a given performance requirement and at an acceptably low or reduced pressure drop.
Preferably, each of the transverse corrugations is sinuously longitudinally profiled in the general plane of the sheet. This sinuous -~
profiling in the direction of fluid flow may lead ~o improved heat transfer, improved sheet rigidity for pressure loading, and prevent nesting of adjacent sheets.
Brief Description of the Drawings Figure 1 is a perspective elevational vieu of a compact primary -~
surface heat exchanger constructed in accordance with the present invention.

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, . . . . ' , Fig. 2 is an enlarged, fragrnentary perspective elevational and sectional view of the central core of the heat exchanger of the present invention taken along line II-II of Flg. 1 and illustrating t~le crest-to-crest stacked nature of the individual sheets thereof.
Fig. 3 is an enlarged, fragmentary top plan view of the central core of the heat exchanger shown in Fig. 1~
showing the longitudinal wave form thereof with a portion of the top sheet broken away to better illustrate the precisely misaligned relationship of the wave form of the second sheet therewith.
Fig. 4 is an enlarged, fragmentary transverse sectional view of two superimposed heat exchanger sheets taken along the line IV-IV of Fig. 2 and showing the laterally shifting nature of the serpentine passages defined therebetween.
Fig. 5 is an enlarged vertical sectional view of a preferred embodiment corrugated sheet of the heat exchanger of the present invention showing the parameters of the repetitive transverse convolutions thereof.
F'ig. 6 is an enlarged vertical sectional view of a first alternate embodiment corrugated sheet at a scale somewhat larger than Fig. 5, showing the parameters of the repetitive transverse convolutions thereof.
Fig. 7 is an enlarged, fragmentary top plan view of a second alternate embodiment corrugated sheet showing an arch-type longitudinal wave form, and which can be compared with the sinuous wave form of Fig. 3.
Fig. 8 is an enlarged, fragmentary-vertical sectional view of several adJacent corrugated sheets of a prior art primary surface heat exchanger with the transverse ~6--.

convollltions thereof havlng a relati~ely flat proflle and de~ining substantlally equal f~low areas between the sheets.
Description of the Preferred Embodime~t Referring in~tially to F-l~. 1, a compact primary surface heat exchanger 10 ls shown as having three principle regions including a centrally disposed rectangular counterflow area or core 12 to which the present invention is particularly directed, and a pair of outer triangulârLy-shaped crossflow zones 14 and 16 flanking the opposite ends of the core. In the illustrated embodiment, the outer zone 14 serves as a manifold to direct hot gas to the core as by way of a plurality of elevationally spaced gas entrance passages 18 which open outwardly on an elongated end 20 thereof, and further to direct heated air from the core as by way of a corresponding plurality of elevationally-spaced and offset air-exit passages 22 opening outwardly on a foreshortened end 24 thereof.
~he opposite outer zone 16 has a corresponding number of air entrance passages 26 along a foreshortened end 28 thereof, and a plurality of elevationally spaced gas-exit passages, not shown, are disposed along an elongated end 30 thereo~ which are respectively in communication with the core of the heat exchanger. In this way, the gas and air are advantageously communicated in opposite directions through the core in an effective counterflowing heat exchanging manner. Such general construction is described in greater detail in U.S. Patent Nos. 3,291,206 and 3,759,323 mentioned above.
As best shown in ~ig. 2, the compact primary surface heat exchanger 10 includes a plurality o r transversely corrugated sheets 32 whi.ch are alternately interleaved in precise superimposed relation to form a vertically aligned ..... . . - . .
::- , ~ - , . . .
;'~ '' ' ~ , .', . '-~1~33 stack thereor. As representatively shown by the top sheet in Fig. l~ each sheet has a central rectangular area 33 collectively making up the ma~or portion o~ the heat exchanger core 12, and a pair of oppositely disposed triangularly-shaped areas 34 and 35 making up the major portion of the heat exchanger outer zones 14 or 16. However, the sheets are not oriented the same way in the stack, but rather are benefi-cially arranged in crest-to-crest facing pairs so that the corrugations of their central areas are alternately longitudi-nally offset or misaligned in a predetermined manner to optimize heat transfer and to prevent nesting as will be subsequently described.
More particularly, the corrugated sheets 32 of the present invention are preferably formed from relatively thin stainless steel and provided with opposite flat side margins or edges 36 as clearly illustrated in ~ig. 2. These side edges are suitably sealed together at their outer side extremities as by being brazed or welded to a plurality of edge bars 38 so that, in general, a path is provided for the relatively hot gas (G) between certain pairs of adjacent sheets, while alternately providing a path for relatively cool air (A) to be heated between such pairs.
Referring now to Fig. 5, and pursuant to the present invention, each of the sheets 32 is formed with a plurality of vertically extended repetitive transverse convolutions 40 disposed at a substantially right angle to the general direction of fluid flow between the side edges 36 thereof and which extend longit,udinally between a pair of borders 41 disposed interme~late the centra:L area 33 and the outer triangular areas 34 and 35, as lndicated ln Fig. 1. Each of , - - , , :

