Page 157 - Introduction to Marine Engineering

Auxiliaries 143

Steam inlet to ejectors

D«up«rhe«tin»_
spray
inlet
Steam inlet
to pre-hewer Preheater

Figure 7.7 Two-stage flash evaporator

A two-stage flash evaporator is shown in Figure 7.7. The feed pump
circulates sea water through the vapour condensers and the preheater.
The heated sea water then passes to the first-stage flash chamber where
some of it flashes off. A demister removes any water droplets from the
steam as it rises and is then condensed in the first-stage condenser.
The heated sea water passes to the second-stage flash chamber, which
is at a lower pressure, and more water flashes off. This steam is demisted
and condensed and, together with the distilled water from the
first-stage, is drawn off by the distillate pump.
The concentrated sea water or brine remaining in the second-stage
flash chamber is drawn off by the brine pump. The preheater uses steam
to heat the sea water and most of the latent heat from the flash steam is
returned to the sea water passing through the condensers. An air ejector
is used to maintain the low pressure in the chambers and to remove any
gases released from the sea water.