DEMINERALIZERSDOE-HDBK-1018/2-93Miscellaneous Mechanical ComponentsDEMINERALIZERSThe cost of corrosion and radioactive contamination caused by poor water qualityin nuclear facilities is enormous. Demineralizers are an intricate part of waterquality control. The chemical theory of demineralizers is detailed in theChemistry Fundamentals Handbook. This chapter will address the mechanics ofhow demineralizers operate.EO 1.11STATE the purpose of a demineralizer.Purposeof DemineralizersDissolved impurities in power plant fluid systems generate corrosion problems and decreaseefficiency due to fouled heat transfer surfaces. Demineralization of the water is one of the mostpractical and common methods available to remove these dissolved impurities.In the plant, demineralizers (also called ion-exchangers) are used to hold ion exchange resins andtransport water through them. Ion exchangers are generally classified into two groups: single-bed ion exchangers and mixed-bed ion exchangers.DemineralizersA demineralizer is basically a cylindrical tank with connections at the top for water inlet andresin addition, and connections at the bottom for the water outlet. The resin can usually bechanged through a connection at the bottom of the tank. The resin beads are kept in thedemineralizer by upper and lower retention elements, which are strainers with a mesh sizesmaller then the resin beads. The water to be purified enters the top at a set flow rate and flowsdown through the resin beads, where the flow path causes a physical filter effect as well as achemical ion exchange. Single-BedDemineralizersA single-bed demineralizer contains either cation or anion resin beads. In most cases, there aretwo, single-bed ion exchangers in series; the first is a cation bed and the second is an anion bed.Impurities in plant water are replaced with hydrogen ions in the cation bed and hydroxyl ionsin the anion bed. The hydrogen ions and the hydroxyl ions then combine to form pure water.The Chemistry Handbook, Module 4, Principles of Water Treatment, addresses the chemistry ofdemineralizers in more detail.ME-05Rev. 0Page 24
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