Before it enters the seawater heater, the auxiliary exhaust steam is desuperheated by water sprayed into the steam inlet piping. The amount of water for desuperheating is adjusted by a manual control valve to maintain the steam temperature 50 to 10F higher than seawater heater shell temperature. In this manner the temperature of the seawater feed (light green) leaving the air ejector condenser/seawater heater assembly is maintained relatively constant. The water supply for the desuperheating water is a portion of the discharge from the seawater heater drain pump. During plant startup, when water from the seawater heater drain pump may not be available, the ships condensate system furnishes the water supply. A two-valve interlock between the supply from the seawater heater drain pump and the supply from the condensate system prevents cross-connecting of these two systems. The two-valve interlock is similar to that described for the distillate circuit.

SEAWATER HEATER DRAIN CIRCUIT.- Water condensed from auxiliary exhaust steam is pumped from the shell of the seawater heater by the seawater heater drain pump. A drain regulator serves as a hotwell and assures a constant suction head for the pump. The drain regulator is a ball float-operated valve attached below the condensate drain connection of the seawater heater. The ball float in the drain regulator operates the valve to maintain a relatively constant water level in the housing, which is indicated by a gauge glass. A decrease in water level will tend to close the valve in the drain regulator. Therefore, the amount of condensate discharged by the drain pump will be throttled until the water level in the float housing rises again. The water level in the regulator maintains a suction head for the seawater heater drain pump and prevents loss of vacuum in the seawater heater by maintaining a water seal between the heater and the pump. The drain pump discharges condensate from the seawater heater to the condensate system (startup only), to the steam drain collecting system (normal lineup), or to the bilge.

SALINITY MONITORING AND INDI-CATING.- Four salinity cells in the plant provide continuous monitoring of water purity: (1) in the loop seal line between the first- and second-stage distillate; (2) in the distillate pump discharge; (3) in the air ejector drain line; and (4) in the seawater heater drain pump discharge. All of the salinity cells are read on a salinity indicating panel. Two of the cells, located at the distillate pump discharge and the air ejector drains, also control three-way solenoid-operated trip valves, which are automatically tripped by a salinity cell to divert the flow of water to the bilge to prevent contamination of the potable or reserve feedwater systems.

Other Applications of the Flash-Type Distilling Unit

Flash-type distilling plants may have any number of stages and output capacities. For example, the Navy uses one type of three-stage, 30,000 gpd plant as well as a five-stage, 50,000 gpd unit. All flash-type plants operate on the same basic principles as those described for the two-stage plant.