Page 192 - Introduction to Marine Engineering

178 Refrigeration, air conditioning and ventilation

outlet terminal. In cold climates the preheater will warm both supplies of
air, resulting in a warm and a hot supply to each space.
The 'single duct with reheat' system is used for vessels operating in
mainly cool climates. The central unit will cool and dehumidify or
preheat and humidify the air as required by outside conditions. In
addition, before discharge into the treated space a local reheating unit
will heat the air if required, depending upon the room thermostat
setting.
The refrigeration system used in the central unit is shown in Figure
9.12. A direct-expansion system is shown using a reciprocating

Cooler Heater

D—#
Compressor
Figure 9.12 Direct-expansion refrigeration system for an air cooler

compressor, sea water cooled condenser and a thermostatically
controlled regulating valve. The air to be cooled passes over the
evaporator or cooler. The cooling effect of the unit may need to be
reduced if there is no great demand and the hot gas bypass system
provides this facility.
Maintenance of the above systems will involve the usual checks on the
running machinery and the cleaning of filters. Air filters in the central
units are usually washable but may be disposable. The filters should be
attended to as required, depending upon the location of the ship.