WO2018033722A1

WO2018033722A1 - Water or evaporative cooler comprising a woven or warp-knitted mesh sheet

Info

Publication number: WO2018033722A1
Application number: PCT/GB2017/052404
Authority: WO
Inventors: Brian Parry Slade
Original assignee: H2O Technology Ltd
Current assignee: H2O Technology Ltd
Priority date: 2016-08-16
Filing date: 2017-08-16
Publication date: 2018-02-22
Anticipated expiration: 2019-02-16

Abstract

An evaporator incorporates an evaporative pad of an openwork sheet of hydrophobic fibrous material, e.g. polyester fibres and in a woven or preferably in a warp knitted pattern e.g. a Rachel knit. Embodiments have upper and lower edges and define flow paths that are inclined with respect to a vertical axis of the sheet so that water flowing continuously along the flow paths moves down and across the sheet e.g towards its lateral edges. The evaporator may form a component of a device for desalinating seawater, or it may form part of an evaporative cooler for producing cooled ventilation air and/or cooled water.

Description

WATER EVAPORATOR AND WATER PURIFICATION APPARATUS OR EVAPORATIVE COOLER INCORPORATING THE EVAPORATOR

FIELD OF THE INVENTION

The present invention relates to an evaporator for evaporating water, to a method of producing water vapour using the evaporator, to desalination apparatus incorporating the evaporator and to a method for desalinating water. It also relates to an evaporator for use in evaporative cooling and to a method of cooling ventilation air and/or cooling water using the evaporator.

BACKGROUND

There is an extensive patent literature concerning methods for producing potable water by evaporation, distillation or reverse osmosis. Evaporative or distillation methods can produce water of a higher quality than reverse osmosis. However, the necessary heating process to boil or vaporise the water is energy- intensive and costly owing to the substantial amount of energy required to boil or evaporate seawater.

US-A-3290231 (Ries et al., Standard Oil) discloses a method for evaporating and/or distilling a volatile liquid, especially at temperatures below the boiling point of the liquid media, and which is said to be especially useful for the economical production of salt and fresh water from saline water by solar evaporation. The evaporating surface of a body of the water is covered with a thin floating layer of discrete, imperforate particles of an inert solid non-absorbent material having the surface thereof wettable by water, which particles have an average diameter of less than one centimetre and a density less than the water, whereby the rate of vaporization of said water is accelerated at a given temperature over the rate of vaporization in the absence of said layer.

US-A-7435317 (Seiji, Biomass Conversions) discloses distilling significant quantities of water at temperatures well below the boiling point. During distillation, a compound is taken from a liquid-phase to a gas-phase and then condensed to the liquid-phase again to get a pure liquid. The Seiji invention is based on the idea that weakening the interaction of water-water and water-other molecules shows that it is possible to alter the heat transfer characteristics of the water molecule. Weakening the water-water interactions can lower the amount of energy needed to convert water from a liquid to a gas. The surface of a small particle is said to be optimal for producing interactions that change the amount of energy needed to evaporate water. In other words, the energy needed to evaporate a water molecule can be reduced by the reducing the water-water interaction. The most effective materials for this purpose are said to be hydrophilic compounds such as natural polysaccharide macromolecules found in plant materials such as wood, paper, bamboo, and rice straw as well as proteins and inorganic metal oxides such as silica (silicon dioxide), alumina (aluminum oxide), titania (titanium dioxide), magnesia (magnesium oxide), iron oxide, other metal oxides, clays, etc. Energy can be supplied as normal convective or conductive heating, or radiative heating by visible, infrared, or microwave radiation or simply by sunshine. A method is described for accelerating the evaporation of water comprising the steps of: (a) making a mixture of water and a quantity of fine particles, the particles having an average diameter of 100 μηι or less; (b) agitating the mixture within a container at a temperature below the boiling point of water whereby water vapor release from the mixture is accelerated; (c) spraying additional water into the mixture to replenish evaporated water; and (d) adjusting a temperature of the additional water to prevent lowering a temperature of the mixture. Apparatus is also described for accelerating evaporation of water comprising: (a) a container holding a mixture of water and fine particles at a temperature below the boiling point of water, wherein the particles have an average diameter of 100 μιη or less;(b) means for agitating the mixture whereby water vapor release from the mixture is accelerated; (c) means for spraying additional water into the mixture to replenish evaporated water; and (d) means for adjusting a temperature of the additional water to prevent lowering a temperature of the mixture.

US 2004/141810 (Lysne) discloses apparatus for desalinating soil including a frame, a distribution pipe and an associated vertically oriented evaporating cloth which may be made from any porous material which absorbs water (or at least which is not hydrophobic) and which presents a relatively large surface area for evaporation.

