Patent Publication Number: US-6702006-B2

Title: Plate pack, flow distribution device and plate heat exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application under 35 U.S.C. 371 of PCT/SE01/01102, filed May 18, 2001 and published in the English language, and claims the benefit of Swedish application 0001887-9 filed May 19, 2000. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a plate pack for a plate heat exchanger, comprising a number of heat transfer plates, each of which has a heat transfer portion and a number of through ports, said plates interacting in such manner, that a first flow duct is formed between them in a plurality of first plate interspaces and a second flow duct is formed in a plurality of second plate interspaces and that the ports form at least one inlet duct and at least one outlet duct for each of the flow ducts, that the inlet duct of at least the first flow duct comprises at least one primary duct, which is arranged to receive a fluid flow for the first flow duct, and at least one secondary duct, which communicates with the primary duct and the first flow duct and which is arranged to receive the fluid flow from the primary duct and to convey the fluid flow to the first flow duct. 
     The invention further relates to a flow distribution device for use in an inlet duct of a plate heat exchanger, and a plate heat exchanger. 
     BACKGROUND ART 
     A plate heat exchanger may comprise a “frame plate”, a “pressure plate” and a number of intermediate heat transfer plates clamped together in a “plate pack”. The heat transfer plates are arranged and designed so that flow paths for at least two heat transfer media are formed between them. Each heat transfer plate is provided with a number of through ports, which together form at least two inlet ducts and two outlet ducts extending through the plate pack. One of the inlet ducts and one of the outlet ducts communicate with each other via some of the flow paths, which form a flow duct for one heat transfer medium, and the other inlet and outlet ducts communicate with each other via the other flow paths, which form a flow duct for another heat transfer medium. 
     The plate heat exchanger works by two different heat exchanging media being supplied, each via a separate inlet duct, to two separate flow ducts, where the warmer medium transfers part of its heat content to the other medium by means of heat transfer plates. The two media can be different liquids, gases, vapours or combinations thereof, so-called two-phase media. 
     The plate heat exchanger concept will be described in more detail in connection with a plate heat exchanger intended for so-called two-phase application and described in the Alfa Laval AB brochure The plate evaporator from 1991 (IB 67068E) (see FIG.  1 ). 
     The medium that is to be completely or partially vaporised, for example juice that is to be concentrated, is supplied to the heat exchanger through an inlet formed by two openings in the frame plate. These two openings lead directly to a common first inlet duct, which extends through the pack of heat transfer plates. Vapour is supplied to the flow ducts formed between the heat transfer plates and intended for this purpose through a second inlet duct. This second inlet duct is formed by ports located in an upper corner portion of the plates and, since the vapour takes up a relatively large volume, it has a relatively large cross-sectional area. 
     When the plate heat exchanger is in operation the vapour flows downwards in its interspaces and is completely or partially condensed. The condensate is discharged through two outlet ducts, which are defined by ports in the two lower corners of the plates and which lead out from the plate heat exchanger via two connecting ports in the frame plate. The second medium is conveyed upwards in its interspaces and is completely or partially vaporised before being finally discharged via an outlet duct, which is formed by ports located in the other upper corner of the plates and which leads out via a connecting port in the frame plate. 
     A problem associated with this technique is that in long plate heat exchangers, i.e. plate heat exchangers with a large number of heat transfer plates in the plate pack, the amount of flow of the two media in the plate interspaces tends to vary along the length of the plate heat exchanger. Therefore, the maximum capacity of the plate heat exchanger cannot be exploited. Even if one or several plate interspaces are utilised at maximum capacity, there is a fairly large number of plate interspaces whose utilisation level is considerably below the maximum capacity. This problem is accentuated in two-phase applications, since the vapour phase of a medium has different characteristics than the liquid phase. This means that the vapour phase and the liquid phase will behave differently in the heat exchanger and thus present a different distribution in the plate interspaces concerned. 
