GB2120768A

GB2120768A - Apparatus for the desalination of sea water

Info

Publication number: GB2120768A
Application number: GB08311155A
Authority: GB
Inventors: Gamal El Din Nasser
Original assignee: DVT BURO fur ANWENDUNG DEUTSCH
Current assignee: DVT BURO fur ANWENDUNG DEUTSCH
Priority date: 1982-05-24
Filing date: 1983-04-25
Publication date: 1983-12-07
Also published as: GB8432108D0; GB2120768B; GB2120720B; GB8311153D0; GB2120950A; GB8432110D0; GB2120720A; GB2120950B; GB8432109D0; GB2160115A; GB8311154D0; GB2160117A; GB2160117B; GB2160116A; GB2160116B; GB8311155D0; GB2160115B

Abstract

Apparatus for the desalination of sea water according to the multieffect evaporation process with an untreated water preheater (VW) comprising stamped heat exchanger plates (30) and a falling film evaporator (FV), which also comprises stamped exchanger plates (30). The heat exchanger plates have uniformly arranged reinforcing grooves (32, 33), which are longitudinally and transversely aligned in grid-like manner and are in each case combined in pairs in mirror-symmetrically superimposed manner in such a way that the beads (32) of a heat exchanger plate pair oriented in one direction form tubular ducts (34) and the beads (33) oriented in the other direction define slot-like ducts (35) with the adjacent heat exchanger plate (30) of the adjacent heat exchanger plate pair. They are subdivided into a plurality of sections (VSt1 to VStn); (FSt1 to FStn) within vertical columns in accordance with the selected pressure and temperature stages. There are also supply lines for the sea water and primary steam and removal lines for the salt solution and clean water, together with a connection to a vacuum system. <IMAGE>

Description

GB2120768A 1

SPECIFICATION

Apparatus for the desalination of sea water TECHNICAL FIELD AND BACKGROUND ART

This invention relates to apparatus for desalinating sea water and is primarily concerned with such apparatus which utilises the multieffect evaporation process. In this latter process 10 there is a requirement for an untreated water (sea water) preheater and a falling film evaporator (subdivided into a plurality of sections within vertical columns in accordance with predetermined and selected atmospheric pres- 15 sure and temperature stages), supply lines for the sea water and for steam, removal lines for the salt solution and the treated, clean, water and a vacuum system. An example of the multieffect evaporation process is to be found 20 in German Patent No. 2,334,481.

In the aforementioned process, sea water heated by the preheater (constructed as a successive stage heater) is passed successively through the individual stages of the failing 25 film evaporator within the first stage of which it is heated with primary steam alone operat ing close to the maximum steam boiling point at the pressure within that stage. In the indivi dual stages of the falling film evaporator there 30 is, in each case, a partial evaporation of the sea water, whilst the unevaporated sea water flows onto the next stage, which is heated with mixed steam, so that after passing through the final stage, the sea water is 35 subdivided into brine and distillate (conveni ently termed 'clean water'). As a function of the boiling point reduction which can be ob tained in each stage in view of the difference in the decreasing atmospheric pressure for the 40 respective stages, the preheater and falling 105 film evaporator are subdivided into identical pressure and temperature stages and are ar ranged as tubular exchangers in the form of vertical columns within a support structure.

Experience has shown that tubular exchangers have an economic length of approximately 7m, so that the number of stages is limited in the case of a temperature difference of approximately 120C available for stage 50 evaporation, because even with only 15 stages, the overall height of the column reaches more than 1 00m. The efficiency of the multieffect evaporation process is dependent on the number of evaporation stages 55 which can be obtained within the temperature range between the first and last stages of the evaporator (and within the temperature range which is available for evaporating sea water).

In addition, when desalinating sea water 60 using the multieffect evaporation process gas residues, particularly inert gases, will collect in the individual stages of the failing film evaporator and these must be removed by suction in order not to impair the condensation pro- 65 cess. This causes considerable problems in the case of tubular exchangers because, due to the spatial arrangement of the individual tubes in bundles, there is neither a precise condensation end of the stage nor the possi- bility of appropriately removing the gases by suction.

