Abstract:
A turbulator for a heat exchanger tube, and a method of manufacturing the heat exchanger tube. The turbulator comprises a mesh, ideally a woven mesh, of material, ideally metallic wires. The mesh is formed into corrugations of a size substantially to fill the heat exchanger tube. The method comprises the steps of: making a mesh of material; forming the mesh into corrugations; and fitting the corrugated mesh into the heat exchanger tube so that the corrugated mesh substantially fills the heat exchanger tube for some or all of its length. The turbulator and method are particularly suited to flat heat exchanger tubes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 of United Kingdom Patent Application No. 0720627.9 filed Oct. 19, 2007. The disclosure of such United Kingdom patent application is hereby incorporated herein by reference in its entirety, for all purposes. 
         [0002]    1. Field of the Invention 
         [0003]    This invention relates to a turbulator for a heat exchanger tube, and to a method of manufacturing the heat exchanger tube. 
         [0004]    2. Background of the Invention 
         [0005]    Often it is necessary to cool a working fluid, and it is known for this purpose to use a heat exchanger. Heat exchangers often comprise one or more metallic tubes suspended between two tube plates. Usually, the working fluid to be cooled, which may for example be water or oil, flows through the tubes, whilst the coolant passes around and between those tubes, the working fluid giving up its latent heat to the tubes and thus to the coolant. 
         [0006]    The effective surface area of a tube can be enlarged in order to increase the heat transfer, as by the addition of one or more extended surface members or fins in thermal contact with the outer surface of the tube. Such finned tubes are particularly useful if the coolant has a low viscosity, and if the coolant is a gas, such as air. 
         [0007]    In addition, the heat exchange can be increased by the use of a turbulator within the tube, the turbulator acting to disturb any laminar flow of the working fluid within the tube, or in other words to induce turbulence into the working fluid as it flows along the heat exchanger tube. Thus, it is recognised that the presence of laminar flow in the working fluid decreases the heat exchange as cooler working fluid remains adjacent to the tube wall whilst hotter working fluid flows along the centre of the tube and gives up less of its heat energy to the tube wall than would be the case with turbulent flow. This is a particular problem when the working fluid is oil, as the viscosity of oil changes significantly over the temperature range typically encountered in the heat exchanger, with the cooler oil forming a substantially stationary surface layer upon the inside of the tube, the stationary layer acting as a heat insulator and reducing the heat transferred from the hotter oil flowing along the centre of the tube. 
       DESCRIPTION OF THE PRIOR ART 
       [0008]    There are several types of turbulator in present use. One type comprise a wire wound around a central shaft, the wire being wound into a shape which has the appearance of the outline of a series of flower petals surrounding the central shaft. The series of “petals” surrounds the central shaft and spans the length of the central shaft in a substantial helical pattern. It is arranged that the “petals” are offset along the length of the central shaft, i.e. a petal is out of alignment with its longitudinal neighbors, so that a continuous path for the working fluid along the tube is avoided. 
         [0009]    Another turbulator comprises a strip of metallic tape having a width similar to the diameter of the tube, the tape being wound into a helix. When the tape is inserted into the tube the working fluid is forced to undertake a helical flow path along the tube. 
         [0010]    Both of these turbulators are limited to use in heat exchanger tubes having a circular cross-section. Not all heat exchanger tubes fulfill that criterion, and in particular oval or flat tubes are known to provide better performance when the coolant is air, for example in the radiators and oil coolers of motor vehicles. For the avoidance of doubt, flat tubes as used in heat exchanger applications have a cross-sectional shape comprising two parallel long sides joined by two curved short sides, and therefore have the cross-sectional appearance of a severely flattened circle. 
         [0011]    A turbulator for flat tubes is also known, and comprises a sheet of metal which has a pattern of slits formed therethrough, the slitted sheet then being pressed so that the slitted parts form many rows of corrugations. In use, the rows of corrugations run perpendicular to the longitudinal axis of the tube, and each row is offset from its neighbors. Despite the offsetting of the neighboring rows, however, a substantially direct path through the turbulator remains for the working fluid, and so this turbulator does not maximize the heat exchange which is available, particularly when the working fluid is oil. In addition, the requirement to form slits into the sheet of metal, and subsequently to press the metal into rows of corrugations, limits the materials which can be used for the turbulator. 
       SUMMARY OF THE INVENTION 
       [0012]    It is the object of the present invention to provide a turbulator for a heat exchanger tube, and in particular for a flat heat exchanger tube, which avoids or reduces the disadvantages of the prior art turbulators described above. 
         [0013]    According to the invention, there is provided a turbulator for a heat exchanger tube comprising a mesh of material, the mesh of material being formed into corrugations. The corrugated mesh will ideally substantially fill the heat exchanger tube for all of part of its length. 
