Enthalpy exchanger element, enthalpy exchanger comprising such elements and method for their production

Abstract

The present invention provides enthalpy exchanger elements (E, E′, PR, PF) and enthalpy exchangers comprising such elements. Furthermore, the invention discloses a method for producing such enthalpy exchanger elements and enthalpy exchangers, comprising the steps of a) providing an air-permeable sheet element (1); b) laminating at least one side (1a, 1b) of the sheet element (1) with a thin polymer film (3, 4) with water vapor transmission characteristics; and c) forming the laminated sheet element (1) into a desired shape exhibiting a three-dimensional corrugation pattern (5, 5, . . . ).

Claims

1. A method for manufacturing one or more enthalpy exchanger elements (E, E′, PR, PL), the method comprising: a) providing an air-permeable sheet element (1); b) laminating at least one side (1a, 1b) of the sheet element (1) with a polymer film (3, 4) having water vapor transmission characteristics; and c) forming the laminated sheet element (1) into a desired shape exhibiting a three-dimensional corrugation pattern (5, 5, . . . ) said corrugation pattern including one or more parallel flow regions or counter flow regions, a first cross flow region upstream of the one or more parallel flow region or counterflow region and one or more second cross flow region downstream of the one or more parallel flow region or counterflow region, wherein said first or second cross-flow region comprises a plurality of air guiding walls (15, 16); thereby producing one or more enthalpy exchanger elements (E, E′, PR, PL) upon completing steps a-c.

2. The method according to claim 1, wherein the sheet material of the sheet element (1) comprises a polymer.

3. The method according to claim 1, wherein the sheet element (1) is a fabric.

4. The method according to claim 3, wherein a fraction of the fibers (6) of the fabric are multi-component.

5. The method according to claim 1, wherein the laminating step b) further comprises at least one of bonding, welding or gluing, of the polymer film (3, 4) to the sheet element (1).

6. The method according to claim 1, wherein the at least one polymer film (3, 4) on the at least one side (1a, 1b) of the sheet element (1) is an air-impermeable polymer film.

7. The method according to claim 1, wherein the polymer film (3, 4) is a multilayer film comprising a sequence of polymer layers of different polymer types.

8. The method according to claim 7, wherein the polymer type of each polymer layer is selected from the group consisting of polyether ester, polyether amide and polyether urethane.

9. The method according to claim 7, wherein the total thickness of the thin polymer multilayer film is between 5 μm and 200 μm.

10. The method according to claim 7, wherein the thickness of each individual polymer layer within the polymer multilayer film is between 1 μm and 20 μm.

11. The method according to claim 1, wherein the forming step c) is one of a thermoforming step, a vacuum forming step, a pleating step, or combinations thereof.

12. The method according to claim 11, wherein said laminated sheet element is formed in step c. by a first mold part having a first corrugation formation defining a predetermined corrugation pattern of the enthalpy exchanger element (E, E′) to be manufactured.

13. The method according to claim 12, wherein a second mold part having second corrugation formations complementary to the first corrugation formations co-defining the predetermined corrugation pattern of the enthalpy exchanger element (E, E′) to be manufactured, is provided for and used in the thermoforming step c).

14. The method according to claim 10, wherein nozzles connected to a pressurized air source provided for and used in the thermoforming step c).

15. The method according to claim 14, wherein the nozzles are provided in the vicinity of the first mold part and/or the second mold part.

16. The method of claim 1 further comprising: d) repeating steps a), b) and c) to produce a plurality of laminated and formed sheet elements exhibiting a three-dimensional corrugation pattern; e) stacking the plurality of laminated and formed sheet elements; and f) fixing the stacked laminated and formed sheet elements to each other.

17. An enthalpy exchanger comprising: at least three sheet or plate enthalpy exchanger elements (E; E′, PR, PL), produced using the method as defined in claim 1; wherein each enthalpy exchanger element includes: an air-permeable sheet element (1); and a predetermined three-dimensional corrugation pattern (5, 5, . . . ); wherein a first polymer film (3) is laminated to a first side (1a) of the sheet element (1) and/or a second polymer film (4) is laminated to a second side (1b) of the sheet element (1), the one or both polymer films (3, 4) having characteristics for selective water vapor transmission; and wherein each enthalpy exchanger element further includes a parallel flow region or counter flow region, a first cross flow region upstream of the parallel flow region or counterflow region and a second cross flow region downstream of the parallel flow region or counterflow region; and wherein the enthalpy exchanger elements are stacked such that right handed enthalpy exchanger elements are alternatingly stacked with left handed enthalpy heat exchanger elements.