     Another problem associated with most plate heat exchangers is that it is difficult, in many cases, to obtain an even distribution of the fluid flow across the whole width of each plate, i.e. across the entire heat transfer portion. One way to try to improve the distribution is to give the plate ports intended to form the inlet duct an elongate shape, as shown in FIG.  1 . To facilitate connection of the heat exchanger to other devices it is possible to use, for instance, two connecting ports in the frame plate, which connect directly to the inlet duct having an elongated cross-section. In general, it is undesirable to have such abrupt dimensional variations in a duct. Because of the dead flow space formed immediately behind the connecting ports of the frame plate, the first interspaces do not get the desired distribution of liquid. Instead any gases present have a tendency to flow in these plate interspaces. 
     The above-related problems arise even if the plate heat exchanger is not being used for two-phase applications, but they are particularly pronounced in two-phase applications. 
     WO97/15797 discloses a plate heat exchanger, which is intended for evaporation of a liquid, for example a refrigerant. This plate heat exchanger has an inlet duct and a distribution duct, which extend through the plate heat exchanger and communicate with each other along the whole length of the plate heat exchanger. The purpose of the distribution duct is, inter alia, to create substantially equal flows in the different plate interspaces by serving as an expansion or equalization chamber between the inlet duct and the plate interspaces. The proposed design does not, however, provide a completely satisfying solution for all operational situations in which conventional industrial plate heat exchangers are used. 
     GB-A-2 052 723 and GB-A-2 054 124 disclose two variants of a plate heat exchanger having a front and a rear section of plate interspaces. To allow the flow to the plate heat exchanger to reach the rear section, the plate heat exchanger is provided with a by-pass duct consisting of a pipe, which is concentrically arranged in the inlet duct. The purpose of the concentric pipe is to convey part of the flow to the rear section. The plate interspaces of the first section communicate directly with the front portion of the inlet duct. The plate interspaces of the second section communicate directly with the rear portion of the inlet duct. 
     Consequently, there are no prior art constructions, which give a satisfactory flow distribution both along the length of the plate heat exchanger and across the width of the plates. Above all, there is no prior art construction that solves these problems in two-phase applications. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a solution, which allows a satisfactory flow distribution along the length of the plate heat exchanger and across the width of the plates, and by means of which it is also possible to avoid the above distribution problems in two-phase applications. 
     The present object is achieved by means of a plate pack of the type described by way of introduction, characterised in that the primary duct and the secondary duct communicate with each other through at least one flow passage portion spanning a plurality of plate interspaces, that the extension of the flow passage portion along the primary duct is substantially smaller than the extension of the primary duct, and that there is substantially no flow passage between the primary and secondary ducts outside said flow passage portion. 
     By providing the plate pack with one primary and one secondary duct, and supplementing this configuration with the flow passage described above, a plate pack in which the fluid flow can be advantageously distributed both along the length of the plate pack and across the width of the plates is obtained. The fluid flow which has flowed from the primary duct through the flow passage will whirl around in the secondary duct, largely because of the limited extension of the flow passage, and will thus be evenly distributed along the length of the plate pack. By controlling in which plate interspaces the fluid will flow between the primary and secondary ducts, the flow in the secondary duct can be controlled and thus the flow distribution across the length of the plate pack. In addition, the limited extension of the flow passage portion along the length of the primary duct means that different fluid phases will not prefer different ways between the primary and secondary ducts, but substantially the same phase distribution as that of the two-phase fluid in the primary duct will flow to the secondary duct and, through this, be distributed between the different plate interspaces. Owing to the use of one primary duct and one secondary duct, the secondary duct may further be designed to spread the fluid flow across the entire width of each plate, whereas the primary duct may be designed to allow conventional, round pipes to be connected to the plate pack. By providing the inlet duct with one primary duct and one secondary duct, the interface between duct and heat transfer surface and the interface between duct and external connections can be designed relatively independently from each other. This means that abrupt dimensional variations in the flow paths can be avoided, and thus also any undesirable turbulence. 
