Note: Descriptions are shown in the official language in which they were submitted.
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escriPtion
Heat Exchanqer Plate Having
Distortion Resistant Uniform Pleats
Technical Field
The invention relates to apparatus for forming
an undulatory pattern of uniform pleates in sheet metal
designed especially for use as a heat transfer plate in
a primary surface heat exchanger.
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
sQme recoverable heat capable of being converted to
useful work. The cost of such exchangers has, however
in the past discouraged widespread use of heat
exchangers in certain applications. One well known
type of low cost heat exchanger employs a plurality of
stacked plates arranged to allow heat donative and heat
recipient Eluids 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 of the total
surface area of the stacked plates and an inverse
function of the wall thickness of the 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
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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
efficiency of corrugated plates of the type illustrated
by U. S. Patent No. 3,759,323, by metal gauge reduction
1~ and increased pleat density, have not always met with
success. The structural rigidity of the corrugation
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
25 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 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 undesired mechanical
or temperature induced pleat wall distortions and has,
up to the present, placed a practical limit on the
efficiency which can be achieved by the use of primary
surface heat exchanyers employing pleated plates.
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Disclosure of the Invention
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According to the present invention an
apparatus for forming a heat exchanger plate has pleat
forming means for forming undulatory pleats in a sheet
of ductile heat transfer material, the pleat forming
means includes Eirst passage forming means for forming
donative fluid flow passages on one side of the sheet
with each donative fluid flow passage being bounded on
opposite sides in plan view by the side walls of a
1~ pleat and having a central axis in plan view extending
along a continuous curvilinear path between separate
points on the sheet perimeter. Second passage forming
rneans forms recipient fluid flow passage~ on the
opposite side of the sheet with each recipient fluid
flow passage being bounded on opposite sides in plan
view by the side walls of a pleat and having a central
axis in plan view extending along a continuous
curvilinear path between separate points on the sheet
perimeter. The first and second passage forming means
are shaped and positioned to cause the slope of each
side wall of each of the pleat to be constant along the
entire length of the flow passage wherein the slope is
measured in a plane perpendicular to the central axis
of the corresponding fluid flow passage.
Brief ~escription of the Drawings
Fig. 1 is an exploded perspective view 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 an
apparatus for forming a heat exchanger 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
apparatu~ 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
portion of a prior art pleating apparatus;
Fig. 5 is a cross-sectional view of the prior
art pleating apparatus illustratecl 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. 3 is a partial exploded, cutaway,
perspective view of one embodiment of a pleat forming
appaxatus designed in accordance 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 Carryin~ Out the Invention
Referring now ~o Fig. 1, a plurality of heat
exchanger plates 2, 4, 6, 8 are illustrated in exploded
perspective view as such plates would be used to form a
stacked plate type heat exchanger~ Heat exchangers of
this general type are disclosed and discussed more
fully in U. S. Patent No. 3,759,323. Each heat
exchanger plate includes a plurality of undulatory
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pleats 12 having a wav~ 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 of fluid flow passages to increase the total
surface area actually contacted by the heat transfer
fluids flowing between the heat exchanger plates.
lv ~dge 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 fluids while allowing heat transfer therebetween.
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.
~lor purposes of this description, the term
"donative fluid" will refer to fluids capable of giving
up heat energy within a heat exchanger and may include
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 from 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.
Within 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~ ~he 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
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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 L6 are arranged to
cause the donative fluid to flow along the C~shaped
flow path illustrated by arrow 22 within alternative
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 remaining alternate
spaces.
~o 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 patent, a
method and apparatus for forming substantially flat,
relatively thin deformable sheet metal into a pleated
heat exchanger plate is disclosed. According 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
subiect invention~ reference 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 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 recipient
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
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sheet material and a second position in which the
ductile sheet material has been deformed so as to forln
a pleat side wall in the clearance space be~ween the
respective passage forming blades.
Fig. 2 is a schematic cross-sectional
illustration of pleating apparatus applicable to both
the prior art as well as the present invention. 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 for~ 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
wherein first and second forming means 26 and 28 have
been displaced upwardly to permit the ductile sheet
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
30 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. 3,892rll9 is shown
including a pair of donative fluid flow passage forming
blades 34' and 36' and a recipient fluid flow passage
9l
forming blades 40'. The prior art blades of Eig. 4
have uniform thicknesses. When equipped with fluid
passage forming blades of this type, the apparatus of
Fig. 2 will form pleats in ductile sheet material 37
having side walls of irregular slope, thus creating an
unstable structure in which the side walls are easily
distorted by outside mechanical force or temperature
induced contractions and expansions. ~ understand
this more fully, reference is made to Fig. 5 wherein a
1~ cross-sectional view taken along lines 5-5 of the
apparatus of Fig. 2 is illustrated as the apparatus
would appear if equipped with the prior art blades of
~ig. 4. In particular, Fig. 5 illustrates donative
fluid passage forming blades 34' and 36l having a
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
40' is also formed with a constant thickness d2 and
is provided with side walls which in cross section are
each formed of successive circular arcs which define a
periodic function having the same phase and wavelength
as the periodic functions 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 configuration of the curvilinear pattern
formed by the blade surfaces, 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
clearance is measured in a direction perpendicular to
the central axis of the clearance space. Eor purposes
of this application, the central axis between two
curvilinear lines will be defined as the loci of all
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poin~s located midway between the two curvilinear lines
as measured along a line normal to one of 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
1~ must be variable as measured in a plane perpendicular
to the central axis of the clearance in plan view.
