Abstract:
A wire spacer ( 20 ) for spacing two adjacent heat transfer plates ( 2, 4 ) of a plate type heat exchanger ( 1 ). The wire spacer ( 20 ) is formed by a bent wire ( 22 ) that is alternatingly abutting the two adjacent heat transfer plates ( 2, 4 ), while extending along the fluid channel ( 6, 8 ) of the heat exchanger ( 1 ). The wire spacer ( 20 ) has first and second support segments ( 24, 26 ) providing a plate supporting function, and spacing segments ( 28 ) for maintaining a minimal vertical spacing (Δy) between the heat transfer plates ( 2, 4 ) during heat exchanger operation. The first lower support segments ( 24 ) are formed by wire paths ( 32 ) that span a plane (S 1 ) coinciding with a top surface of the lower heat transfer plate ( 2 ), and allow the wire spacer ( 20 ) to keep its orientation fixed with respect to the heat transfer plates.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a wire spacer for a plate type heat exchanger, and to a heat exchanger provided with a plurality of such wire spacers. Furthermore, the invention relates to a method for upgrading existing plate type heat exchangers. 
       BACKGROUND ART 
       [0002]    A conventional plate type heat exchanger generally consists of a plurality of heat transfer plates, forming spatially separated but thermally connected fluid channels through which fluid streams with a different temperature are allowed to flow. This enables heat transfer to take place from the hotter fluid to the colder fluid. 
         [0003]    From U.S. Pat. No. 2,595,457, various plate type heat exchanger configurations are known, having a heat exchanger plate assembly or plate core comprising heat transfer plates between which heat exchanger elements in the form of periodically (e.g. sinusoidally) bent wires are mounted. One function of these bent wires is to provide extended wire fin surfaces that increase the effective heat transfer area of the heat transfer plates. A second function of the bent wires is to maintain a parallel orientation of the adjacent heat transfer plates, so that the fluid channels will not deform as a result of the thermal gradients occurring during use. As shown in U.S. Pat. No. 2,595,457, the periodically bent wire spacers generally comprise first support segments (e.g. a yoke portion of a U-shape) for abutting the first heat transfer plate along a first support line in the direction of the fluid flow, second support segments (e.g. a yoke portion of an inverted U-shape) for abutting the second heat transfer plate along a second support line and at a spacing distance from the first support line, and spacing segments (e.g. legs of the U-shape) interconnecting the first support segments and the second support segments in an alternating manner. 
         [0004]    The periodical wire spacers from U.S. Pat. No. 2,595,457 have to be welded or brazed with their yoke portions to at least one of the heat transfer plates, in order to fix the orientation of the wire spacer with respect to the plates, and to provide sufficient thermal bonding for obtaining the increased effective heat transfer area. The required welding or brazing complicates manufacturing of such a heat exchanger. 
       SUMMARY OF INVENTION 
       [0005]    It would be desirable to provide a wire spacer for a plate type heat exchanger, which simplifies heat exchanger construction and/or maintenance. 
         [0006]    Therefore, according to a first aspect, there is provided a wire spacer according to claim  1 . 
         [0007]    The wire spacer according to this aspect of the invention is formed by bending a stiff wire into a desired elongate shape. The mechanical stiffness (or rigidity) of the wire material is sufficient to maintain the predefined wire shape during use of the heat exchanger, to properly space (i.e. maintain the desired space between) the heat transfer plates. The minimally required stiffness is determined by the allowed deformations of the wire spacers and the heat transfer plates under the typical thermal gradients and mechanical stresses occurring during heat exchanger operation. The wire portion perpendicular to the plates must be rigid with respect to the stresses imposed on the wire, and the wire thickness must be selected to meet this condition. 
         [0008]    Typically, the heat transfer plates are 1-2 millimeter thick, so that the wire cross-section (or diameter, in case of a cylindrical wire) typically needs to be in the range of 2-4 millimeter. For such a configuration, height of the vertical portion must be 0.1 to 0.2 mm smaller than the spacing distance between the heat transfer plates, both in cold condition and during shop assembly, in order to accommodate the small deformation due to thermal gradients during operation. 
