The submerged-tube distilling plant in figure 15-2 contains a single evaporating stage in a shell, an air ejector assembly, a brine eductor assembly, a ratosight meter, a distillate pump, a distillate sterilizer, a distillate sterilizer temperature control system, and a distillate cooler. For the operation and monitoring of the distilling plant, the salinity indicating system, a three-way solenoid trip valve, a water meter, pressure gauges, thermometers, and associated piping, valves, and fittings are all provided. All the components, except the evaporator feed pump and the feed-water strainer, are mounted on a common base. As we discuss the main components of the submerged-tube heat recovery distilling plant, refer to figure 15-2. You may also find it helpful to refer to figure 15-9 (at the end of this chapter), which shows the major flow paths and relative location of the components within each flow path.

EVAPORATOR ASSEMBLY.-The evap-orator assembly is an evaporator-condenser effect contained within a single shell mounted on the distiller base. The distilling process is shown by the large arrows in figure 15-3. Feedwater (sea-water) that is pumped into the bottom of the evap-orator will eventually be converted into steam. Jacket water from the diesel engine(s) is pumped through the heating tube bundle and serves as a source of heat for the tube bundle.

Figure 15-3.-Evaporator feed/distillate assembly.

Submerged in the feedwater, the heating tube bundle transfers the heat by conduction from the hot jacket water to the feedwater. The heat from the jacket water causes the feedwater to boil and give off steam. (Because of the low pressure created by a vacuum inside the evaporator, the feedwater boils at a lower temperature.) As the steam rises, it first passes through the suppression baffles. While the suppression baffles allow the steam to pass through to the demister, they also block the larger moisture-laden droplets of steam from entering the demister. The steam then passes through the demister (vapor separator). The demister consists of staggered layers (or pads) of densely woven Monel mesh. By virtue of its design, the demister causes the vapor to change its direction of motion many times as it passes through. The demister works by separating the moisture from the vapor, thereby producing a dry saturated steam. The separated moisture, which may contain salt particles, drips back to the feedwater as brine. As the saturated steam leaves the demister, it is drawn to the condenser tube bundle. Seawater pumped through the condenser tubes cools the tubes. As the steam passes over the condenser tube bundle, vapor condenses on the tubes as distillate (fresh water). The distillate falls from the condenser tubes into a collection trough located under the condenser and is then channeled to the distillate water outlet.

A pressure relief valve located on the back of the evaporator shell prevents the shell from being damaged by high internal pressure that may result from improper operation of the unit. A brine-out connection, installed in the lower section of the shell, maintains the proper level of feedwater while drawing off the brine. The connection is located at a height that allows it to maintain the feedwater level slightly above the heating tube bundle. This design assures adequate and continued submersion of the heating tube bundle. A screen in the brine-out connection blocks any debris that might foul the brine eductor.

Observation windows are installed in the evap-orator shell. These windows will allow you to monitor the system by viewing water level condi-tions in the vapor and distillate collection areas.

AIR EJECTOR.-The air ejector is mounted on the upper side of the evaporator shell near the top of the distillate condensing area.

Figure 15-5.-Ratosight meter functional diagram.

Figure 15-4.-Air ejector.

(Refer to fig. 15-4.) The air ejector is a single-stage, seawater-driven jet pump. Firemain pres-sure is applied to the pressure side of the ejector and flows through the constricted nozzle, creating a high vacuum at the vent connection of the evap-orator shell. This method creates the initial vacuum in the evaporator. Once the initial vacuum is drawn, the main function of the ejector is to remove noncondensable gases such as air and carbon dioxide from the evaporator. If allowed to accumulate, noncondensable gases would insulate the heat transfer surfaces. The accumulation of gases would also raise the pressure and upset the temperature and pressure operating conditions of the distilling plant.