Note: Descriptions are shown in the official language in which they were submitted.
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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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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
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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
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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
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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
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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
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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,
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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
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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
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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
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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
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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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