     Preferred embodiments of the invention are apparent from the dependent claims. 
     According to a preferred embodiment, the primary duct communicates with the secondary duct through at least two flow passage portions located at a distance from each other along the primary duct. This means that a fluid flow can be distributed across long plate packs while maintaining the positive distribution properties described above. This embodiment also provides a large amount of flexibility as regards different forms of sectioning of the plate pack. 
     According to a further preferred embodiment, a flow distribution device is arranged in the primary duct for deflecting part of the fluid flow in the primary duct via said flow passage portion. By arranging a flow distribution device in the primary duct, the amount of fluid flow deflected from the primary duct in different places along the primary duct can be regulated in a simple and reliable way. The deflecting property of the flow distribution device also stimulates the equalizing fluid flow in the secondary duct. 
     The primary duct advantageously extends through the whole plate pack, since this is a simple way of supplying the whole plate pack with fluid. 
     According to a preferred embodiment, the secondary duct extends through the whole plate pack. Owing to this design only one secondary duct is needed for the whole plate pack. 
     According to a preferred embodiment, the secondary duct is divided into a number of separate sections, each extending only through part of the plate pack. This design is particularly suitable in plate packs consisting of a large number of plates, and it makes it possible to obtain an equalization of the fluid flow for a determined number of plate interspaces in each secondary duct. By distributing the equalizing function among a number of separate secondary ducts, a slightly lower degree of equalization for each of the secondary duct sections can be tolerated, while still obtaining a satisfactory distribution along the whole length of the plate pack, than what would have been possible with a single long secondary duct with the same degree of equalization. This division means that the plate pack can be used in more varying applications without major performance losses. 
     The flow distribution device suitably delimits a section of the cross-sectional area of the primary duct, which section is reduced along the primary duct in the flow direction of the fluid flow. The flow deflected from the primary duct is thereby supplied to the secondary duct in a way that is consistent with fluid technology. 
     According to a preferred embodiment, the flow distribution device comprises a tubular body surrounding an inclined ramp. The tubular shape of the body allows it to be easily arranged and fixed in the inlet duct of the plate pack. The inclined ramp provides a good deflecting action, since it allows the fluid to flow along the ramp in such manner that its flow direction is gradually redirected. 
     The front portion of the inclined ramp is advantageously located at a distance from the duct wall of the primary duct. This ensures that the ramp extends into the fluid flow of the duct and deflects part of the flow. 
     The rear portion of the inclined ramp suitably connects to the duct wall of the primary duct adjacent to the flow passage between the primary duct and the secondary duct. This results in the deflected fluid flow being conveyed directly to the secondary duct. 
     An appropriate way of reliably deflecting a correct share of the fluid flow is to provide the inclined ramp of the flow distribution device with a deflecting edge, which is oriented in a direction opposite to the fluid flow. 
     According to a preferred embodiment, the deflecting edge extends essentially vertically. This orientation of the deflecting edge is advantageous in that also two-phase flows, such as annular or stratified flows, are divided into approximately equal shares of each of the different phases. This is important since an uneven distribution of vapour and liquid, respectively, both reduces the capacity of the plate heat exchanger and increases the risk of the heat exchanger “running dry”, i.e. that the fluid flow between one or several plates is not sufficient, which may cause solid particles in the fluid flow to get burnt and stick to the plates. 
     The inclined ramp suitably comprises an essentially flat, semi-elliptical sheet. This is a simple way of ensuring the deflecting action of the flow distribution device. 
     The extension of the inclined ramp along the primary duct is advantageously larger than its largest extension across the primary duct. As a result, the deflection obtained does not cause any extensive turbulence. 
     According to a preferred embodiment, the flow distribution device comprises a number of outwardly extending connecting means arranged to be fixed between the plates in their abutment against each other round the primary duct. By fixing the flow distribution device in this way no supplementary means for fixing the flow distribution device in the duct are needed. The forces of the tie bars acting to compress the plate pack are thereby also used to fix the flow distribution device. 
