Throughout the specification, unless the context requires otherwise, the word "comprise"or variations such as"comprises"or"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

BACKGROUND ART Notwithstanding that the invention has broader application than in the desalination of saline water, the latter at this stage represents the chief commercial application of the invention and certain is the most developed in terms of relevant prior art.

Accordingly, the present discussion of relevant background art will focus on known desalination processes. However, it needs to be appreciated that this is not intended to be a limitation on the scope of the present invention, and that it has equal application to the removal of dissolved solids from liquids in areas other than those concerned with desalination alone.

There are three main recognised types of desalination process.

The first of these is known as the multiple effect process, a schematic of a plant of such being shown in figure 1A of the drawings. In this process, the saltwater/brine stream A1 is passed through a series of heating stages or effects A2'-A2"of

different temperature and pressure to generate distillate. This is initially done by pumping the saltwater/brine stream through a series of preheater/condenser tubes A3 in each effect A2 to condense vapour on the outside of the tubes in a condensate or distillate-collecting chamber A4 and preheat the brine A1 passing through on the inside of the tubes. The temperature in the tubes A3 is increased progressively from effect A2 to effect, the brine being passed through the preheater/condenser tubes A3 of the last effect A2"first, where the chamber temperature and vapour pressure is at the lowest, and then is pumped on to the preceding effect where the temperature and pressure is higher.

After leaving the preheater/condenser tubes A3 of one effect A2 and before passing through the preheater/condenser tubes of the preceding effect, a portion of this brine is sprayed over an evaporator heat exchanger tube bundle A5 of that effect. A fraction of this water evaporates on the outer surface of the tube bundle A5, which is heated by vapour condensing inside the tube bundle that was produced in the previous effect at a higher temperature and pressure.

In the first effect A2', the evaporator tubes A5 are heated inside by an external heat source, normally steam, from a boiler A6, to create the high temperature and pressure in this effect, and open into the condensate or distillate-collecting chamber A4 of this effect. This chamber A4 is not connected to the succeeding distillate-collecting chamber of the next effect, as is the case with the other chambers A4 of the subsequent effects, but is recirculated back through the heat source A6, to maintain the steam source for heating the evaporator tubes A5 of the first effect A2'.

The residual sprayed brine A7, pools in the bottom of this effect, and is progressively passed down each effect, gathering the residual sprayed brine of succeeding effects, until it is discharged from the last effect A2n. The vapour generated from the heating of the sprayed brine over the evaporator tubes A5 is passed through a demister A8 of the particular effect and into the distillate- collecting chamber A4 in communication with the inside of the evaporator tubes of that effect. The distillate A9 generated by the condensing of the vapour from within the evaporator tubes A5 is collected at the bottom of the chamber A4 and is

transferred to the chamber of the next effect, flashing as it enters, and so on until the last effect A2". It is then pumped out of the last effect A2"into some form of storage system for subsequent use.

The principal characterising feature of the multiple effect process is that the condensate generated from the condensing of the vapour in the evaporator tubes is the heat carrier.

The second type of desalination process is known as the multi-stage flash process and the schematic of a plant involving such is shown in figure 1 B. In this process the saltwater/brine stream B1 is passed through a series of flashing stages B2 of different pressures to generate distillate in a converse manner to the multiple effect process. This is initially done via condenser tubes B3, progressing from the last stage B2"to the first stage B2', where the brine B1 is initially and progressively preheated by vapour condensing on the outer surfaces of the tubes.

The brine A1 is then passed through a brine heater B4 where its temperature is elevated again, before being directed through the flash chambers B5 of each of the stages, commencing with the first stage B2', where, due to the pressure reduction, it undergoes a small amount of evaporation (flashing).

The vapour condenses on the outer surface of the condenser tube B3 passing through that stage, as described above, and the condensate is collected as distillat B6.

The remaining brine B1 and the collected condensate or distillate B6 is then separately directed successively through the remaining lower pressure stages, where the brine is further evaporated (flashing) and the vapour condenses (cooled) on the outer surface of the condenser tube B3 passing through each stage B2. The condensate is similarly collected as distillate B6, pooled with the distillate collected and directed to that stage B2 from the previous stage, and then is directed to the next stage. The process is repeated through each of the stages until the residual brine B7 and collected distillate B8 is pumped out from the last stage B2".

This process is characterised by the brine carrying the heat and that the main heating of the brine is effected by some external source.

The third type of desalination process of pertinence is known as vapour compression distillation and the schematic of a plant involving such process is shown at figure 1C. In this process the saltwater C1 is preheated in a heat exchanger C2 by the outgoing distillate C3 and brine C4 before entering the vapour compression plant. It is then sprayed via a series of spray nozzels C5 over an evaporator tube bundle C6, where part of it is evaporated. The vapour is passed through a demister C7, and its pressure and temperature are increased in a compressor C8. The warmer steam is then passed through the evaporator tube bundle C6, where it condenses on the inside of the tube bundle, releasing its heat to the cooler evaporating saltwater being sprayed over the tube exterior. The condensate or distillate C3 is cooled in the heat exchanger C2 and is then pumped to storage. The remaining brine C9, which has not been evaporated, is partly recirculated to be sprayed again over the evaporator tube bundle C6, and partly cooled in the heat exchanger C2 to be pumped back into the saltwater supply.

An external heat source, such as a boiler C10, is provided to maintain the temperature and pressure of the plant.

This process is characterised by requiring an external source of energy, generally applied in the form of shaft power, to cover the heat loss of the plant and recirculate steam back to the process.

The plants that need to be constructed for implementing each of these processes using known and proven configurations are relative expensive and complex, requiring elaborate piping and tubing schemes in the heat exchanger designs used therein. In this respect, the costs of constructing a plant are generally driven by the particular material used for constructing the heat exchangers, the particular configuration of the heat exchangers chosen, condensation surfaces and fabrication costs. In addition, costs are contributed to by plant properties such as

dimensions and brine flow rates, and performance requirements such as specific thermal energy and electrical power requirements.

