Note: Descriptions are shown in the official language in which they were submitted.
1~52~317?~
DESCRIPTION
HEAT EXCHANGER PLATE HAVING
DISTORTION RESISTANT UNIFORM PLEATS
Technical Field
This invention relates to a low cost, distortion
resistant heat transfer plate for use in a heat ex-
changer such as a gas turbine recuperator or other
type of primary surface heat exchanger. The inven-
tion also relates to a metal working method for
10 efficiently and easily forming a heat transfer plate
out of ductile sheet metal and to apparatus for
forming an undulatory pattern of uniform pleats in
sheet metal designed especially for use as a heat
transfer plate in a primary surface heat exchanger.
15 ~ackground Art
Rising energy costs have significantly increased
the need for low cost, yet effective, heat exchangers
since virtually every type of fuel consuming engine,
power plant or industrial process gives off some
20 recoverable heat capable of being converted to useful
work. The cost of such exchangers has, however,
in the past discouraged wide spread use of heat ex-
changers in certain applications. One well known
type of low cost heat exchanger employs a plurality
25 of stacked plates arranged to allow heat donative
and heat recipient fluids to flow in heat exchange
relationship on opposite sides of each plate. It
; has long been recognized that the efficiency of such
primary surface heat exchangers is a direct function
30 of the total surface area of the stacked plates and
an inverse function of the wall thickness of the
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plates which separate the heat exchange fluids.
One technique for forming such heat exchanger
plates, thus, includes forming a large number of
corrugations or pleats in ductile sheet metal of
relatively thin gauge. In order to prevent nesting
of the plates when stacked, the corrugation pleats
are given a wavy (or curvilinear) configuration in
plan view. When thus constructed the pleat crests
of one plate form at least some points of contact
with the crests of the adjacent plates. An example
of this type of corrugated heat exchanger plate is
illustrated in U. S. Patent No. 3,759,323, to Dawson
et al which issued September 18, 1973.
Attempts to increase the heat transfer effi-
ciency of corrugated plates of the type illustratedby U. S. Patent No. 3,759,323, by metal gauge reduction
and increased pleat density, have not always met
with success. The structural rigidity of the cor-
rugation pleats is decreased upon reduction in the
gauge of metal forming the plate, and when such
weakening is combined with an increase in the density
of pleats, the chances of a flow passage becoming
restricted or obstructed dramatically increases.
In particular, weak walled, high density pleats can be
subject to mechanical distortion during the process
of manufacture and are also subject to distortion
and/or collapse from uneven temperature induced
expansions and contractions.
In U. S. Patent No. 3,892,112 which issued July 1,
1975 to Miller, et al it is noted that cost savings
without reduced efficiency can be realized
in the manufacture of heat exchangers formed
of plates such as illustrated in U. S. Patent No.
; 3,759,323 by increasing the height and number of
pleats in each plate to permit reduction in the
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number of plates required for a given heat exchange
capacity. An increase in the height of each pleat,
however, has further aggravated the problem of un-
desired mechanical or temperature induced pleat wall
5 distortions and has, up to the present, placed a
practical limit on the efficiency which can be ach-
ieved by the use of primar`y surface heat exchangers
employing pleated plates.
Disclosure of the Invention
The present invention is directed to a low cost,
structurally rigid heat transfer plate for use in
a heat exchanger wherein the plate is designed to
overcome the deficiencies of the prior art as des-
cribed above. In particular, the heat exchanger
15 plate of the present invention is provided with an
undulatory pattern of pleats for forming fluid flow
passages on opposite sides of the plate, wherein
the side wall of each pleat has a constant slope
throughout the length of each fluid flow passage.
20 This uniformity in slope provides greater structural
r~gidity and over-all uniformity to the heat exchanger
plate. Moreover, restriction and/or obstruction
of fluid flow passages due to mechanical or tempera-
ture induced distortions in the walls foEming the
25 fluid flow passages can be reduced by this arrange-
ment without sacrificing the efficiency and low cost
manufacturing advantages of prior art pleated heat
exchanger plates.
The present invention further provides a method
30 and apparatus for forming a heat exchanger plate
having an extremely rigid, uniform characteristic.