US 8287735 (Hanemaaijer, TNO Delft) is concerned with the production of desalinated water from seawater

US 9186597 (Ramon, Lesico Technologies) discloses a modular evaporation assembly for use in outdoor ponds where wastewater is stored and where evaporation is needed to concentrate the waste for further treatment. These ponds present an environmental challenge, as leakage from such ponds can lead to serious groundwater contamination. The evaporation surface of the evaporation member can be wettable by liquid and at least partially exposable to wind when wetted, to allow evaporation of said liquid from said evaporation surface. The evaporation surface of the evaporation element can be hydrophilic. For example, the evaporation surface can be sheets of fabric of a hydrophilic material or of a non- hydrophilic base material, the base material being coated with a hydrophilic coating or chemically treated to make the evaporation surface thereof hydrophilic. An example of a suitable base material an 85% shade net produced and sold under the trade name TamaShade™ by Tama Plastic Industry, Kibbutz Mishmar Ha'Emek, Israel.

Evaporators are also used in evaporative coolers. Such coolers are described e.g. in US 9383142 (Habeebullah, Umm Al-Qura University) in which it is explained that water is passed through wet porous air filters and air is drawn through those filters. The moisture evaporates in the air as it is drawn through the filters. The air is cooled by the latent heat of evaporation absorbed from the air as the water changes phase from liquid to vapor. This principle works well in drier climates with relatively low ambient humidity, decreasing the temperature of the air passing through the cooler by perhaps as much as 15° C. It is further explained that relatively few materials have been developed for forming the porous pads, with those materials not necessarily being particularly efficient at distributing the moisture as fine droplets to promote evaporation. It is proposed that such pads should be made from luff a sponge material, sheets of such material being pressed together to form a single, cohesive, porous evaporator pad. Luffa sponges are the fibrous interiors of the fruits of the luffa sponge gourd plant (Luffa aegyptiaca), which is grown in a number of countries throughout the world, including the United States. The specific species of luffa, i.e., Luffa aegyptiaca, was selected due to its superior air and liquid porosity when dried and when moistened with water in comparison with other evaporator pad material and compared with other luffa plant species, see also WO 01/79519 (Mohamm). The use of an evaporative cooler pad of woodwool is disclosed in US4460394 (Wrightson), similarly 4833895 (Carlson), 4902449 (Hobbs, Hobbs Bonded Fibers) and 6183579 (Ware, PPS Packaging). Glass fiber cooling pads for evaporative coolers are disclosed in US 5340651 and 5731081 (both Esu, Hollinee Corporation).

There is a need for a process to separate the salt and minerals from seawater that does not require substantial amounts of energy, and which also allows the separate collection of the salt and minerals from pure water. It is the purpose of the present invention, to provide a less energy-intensive means, using gravity and osmotic pressure for processing pure water and minerals from seawater, to substantially reduce process costs, thereby making fresh water readily available across the world at a more affordable price. The present invention has the added benefit of reclaiming valuable and scarce minerals in the process. There is also a need for an improved pad structure for an evaporative cooler.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a water evaporator including: an openwork sheet of hydrophobic fibrous material having upper and lower edges and openings defining flow paths that are inclined with respect to a vertical axis of the sheet so that water flowing continuously along the flow paths moves both down and across the sheet; and a supply for the water containing the dissolved minerals leading to the upper edge of the openwork sheet.

Movement of water across the sheet may be such that the openings in the sheet occur in a pattern configured to direct water flow towards lateral edges of the sheet. In such a pattern, the flow of water across the sheet may be bidirectional so that water flowing down the centre of the sheet is directed towards its edges, and water flowing near its edges is directed towards its centre. For that purpose, openings in the sheet may occur in diagonally offset rows, openings in the sheet being generally polygonal or oval with their largest dimension coinciding with the vertical direction or axis of the sheet. The sheet may have a length greater than its width, and in embodiments the sides diverge at an angle of 10-30° e.g. about 20°. The sheet may be arranged so that water will flow down it, e.g. at >45° to the horizontal and in embodiments vertically or substantially so.

As discussed in more detail below, the use of an openwork sheet as described above has been found to be particularly effective for evaporation of water from seawater or other saline solutions. It is also believed effective for evaporation of fresh water, e.g. in evaporative coolers, where water travels along the lattice-like structure of the sheet between the openings, and (although the invention does not depend on the correctness of this theory) it is believed that the hydrophobic nature of the sheet promotes formation of water droplets, increasing surface area and promoting evaporation. Where the evaporator is used in an air current e.g. from a fan, the velocity of the current is desirably such that water droplets do not become entrained in the air current or that any such entrainment is within small and acceptable quantities.

