Patent Publication Number: US-2011056663-A1

Title: Shell-and-Tube Heat Exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of priority of German Application No. 102009040558.5, filed Sep. 8, 2009. The entire text of the priority application is incorporated herein by reference in its entirety. 
     FIELD OF THE DISCLOSURE 
     The disclosure relates to a shell-and-tube heat exchanger of the type used in beverage bottling operations with juices and juice-type foods, and an inner and/or jacket tube of a shell-and-tube heat exchanger of this type. 
     BACKGROUND 
     A shell-and-tube heat exchanger for the treatment of juices with fibers and/or particles in which simply twisted inner and/or outer tubes are used is known from DE 600 19 635 T2. In the inner tube the angle of twist is shown at approximately 45°, whereas the angle of twist in the jacket tube is illustrated with about 75°, referred to the tube axis in each case. With this shell-and-tube heat exchanger a disadvantage is that to increase the heat transfer very substantial pressure loss is accepted for a constant area presented to the flow and the boundary layer of the flow is induced to strong rotation at the spirals which run with the same angle of twist to the tube axis. This requires very high feed pressure, which may be disadvantageous for the product, for the treatment of the product in the shell-and-tube heat exchanger. 
     From DE 102 56 232 B4 a shell-and-tube heat exchanger is known for the UHT treatment of milk and milk products for pasteurization. The jacket tube is a smooth tube. Only the inner tubes can be crossed and twisted tubes in which the multiple-start, mutually opposed spirals cross at angles of incidence between 25° and 35° longitudinally to the tube axis, i.e. at angles of twist between 65° and 55°. This angular range is specially matched to milk and milk products which have a very strong tendency to form deposits on the surface in contact. Therefore, the spirals are in addition polished electrochemically. The combination of the two measures results in optimum inhibition of the formation of deposits of milk and milk products, giving a longer service period of the shell-and-tube heat exchanger before a cleaning cycle is needed. An angle of twist of 45° is also mentioned, which is regarded as optimum for the heat transfer, but which however leads to an inadmissible heavy formation of deposits with milk and milk products. Angles of incidence between 25° and 35° and the additional surface treatment are consequently optimum for milk and milk products with regard to the inhibition of the product-specific formation of deposits, but, as with the current angle of 45° to the tube axis, they give an unfavorable relationship between the increase in the heat transfer obtained and an excessive increase in the loss of pressure in the flow. 
     SUMMARY OF THE DISCLOSURE 
     An aspect of the disclosure is to provide a shell-and-tube heat exchanger of the type mentioned in the introduction as well as a jacket tube and/or inner tube for a shell-and-tube heat exchanger of this nature, which, when processing particularly juices or juice-type foodstuff products with low, medium or high viscosity, facilitates optimally short dwell times and small heat exchange surfaces despite only moderate feed pressure. This applies to all liquid foodstuffs in this viscosity range. 
     Through the use of at least one crossed and twisted tube in the shell-and-tube heat exchanger (for the jacket tube and/or each inner tube) a substantial increase in the heat transfer is achieved due to the crossing spirals. However, with the increase of the pressure loss associated with the cross-twisting, the achievable increase in the heat transfer is brought to an optimum relationship in the through-flow by restricting the angle of incidence α to 72° to 67° longitudinally to the tube axis and the resulting angle of twist of 18° to 23° perpendicular to the tube axis, i.e. to a relatively acute angle, which, despite only moderate feed pressure, results in a smaller heat exchange surface. In other words, due to the use of crossed and twisted tubes of this nature, the shell-and-tube heat exchanger needs a relatively small heat exchange surface and therefore a short conveying distance. With juices and juice-type foodstuff products the tendency to form product deposits is of secondary importance, because with the use of crossed and twisted tubes with the stated flat angles of twist the primary factor is that pulp, fibers or particles as constituents of juices or juice-type foodstuff products do not tend to cling and collect due to the flat angle of twist, but are instead rapidly flushed further. Moreover, in this way cleaning to a hygienically flawless condition is possible. 