The method includes the steps of successively bending
a sheet of ductile heat conducting material to pro-
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duce a series of undulatory pleats forming two setsof curvilinear fluid flow passages on opposite sides
of the heat exchanger plates wherein the bending
steps are controlled in a way to cause the slope
of the side walls of each pleat to be constant along
the entire length of the corresponding flow passages.
The apparatus for forming the heat exchanger plate
has uniformly sloped pleats including a plurality
of cooperating fluid passage forming blades wherein
at least one blade has a curvilinear configuration
in plan view and a uniform thickness. This blade
is positioned for relative reciprocal movement be-
tween second and third fluid passage forming blades
each having a non-uniform cross-sectional area.
The clearance between the first blade and each of
the second and third blades in uniform throughout
the operative length of the blades to insure a con-
stant slope in the pleats of a plate formed by the
apparatus.
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Brief Description of the Drawings
Fig. 1 is an exploded perspective view of one aspect
of the present invention showing a plurality of heat
exchanger plates as such plates would be employed in
a primary type heat exchanger;
Fig. 2 is a cross-sectional view of another aspect
of the present invention showing an apparatus for
forming a heat exhanger plate having distortion resistant
uniform undulatory pleats;
Fig. 3 is a cross-sectional view of the apparatus
illustrated in Fig. 2 wherein portions of the apparatus
have been moved to an open position in preparation
for a pleat forming operation;
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Fig. 4 is a cut-away perspective view of a prior
art pleating apparatus;
Fig. 5 is a cross-sectional view of the prior
art pleating apparatus illustrated in Fig. 4 as such
apparatus would appear when moved to the position
illustrated in Fig. 2, the cross-sectional view being
taken along lines 5-5 of Fig. 2;
Fig. 6 is a partial cross-sectional view of
the pleat forming apparatus of Fig. 5 as taken along
lines 6-6;
Fig. 7 is a partial cross-sectional view of
the pleat forming apparatus of Fig. 5 taken along
lines 7-7;
Fig. 8 is an exploded, cutaway, perspective
viewof a pleat forming apparatus designed in accor-
dance with the subject invention;
Fig. 9 is a cross-sectional view of the pleat
forming apparatus illustrated in Fig. 8 as such would
appear when moved to the position illustrated in
Fig. 2, the cross-sectional view being taken along
lines 5-5;
Fig. 10 is a partial cross-sectional view of
the pleat forming apparatus of Fig. 9 as taken along
lines 10-10; and
Fig. 11 is a partial cross-sectional view of
the apparatus of Fig. 9 as taken along lines 11-11.
Best Mode for Carrying Out the In~ention
Referring now to Fig. 1, a plurality of heat
exchanger plates 2, 4, 6, 8, designed in accordance
with the subject invention, are illustrated in ex-
ploded perspective view as such plates would be used
to for~ a stacked plate type heat exchanger. Heat
exchangers of thisigeneral type are disclosed and
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discussed more fully in U. S. Patent No. 3,759,323.
Each heat exchanger plate includes a plurality of
undulatory pleats 12 having a wavy pattern in plan
view designed to prevent nesting of the respective
plates by causing the crowns or crests of each pleat
to contact the crowns of the pleats formed in an
adjacent heat exchanger plate. The side walls of
each pleat subdivide the space between adjacent
plates into a plurality offluid flow passages to
increase the total surface area actually contacted
by the heat transfer fluids flowing between the heat
exchanger plates.
As more fully explained in U. S. Patent No.
3,759,323, edge bars 14 are positioned at selected
peripheral positions between successive heat exchanger
plates to direct the flow of heat exchange fluids
through the heat exchanger and prevent commingling
of the fluid9 while allowing heat transfer there-
between. Inlet sections 15 and outlet sections 16
are attached to opposed sides of each heat exchanger
plate to assist in directing the heat exchange fluids
into the interplate spaces.
For purposes of this description, the term "don-
ative fluid" will refer to fluids capable of giving
up heat energy within a heat exchanger and may in-
clude either gas or liquid. The term "recipient
fluid" will refer to any fluid, gas or liquid, which,
when introduced into a heat exchanger, is capable
of receiving heat energy fro~ the donative fluid.
In Fig. 1, heat exchanger plates 2 and 4 are designed
to define a recipient fluid flow chamber when the
respective plates are positioned adjacent one another.