Accordingly, various construction alternatives have been developed in order to bring down costs without unduly impairing efficacy. These have included the use of horizontal tube heat exchangers, vertical tube heat exchangers, helical wound pipe heat exchangers, vertical plate heat exchangers, horizontal plate heat exchangers and vertical tube fluidised bed heat exchangers.

It is believed that plate heat exchangers offer significant cost benefits over all of these alternatives. Moreover, they can be mass produced, are quite versatile in that they can be made to replace most existing types of heat exchangers with minor modifications, and are tried and proven performers.

With respect to the two types of known plate designs, vertical plate evaporation has certain problems associated with it that are not necessarily associated with horizontal plate evaporation. Moreover, vertical plate evaporation can have static pressure losses, high vapour velocities that can cause splashing, and high foaming requiring the use of anti-foaming additives.

Notwithstanding these considerations, existing designs of plants using horizontal plate evaporation heat exchangers are still considered to be less than optimum because fairly significant power sources are required to supply the energy necessary to maintain the operation of these plants. The production of this power principally involves the use of oil and/or gas for heating and electrical energy for pumping purposes, which is not considered to be ideal in these days of environmental awareness and in situations where these plants need to be operated in remote locations.

DISCLOSURE OF THE INVENTION It is an object of the invention to reduce the net cost of removing dissolved solids from a liquid, such as purifying or desalinating water, using plate design heat exchanger principles.

Generally, the invention is concerned with arranging a plurality of modularly configured plates in a parallel, spaced arrangement to form a plurality of heat exchanger means, whereby the plates incorporate apertures and seals that may be arranged to provide for discrete passageways, integral with the heat exchanger surfaces, to enable different flow paths for liquid and vapour evaporated therefrom, through the plates using known heat exchange processes for the removal of dissolved solids from the liquid.

In such an arrangement, a highly compact system may be formed, providing considerable savings in manufacturing and construction costs, as well as in operating costs compared with prior art systems.

As a result of the compactness of the systems that can be constructed using the present invention, other improvements have been developed to overcome certain shortcomings associated with this high degree of compactness that are quite inventive in their own right. In particular, one aspect of the invention entails the using of an interleaving of different groupings of heat exchanger means run in isolation and in parallel with each other, but disposed integrally within the one system to permit the location of valves or other flow regulating devices in an optimal and space saving manner.

The application of the invention finds particular utility in using the multiple effect heat exchange process. Consequently, in this specification, the term"multiple effect machine"is defined to mean: (i) a plurality of effects for the treatment of a liquid, the effects being at differing temperatures and pressures graduating from a first effect at a comparatively high temperature and pressure to a final effect at a comparatively low temperature and pressure; (ii) a first heat exchanger means associated with each effect for heating liquid passed therethrough on one side with vapour passed therethrough on the other side, evaporating a fraction of the liquid and condensing some of the vapour on the respective sides of the heat exchanger;

(iii) a second heat exchanger means also associated with each effect for preheating the liquid passed therethrough on one side with vapour passing therethrough on the other side during its passage from the supplying means to the first effect progressively via each of the succeeding effects from the last effect to the first effect and using the liquid to facilitate the condensing of vapour on said other side of said first heat exchanger and the vapour correspondingly preheating the liquid on the one side of the second heat exchanger means in each of the effects during the passage of the liquid to the first effect.

In accordance with one preferred aspect of the present invention, there is provided in a system for removing dissolved solids from a liquid, the system comprising: a multiple effect machine, means for supplying liquid to be treated to the multiple effect machine, and means for heating and initially supplying vapour to the other side of the first heat exchanger means of the first effect, wherein the liquid is firstly supplie to the one side of the second heat exchanger means of the last effect and then is directed progressively through the second heat exchangers of the preceding effects via a liquid supply passageway until it reaches the first effect, whereupon it is percolated through the one side of each first heat exchanger means of the effects, progressively from the first effect to the last effect via a liquid treatment passageway, wherefrom it is discharged, and the vapour is condensed and collected as distillate at the other side of the first heat exchanger means and second exchanger means of each effect and is passed progressively from the first effect to the last effect via a distillate flow passageway whereupon it is output for use, without allowing the passage of vapour nor any pressure loss between the effects; a plurality of discrete groups of effects having the same number of effects in each group; wherein the groups are physically interleaved and aligned in a sequential manner, such that the first effect of each of the groups are disposed in sequence, followed by the second effect of each group, and so on until the last effect; and wherein the passageways for the percolating liquid and vapour are disposed in a parallel and dedicated arrangement in open communication with the group thereof and closed from the other group (s).

By virtue of this arrangement, the heat exchanger means and passageways for transferring the liquid and vapour can be arranged optimally in a compact structure so that space is still provided between successive effects of the same group to accommodate valving or other control means for regulating the flow of liquid from one effect to the next. This can be done whilst still making use of this space in effectively multiplying the effects operating simultaneously within approximately the same overall volume that a single group of effects will operate.

Preferably, the effects are formed by a plurality of stackable, substantially planar trays of corresponding and modular configuration arranged into two types, one being of thermally conductive material and the other being of thermally insulating material, the trays comprising a central expansive portion for defining the sides of the heat exchanger means and an outer network of apertures for defining a transverse portion of said passageways, said trays being symmetrical about orthogonal axes in the plane of the tray to permit corresponding alignment of the apertures when stacked in a corresponding disposition or in a relative orthogonal disposition to each other, a heat exchanger means being defined by alternating layers of trays of thermally conductive material and thermally insulating material.

In this manner, the trays themselves combine to form the walls of the passageways for the liquid and vapour flow through and along the heat exchanger means, as opposed to the discrete provision of pipes and tubes. This in itself provides a huge cost saving in the construction of the system compared with prior art designs.