It is also difficult in the case of tubular exchangers, in each case, uniformly to distribute the sea water to be evaporated over the surfaces of the inner walls of the individual tubes.

Only as a result of a uniform distribution of the sea water at the start of each stage is it possible to obtain the desired homogeneous liquid films covering the inner circumferential surfaces of the tubes as is required for effective evaporation. However, such liquid films cannot be maintained over tube lengths of approximately 7metres.

Finally, considerable difficulties are encountered in the assembly and maintenance of tubular exchange columns due to the fact that residues are unavoidable in evaporation processes for obtaining fresh water from sea water and, apart from energy costs, the cost balance is largely influenced by maintenance costs.

An object of the present invention is to provide a novel apparatus for desalinating sea water on the basis of the multieffect evaporation process in which, without the use of tubular exchangers, it is possible to increase the efficiency by increasing the number of stages and in which the construction and arrangement of the necessary heat exchangers are chosen in such a way that the operating costs are considerably reduced, both with respect to the energy requirement and the maintenance requirement.

STATEMENT OF INVENTION AND ADVANTAGES

According to the present invention there is provided apparatus for desalinating sea water by the multieffect evaporation process which comprises a sea water preheater and a failing film evaporator, subdivided into a plurality of sections within vertical columns in accordance with predetermined and selected pressure and temperature stages; supply lines for the sea water and for primary steam; removal lin6s for the salt solution and clean water, and a vacuum system, and wherein each of the sea water preheater and the failing film evaporator comprises heat exchanger plates (preferably pressed or stamped from sheet material) having uniformly arranged depressions forming reinforcing beads which are longitudinally and transversely aligned in grid-like form, said plates being combined in pairs in mirrorsymmetrically superimposed manner so that the beads of a pair of heat exchanger plates form tubular ducts between those plates which ducts are orientated in one direction and the beads of two adjacent and similarly GB2120768A 2 disposed heat exchanger plates derived one from each of two adjacent pairs of such plates form slot-like ducts which are oriented in a direction perpendicular to said one direction.

5 Further according to the present invention there is provided apparatus for desalinating sea water and having a heat exchange assem bly which comprises an array of similarly formed heat exchange plates each of which is 10 stamped or pressed from sheet material to have a plurality of depressions forming beads in the sheet, said beads being disposed as a sl3aced array in substantially parallel longitudinally extending and transversely spaced rows 15 so that the beads in the respective rows are in alignment to provide a corrugated grid-like structure, and wherein a pair of said plates are secured together in adjacent overlying relationship and in mirror image of each other 20 and sealed along opposed longitudinally extending side edges of those plates to provide tubular ducts between the pair of opposed plates through which fluid can flow longitudinally, and a further plate is secured to one plate of said pair in adjacent overlying relationship and in mirror image of said one plate and sealed along opposed transversely extending side edges of the further plate and said one plate to provide slot-like ducts between 30 the further plate and said one plate through which fluid can flow laterally.

Preferably the heat exchanger plates forming the untreated water preheater are arranged to be supplied with steam through 35 substantially horizontally disposed tubular ducts and water through the slot-like ducts while the heat exchanger plates which form the falling film evaporator are arranged to be supplied with water through substantially ver- 40 tically disposed tubular ducts and steam through the slot-like ducts. The heat exchange plates which form the water preheater may have opposed substantially flat (unstamped) areas the arrangement of which is such that 45 when subjected to pressure from water flowing through the slot-like ducts said areas are urged into abutment and subdivide the tubular ducts of the water preheater into sections corresponding to the selected number of pres- 50 sure and temperature stages.

Preferably the heat exchanger plates which form the failing film evaporator are arranged so that the tubular ducts are vertically aligned and form substantially circumferential surfaces for the film evaporation of untreated water supplied thereto, while the transversely directed beads form successively arranged cross connections for said circumferential surfaces of the tubular ducts for uniformly distri- 60 buting the sea water.

Desirably the number of heat exchanger plates of the failing film evaporator varies between the pressure and temperature stages so that by reducing the number of heat ex- 65 changer plates and increasing the vertical spacing between the stages, the flow crosssection for the steam is increased in stagewise manner, starting from the inlet stage to the falling film evaporator for the preheated sea water and extending to the final stage of that evaporator. The heating surface for preheating the sea water required for a particular pressure and temperature stage may be determined by selecting the number of pairs of heat exchanger plates forming the tubular ducts associated with that stage.