Such variation in side wall slope greatly affects the
lateral sti~fness of the pleats and causes them to
close up in some areas, thus restricting the total flow
area of a heat exchanger formed with pleated heat
exhanger plates. To understand this more clearly r it
should be noted that the total effective
cross-sectional area for the flow of gaseous donative
fluid is normally made larger than the e~fective
cross-sectional area of the 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 wavelength section of blade 40' is
similarly formed to provide blade surfaces having radii
o~ 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
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rl=rl' and r2=r2' J each wavelength portion of
donative fluid forming passage blades 34' and 36'
simil~rly includes surfaces which define circular arcs
having radii of curvature Rl and R2 with a
coincident center 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 coincident 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 k2=R2'.
Since the wave patterns defined by the blades
are symmetricalt the centers of curvature of the blade
surfaces are also symmetrical and are displaced by an
amount equal to the double amplitude H of each wave
plus r2 ~ rl. This relationship facilitates the
construction and reproduction of the heat exchanger
plate. As can be understood by reference to Fig. 5,
the clearance between the blades varies from a maximum
2~ 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 blades 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 ~learance m
between the respective passage forming blades will have
a substantially vertical slope. Side walls formed in
this manner have very little lateral rigidity which
causes shifting of the pleating and uncontrolled
obstruction of the fluid flow passages. Some shifting
of the side walls forming the donative fluid flow
passages may be tolerated since these passages have a
substantial larger cross-sectional area. HoweYer, a
shift in the side walls forming each of the recipient
fluid flow passages can be highly detrimental due to
their smaller cross-sectional 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 6-6 of
Fig. 5 loca~ed a~ a point of minimum clearance 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 51 in Fig. 6 are representative of the slope of
both side walls 38 and 42. AS is apparent, the slope
of these side walls is virtually perpendicular to the
plan surface of the heat exchanger plate being pleated.
Contrasting with the configuration of Fig. 6
is a cross-sectional view of Fig. 7 of a portion of a
heat exchanger plate being formed by the assembly
illustrated in Fig~ 5 as taken along line 7-7. In
particular, note the slope of side wall 38 as
represented by line s2 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 Fig. 5 results
from variation in the clearance between the blade
surfaces.
Reference is now made to Fig. 8, wherein a
perspective view if shown of a portion of the heat
exchanger plate forming apparatus of the subject
invention. As clearly illustrated in Fig. 8, donative
fluid flow passage forming blades 34" and 36" have been
substituted for the corresponding blades of the prior
art illustrated in Fig. 4. As is apparent by a
35 comparison of Figs. 4 and 8, blades 34" and 36" have a
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non-uniform cross-sectional configuration. 'rO
understand the precise function of the modified blades
34" and 36", reference is made to Fig. 9, which is a
cross-sectional view of the apparatus illustrated in
Fig. 8 when positioned by the for~ning assembly,
illustrated in Fig. 2 taken along lines 5-5.
~ eferring now particularly to E`ig. 9, the
donative fluid passage forming blades 34" and 36" are
shown as having a substantial blade thickness variation
lV along the longitudinal extent of each blade from a
minimum of Pl to a maximum of P~. In contrast to
this, the recipient fluid passage f~rming blade 40" is
provided with a uniform thickness as measured in the
direction of a piane passing perpendicularly through
the centra~ axis of the blade in plan view along the
entire longitudinal length of the central axis.
Variations in the width of the donative fluid flow
passayes are significantly more acceptable in view of
the substantial width of such passages as compared with
the narrower cross-sectional width 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
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 formation
of pleat side walls having a constant uniform slope as
measured in a plane passing perpenticularly through the
central axis of each flow passage.
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Achieving both uniform cross section in each
recipient flow passage and uniform slope in the
orientation of the side walls of all pleats having a
curvilinear plan view configuration requi.res 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" ~herein the
general case required for forming a recipient flow
passage of uniform cross-sectional area combined wi.th
1~ pleat side walls having a constant slope throughout the
heat exchanger plate is illustrated. In particular,
the wavelength portion W of blades 34", 36" and 40"
spanning between the lines marked wl and w2 can
each be divided into a first arcuate section wherein
the radii of curvature of the respective side walls of
blade 40" are indicated by 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
curvature of each of the circular arcs identified by
arrows Sl through S4 are coincident at point SC.
Similar1y, the remaining side surfaces of each of the
25 blades 34", 36" and 40" form in plan view circular arcs
touched by arrows Yl, Y2, Y3 and Y4 havin9 a
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 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"
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adjacent blade 40" is for~ned by the circular arcs
touched by arrows 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
equal radii since the waves need not be symmetrical
when viewed from opposite sides of the heat exchanger
plate. Moreover, the wavelength W along the
longitudinal extent of each blade need not be identical
lV nor is it necessary for the amplitude of successive
wavelength portions W of each of the blades to be
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 - Y4r 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. lO, a partial
cross-sectional view of blades 34", 36" and 40" is
illustrated as taken along lines lO-lO of Fig. 9
wherein the slopes of side walls 38, 42 and 44 are
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illustrated by lines 46, 48 and 50. As can be seen in
Fig. 10, 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 11-11 of Fig. 9. Note that the
cross-sectional view of Fig. 11 has been taken at a
point of maximurn 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. 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 identical
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
constan~ cross-sectional areas while the slope of the
side walls of the pleats forming the respective fluid
flow passages is constant throughout the entire
longitudinal 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 illustrated.
Other aspects, objects and advantages of this
invention can be obtained from a study of the drawings,
the disclosure and the appended claims.