Preferably, the central expansive portion of each thermally conductive tray is grooved in a contiguous manner to define a continuous passageway communicating with one aperture at one periphery of the tray and traversing the entire central expansive portion of the tray in order to maximise surface area contact with the tray, to communicate with an opposing aperture at the opposite periphery of the tray.

By this arrangement, a flow path is provided from one of the apertures defining an inlet, through the heat exchanger means alongside one or the other sides thereof, and to the opposing aperture defining an outlet.

Preferably, a self-regulating float valve is provided within the apertures forming the passageway for liquid flow from one effect to the next effect, whereby liquid flow from the one effect to the next effect is controlled by the float valve maintaining the liquid in the one effect to a prescribed level and allowing flow of the liquid to the next effect without being affecte by the pressure difference between either effect.

Preferably, the major portion of the float valve controlling liquid flow between the one effect and the next effect is disposed in the passageways provided in an adjacent, succeeding effect of another group interleaved with the group of the one effect and the next effect.

Preferably, a vapour trap is provided within the apertures forming the passageway for distillate flow from one effect to the next effect, whereby distillate flow from the one effect to the next effect is controlled by the vapour trap maintaining the distillat in the one effect to a prescribed level and allowing flow of the distillate to the next effect without being affected by the pressure difference between either effect.

Preferably, the major portion of the vapour trap controlling distillate flow between the one effect and the next effect is disposed in the passageways provided in an adjacent, succeeding effect of another group interleaved with the group of the one effect and the next effect.

In accordance with another preferred aspect of the present invention, there is provided a tray for forming part of a heat exchanger means of a multiple effect machine comprising a central expansive portion for defining the sides of the heat exchanger means and an outer arrangement of apertures for defining a transverse portion of passageways for directing the flow of liquid and distillate to

one side or the other side of the central expansive portion or through the tray, said tray being symmetrical about orthogonal axes in the plane of the tray to permit corresponding alignment of the apertures when a plurality of trays are stacked in a corresponding disposition or in a relative orthogonal disposition to each other.

By virtue of this arrangement, a heat exchanger means can be formed by stacking a plurality of these trays in alternating layers of trays formed of thermally conductive material and thermally insulating material. The peripheral edges of the trays and the apertures are then sealed and closed so as to direct flow in a desired manner.

Preferably, the central expansive portion of each thermally conductive tray is grooved in a contiguous manner to define a continuous passageway communicating with one aperture at one periphery of the tray and traversing the entire central expansive portion of the tray in order to optimise surface area contact with the tray, to communicate with an opposing aperture at the opposite periphery of the tray.

In accordance with a further preferred aspect of the present invention, there is provided a method for optimising the structure of a multiple effect machine, comprising:- interleaving the effects of one group of effects of the machine with the same number of effects of another group of effects; and aligning the effects in a sequential manner so that the first effect of each of the groups are disposed in sequence, followed by the second effect of each group, and so on until the last effect; wherein the passageways for the percolating liquid and vapour are disposed in a parallel and dedicated arrangement in open communication with the group thereof and closed from the other group (s).

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in accordance with one specific embodiment. The description will be made with reference to the following

drawings, which also include the drawings considered with the discussion of the prior art above wherein:- Fig 1A is a schematic drawing of a typical plant using the multiple effect process as previously known; Fig 1B is a schematic drawing of a typical plant using the multi-stage flash process as previously known; Fig 1C is a schematic drawing of a typical plant using the vapour compression distillation process as previously known; Fig 2 is a schematic block diagram showing a stack for a two group, four effect plant incorporating the multiple effect machine of the present invention and the relevant saltwater, brine, steam and vapour flows with respect to same; Fig 3 is a schematic conceptual diagram showing the internal arrangement of trays and flow paths for the brine, steam and vapour with respect to a single group of effects for a five stage or effect plant; Fig 4 is an exploded schematic view of the trays of the first and second effects of an actual stack incorporating interleaved groups showing the feed water flow path through the second effects and first effects of both groups; Fig 5 is an exploded schematic view of the same trays of Fig 4 but showing the brine flow path for the first group of effects through the first effects and second effects of both groups; Fig 6 is an exploded schematic view of the same trays of Figs 4 and 5 but showing the vapour flow path for the first group of effects through the first effects and second effects of both groups and the start of the vapour flow path for the second group of effects;

Fig 7 is a plan view of a tray; Fig 8 is a cross-sectional elevation of a self-regulating float valve for controlling brine flow between effects; Fig 9A is a cross-sectional elevation of a self-regulating, passive, multi-stage vapour trap for controlling distillate flow between the effects; and Fig 9B is a schematic diagram helping illustrate the conceptual operation of the vapour trap.

BEST MODE (S) FOR CARRYING OUT THE INVENTION The preferred embodiment of the invention is directed towards a plant using a multiple effect machine for distilling saltwater, which is sourced from either the ocean as seawater or from underground as bore water.

The plant 11 comprises one or more multiple effect machines in the form of stacks 13, each made up of an even number of identical effects or modules 15, each module in turn being made up of a plurality of trays or plates 17 forming heat exchanger means. The plates 17 are divided into thermally conductive (typically metallic) plates 17'alternating with thermally insulating (typically thermoplastic) plates 17". The plates 17 are oriented horizontally and stacked vertically.

Seals are provided so that once the stack 13 is assembled, it defines within itself vacuum-tight spaces 19 in which the distillation process can take place, as well as passageways for fluid transfer. The passageways are divided into three main types: (i) a common liquid supply passageway 21 a for delivering feed water to the top of the stack and preheating it through each of the effects on the way; (ii) a dedicated liquid treatment passageway 21 b for directing brine flow downwardly through each group of effects; and (iii) a dedicated distillate flow passageway 21 c for directing distillate flow downwardly through each group of effects as well as steam/vapour flow within each of the effects.