The slot-like duct of the heat exchanger plates which form the falling film evaporator may be interconnected in each case by an unstamped zone (preferably located in the geometrical centre) of the plates and which is arranged so that non-condensible inert gas components can be removed from the bottom slot-like duct of the pairs of heat exchanger plates in the sections.

The pairs of heat exchanger plates in the failing film evaporator may have upwardly directed channels at the edges of those plates through which channels the sea water to be evaporated is supplied to the tubular ducts and rods may be loosely located in those channels which rods act as flow distributors.

Desirably a heat exchanger is provided which condenses the steam collecting beneath the final stage of the falling film evaporator.

The use according to the invention, of the stamped heat exchanger plates with the beads arranged in a corrugated grid-like manner both for the sea (untreated) water preheater and for the failing film evaporator (to each of which is supplied, in a so-called cross-flow process, stream and sea water or sea water and steam) leads to a number of advantages. For example, the heat exchanger plates form- ing the water preheater and having the opposed flat (unstamped) areas which are responsive to water pressure in the slot-like ducts to abut each other and thereby subdivide the tubular ducts of the preheater into closed sections provide a very simple and convenient means of achieving the required number of stages for the available temperature range.

Preferably each stage of the untreated water preheater is associated with a corresponding stage of the falling film evaporator (which also comprises a group of identically stamped heat exchanger plates). The transversely directed reinforcing beads of the heat exchanger plates of each stage in the failing film evaporator form the tubular (generally and preferably circumferential) surfaces for the film evaporation of the supplied untreated water, whilst the longitudinally directed reinforcing beads of the heat exchanger plates form cross connections for the tubular surfaces. As a result of this latter arrangement the untreated water supplied to the tubular ducts will, after each passage over a tubular surface between the heat exchanger plates, be redistributed on the n z following tubular surfaces. As the length of the longitudinally directed beads can be made relatively small, for example 35mm, it may be ensured that in each stage where two opposing beads of a pair of heat exchanger plates together form a tubular (generally circumferential) surface, that surface can carry an uninterrupted homogeneous liquid film-this lends itself to efficient liquid evaporation in 10 each stage. As the heat exchanger plates both for the sea water pre- heater and for the failing film evaporator preferably have the same stamping or pressing configuration, it is possible to adapt, in an optimum and extremely 15 simple manner, the heating surfaces required per stage and also the cross-sections required in each stage for supplying the steam in accordance with the particular prevailing pressure and temperature conditions. Furthermore, 20 the construction of the failing film evaporator from pairs of heat exchanger plates, makes it possible by arranging for an unstamped area in (preferably the geometrical centre of) each heat exchanger plate, to place a suction point 25 at the bottom of each stage and in the actual condensation zone so that such inert gases as are liberated by the process are simply removed at the lowermost slot of each stage.

Thus, the use of heat exchanger plates 30 makes it possible in an extremely simple and commercially reliable manner, to optimize the number of stages in the multieffect evaporation process with respect to the available temperature range, as well as a modular con- 35 struction for each stage of the untreated water preheater and the failing film evaporator. The geometrical dimensions of the individual modules can be selected in such a way that easily manipulated and maintained heat exchanger units are formed so that, apart from a 105 considerable reduction of the energy require ment for the evaporation process, it is also possible to significantly reduce assembly and maintenance costs.

FIGURES IN THE DRAWINGS One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying il- 50 lustrative drawings, in which:- Figure 1 is a flow chart of a sea water desalination apparatus constructed according to the invention; Figure 2 is a perspective view of part of an 55 untreated water preheater column according to the apparatus of Fig. 1; Figure 3 is a perspective view of part of a failing film evaporator column according to the apparatus of Fig. 1; 60 Figure 4 is a diagrammatic view of the 125 columns in Figs. 2 and 3 with, in each case, only the first and last stages of the columns in the apparatus being shown; Figure 5 is a perspective view of part of a 65 heat exchanger plate assembly incorporated in 130 GB2120768A 3 the prehenter and failing film evaporator, the plates in ihe assembly being shown in an exploded condition to facilitate the description, and

Figure 6 is a section through part of the assembly of Fig. 5 to show tubular ducts and slot-like ducts formed between the plates.

