This invention relates to an improved method of treating water to eliminate undesired solutes and bioforms, and particularly to the treatment of cooling water.

The cooling of a great many devices involves transfer of heat from the device to a flow of water which is later cooled by a contraflowing stream of air. Cooling of the water often takes place in a so-called cooling tower in which the hot water from the device is sprayed or allowed to cascade in a manner that increases its surface area while it falls under the action of gravity into a sump. Cooling takes place through heat exchange with the contraflowing air and by evaporation. The cooled water that accumulates in the sump is subsequently re-used and pumped back into the device to be cooled. In factories and offices the ambient air is often maintained at a substantially constant temperature and humidity by a process called 'air conditioning'. The necessary cooling of ambient air to remove moisture and, when necessary, lower temperature requires a refrigeration plant which cannot operate without cooling for the refrigerant. Except in the case of very small plants external water cooling is used. The temperature of the water used for cooling is often ideal for the growth of bacteria and other bioforms. Sufficient nutrients are usually present for large colonies of pathogenic bacteria to be created and inevitably dispersed in droplets of cooling water. In the event that εuch droplets are ingested through the lungs of humans or animals serious and even fatal infections may result.

It has been proposed that the growth of bacteria and other unwanted bioforms may be eliminated by the addition

of biocidal chemicals to cooling water. This solution to the problem is not totally satisfactory as there is consid¬ erable cost involved. Careful monitoring must be carried out to ensure adequate amounts of biocide are present in the cooling water to prevent bacterial build-up avoiding excessive usage. Many of the biocidal chemicals are highly toxic in their undiluted form, hazardous to handle and their residueε are environmentally undesirable.

A further problem that occurs when treating cooling water is the build-up of solid residues on the interior surfaces of pipes, valves and other water carrying parts of the system. The solids arise from precipitation of insoluble calcium and magnesium compounds which were present initially as soluble salts. Removal of these salts prior to use by ion exchange processes is expensive and increases the total volume of water consumed by the system due to the water used in the regeneration of the ion exchange medium.

The present invention provides a system for treating water which eliminates undesired solutes and bioforms without the use of chemical additives for either purpose and substantially eliminates the build up of solid residues in the system.

According to the present invention there is provided a method of treating water supplied to an evaporative cooler comprising subjecting the water to water conditioning treatment by the application of an electromagnetic radiat¬ ion field, passing the water through a heat exchanger in which it extracts thermal energy from the heat exchanger or fluid passing through it, passing the water to a first dispersion means whereby the water is formed into small droplets which fall under the action of gravity directly or

through one or more baffles to a collection reservoir where the suspended solids are able to settle under the influence of gravity to a sump, returning the supernatant water from the reservoir to the water heat exchanger and passing the sump contents to a series of filters and through an electrolytic biocidal cell to a second dispersion means whereby the water is formed into small droplets which fall under the action of gravity directly or through one or more baffles to the collection reservoir.

The water conditioner is preferably one using an electromagnetic field in the radio frequency band. The power may be varied in accordance with the rate of flow of the water being treated; such water conditioners are described in GB-A-2 246 725 or WO-A-92 00916. The effect of such conditioners is to change certain salts in solution as ions in the water flowing through the conditioner field into colloidal suspensions. It is possible for the suspensions to re-dissolve however by careful manipulation of the conditioned water the suspended particles can be caused to agglomerate and precipitate. In the process of the present invention the solid suspended particles are removed by filtration prior to recirculation of the treated cooling water.

After the water has been conditioned it is passed through a heat exchanger in which the thermal energy from a hot fluid pasεing through the heat exchanger is trans¬ ferred to the cooling water passing through it. Such heat exchangers are well known and their design is dictated by the rate of flow of the two fluids, the contact surface area and the thickness and composition of the material, normally a metal or alloy, from which the exchanger is constructed.

The heated water leaving the heat exchanger is passed to a firεt dispersion means whereby the water is formed into small droplets by spray jets or passage through a perforated pipe. The droplets of heated water fall under the action of gravity directly or through one or more baffles to a collection reservoir. During their fall the droplets lose thermal energy by direct transfer to the air and by partial evaporation. To assist the thermal energy loss the structure in which the droplets fall is formed with an outer shell which entraps the air entering at the lower end so that it rises at ever increasing velocity within a venturi. Such cooling systems are well known and emit a mixture of heated air and water vapour at their upper end giving rise to visible vapour clouds when cooled by the external air. The structureε are often referred to as 'cooling towers'.

Due to the evaporative loεses the solids suspended are concentrated and able to settle, without re-solution, in a sump located at the lower end of a cooling tower. The supernatant water above the sump is passed through a series of filters and through an electrolytic biocidal cell before reuse by pumping to a second disperεion meanε. In the second dispersion means the water is formed into small droplets as described previously and allowed to fall as previously described through one or more baffles to the collection reservoir.

The first filter into which the supernatant water is passed has a coarse structure to remove debris, such as insects, leaves, etc., picked up by the water droplets as they fall through the cooling tower. This is followed by a second filter which removeε smaller particleε such aε aggregated solids released from εolution initially by the water conditioner. Such filter systems are known and in

uεe with εwimming pool cleaning εystemε.