     According to a preferred embodiment of the body, it comprises an open, tubular cage structure, which surrounds and supports the inclined ramp. The body thus surrounding the ramp facilitates a correct positioning of the ramp in the duct. 
     According to a preferred embodiment, the body comprises a pipe, which surrounds the inclined ramp and which is provided with an opening in its circumferential surface, the inclined ramp being connected to said opening. This body design is very robust and does not affect the fluid flow in the duct very much. It also ensures that correct shares of the fluid are conveyed to the secondary duct. The tubular shape ensures that unwanted leaks between primary and secondary ducts are avoided. 
     The external shape of the flow distribution device suitably corresponds to the internal shape of the primary duct. This means that the flow distributor interferes only to a very small extent with the fluid flow, and because more or less coincident surfaces can be used, that it is easier to obtain a correct positioning. 
     According to a preferred embodiment, the flow passage between the primary duct and the secondary duct has an extension length along the primary and secondary ducts that is smaller than the extension length of each of the ducts along each other. This construction enhances the tendency of the fluid flow to present an equalizing, circulating flow in the secondary duct, resulting in an excellent distribution across the different plate interspaces communicating with the secondary duct. 
     According to a preferred embodiment, there is only one flow passage between the primary and the secondary duct. This enhances the tendency of the fluid flow to present an equalizing, circulating flow in the secondary duct. 
     By using a plate pack of the kind described above in a plate heat exchanger, a plate heat exchanger in which the fluid flow is evenly distributed across the different plate interspaces is obtained. The even distribution will also be obtained in two-phase applications, i.e. when the fluid has both liquid and gas phases. The primary duct, with its flow distribution device conveys the fluid flow to the secondary duct, where the fluid flow is equalized. 
     According to a preferred embodiment, the plate heat exchanger comprises at least two plate packs, wherein the primary duct of the first plate pack is connected to and substantially coincides with the primary duct of the second plate pack, and the secondary duct of the first plate pack is separated from the secondary duct of the second plate pack. This construction gives a very favourable distribution of the fluid flow along the length of the plate heat exchanger even if a somewhat less satisfactory distribution would be obtained locally in a plate pack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below with reference to the accompanying schematic drawings, which by way of example show currently preferred embodiments of the invention according to its different aspects. 
     FIG. 1 is a schematic illustration of the operation of a plate heat exchanger according to prior art. 
     FIG. 2 shows a heat transfer plate for use in a plate pack according to the invention. 
     FIG. 3 shows a heat transfer plate and schematically suggests the placement and orientation of a flow distribution device in the primary duct. 
     FIG. 4 is an exploded view of a preferred embodiment of a plate heat exchanger according to the invention. 
     FIG. 5 shows a flow distribution device according to a first preferred embodiment. 
     FIG. 6 shows a variant of the flow distribution device shown in FIG.  5 . 
     FIG. 7 shows a flow distribution device according to a second preferred embodiment. 
     FIG. 8 shows part of the flow distribution device in FIG.  7 . 
     FIGS. 9-11 illustrate the function of the preferred embodiments of the flow distribution device in different two-phase flows. 
     FIGS. 12-15 illustrate how the flow is distributed along the length of the plate heat exchanger according to prior art (FIGS. 12-13) and according to a preferred embodiment of the invention (FIGS.  14 - 15 ). 
     FIG. 16 is a top view illustrating how flow distribution devices are arranged in the primary ducts according to an embodiment of the invention. 
     FIG. 17 is a top view of an alternative embodiment with an alternative configuration of the primary and secondary ducts. 
     FIGS. 18 and 19 are two schematic illustrations of different gasket configurations between a primary duct and a secondary duct. 
     FIG. 20 shows an embodiment of the invention, in which the inclination of the deflecting ramps may be varied. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in FIG. 2 each of the heat transfer plates  100  comprises an upper port portion A, a lower port portion B and an intermediate heat transfer portion C. 