These passageways include spaces for regulating devices in the form of vapour traps 23 and float valves 25. These are particularly important to the stable and efficient operation of the stack.

The stack 13 is the basic functional unit of the desalination plant. Its size will vary with the number of effects or stages incorporated to achieve the efficiency that is desired. Additional stacks 13 can be assembled and added to the plant when addition capacity is required.

While modular construction and the use of horizontal plates occur in prior art in this field, full advantage has not been taken hitherto of the potential for extreme compactness inherent in the stacked-plate configuration. In this respect, one problem that has eluded solution by other workers has been that of providing sufficient depth for vapour traps and float valves which are needed to make the equipment self-regulating. Self-regulation is highly desirable in modular equipment, because the multiplicity of units in a large plant makes external, active controls expensive to implement. Providing regulation internally also minimises external plumbing and wiring, which are also onerous in a large-capacity plant assembled from multiple standard units of convenient size.

The present embodiment overcomes this problem by including self-regulating organs (the float valves and vapour traps) without sacrificing the narrow inter-plate spacing that gives a compact, lightweight plant.

Moreover, each of the aforementioned stacks 13 actually contains two groups of interleaved distillation effects A and B operating in parallel. Each stage or effect of group A has a corresponding effect of group B adjacent to it in the stack, operating at the same pressure but independently, with its own distillate/steam and brine flow. The only passageways that are common to both the A and B effects are a vacuum manifold 27 connected to a deaerator 29 that removes air and non-condensable vapours, and the liquid supply passageway 21 a that preheats incoming saltwater (feed water) before it descends through the stack 13.

Fluid flows in A and B groups are at right angles to one another, employing two independent sets of dedicated liquid treatment passageways 21 b and distillate flow passageways 21 c defined by the plates when stacked together.

The reason for interleaving the two groups is that the fluid passageways, passing through A but assigned to B provide spaces in which the regulatory organs of B can be installed and operated, and vice versa. This arrangement allows volume that would be wasted by B to be used by A, and vice versa, increasing the amount of capacity that can be accommodated in a given total volume.

Another problem that has hitherto prevented modular groups of effects that use trays from realising their full potential is the necessity of providing a vacuum-tight enclosure and internal baffling.

The vacuum-tight enclosure is necessary because multi-stage distillation re-uses heat by progressively reducing the pressure at which boiling occurs, thereby lowering the boiling temperature of the brine. This requires that a partial vacuum be maintained in the working parts of the stack, at different levels corresponding to the individual effects or stages. This in turn means that the stack as a whole must be sealed to prevent air from entering, and that it be internally partitioned so that each effect maintains its assigned pressure level with a minimum of pumping work expended. Internal partitioning is usually also needed to prevent salt spray from boiling brine contaminating the distillate or purified water.

In the present embodiment, both the vacuum sealing and the internal partitioning (not shown), as well as all passageways, are integrated into the design of the plates themselves. When the plates 17 are assembled into a stack, with their o- rings (or rings with any other adequate cross-section) and spacers, the distillation section containing all of the heat exchanger means of the stack is complete.

Consequently, maintenance time is significantly reduced by not requiring an outside shell to be opened and resealed, and the plant designer has considerable freedom in configuring a stack because the design need not fit a fixed"envelope."

The design of the plates 17 is best shown in figure 7. All of the plates comprising the stack are modular and symmetrical about planar orthogonal axes thereof.

Thus the plates 17 are essentially square in plan and comprise three circular apertures 30a, 30b and 30c at each of three of the four corners thereof and four substantially rectangular apertures 32a, 32b, 32c and 32d along each of the sides thereof. The apertures 30 and 32 are disposed around the periphery of the plate and circumscribe a central expansive portion 34 which is partitioned to define a flow path therealong from one of the circular apertures to another and from one of the rectangular apertures to another. The blank corner 30d, without an aperture, is closed but can be aligned with open apertures of adjacent plates to either: direct axial flow laterally to the expansive portion 34 from an axial passageway formed by the adjacent open apertures; or direct lateral flow from the expansive portion, axially along the adjacent open apertures; depending upon the source of flow to the corner.

The modular design of the plates enables circular o-rings 36a and rectangular o- rings 36b to be fashioned and positioned around selected circular and rectangular apertures 30 and 32 respectively, in a manner so as to selectively open or close communication with the flow path of the central expansive portion 34, allowing considerable flexibility in channelling flow and forming passageways 21 through the stack. The effect is complete by the positioning of a rectangular circumferential seal 36c around the outer peripheral edge of the plate 17. The o- rings and seals 36 extend laterally of the plate to engage corresponding locations of an adjacent plate and in this manner seal off external communication to selectively define flow paths.

For example, figure 7 shows the arrangement of o-rings and seals to form one of the thermally conductive plates 17'. In this arrangement, brine flow moving down from adjacent circular apertures 30 and striking the blank corner 30d, is directed laterally and along the central expansive portion 34 to the diagonally opposite circular aperture 30b not provided with an o-ring. This lateral flow is then directed axially through the aperture 30b, downwardly to the next plate. The circular apertures 30a and 30c, with o-rings 36a, are sealed from communicating with the

central expansive portion 34 of the plate. Thus one can allow for the downward flow of brine through the plate between effects along the passageway 21 b of another group, and the other can allow for the upward flow of feed water along passageway 21 a through the effect to the thermally conductive plate 17'between effects.