         [0009]    The material of the wire should be selected based on the expected heat exchanger operating temperature and thermal gradient forces. Among the possible wire materials are carbon steel wire (uncoated or aluminized or galvanized), various grades of austenitic stainless steel. A circular wire cross-section is preferred, but other wire cross-section shapes (e.g. polygonal) are possible. 
         [0010]    The wire paths jointly span the support plane parallel to and abutting the first (lower) heat transfer plate. “Spanning of the plane” refers herein to providing at least three points that are non-coinciding and not co-linear, and that together define the support plane. The term “wire path” refers herein to a continuous portion of the bent wire that traces out a curved or bent wire trajectory spanning at least a line but preferably spanning a portion of the support plane (e.g. by forming a semi-circular shape, a U-shape, an S-shape, or a W-shape, within the support plane). In any case, the wire paths are formed to at least jointly span the support plane comprising the first support line. 
         [0011]    The first support line forms a path along the first heat transfer plate, which path may for example be linear along a first direction corresponding to the main flow direction of the fluid within the channel enclosed by the adjacent heat transfer plates, e.g. in a cross-flow plate type heat exchanger with linear fluid channels. Alternatively, the first support line may be slightly curved, or even substantially curved so as to follow a more sophisticated trajectory, e.g. the trajectory defined by the curved fluid channels in a Z-type concurrent- or counter-flow plate type heat exchanger. 
         [0012]    Each wire path extends at its path ends into respective spacing segments. The spacing segments interconnect the first support segments and the second support segments in an alternating manner, and have a mechanical stiffness that is sufficiently large for keeping the first and second heat transfer plates at the desired spacing distance, as was described herein above. 
         [0013]    The co-planar shapes of the wire paths cooperate to form the support plane, such that the wire spacer can be positioned between the heat transfer plates with this common support plane parallel to and abutting a heat transfer surface of the first heat transfer plate. In the intended orientation, the spacing segments fix the orientation of the second support segments at the desired spacing distance with respect to the base wire portions and the support plane. So effectively, each wire path in the wire spacer according to this aspect of the invention functions as a base for supporting the wire spacer, and for fixing the orientation of the wire spacer with respect to the heat transfer plates. 
         [0014]    According to embodiments of the wire spacer, the spacing segments extend at least partially and preferably entirely along a second direction that is locally perpendicular to the first support line. 
         [0015]    In case of a straight first support line, the second direction is perpendicular to the first direction. For adjacent parallel heat transfer plates, this second direction will be perpendicular to the heat transfer surfaces of both heat transfer plates. 
         [0016]    In general, the proposed wire spacer according to this aspect allows for easy placement between two adjacent heat transfer plates, without needing further means for holding the wire spacer in the correction upright orientation. 
         [0017]    The proposed wire spacers may be configured as relatively thin elongated structures, which do not occupy a significant volume. In contrast to this, bar spacers (e.g. known from patent document U.S. Pat. No. 5,383,516) occupy a larger volume, significantly reduce the effective cross-section of the fluid channels, and undergo substantial differential thermal expansion and thermal stresses during heat exchanger operation. Similarly, plate-embossing spacers (e.g. known from patent document U.S. Pat. No. 2,281,754) create large flow obstructions and corresponding pressure drops. 
         [0018]    According to an embodiment, the wire paths of the wire spacer jointly extend bi-directionally from the first support line and along a transversal direction, which is perpendicular to the first support line and is within in the support plane. 
         [0019]    According to further embodiments, the wire paths may comprise smoothly curved portions and/or connected linear segments, or various other shapes spanning the support plane. For example, any of the wire paths may comprise interconnected linear path segments that are oriented in the support plane and perpendicular to the first support line. Alternatively, any wire path may be formed as a smoothly curved shape with its curvature spanning the support plane, e.g. a semi-circular wire path. 
