Page 177 - Introduction to Marine Engineering

Chapter 9

Refrigeration, air conditioning

and ventilation

Refrigeration is a process in which the temperature of a space or its
contents is reduced to below that of their surroundings. Air conditioning is
the control of temperature and humidity in a space together with the
circulation, filtering and refreshing of the air. Ventilation is the
circulation and refreshing of the air in a space without necessarily a
change of temperature. With the exception of special processes, such as
fish freezing, air is normally employed as the heat transfer medium. As a
result fans and ducting are used for refrigeration, air conditioning and
ventilation. The three processes are thus interlinked and all involve the
provision of a suitable climate for men, machinery and cargo.

Refrigeration

Refrigeration of cargo spaces and storerooms employs a system of
components to remove heat from the space being cooled. This heat is
transferred to another body at a lower temperature. The cooling of air
for air conditioning entails a similar process.
The transfer of heat takes place in a simple system: firstly, in the
evaporator where the lower temperature of the refrigerant cools the
body of the space being cooled; and secondly, in the condenser where
the refrigerant is cooled by air or water. The usual system employed for
marine refrigeration plants is the vapour compression cycle, for which
the basic diagram is shown in Figure 9.1.
The pressure of the refrigerant gas is increased in the compressor and
it thereby becomes hot. This hot, high-pressure gas is passed through
into a condenser. Depending on the particular application, the
refrigerant gas will be cooled either by air or water, and because it is still
at a high pressure it will condense. The liquid refrigerant is then
distributed through a pipe network until it reaches a control valve
alongside an evaporator where the cooling is required. This regulating
valve meters the flow of liquid refrigerant into the evaporator, which is

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