DESCRIPTION WITH REFERENCE TO THE

DRAWINGS As can be gathered from the flow chart of Fig. 1, cold sea water supplied by means of a pump 10, is preheated by means of a multiple stage (stages VStn to VStJ vertical un- treated (sea) water preheater VW heated with condensing steam and is subsequently evaporated by means of a multiple stage (stages FSt, to FStJ vertical failing film evaporator FV which is also heated by means of condensing steam. For this letter purpose primary steam is supplied to the first uppermost stage FStl by means of a lino 11 and this steam is also used for heating the uppermost stage VSt, of the preheater. The preheated sea water passes via an overflow 12 into the first stage FSt, of the falling film evaporator FV and from there to the adjacent stage and so on, until it has passed through all the stages FStl to FStnThe steam formed in the first stage FS, and the steam component formed through the flash evaporation of the condensate from the first stage at 14, is used for heating the second stage FSt2of the falling film evaporator and the associated stages VSt, to VSt. of the preheater (as is represented by connections 15, 16 and 17). In the same way, the following and in each case associated stages of the failing film evaporator and untreated water preheater are interconnected, so that the above described process is repeated in each stage. The steam remaining in the final stage FSt,, is condensed at 21 and is evacuated by a pump 23, together with the clean water. Thus, after passing through all the stages, the clean water can be removed at 23 and the brine collected at 25 below the final stage can be removed at 24.

The untreated water preheater VW as shown in Fig. 2 comprises identical, pairwise- combined rectangular heat exchanger plates 30 (see Fig. 5). Each heat exchanger plate 30 is a stamped or pressed shaped sheet and has uniformly arranged depressions forming reinforcing beads 32, 33 which are longitudinally (with respect to the column) and transversely aligned in rows as a corrugated grid-like structure. The heat exchanger plates are mounted in pairs with the plates in each pair being mirror images, and symmetrically superimposed over each other with respect to their beads 32. In Figs. 5 and 6 four identical plates 30 are shown, conveniently referenced 30A to 30D and of these 30A and 30B may be regarded as one pair of plates and 30C and 30D as another pair. Alternatively 30B GB2120768A 4 and 30C may be regarded as one pair while plates 30A and 30D are adjacent plates to that pair but which form part of other pairs of plates which are respectively adjacent to and 5 on opposite sides of the said one pair of 70 plates. Assuming that the plates 30B and 30C form a pair, these are seam welded together along their adjacent transverse side edges 40 for both of the opposed transverse edges of 10 those plates. As shown in Fig. 6 the beads 32 in the plates 30B and 30C are arranged with their respective concave surfaces opposing each other so that these beads 32 of the adjacent heat exchanger plates 30B and 30C 15 form an array of generally tubular ducts 34 extending transversely between the plates.

The transverse side edges 40 of the plates 30A and 30D are similarly welded to respec tively adjacent plates 30 (not shown) to form 20 therewith respective pairs which are located on opposite sides of the pair 30B and 30C.

The plates 30 each have their longitudinally extending marginal edge regions 401 dog legged so that these edge regions diverge 25 from each other for the welded together pair of plates 30B and 30C and form between those plates a channel 42 which communi cates with the tubular ducts 34. The longitudi nal edge regions 401 of the adjacent plates 30 30A and 30B and of the plates 30C and 30D are located substantially in abutment and are seam welded together so that longitudinally extending slot-like ducts are formed between the beads 33 of the plates 30B and 30C and 35 the respectively adjacent plates 30A and 30D. By this welding together technique a pack or unit of the heat exchanger plates can be built up to an appropriate thickness so that the -plates give the required surface area of the 40 ducting for the heat exchange. Within such a pack or unit of plates there will be formed, in one direction, a plurality of juxtaposed, tubular ducts 34 and in a direction perpendicular thereto a plurality of juxtaposed slot-like ducts 45 35, so that the heat exchanging fluid of steam and water can flow through the respective ducts across one another. It will be seen from Figs. 5 and 6 that the depressions of the beads 32 are themselves provided with corru- 50 gated surfaces 32' to increase the surface area for the heat exchange.