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~ithin this recipient fluid flow chamber, a plurality
of recipient fluid flow passages 18 are defined by
adjacent side walls of the pleats 12 projecting into
the recipient fluid flow chamber from plates 2 and
4. Similarly, the space between plates 4 and 6 is
designed to form a donative fluid flow chamber with
the area between pleats 12 opening into the chamber
forming a plurality of donative fluid flow passages
20. In the specific embodiment of Fig. 1 the edge
bars 14 and inlet and outlet sections 15 and 16 are
arranged to cause the donative fluid to flcw along
the C-shaped flow path illustrated by arrow 22 within
alternate spaced formed by the stacked plates while
the recipient fluid is caused to flow in a reverse
C-pattern illustrated by arrows 24 within the re-
maining alternate spaces.
To understand more fully the unique advantages
of the subject invention, a previously known pleated
heat exchanger plate as disclosed in U.S. Patent
No. 3,892,119 will first be discussed. In this pat-
ent, a method and apparatus for forming substantially
flat, relatively thin deformable sheet metal into
a pleated heat exchanger plate is disclosed. Accord-
ing to the patent, progressive single fold forming
steps are performed on the sheet material as it
advances between oscillating pleat forming blades
mounted on two pairs of opposed forming members.
Since the exact purpose and sequential movement of
each of the four forming members is not critical
to an understanding of the subject invention, re-
ference is made to U.S. Patent No. 3,892,119 for
a more complete description of the movement and
purpose of each of the four forming members employed
to form a pleated heat exchanger plate of the type
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to which the subject invention is directed. For
purposes of this invention, it is sufficient to note
that an upper donative fluid flow passage forming
blade is mounted for relative oscillatory movement
with respect to a lower recipent fluid flow passage
forming blade. The blades are designed to move
between a first position in which the blades are
separated to receive an unpleated ductile sheet
material and a second position in which the ductile
sheet material has been deformed so as to form a
pleat side wall n, the c'ea an.ce space be' ~?een ' he
respective passage forming blades.
Fig. 2 is a schematic cross-sectional illus-
tration of pleating apparatus in which both the
method and apparatus of the prior art as well as
that of the present invention may be employed. In
particular, two pairs of relatively movable forming
means 26, 28, 30 and 32 are illustrated. First
forming means 26 and second forming means 28 each
carry an identical donative fluid passage forming
blade 34 and 36, respectively. Third forming means
30 is positioned to cooperate with blade 34 in order
to properly position the incoming ductile sheet
material 37 and to form one side wall 38 of each
pleat. Fourth forming means 32 supports a recipient
fluid passage forming blade 40 adapted to enter the
; space between blades 34 and 36 as illustrated in
Fig. 2, thereby causing a second side wall 42 to
be formed in the clearance space between blades 34
and 40 and a third side wall 44 to be formed in the
clearance space between blades 40 and 36.
Fig. 3 illustrates the apparatus of Fig. 2 where-
in first and second forming means 26 and 28 have
- been displaced upwardly to permit the ductile sheet
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material 37 to be displaced by a distance equal to
the wavelength of the pleat wave in plan view in
preparation for forming a successive pleat by forming
means 26 through 32 all as described in greater
detail in U.S. Patent No. 3,892,119.
Turning now to Fig. 4, a perspective view of
prior art fluid passage forming blades of the type
used in the apparatus of U.S. Patent No. 3,892,11
is shown including a pair of donative fluid flow
passage forming blades 34' and 36' and a recipient
fluid flow passage forming blades 40'. The prior
art blades of Fig. 4 have uniform thicknesses. When
equipped with fluid passage forming blades of this
type, the apparatus of Fig. 2 will form pleats in
ductile sneet material 37 having side walls of ir-
regular slope, thus creating an unstable structure
in ~7hich the side walls are easily distorted by
outside mechanical force or temperature induced
contractions and expansions. To understand this
more fully, reference is made to Fig. 5 wherein a
cross-sectional view taken along lines 5-5 of the
apparatus of Fig. 2 is illustrated as the apparat~s
would appear if equipped with the prior art blades
of Fig. 4. In particular, Fig. 5 ill~trates--dona-
tive fluid passage forming blades 34' and 36' havinga constant thickness dl and a pair of curvilinear
side walls each of which consists of alternating
circular arcs arranged in a path which defines a
periodic function. The recipient fluid passage
forming blade ~0' is also formed with a constant
thickness d2 and is provided with side walls wllich
in cross section are each formec of successive cir-
; cular arcs which define a periodic function having
- the same phase an~ wavelength as the periodic func-
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tions defined by the surfaces of blades 34' and 36'.
