9 Evaporators and other means of water treatment
The traditional method of producing high quality water for boiler feed was to distil water by the evaporation of treated raw water. Treated water is boiled in the evaporators by the application of heat in the form of LP bled-steam from the turbine or a steam supply from the auxiliary boiler.
The resultant vapour is condensed and can be used to replenish the reserve feedwater tanks or fed directly to the condenser as a make-up water; however, the lower cost of water treatment by the deionisation method has made it uneconomic to install bled-steam evaporators for the latest plant.
Current practice is to use a chemical raw water treatment plant followed by filters and mixed-bed deionisation plant. For the AGRs and for the projected PWR, a full-flow polishing plant in the feed train is provided to ensure the high quality of boiler feedwater.
A full description of the raw water treatment plant, filters and mixed-bed deionisation plant, and a full-flow condensate polishing plant is given in Volume E. However, to provide a complete description of modern plant, the following section gives details of typical steam evaporating equipment which is currently in use in large CEGB power stations.
To provide the operational water needs of modern power stations, a raw water treatment plant is needed with a daily output of about 3% CMR steam flow. The water treatment plant has also to provide water to meet the additional losses of two-shift operation, the filling of boilers and the feed system after maintenance, and the special needs of chemical cleaning and commissioning.