The heat exchanger plate pairs are mounted as packs or units in a sealed container and are held in position by means of the container 55 side walls 205, 206 (which are not shown in detail).

As can be gathered from Fig. 2, the untreated water preheater VW extends over the entire height of the column and its heat 60 exchanger plates are arranged with their tubular ducts 34 extending horizontally for the passage therethrough of steam and with their slot- like ducts 35 extending vertically for the passage therethrough of sea water. The heat 65 exchanger plates of the preheater column are subdivided into sections corresponding to the desired number of stages by transversely extending, unstamped areas 37 in the individual heat exchanger plates 30. This subdivision is effected when the unstamped areas of adjacent heat exchanger plates in a pair are pressed against each other by the pressure of water in the slot-like ducts 35. The total number of tubular ducts 34 of the preheater is therefore subdivided into a plurality of groups in each case comprising several tubular ducts 34.

The disposition of the unstamped areas 37 is determined in accordance with the pressure and temperature stages as predetermined and selected for the multieffect evaporation process in the light of the temperature range which is available. In the present example, 55 stages are provided and the vertical spacing between the unstamped areas 37 becomes progressively smaller from stage to stage, starting with the inlet stage and extending up to the outlet stage, as does the number of tubular ducts 34 per stage.

The slot width of the slot-like ducts 35 is obtained on the basis of the spacing between the stamped beads 33, (that is on mounting the pairs of heat exchanger plates there are several separate slots in the slot-like ducts). If the stamping action is omitted at a particular point (such as in zone 39) a bypass is conveniently provided between the slot-like ducts.

The cold sea water is fed into the slot-like ducts 35 at the first, bottom stage VStu of the untreated water preheater VW and passes through the latter and up to the final top stage, whilst heating steam is fed through the tubular ducts 34 at right angles to the flow of the untreated water. Hot primary steam alone is fed into the final top stage VSt, whilst secondary steam (that is mixed steam derived from the stages of the failing film evaporator FV) is fed into all the other stages of the preheater VW. The lowest steam temperature occurs in the first stage VSt,, corresponding to the lowest temperature of the first stage of the pre-heater and the lowest pressure in the sealed container.

The failing film evaporator FV (partly shown in Fig. 3) is constructed from units or packs of heat exchanger plates 30 combined into pairs in the same manner as previously described with reference to Figs. 5 and 6. Unlike the case of the untreated water preheater VW, the packs of heat exchanger plates in the evaporator FV are oriented in such a way that the tubular ducts 34 are positioned vertically and the slot- like ducts 35 horizontally.

The tubular ducts 34 comprising individual sections along the length of the identical beads 32 consequently form the evaporation surface for the film evaporation of the sea water to be fed in, whilst the requisite steam is supplied to flow through the slot-like ducts 35. However, the beads 33 of the heat ex- 4F GB2120768A 5 changer plates form cross connections or branch channels extending over substantially the entire width of the plates for the sea water flowing through the tubular ducts 34. The 5 water-steam mixture leaving an individual sec tion of the tubular ducts 34 (that is from within a corrugated bead 32) is consequently immediately uniformly distributed again within following beads 32 as a result of the cross 10 connections formed by the beads 33. Thus, the steam pressure within the individual tubu lar ducts and differences in the salt concentra tion can be equally distributed before there is further flow downwardly into a following stage 15 having individual sections or beads 32 of 80 ducts 34.

Figs. 3 and 4 show the stagewise arrange ment of the packs or units of heat exchanger plates of the failing film evaporator. Whilst the 20 vertically arranged untreated water preheater VW extends over the entire length of the column in the closed container and is an assembly subdivided into stages by the un stamped areas 37, the failing film evaporator 25 FV is subdivided into a vertical array of as semblies or sections (corresponding to the number of stages in the preheater column) arranged in planes E, to E,, having different vertical spacings. The number of planes E, to 30 En consequently corresponds to the number of stages of the preheater, so that a plane E is associated with each of the selected pressure and temperature stages St, to St,, for the multieffect evaporation process and comprises 35 both one stage of the preheater and one stage 100 of the failing film evaporator, (see Figs. 1 and 4). For the purpose of carrying the exchanger plates of the evaporator FV a frame structure 301 is fixed to the opposed walls 205, 206 40 of the closed container (see Fig. 4). The frame 105 structure 301 forms a number of planes El to En having different vertical spacings and corre sponding to the selected number of stages in the preheater column. On each of these 45 planes is arranged one of the film evaporator units or packs shown in greater detail in Fig.

