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WATER PRODUCTION
There is a high water demand for many other uses besides
the obvious crew consumption. It is impossible to
carry enough water to last months at a time without making
the submarine impractically larger, so submarines create
it.
TURNING SEAWATER INTO FRESHWATER
The production of water is facilitated by purifying
seawater with reverse osmosis (RO). Normal osmosis
is a natural process in which a solvent (in this case,
water) passes through a semipermeable membrane from a
lower concentration solution to a higher concentration
solution, diluting the higher side until both are
equal. For seawater, the most important solute is
salt. The driving force behind this process is
called the osmotic pressure.
Reverse osmosis is the process of pressurizing the higher
concentration side to overcome osmotic pressure and force
water to flow to the low concentration side. This
would increase freshwater supply, while concentrating the
seawater side into brine, which is disposed of back into
the ocean. This pressure is supplied with
pumps. Osmotic pressure increases as the difference
in concentrations increase, and seawater normally has a
salt concentration of about 35 parts per thousand.
The output quality of the machines on the Ohio class is
less than 1 part per million. Therefore, the pumps
are operating at quite a high pressure.
IMAGE SOURCE: Puretec
Water
Even the multi-pass RO units are not enough to make pure
water, which is required for various functions, such as
the steam system and reactor coolant systems.
Deionized (DI) water is produced by sending the
potable-grade water output of the RO units through an ion
exchanger when such water is needed.
WHERE DOES IT GO?
Like most other systems that have two of everything,
there are two RO units with a combined output of 12,000
gallons per day. Under normal operating conditions,
the ship's approximate usage is in the range of
5,000-6,000 gallons per day. Potable water is the
largest demand, serving not only as drinking water, but
also is used for showers, sinks, laundry, cooking, and
dish washing. The next largest demand is the steam
and condensate systems due in part to design features that
use the internal fluids as its own seal and
lubrication. For instance, the feed pumps have
rotating shafts supported by bearings. These
bearings will need to be kept cool and lubricated, so as
the pump is pumping, some of the water is designed to leak
out through these spaces. The water is considered
contaminated the moment it leaves the system, so it is not
reused, making the system need frequent replenishment.
Other Less Frequent Uses
- Reactor coolant
- Reactor plant freshwater cooling system
- Engine room freshwater cooling system
- Chill water
- Diesel fresh water cooling system
- Water for generating oxygen
One surprising thing that freshwater is not used for is
toilets, which are flushed with seawater. Just one
of many examples where seawater usage is maximized to
reduce demand on the RO units.