3~3 these tr-ansverse convolutions extends upwardly and depends downwardly a similar and relatlvely large distance (D) from a central plane 42 thereo~, and provides a greatly vertically extended uniform shee~ height when compared with the relatively thin sheet thickness of from two to eight mils. In the particular sheet illustrated, the overall sheet height (2D) is approximately 3.9 mm (0.155") and the sheet thickness (T) is approximately 0.076 mm (0.003"). Further, each transverse convolution has a generally vertically extended sinuous wave profile providing a cycle width (C) of approximately 1.27 mm (0.050"). Thus, it is apparent that the convolution height is significantly greater than the cycle width. Pre~erably, the uniform convolution height is approximately two times or more greater than the cycle width, and in the illustrated embodiment such ratio is approximately 3:1.
In accordance with one aspect of the invention, each of the corrugated sheets 32 is also sinuously profiled in the general direction of fluid flow as best shown in Figs. 2 and 3, in order to increase the stif~ness of an individual sheet and to provide certain other advantages as will be sub-sequently described. ~or example, each of the transverse convolutions 40 of the top sheet includes a repetitive longi-tudinal convolution or sine wave 43 with a wave pitch (P) as -.
indicated in ~ig. 3 of approximately 9.65 mm (0.38"), and a wave amplitude (A) of approximately 1.57 mm (0.062"). On the other hand, the second sheet is substantially identical to it, but for the ract that its repetitive longitudinal wave form is beneficially arranged symmetrically out of phase with the top sheet ~or improved heat exchanging effectiveness and for improved criss-crossed stacking thereof. Speci:~ically, : , .

as clearly shown by ~'ig. 3, the r~spectively aseocia~ed apexes of undulation of each o~ the ad~acen~ sheets are extended in transversely opposite directions. This pre-determined longitudinally offset or symmetrically out of phase misalignment holds true for the remaining sheets in the heat exchanger core 12 in an alternating manner.
In the partlcular heat exchanger 10 illustrated, the sheets 32 are formed with the longitudinal waves 43 oriented in such a manner with respect to the borders 41 thereof that it is only necessary to turn alternate ones of the sheets over to obtain the precise misalignment required.
It is necessary to provide two different basic sheets to achieve askewed orientation between them for other primary surface heat exchangers, such as a heat exchanger having one f the elongated ends 20 or 30 illustrated in Fig. 1 positioned diagonally oppositely to the other elongated end or having one of the crossflow zones 14 or 16 reversed without departing from the spirit of the present invention.
As indicated generally above, the major portion of each transverse convolution 40 is so arranged as to provide a plurality of substantially vertically extending and sub-stantially parallel, but longitudinally undulating walls as indicated generally by the reference numerals 44, 45 and 46 in Fig. 5. These walls are integrally joined by a corres-ponding plurality of upper and lower semicylindrical walls orcrest members as respectively indicated by the numerals 47 and 48. However, in accordance with another aspect of the present invention, the vertically extending walls are advan-tageously unequally laterally spaced and smoothly blended with the crest members to produce a transversely unsymmetrical . ' , ;' .

