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~ <0 CJl -.:I V VERTICAL TUBE EVAPORATOR-MULTISTAGE FLASH PROCESS The vertical tube evaporator (VTE) distillation process is being considered for the design of the largest seawater conversion plants. There 1s a considerable background of experience on the basic VTE process, "-'I' since it was one of the earliest separation techniques used by industry. However, much of this experience involved the concentration of high value products, and it was not necessary to minimize the cost of the process to the extent required in desalting water. There are a number of configurations of this process involving various methods of feed addition, vapor and fluid movement, and heat recovery. However, the scaling properties of seawater place certain restrictions on the process, and as a result most designs have followed the configuration that was used for the Freeport long tube vertical (LTV) 18-22 evaporator. The Freeport configuration used forward feed addition, heat recovery to the feed stream by condensing a portion of the vapor strea~, and heat recovery from the product stream by flash chambers. Heat transfer ~o the boiling brine occurred inside vertical smooth tubes to a falling brine film. Vapor from the preceding stage was condensed on the outside of the vertical tubes to produce the required heat and product water, Since the major cost items in VTE plant investment are the vertical tubes, considerable effort'has been directed toward increasing the effec- tiveness of these heat transfer surfaces and reducing costs. The most promising development in this area has been the double fluted tube originated by General Electric and further developed by Oak Ridge National 23-27 Laboratory. Heat transfer coefficients in the double fluted tubes 27