BACKGROUND TO THE INVENTION Various methods can be employed to desalinate sea water. One such method is to pump sea water at high pressure through a reverse osmosis membrane, or a plurality of reverse osmosis membranes. These membranes become saturated with salt and other contaminants and have to be purged with chemicals. The additional expense and the down-time required while the membranes are being purged make the water being recovered too expensive for most purposes.

Another method is to heat the sea water and condense the resulting water vapour. This method consumes a large amount of energy, as well as the disadvantage that heating of sea water increases its corrosive properties. Heating of the sea water also results in an increase of scaling which necessitates the periodical shutting down of the installation for de-scaling, which is both an expensive and time- consuming process.

The above methods require large amounts of energy which result in high costs to the consumer. These methods are therefore uneconomical in third world countries and uneconomical for farming and agricultural usage.

In order to overcome the difficulties encountered with the above- mentioned methods, a method which involves cooling and subsequently freezing the sea water rather than heating it, has been proposed. This method is more successful at inhibiting the corrosive and scaling properties of the sea water than the method of heating the sea water. Furthermore, less energy is required to cool the sea water to below freezing than it takes to raise the temperature thereof to boiling point.

The freezing of sea water results in the formation of a sludge, which comprises approximately 80% ice crystals, containing pure water, and 20% concentrated brine. However, the difficulty in the past has been in finding a suitable method of separating the ice crystals from the brine in order to harvest the ice crystals containing desalinated water.

One such method employed is to use a gravity-fed rising column so that the sludge within the column essentially comprises a layer of ice crystals located on top of a layer of concentrated brine. A fine spray of water at the top of the column assists in washing off any brine that has adhered to the ice crystals.

Thereafter, ice crystals free of brine can be raked off the upper surface of the column. However, this method is inefficient and can result in a large quantity of the ice crystals being washed away with the brine. Furthermore, the build up of ice crystals on components used to cool down the sea water, slows down the freezing process and inhibits the formation of the sludge.

The present invention therefore seeks to provide an improved process for forming of the sludge and for separating of the resultant ice crystals from the concentrated brine.

It is a further object of the present invention to provide a cost and energy efficient installation for such a process, so that the resultant desalinated water can be used economically, especially for farming and agricultural purposes.

BRIEF DESCRIPTION OF THE INVENTION According to one aspect of the present invention there is provided an installation for desalinating sea water, said installation comprising means for reducing the temperature of the sea water to form a mixture of ice crystals and concentrated brine, a centrifuge for separating the ice crystals from the concentrated brine and means for washing the residual concentrated brine from the ice crystals.

According to a further aspect of the present invention there is provided an installation for desalinating sea water, said installation comprising means for generating power to drive said installation, means for reducing the temperature of the sea water to form a mixture of ice crystals and concentrated brine, a centrifuge for separating the ice crystals from the concentrated brine and means for washing the residual concentrated brine from the ice crystals.

The means for generating power can comprise a sea-swell activated pump for providing a supply of water under pressure.

The means for generating power can further include a turbine to which the supply of water under pressure is fed and an electrical generator driven by the turbine.

The means for generating power can comprise a sea-swell activated air compressor for providing a supply of air under pressure.

The means for generating power can further include an air turbine to which the air under pressure is fed and an electrical generator driven by the turbine.

The centrifuge has a wall which is of downwardly converging conical form and which has an upper rim, there being holes in said wall through which, when said centrifuge is rotated, the concentrated brine passes, the ice crystals passing over the rim of said centrifuge to said washing means.

The washing means can comprise a conveyor, said conveyor having holes therein to allow brine to pass therethrough whilst retaining the ice crystals.

The washing means can further include water spray means for washing the brine from the ice crystals on the conveyor.

The conveyor can be inclined and there can be means for vibrating the conveyor so that the vibrating action assists in moving the ice crystals up the incline of the conveyor.