3. Deflectors or baffles LB in each stage (and which are not shown in detail here) ensure that in each case steam and clean water are 50 supplied to the individual stages. Much the same applies with regards to a suction or vacuum system (not shown) for the removal of non-condensable inert gas obtained in the condensation area of each stage of the failing 55 film evaporator. This condensation area is formed by an unstamped zone 39, preferably in the geometrical centre (see Fig. 3), of the heat exchanger plates 30 associated with the failing film evaporator. The zone 39 connects 60 all the slot-like ducts 35 of each plate pair and the inert gas components can be removed at the bottom slot-like duct of the heat ex changer plate pairs of each section or stage of the falling film evaporator.

65 In order to distribute uniformly the water within the individual tubular ducts 34, rods 41 corresponding to the length of the failing film evaporator plates are placed in the upwardly directed channels 42 which are formed by the aforementioned dog-legged edge regions 40' of two heat exchanger plates 30 welded together by means of a seam weld along their transverse edges 40 (see plates 30B and 30C in Fig. 5). These channels 42 are directed towards the overlying stage of the failing film evaporator. The rods 41 also serve (a) to mix together possible different brine concentrations which may flow towards their respectively associated tubular ducts to distribute uniformly the brine strength, and (b) to restrict the flow of brine from the respective channels 42 to the immediately underlying tubular ducts 34 so that reservoirs of the brine form in the channels 42-these reser- voirs act as seals to assist in maintaining the pressure differentials between the various stages of the failing film evaporator.

All the stages of the untreated water preheater and of the falling film evaporator,-to- gether with the frame construction carrying them are surrounded by the partly shown closed pressure container secured by a support structure (not shown) and which is connected to a vacuum system (not shown) for progressively reducing the pressure below atmospheric and as required in the respective progressive stages to effect the progressive desalination. An example of an appropriate container is disclosed in our co-pending Application No. (Ref. FJW/GDG/PB3201).

By way of example, in a practical embodiment of the multieffective evaporation process for sea water desalination with the aid of the aforementioned stamp heat exchanger plates and when using 55 stages for the untreated water preheater VW and the failing film evaporator FV, the individual reinforcing beads 32 of each heat exchanger plate 30 have a length of 35mm, the tubular ducts 34 in the untreated water preheater a length of 350mm, the slot-like ducts 35 of the failing film evaporator a length of 2160mm, the horizontal thickness of a pack or unit of plates for the failing film evaporator (constructed as a slide-in unit) was 500mm and the height of the column 3400 mm. The sea water enters the bottom stage of the untreated water preheater with a water inlet pressure of approximately 7 bar. Flow through the individual stages of the failing film evaporator takes place under gravity.

The heat exchanger plates forming the untreated water preheater are supplied with steam through the tubular ducts 34 and water through the slot-like ducts 35, whilst the heat exchanger plates forming the failing film evaporator are supplied with water through the tubular ducts 34 and with steam through the slot-like ducts 35.

GB2120768A 6

Claims

1. Apparatus for desalinating sea water by the multieffect evaporation process which 5 comprises a sea water preheater and a failing film evaporator, subdivided into a plurality of sections within vertical columns in accordance with predetermined and selected pressure and temperature stages; supply lines for the sea 10 water and for primary steam; removal lines for the salt solution and clean water, and a vacuum system, and wherein each of the sea Water preheater and the failing film evaporator comprises heat exchanger plates having uni- 15 formly arranged depressions forming reinforcing beads which are longitudinally and transversely aligned in grid-like form, said plates being combined in pairs in mirrorsymmetrically superimposed manner so that 20 the beads of a pair of heat exchanger plates form tubular ducts between those plates which ducts are orientated in one direction and the beads of two adjacent and similarly disposed heat exchanger plates derived one 25 from each of two adjacent pairs of such plates form slot-like ducts which are oriented in a direction perpendicular to said one direction.