         [0014]    Preferably the mesh is of a heat conductive material. Whilst it is preferable for the mesh to be heat conductive so as to facilitate the transfer of heat from the working fluid to the tube, it has been discovered that this is not always necessary, particularly with flat tubes, and a turbulator of a thermally insulating material can increase the heat exchange merely by inducing turbulence into the fluid. 
         [0015]    A mesh material can be made from many suitable materials and so there are few limitations upon the material from which the turbulator can be made. Ideally the mesh material is a metal, and most of the metals which might be suitable for the use as a turbulator in heat exchanger applications can be formed into wires and subsequently formed into a mesh. Alternatively the mesh material could be moulded or sintered plastic for example, a suitable sintered nylon material being produced by selective laser sintering. 
         [0016]    Preferably, the mesh is woven from wires or strands of the material. Preferably also, respective wires or strands of the woven material are arranged substantially perpendicular to each other. The present invention takes advantage of the fact that weaving with substantially perpendicular wires or strands is well established technology, and there are many manufacturers of woven metal wire mesh for example. 
         [0017]    Ideally the corrugations are substantially sinusoidal. Sinusoidal corrugations are not essential, however, and corrugations of other forms can be used. However, curved corrugations are preferred, i.e. it is not presently preferred to use rectangular corrugations. 
         [0018]    Preferably, the axis of the corrugations, i.e. that axis along which the distance between successive peaks of the corrugations is minimized, is at an acute angle to the axes of the mesh, i.e. at an acute angle to the longitudinal axes of the wires forming the mesh. Angling the corrugations relative to the mesh in this way reduces the likelihood that there is a substantially straight path through the turbulator. 
         [0019]    There is also provided a heat exchanger tube fitted with a turbulator as defined herein. 
         [0020]    Desirably, the turbulator is a sliding fit within the tube. A sliding fit is preferred so as to minimize the likelihood of the turbulator becoming distorted or damaged during insertion into the tube. 
         [0021]    Alternatively (or additionally), the turbulator can include a substantially linear wire which is used to pull the turbulator into the heat exchanger tube. The substantially linear wire is preferably secured to the mesh at multiple positions along the length of the mesh, so that as the substantially linear wire is pulled through the tube the mesh is pulled thereby. 
         [0022]    In certain embodiments the corrugated mesh of material is resilient, so that the corrugations in the mesh can be significantly flattened during insertion into the heat exchanger tube, and once inserted the corrugations can move into tight (or tighter) contact with the tube wall. This is particularly advantageous with heat conductive mesh as a tighter contact with the tube wall will usually lead to an increase the heat exchange capability therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
         [0023]    The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1  shows a plan view of a mesh material prior to corrugating; 
           [0025]      FIG. 2  shows a sectional view along the line II-II of  FIG. 1 , after the mesh material has been corrugated; and 
           [0026]      FIG. 3  shows a representation of a heat exchanger tube according to the invention, in cross-section. 
       
    
    
     DETAILED DESCRIPTION  
       [0027]    The mesh material  10  of  FIG. 1  is made of metal, and is formed from a first set of wires  12  and a second set of wires  14 , the wires in the set of wires  12  all being substantially parallel with each other, as are the wires in the set of wires  14 . The wires  12 ,  14  are interlaced or woven in known fashion. 
         [0028]    The mesh  10  is formed as a strip having a longitudinal axis L which will be aligned with the longitudinal axis of the heat exchanger tube  16  ( FIG. 3 ) when fitted. The wires  12  are arranged at an angle a to the axis L, and the wires  14  are arranged at an angle β to the axis L. 
         [0029]    In this embodiment both of the angles α and β are 45°, so that the wires  12  and  14  are perpendicular to one another. In another embodiment the angle α is 0° and the angle β is 90°, and in yet other embodiments the angles fall between these values. In yet other embodiments the wires  12  and  14  are not perpendicular, the angles α and β being chosen to suit a particular material from which the mesh is made, or to meet a desired manufacturing or performance criterion. 
         [0030]    The wires  12  and  14  in this embodiment are of aluminum with a circular cross-section having a diameter of 0.1 mm and a mesh pitch of 1 mm. Such a mesh material is available from Potter &amp; Soar Limited, of Beaumont Road, Banbury, OX16 ISD, UK, for example. The wires  12  and  14  can be coated with a protective material such as epoxy, which will reduce the tendency of the wires to break during corrugation or during insertion into the heat exchanger tube  16 , it being recognised that small fragments of wire which break off from the mesh  10  could interfere with other components within the circuit of the working fluid. 
         [0031]    Notwithstanding the use of woven wire in the preferred embodiment described, the present invention could alternatively utilize a mesh formed of wires which are bonded at their junctions, the bonding perhaps being achieved by a coating material which serves both to bond the wires together and also to protect the wires during corrugation and insertion into the tube. 
