Page 190 - Introduction to Marine Engineering

176 Refrigeration, air conditioning and ventilation

Air conditioning

Ships travel the world and are therefore subject to various climatic
conditions. The crew of the ship must be provided with reasonable
conditions in which to work regardless of the weather. Temperature
alone is not a sufficient measure of conditions acceptable to the human
body. Relative humidity in conjunction with temperature more truly
determines the environment for human comfort. Relative humidity,
expressed as a percentage, is the ratio of the water vapour pressure in
the air tested, to the saturated vapour pressure of air at the same
temperature. The fact that less water can be absorbed as air is cooled and
more can be absorbed when it is heated is the major consideration in air
conditioning system design. Other factors are the nearness of heat
sources, exposure to sunlight, sources of cold and the insulation
provided around the space.
An air conditioning system aims to provide a comfortable working
environment regardless of outside conditions. Satisfactory air treatment
must involve a relatively 'closed' system where the air is circulated and
returned. However, some air is 'consumed' by humans and some
machinery so there is a requirement for renewal. Public rooms and
accommodation will operate with a reduced percentage of air renewal
since the conditioning cost of 100% renewal would be considerable.
Galleys and sanitary spaces, for instance, must have 100% renewal, but
here the air quantities and treatment costs will be much smaller. Systems
may however be designed for 100% renewal of air although not
necessarily operated in this way. Noise and vibration from equipment
used in the system should be kept to a minimum to avoid a different
kind of discomfort. Three main types of marine air conditioning system
are in general use, the single duct, the twin duct and the single duct with
reheat,
The single-duct system is widely used on cargo ships (Figure 9.10).
Several central units are used to distribute conditioned air to a number
of cabins or spaces via a single pipe or duct. In warm climates a mixture
of fresh and recirulated air is cooled and dehumidified (some water is
removed) during its passage over the refrigeration unit. In cold climates
the air mixture is warmed and humidified either by steam, hot water or
electric heating elements. The temperature and humidity of the air is
controlled automatically at the central unit. Within the conditioned
space control is by variation of the volume flow of air.
The twin-duct system provides increased flexibility and is mainly used
on passenger ships (Figure 9.11). A central unit is used with cooled
dehumidified air provided through one duct. The other duct is supplied
with cooled air that has been reheated. Each treated space is provided
with a supply from each duct which may be mixed as required at the