         [0020]    According to an embodiment, each wire path extends to at least one side of the first support line along the transversal direction, which is oriented perpendicular to the first support line. The wire paths may extend toward opposite sides in an alternating manner along the wire spacer. For example, one particular wire path may extend to the positive transversal direction, while the preceding and subsequent wire paths extend to the negative (i.e. opposite) transversal direction. This alternating configuration allows fixing the orientation of the wire spacer between the heat transfer plates, while requiring a minimal amount of wire. 
         [0021]    According to a further embodiment, each wire path may be individually bent to extend bi-directionally along both the positive and negative transversal directions from the first support line, to span a total base width. 
         [0022]    A wire path that by itself extends to both directions from the first support line allows stabilization of its orientation with respect to the first heat transfer plate. Furthermore, if each wire path is identically shaped to extend in both transversal directions, then the wire spacer may be formed as a periodical structure of consecutive identical units that each comprise an interconnected quadruplet formed by a first support segment, a spacing segment, a second support segment, and a further spacing segment. This periodicity greatly simplifies the manufacturing process wherein the wire is bent to form the proposed wire spacer. 
         [0023]    According to a further embodiment, the total base width equals the spacing distance. 
         [0024]    If the total base width equals the spacing distance, then an incidental occurrence of local twisting of the wire spacer about an axis parallel to the support line (i.e. a rotation of wire paths in the plane spanned by the spacing direction and transversal direction) will not have a detrimental effect on the spacing function. At the site of the twisting, the rotated wire paths with a total base width matching the desired spacing distance will still locally provide a spacing function. 
         [0025]    According to an embodiment, the wire path may be formed by multiple interconnected linear path segments that are arranged with their long axes directed along the transversal direction. These linear path segments may be abutting as viewed along the support line, and interconnected by short curved portions at the respective segment end points. In this configuration, a length of the wire path viewed along the support line (i.e. the first or fluid flow direction) can be minimized, while the support width in the transversal direction will be maximized. Hence, stabilization in the spacing direction and transversal direction is optimized, while an effective thermal contact area between the wire spacer and the heat transfer plate is kept relatively small. 
         [0026]    For example, the wire path may be formed as one of a contracted U-shape, a contracted S-shape, or a contracted W-shape. Any one of these shapes is easily formed in a manufacturing process involving wire bending actions in the positive and negative transversal directions only. Hence, wire bending actions in the direction along the support line, which complicate the manufacturing process, are avoided. 
         [0027]    According to another embodiment, the wire path is smoothly curved in the support plane. According to further embodiments, the wire path forms one of a U-shape, an S-shape, or a curved W-shape. 
         [0028]    A smoothly curved wire path is easily formed by bending the wire into the desired shape, without creating sharp turns or folds. Smooth curves minimize the risk of breaking the wire during construction. The smoothly curved wire path provides a considerable structural support area in the support plane, while minimizing the thermal contact area between the wire spacer and the first heat transfer plate. Any one of a smoothly curved U-shape, an S-shape, or a curved W-shape, is easily formed in a manufacturing process involving wire bending actions in the transversal directions only. Hence, wire bending actions in the direction along the first support line, which would complicate the manufacturing process, are avoided. 
         [0029]    According to an embodiment, the second support segments are formed by linear support segments with support lengths along the first support line. 
         [0030]    The first support segments are effectively spaced along the first support line by the linear support segments. The preferred lengths of these linear support segments are determined by the expected differential pressures between the adjacent channels and the operating temperatures occurring in the heat exchanger. In particular, by forming the support segments with equal lengths, the wire spacer possesses a linear symmetry that will provide a nearly uniform linear supporting capability along the first support line. Correspondingly, the wire spacer manufacturing process is greatly simplified. 
         [0031]    According to a further embodiment, the support lengths are equal support lengths in the range of 100 mm-200 mm. 
         [0032]    Support lengths in the range of 100 mm-200 millimeter allow robust spacing in a heat exchanger having heat transfer plates of 1-2 millimeter thickness, and operating at a differential pressure of 500-1000 Pa in a temperature range of 100-300 C. 
         [0033]    According to an embodiment, the spacing segments are formed by perpendicular linear segments with spacing heights equal to the spacing distance. Optionally, the wire could extend transversely at the end of the perpendicular linear segments in addition to or in alternative to support segments along the first support line. 
