The flash-type distilling plant is widely used throughout the Navy. Flash-type plants have some unique features when they are compared to the submerged-tube type of plant. One is that the flash-type flashes the feed into vapor (steam) rather than boiling it inside the evaporator shell. The flashing process involves heating the feed before it enters the evaporator shell. The shell is under a relatively high vacuum. The feed is heated to a temperature at which it will flash into vapor when it enters the first stage, which is maintained at a high vacuum. With this design, there are no submerged heat transfer surfaces within the evaporator shell, such as the tubes in the submerged-tube unit. The elimination of these surfaces greatly reduces the scale formation problem of evaporators and allows prolonged operation at maximum efficiency. Any scale that may form on heat transfer surfaces of a flash-type plant is composed mainly of soft calcium carbonate compounds that are relatively easy to remove by a chemical cleaning process.

Two-Stage Flash

Figure 15-10 is an illustration showing the major components of the two-stage flash distilling plant that we are going to discuss. Figure 15-11 (at end of this chapter) shows the major flow paths through a two-stage, 12,000 gpd flash distilling plant. Follow along on the diagram as we discuss the operation of this plant. Notice the color coding as we continue this discussion.

SEAWATER FEED CIRCUIT.-The sea-water feed pump (upper left of fig. 15-11) takes a suction through a sea chest and strainer and discharges seawater (light green) into the tubes of the condensing section of the second stage of the evaporator. The seawater feed then flows through

Figure 15-10.-Two-stage, 12,000 gpd flash-type distilling plant.

the tubes of the heat exchanger in the first-stage condensing section. The condensing sections of the evaporator are tube bundles, one in each stage of the evaporator shell.

As it passes through the second-stage condenser section, the seawater feed carries heat away from the surrounding vapor and, as a result of the heat exchange process, the feedwater increases in temperature. When the incoming seawater feed has an injection temperature of 85F (29C), the feed leaving the first-stage condensing section of the evaporator should be approximately 138F (59C), an increase in sensible heat of 53F (11C).

Upon leaving the first-stage condensing section, the feed enters the air ejector con-denser/ seawater heater assembly. As it passes through this double-flow, shell and tube heat exchanger, the feed picks up more heat by condensing exhaust steam from the air ejectors and from steam admitted to the seawater heater from the auxiliary exhaust steam system. The feedwater leaves the air ejector con-denser/ seawater heater assembly at a temperature of approximately 170F (77C) and is fed into the first stage of the evaporator shell. (A feedwater temperature of 170F (77C) is required to ensure that all harmful organic suspended matter is killed.) The feed enters the bottom of the shell through two spray pipes. In conjunction with the high vacuum in the first stage, these spray pipes atomize the water, an action that helps to flash the incoming feed water into vapor. Feed that does not flash to steam in the first stage is directed to the second stage through an internal loop seal located at the bottom of the shell. Flow control of the feedwater is accomplished by manual operation of the feed valve in the line just before the feedwater enters the first stage. The feedwater enters the second stage in a manner similar to the way it entered the first-stage shell. The force that moves the feed from the first to the second stage is the difference in pressure between the two stages. The first-stage shell pressure is maintained at approximately 23 in.Hg, while the second-stage shell pressure is maintained at an even higher vacuum (lower pressure) of approximately 27 in.Hg. A loop seal arrangement at the bottom between the first and second stage prevents the pressure from equalizing between the stages. Feedwater that does not flash into vapor in the second stage becomes brine, which is pumped overboard.