18. The enthalpy exchanger element (E, PR, PL) according to claim 17, wherein the first polymer film (3) and the second polymer film (4) are identical to each other.

19. The enthalpy exchanger element (E′, PR, PL) according to claim 17, wherein the first polymer film (3) and the second polymer film (4) are different from each other.

20. The method according to claim 3, wherein the sheet element (1) is a nonwoven fabric.

21. The method according to claim 9, wherein the total thickness of the polymer multilayer film is between 10 μm and 150 μm.

22. The method according to claim 4, wherein at least 50% by weight of the fibers (6) of the fabric are bi-component fibers.

23. The method according to claim 10, wherein the thickness of each individual polymer layer within the thin polymer multilayer film is between 4 μm and 20 μm.

24. The method according to claim 23, wherein the thickness of each individual polymer layer within the thin polymer multilayer film is between 4 μm and 15 μm.

Description

(1) In the following description, two non-limiting embodiments of the invention are described in further detail below with reference to the drawings, wherein:

(2) FIG. 1 is a schematic representation of the method for producing enthalpy exchanger elements according to the invention;

(3) FIG. 2 is a schematic representation of an enthalpy exchanger according to the invention or a portion thereof including a plurality of enthalpy exchanger elements according to the invention;

(4) FIG. 3 is a SEM (scanning electron microscope) micrograph of a cross-sectional view of a portion of an intermediate product produced during the method for producing an enthalpy exchanger element according to the invention;

(5) FIG. 4 is a SEM micrograph of a cross-sectional view of a portion of an enthalpy exchanger element produced by the method according to the invention;

(6) FIG. 5 is a SEM micrograph similar to the one of FIG. 3 showing of a larger scale cross-sectional view of a smaller portion of an intermediate product produced during the method for producing an enthalpy exchanger element according to the invention;

(7) FIG. 6 is a SEM micrograph similar to the one of FIG. 4 showing of a smaller scale cross-sectional view of a larger portion of an enthalpy exchanger element produced by the method according to the invention;

(8) FIG. 7 is a schematic perspective view showing a right-handed enthalpy exchanger sheet element of a first embodiment according to the invention;

(9) FIG. 8 is a schematic plan view showing the right-handed enthalpy exchanger sheet element of the first embodiment according to the invention;

(10) FIG. 9 is a schematic perspective view showing a left-handed enthalpy exchanger sheet element of a second embodiment according to the invention;

(11) FIG. 10 is a schematic plan view showing the left-handed enthalpy exchanger sheet element of the second embodiment according to the invention;

(12) FIG. 11 is a schematic perspective view showing a right-handed enthalpy exchanger sheet element of the second embodiment according to the invention;

(13) FIG. 12 is a schematic plan view showing the right-handed enthalpy exchanger sheet element of the second embodiment according to the invention; and

(14) FIG. 13 is a schematic plan view showing a pair of enthalpy exchanger sheet elements stacked one on top of the other, one being the left-handed enthalpy exchanger sheet element as shown in FIG. 10 and the one other being the right-handed enthalpy exchanger sheet element as shown in FIG. 12.

(15) In FIG. 1, a schematic representation of the method for producing enthalpy exchanger elements according to the invention is shown. Cross-sections of the intermediate products, i.e. the results of each of steps S1, S2 and S3, are shown.

(16) In a first step S1, an air-permeable sheet element 1 having voids or openings 2 is provided.

(17) In a second step S2, both sides 1a, 1b of the sheet element 1 are laminated with a thin polymer film 3, 4 with water vapor transmission characteristics.

(18) In a third step S3, the laminated sheet element 1 is formed into a desired shape exhibiting a three-dimensional corrugation pattern 5.

(19) The sheet element 2 is a non-woven fabric including thermoplastic fibers only or a combination of thermoset fibers and thermoplastic fibers. The fabric may include bicomponent fibers together with standard thermoset and/or thermoplastic fibers.

(20) The thin polymer film 3, 4 is a multilayer film which may comprise a sequence (not shown) of polymer layers of different polymer types.

(21) The forming step S3 is a thermoforming step, preferably a vacuum forming step. At least a first mold part (e.g. lower tool, not shown) having first corrugation formations co-defining the predetermined corrugation pattern 5 of the enthalpy exchanger element E, E′ to be manufactured, is used in the thermoforming step S3. In addition to the at least first mold part, a second mold part (e.g. upper tool, not shown) having second corrugation formations complementary to the first corrugation formations and/or a forming vacuum co-defining the predetermined corrugation pattern of the enthalpy exchanger element E, E′ to be manufactured, is/are used in the thermoforming step S3.

(22) The resulting enthalpy exchanger element E having a first thin polymer film 3 on the first side 1a of the sheet element 1 and a second thin polymer film 4 on the second side 1b of the sheet element 1 comprises a corrugated structure 5 with alternating squeezed portions 5a and squeezed/stretched portions 5b. The squeezed portion 5a extend in a first direction (horizontal direction in FIG. 1) and the squeezed/stretched portions 5b extend in a second direction different from the first direction. Preferably, the angle α between the first direction and the second direction in the corrugation pattern 5 of the enthalpy exchanger element E is between 90° and 120°, preferably between 95° and 105°, an example of which is shown in FIG. 1. Alternatively, unlike the example shown in FIG. 1, the angle α between the first direction and the second direction in the corrugation pattern 5 of the enthalpy exchanger element E is between 80° and 90°, preferably between 85° and 90°.

(23) In FIG. 2, a schematic representation of a first type enthalpy exchanger E1-E2-E3 or second type enthalpy exchanger E1′-E2′-E3′ according to the invention is shown. The first type E1-E2-E3 includes a plurality of enthalpy exchanger elements E1, E2, E3 where the first thin polymer film 3 and the second thin polymer film 4 (FIG. 1) are films of the same type. The second type E1 ‘-E2’-E3′ includes a plurality of enthalpy exchanger elements E1′, E2′, E3′ where the first thin polymer film 3 and the second thin polymer film 4 (FIG. 1) are films of different types including the case where one of the two films 3, 4 has zero thickness, i.e. the enthalpy exchanger element has only one thin polymer film 3 or 4 on one side 1a or 1b of the sheet element 1.

(24) In FIG. 2, the outer walls of the housing/packaging of the enthalpy exchanger E1-E2-E3 or E1 ‘-E2’-E3′ is not shown. The air inlet/outlet portions (not shown) of the enthalpy exchanger E1-E2-E3 or E1 ‘-E2’-E3′ are provided with air distribution patterns such that the air flow direction in adjacent air ducts in the enthalpy exchanger E1-E2-E3 or E1 ‘-E2’-E3′ are in opposite directions, as shown by the O symbol indicating air flow towards the viewer and the X symbol indicating air flow away from the viewer.

(25) FIG. 3 shows a SEM (scanning electron microscope) micrograph of a cross-sectional view of an air-permeable sheet element 1 laminated on its upper side 1a with a first thin polymer film 3 and laminated on its lower side 1b with a second thin polymer film 4 as a result of step b) of the method according to the invention.

(26) The laminating step b) may comprise bonding, preferably heat bonding and/or gluing, of the thin polymer films 3, 4 to the sheet element 1. A thermoplastic adhesive (hot melt adhesive) may be used for the bonding between the polymer film 3 and 4 and the sheet element 1.

(27) The sheet element 1 is a nonwoven fabric comprising a plurality of fibers 6. The fibers 6 may be thermoplastic fibers only or a combination of thermoset fibers and/or mineral fibers on the one hand and thermoplastic fibers on the other hand. Most preferably, the fabric includes multicomponent or bicomponent fibers together with standard thermoset and/or thermoplastic fibers. As can be best seen by comparing FIG. 3 with FIG. 4, the fibers 6 of the nonwoven fabric sheet element 1 shown in FIG. 3 are less densely packed than the fibers 6 of the nonwoven fabric sheet element 1 of the enthalpy exchanger element shown in FIG. 4.