The sheet may be a woven mesh or preferably a warp-knitted mesh e.g. a raschel warp-knitted mesh. It may comprise >50% polyester fibres, the balance also being hydrophobic fibres, e.g. >75% polyester fibres and more preferably 100% polyester fibres.

Supply of water to the sheet may be by pumping means or may be syphonic. A drain may be provided for collecting water from a lower edge of the sheet.

Embodiments of the invention provide an evaporator for evaporating the water content of seawater, brackish water or other water containing dissolved minerals, said evaporator including an openwork sheet of hydrophobic fibrous material having upper and lower edges and defining flow paths that are inclined with respect to a vertical axis of the sheet so that the mineral-containing water flowing continuously along the flow paths moves down the membrane and towards its lateral edges, a supply for the water containing the dissolved minerals leading to the upper edge of the openwork sheet. Further embodiments of the invention provide desalination apparatus comprising an evaporator as defined above, a condenser for condensing water vapour from the evaporator, a heater for heating air from which water vapour has been condensed and a flow path including a fan for directing air through the evaporator, directing air through the condenser and through the heater, and redirecting warmed air of reduced water vapour content to the membrane. The condenser and heater conveniently form components of a refrigeration circuit.

Yet further embodiments of the invention provide method of desalinating water, comprising:

supplying water to be desalinated to an upper edge of an openwork sheet of hydrophobic fibrous material having upper and lower edges and defining flow paths that are inclined with respect to a vertical axis of the sheet so that the saline water flowing continuously along the flow paths moves down the membrane and towards its lateral edges,

directing air through the openwork sheet;

directing water vapour evaporated from the openwork sheet to a condenser; reheating cooled air of reduced water content from the condenser to a heater; redirecting air from the heater towards the openwork sheet; and

recovering desalinated water from the condenser.

The constant flow of seawater irrigates the membrane and controls the amount of minerals concentrated on the membrane. The method may comprise an additional step of collecting the concentrated minerals from the membrane. The method may also comprise an additional step of collecting unevaporated mineral- containing water at the bottom of the membrane. In some embodiments, the unevaporated mineral-containing water may drip from the bottom of the membrane at a flow rate such that the minerals are concentrated into stalactites that are suspended from the bottom of the membrane. For continuous operation, in other embodiments, the flow of seawater or other mineral-containing water is adjusted so that solid crystals do not form on the membrane, and instead the salt or other minerals are recovered in more concentrated form in the flow of water from the bottom of the membrane. In a further embodiment there is provided an evaporative cooler comprising a housing, an air inlet to the housing, one or more evaporative pads within the housing, a reservoir for water, a pump arrangement or syphon for supplying water to the or each evaporative pad and an air outlet, and a fan for drawing air into the housing and impelling it through the or each evaporative pad and through the outlet, wherein the or each evaporative pad is of an openwork sheet of hydrophobic fibrous material. The openwork sheet may have a latticelike nature or show openings through its substance, and may a woven or warp-knit material, e.g. a raschel-knit material. The cooler may be configured for production of a stream of cool ventilation air for residential, office or industrial room cooling, or it may be configured as part of an evaporative cooling tower for producing cooled water.

Also provided is a method for evaporative cooling comprising: supplying a housing and one or more evaporative pads within the housing; maintaining a supply of water to the evaporative pad(s); maintaining a forced flow of air into the housing, through the evaporative pad(s) and through an outlet from the housing; and either recovering cooled ventilation air and/or recovering cooled water; wherein the or each evaporative pad comprises an openwork sheet of hydrophobic fibrous material.

BRIEF DESCRIPTION OF THE FIGURES

How the invention may be put into effect will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a side view of water evaporation apparatus;

Figure 2 is a front view of a membrane forming part of the apparatus of Figure 1 and with arrows representing the structure of the woven material;

Figure 3 is a photograph on an enlarged scale of the membrane of Fig. 2 showing a single opening and the adjoining fibre structure;

Figure 4 is a further photograph of a single opening at a lower edge of the membrane and towards a side edge thereof, showing salt accumulation following membrane use; Figure 5 is a diagram of the membrane of Figure 2 showing the distribution of salinity levels across the membrane;

Figure 6 is a diagram showing a further embodiment of the membrane incorporating at its upper end hydrophilic material for providing a symphonic feed of saline water and incorporating at its lower end hydrophilic concentrated brine collection material;

Figure 7 is a simplified block diagram of desalination apparatus incorporating a membrane of the general type previously shown; and

Figure 8 is a simplified block diagram of an evaporative cooler.