     In its lower port portion, the plate  100  has two primary inlet ports  110   a-b  and a secondary inlet port  110   c  for a first fluid as well as two outlet ports  120   e-f  for a second fluid. The two outlet ports  120   e-f  are located at the plate corners. The two primary inlet ports  110   a-b  are located inwardly of the outlet ports  120   e-f . The secondary inlet port  110   c  has an elongate shape and is located partly between the two primary inlet ports  110   a-b  and between the primary inlet ports  110   a-b  and the heat transfer portion C. The secondary inlet port  110   c  has an elongate shape and extends across the major part of the width of the heat transfer portion C. 
     In the upper port portion, the plate  100  has two double inlet ports  120   a-b ,  120   c-d  located in the two corners, said ports forming a continuous inlet duct in each of the two corners for the second fluid and a central outlet port  110   d  for the first fluid. 
     The plate  100  is intended to be arranged in a plate heat exchanger in the way illustrated in FIG.  4 . The plate heat exchanger comprises a frame plate  210 , a pressure plate  220  and a number of intermediate heat transfer plates  100 , which are arranged to be clamped together by means of conventional tie bars (see FIG.  1 ), which engage the frame plate  210  and the pressure plate  220  and pull them towards each other. The ports  110   a-d ,  120   a-f  of the different heat transfer plates  100  coincide to form inlet and outlet ducts extending through the plate heat exchanger. 
     The heat transfer plates  100  have gaskets  131  in gasket grooves  130  or elevated beads (not shown) arranged to abut against the adjacent heat transfer plate  100 , thereby delimiting the plate interspaces  250  relative to the surroundings. The heat transfer plates  100  also have gaskets or the like, which extend round some of the ports  110   a-d ,  120   a-f  described above. The gaskets round the ports  110   a-d ,  120   a-f  have a different shape on the respective sides  100   a-b  of the plates  100  to allow some of the ports  100   a-d  to communicate with each other along a first side  110   a  of the heat transfer portion C of the plates  100 , while the other ports  120   a-f  communicate with each other along the other side  100   b  of the heat transfer portion C of the plates  100 . 
     In addition, the plates  100  have some form of corrugation (not shown), which allows them to abut against each other in a large number of points, so that an inter space is formed between the plates  100  even when they are compressed between the frame plate  210  and the pressure plate  220 . 
     As shown in FIG. 4, the first fluid is supplied to the plate heat exchanger via two connecting ports  211   a-b  extending through the frame plate  210  and coinciding with the primary inlet ports  110   a-b  of the plates  100 . The primary inlet ports  110   a-b  form two primary inlet ducts  230   a-b ,  330   a-b  (see FIGS. 4,  16  and  17 ) extending through the plate heat exchanger. The first fluid flows from the primary ducts  230   a-b ,  330   a-b  to a secondary duct  240 ,  230  formed by the secondary ports  110   c . The primary ducts  230   a-b ,  330   a-b  and the secondary duct  240 ,  340  communicate with each other via flow passages having a limited extension along the primary and secondary ducts  230   a-b ,  330   a-b ,  240 ,  340 . The secondary duct  240 ,  340  communicates, in turn, with the plate interspaces  250  that form the first flow duct  250   a.    
     Different ways of providing the flow passage having a limited extension will be described below. The limited extension of the flow passage(s) between the primary and secondary ducts  230   a-b ,  330   a-b ,  240 ,  340  causes a circulating, equalizing fluid flow to form in the secondary duct  240 ,  340 , which results in an even flow distribution across the different plate interspaces  230  along the length of the secondary duct  240 ,  340 , and thereby along the length L of the plate heat exchanger. 
     The limited extension of the flow passage between the primary ducts  230   a-b ,  330   a-b  and the secondary duct  240 ,  340  may be achieved for example by means of a flow distribution device  400   a-b ,  500  (see FIGS.  5 - 8 ), which is arranged in the primary ducts  230   a-b ,  330   a-b  and which deflects part of the fluid flow in the primary ducts  230   a-b ,  330   a-b  and conveys this part to the secondary duct  240 ,  340  at certain locations along the extension of the ducts (see FIGS.  16 - 17 ). 
