It is quite possible to prepare sea water in such a way as to
practically prevent any serious deposit forming from it.
The process employed is to add to the sea water a known quantity of
precipitator powder consisting chiefly of soda ash, and having done
this in a closed vessel, to heat the mixture by blowing into it waste
steam, until a pressure of from 5lbs. to 10lbs. is created; under these
circumstances practically all the magnesium and calcium salts separate
from the water and are easily got rid of by filtering it under pressure
into the hot-well.
A precipitator 6 ft. 4 in. high and 3 ft. in diameter, holds a ton of
water, and the time taken, from the first running the sea water in, to
its delivery into the hot-well, need not exceed 1 hour and 15 minutes,
so that in practice, giving plenty of time between the makes, it would
be perfectly easy to prepare 8 to 12 tons in the 24 hours with a small
precipitator of the size named. The prepared water has a density of
l/32nd, and may with safety be evaporated until its density is 5/32nds,
the salts present not crystalizing out until a density of from 6/32nds
to 7/32nds is reached.
In preparing sea water in the way proposed, every precaution must
be taken to add slightly less of the precipitant than is necessary
to entirely throw down the calcium and magnesium salts, as it is
manifestly impossible in practice to guard against small quantities of
sea water finding way into the boiler either from leaky condensers or
else being fed in by the engineer during some emergency, and if under
these conditions any excess of the precipitant were present in the
boiler, a bulky precipitate would be thrown down and cause trouble,
although it would not bind into a solid scale.
Briefly recapitulated the means which are best adapted for preventing
the formation of the dangerous organic and oily deposits considered are: