7.2.1 A refrigeration system
should consist of one or more units capable of maintaining the required
cargo pressure/temperature under conditions of the upper ambient design
temperatures. Unless an alternative means of controlling the cargo
pressure/temperature is provided to the satisfaction of the Administration,
a stand-by unit (or units) affording spare capacity at least equal
to the largest required single unit should be provided. A stand-by
unit should consist of a compressor with its driving motor, control
system and any necessary fittings to permit operation independently
of the normal service units. A stand-by heat exchanger should be provided
unless the normal heat exchanger for the unit has an excess capacity
of at least 25% of the largest required capacity. Separate piping
systems are not required.
7.2.2.1 Where two or more
refrigerated cargoes which may react chemically in a dangerous manner
are carried simultaneously, special consideration should be given
to the refrigeration systems to avoid the possibility of mixing cargoes.
For the carriage of such cargoes, separate refrigeration systems,
each complete with a stand-by unit as specified in 7.2.1, should be provided for each cargo.
However, where cooling is provided by an indirect or combined system
and leakage in the heat exchangers cannot cause mixing of the cargoes
under any envisaged condition, separate refrigeration units need not
be fitted.
7.2.2.2 Where two or more
refrigerated cargoes are not mutually soluble under the conditions
of carriage, so that their vapour pressures would be additive on mixing,
special consideration should be given to the refrigeration systems
to avoid the possibility of mixing cargoes.
7.2.3 Where cooling water is
required in refrigeration systems, an adequate supply should be provided
by a pump or pumps used exclusively for this purpose. This pump or
these pumps should have at least two sea suction lines, where practicable
leading from sea-chests, one port and one starboard. A spare pump
of adequate capacity should be provided, which may be a pump used
for other services so long as its use for cooling would not interfere
with any other essential service.
7.2.4 The refrigeration system
may be arranged in one of the following ways:
-
.1 a direct system where evaporated cargo is
compressed, condensed and returned to cargo tanks. For certain cargoes specified
in chapter 17 this system should not be used;
-
.2 an indirect system where cargo or evaporated cargo
is cooled or condensed by refrigerant without being compressed;
-
.3 a combined system where evaporated cargo is
compressed and condensed in a cargo/refrigerant heat exchanger and returned to the
cargo tanks. For certain cargoes specified in chapter
17 this system should not be used.
7.2.5 All primary and secondary
refrigerants must be compatible with each other and with the cargo
with which they come into contact. The heat exchange may take place
either remotely from the cargo tank or by cooling coils fitted inside
or outside the cargo tank.