Taking advantage of 3-in-1 plate technology

Economic and environmental benefits as well as space savings are promised from a new Alfa Laval freshwater generator requiring only half as much sea water as other designs

Aspecialist in developing vacuum desalination equipment, Alfa Laval introduced the first plate-based freshwater generator in 1955, achieving a significant reduction in space requirements and advances in operating efficiency and maintenance. Its latest AQUA generator is claimed to offer further performance, installation and operational benefits for yards and shipowners over existing designs.

A 3-in-1 plate technology is adopted for the first time, enabling the system to handle the entire desalination process with a single plate pack using only one type of titanium plate. The result is a more compact generator with half the seawater requirements of other designs and which provides fresh water with a guaranteed maximum of 2 ppm salt and other dissolved solids in all conditions.

Unlike most plate-based freshwater generators, which house two separate plate packs within a large vacuum chamber, the AQUA system uses a single plate pack to contain the process vacuum. Evaporation, separation and condensation processes are handled by the 3-in-1 technology.

Apart from eliminating the need for an outer shell – thus saving weight and reducing the footprint –the technology is said to underwrite an optimised process with fluid distribution allowing the flow of sea water to be reduced by half. Limited sea water requirements allow smaller pumps to be installed, thus reducing electricity consumption and operating costs as well as CO₂ emissions.

Minimal maintenance and service inspections are fostered by the corrosion- and erosion-resistant titanium plates, while optimised distribution of fluid across the plates eliminates dry spots and inhibits natural scaling. The plate pack slides open for easy access to the interior.

Feed water taken from the flow of seawater coolant enters the lower (evaporator) section of the plate pack, in which the plates are warmed by the heating medium. The water is evaporated at around 40-60oC in a vacuum of 85-95 per cent which is maintained by the brine/air ejector.

The vapour produced rises between the plates into the middle (separator) section of the plate pack, where any droplets of entrained sea water are removed. Gravity causes the droplets to fall back into the brine sump at the bottom of the generator.

Only clean freshwater vapour reaches the top (condenser) section of the plate pack, which is cooled by a flow of sea water. Here the vapour is condensed into fresh water, which is pumped out of the generator by the freshwater pump.

A flexible number of titanium process plates in the plate pack are suspended within a frame comprising a carrying bar, frame plate and pressure plate. Among the features of the plate pack are gaskets with rubber flaps that indicate correct plate assembly, and distance pipes that ensure proper plate alignment and correct tightening.

Connected to the plate pack is a combined system for feed water, condenser cooling water and ejector water. The freshwater system consists of a freshwater pump and a freshwater control sensor that ensures a stable outgoing flow.

Compact installations are fostered as there is no need for an extra service area; and all sections can be accessed from within the generator footprint. The heating medium can be engine jacket cooling water, a hot water loop or a closed circuit heated by steam.

An ejector pump supplies seawater coolant for the condenser, feed water for evaporation and water for the combined brine/air ejector. The pump is installed separately and connected to its own seawater intake. The fresh water produced by the generator is pumped into the storage tank by a built-in pump.

A control panel, incorporating motor starters and a salinometer, supplies electrical power to the ejector and freshwater pumps, as well as control voltage to the salinometer and dump valve. The salinometer system continuously monitors the quality of the outgoing fresh water.

A capacity range from 3.1 to 60 m³/24h – depending on the heating medium and cooling water temperatures – is covered by four models in the series (AQUA-65, 80, 100 and 125). The generators can be dimensioned to suit any jacket water temperature from 55-95oC at any cooling water temperature. MP