With respect to the rectangular apertures 32, three of these will be completely closed from communicating with the central expansive portion 34 by the provision of full height rectangular o-rings 36b, but one of these, for example 32b, will be provided with an intermediate height o-ring 36b', to allow still communication with the central expansive portion. Thus the passage of vapour, evaporated from the brine flowing along the central expansive portion 34, is permitted through the space between the top of the intermediate height o-ring 36b'and the bottom of the next plate, and through the adjacent rectangular aperture to define the vapour/distillate passageway 21 c and one of the vacuum-tight spaces, for example 19', of the effect. Simultaneously, the intermediate height o-ring 36b' prevents passage of the brine through the rectangular aperture 32b. The directly opposite closed, rectangular aperture 32d is arranged to provide for the other vacuum-tight space 19"of the same effect to be sealed from the central expansive portion 34 of the plate 17'and the space 19'. This allows vapour/distillate flow on the other side of the effect, through the plate 17', maintaining the pressure differential between the spaces 19'and 19". The other adjacent apertures 32a and 32c are fully sealed to maintain isolation from the effect of which the plate 17' forms part, and thus can be used for forming the passageway 21 c for transferring vapour/distillate flow of the other group of effects, through the plate.

The particular arrangement of the plates in forming adjacent effects is complex and will be described in more detail with reference to figures 4 to 6 later.

The specific arrangement of the partitioning of the central expansive portion of the plates is optional and will not be further described, suffice it to state that it is arranged in a repeating and parallel manner, allowing adjacent plates to be stacked orthogonally with respect to each other. This creates a flow space

between the partitions, whereby the plates cannot be stacked correspondingly due to their identical profile.

The operation of the stack will now be described with respect to figures 2 to 6. It should be noted, however, that specifically with respect to figure 3, only one of the groups is shown and the arrangement of the feed water flow and preheating by virtue of the secondary heat exchanger means is not shown.

Fluid flow directions for this embodiment are as follows: steam/vapour/distillate: top to bottom; saltwater/feed water: bottom to top through the preheater section of each stage; saltwater/brine: top to bottom down through the trays.

In an alternative embodiment, it is possible to design for fluid flows in the opposite direction by re-arranging the effects so that the lowest pressures occur at the top of the stack. This results in a saving in pump power (because the pressure gradient assists in raising the fluids), at the cost of some difficulty in starting the machine. The preferred embodiment, however, is that shown and discussed further below.

In figure 2, the convention is to number the effects from lowest (#1, at or near atmospheric pressure) to highest (#4 in the diagram, but of course a larger or smaller number of effects could be used) and to designate the two groups operating in parallel and independently, A and B.

The incoming stream of saltwater is piped to the base of the stack at 31, where it enters a condenser 33 and is used to cool the still uncondensed vapour issuing from the final effect of the stack at 35. The saltwater exit stream from the condenser at 37 is split, some of the water going to waste at 37a to carry off the waste heat of the process, the remainder entering the stack as feed water at 37b

to undergo desalination. This stream of feed water rises through the stack to the top via the liquid supply passageway 21 a and the one side of the second heat exchanger means PH formed by the preheat plates 39 in each of the effects (see figures 2 and 4 to 6, but not shown in figure 3). After reaching and passing through the first effect, it is deaerated via the deaerator 29 (not shown in figure 2) at 43 and then is allowed to enter the saltwater/brine circuit at 45 as brine, commencing at the first effect. The saltwater/brine circuit is actually split at this stage into dedicated streams for each of the groups of effects (not shown in figure 3), whereupon it percolates through the successive effects via the liquid treatment passageways 21 b for each group. The brine becomes more concentrated as fresh water is evaporated from it. The subsequent vapour from the evaporated fresh water fraction is condensed within each effect to form the distillate or product stream.

In figure 3 showing a single group of effects, and in figures 4 to 6 showing an interleaved pair of groups A and B down to the first and second effects, the various flow paths through a group of effects are shown, still schematically but in more detail. Each effect (#1A, #1B, #2A etc) is made up of two first heat exchanger means, essentially operated in series. Each first heat exchanger means is formed by an intermediate thermally conductive plate 17'layered between two thermally insulating plates 17", which are disposed on either side.

The pair of first heat exchanger means are stacked one on top of the other, so that the bottom plate 17"of the first of the two first heat exchanger means also forms the top plate 17"of the second of the two first heat exchanger means. The thermally conductive plates 17'constitute the interface for the one side and the other side of the first heat exchanger means, accommodating brine flow and evaporation of same on the one side and providing a transition surface for forming condensate from the vapour space on the other side. The thermally insulating plates 17"insulate and separate adjacent first heat exchanger means from each other within an effect and insulte and separate the second heat exchanger means from the first exchanger means between effects.

As best shown in figures 4 to 6, each second heat exchanger means is formed by a preheat plate 39 layered between the bottom thermally insulating plate 17"of one effect, and the top thermally insulating plate 17"of the next effect. The preheat plate 39 is also formed of thermally conductive material and constitutes the interface for the one side and the other side of the second heat exchanger means, accommodating feed water flow and preheating of same on the one side, and providing a transition surface for forming condensate from the vapour space on the other side.

It should be noted that consecutive effects have a common vapour/distillate space 19'or 19", provided by the vacuum-tight space 19, in which vapour from the brine heated in the first effect (higher pressure and boiling point), on the one side of the first heat exchanger means, serves as process steam-and is condensed-on the other side of the first heat exchanger means of the next effect (lower pressure and boiling point). Thus the vapour passageway 21 c between consecutive effects is at constant pressure and needs no pressure-reducing valves. However, the vapour spaces 19'and 19"on opposite sides of the heat exchanger means of the same effect are at different pressures and temperatures to each other and remain separate from each other by virtue of the walls formed by the plates 17', 17"and 39, as can be seen in figure 3.

The fluid flows are perhaps best shown in figures 4 to 6. Although these drawings are still schematic in the sense that details of plate contours are not shown, and spacers, seals and orifices are omitted to avoid visual cutter, nevertheless the actual flow paths are shown correctly, instead of being"unfolded"into a common plane as in figures 2 and 3. It will be immediately obvious that the plates are square and that two of the three types of plate have fourfold symmetry with regard to the provisions for fluid passages (the round and oblong holes) and bilateral symmetry when channels and corrugations are taken into account.

