Distilling plants installed in naval ships are of three general types: (1) vapor compression, (2) low-pressure steam, and (3) heat recovery. The major differences between the three types are the kinds of energy used to operate the units and the pressure under which distillation takes place. (NOTE: A relatively new process of producing potable water from seawater is currently under-going evaluation on three ships by NAVSEA. The reverse osmosis (RO) unit uses a completely different operating principle which does not require the heating of seawater. In the RO unit, reciprocating pumps force the seawater at high pressure against a membrane, where the liquid is permitted to pass but the salts are entrained. The salts are pumped back to the sea as brine. At the present time, RO units are not capable of producing water of a purity that is safe to use in boilers without additional processing.)

VAPOR COMPRESSION UNITS use elec-trical energy for heaters (the heat source) and a compressor. In addition, vapor compression units boil the feedwater at a pressure that is slightly above atmospheric pressure. Although used on submarines, the vapor compression type of distilling plant has all but been replaced on surface craft by the heat recovery distilling units. For this reason, vapor compression distilling units will not be covered in this manual. Chapter 9580, section II (53 1) of the Naval Ships Technical Manual contains information on these plants.

LOW-PRESSURE STEAM UNITS use low-pressure steam from either the auxiliary exhaust systems or the auxiliary steam system. Low-pressure steam distilling units are so called for two reasons. First, they use low-pressure steam as the heat source. Second, their operating shell pressure is less than atmospheric pressure (vacuum).

HEAT RECOVERY DISTILLING UNITS use low-pressure hot water from the diesel jacket-water system as a heat source. Heat recovery units

Table 15-1.-Properties of Saturated Steam

are also of the low-pressure design in that they also operate with a relatively high vacuum like the low-pressure steam distilling plants. There are two reasons why naval distill-ing plants are designed to operate at low pressures and low boiling temperatures. One is that low temperature operation helps to prevent the formation of harmful scale on heat transfer surfaces. Scale is formed when sea salts precipitate out of seawater at high temperatures. The other reason is that heated seawater will flash into steam when the pressure is lower than the saturation pressure corre-sponding to the temperature.