Page 178 - Introduction to Marine Engineering

164 Refrigeration, air conditioning and ventilation

«. i r • ~
9 Sensing bulb
parator
t
Compressor
> '
Evaporator 'Oil Condenser
t

Solenoid
valve
! '
\ * Drier '
4
Expansion
valve
regulator
Figure 9.1 Vapour compression cycle

at a lower pressure. Air from the cooled space or air conditioning system
is passed over the evaporator and boils off the liquid refrigerant, at the
same time cooling the air. The design of the system and evaporator
should be such that all the liquid refrigerant is boiled off and the gas
slightly superheated before it returns to the compressor at a low
pressure to be recompressed.
Thus it will be seen that heat that is transferred from the air to the
evaporator is then pumped round the system until it reaches the
condenser where it is transferred or rejected to the ambient air or water.
It should be noted that where an air-cooled condenser is employed in
very small plants, such as provision storerooms, adequate ventilation is
required to help remove the heat being rejected by the condenser. Also,
in the case of water-cooled condensers, fresh water or sea water may be
employed. Fresh water is usual when a central fresh-water/sea-water
heat exchanger is employed for all engine room requirements. Where
this is the case, because of the higher cooling-water temperature to the
condenser, delivery temperatures from condensers will be higher than
that on a sea water cooling system.

Refrigerants
Generally speaking these are sub-divided into primary and secondary
refrigerants.

Primary refrigerants
This is the refrigerant employed in the compressor, condenser and
evaporator system and certain properties are essential requirements. For