(28) FIG. 4 shows a SEM micrograph of a cross-sectional view of a portion of an enthalpy exchanger element E produced by forming the laminated sheet element 1 of FIG. 3 into a desired shape exhibiting a three-dimensional corrugation pattern as a result of step c) of the method according to the invention.

(29) The forming step c) may be a pleating step or thermoforming step, preferably a vacuum forming step. At least a first mold part (e.g. lower tool, not shown) having first corrugation formations defining or co-defining the predetermined corrugation pattern of the enthalpy exchanger element E, E′ to be manufactured, is provided for and used in the thermoforming step. In addition to the at least first mold part, a second mold part (e.g. upper tool, not shown) having second corrugation formations complementary to the first corrugation formations and/or a forming vacuum co-defining the predetermined corrugation pattern of the enthalpy exchanger element E, E′ to be manufactured, may be provided in the thermoforming step.

(30) The first mold part (e.g. lower tool) may comprise nozzles or through holes pneumatically connected to a vacuum source providing a vacuum for the vacuum forming step.

(31) In addition to the first mold part and/or the second mold part used in the forming step c), preferably for supporting the vacuum action in the vacuum forming step, nozzles connected to a pressurized air source may be provided. These nozzles may be provided in the vicinity of, preferably adjacent to, the first mold part and/or the second mold part. Preferably, the pressurized air source comprises an air heating device for heating the pressurized air.

(32) The combined use of the first tool and the vacuum source in the thermoforming step c) can be supplemented by the second tool and/or the pressurized air source, preferably with an air heating device. As a result, using at least some of these supplements, a sheet element 1 laminated with a first thin polymer film 3 and an optional second thin polymer film 4 can be pressed more strongly against the first corrugation formations of the first mold part, thus producing an enthalpy exchanger element E with a better copy of the first corrugation formations of the first mold part defining or co-defining the predetermined corrugation pattern of the enthalpy exchanger element E to be manufactured.

(33) The sheet element 1 of the enthalpy exchanger element E has its fibers 6 much more densely packed than the sheet element 1 of FIG. 3. During the pleating or thermoforming step c), the fabric sheet element 1 with its first thin polymer film 3 and its second thin polymer film 4 is compressed and heated. At least the thermoplastic fibers or the multicomponent or bicomponent fibers of the plurality of fibers 6 are softened or partly melted during the pleating or thermoforming step c).

(34) As a result, after cooling and hardening of the thermoplastic fibers or the multicomponent or bicomponent fibers of the plurality of fibers 6, the fabric sheet element 1 with its first thin polymer film 3 and its second thin polymer film 4 is transformed into an enthalpy exchanger element E according to the invention with a more compact fiber structure in the fabric sheet element 1 and with a three-dimensional corrugation pattern.

(35) FIG. 5 shows a SEM micrograph similar to the one of FIG. 3 showing of a larger scale cross-sectional view of a smaller portion of the air-permeable fabric sheet element 1 laminated on its upper side 1a with the first thin polymer film 3 and laminated on its lower side 1b with the second thin polymer film 4 as a result of step b) of the method according to the invention.

(36) FIG. 6 shows a SEM micrograph similar to the one of FIG. 4 showing of a smaller scale cross-sectional view of a larger portion of the enthalpy exchanger element E produced by the method according to the invention.

(37) FIG. 7 and FIG. 8 are a schematic perspective view and a schematic plan view, respectively of a right-handed enthalpy exchanger sheet element PR of a first embodiment according to the invention. The plate-like enthalpy exchanger element PR has a parallel flow/counter-flow region PF comprising the corrugation 5, a first cross-flow region CF1 upstream of the parallel flow/counter-flow region PF, and a second cross-flow region CF2 downstream of the parallel flow/counter-flow region PF. A plurality of such right-handed enthalpy exchanger sheet elements PR are stacked with a plurality of left-handed enthalpy exchanger sheet elements PL (not shown) to form an enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL.