     In an expedient embodiment, in the axial section of the tube each spiral has a trough-shaped indentation with ribs on both sides having an approximate wedge shape in cross-section, between which a spiral depth of between about 0.8 mm to 1.2 mm is present in the indentation. The interaction between the relatively flat angles of twist and the moderate spiral depth results, even with a moderate feed pressure, in a relationship between the increase in the heat transfer and the resulting increase in the pressure loss which is optimum for juices and juice-type foodstuff products of low, medium and high viscosities. Preferably, the indentations and the ribs are arranged on the surface which is in contact with the product. 
     In an expedient embodiment the width in the indentation, viewed in the direction of the tube axis, is a multiple of the spiral depth. It should be between about 5.0 and 20.0 mm. The indentations are relatively wide cavities, on their adjacent ribs of which constituent parts of the product are rapidly flushed further and which also facilitate hygienically flawless cleaning, e.g. for a change of product. 
     In order to be able to optimally exploit the effect of the crossed spirals, both multi-start spirals fully cover the tube over all the surface. 
     In an expedient embodiment the jacket tube as a crossed and twisted tube with multiple crossed and twisted inner tubes forms a module of the shell-and-tube heat exchanger. This module can extend in an expedient manner over 3.0, 6.0 m or more and is expediently combined with several modules in series for the treatment of the product in the shell-and-tube heat exchanger. 
     In another embodiment only the inner tubes are crossed and twisted tubes with relatively flat angles of twist, for example, if a heat transfer medium is used in the flow channel between the jacket tube and the inner tubes. 
     In another embodiment the jacket tube has the indentations of the two spirals on the inner side of the tube, whereas the respective crossed and twisted inner tube has the indentations of the spirals on the outer side or the inner side, for example, depending on along which tube surface the product flows. 
     The shell-and-tube heat exchanger is particularly well suited to the treatment of juices or juice-type foodstuff products with viscosities&gt;about 5 mPas. 
     For the treatment of the relevant product two different methods present themselves, i.e. either a recuperative method in which product is processed against product separated by the respective crossed and twisted tube, e.g. in counterflow, or a method with a heat transfer medium against the product separated by the respective crossed and twisted tube, whereby then, preferably, the indentations of the two spirals of the crossed and twisted tube face the product. 
     The crossed and twisted tubes used are expediently stainless steel tubes on which both spirals act on the flows on the inner and outer tube surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure are explained based on the drawings. The following are shown: 
         FIG. 1  a shell-and-tube heat exchanger for the heat treatment of juices or juice-type foodstuff products with an example of a module with a crossed and twisted tube as jacket tube, 
         FIG. 2  another embodiment of a module with at least one inner tube as a crossed and twisted tube in, for example, a smooth jacket tube, 
         FIG. 3  a further embodiment of a module in which the jacket tube and each inner tube are formed as crossed and twisted tubes. 
         FIG. 4  a detailed section of the tube wall of a crossed and twisted tube with indentations of both spirals facing the inside of the tube, and 
         FIG. 5  a cross-section of the tube wall of a crossed and twisted tube with indentations of both spirals located on the outer side of the tube. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a module M of a shell-and-tube heat exchanger W for the heat treatment of juices or juice-type foodstuff products with medium to high viscosities, for example, a viscosity of more than about 5 mPas, which comprises a jacket tube  1  and at least one approximately coaxial inner tube  2  accommodated inside the jacket tube  1  and spaced from the inner wall of the jacket tube  1 . In the shell-and-tube heat exchanger W the module M is, for example, combined with further modules of the same type or similar which are not illustrated in order to form a treatment section of a certain conveying length. The product is either treated according to a recuperative method, i.e. product separated against product, for example, by the inner tube  2 , or according to a method in which a heat transfer medium (steam or water) is used, whereby the heat transfer medium is separated against the product, for example, by the relevant inner tube  2 . The relevant method is preferably operated in counterflow or uniflow. 
     The jacket tube  1  in  FIG. 1  is formed as a crossed and twisted tube with multi-start spirals D 1 , D 2  crossing one another essentially symmetrical to the tube axis X, whereby the angle of incidence α to the tube axis X is between 67° and 72° and the resulting angle of twist β is 18° to 23° perpendicular to the tube axis. The crossed and twisted tube is, for example, similar to that illustrated in  FIG. 4  with the indentations  3  facing the inside of the tube. In  FIG. 1  the respective inner tube  2  is either a smooth tube or similarly a crossed and twisted tube with essentially multi-start, mutually crossing spirals symmetrical to the tube axis X and with angles of twist β 1  between 18° and 23°. 
