PLATE EVAPORATOR

Abstract: The present invention refers to a plate heat exchanger for evaporation of a fluid, comprising a number of towards each other abutting heat transfer plates (1, 2), delimiting flow spaces (3) between each other and being provided with corrugation pattern in the shape of ridges and grooves, said ridges, in the flow spaces, abutting towards each other and forming a number of supporting points between adjacent heat transfer plates (1, 2) wherein alternate flow space forms a passage (4) for said fluid and remaining flow spaces (3) forms passages (7) for a heating medium, which passages each has an inlet and an outlet. The importance with the invention is that at least alternate heat transfer plate has a number of zones (17, 18, 19) with different corrugation pattern arranged after each other along the flow path for the fluid in each of the passages therefor, and that said ridges and grooves of the heat transfer plates is so directed that they cooperate to achieve a flow resistance for the fluid and generated vapour, in each passage (4), which decreases gradually along essentially the entire passage from its inlet to its outlet.

Description: Plate Evaporator The present invention relates to a plate heat exchanger for evaporating a fluid, comprising a number of towards each other abutting heat transfer plates, defining flow spaces between each other and being provided with corru¬ gation pattern in the shape of ridges and grooves, said ridges abutting each other in the flow spaces and forming a number of supporting points between adjacent heat transfer plates, wherein alternate flow space forms a passage for said fluid and the remaining flow spaces form passages for a heating fluid, which passages each has an inlet and an outlet. At a known plate heat exchanger of this kind, described in DE-A1 3721132, the main part of the heat transfer portion of each heat transfer plate has one and the same kind of corrugation pattern over its entire surface. This has shown ineffective referring to the heat transfer capacity of the plate heat exchanger. An object of the present invention is to increase the efficiency of a plate heat exchanger of the introductory described kind. This object is obtained according to the invention by that at least alternate heat transfer plates have a number of zones with different corrugation pattern arranged after each other along the flow path of the fluid in each of the passages therefor, and that said ridges and grooves of the heat transfer plates are directed in such a way that they in each passage for the fluid cooperates to provide a flow resistance to the fluid and generated vapour, which decreases gradually along essentially the entire passage from its inlet to its outlet. When a fluid flows through an evaporating passage and gradually evaporates, an increasing space is required for the produced vapour. It is difficult to provide for such increasing space requirement in a plate interspace, therefore a gradually increasing flow rate of the produced vapour must be accepted. The above mentioned corrugation pattern of the heat transfer plates, which has, among others, a purpose of improving the efficiency of the plate heat exchanger, causes, however, a great flow resistance to the produced vapour. This involves that a considerable pressure drop of the rapidly flowing vapour is achieved, and therefore the absolute pressure in the section of the evaporating passage in which the evaporation is intended to start, must be maintained rather high, i.e. the boiling point of the fluid must be maintained rather high. A natural desire is that said boiling point should be kept as low as possible, so that a heating fluid may have as low temperature as possible. At the same time it is a desire that the heat transfer plates should have as efficient corrugation pattern as possible for the heat transfer. At a plate heat exchanger according to the invention a gradually decreasing flow resistance in the flow direction is obtained in the evaporating passages for the fluid and generated vapour. A very efficient heat transfer to the fluid is thereby achieved in the beginning of the evaporating passages, despite the relatively low flow rate of the fluid, simultaneously as a far too great flow resistance for produced vapour, i.e. far too great pressure drop of said vapour, is avoided in the end of the evaporating passages. In practice this leads to that if a certain, previously used, plate size is maintained, it will be possible to reduce the total heat transfer surface and thereby the number of plates in a plate heat exchanger to solve a certain evaporating task. With other words, through the invention a plate heat exchanger may be designed to provide an optimal heat transfer at a given saturation pressure for generated vapour. This is applicable both in a falling film evaporator and in a climbing film evaporator. It is previously known that the heat transfer between each of two heat transfer plates and a heat transfer fluid, flowing through a passage between the plats, is influenced of how the towards each other abutting corrugation ridges of the heat transfer plates cross each other and extend in relation to the flow direction of the heat transfer fluid. If the ridges cross each other with an obtuse angle, directed against the flow direction of the fluid, a greater pressure drop of the fluid is obtained and a more efficient heat transfer is obtained than if the ridges cross each other with an acute angle, directed against the flow direction of the fluid. In accordance with the invention the design of corrugation pattern