(38) In plan view, the first and second cross-flow regions CF1 and CF2 are delimited by a triangular contour line, i.e. they have a triangular shape. Each cross-flow region CF1 and CF2 comprises a plurality of air guiding walls 15 and 16, respectively. The air guiding walls 15 are substantially parallel to each other and substantially parallel to one side of the triangular shape of the first cross-flow region CF1. Similarly, the air guiding walls 16 are substantially parallel to each other and parallel to one side of the triangular shape of the second cross-flow region CF2. The air guiding walls 15 of the first cross-flow region CF1 and the air guiding walls 16 of the second cross-flow region CF2 both extend in a direction forming an angle β of about 45° with the direction of the corrugations 5 of the parallel flow/counter-flow region PF. As a result, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, the air guiding walls 15 of adjacent right-handed and left-handed sheet elements PR and PL extend in directions forming an angle of about 90° with respect to each other, thus defining the first cross-flow region CF1 of the enthalpy exchanger stack. Similarly, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, the air guiding walls 16 of adjacent right-handed and left-handed sheet elements PR and PL extend in directions forming an angle of about 90° with respect to each other, thus defining the second cross-flow region CF2 of the enthalpy exchanger stack.

(39) A first transition region 18 extends between the first cross-flow region CF1 and the parallel flow/counter-flow region PF. A second transition region 19 extends between the second cross-flow region CF2 and the parallel flow/counter-flow region PF. Both transition regions 18 and 19 comprise open ends and closed ends of the corrugations 5 and the parallel air ducts formed by the corrugations such that each closed-ended duct has an adjacent open-ended duct. The end wall of each closed-ended duct is sloped such that the wall forms an angle of 5° to 60° with respect to the longitudinal direction of the parallel air ducts. As a result, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, resistance to air flow is minimized in the first transition regions 18 of the enthalpy exchanger stack and in the second transition regions 19 of the enthalpy exchanger stack.

(40) The plate-like enthalpy exchanger element PR has a plurality of steps along its outer contour line. In the example shown in FIGS. 7 and 8, the plate-like enthalpy exchanger element PR has a first step 11 located at the corner of the first triangular cross-flow region CF1, a second step 12 located about halfway along the parallel flow/counter-flow region PF on one side thereof, a third step 13 located at the corner of the second triangular cross-flow region CF2 and a forth step 14 located about halfway along the parallel flow/counter-flow region PF on the other side thereof. As a result, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, adjacent enthalpy exchanger sheet elements can be correctly positioned by fitting the steps 11, 12, 13, 14 of a right-handed enthalpy exchanger sheet element PR with the corresponding complementary steps 11, 12, 13, 14 of an adjacent left-handed enthalpy exchanger sheet element PL. In other words, a right-handed sheet element PR goes “hand in hand” and fits snugly with an adjacent left-handed sheet element PL before permanently fixing the stacked sheet elements to each other.

(41) Also, the plate-like enthalpy exchanger element PR has an offset 17 located about halfway along the length of each duct formed by the corrugations 5. As a result, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, adjacent enthalpy exchanger sheet elements can be correctly positioned and are prevented from slipping into each other during the stacking operation.

(42) FIG. 9 and FIG. 10 are a schematic perspective view and a schematic plan view, respectively of a left-handed enthalpy exchanger sheet element PL of a second embodiment according to the invention. The plate-like enthalpy exchanger element PL has a parallel flow/counter-flow region PF′ comprising the corrugation 5, a first cross-flow region CF1′ downstream of the parallel flow/counter-flow region PF′, and a second cross-flow region CF2′ upstream of the parallel flow/counter-flow region PF′.

(43) FIG. 11 and FIG. 12 are a schematic perspective view and a schematic plan view, respectively of a right-handed enthalpy exchanger sheet element PR of the second embodiment according to the invention. The plate-like enthalpy exchanger element PR has a parallel flow/counter-flow region PF comprising the corrugation 5, a first cross-flow region CF1 upstream of the parallel flow/counter-flow region PF, and a second cross-flow region CF2 downstream of the parallel flow/counter-flow region PF.

(44) The difference between the second embodiment and the first embodiment is the type and number of offsets. Other than that, the right-handed plate-like enthalpy exchanger elements PR and left-handed plate-like enthalpy exchanger elements PL of the first and second embodiments are identical. In the drawings of the first and second embodiments, the same reference numerals are used for identical features in both embodiments.