As long as the passage forming blades have a constant
thickness, the clearance space between the blades
in plan view, regardless of the shape or configura-
tion of the curvilinear pattern formed by the bladesurfaces, cannot be constant. Even if the surfaces
of each blade were formed by identical sine waves
displaced laterally, the clearance spacing between
the blade surfaces would still vary when the clear-
ance is measured in a direction perpendicular tothe central axis of the clearance space. For pur-
poses of this application, tne central axis belween
two curvilinear lines will be defined as the loci
of all points located midway between the two curvi-
linear lines as measured along a line normal to oneof the curvilinear lines at each point along such
line. Obviously, this definition presupposes the
absence of any discontinuities in the two curvilinear
lines in order for there to be a continuous central
axis.
When the height of the pleats is constant and
the clearance between blade surfaces is variable,
it is obvious that the slope of the side walls of
the pleats must be variable as measured in a plane
perpendicular to the central axis of thè clearance
in plan view. Such variation in side wall slope
greatly affects the lateral stiffness of the pleats
and causes them to close up in some areas, thus re-
stricting the total flow area of a heat exchanger
formed with pleated heat exchanger plates. To under-
stand this more clearly, it should be noted that
the total effective cross-sectional area for the
flow of gaseous donative fluid is normally made lar-
ger than the effective cross-sectional area of the
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flow of recipient fluid since the higher temperature
donative fluid will normally be available in larger
volume in the heat exchange process. Thus, given
the requirement that the number of donative fluid
passages and recipient fluid passages must be equal,
it follows that each donative fluid flow passage
must be larger in cross-sectional area than is each
of the recipient fluid flow passages. As illustrated
in Fig. 5, each wavelength portion W of blade 40'
is constructed in a first section with side walls
which sweep out circular arcs having radii rl and
r2 with both arcs having a coincident center of
curvature Cl. The remaining portion of the wave-
length section of blade 40' is similarly formed to
provide blade surfaces having radii of curvature
rl' and r2' with a coincident center of curvature
C2 located on the opposite side of the blade. If
the blade is made symmetrically so that rl=rl' and
r2=r2', each wavelength portion of donative fluid
forming passage blades 34' and 36' similarly includes
surfaces which define circular arcs having radii
of curvature Rl and R2 with a coincident centQr of
curvature C3. A second section of each wavelength
portion of blades 34' and 36' has corresponding radii
of curvature Rl', and R2' with a coi~cident-center
of curvature C4 located on an opposite side of blades
34' and 36' from center of curvature C3. Since these
blades are normally made to be symmetrical, Rl=Rl'
and R2 2
Since the wave patterns defined by the blades
are symmetrical, the centers of curvature of the
blade surfaces are also symmetrical and are displaced
by an amount equal to the double amplitude H o~ each
wave plus r2 - rl. This relationship facilitates
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the construction and reproduction of the heat ex-
changer plate. As can be understood by reference
to Fig. 5, the clearance between the blades varies
from a maximum of M to a minimum of m. The minimum
clearance m is normally made only slightly larger
than the thickness of the plate material plus a small
amount allowed for ease of withdrawing the bla~es
of the pleating apparatus. This arrangement allows
the greatest number of pleats per unit length of
plate as possible.
When spaced in this manner, the slope of the
side walls formed in the areas of minimum clearance
m between the respective passage forming blades will
have a substantially vertical slope. Side walls
formed in this manner have very little lateral rigid-
ity which causes shifting of the pleating and ~n-
controlled obstruction of the fluid flow passages.
Some shifting of the side walls forming the donative
fluid flow passages,may be tolerated since t'nese
passages have a substantial larger cross-sectional
area. However, a shift in the side walls forming
each of the recipient fluid flow passages can be
highly detrimental due to their smaller cross-sec-
tional area.