         [0034]    Spacing segments formed from linear segments that are oriented along the spacing direction provide maximal structural integrity and support. 
         [0035]    According to an embodiment, a cross-section of the bent wire is circular. 
         [0036]    A circular wire is easy and cheap to construct. Due to its cylindrical symmetry, the circular wire is easily bent into any desired elongated wire spacer shape. In addition, the isotropic bending resistance of the circular wire allows bending in the transversal and spacing directions required for forming the inherently three-dimensional configuration of the proposed wire spacer according to the first aspect. The circular cross section also minimizes the contact area between the wire spacer and the heat transfer plates, thereby avoiding excessive thermal gradients and resulting stresses occurring during operation of the heat exchanger (as is the case with pins or stud spacers welded to heat transfer plates, e.g. known from patent document WO96/19708). Furthermore, any thermal insulation coating can be applied to the smooth surface of the circular wire spacer in an easy and durable manner. 
         [0037]    According to a further embodiment, a wire diameter of the bent wire is in a diameter range of 2-4 mm. 
         [0038]    At the expected heat exchanger design temperatures and thermal gradient forces described herein above, this preferred diameter range yields a wire spacer that is sufficiently rigid to prevent deformation, while still allowing the wire to be manufactured without difficulty. A thicker wire would be difficult to process, while a thinner wire would not be able to prevent deformation. 
         [0039]    According to an embodiment, the bent wire has a first end and a second end, wherein each end is provided with attachment means for connecting the wire spacer to the first heat transfer plate and/or the second heat transfer plate. Optionally, this connection could be through electrical resistance welding or through a pin welded to the first and/or second heat transfer plates. 
         [0040]    By providing the wire spacer with attachment means at the opposite wire ends, the wire spacer may be easily fixed with respect to the heat exchanger by attachment to externally accessible regions of the heat exchanger, for example to the plate edges near fluid channel apertures, or to flow guiding elements (ferrules) located at the plate edges. 
         [0041]    According to a second aspect, and with corresponding effects and advantages as described herein above, there is provided a plate type heat exchanger as defined by claim  10 . 
         [0042]    As described herein above with respect to the first aspect of the invention, the first direction corresponding to the main flow direction of the fluid within the channel enclosed by the adjacent heat transfer plates may define a straight first support line along the first heat transfer plate (like in a cross-flow plate type heat exchanger with linear fluid channels). Alternatively, this first direction may also be construed as a local direction, which may change along the first support line. This allows the first support line to be curved along the fluid channel of the heat transfer plate (like in curved fluid channels in a Z-type concurrent- or counter-flow plate type heat exchanger). 
         [0043]    According to an embodiment, the at least one wire spacer of the plate type heat exchanger is releasably positioned between the adjacent heat transfer plates, and wherein the first end and second end of the wire spacer are fixed to respective outer edges of the heat transfer plates. Optionally, this attachment can be made by electrical resistance welding or by welding a pin to the heat transfer plates. 
         [0044]    According to a third aspect of the invention, there is provided a method for upgrading an existing plate type heat exchanger as defined by claim  14 . 
         [0045]    The effects and advantages of a heat exchanger resulting from the upgrading method according to this aspect have already been described herein above in view of the other aspects. In addition to a method for upgrading an existing heat exchanger, the method according to this aspect may also represent the reassembly phase after cleaning or repairing any heat exchanger provided with the wire spacers according to the first aspect of the invention. In such cleaning or repairing methods, the initial phase comprises removing the wire spacers from the fluid channels of the plate type heat exchanger. Subsequently, the heat transfer plates having the wire spacers removed are easily cleaned or repaired by suitable methods without obstruction from the wire spacers. Subsequently, the original wire spacers or repaired substitutes are re-inserted into the fluid channels, as defined by this third aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0046]    Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
           [0047]      FIG. 1  schematically shows a heat exchanger according to an embodiment, and 
           [0048]      FIG. 2 a -2 d    present perspective views of wire spacers according to embodiments; 
           [0049]    The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0050]      FIG. 1  shows a perspective view of a plate type heat exchanger  1  for exchanging thermal energy between a first fluid  14  and a second fluid  16  having a different temperature. The shown heat exchanger  1  comprises a number of stacked heat transfer plates  2 ,  4 . Between each two adjacent heat transfer plates  2 ,  4  first fluid channels  6  and second fluid channels  8  are formed, for transporting the first fluid  14  and second fluid  16  respectively. 
