Now that you have read about the major components and their respected functions and locations within the system, you should be able to study and understand the flow diagram in figure 15-9 (a foldout at the end of this chapter). This diagram illustrates the operation of a submerged-tube distilling plant.

FEEDWATER CIRCUIT.-Feedwater is piped to the suction of the feedwater pump. This pump discharges the feedwater through a duplex feedwater strainer, through the tube side of the distillate cooler, and through the tubes of the condenser tube bundle. The major portion of the feedwater is piped overboard. A portion is piped through the ratosight meter and into the shell of the evaporator. The heating water, flowing through the heating tube bundle, heats the feed-water to boiling, causing water vapor to be released. The vapor is drawn up and passes through demister pads in the condensing area. The demisters aid in purifying the resultant distillate by mechanically blocking out salt-entrained drops of feedwater, thus allowing only pure water vapor to pass through.

BRINE CIRCUIT.-The feedwater fills the evaporator to a level above the heating tube bundle. Because there is less distillate produced than feedwater introduced, the overflow is piped to the suction of the brine eductor. The brine eductor draws this brine out of the evaporator and discharges it overboard.

DISTILLATE CIRCUIT.-The water vapor condenses on the condensing tube bundle into fresh water (distillate). This condensing action maintains the vacuum in the effect, which was established initially by the air ejector. The distillate collects in the distillate trough, and flows to the suction of the distillate pump. The distillate pump moves the distillate through the shell side of the distillate sterilizer. The distillate sterilizer heats the distillate to 170F. (The temperature must be high enough to kill all harmful bacteria and other organisms.) The distillate then flows through the shell side of the distillate cooler, where it is cooled to 95F. From the cooler, the distillate flows through a three-way solenoid trip valve. If it is pure, the distillate flows through a water meter to storage or to the brominator for treatment (disinfection), which serves to maintain purity, and then to the potable water system. If it is impure, it flows to waste.