Figure 9 is a photograph of a polyester mesh, said to be woven, downloaded from the web.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows a proof of concept apparatus 100 for separating water from dissolved minerals. The apparatus 100 comprises a vertically orientated membrane 102, a reservoir 104, a stand 106 and a collecting dish 108. It will be appreciated that the membrane may be at an inclination to the vertical provided that its inclination does not interfere with flow rate or other operation of the evaporator, although verticality is preferred. A reservoir 104 is supported in an elevated position by the stand 106, while a collecting dish 108 forms the base of the apparatus 100. An upper region 110 of the membrane 102 is attached to the reservoir 104 and extends into it, so that in use an end region 112 of the membrane 102 contacts with water contained in the reservoir 104. The end portion 112 is secured to the reservoir 104 by attachment means 114 which may comprise a clip, bracket, adhesive or any other suitable means.

The reservoir 104 may comprise any suitable material for containing water. In the embodiment illustrated in Figure 1, the reservoir 104 is a glass vessel. The end portion 112 of the membrane 102 within the reservoir 104 and the upper region 110 of the membrane 102 in contact with the glass surface of the reservoir 104 when wetted with water act as cooperating laminae through which water may be siphoned from within the reservoir 104, for which purpose the reservoir is periodically refilled so as to maintain the end portion 102 at least partially in contact with the water.

Figure 2 shows a more detailed view of the membrane 102. In the present embodiment, it comprises a 100% polyester fibre network raschel warp knitted into an openwork or cellular structure of thickness in this instance about 38 mil (1mm) that is highly permeable to air and defines flow paths for directing water flowing down the membrane 102. It will be understood that the thickness of the membrane can be changed according to the mechanical strength required, e.g. if a larger membrane is used. The woven fibrous membrane is, as previously explained, made from 100% polyester which is known to be a hydrophobic material and therefore capillary action does not disturb or play a part in influencing the flow of liquid like water, whereas gravity does. The feed water encloses the fibrous structures of the membrane by encircling them, so that the affinity of "water for water" uses the structures to direct the feed water down, and across the membrane. This is important because the support membrane is hydrophobic; it allows the water molecules to easily escape from the membrane when subjected to opposing osmotic forces. If the support membrane was made from a hydrophilic material there would be a tendency for the membrane to hold on to the feed water. Although 100% polyester fibres are presently preferred, it is believed that similar results may be obtained using membranes of other synthetic hydrophobic polymer fibres, e.g. polyamide, aramid, acrylic, modacrylic and polypropylene fibres, blends of these, and blends with polyester fibres.

In an embodiment, the membrane structure comprises somewhat polygonal or oval openings that allow a rod of 1.75 mm diameter to pass through and have largest dimension approximately 3mm and pattern repeat 7 mm formed in rows with the openings of each row offset from those in the immediately preceding and succeeding row so that they lie in a diagonal alignment as shown. In use, the largest dimension of the holes is aligned with the intended flow direction of the water as shown in Fig. 2. The generally oval shape of the holes is due to the manner of warp knitting in forming the sheet structures, which are suitably aligned for creating fluid pathways in the membrane as shown in Fig. 2 and make it highly permeable to air and water. These holes are created and enclosed by fibre structures that travel top to bottom and from side to side in such a way that they also create structures that travel diagonally. These diagonal structures that have been created by the design of the weave are very well defined, (see Fig 3) and play an important part in providing fluid pathways for water to travel, despite the fact the fibrous membrane is made from a hydrophobic material. The diagonal structures can however support the downward flow of water because the fibrous structure has a vertical component in its direction, and because the water is fed to both sides of the membrane. The openwork structure of the hydrophobic membrane provides paths that permit the water on one side of the membrane to contact the water on the opposite side of the membrane. Because the membrane is in use fed with water from both sides, both sides provide areas for evaporation.

In an alternative embodiment, the fabric may be a woven openwork netting e.g. in fine woven polyester mesh with holes of size e.g. 3-4mm and of hexagonal or oval shape, the holes being formed in rows with each row staggered so that the holes also form a diagonal pattern tending, as before, to direct flow to the sides of the sheet. As before the strands of the mesh advantageously are formed of multifilaments woven with small holes between them to promote water flow and evaporation. Woven netting of this type is shown in Fig. 9 which is a macro-photo said to be of fine woven polyester mesh/netting, the photo being downloaded from Alamy.