2. Apparatus as claimed in claim 1 in which the heat exchanger plates forming the 30 water preheater are arranged to be supplied with steam through substantially horizontally disposed tubular ducts and water through the slot-like ducts and in which the heat exchanger plates forming the failing film evapo- 35 rator are arranged to be supplied with water through substantially vertically disposed tubular ducts and steam through the slot-like ducts.

3. Apparatus as claimed in claim 2 in 40 which the heat exchanger plates forming the water preheater have opposed substantially flat areas the arrangement of which is such that when subjected to pressure from water flowing through the slot-like ducts, said areas 45 are urged into abutment and subdivide the tubular ducts of the water preheater into sections corresponding to the selected number of pressure and temperature stages.

4. Apparatus as claimed in either claim 2 50 or claim 3 in which the heat exchanger plates forming the failing film evaporator are arranged so that the tubular ducts are vertically aligned and form substantially circumferential surfaces for the film evaporation of untreated 55 water supplied thereto, and wherein the transversely directed beads form successively arranged cross connections for said circumferential surfaces of the tubular ducts for uniformly distributing the sea water.

60

5. Apparatus as claimed in any one of the 125 preceding claims in which the number of heat exchanger plates of the failing film evaporator varies between the pressure and temperature stages so that by reducing the number of heat 65 exchanger plates and increasing the vertical spacing between the stages, the flow crosssection for the steam is increased in stagewise manner, starting from the inlet stage to the failing film evaporator for the preheated sea water and extending to the final stage of that evaporator.

6. Apparatus as claimed in any one of the preceding claims in which the heating surface for preheating the sea water required for a particular pressure and temperature stage is determined by selecting the number of pairs of heat exchanger plates forming the tubular ducts associated with that stage.

7. Apparatus as claimed in claim 2 or in any one of claims 3 to 6 when appendant thereto in which the slot-like ducts of the heat exchanger plates forming the failing film evaporator are interconnected in each case by an unstamped zone of the plates and which is arranged so that at least non-condensible inert gas components can be removed from the bottom slot-like duct of the pairs of heat exchanger plates in the sections.

8. Apparatus as claimed in claim 2 or in any one of claims 3 to 7 when appendant thereto in which the pairs of heat exchanger plates in the falling film evaporator have upwardly directed channels at the edges of those plates through which channels the water to be evaporated is supplied to the tubular ducts and wherein loosely located in the channels are rods acting as flow distributors.

9. Apparatus as claimed in any one of the preceding claims in which a heat exchanger is provided for condensing the steam which passes together with the brine through the final stage of the falling film evaporator.

10. Apparatus as claimed in any one of the preceding claims in which the plates are stamped or pressed to form said beads.

11. Apparatus for desalinating sea water and having a heat exchange assembly which comprises an array of similarly formed heat exchange plates each of which is stamped or pressed from sheet-material to have a plurality of depressions forming beads in the sheet, said beads being disposed as a spaced array in substantially parallel longitudinally extending and transversely spaced rows so that the beads in the respective rows are in alignment to provide a corrugated grid-like structure, and wherein a pair of said plates are secured together in adjacent overlying relationship and in mirror image of each other and sealed along opposed longitudinally extending side edges of those plates to provide tubular ducts between the pair of opposed plates through which fluid can flow longitudinally, and a further plate is secured to one plate of said pair in adjacent overlying relationship and in mirror image of said one plate and sealed along opposed transversely extending side edges of the further plate and said one plate to provide slot-like ducts between the further plate and said one plate through which fluid _1 li z 3, 7 GB2120768A 7 can flow laterally.

12. Apparatus as claimed in any one of the preceding claims in which at least the depressions forming the beads for the tubular 5 ducts are corrugated to increase the surface area of said ducts.

13. Apparatus for desalinating sea water substantially as herein described with reference to the accompanying drawings.

Printed for Her Majesty's Stationary Office by Burgess Et Son (Abingdon) Ltd.-1 983. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.