This bilateral plate symmetry, combined with the fact that adjacent A and B stages in the stack have flow paths at right angles to one another, achieve an important object-that of compensating for dimensional variations in manufacture. This

compensation is necessary because, while the manufacturing processes that allow the plastic plates to be made inexpensively give excellent dimensional accuracy in the plane of the plate, some variation in thickness is unavoidable. If one side were thicker than the other and the plate design did not allow the high sides of consecutive plates to be rotated with respect to one another, the cumulative error over the height of the stack might be sufficient to tilt the top plates enough to make the machine inoperable.

In figure 4, the arrows 47 show the feed water flow through the liquid supply passageway 21 a. Starting with the bottom of the effect #2B, the feed water flows through the corner apertures 49 of the stack of plates forming the effect #2B. The seals surrounding these apertures are arranged so that no communication is provided with the central expansive portion thereof. In this manner, the feed water flow bypasses the first heat exchanger means of the effect #2B and reaches the circular aperture 50 of the plate 39.

This aperture 50 is sealed in a manner so as to permit feed water flow to the central expansive portion of the preheat plate 39. Moreover, the plate 17'above it is disposed so that its blank corner 51 overlies the aperture 50, preventing further axial flow along the stack, the aperture 50 is not sealed with an o-ring, allowing communication with the flow path along the central expansive portion of the plate to the blank corner 52 of the plate. This blank corner 52 constitutes the bottom of another set of apertures 49 provided at the corners of the plates forming the next effect #2A. The apertures 49 are sealed so as to permit no communication with the central expansive portions thereof, consequently directing feed water flow axially of the stack once more and through the effect #2A.

This arrangement of apertures and sealing is repeated through successive effects #1 B and #1A, whereupon the feed water is directed through the deaerator 29 and returned as brine to flow through the liquid treatment passageway 21 b, which is shown in figure 5.

This brine flow path of one group of effects is shown by the arrows 53 and commences at the first effect #1A. The flow is initially axial of the stack from the deaerator 29 through the corner aperture 55 of the top insulating plate 17". It is then directed towards the central expansive portion of the first thermally conductive plate 17'of the effect, by placement of the plate with its blank corner 57 in alignment with the aperture 55, in a similar manner to the arrangement previously described with the feed water flow.

The brine is directed along the flow path to the diagonally opposite corner aperture 59, whereupon it is directed axially again via appropriate sealing and aligned corner apertures to the second thermally conductive plate 17'. The flow is then directed towards the central expansive portion of this plate in a similar manner as to the first plate and passes through to the diagonally opposite corner aperture 61.

This aperture 61 is aligned with a series of corner apertures 63 of the preheat plate 39, between the effects #1 A and #1 B, and the plates forming the next effect #1 B. These apertures 63 are sealed so as to not permit communication with the central expansive portion of the plates thereof, thereby directing the brine flow axially of the stack and through the effect #1 B.

The corner aperture 65 of the preheat plate 39 between the effects #1 B and #2A, which is aligned with the apertures 63, is sealed in a particular manner so as to form a seat 65a for a float valve 23 reposed within the apertures 63. This valve regulates brine flow from the first effect to the second effect in a manner to be described in more detail later.

The aperture 65 is also aligned with the corner aperture 67 of the top thermally insulating plate 17"of the second effect #2A and is arranged in conjunction with the first thermally conductive plate 17'of the effect #2A in a similar to the corresponding plates of the first effect #1A, to direct brine flow to the central expansive portion of the plate 17'.

The brine flow continues through the subsequent effects in a similar manner to the previous effects, with flow being regulated by the float valves 23 disposed within the appropriate apertures of the B effects.

Although not shown, the brine flow for passing along the central expansive portions of the B effects, is conversely arranged to that for the A effects, whereby the float valves for regulating flow between subsequent B effects are disposed within the appropriate apertures for the A effects.

Now describing the steam/vapour flow for the stack and the distillate flow passageway 21 c, reference is made to figure 6 and the arrows 69 and 70.

Steam is initially directed from a boiler 71, which may be heated via a thermal loop 72 from a solar pond, as shown in figure 3. The boiler is typically a shell and tube or a plate boiler, both of which are of known design.

The steam is directed from the boiler 71 at 73 to the vapour space 19"of the first effect #1A, as shown by the arrows 69, via the aligned rectangular apertures 75 of all of the plates forming both of the first effects #1A and #1 B. The sealing of the apertures is alternately arranged, however, so as to prevent communication with the central expansive portion of the top or the one side of each of the thermally conductive plates 17', but to allow communication with the central expansive portion on the underside or other side of these plates 17'. In this manner, a vapour/distillate space within which vapour may condense is provided on the other side of the plate 17'and distillate can collect and flow along the top surface of the succeeding thermally insulating plate 17", back to the vapour/distillate flow passageway 21 c formed by the aligned apertures 75.

The arrangement is continued with the aligned aperture 75 of the preheat plate 39 between the effects #1A and #1 B, whereby communication is prevented with the central expansive portion on the top or one side thereof, but communication is allowed, through the agency of the top thermally insulating plate 17"of the effect #1 B, with the underside/other side of the plate 39.

The succeeding rectangular apertures 75 of the next effect #1 B in axial alignment with the rectangular apertures 75 of the effect #1A, continue the formation of the distillate flow passageway 21 c, but are all sealed so as to prevent communication with the central expansive portions of the plates thereof. These apertures, however, accommodate a vapour trap 25, which sits on a seat 77 formed within the aligned rectangular aperture 75 of the preheat plate 39 disposed between the effect #1 B and the effect #2A. The design of the vapour trap 25 will be described in more detail later. Essentially, the vapour trap 25 allows flow of distillate therethrough but does not allow vapour flow, and hence maintains the pressure differential that exists between the first and second effects on this side of the distillate passageway 21 c.