The membrane 102 is flared outwardly from the top to the bottom, being narrower at the top and wider at the bottom. In a practical embodiment used for proof of principle, a membrane as shown in Fig 2 had a height of 16 cm, a width at its top end of 2.5 cm, a width at its bottom end of 8 cm and sides diverging at an angle of 20°. The arrows A-D indicate the directing influence caused by the structure of the woven material. The middle vertical arrows A represent the initial downwards direction of seawater driven by the force of gravity. The middle line indicated by A also denotes an axis of symmetry of the structural design and shape of the membrane 102. The outwardly-directed diagonal arrows B represent the diverging structural fibre elements radiating out from the middle of the membrane 102. The pair of inwardly-directed diagonal arrows C, near the top of the membrane 102, represent fibre structures that converge towards the middle. The horizontal arrows D represent fibre structures that oppose each other in a horizontal direction, perpendicular to the seawater supply flowing down the middle of the membrane 102.

The structural lines created by the design of the weave each carry out different functions which are important to the separation process. The converging structures help to maintain the central column of the seawater on the membrane 102, and thus maintain a constant salinity level as a reference. It may be appreciated that, in the absence of capillary action or wicking, the route that the bulk of the seawater may take will be directed by the convergent structures, driven by gravity and the hydrophilic attraction of water molecules to each other. The diverging structures create increasingly longer pathways from the middle of the membrane 102 to its outside edges, progressively reducing the thickness of the layer of seawater travelling along the flow paths and thereby promoting evaporation of the water. This process causes an increase in the concentration of salinity on the two opposing sides of the membrane 102. Without being bound by theory, it is believed that this creates an osmotic pressure gradient across the membrane 102. The fibre structures at 90 degrees to the axis of symmetry of the membrane 102 provide lateral pathways that convert the osmotic pressure gradient into opposing force vectors that overcome the surface tension of the seawater to pull apart neighbouring water molecules and thus promote "super evaporation" of the water.

A photographic enlargement of the knitted membrane material showing one of the openings and its surrounding knitted structure is shown in Fig. 3. It will be seen that the weave is in fact a structure formed by knitting together multiple individual polyester fibres, with the warp knitted structure providing a multitude of smaller openings which may also facilitate evaporation. Figure 4 is similar photographic view showing an opening at the lower edge of the membrane towards one side thereof after the apparatus has been in use for 24 hours and salt has accumulated.

According to a method of the present invention, seawater is delivered at the top of the membrane 102 and continuously flows from the top to the bottom, driven by gravity. As the seawater flows down the membrane 102, the angular inclination of the fibrous structures relative to the vertical directs the encircling seawater along flow paths. There is no surface attraction between the seawater and the fibrous structures. The relationship can be visualised as forming a sleeve of liquid that is driven by gravity, and continually travels along the structural guide of the woven fibres.

While osmosis normally happens in a passive environment at a single interface (i.e. a semi-permeable membrane) between two columns of water of different salinity, it is thought that the structured membrane 102 of the present invention, acting in a single vertical plane, creates many interfaces of seawater flowing along angled pathways of different lengths, immediately alongside each other, across the surface of the membrane 102. These discrete bands of water move at different rates, driven by gravity, as seawater of a constant salinity is delivered down the middle of the membrane 102, so that they are proportionally divided either side of the vertical.

Figure 5 shows a representation of the distribution of salinity across the membrane 102. To create opposing vector forces 302, the fibrous structure is designed to have an axis of symmetry aligned to the vertical, in order to be able to use gravity to drive the system. Seawater of a nominal level of salinity is supplied to the middle of the membrane 304, where the thickness of the layer of seawater is greater and the normal evaporation rate is low. As the seawater travels from the middle 304, along the angled fibre structures of the membrane 102, towards the angled sides of the membrane 306, the layer becomes thinner due to normal evaporation over a distance from the source.

As diagrammatic ally shown in Figure 5, the resulting difference between the salinity in the middle of the membrane 304 and its opposite angled sides 306 creates balanced, opposing osmotic pressure differentials across the horizontally opposed structures of the membrane 102, which can exert opposing force vectors 302 to pull apart neighbouring molecules of water from each other. The process starts at the top of the membrane 102, where the diverging pathways are shorter, and progresses down towards the bottom where the divergent pathways are longer. The "super evaporation" process begins when the osmotic pressure becomes great enough to overcome the hydrophilic attraction of water molecules to each other. The process becomes self-sustaining due to precipitation of the salt ions travelling to the angled sides of the membrane 306, which maintains the mineral concentration gradient.

Due to its hydrophobicity, the membrane 102 cannot prime itself to initiate the flow of water to begin the separation process. Priming is therefore achieved by forming the laminate in contact with the upper region 110 of the membrane 102 using a hydrophilic material, in this case the glass of the reservoir 104, as shown in Figure 1. Lamination of the hydrophobic and hydrophilic materials enables seawater to be siphoned from the seawater reservoir 104 and create a hydrostatic head at the upper region 110 of the membrane 102 that is directly proportional to the vertical length of the laminated part of the membrane 102. The amount of hydrostatic pressure can be controlled by varying the height of the contact between the hydrophobic surface and the hydrophilic surface of the glass wall of the reservoir 104. The level of seawater in the reservoir 104 must be slightly lower than the top of the membrane 102, so that the water in the reservoir 104 does not exert too high an external hydrostatic pressure on the membrane 102 and thus prevent siphoning through the laminate.