9~3 sinuous wave pattern. For example, the dlstance (~) between the walls 1~5 and 46 is approximately 0.83 mm (0.03Z~) and the distance (F) between the walls 41l and ~5 is in contrast only approximately 0.30 mm (0.012").
Because the sheets 32 are stacked in facing pairs in precisely superimposed crest-to-crest askewed bridging rela-tion as is clearly illustrated in Figs. 2 and 4, a plurality of somewhat larger and generally serpentine fluid flow passages 50 are provided internally between them for the hot gas (G), and a plurality of somewhat smaller serpentine fluid flow passages 52 are provided alternately exteriorly between them for the air (A) to be heated. The serpentine character of the fluid flow paths is best visualized by noting that in the transverse sectional view of Fig. 2, the convolutions 40 of adjacent sheets are vertically aligned to present a mirror image of each other~ whereas at the slightly longitudinally displaced transverse sectional view of Fig. 4, the same con-volutions are laterally offset with respect to each other.
This provides serpentine passages which are intertwined in elevationally overlapping relation throughout the heat exchanger core 12. And, as a result of the unequal lateral spacing of the represantative walls 44, 45 and 46, the total transverse cross sectional area between one pair of sheets is considerably higher for the gas than the adjacent pair of sheets provides for the air. In the instant embodiment, the unsymmetrical transverse ccnvolution pattern results in a gas-to-air flow area ratio of l.8:l. This predetermined ratio directly minimizes the overall pressure losses across the heat exchanger core, and is preferably within a range of from l.5:l to 3.0:l in connection with transferr:l.ng heat from a relatively hot exhaust gas ~om a gas turbine engine, not shown, and to relatively cool inlet air. This ratio is desira~le because the exhaust gas specific volume is greater tllan that for air and its pressure ~rop is consequently greater, and the sizing of the areas between the sheets can be tailored specifically to provide a pressure drop or fluid velocity therein at the level desired.
It should be appreciated that the heated air is subsequently utilized in the gas turbine engine with greatly improved efficiency thereof and at a reduction of ~uel con-sumption. The aforementioned ratio further directly allows minimizing the overall volume of the heat exchanger at a savings in space~ weight and cost.
Description of First Alternate ~mbodiment Referring to Figure 6, a firs~ alternate embodi-ment corrugated sheet 54 is shown which is somewhat easier to manufacture because of its shallower overall height utilizing for example, the sheet material fo~ming apparatus of United States ~atent No. 3,892,119 and mentioned previously above.
In this embodiment, the thickness (T) is also 0.076 mm C0.003'l)~ the ove~all sheet height (2D) is 2.36 mm (0.093"), the cycle width CC) is 1.37 mm ~0.058") and the gas-to-air ratio is approximately 1.78:1. Such sheet construction involves a vertically extended unsymmetrical convolution height which is less than that o~` the pre~erred embodiment, so that more sheets are required for the same overall heat exchanger heigh~.
However, this configuration is sati.sfactory for general use.
Incidentally, it is noted that while the vertically extended walls of these sheets are slightly inclined relative to the central plane thereof, they are still considered substantially para:llel, even though they also longitu(linally undulate.
Descriptlon of Second Alternate Embodiment A second alternate embodiment corrugated sheet 56 is shown in Fig. 7 which incorporates an unsymmecrical convolution 40 transverse to the direction of fluid flow similar to that of the preferred embodiment, but alternately has an arch-type repetitive wave form 57 in the general direction of fluid flow. Specifically, the longitudinally extending arch-type wave pattern is nearly all of equal radius of curvature as representatively illustrated by the relatively large radius (Rl) shown. Of course, a small radius (R2) is also needed for blending purposes between each repetitive large radius wave. It is theorized that this construction stiffens the individual sheets by substantially eliminating the flat areas which interconnect the substantially equal radii of curvature in the preferred sinuous wave form of Fig. 3 and as indicated by the flat area reference numeral 58 shown in the referenced figure. It is believed that each of these flat areas 58 must react to pressure loads by bending or curving, and this extra deflection could eventually deleteriously restrict flow to some degree in specific cases in the serpentine passages formed between the individual sheets. On the other hand, the arch-type construction of this embodiment could reduce such sheet deformation and thereby minimize heat exchanger pressure loss.
In view of the foregoing, it is apparent that the high surface to volume ratio primary surface heat exchanger 10 of the present invention provides a highly effective heat exchanger by utilizing a plurality of thln, corrugated sheets 32 arranged in a stack, and with each of the sheets having a plurality of substantially vertically extended unsymmetrical convolutions Llo transverse to the general direction of fluid flow. Such an effective profile which is particularly valuable for a heat exchanger having a total sur~ace area in the range of approximately 93 square meters (100 square feet) of surface area per 0.028 cubic meters (1 cubic foot) of volume allows the total number of sheets to be significantly reduced for a given heat exchanger performance requirement. For example, the preferred embodiment of Fig. 4 ]0 requires only 6 1/2 sheets per inch of stack.
This is in marked contrast to a typical prior art sheet arrangement represented by the equally folded transverse convolution illustrated in Fig. 8, wherein the individual sheets are relatively flat and structurally weak under pressure loading, and a relatively large number of sheets would be required to provide a heat exchanger with a given capacity. For example, 20 or more sheets per inch of stack have been heretofore required.
In addition, each of the sheets 32 of the present invention is of corrugated form with repetitive waves 43 in the direction of fluid flow, and with adjacent sheets having the waves offset or out of phase with each other. This non-nesting sheet combination provides a plurality of serpentine fluid flow passages 50 for the hot gas, as well as an alternating plurality of intertwined fluid passages 52 for the air to be heated, and with the transversely unsymmetrical convolutions 40 allows their respective flow areas to be proportioned for low pressure drop and maximum overall effec-tiveness. Further, the undulating pattern of the sheets increases the structural strength and integrlty of the heat ~14-.