         [0051]    The first and second fluid channels  6 ,  8  are oriented mutually perpendicular, along a first direction X and a transversal direction Z respectively. The first and second fluid channels  6 ,  8  are alternatingly provided in the heat exchanger  1  in a second (vertical) direction Y, which is perpendicular to the first direction X and the transversal direction Z. This plate configuration forms a so-called cross flow plate type heat exchanger. The adjacent heat transfer plates  2 ,  4  are spaced apart at a spacing height Δy in the second direction Y. Several of the first fluid channels  6  are provided with a plurality of wire spacers  20 . The shown wire spacers  20  comprise first support segments  24 , second support segments  26 , and spacing segments  28  that interconnect the first support segments  24  and the second support segments  26 . 
         [0052]    The first support segments  24  are formed so as to abut the first heat transfer plate  2  along a first support line C 1 , and the second support segments  26  are formed so as to abut the second heat transfer plate  4  along a second support line C 2 . 
         [0053]    The first support segments  24  are curved into wire paths  32 , which jointly define a first support plane S 1  that comprises the first support line C 1 . As shown in  FIG. 1  and further illustrated by  FIG. 2   a,  each wire path  32  comprises three linear path segments  34   a - c  that are interconnected via sharply curved segments, jointly forming a contracted S-shape. As a result, the linear path segments  34   a - c  of each wire path  32  jointly extend bidirectionally from the first support line C 1 , and span the first support plane S 1  along both the first support line C 1  and a transversal direction Z. Consequently, each wire path  32  forms a support portion that spans a total base width Δx and effectively holds the wire spacer  20  steady between the heat transfer plates  2 ,  4 , with the spacing wire segments  28  in an upright orientation. Preferably, the total base widths Δx of the wire paths  32  equal the spacing distance Δy between the second support segments  26  and the first support line C 1 . 
         [0054]    The second support segments  26  are formed as linear wire segments with second support lengths Δx 2  along the second support line C 2 . Here, the second support lengths Δx 2  of subsequent second support segments  26  are shown to be equal. A typical value for the second support lengths Δx 2  may be in the range of 100 mm-200 mm. 
         [0055]    The spacing segments  28  interconnect the first support segments  24  and the second support segments  26  in an alternating manner. The spacing segments  28  in the shown embodiments are formed as linear wire segments that are perpendicular to the first support plane S 1 , and which hold the second support segments  26  at a spacing distance Ay from the first support line C 1 . For a steel heat transfer plates of 1-2 mm thickness, the spacing distance Δy is preferably 0.1 to 0.2 mm smaller than a plate distance between the heat transfer plates  2 ,  4  of a fluid channel  6 ,  8  in cold condition. 
         [0056]    The wire spacer  20  is manufactured from bending a wire  22  having a circular cross-section, into a periodical structure having a multiplicity of the described segments. A typical wire diameter Ø of the bent wire  22  (see  FIG. 2 a   ) is in a diameter range of 2-4 mm. 
         [0057]      FIG. 1  shows that each bent wire  22  has a first wire end  6  located at one side of the first fluid channel  6 . At this first wire end  6 , the bent wire  22  is provided with attachment means  44  for connecting the wire spacer  20  to the first heat transfer plate  2 . At the opposing side of the first fluid channel  6 , the wire spacer  20  terminates in a second end (not shown), wherein also the second end is provided with similar plate attachment means  44 . The wire spacers  20  are releasably positioned between the adjacent heat transfer plates  2 ,  4 , by temporarily fixing each wire spacer  20  with its first and second ends via the attachment means  44  to respective outer edges of the first heat transfer plate  2 . Alternatively, the attachment means  44  may also be provided on an upwardly bent portion of the wire spacer  20 , so as to attach the wire spacer  20  to the second heat transfer plate ( 4 ). The attachment means  44  can include ends of wire being attached through electrical resistance welding or through a pin welded to the heat transfer plate  2  and/or  4 . 