of the heat transfer plates can be varied in such a way that the crossing ridges of co- operating heat transfer plates provide flow resistance of different value in different sections of the passages between the plates. In a plate heat exchanger arranged for climbing film evaporation, i.e. in which the heat transfer plates are arranged vertically and fluid to be evaporated is supplied to alternately plate interspace at its lower portion, the height of the heat transfer plates has a crucial importance for the saturation pressure of the produced steam. It depends on the static pressure of the fluid in said plate interspace. Through the present invention maximum heat transfer can be effected at optimum height of the heat transfer plates. The invention will be described in the following in more detail with reference to the accompanying drawings, in which fig 1 shows a schematically exploded view of a plate heat exchanger formed in accordance with the invention and comprising two units of heat transfer plates, fig 2 shows a schematically front view of a first kind of heat transfer plate, and fig 3 shows a schematical front view of a second kind of heat transfer plate. The plate heat exchanger shown in fig 1 comprises two kinds of heat transfer plates with different corrugation pattern, one first kind of heat transfer plates 1 and a second kind of heat transfer plates 2, which heat trans¬ fer plates are intended to be kept together in a frame (not shown) in a conventional manner. The heat transfer plates 1 and 2 may in a conventional manner be provided with rubber gaskets along its edges to delimit flow spaces 3 between themselves, but as an alternative they could be permanently joined to each other, e.g. through soldering, welding or glueing. The heat transfer plates 1 and 2 have been provided with a corrugation pattern in shape of ridges and grooves by means of pressing, and the ridges of two adjacent heat transfer plates in the flow spaces 3 crossing and abutting each other to form a number of supporting points between the heat transfer plates. Each second of such flow space 3 forms a passage 4 for evaporating of a fluid, which passage communicates with a fluid inlet 5 extending through a lower portion of the heat transfer plates and an outlet 6 for fluid and generated vapour, which extends through an upper portion of the heat transfer plates. Remaining flow spaces form passages 7 for a heating fluid, which passages communicate with a steam inlet 8 extending through the upper portion of the heat transfer plates, and two condensate outlets 9 extending through the lower portion of the heat transfer plates. The heat exchanger shown in figure 1 is principally intended for evaporation or concentration of various liquid products by means of climbing film evaporation. The heat- transfer plates 1 and 2 are arranged verti¬ cally, and fluid to be evaporated is intended to be supplied to the passages 4 at their lower portion and discharged at their upper portion. When counterflow heat exchanging is preferred the plate heat exchanger is arranged for falling film evaporators and steam as heating medium is intended to be supplied at the upper portion of the passages 7 and produced condensate is intended to be discharged at the lower portion of the passages 7. Each of the heat transfer plates 1 and 2 has a lower distribution area 15, a heat transferring portion 16, divided into discrete zones 17, 18 and 19 having different corrugation pattern, and an upper distribution area 20. The lower distribution area 15 is arranged to convey fluid in each passage 4, essentially vertically upwards from the inlet 5 to the heat transfer portion 16, and in each passage 7, to convey the condensate both vertically downwards and horizontally towards the outlets 9 on each side of the plate. The upper distri¬ bution area is formed in a manner which appears more closely in US 3783090. The heat transfer plates 1 and 2 shown in figures 2 and 3 have an elongated rectangular shape and punched holes at each of its ends. These holes form a port 10A and 10B, respectively, at the bottom, for the fluid to be evaporated, a port 11A and 11B, respectively at the top, for concentrated fluid and generated vapour, a port 12A and 12B, respectively at the top for heating steam, and two ports 13A, 14A and 13B, 14B, respectively, at the bottom, for condensate and eventually uncondensated steam of the heating medium. The heat-transfer plates 1 and 2, show lower distri¬ bution areas 15A and 15B, respectively, upper distri¬ bution areas 20A and 20B, respectively and heat transfer portions 16A and 16B, respectively, which are divided into distinct zones 17A, 18A, 19A and 17B, 18B, 19B, respectively, having different corrugation pattern. The heat transfer plate 1 (fig 2) has on one if its sides a number of grooves housing a gasket 21 formed in one piece. The gasket extends around each of the ports 10A and 10B and around the whole periphery of the plate. Similarly, the heat exchange plate 2 (fig 3) has a number of grooves and placed therein, a gasket extending around each of the ports 12B, 13B and 14B and around the whole periphery of the plate. The gaskets 21 and 22 are arranged to seal between adjacent heat transfer plates 1 and 2. The gasket grooves can, as an alternative, be formed such that two adjacent plates may be welded together having the bottom of the grooves turned against each other, wherein only alternate plate interspace is provided with a gasket, which in such case occupies two grooves facing each other in adjacent heat transfer plates. In the corrugation zones 17A-19A and 17B-19B respec¬ tively, the ridges and the grooves incline differently against the intended main flow direction of the fluid. Accordingly, the gradient decreases from one zone to another from below and upwards. In the embodiment of the invention, as shown in the drawing of the plate 1 (fig 2), an angle α between the main flow direction of the fluid and the extension of the corrugating ridges is about 30° in zone 17A, 25° in zone 18A and 20° in zone 19A. In plate 2 (fig 3) the ridges of the zones 17B-19B form as great angles with the main flow direction of the fluid, however, they extend in another direction than the ridges of zones 17A-19A of plate 1, so that the ridges of plate 1 will intersectingly abut against the ridges of the plate 2. The value given on the angle α has been chosen with reference to a certain heat exchange task for the present heat exchanger. Other values can of course be chosen for other heat exchange tasks. Fluid which is to be completely or partly evaporated is supplied into the plate heat exchanger through the fluid inlet 5 which is located in the lower portion of the heat transfer plates, and then flows upwards through the passage 4. Fluid is even distributed across the width of the heat transfer plates between the lower distribution areas 15A and 15B. In the heat transfer portion 16A and 16B the fluid first passes the zones 17A and 17B, which have a corrugation pattern providing a relatively great flow resistance, i.e. the ridges cross each other with a comparatively large intervening angle directed against the flow direction of the fluid. Owing to the fact that this corrugation pattern provides a relatively large flow resistance vapour is generated relatively soon in the passages 4. Fluid and generated vapour continue between the zones 18A and 18B and between the zones 19A and 19B, which have gradually decreasing angle between the crossing ridges, i.e. a more and more acute intervening angle is formed by the ridges, directed against the flow direction. The flow resistance for the fluid and generated vapour thus decreasing gradually in the flow direction of the passages 4. Fluid and generated vapour then continue to the upper distribution areas 20A and 20B and further through the outlet 6. In the passages 7 for the heating medium, the flow is performed in opposite direction. Steam is supplied through the steam inlet 8 and is in the passages 7 subjected to a gradually increasing flow resistance. In the figures two condensate outlets 9 are shown, however, only one may be used if desired. In the embodiment of the invention shown in the drawings, both of the heat transfer plates 1 and 2 have several zones with different corrugation pattern. However, only one of the heat transfer plates could have several different zones, while the whole of the other heat transfer plate has one and the same corrugation angle. In addition, the zones of the different heat transfer plates have been shown located right opposite to each other, but as an alternative they could be located so that they only partly overlap each other. Also the number of zones and the size of the zones could of course vary.

Claims: Claims 1. Plate heat exchanger for evaporating a fluid, comprising a number of towards each other abutting heat transfer plates (1, 2), delimiting flow spaces between each other and being provided with corrugation pattern in the shape of ridges and grooves, said ridges abutting each other in the flow spaces and forming a number of supporting points between adjacent heat transfer plates (1, 2), wherein alternate flow space forms a passage (4) for said fluid and remaining flow spaces form passages (7) for a heating fluid, each passage having an inlet and an outlet, c h a r a c t e r i z e d i n that at least alternate heat transfer plate has a number of zones (17, 18, 19), with different corrugation pattern, arranged after each other along the flow path of the fluid in each of the passages (4) therefor, and that said ridges and grooves of the heat transfer plates are directed in such a way that they, in each passage (4) for the fluid, cooperate to provide a flow resistance to the fluid and generated vapour, which decreases gradually along essentially the entire passage from its inlet to its outlet. 2. Plate heat exchanger according to claim 1, c h a ¬ r a c t e r i z e d i n that the ridges of adjacent heat transfer plates intersectingly abut towards each other. 3. Plate heat exchanger according to claim 1 or 2, c h a r a c t e r i z e d i n that the ridges and grooves of said zones (17, 18, 19) incline with an angle (α) against the intended main flow direction of the fluid in each passage (4) for the fluid, wherein the angle (a) decreases from one zone to another along the passage (4) from its inlet to its outlet. 4. Plate heat exchanger according to any of the claims 1-3, c h a r a c t e r i z e d i n that at least alternate heat transfer plates have at least two different zones (17, 18, 19), located in a heat transfer portion (16) of the heat transfer plate. 5. Plate heat exchanger according to any of the claims 1-4, c h a r a c t e r i z e d i n that each of two adjacent heat transfer plates has zones with different corrugation pattern. 6. Plate heat exchanger according to claim 5, c h a ¬ r a c t e r i z e d i n that each of the zones (17A, 18A, 19A) of one of the heat transfer plates are located right in front of a zone (17B, 18B, 19B) of the other heat transfer plate.

Inventors: HALLGREN LEIF, HALLGREN, LEIF
Applicants: ALFA LAVAL THERMAL, ALFA-LAVAL THERMAL AB
Application year: 1991
Main classification: F28F13
IPC codes: B01D1, F28D9, F28F3