ENGINEMAN 1 & C
line of the brine overboard pump. In plants
equipped with wire control valves and in basket
type plants, the brine density is controlled by ad-
justing the first-effect feed valve. Increasing the
rate of feed decreases the brine density, and
decreasing the rate of feed increases the brine
density.
Frequent changes of brine density have a
tendency to disrupt steady performance of the
plant; therefore, only very small changes should
be made. The proper setting for a specific plant
should be learned from experience, and this
setting should be maintained as practicable.
Use of the Salinometer
The salinometer is an instrument (on the
principle of a float) for measuring the degree of
salinity or the concentration of the brine. It is a
hollow, metal vessel weighted at the bottom, and
having a projecting stem which is graduated in
four scales to read the salinity for various
temperatures of the brine. The graduations are
marked in thirty-seconds. When the salinity of a
sample of brine is to be measured, the temperature
of the sample should be brought to a temperature
corresponding to that of one of the scales on the
instrument in order that an accurate reading may
be obtained. The accuracy of the salinometer
should be checked occasionally by placing it in
distilled water; if it is accurate, it should sink to
the zero mark on the scale corresponding to the
temperature of the water.
A pot is provided for holding the sample of
brine. The pot must be amply deep so that there
is no danger of breaking the bottom of the
salinometer when it is placed in the pot. To use
the salinometer, proceed as follows:
1. Draw off a sample of brine, from the test
cocks on the discharge side of the brine pump,
then insert the thermometer into the sample.
2. Allow the sample to cool to the
temperature of one of the scale temperatures.
3. Put the salinometer in the pot and read the
degree of salinity.
4. Remove the salinometer and wipe off all
moisture since accumulations of salt or dirt will
result in false readings.
FLASH TYPE DISTILLING
PLANTS
The flash type evaporator, like all distilling
plants, removes salts and other impurities from
raw seawater by the process of evaporation and
condensation. This is accomplished by boiling the
water to convert it to steam, and condensing this
steam to form distilled water. The flash
evaporator is different from other distilling plants,
because evaporation takes place at temperatures
well below the normal boiling point of water and
without the use of submerged heat transfer
surfaces.
In the flash type distilling plant, the
temperature of the water is never raised beyond
175 °F, and is only raised to this temperature
within the last pass of tubes of the saltwater
heater. Flash evaporation takes place at
temperatures as low as 104°F. In addition, no
boiling occurs on heat transfer tube surfaces; as
a result, the scale formation is greatly reduced and
operation at maximum efficiency is prolonged.
The term “flash evaporation” means that
water is converted to steam as it enters an
evaporating chamber, without further addition of
heat. Flashing at low temperatures is possible only
when a vacuum is maintained in the chamber,
since the boiling point of water decreases as the
pressure in the chamber is reduced. As in other
methods of distillation, a portion of the water re-
mains behind in the evaporating chamber and is
taken off as a concentrated waste (brine).
Principal Components
The unit discussed in this section is a five-stage
plant in which feedwater is flashed to vapor in
five evaporator stages at successively lower
pressures.
Connections (or passages) that exist between
the evaporator stages are the feedwater and its
distillate loop seals, which permit the flow of feed-
water and distillate from stage to stage while
preserving the varying degrees of vacuum in each
stage.
Condensers are mounted on top of each stage
between the front and rear water boxes. Feedwater
flows through the tubes in six passes, entering at
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