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~)6~ J3 exchanger, particularly ln the ver~lcal dlrectlon, and controls the turbulance of the fluids passing therebetween in order to break up the boundary layer ad~acent the sheets and to establish a relatively high heat transfer coefficient thereat without excessively increasing such pressure drop.
Such improved structural strength also eliminates the need for brazing or welding the sheets at the central areas thereof.
While the invention has been described and shown with particular reference to a preferred embodiment, and two alternate embodiments, it will be apparent that variations might be possible that would ~all within the scope of ~he present invention~ which is not intended to be limited except as defined in the following claims.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A primary surface heat exchanger comprising a plurality of transversely corrugated sheets which are arranged in a stack with the corruga-tion crests of adjacent sheets in contact with one another and crossing over one another at their points of contact to provide first and second tortuous fluid passageways alternately between them, the corrugations of each sheet presenting a repetitive pattern with a plurality of substantially parallel wall portions extending substantially perpendicularly to the general plane of the sheet and integrally joined by a crest portion, the transverse corrugations of the sheets being so profiled that the first and second fluid passageways have different cross-sectional areas.

2. A heat exchanger according to claim 1, wherein the first and second fluid passageways have cross-sectional areas in a ratio of between 1.5:1 and 3.0:1.

3. A heat exchanger according to claim 1, wherein the repetitive pattern comprise a wave pattern of which the substantially parallel wall portions of one half wave cycle are separated by a different distance than those of the other half wave cycle.

4. A heat exchanger according to claim 3, wherein the wave pattern has an amplitude which is greater than the wave length.

5. A counterflow heat exchanger according to claim 1, wherein the corrugations of adjacent sheets extend between the same pair of opposite sides of the stack.

6. A heat exchanger according to claim 5, wherein each of the trans-verse corrugations is sinuously longitudinally profiled in the general plane of the sheet.

7. A heat exchanger according to claim 6, wherein the longitudinal sinuous profile comprises a repetitive wave pattern, the majority of each cycle of which presents a constant radius of curvature.

8. A heat exchanger according to claim 6 or claim 7, wherein the sinuous longitudinal profiling of adjacent sheets are substantially out of phase with one another.

9. A heat exchanger according to claim 1, 2 or 3, wherein adjacent sheets have the same constructional shape and are stacked with one in an inverted orientation relatively to the other.

10. A heat exchanger according to claim 1, 2 or 3, wherein each sheet has a pair of opposite side edges which are sealed to corresponding side edges of adjacent sheets to define the flow passageways.

11. A heat exchanger according to claim 1, 2 or 3, wherein each sheet is made of stainless steel.