Page 182 - Introduction to Marine Engineering

168 Refrigeration, air conditioning and ventilation

Reciprocating compressors cover the whole spectrum of refrigeration
requirements at sea, from air conditioning to low temperature cargo
installations. They are normally of a compact design and may be of an
in-line, V or W configuration. Figure 9.2 shows a 4-cylinder W
configuration. The construction arrangement can be seen and the
principle of operation is similar in many respects to an air compressor,
For low-temperature applications the machine may be arranged as a
two-stage compressor and some machines are made so that they can be
changed from single to two stage, depending on cargo requirements. As
the crankcase is subject to refrigerant pressure, the drive shaft seal is
required to prevent a flow of refrigerant out of the compressor or
ingress of air. In semi-hermetic or hermetic machines this problem is
obviated as the motor and compressor are in one casing.
Screw compressors have replaced reciprocating compressors in large
installations for two reasons. Firstly, fewer and more compact machines
are used; secondly, a reduced number of working parts results in greater
reliability with reduced maintenance requirements. There are two types
of screw compressor; one employs two rotors side by side and the other,
which is a more modern development, is a single rotor with two star
wheels, one on either side. As the star wheels compress the gas in
opposite directions, the thrust on this type of rotor is balanced. Such a
compressor in shown in Figure 9.3, The principle of operation for both
types is similar to a screw-type positive displacement pump (see Chapter
6).
To achieve a seal between the rotors, oil is injected into the
compressor: to prevent this being carried into the system, the oil
separator is larger and more complex than the normal delivery oil
separator associated with a reciprocating compressor. Also, because
some of the heat of compression is transferred to the oil, a larger oi!
cooler has to be fitted, which may be either water or refrigerant cooled.
Since a.c. motor driven compressors are usually single speed, some
form of cylinder unloading gear is necessary to reduce the compressor
capacity. This unloading gear usually comprises a means of holding the
suction valves open.

Condensers
Condensers are generally water cooled, as mentioned previously, and
are of the shell and tube type. A typical modern unit is shown in Figure
9.4 in which it will be seen that the refrigerant passes over the tubes and
the cooling water is passed through the tubes. In the case of sea water
cooled condensers it is usual to have a two-pass arrangement through
the tubes. The sea water side maintenance mentioned for coolers in
Chapter 7 applies also to this condenser.