         [0058]    In an alternative embodiment shown in  FIG. 2   b,  each wire path  32  comprises only two linear path segments  34   a,    34   b  that is interconnected via one sharply curved segment, and which jointly form a contracted U-shape. Here, each wire path  32  only extends in a single transversal direction Z within the first support plane Si from the first support line C 1 . Consecutive wire paths  32 ,  32 ′ transversally extend in opposite transversal directions, so that the wire paths  32  jointly extend bidirectionally from the first support line C 1 . 
         [0059]    Many alternative wire spacers  20  provided with wire paths  32  formed from linear path segments and sharply curved connection segments may be conceived. For example, the wire path  32  may be formed from a contracted W-shape with four linear path segments joined by three sharply curved interconnection segments. 
         [0060]      FIG. 2 c    shows an alternative embodiment of the wire spacer  20 , wherein the wire paths  32  are smoothly curved within the support plane S 1 , so as to form a smooth S-shape. Also in this embodiment, each wire path  32  extends bi-directionally from the first support line C 1 , and spans a total base width Δz. The wire spacer  20  in  FIG. 2 c    has a smoothly curved first support segment  24  with a first support length Δx 1  that is considerably larger than in the previous embodiments. 
         [0061]    Many alternative embodiments provided smoothly shaped wire paths  32  may be conceived. For example, the wire path  32  may be formed as a U-shape, or a curved W-shape. 
         [0062]      FIG. 2 d    shows a further alternative embodiment of wire spacer  20 , wherein each wire path  32  comprises three linear path segments  34   a - c  that are interconnected via sharply curved segments, jointly forming a contracted S-shape, similar to the embodiment shown in  FIG. 2A . However, in  FIG. 2 d   , wire path  32  alternatingly forms first support segments  24  along line C 1  and C 2 , with second support segments  26  and spacing segments  28  between. 
         [0063]    Any of the wire paths  32  described above have the property that wire spacer  20  is formed by bending the wire  22  using only bending operations in directions transversal to a main direction along the wire spacer  20 , e.g. along the first direction X (or any of the support lines C 1 , C 2 ). 
         [0064]    Alternatively, the wire path  32  may also be formed from backward or forward wire bending operations along this main direction along the wire spacer  20 , although this will complicate the manufacturing process. By such a process, a wire spacer  20  having a more complex wire path  32  configuration may be obtained. An example of such a complex wire path  32  is a (nearly) circular wire path (not shown) that starts at an end of a spacing wire  28 , extends perpendicular along the transversal direction Z, curves backward along the negative first direction −X, toward the negative transversal direction −Z, toward the positive first direction X, and back along the transversal direction Z to extend into a subsequent spacing wire. 
         [0065]    The descriptions above are intended to be illustrative, not limiting. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice, without departing from the scope of the claims set out below. 
         [0066]    For example, any combination of the wire paths  32  described above may be provided within a single wire spacer  20 . 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  plate type heat exchanger 
           2  first heat transfer plate 
           4  second heat transfer plate 
           6  first fluid channel 
           8  second fluid channel 
           10  first fluid aperture 
           12  second fluid aperture 
           14  first fluid 
           16  second fluid 
           18  plate edge 
           20  wire spacer 
           22  bent wire 
           24  first support segment 
           26  second support segment 
           28  spacing wire segment 
           30  base wire portion 
           32  wire path 
           34   a - c  linear path segment 
           40  first end 
           42  second end 
           44  wire spacer attachment means 
         Ø wire diameter 
         X first direction 
         Y second direction 
         Z transversal direction 
         S 1  first support plane 
         S 2  second support plane 
         C 1  first support line 
         C 2  second support line 
         Δx 1  first support length 
         Δx 2  second support length 
         Δy spacing distance 
         Δz base width