     According to a first embodiment of the flow distribution device  400   a-b  (see FIGS.  5 - 6 ), the device comprises a body in the form of a tubular, elongate, open cage structure. The two flow distribution devices in FIG. 5 and FIG. 6, respectively, are variants of each other and the same reference numerals have been used to designate corresponding elements in the two variants. The open cage structure surrounds and supports an inclined ramp  410 . The open cage structure comprises a number of rings  411  and a number of elongate struts  412 , which serve to interconnect the rings  411 . According to both variants, the flow distribution device  400   a-b  comprises three rings  411 . In one variant, the flow distribution device  400   a  comprises three struts  412  and in the other the flow distribution device  400   b  comprises four struts  412 . 
     According to a second embodiment of the flow distribution device  500 , the device comprises a pipe  501 , which has an opening  502  in its circumferential surface. The flow distribution device  500  further comprises an inclined ramp  510 , which is arranged to cover the opening  502 . 
     The opening  502  is shaped in such manner that it is defined, in one direction (opposite to the direction F in FIG.  8 ), by two edges  503  a,b, which extend from a point on the circumferential surface  501  and whose relative distance then increases as the edges  503   a-b  are located at an increasing distance from each other in the circumferential direction. This means that, at a first end (according to the direction F, the opening  502  encompasses almost half of the circumference of the circumferential surface  501  and, at a second end, the opening  502  is terminated by its edges  503   a-b  converging and connecting to the circumferential surface  501 . At the first end of the opening  502 , the edge  503  of the circumferential surface  501  as defined by the opening  502  is located at a first radial distance H from the original circumferential surface  501 . 
     By designing the opening  502  in this way and arranging an inclined ramp  510  that covers the recess, a whistle-like structure is obtained. The distance H determines the amount of the flow F in the pipe  501 , which is deflected. 
     Both embodiments of the flow distribution devices  400   a-b ,  500  are intended to be used in the same way. One or more flow distribution devices are arranged in the primary duct in different places along the length of the duct as shown in FIGS. 4,  16  and  17 . 
     The inclined ramp  410 ,  510  serves the purpose of deflecting part of the fluid flow in the primary duct to the secondary duct. FIG.  3  and FIGS. 9-11 show how the inclined ramp  410 ,  510  is arranged to be oriented. FIG.  3  and FIGS. 9-11 show the flow distribution device as seen from the flow direction F (see FIGS.  5 - 8 ). The deflecting edge  410   a ,  510   a  of the inclined ramp, located in the front portion of the ramp, is located at a radial distance H from the duct wall, through which the flow distribution device is arranged to deflect a partial flow. The deflecting edge  410   a ,  510   a  divides the flow in the primary duct into a main flow F H  and a secondary flow F S , which is intended for the secondary duct. 
     The deflecting edge  410   a ,  510   a  is vertically arranged, which means that it has a favourable distribution function also in two-phase applications (see FIGS.  10 - 11 ). Both in a “stratified flow” (where the gas phase is located above the liquid phase) and in an “annular flow” (where a liquid film surrounds the gas phase) the flow distribution devices will deflect substantially the same proportion of the two phases as is present in the main flow F H , which means that distribution problems that otherwise are common in two-phase applications can be avoided. In a traditional plate heat exchanger, the gas phase has a tendency to flow upwards to a great extent through the first plate interspaces. The radial placement of the deflecting edge  410   a ,  510   a  determines to a high degree how much of the fluid flow is deflected. 
     In addition to the radial distance H of the inclined ramp  410 ,  510 , it is also possible to vary the angle of inclination and its extension along the primary duct. The extension is determined, inter alia, by the extension of the flow passage between the primary and the secondary duct. The extension is also determined by the maximum angle of inclination that can be used without undesirable turbulence and pressure drops being introduced. The inclination in turn is dependent on the radial placement of the deflecting edge and the extension of the ramp. Each selection of parameter value is thus influenced by the other parameter value selections and by the application in which the plate heat exchanger is to be used. According to a preferred embodiment, the inclined ramp  410 ,  510  has an angle of inclination α of 15° (See FIG.  16 ). 