After the filtered water leaves the filter system it enters an electrolytic biocidal cell in which biocidal metal ionε are released into the water. Cells of thiε nature are known in which the electrodeε are formed from an alloy compriεing copper and εilver; the ratio of the metals preferably lies in the range 90/10 to 70/30. In a given installation electrodes of different compositions may be used according to the history of the cooling water in the system. By appropriate construction of the electrode system and control of the magnitude and polarity of the potential applied to the electrodes even wear can be achieved aε the metalε are releaεed into the water passing through. It is possible to monitor the level of released ions downstream of the electrode system and ensure that the concentration is maintained at the desired level. A self- compenεating system providing a regular reversal of electr¬ ode polarity is preferred in which the electrodes are maintained at a preset conεtant potential. Both copper and εilver are toxic to moεt bacteria and fungal species so that any such undesirable life forms picked up during the passage of the water through the cooling tower will be inactivated and, provided sufficient ionε are released into the εystem, bacterial and fungal growth in the exposed areas of the cooling tower and within the fluid system will be prevented.

In order that the invention may be clearly understood, one form thereof will be described with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a water treatment system according to the invention, and

Figure 2 is a cross-εectional view of a biocidal electrode used in the system of Figure 1.

An evaporative cooling system incorporating the water treatment method of the present invention, see Figure 1, consists of a heat exchanger l which is supplied with heated fluid, such as steam or refrigerant condensate, from a thermal engine, not shown. The heated fluid flows through tubes 2 in thermal contact with coolant water in a manifold 3. The coolant water is supplied to the heat exchanger 1 from a reservoir 4 by means of a pump 5. The pumped water passes through a pipe 6 which is fitted with a water conditioner 7. Af er passage through the heat exchanger 1 the water enters a first dispersion means 8 consisting of a pipe 9 carrying a series of dispersion roses 11 placed at the top of an enclosed cooling tower 12. The water leaves the roses 11 in the form of fine droplets which fall on a series of baffles 13 so as to increase the effective path through the cooling area. At the bottom of the tower the droplets accumulate in the reservoir 4.

In order to ensure a steady and rapid air flow through the tower 12 the air that enters the lower end of the tower through apertures 14 is drawn upwards by means of a fan 15 located at the upper end of the tower. A baffle 16 acts as a drift suppressor and prevents water droplets being sucked out of the cooling system. The reservoir 4 is kept at a steady level by the addition of further water from a mains supply controlled by a float valve 17. The lower portion of the reservoir 4 forms a sump 18 where a mixture of water and solids accumulate.

The residue in the sump 18 is sucked through a coarse filter 19 by means of a second pump 21. The output of the pump 21 is directed into a sand filter 22 where the majority of solid particles are removed from the water passing through it. The pump 21 and filter 22 are fitted with a control system, not shown, which causes an

intermittent reversal of flow to prevent the filter 22 becoming clogged. The effluent from the filter 22 flows through an electrolytic biocidal cell 23 which releases copper and silver ions into the water. After the biocidal treatment the water enters a second dispersion means 24 conεiεting of a pipe 25 carrying a series of disperεion roses 26 placed above the first dipersion meanε 8. The water leaves the roses and pasεes through the cooling tower 12, in the manner previously described, and accumulateε in the reservoir 4.

It will be seen that any 'hardness' or dissolved metals, such as calcium and magnesium, from the mains supply water used to top up the reservoir 4 will be converted to a solid suspension before entering the heat exchanger 1 so that the build-up of scale deposits will be prevented. Aε the water iε re-cycled and after filtration any bioformε picked up from the air entering the cooling εyεtem or from other εources are killed or rendered impotent by the copper and εilver ionε from the electro¬ lytic cell 23.

Aεεociated with the water treatment εystem described will be control systemε. The water conditioner 7 will require a control system to alter the magnetic field in accordance with the turbulence of the water pasεing through it if it is of the type deεcribed in EP-0 493 559. The electrolytic cell requireε a control εystem to ensure the correct level of biocidal ions is maintained. Further control systems are required to control the water flow and filtration in accordance with the thermal load placed on the cooling system. The design and operation of such systems is known and the control features may be conven¬ iently incorporated in a suitably programmed microprocessor controller.

The biocidal electrode, see Figure 2, comprises a pipe 31 having a constriction 32 which, in conjunction with a deflector plate 33, ensures that water flowing through the pipe circulates around the electrode εtructure 34. The electrode structure is formed from a number of electrodes of an alloy in which the ratio of copper to silver iε 80 to 20. The electrode εtructure iε held in a branch pipe 35 by meanε of a screw fitted plug 36 sealed with an 'O'ring 37. The rear of the electrode 34 is attached to the plug 36 by means of bolts 38 which also act as terminals for connecting the neceεsary electrical supply to the electr- odeε. As the electrolytic syεtem operateε the electrodeε dissolve and must be replaced at infrequent intervalε. Replacement is carried out simply by unscrewing the plug 36 and the bolts 38.

In a practical test a water treatment installation operating in accordance with the invention was fitted to a cooling tower having a flow rate of approximately 2000 litres per minute and a sump or reservoir of approximately 2000 litres. The water inlet temperature was about 35 degrees Celcius which dropped to 24 degrees on cooling. In spite of the ideal conditions for forming scale in the heat exchanger and for the growth of bioforms neither occurred.