-The disadvantages of varying side wall slope
are illustrated more graphically in Fig. 6 which
is a partial cross-sectional view taken along lines
; 5-6 of Fig. 5 located at a point of minimum clear-
ance between respective pleat forming blades. In
particular, lines 6-6 indicate a cross-section taken
along a plane perpendicular to the central axis of
blade 34' and thus lines sl in Fig. 6 are represen-
, tative of the slope of both side walls 38 and 42.
; As is apparent, the slope of these side walls is
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virtually perpendicular to the plan surface of the
heat exchanger plate being pleated.
Contrasting with the configuration of Fig. 6
is the cross-sectional view of Fig. 7 of a portion
of a heat exchanger plate being formed by the assem-
bly illustrated in Fig. 5 as taken along line 7-7.
In particular, note the slope of side wall 38 as
represented by line 52 and yet another slope angle
represented by line s3 of side wall 42. As can now
be readily appreciated this varying slope of the
pleat side walls 38 and 42 along the longitudinal
extent of each pleat formed by the assembly of Flg.
5 results from variation in the clearance between
the blade surfaces.
Reference is now made to Fig. 8, wherein a per-
spective view is shown of the heat exchanger plate
forming apparatus o the subject invention. As
clearly illustrated in Fig. 8, donative fluid flow
passage forming blades 3~" and 36" have been sub-
sti~uted for the corresponding blades of the prior
art illustrated in Fig. 4. As is apparent by a com-
parison of Figs. 4 and 8, blades 34" and 36" have
a non-uniform cross-sectional configuration. To
understand the precise function of the modified
blades 34" and 36", reference is made~~o Fig.~ ~,
which is a cross-sectional view of the apparatus
illustrated in Fig. 8 when positioned by the forming
assembly, illustrated in Fig. 2 taken along lines
5-5.
Referring now particularly to Fig. 9, the don-
ative fluid passage forming blades 34" and 36" are
shown as having a substantial blade thickness vari-
ation along the longitudinal extent of each blade
from a minimum of Pl to a maximum of P2. In contrast
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to this, the recipient fluid passage forming blade
40" is provided with a uniform thickness as measured
in the direction of a plane passing perpendicularly
through the central axis of the blade in plan view
5 along the entire longitudinal length of the central
axis. Variations in the width of the donative fluid
flow passages are significantly more acceptable in
view of the substantial width of such passages as
compared with the narrower cross-sectional width
10 of the recipient fluid flow passages. Any variation
in the cross-sectional width of such recipient fluid
flow passages could obviously be more detrimental
to the efficient operation of a heat exchanger formed
from pleated plates than would variations in the
15 cross-sectional area of a donative flow passage.
More significantly, however, is the fact that a
uniform clearance space between the surfaces of blade
40" and each of the blades 34" and 36" results in
the format~on of pleat side walls having a constant
20 uniform slope as measured in a plane passing perpen-
dicularly through the central axis of each flow
passage.
Achieving both uniform cross section in each
recipient flow passage and uniform slope in the orien-
25 tation of the side walls of all pleats having a curvi-
linear plan view configuration requires very careful
design of the respective blades 34", 36" and 40".
Reference is now made to a wavelength W section of
each of the blades 34", 36" and 40" wherein the
30 general case required for forming a recipient flow
passage of uniform cross-sectional area combined
with pleat side walls having a constant slope through-
out the heat exchanger plate is illustrated. In
particular, the wavelength portion W of blades 34",
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36" and 40" spanning between the lines marked
and w2 can each be c9ivided into a first arcuate
section wherein the radii of curvature of the re-
spective side walls of blade 40" are indicated by
5 Sl and S2, respectively. The adjacent surfaces of
blades 34" and 36" facing the corresponding surfaces
of blades 40" are shown by arrows indicated at S3
and S4, respectively.
As illustrated in Fig. 9, the center of curva-
10 ture of each of the circular arcs identified by
arrows Sl through S4 are coincident at point SC.
Similarly, the remaining side surfaces of each of
the blades 34", 36" and 40" form in plan view circular
arcs touched by arrows Y1, Y2, Y3 and Y4 having a
15 coincident center of curvature YC located on a side
of blade 40" opposite to center of curvature SC.