     FIG.  5  and FIG. 6 show two different variants of the flow distribution device  400  deflecting different amounts of the flow in the primary duct. 
     Another way of providing the limited extension of the flow passage between the primary and secondary ducts is to arrange gaskets  131  around the primary ports  110   a-b  in a number of plate interspaces  250  (see FIG. 18) and only allow the first fluid to flow between the primary port and the secondary port in a limited number of plate interspaces. By using partially recessed or cutout gaskets  131 ′ (see FIG. 19) adjacent to the flow passage portion, the flow in the flow passage between the primary duct and the secondary duct can be regulated. The level of recessing or the amount of cutout gasket  131 ′ determines the deflection and thus corresponds in terms of function to the selection of inclination, extension and degree of radial insertion for the inclined ramp in the flow distribution device. Because the flow passage only extends across a flow passage portion of a relatively limited extension, this construction can also be used in some two-phase applications. 
     As appears from FIGS. 14-17, it is preferred that the plate pack of the plate heat exchanger is divided into a number of sections. The sectioning is done by the secondary duct  240 ,  340 ,  640  being divided into a number of sections, each communicating with a number of plate interspaces. Each section of the secondary duct serves a certain number of plate interspaces. One way of performing the division of the secondary duct  240 ,  340 ,  640  is to occasionally arrange a plate  100 , in which the secondary port  110   c  has not been stamped out. 
     This design is particularly suited for long plate heat exchangers. The division of the secondary duct means that the tendency of the flow passage and the flow distribution device to create an equalizing flow in the secondary duct can be used also in long plate heat exchangers. 
     A conventional plate heat exchanger, which is not sectioned, is shown in FIG.  12 . FIG. 13 illustrates the distribution tendency of the liquid flow along the plate heat exchanger, particularly in two-phase applications. The corresponding tendency in a sectioned plate heat exchanger is shown in FIGS. 14 and 15. Owing to the sectioning, an altogether better flow distribution along the length of the plate heat exchanger is obtained. 
     In addition, the sectioning means that you can allow a less satisfactory distribution in each of the sections and still obtain a better overall distribution. However, owing to the sectioning it becomes easier to obtain a satisfactory distribution for each of the sections, which means that the overall distribution is considerably better than in a non-sectioned long plate heat exchanger. 
     FIG. 16 shows a configuration of two primary ducts  230   a-b  and a secondary duct  240  supplemented with flow distribution devices  231  and sectioning of the secondary duct  240  in two sections  240   a-b . In this embodiment, each of the primary ducts  230   a-b  communicates with each of the secondary duct sections  240   a-b  via two flow passage portions, adjacent to which flow distribution devices are arranged in the primary ducts  230   a-b . It is worth noting that the different passage portions leading from a primary duct are located at a distance P from each other. In addition, the flow passage portions leading from one primary duct  230   a  are displaced relative to the corresponding flow passage portion leading from the other primary duct  230   b . This allows an equalizing flow in the different sections  240   a-b  of the secondary duct  240  to be obtained. 