     In the embodiment in  FIG. 2  of the module M the jacket tube  1  is a smooth tube. In contrast each inner tube  2  contained in the jacket tube  1  is a crossed and twisted tube with multi-start, mutually crossing spirals D 1 , D 2  essentially symmetrical to the tube axis X and with angles of incidence α 1  between 67° and 72° to the tube axis. The spirals D 1 , D 2  are multi-start, so that despite the relatively steep angle of incidence α 1  (angle of twist β 1  between 18° and 23°), the complete tube surface is offered to the product, primarily the macro-structures which intensify the heat transfer, and the optimum is obtained between the increase in the heat transfer and the pressure loss due to the spirals. 
     In the embodiment in  FIG. 3  of the module M crossed and twisted tubes with multi-start, mutually crossing spirals D 1 , D 2  essentially symmetrical to the tube axis X are used for the jacket tube  1  and each inner tube  2 , whereby here too the angle of twist β, β 1  is between 18° and 23° perpendicular to the tube axis X. 
     An angle of twist β, β 1  of 18° to 23° to the tube axis means an angle of incidence α, α 1  of each spiral of β, β 1 =90°−angle of twist=α, α 1 =67° to 72° referred to a plane longitudinal to the tube axis X. 
       FIG. 4  illustrates the macro-structures formed as the jacket tube and/or inner tube  1 ,  2  by the crossing spirals D 1 , D 2  of the respective crossed and twisted tube, which are present on the inner side and the outer side of the tube. In  FIG. 4  the cavity-type indentations  3 , which are in each case bounded by essentially wedge-shaped ribs  5  and have a spiral depth T between 0.8 mm and 1.2 mm, are provided facing the tube axis X and following one another in the axial direction. The width B of each indentation  3  is a multiple of the spiral depth T, preferably between 5.0 mm and 20.0 mm. On the outer side of the tube and corresponding to the indentation  3 , a rounded dome  4  is provided which is bounded in the axial direction by approximately V-shaped grooves  6 . In an alternative which is not illustrated the ribs  5  and the grooves  6  can be rounded, for example with a view to easy tube cleaning. The ribs  5  or the grooves  6 , as well as the indentations  3  and the domes  4 , run over the complete inner, respectively outer tube surface at an angle of incidence α, α 1  like a thread and cross one another periodically. 
     In the embodiment in  FIG. 5  a crossed and twisted tube is shown as a jacket or inner tube  1 ,  2  on which the indentations  3  are present on the outer side of the tube (i.e. facing away from the tube axis X). In this respect the rounded domes  4  and the grooves  6  face the tube axis X. The spiral depth T is between 0.8 mm and 1.2 mm. The angle of twist β, β 1  is between 18° and 23° to the tube axis X. The crossed and twisted tube illustrated in  FIG. 5  can be expediently used as inner tube  2 , if, for example, the product flows between the jacket tube, which is designed as in  FIG. 4 , and the outer side of the inner tube  2 . If a heat transfer medium is being used, which flows in the flow channel between the jacket tube  1  and the inner tube  2 , the crossed and twisted tube of the inner tube  1 ,  2  is expediently formed analogously to  FIG. 4 . 
     From the use of the crossed and twisted tubes with an angle of incidence α, α 1  of 67° to 72° and a spiral depth T between 0.8 mm and 1.2 mm for a spiral width B between about 5.0 mm and 20.0 mm, an optimum relationship results between the increase in the heat transfer achievable by the crossed spiral technique or the heat transfer coefficient and the increase in pressure loss which has to be accepted for the through-flow for medium or highly viscous juices or juice-type foodstuff products, such that the respectively applied method (recuperative or with heat transfer medium) requires a relatively small heat exchange surface for only moderate feed pressure with short dwell times in the shell-and-tube heat exchanger, or a relatively short conveying section is sufficient for the shell-and-tube heat exchanger W.