As the membrane 102 is primed with seawater, the hydrostatic pressure formed in the laminate at the upper region 110 of the membrane 102 causes the seawater to flow down the membrane 102 and enables continuous siphoning from the reservoir 104. Seawater is maintained in the reservoir at a constant height.

In an alternative embodiment (not shown), the reservoir 104 is made of a material such as plastic or metal and the hydrophilic material is provided as a separate layer of material. In this case, the layer of hydrophilic material may be sewn together with the membrane 102 to ensure good contact.

In certain embodiments (not shown), several membranes 102 can be arranged in parallel with each other around the reservoir 104 to form an efficient, compact seawater membrane battery system. The evaporated water can be collected in a condenser (not shown) and a separate supply of seawater can be used as a cooling water supply for the condenser. The system can be fully automated to provide a continuous process to remove the sodium chloride and other minerals from the seawater, at the same time as the process of collecting the evaporated water into a condenser. Moreover, the cooling effect of seawater can be used to facilitate the removal of heat from the water vapour collected in the condenser. This system offers greater flexibility in designing for large plants or for small modular systems due to the compactness of the design of the present invention.

Although not bound by this theory, it is believed that the material of the membrane 102 is hydrophobic and does not have a wicking action with seawater against gravity. It has a structural design and shape which combine together to create flow paths for seawater, along the weave, of progressively increasing lengths. In use, a layer of seawater encloses the fibrous structure— not because of an affinity between the seawater and the fibres, but due to the surface tension of the seawater and the surrounding atmospheric pressure, which maintains the contact between the fibres and the layer of seawater.

The membrane structure is believed to cause the feed water to be distributed in such a way as to create "osmotic forces" to be generated across the surfaces of the fibrous membrane. The feed water travels down both sides "front and back", comprising the sheet of the support membrane and not through it. Although air can move through the holes in the support membranes surface to help facilitate the removal of water vapour that is released by the "super evaporative" means that is driven by osmosis. In embodiments, the support membrane is vertically aligned so that gravity can be used to the greatest effect, to drive the feed water flowing down, across, and on both sides back and front, of the membrane. The support membrane in receiving the feed water at the top of its surface directs and divides the flow in such a way, as to create opposing force vectors that are created by the effect of osmosis acting upon the feed water being continuously supplied to the support membranes surface.

"Super evaporation" of the seawater is believed to be initiated by the "normal evaporation" process that applies to volatile liquids at atmospheric pressure. The rate of evaporation from the surface of a fibrous structure can change over distance, by progressively reducing the thickness of a layer of seawater on the fibrous structure and exposing more surface area to evaporation. The main body of seawater being supplied to the middle of the membrane forms a thicker layer with a smaller exposed surface area, and thus experiences a lower rate of evaporation. The effect of this is that the thinner layer of seawater has a proportionally higher level of salinity than the thicker layer.

In early experiments, a quick result was desired to test the concept. The initial objective was to focus on "seawater" and so to that end a saline solution was made by adding table salt to tap water. At this point it was not certain whether there would be a discernible effect over such a small area of the membrane at 90cm², so it was decided to make a solution of very high near saturated salinity. Also, a long thin wick 2.6mm diameter was fitted horizontally across the bottom of the membrane 102, the purpose of which was to remove highly salinized water produced by the process so that salt could crystalize away from the membrane. In an initial run the reservoir 104 was fully loaded and left overnight. The next day there were clear signs that salt had crystalized at the bottom corners and sides of the membrane 102. Water dripped from the middle of the membrane with a much lower salt cotent than att he lower corners of the membrane. In a further run, the reservoir 104 was topped up with highly salinized water and the apparatus was weighed, after which saline water was run for 20 hours with maintenance of water in contact with the syphonic region 112 of the membrane and the apparatus was re-weighed. It was found that the rate of evaporation achieved from the membrane was about 30g per 24 hours representing water vapour that had evaporated from the membrane 102, the rate of weight loss being substantially constant with time.