The aligned rectangular apertures 79 on the opposite side of the first and second effects of both groups, form the vapour space 19'for the first and second effects of group A, where vapour/distillate flow is shown by the arrows 70.

As best illustrated in figure 3, the vapour space 19'is different from the vapour space 19", whereby it is in communication with the brine 80 and provides a passageway 21 c for carrying vapour evaporated from the same in one effect to the next effect. In this next effect, the passageway 21 c is used to heat the brine of the next effect and condense on the other side of the thermally conductive plates 17'thereof. After the first effect, the vapour spaces 19'and 19"mirror each other, bridging successive effects of the same group on different sides of the heat exchanger means formed thereby. For example, the vapour space 19'bridges the one side of the thermally conductive plates 17'of the first effect #1A at the brine boundary, with the other side of the thermally conductive plates of the second effect #2A. Simultaneously, the vapour space 19"bridges the one side of the thermally conductive plates 17'of the second effect #2A at the brine boundary, with the other side of the thermally conductive plates of the third effect #3A.

Thus, as shown in figure 6, the vapour space 19'communicating with the one side of the thermally conductive plate 17'is formed by the aligned rectangular

apertures 79 of all of the effects #1A, #1 B, #2A and #2B. These are sealed in an alternate manner, as with the case of the apertures 75, but this time so as to communicate with the central expansive portion on the top or one side of the thermally conductive plates 17'of the first effect #1A, but not with the underside or other side of these plates. The intermediate height o-ring, as previously described, effectively provides a wall or baffle preventing the flow of brine 80 on the one side from entering the vapour passageway formed by the apertures 79, but allowing vapour flow thereto.

For applications requiring product water of very high purity, the vapour/distillate passageway 21 c can be provided with baffles to prevent carryover of salt spray from the brine into the vapour/distillate stream. In applications where only ordinary potable-water standards apply, the baffles may be omitted and the machine thus simplified.

With respect to the aperture 79 of the preheat plate 39 between the effects #1A and #1 B, this is sealed so as to prevent communication with the one side of this plate, thereby keeping the feed water circuit closed from the vapour circuit.

The vapour flow, represented by arrow 70, continues through the apertures of the various plates of the effect #1 B, which are all sealed from communication with the central expansive portions thereof, so as to bypass the same.

With respect to the aperture 79 of the preheat plate 39 between the effects #1 B and #2A, this is sealed so as to prevent communication with the top or the one side thereof, but the alternating sealing is provided again with successive apertures 79 of the plates of the effect #2A, so as to allow open communication with the underside/other side of this preheater plate. Similarly, communication is permitted with the central expansive portion of the underside/other side of each of the thermally conductive plates 17'of the second effect #2A, but is closed with the central expansive portion of the top/one side of each of these plate 17'. Thus vapour is permitted to enter the vapour spaces so formed on the underside of the

plates 17'to condense and allow collection of distillate, in the same manner as previously described with respect to the vapour space 19".

The apertures 79 of the next preheat plate 39 between the effects #2A and #2B, and the plates forming the effect #2B are all closed from communication with the central expansive portions thereof. Accordingly, these apertures 79 accommodate another vapour trap 25, which is arranged correspondingly to that described with respect to the vapour space 19", to provide for distillate flow from the vapour space 19'to the next space 19', bridging the effects #3A, #3B, #4A and #4B.

In progressing from one effect to the next, both brine and distillate must pass through regulating organs that reduce their pressure to that of the next effect and allow only the right quantity of fluid to pass-neither flooding nor drying out either effect. These pressure reducers are the float valves 23 and vapour traps 25 in figure 3.

In the preferred embodiment, the regulators are: (i) Brine: modular float valves; (ii) Condensate/distillate: passive multistage vapour traps.

Both the float valves and the traps incorporate original features, and will therefore be described in more detail below. It should be noted, however, that these organs, although represented by tiny symbols in figure 3, take up a great deal of volume in the real machine. They are therefore lodged in dedicated passages through, but not communicating with, the B machine.

The purpose of the float valve 23 is:- (i) to allow liquid to flow from one space, in which the valve mechanism is located, into a space where a lower pressure prevails; (ii) to regulate the flow of the liquid so that liquid level in the space occupied by the valve remains within limits chosen by the designer;

(iii) to permit a small, constant and controlled leakage between the two spaces, even when the valve is not operating; and (iv) to do all the foregoing without intervention from a human operator or an active control system.

The present embodiment utilises a modular design because, in multi-stage or multiple effect desalination apparatus, each inter-stage transition requires different values for the key valve characteristics, namely float volume and orifice size.

When the system in question is modular, however, and can be configured for a wide range of capacity, product purity and specific energy consumption, the potential number of different valve configurations becomes very large. If a different valve design were designed, tooled for and manufactured for every possible need, the result would be so uneconomical as to void much of the economic advantage of modular distillation equipment.

As shown in figure 8, it is readily seen that the valve 23 has four principal components: A bolt or machine screw 81 extending lengthwise (vertically, when the valve is in its operating position). In the preferred embodiment of my invention, the screw 81 is preferably made of a suitable polymer material, such as polyamide. This material is light, easily cut to length and is not attacked by brine at the usual operating temperatures of the machine.

* One or more toroidal floats 83, through which the screw 81 extends with a sliding fit. These floats 83 are shaped so as to fit together snugly, without waste of volume or vertical height, and to have sufficient clearance from the plates 17 which delimit the space in which the valve 23 operates to ensure that the valve does not stick in its bore. The floats 83 may be made of any suitable material, the only criteria being density lower than that of water and immunity to chemical or thermal attack under the prevailing operating conditions. In the

present embodiment, they are moulded from a closed-cell foamed polymer material.