The installation can include means for removing ice build-up on the exterior of a component of said temperature reducing means. Said component being a refrigeration expansion unit located within a refrigeration unit.

Said removing means can comprise of a plurality of rings which encircle said refrigeration expansion unit, said rings being spaced apart along the length of said unit, and activating means for moving said unit back and forth relative to one another.

Said actuating means can comprise a pneumatic cylinder, the piston of which is attached to said unit via a first rod, a second rod and a swivel joint located between said first and second rods.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- Figure 1 is a schematic layout of an installation, powered by electricity, in accordance with the present invention, Figure 2 is a schematic layout of an installation, powered by compressed-air, in accordance with the present invention, Figure 3 is a schematic layout of an installation, powered by water, in accordance with the present invention; Figure 4 is a front elevation of a refrigeration expansion chamber which is part of the refrigeration unit of the installations of Figures 1 to 3; and Figure 5 is a section of Figure 4 along the line V-V.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Figure 1, an installation which is powered by electricity is illustrated. The installation is generally designated 10.

Water, either fresh or sea water, stored in a circulating tank 12 is pumped by means of sea-swell activated pumps 14 to a turbine 16 which drives an electrical generator 18. The flow-rate and pressure of the water pumped from the pumps 14 to the turbine 16 is regulated by a flow-rate control valve 20 and a pressure control valve 22 respectively.

The electricity generated by the generator 18 is used to drive pumps 24 and 26. The electricity is also used to power a refrigeration unit 28, an extractor fan 30 and a centrifuge 32 via electric motors 34,36 and 38 respectively. An inclined vibrating conveyer 40 also receives power from the generator 18.

A sea-swell pump 42 is used to pump sea water from the sea which is, for example, at a temperature of between 15°C and 20°C, into the coil of a vessel 44 which contains ice crystals. The sea water melts the ice crystals and, as a result, the sea water exits the coil of the vessel 44 at a temperature of between 10°C and 15°C.

The sea water enters the coil of a heat exchanger 46 where heat exchange occurs with concentrated brine, which enters from the centrifuge 32, at a temperature of between 0°C and 2°C. This results in the sea water being cooled to a temperature between 5°C and 10°C before it exits the heat exchanger 46 and enters the refrigeration unit 28. The concentrated brine, as a result of heat exchange occurring with the sea water, has its temperature raised to between 5°C and 10°C. The concentrated brine is pumped back into the sea by the pump 26.

The sea water passes through the refrigeration unit 28 and is cooled to a temperature of between-1. 6°C and-1. 8°C before entering an ice-formation vacuum chamber 48 which is maintained at sub-atmospheric pressure by the extractor fan 30. The sea water under pressure enters the chamber 48 through spray nozzles (not shown) which, together with the sub-atmospheric pressure of the vacuum chamber 48, serves to cool the sea water still further to a temperature of between-1. 6°C and-2. 3°C. At this temperature, the sea water does not freeze, but forms a sludge consisting of ice crystals, comprising pure water, and concentrated brine. The ratio of ice crystals to brine is approximately 8: 2.

The sludge exits the ice-formation chamber 48 via a gravity feed pipe 50 and enters the centrifuge 32.

The centrifuge 32 is conical in shape and has holes (not shown) in its inclined surface. The rotation of the centrifuge 32 causes the concentrated brine to exit the centrifuge 32 via the holes and enters a vessel 52. From the vessel 52, the concentrated brine passes through the heat exchanger 46 and is pumped into the sea by the pump 26.

Furthermore, the rotation of the centrifuge 32 causes the ice crystals to rise up the inclined surface thereof, pass over the rim and onto the inclined vibrating conveyer 40. Fine spray nozzles 54 are positioned above the conveyor 40 so that, as the ice crystals rise up the conveyor 40, water sprayed from the nozzles 54 washes the ice crystals and removes any remaining brine adhering thereto. Holes (not shown) in the surface of the conveyor 40 allows the mixture of brine and water to pass therethrough and run down an inclined surface 56 and into the vessel 52, where it mixes with the brine from the centrifuge 32.

