9.2 Types of bled-steam evaporator
Two types of evaporator have been used by the CEGB (discounting the horizontal submerged coil type as inappropriate for large modern power station feed-water treatment).
The first is the surface 'Vertical-tube' type invented by Kestner in 1909. Figure 3.65 shows the arrangement of a typical vertical-tube evaporator. The water inside the tubes is boiled by the heating action of the bled-steam on the outside of the tubes and a mixture of vapour and water is expelled from the top of the tubes. The water spills into the central downcomer and the process continues by natural circulation. The rising vapour passes through baffling to reduce the quantity of entrained droplets thrown up by ebullition and is then condensed in an external vapour condenser. Any non-condensable gases carried out with the vapour are vented.
The second type is the 'Flash' evaporator. A schematic diagram of a two-stage flash evaporator is shown in Fig 3.66. The raw water is heated near to saturation temperature of the heating steam and is then cascaded through the evaporator in series, which is at a lower pressure than the water. A proportion of the hot raw water flashes in each stage and is condensed by the raw water on its way to the heat input section. The distillate cascades from stage to stage and is finally removed by means of an extraction pump.