CA1221819A - Heat exchanger plate having distortion resistant uniform pleats - Google Patents

Abstract

Heat Exchanger Plate Having Distortion Resistance Uniform Pleats Abstract A heat exchanger plate has undulatory pleats for forming fluid flow passages on opposite sides of the plate. The heat exchanger plate is improved by providing the side walls of each pleat with a uniform slope. By this arrangement, the plate is provided with improved distortion resistance and fluid flow capacity. Apparatus for forming the pleated plates is disclosed using a pair of non-uniform donative fluid passage forming blades mounted for relative oscillatory movement with respect to a recipient fluid passage forming blade wherein the clearance between each blade is uniform throughout the entire length of each blade.
By making the blade clearance constant the slope of pleat sidewalls may be constant even though each pleat follows a curvilinear path.

Description

~;~Z~8~9 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 lZ2~ 819 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 -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;

8~3 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 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 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 ~;2Z~&~

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 ~l22~ 9 g 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 ~22~8~9 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 ~2Z18~

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.

~z~ 9 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"

~2S~ 9 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 ~2~ 9 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.

Claims (6)

Claims:

1. Apparatus for forming a heat exchanger plate including pleat forming means for forming undulatory pleats in a sheet of ductile heat transfer material, said pleat forming means, including first passage forming means for forming donative fluid flow passages on one side of the sheet with each donative fluid flow passage being rounded 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 and second passage forming means for form-ing recipient fluid flow passages 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, characterized in that the first and second passage forming means are shaped and positioned to cause the slope of each side wall of each said 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.

2. Apparatus as defined in claim 1, wherein said first passage forming means includes a first blade having an operative portion with a plan view configuration corres-ponding to the desired plan view configuration of each donative fluid flow passage and said second passage forming means including a second blade having an operative portion with a plan view configuration corresponding to the desired plan view configuration of each recipient fluid flow passage, one of said blades having a cross-sectional thick-ness which varies along its longitudinal length.

3. Apparatus as defined in claim 2, wherein said second blade has a uniform cross-sectional thickness.

4. Apparatus as defined in claim 3, further including mounting means for reciprocally moving said first and second blades relative to one another between a first position in which the blades are separated to receive an unpleated ductile sheet and a second position in which the ductile sheet is bent to form the side wall of a pleat within the clearance space between said first blade and a second blade, said clearance space having a uniform thick-ness as measured in a direction perpendicular to a center-line passing in plan view between said first and second blades.

5. Apparatus as defined in claim 4, wherein each said first and second blades includes a central axis in plan view which follows a curvilinear path defined by a periodic function having a constant amplitude and constant wavelength.

6. Apparatus as defined in claim 4, wherein said clearance space may be divided into a plurality of wave-length portions, each wavelength portion being defined by corresponding wavelength portions of adjacent surfaces of said first and second blades, a first section of each wavelength portion of said first blade surfaces includes a first circular arc in plan view having a first center of curvature on one side of said clearance space and a first section of each wave length portion of said second blade surface includes a second circular arc in plan view having a center of curvature coincident with said first center of curvature, and a remaining section of each wave length portion of said first blade surface includes a third circular arc in plan view having a second center of curvature on the side of said clearance space opposite to said first center of curvature and a remaining section of each wave length portion of said second blade surface includes a fourth circular arc in plan view having a center of curvature coincident with said second center of curvature.