The water supplied to the nozzles 54 can initially be in the form of fresh or distilled water until sufficient desalinated water has been produced and stored.

Thereafter, a supply of desalinated water can be fed to the nozzles 54.

The washed ice crystals on the inclined surface pass over the upper edge of the conveyor 40 and into the vessel 44 via a pipe 58.

Cold air and water vapour drawn off the chamber 48 by the extraction fan 30 is fed, via a pipe 60, into the vessel 44 via moisture traps (not shown). A pressure above atmospheric pressure is maintained in the vessel 44 and this assists in melting the ice crystals. The resultant desalinated water is pumped by the pump 24 to a storage tank 62.

Referring now to Figure 2, an installation which is powered by compressed-air is illustrated. The installation is generally designated 66. Where applicable, the same reference numerals of Figure 1 have been used.

Under the wave action of the sea, air-compressors 68 draw air in through an air intake 70. The air is compressed by the compressors 68 and the resultant compressed air is pumped into a storage tank 72. The compressed air is used to drive pumps 24,26, 42, the refrigeration unit 28, the extractor fan 30, the centrifuge 32 and the inclined vibrating conveyer 40 via air turbines 74,76, 78,80, 82,84 and 86 respectively.

The flow-rate and pressure of the compressed air used to drive the installation 66 is regulated by flow-rate and pressure control valves 88.

The installation 66 desalinates sea water in the same way as for the installation 10.

Referring now to Figure 3, an installation which is powered by water is illustrated. The installation is generally designated 92. Where applicable, the same reference numerals of Figure 1 have been used.

Water, either fresh or sea water, stored in the circulating tank 12 is pumped by means of the sea-swell activated pumps 14 to drive the installation 92.

The flow-rate and pressure of the water pumped from the pumps 14 is regulated by the flow-rate and pressure control valves 20,22 respectively.

The water is used to drive pumps 24,26, 42, the refrigeration unit 28, the extractor fan 30, the centrifuge 32 and the inclined vibrating conveyer 40 via water turbines 94,96, 98,100, 102,104 and 106 respectively.

The installation 92 desalinates sea water in the same way as the installations 10 and 66.

Referring now to Figures 4 and 5, the refrigeration unit 28 of installations 10,66 and 92 comprises a compressor (not shown), a motor (not shown) for running the compressor and a refrigeration expansion unit 108, which is located within a chamber (not shown) through which the sea water passes. A gas inlet and a gas outlet for the refrigeration expansion unit 108 are designated 110 and 112 respectively.

A plurality of rings 114 are each attached via a pair of bars 116 to support struts 118, which are secured to the sea water chamber (not shown). The diameter of the each ring 114 is slightly larger than the diameter of the unit 108. A pneumatic cylinder 120 having a rod 122 protruding from the piston (not shown) is connected to the unit 108 via a swivel joint 124 and a rod 126. A supply of compressed air for the piston 120 is designated 128.

During the desalination process, the formation of a layer of ice crystals on the exterior of the refrigeration expansion unit 108 inhibits the ability of the unit 108 to cool the sea water passing through the chamber down to the required temperature for the formation of the sludge. To periodically remove this layer of ice crystals from the exterior of the unit 108, the piston 120 is activated, causing the piston rod 122 to extend and retract and thus moving the unit 108 up and down, as indicated by the arrow A in Figure 4. This movement of the unit 108 allows the ice crystals to be"scraped off"by the rings 114.

Applicant believes that the process as described and an installation for implementing such a process as described, allows sea water to be desalinated and allows such desalinated water to be economically viable for domestic, industrial and agricultural purposes, due to the low energy requirements of the process and the fact that the energy required is obtained from a freely available natural resource, that being the action of sea-swell.