(45) Unlike the first embodiment where the right-handed plate-like enthalpy exchanger elements PR (FIGS. 7 and 8) and left-handed plate-like enthalpy exchanger elements PL (not shown) each have one offset 17 located about halfway along the length of each duct formed by the corrugations 5 in the plate elements PR and PL, the right-handed plate-like enthalpy exchanger elements PR (FIGS. 11 and 12) and left-handed plate-like enthalpy exchanger elements PL (FIGS. 9 and 10) of the second embodiment each have a first offset 171 and a second offset 172 spaced with respect to each other along the length of each duct formed by the corrugations 5 in the plate elements PR and PF. Again, as a result, in the enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL, adjacent enthalpy exchanger sheet elements PR and PL can be correctly positioned and are prevented from slipping into each other during the stacking operation when mechanical force is exerted on the sheets and/or in operation when pressure differences between opposite air flows in adjacent parallel flow/counter flow regions PF and PF′ in the enthalpy exchanger stack or block may build up.

(46) The first offset 171 and the second offset 172 each are formed as a curved longitudinal section or as an arcuate longitudinal section within the longitudinally extending corrugations 5 in the plate elements PR and PF. As shown in FIGS. 9, 10, 11, 12 and 13, the first offsets 171 of the right-handed plate elements PR extend in a first lateral direction with respect to the longitudinal direction of the corrugations 5, and the first offsets 171 of the left-handed plate elements PL extend in a second lateral direction opposite to the first lateral direction with respect to the longitudinal direction of the corrugations 5. Similarly, the second offsets 172 of the right-handed plate elements PR extend in the second lateral direction opposite to the first lateral direction with respect to the longitudinal direction of the corrugations 5, and the second offsets 172 of the left-handed plate elements PL extend in the first lateral direction with respect to the longitudinal direction of the corrugations 5.

(47) Again, a plurality of such right-handed enthalpy exchanger sheet elements PR (as shown in FIGS. 11 and 12) are stacked with a plurality of such left-handed enthalpy exchanger sheet elements PL (as shown in FIGS. 9 and 10) to form an enthalpy exchanger stack with alternating right-handed enthalpy exchanger sheet elements PR and left-handed enthalpy exchanger sheet elements PL. In such a stack, oppositely laterally extending first offsets 171 of adjacent sheet elements PR and PL are positioned on top of each other. Similarly, oppositely laterally extending second offsets 172 of adjacent sheet elements PR and PL are positioned on top of each other. As a result, all adjacent first offsets 171 and all adjacent second offsets 172 in the stack prevent the corrugations 5 of adjacent stacks from slipping into each other during the stacking operation and/or in operation when pressure differences between opposite air flows may build up.

(48) FIG. 13 is a schematic plan view showing a pair of enthalpy exchanger sheet elements stacked one on top of the other, the one shown with continuous lines being the left-handed enthalpy exchanger sheet element PL as shown in FIG. 10 and the one shown with discontinuous lines being the right-handed enthalpy exchanger sheet element PR as shown in FIG. 12. Such pairs of left-handed enthalpy exchanger sheet elements PL and right-handed enthalpy exchanger sheet elements PR are stacked on top of each other to form a complete enthalpy exchanger stack or block.

REFERENCE NUMERALS

(49) 1 fabric sheet element 1a first surface 1b second surface 2 voids or openings 3 first thin polymer film 4 second thin polymer film 5 corrugation(s) 5a squeezed portion 5b squeezed and/or stretched portion S1 providing step S2 laminating step S3 forming step (co-forming) O air flow direction towards viewer X air flow direction away from viewer 6 fiber α angle (in corrugation pattern) PR plate-like enthalpy exchanger element, right-handed PL plate-like enthalpy exchanger element, left-handed 11 step 12 step 13 step 14 step CF1 first cross flow region of PR CF2 second cross flow region of PR PF parallel flow/counter flow region of PR CF1′ first cross flow region of PL CF2′ second cross flow region of PL PF′ parallel flow/counter flow region of PL 15 air guiding walls (in CF1) 16 air guiding walls (in CF2) 17 offset 171 first offset 172 second offset 18 first transition region (at CF1 side) 19 second transition region (at CF2 side) β angle between directions of corrugations and air guiding walls