Spray generation

Provided that salt residues are of sufficient size to achieve nucleation, it is the number of drops rather than the mass of spray which matters. The aim is a monodisperse spray with a diameter of 0.8 |xm but with the option for some controlled diameter variation. Spinning discs, ultrasonic excitation of Faraday waves and colliding jets of high-pressure water/air solutions have all been studied. The final choice uses silicon micro-fabrication technology. A hexagonal array of 1483 submicron holes will be etched through an 8-^m layer of silicon to meet a 50-^m hole through the thickness of a 0.5-mm wafer. This will be repeated 1345 times within the area of a 3.2-mm hole in a Yokota YST130N stainless steel disc. This hole will be one of 499 spread across a 100-mm wafer to give nearly 109 micro-nozzles in each of the 18 wafers of a spray vessel. Ultrasonic excitation can be used to reduce drop size from the value predicted by Rayleigh (1878). The silicon will be protected by an oxide layer sealed with vapour-deposited Parylene. Energy losses are dominated by the viscosity through the 8-^m layer. The nozzle banks must be drenched in desalinated water when the system is idle.

A weakness of the micro-nozzle approach is that particles much smaller than a nozzle can form an arch to clog it. Fortunately, the need to remove viruses from ground water for drinking purposes has produced a good selection of ultrafiltration products that can filter to a better level than is needed. Suppliers guarantee a life of 5 years provided that back-flushing can be done at the right intervals. Each rotor has a Grundfoss down-hole pump that feeds 17 m3 s-1 to a bank of eight filters with blister valves to allow any filter to be back-flushed. Norit X-flow filters have an excellent record for pre-filtration in reverse osmosis desalination plant (van Hoof et al. 1999). A trash-grid made from titanium mesh will prevent jelly fish and plastic bags from jamming the pump. If it is fed with a current of 90 A, it can also produce 2 ppm electrolytic chlorine to prevent biological growths.

Electrical energy for spray and rotor drive will be generated by a pair of 2.4-m diameter axial-flow turbines on either side of the hull as shown in Figure 11.10. These are very much larger than any propellers needed for a vessel of this size but can act as propellers for 10 hours in windless conditions using energy from a bank of Toshiba SCiB batteries. The vessels will also carry a liquid-cooled version of the Zoche ZO 01A radial diesel aero engine to give trans-ocean range in emergency. The turbine rotation speed will be limited by cavitation to approximately 80 rpm. This is fast enough for the use of polyphase permanent-magnet rim generators built into the turbine ducts. Tiles of neodymium-boron magnets will be moved past wet

Figure 11.10 A cross'

¡-sectional view through rotor and turbines.

Figure 11.10 A cross'

¡-sectional view through rotor and turbines.

printed-circuit pancake stator windings sealed in glass-flake epoxy Parylene. The axial thrust on the rotor is taken by a pair of 45° SKF spherical roller thrust bearings aft of the rotor and an SKF CARB bearing at the forward end.

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