     FIG. 17 shows a configuration of two primary ducts  330   a-b  and a secondary duct  340 , which is divided into two sections  340   a-b . The first section  340   a  of the secondary duct  340  is supplied with a fluid from one primary duct  330   b , and the second section  340   b  of the secondary duct  340  is supplied with a fluid from the other primary duct  330   a . In this embodiment, flow passage portions  331  are shown, which are defined by the absence of fully sealing gaskets (see FIG.  19 ). The flow passage portions  331  are located in the rear part of the secondary duct sections  340   a-b , relative to the flow direction F, to provide a satisfactory equalization of the flow in the secondary duct sections  340   a-b . The primary duct  340   a  serving the rear section  340   b  of the secondary duct is separated from the front section  340   a  of the secondary duct by means of gaskets  332  in the plate interspaces. The sections  340   a-b  of the secondary duct  340  are separated from each other by means of a plate  100 ′, in which no secondary port has been stamped out (cf. secondary port  110   c  in FIG.  2 ). The rear portion of the primary duct  330   b  serving the front section  340   a  of the secondary duct is partly separated from the rear section  340   b  of the secondary duct by means of gaskets  332  and partly separated from the front portion of the primary duct  330   b  by means of the plate  100 ′. To ensure that the plate pack supports the fluid pressure, a small flow is conveyed to the rear portion through small openings in the plate  100 ′ as well as from the secondary duct  340   b  that runs parallel to said portion. Alternatively, all gaskets between the primary duct  330   b ′ and the secondary duct  340   b  may be removed. 
     Without this delimitation relative to the secondary duct  340  and the front portion of the primary duct  330   b  there would be a stagnant fluid in the rear portion  330   b ′ of the primary duct  330   b.    
     FIG. 20 shows a configuration of a primary duct  630  and a secondary duct  640 , said secondary duct being divided into three sections  640   a-c , each serving a number of plate interspaces. This configuration comprises three flow distribution devices  631   a-c , which are arranged in the primary duct  630  and which are each intended to deflect part of the fluid flow in the primary duct  630  to the respective sections  640   a-c  of the secondary duct. 
     As illustrated in the figure, each of the inclined ramps of the flow distribution devices  631   a-c  has a different extension into the primary duct. The distance by which the different inclined ramps extend into the primary duct  630  increases in the direction of the flow F in the plate heat exchanger. The first flow distribution device  631   a  deflects a certain amount of the fluid flow in the primary duct  630 . To ensure that the same flow amount is conveyed to the second section  640   b , the second flow distribution device  631   b  deflects a larger share of the remaining fluid flow in the primary duct  630 . The next flow distribution device  631   c  deflects in turn an even larger share of the further reduced remaining flow in the primary duct  630 . 
     This action obtained by means of different insertion distances of the flow distribution device can also to some extent be obtained in the gasket variant by varying the size of the flow passage portions along the length of the plate heat exchanger. A small flow passage portion thus corresponds to a small insertion distance and a large flow passage portion corresponds to a larger insertion distance. 
     In the embodiment shown in FIG. 20, the flow distribution devices may be set or adjusted. This adjustability is achieved for example by the inclined ramps having a variable angle of inclination. The plate heat exchanger comprises a control unit  700 , which includes the necessary control equipment, and actuating means  632   a-c . In FIG. 20, the actuating means  632   a-c  are shown as elongate struts that are actuated by some kind of motor or piston in the control unit. It is possible to achieve the adjustability in a number of other ways, for example by using servomotors supporting the inclined ramps or by using wire ropes instead of the struts shown, combined with some kind of back spring suspension of the ramps allowing them to assume a certain angle of inclination α. 
     By making the flow distribution devices adjustable, one and the same plate heat exchanger may be used within a considerably larger capacity range than conventional plate heat exchangers. Depending on the total incoming fluid flow, smaller or larger amounts can be deflected to the different sections of the plate heat exchanger. It is even possible to shut off one or more sections of the plate heat exchanger in order to handle a different capacity requirement or to clean them by closing the flow distribution devices  631   a-c  completely. In a conventional plate heat exchanger, which is not provided with primary/secondary ducts or sections, the fluid flow otherwise tends to be unevenly distributed if the fluid flow supplied does not correspond to the fluid flow for which the heat exchanger was designed. 
     It will be appreciated that a number of modifications to the embodiments described herein are possible within the scope of the invention, as defined in the following claims. 
     For example, the different configurations of primary and secondary ducts, flow distributors (fixed and adjustable) whose insertion distance may or may not be increased along the length of the plate heat exchanger, recessed or partially cutout gaskets, may be varied according to current requirements for different applications.