The circular arcs touched by arrows Yl and Sl complete
a full wavelength of one side of blade 40". Similarly,
arrows Y2 and S2 complete a wavelength of the opposite
20 side of blade 40". A full wavelength of the surface
of blade 34" adjacent blade 40" is formed by circular
arcs touched by arrows Y4 and S4. Finally, a full
wave length of the side of blade 36" adjacent blade
40" is formed by the circular arcs touched by arrows
25 Y3 and S3. By this arrangement, the clearance- space
between blades 34", 36" and 40" is uniform. It is
not, however, necessary for the first and second
circular arcs of each blade surface to have eaual
radii since the waves need not be symmetrical when
30 viewed from opposite sides of the heat exchanger
plate. Moreover, the wavelength W along the longi-
tudinal extent of each blade need not be identical
nor is it necessary for the amplitude of successive
~; wavelength portions W of each of the blades to be
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equal. By merely maintaining coincidence of the
center of curvature of each of the circular arcs
touched by arrows identified by Sl - S4 and similarly
maintaining the coincidence of the center of curvature
of each of the circular arcs touched by the arrows
Yl - Y4, the cross-sectional area of the recipient
fluid flow passages formed by blade 40" will remain
constant throughout their longitudinal length. At
the same time the slope of all of the side walls
forming the pleats within the heat exchanger plate
will remain uniformly constant and equal throughout
the full longitudinal extent of each pleat. The
side walls 42 and 44 similarly include wavelength
sections W having concentric circular arc sections
having radii of curvature corresponding to the radii
Sl through S4 and Yl through Y4. Each such radius
is less or greater than the corresponding radius
by an amount equal to the spacing of the blade surface
from the corresponding side wall surface.
Turning now to Fig. 10, a partial cross-sectional
view of blades 34", 36" and 40" is illustratec as
taken along lines 10-10 of Fig. 9 wherein the slopes
of side walls 38, 42 and 44 are illustrated by lines
46, 48 an~ 50. As can be seen in Fig_~lOj lines
46, 48 and 50 form an equal angle relative to a plane
formed by the outer plan surfaces of the pleated
heat exchanger plate.
Fig. 11 similarly discloses a partial cross-
sectional view of blades 34", 36" and 40" taken along
lines ll-ll of Fig. 9. Note that the cross-sectional
view of Fig. ll has been taken at a point of maximum
width of blade 34" as compared with the position
of the cross-sectional view illustrated in Fig. 10
wherein the thickness of blade 34" is at a minimum.
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Despite this variation in the cross section width
of blade 34", the slopes of side walls 38, 42 and
44 as represented by lines 52, 54 and 56 are iden-
tical to the slopes of the corresponding lines 46,
48 and 50 of Fig. 9.
It should now be amply apparent that the method
and apparatus of forming a pleated heat exchanger
plate as illustrated in Figs. 8-11, is capable of
providing a heat exchanger plate wherein the recipient
fluid flow passages include uniform and constant
cross-sectional areas while the slope of the side
walls of the pleats forming the respective fluid
flow passages is constant throughout the entire long-
itudinal extent of each fluid flow passage. By this
arrangement, a highly efficient, compact and rigid
heat exchanger can be formed by stacking plural
pleated heat exchanger plates of the type formed
by the apparatus and method illustrated in Figs.
2, 8 and 9.
Industrial Applicability
Heat exchangers formed by the method and apparatus
disclosed herein, as well as the heat exchanger
plates designed in accordance with this invention,
can be employed in a vast number of applications
wherein the transfer of heat from one fluid to a
second fluid is desired. For example, the exhaust
gases from a gas turbine may form the donative fluid
for heating the compressed intake air leading to
the combustor and then to the turbine whereby the
intake air becomes the recipient fluid referred to
above. Alternatively, a heat exchanger formed in
accordance with the subject invention and including
i the pleated plates described above can be used in
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the boiler of a steam generation device wherein hot
gases from fuel combustion forms the donative fluid
while the recipient fluid is the return water or
make-up water from which steam is to be generated
in the heat exchanger. Still other applications
include the use of a heat exchanger formed in accord-
ance with the subject invention wherein the recipient
fluid is the cooling water of an internal combustion
engine and the donative fluid is the hot oil. Addi-
tional applications include the use of heat exchangersof the subject type employed in heat treatment furnaces
and other industrial applications wherein it is
desired to transfer heat from one fluid to another.
Other aspects, objects and advantages of this
invention can be obtained from a study of the drawings,
the disclosure and the appended claims.
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