It was appreciated that the rate of delivery of saline water to the top of the membrane could be limiting the rate of evaporation achieved. The rate of throughput of saline water to the membrane 402 was increased using glass fibre wicks to provide they syphonic action and giving an evaporation loss of about 66g per 24 hours with the same membrane area. In further embodiments, to improve water delivery to the membrane a larger reservoir of plastics material was employed and as shown in Fig. 6 a strip 404 of cellulosic or other hydrophilic material is attached to membrane 402 as previously described, the attachment being a line of stitching 406. In use, the strip 406 is immersed in the reservoir 104, taking the positions previously occupied by immersed end region 112 and end region 110, and providing sy4phonic delivery to the membrane 402 at an enhanced flow rate. At the base corners of the membrane 402, attached long wicks of hydrophilic material 408 leads concentrated saline water from the membrane, whilst still collecting less concentratedly saline water from the middle of the membrane. The benefits of this change are expected to be increase the capacity / performance for a membrane of a given area.

Desalination apparatus 500 is shown diagrammatically in Fig. 7. Membrane 502 of polyester mesh generally like that described in previous embodiments but at least of increased area is supported vertically and fed at its top with seawater or other brine from supply 504 through tap 506. The apparatus also includes a refrigeration circuit 508 connected to electrical power supply 510 via switch 512. It includes fan 514, refrigerant evaporator 516 and refrigerant condenser 518. Potable water condensed by contact with the relatively cold evaporator 516 is collected at drain 520 and discharged at line 522. Concentrated saline water reaching the bottom of the membrane 502 is collected at drain 525 and is discharged at line 528. Fan 514 circulates air through the apparatus 500, air warmed by condenser 518 circulating as indicated by arrows 524a, 524b to one side of membrane 502, and water-laden air which has passed through the membrane 502 recirculating to evaporator 516 and fan 514 as indicated by arrows 524c and 524d. Evaporation of water from membrane 502 is now no longer in still air but is in the air current indicated by arrows 502, 504 so that higher evaporation rates can be achieved. Heat from the circulating air 524b replaces the heat loss at membrane 502 from water evaporation and maintains the membrane 502 at preferably at least ambient temperatures and more preferably at an elevated temperature to further promote evaporation of water vapour from the membrane 502, the temperature within the apparatus conveniently being between ambient temperature and water boiling temperature. The saturation vapour pressure of water at 15° is 12.8 mm, at a normal room temperature of 20°is 17.5 mm Hg, warmed to 30°C is 32 mm Hg, warmed to 40°C, further warmed to 50°C is 92 mm Hg, and yet further warmed to 70°C is 238 mm Hg. It will be appreciated that operation of the apparatus with even mildly elevated temperatures at membrane 502 and downstream thereof significantly increases the rate of water evaporation and the ability of the airstream to carry the water vapour evaporated from the membrane assuming that relative humidities >,30% and preferably > 60-80% RH resulting in significant improvements in the possible throughput of the apparatus and the rate of production of desalinated water.

Although the apparatus is described in relation to the purification of seawater, any source of water can be successfully treated providing the mineral content is sufficient to effect the osmosis/evaporative processes. Where the source water has a low mineral content (e.g. municipal fresh water), minerals such as salt can be added to the reservoir.

In a further embodiment illustrated in Fig. 8 the openwork sheet is used as the evaporative element of an evaporative cooler. A cooler housing 600 has a vertical membrane 602 of the fabric described above which acts as a cooling pad. A water reservoir 604 is filled with water 606 from which pump 618 pumps water via conduit 620 to an outlet at the upper edge of the membrane 602 to maintain it in a moist state. Air 610 enters housing 600 through inlet 608 and is pumped by fan 618 through the membrane 602 to outlet 614 from which it emerges as cooled stream 616. Because of the rapid evaporation provided by the present polyester openwork sheet, good cooling is achieved. If desired, more than one membrane 602 may be provided, arranged in series for successive cooling of the air passing through them. The temperature of dry air drops significantly through the transition of liquid water to water vapor which is essentially evaporation. This process can cool air with much less energy than with refrigeration, and the present evaporative cooler may be even .

In an alternative embodiment, the reservoir could be located towards the top end of the membrane, and the openwork membrane(s) could be fed with water symphonically as described above.

Claims

1. A water evaporator including:

an openwork sheet of hydrophobic fibrous material having upper and lower edges and openings defining flow paths that are inclined with respect to a vertical axis of the sheet so that water flowing continuously along the flow paths moves both down and across the sheet; and

a supply for the water containing the dissolved minerals leading to the upper edge of the openwork sheet.

2. The evaporator of claim 1, wherein the openings occur in a pattern configured to direct water flow towards lateral edges of the sheet.

3. The evaporator of claim 1 or 2, wherein strands surrounding and interconnecting the openings comprise a multiplicity of filaments providing flow paths and smaller openings for promoting evaporation.

4. The evaporator of claim 1, 2 or 3, wherein openings in the sheet occur in diagonally offset rows.

5. The evaporator of any preceding claim, wherein the openings in the sheet are generally polygonal or oval with a largest dimension coinciding with the vertical axis of the sheet.

