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are roughly twice as great as those attainable in smooth tubes. This significantly reduces the tube surface area requirements and also lowers the cost of other evaporator components such as tube sheets, shells, and ~ pumps. to ~ 00, The second major development in recent VTE conceptual designs has 'been the use of the multistage flash evaporator (MSF) to recover the heat from the b~ine and product streams and heat the incoming feed stream. This apProach has the advantage of reducing the cost of heat recovery surfaces and producing an additional 15 to 20 percent water in the MSF ti 3,23,27 sec on. Plant Design The VTE-MSF plant considered in this study was based on conceptual designs developed by the Oak Ridge National Laboratory and described in references 3 and 23. A flowsheet for the process is shown in Fig. 7 and the energy and mass flows for a 10-mgd plant are given in Table 8. The values of variables used in evaluating the process, in addition to those listed in the general ground rules, are noted in Table 9. The equipment layout calls for 15 vertical tube evaporator effects with double fluted tubes located above 50 multistage flash chambers. The shell costs are reduced by using single pressure-containing walls between adjacent VTE effects and also between the VTE and MSF sections. The volume of the VTE effects and MSF chambers is increased from the high to the low pressure ends of the plant to allow for the reduction in vapor density. Heat is provided for the process by condensing steam outside the tubes in the first VTE effect and the condensate is returned to the steam plant. The vapor from the fifteenth VTE effect is condensed in the final condensor and part of the con dens or cooling water is rejected to 28