         [0032]    Prior to insertion into the tube  16  the mesh  10  is corrugated into a turbulator  20  ( FIG. 2 ). The corrugations (which are shown in  FIG. 2 ) are preferably achieved by passing the strip of mesh  10  through a set of corrugating rollers (not shown). The form of the rollers, and the resulting wavelength, amplitude and orientation of the corrugations, can be determined to suit a particular application, but it is presently preferred that the corrugations be curved rather than rectangular, so as to avoid the need to form sharp corners in the wires  12 ,  14 . Corrugating rollers which form sinusoidal corrugations for example are readily available. 
         [0033]    Importantly, the axis of the corrugations (i.e. the shortest line joining successive peaks  22  (or successive troughs) of the corrugations, should desirably not be parallel with the axis of the wires  12  or  14 . If the axis of the corrugations is parallel to the axis of the wires  12  or  14  it is possible that the working fluid would be presented with one or more substantially linear paths through the turbulator  20 , and this should be avoided, especially if the heat exchanger tube is to be used in a heat exchanger in which the working fluid is oil. 
         [0034]    In this embodiment the axis of the corrugations lies along the line II-II, at an angle δ to the longitudinal axis L, where the angle δ differs from the angles α and β preferably by at least 15°. In another embodiment (in which the angles α and β are 45°) the axis of the corrugations is parallel to the longitudinal axis L. In all embodiments, the peaks and troughs of the corrugations should run generally across the mesh rather than generally along the mesh, i.e. the angle δ is preferably significantly less than 90°, and ideally less than 45°, so that a linear path through the turbulator  20  (i.e. along a trough) is not available. 
         [0035]    The turbulator  20  is intended to substantially fill the heat exchanger tube  16 , so that there are preferably no direct paths for the working fluid between the turbulator  20  and the tube wall. In common with other flat heat exchanger tubes, the tube  16  in cross-sectional view as seen in  FIG. 3  has two parallel long walls  24  and two curved short walls  26 . 
         [0036]    It will be recognised that it may not be possible to form the mesh  10  to completely fill the cross-sectional area of the tube  16 , and there may be small gaps present between the turbulator  20  and the tube wall, for example adjacent to the curved short walls  26 . That is not too disadvantageous, however, as the resulting direct path for the working fluid lies directly adjacent to the tube wall so that the working fluid will nevertheless give up much of its heat to the tube wall. Alternatively, the form of the corrugating rollers can be chosen to form the longitudinal edges of the turbulator into a curved form closely matching the curved shape of the short walls  26 , so that the presence of gaps is reduced or avoided. 
         [0037]    It will be understood that  FIG. 3  represents a cross-section very close to, and viewed towards, the end of the heat exchanger tube  16 , so that only around a half of one corrugation of the mesh  10  is visible for ease of understanding. In an end view of an actual heat exchanger tube made according to the invention, the tube would be totally (or at least substantially) filled by the turbulator  20 . 
         [0038]    When the corrugations are formed in the mesh  10 , it is arranged that the amplitude closely matches the distance between the tube walls  24 , so that substantially no gap lies adjacent to the tube walls  24 . However, it will be appreciated that in these circumstances there will be a frictional resistance to the passage of the turbulator  20  along the tube  16 . If the frictional resistance is too great the turbulator  20  may become distorted or damaged, leading to a larger or smaller pressure drop within the tube  16 , and a better or worse heat exchange performance, than was expected. Accordingly, it may be preferable to make the amplitude of the corrugations very slightly smaller than the distance between the tube walls  24 , so that the turbulator  20  can be slid easily into the tube  16  without the likelihood of distortion or damage. Whilst that would increase the likelihood of a gap between the turbulator  20  and one or both of the tube walls  24 , which gap would provide a direct path for working fluid through the tube  16 , once again that is not too disadvantageous because that direct path lies immediately adjacent to the tube wall  24 . 
         [0039]    In a preferred embodiment the turbulator  20  is sufficiently resilient (because of the material from which it is made and/or the way the corrugations are formed) to allow the corrugations to be flattened to an amplitude smaller than the distance between the tube walls  24  as the turbulator  20  is pulled through the tube  16 , and when released the amplitude will increase so that the turbulator  20  engages both of the tube walls  24 . 
         [0040]    In another preferred embodiment a substantially linear fitting wire (not shown) is secured along the turbulator, usefully being secured at each of the peaks  22  of the corrugated mesh. The fitting wire is provided so that it can be pulled through the tube  16  and thereby pull the turbulator. Because the fitting wire is secured along the corrugated mesh, the tensile force upon the fitting wire as the turbulator is pulled through the tube  16  is spread out over the length of the turbulator so reducing the likelihood of any part of the mesh becoming damaged or distorted. 
         [0041]    The fitting wire may be bonded to the peaks of the corrugations, suitably by an adhesive or the like which also acts to coat and protect the wires of the turbulator. 
         [0042]    In other embodiments the mesh is non-metallic, and can for example comprise a moulded mesh of plastics material, or a sintered mesh from a suitable base material such as nylon.