9.4 Flash type evaporators
Flash evaporators have been available for many years. They have only emerged within the last two decades as a viable proposition for large scale distillation, due to the fact that multiple-effect or multistage plant can now be built in the very large sizes required for potable water in arid countries.
Thanks to modern methods of raw water pre-treatment they can also operate continuously for long periods.
Large multistage evaporators can be built economically on the 'egg crate' principle, each stage consisting of a narrow vertical compartment in the crate with raw water on its way to the heat input section traversing each compartment in straight tubes. Figure 3.69 illustrates the construction of an experimental 10-stage flash evaporator which has been in service with the CEGB.
Since heat consumption varies inversely with the number of stages, the flash evaporator can compete successfully with other forms of evaporation plant by having multiple compartments or stages in a single vessel. However, even this economic form of water treatment plant is not as cost effective as a modern polishing plant for the production of boiler feedwater and no further installations are planned.