A new multiple-effect distiller system with compact heat exchangers

Abstract One of the goals of development in desalination distillation is to reduce the dimensions of the heat exchangers. Particularly in multiple-effect distillation the size of the evaporator tube bundle affects other design aspects like shell configuration, film generation, height of a stack-mounted unit. Heat transfer areas are essentially determined by energy and fouling considerations. All previous attempts to reduce sizes by sacrificing the smooth tube design had to be abandoned in large scale plants for cost reasons. Some years back DVT developed a compact plate-type heat exchanger — the “bavex” heat exchanger. The plates of this heat exchanger are corrugated and fitted together such as to make it particularly suited for film evaporation. The heat transfer coefficients are higher than with tubes and the evaporator volume per unit heat transfer area is less than 60% of the horizontal or vertical tube film evaporator. The standard plate material for desalination application is titanium. The “bavex” heat exchanger was adopted to provide compact evaporator cells for a low energy desalination distiller. A first standard unit with a capacity of 2500 m 3 /d has now been fully developed and designed. It consists of a 12-effect stack-mounted module in series at the warm end with a 4-effect horizontal package arrangement operating with heat recovery by thermo-compression across these first four effects. With top brine temperatures below 85°C, this configuration achieves a performance ratio of 20. The philosophy of the development was to provide a variable system that can also be operated with low temperature heat recovery or with waste heat from diesel generators or gas turbines. The two modules can also be employed individually with a performance ratio 10. Another configuration which integrates an absorption heat pump into the stack-mounted module can be shown to achieve performance ratios between 20 and 24 with max. brine temperatures below 70°C.