At the bottom of the float assembly is a trunconical plug 85 which is also internally threaded to serve as the nut holding the lower end of the float stack in place on the screw 81. The plug incorporates a plurality of vanes 85a, which provide a spacing function. The plug 85 should be made of a dense material, so as to give the assembly a tendency to float upright.

The above assembly constitutes the moving part of the valve.

Incorporated into the seat 65a, separating the space occupied by the valve from the space into which it exhausts, is an orifice 87 mating with the plug 85, and more particularly, the vanes 85a mentioned above. The vanes engage the edge of the orifice 87 in a manner so as to space the trunconical wall of the plug from the edge of the orifice, so as to provide a controlled leakage path of the brine through the orifice. This ensures that there is always some liquid flowing through the valve assembly, even when the valve 23 is seated in the orifice 87.

The operation of the valve 23 is conventional. As liquid accumulates in the space surrounding the valve at a rate higher than that at which it escapes through the leakage path, the liquid level rises around the float stack 83 until buoyancy overcomes the pressure difference holding the valve to its seat 65a. The valve lifts off its seat, allowing liquid to enter the space below at a more rapid rate. The liquid level in the valve space drops in consequence, and the valve returns to its seat.

In assembling a valve 23, after calculating the requirements, an orifice of the required size is chosen. The required number of floats 83 are then threaded over the screw 81, the mating plug 85 is screwed on and the screw is flushed with the end of the plug 85. The orifice 87 is then inserted into the corresponding aperture in the plate 17 and held there by, for example, silicone sealant. After the half-

stage is assembled, the float/plug assembly is dropped into the appropriate circular passageway in the stack.

The vapour trap 25 serves to regulate the pressure of distille water (the product) as it moves from stage to stage.

The purpose of the vapour trap 25 is to allow liquid to flow from one space, with which the trap communicates, into a space where a lower pressure prevails, with low losses, while maintaining the pressure difference between the two spaces, without intervention from a human operator or an active control system. In other words, it is a passive control organ or passive regulator.

The trap 25 also accommodates a range of pressure differences without modification or adjustment.

As shown in figure 9, it can readily be seen that the trap 25 has three principal components: . The oblong housing or shell 89, with outlet opening 89a at one end communicating with the downstream (lower pressure) stage.

The cover 91, with inlet opening 91 a at the other end, communicating with the upstream (higher pressure) stage. interna transverse baffles 93 spaced along the length of the trap, with two types alternating: one 93a with a gap at the top, the other 93b opening at the bottom.

In operation, the trap 25 behaves like a multiplicity of hydraulic traps assembled in series. The individual traps are defined by pairs of bottom-opening baffles 93a and top-opening baffles 93b.

As best illustrated in figure 9B, under the influence of a pressure drop between one space 95a and the next 95b, two water columns 97a and 97b form, communicating at their bottom ends 97c, the upstream column 97a shorter than the downstream one 97b. The pressure difference between the successive spaces 95 is equal to: the weight density of the water 99 disposed in the trap, multiplied by the difference in the height of the two water columns 97a and 97b.

When a quantity of water 99 is added to the upstream column 97a, an equal volume spills from the top of the downstream column 97b into the succeeding trap, where the same process occurs.

The maximum allowable pressure drop per stage is determined by the difference in height between the bottom edge of the bottom-opening baffles 93b and the top edge of the top-opening baffles 93a.

The bottom-opening baffles 93b, except for the one farthest upstream, are provided with small orifices 101 near their top edges, allowing a small leakage of vapour from the traps into the downstream stage. Their purpose is to allow rapid starting and smooth operation of the machine by scavenging air and other non- condensable gases from the spaces 95 overlying the water columns 97. This is advantageous, because the machine as a whole cannot function as designed until the traps 25 are filled with distillate, as until then, no pressure difference can be maintained in successive stages or effects.

It has already been noted that incoming saltwater, after passing through the external condenser 33, enters the stack and rises through the components labelled"PH" (preheater), arriving at last at the top of the stack, where the flow is split between the A and B groups for desalination. Including these preheaters deviates from the normal multiple effect distillation cycle, in which both the vapour/distillate and the saltwater/brine streams flow from the lowest to the highest effect. In the preheaters PH, and only in the preheaters, is saltwater flow countercurrent to the flow of vapour and distillate, progressing from the highest to the lowest stage without pressure change, as in a multi-stage flash process. In this respect the preheaters implement one-half of the condensation side of the

multiple effect process in the present embodiment. In them, the incoming saltwater is preheated. The source of that heat is the condensation of vapour in the stage or effect. Thus, seen from the distillate (freshwater/vapour) side, the preheater serves as an end-condenser for that stage.

The preferred embodiment of the invention has several advantages over prior art systems: 1. It reduces the operating cost of the plant by lowering the cost of each"effect"or stage of distillation; this allows more effects to be employed, giving a higher yield of fresh water per unit of energy used.

2. It lowers the capital cost of the plant by reducing the cost per unit of heat transfer area (upon which the capacity of the plant depends). A further effect is to lower the plant volume per unit of heat transfer area, reducing construction and real estate costs.

3. It provides equipment that is modular and self-sealing, thus allowing plants to be easily modified, reconfigured and repaired, reducing maintenance costs and "down"time.

It should be appreciated that the scope of the present invention is not limited to the particular embodiment herein described. In particular, whilst the emphasis of this description has been on the purification of saline water to make it potable, the improvements to the system provided by the invention are equally applicable to the removal of any dissolved solids. What is more, the use to which the purified water may be put is not limited to drinking water. By suitable configuration of components disclosed herein, a plant providing very high purity water can be assembled; such water has both medical and industrial uses.

In addition, whilst the emphasis of the invention has been directed to application of the multiple effect process, aspects of the invention are equally applicable to other distillation processes. For example, the general interleaving and modularity of stack construction principes using trays or plates of similar design to those described herein may be applied to multi-flash processes and vapour compression processes, without departing from the spirit of the invention.