6. The evaporator of claim 5, wherein the largest dimension of the openings is about 3 mm.

7. The evaporator of claim 6, having a pattern repeat of about 7 mm.

8. The evaporator of any preceding claim, wherein sides of the sheet flare downwardly and outwardly.

9. The evaporator of claim 9, wherein the sheet has a length greater than its width and the sides diverge at an angle of 10-30°.

10. The evaporator of claim 9, wherein the sides diverge at about 20°.

11. The evaporator of any preceding claim, wherein the sheet is vertically directed.

12. The evaporator of any preceding claim, wherein the sheet is a woven mesh.

13. The evaporator of any of claims 1-11, wherein the sheet is a warp-knitted mesh.

14. The evaporator of claim 13, wherein the sheet is of raschel warp-knitted mesh.

15. The evaporator of any preceding claim, wherein the sheet comprises >50% polyester fibres.

16. The evaporator of claim 15, wherein the sheet comprises >75% polyester fibres.

17. The evaporator of claim 15, wherein the sheet comprises 100% polyester fibres.

18. The evaporator of any preceding claim, further comprising pumping means for delivering water to the sheet.

19. The evaporator of any of claims 1-17, further comprising syphonic means for delivering water to the sheet.

20. The evaporator of claim 19, further comprising a water reservoir located adjacent an upper region of the sheet.

21. The evaporator of claim 20, wherein the upper region of the sheet overlaps a rim of the reservoir and contacts water within the reservoir.

22. The evaporator of claim 20, wherein a hydrophilic material is provided directly adjacent to the upper region of the sheet, and the hydrophilic material contacts the water in the reservoir and provides a syphon for water from the reservoir that leads to the upper region of the sheet.

23. The evaporator of any preceding claim, further comprising a drain for collecting water flowing from the lower edge of the sheet.

24. Desalination apparatus comprising an evaporator as defined in any preceding claim for evaporating saline water, a condenser for condensing water vapour from the evaporator, a heater for heating air from which water vapour has been condensed and a flow path including a fan for directing air through the sheet of the evaporator, directing air through the condenser and through the heater, and redirecting warmed air of reduced water vapour content to the evaporator.

25. The apparatus of claim 24, wherein the condenser and heater form components of a refrigeration circuit.

26. A method of desalinating water, comprising:

supplying water to be desalinated to an upper edge of an openwork sheet of hydrophobic fibrous material having upper and lower edges and defining flow paths that are inclined with respect to a vertical axis of the sheet so that the saline water flowing continuously along the flow paths moves both down and across the sheet, directing air through the openwork sheet;

recovering desalinated water from the condenser.

27. The method of claim 26, further comprising recovering water of increased saline content from the lower edge of the sheet.

28. The method of claim 26 or 27, further comprising recovering solidified salt from the sheet.

29. An evaporative cooler comprising a housing, an air inlet to the housing, one or more evaporative pads within the housing, a reservoir for water, a pump arrangement or syphon for supplying water to the or each evaporative pad and an air outlet, and a fan for drawing air into the housing and impelling it through the or each evaporative pad and through the outlet, wherein the or each evaporative pad comprises an openwork sheet of hydrophobic fibrous material.

30. The cooler of claim 29, wherein the openings occur in a pattern configured to direct water flow towards lateral edges of the sheet.

31. The cooler of claim 29 or 30, wherein strands surrounding and interconnecting the openings comprise a multiplicity of filaments providing flow paths and smaller openings for promoting evaporation.

32. The cooler of claim 29, 30 or 31, wherein openings in the sheet occur in diagonally offset rows.

33. The cooler of any of claims 29-32, wherein the openings in the sheet are generally polygonal or oval with a largest dimension coinciding with the vertical axis of the sheet.

34. The cooler of claim 33, wherein the largest dimension of the openings is about 3 mm.

35. The cooler of claim 34, wherein the sheet has a pattern repeat of about 7 mm.

36. The cooler of any of claims 29-35, wherein the sheet is of raschel warp- knitted mesh.

37. The cooler of any of claims 29-36, wherein the sheet is of polyester fibres.

38. A method for evaporative cooling comprising

supplying a housing and one or more evaporative pads within the housing; maintaining a supply of water to the evaporative pad(s);

maintaining a forced flow of air into the housing, through the evaporative pad(s) and through an outlet from the housing; and

either recovering cooled ventilation air and/or recovering cooled water; wherein the or each evaporative pad comprises an openwork sheet of hydrophobic fibrous material.

39. The method of claim 38, wherein the or each evaporative pad has the features of one or more of claims 30-37.