Page 188 - Introduction to Marine Engineering

174 Refrigeration, air conditioning and ventilation

twin circuit arrangement for each cooler (evaporator) provides flexibility
and duplication in the event of one system failing. The back pressure
valve maintains a minimum constant pressure or temperature in the
evaporator when working a space in high-temperature conditions to
prevent under-cooling of the cargo. If one space is operating at a
low-temperature condition at the same time the back pressure valve
would be bypassed. The liquid cooler illustrated in the diagram is
necessary where an abnormal high static head has to be overcome
between the machinery and the coolers. In this vessel the liquid is
sub-cooled to prevent it flashing off before reaching the thermostatk
expansion valve.
Containers which require refrigeration present particular problems.
Where only a few are carried or the ship has no built-in arrangement for
refrigerating containers, then clip-on or integral refrigeration plants
would be provided. The clip-on or integral unit may be either air or
water cooled. In the case of air cooled units adequate ventilation has to
be supplied if they are fitted below decks. For water cooled units some
sort of cooling water arrangement must be coupled up to each unit. Also
an electrical supply is required for each type.
Vessels designed for specific refrigerated container trades have
built-in ducting systems. These can be in two forms: a horizontal finger
duct system in which up to 48 containers are fed from one cooler
situated in the wings of the ship or, alternatively, a vertical duct system in
which each stack of containers has its own duct and cooler. This type of
system is employed for standard containers having two port holes in the
wall opposite the loading doors. Air is delivered into the bottom opening
and, after passing through a plenum, rises through a floor grating over
the cargo and returns via another section of the plenum to the top port.
The connection between the duct arid containers is made by couplings
which are pneumatically controlled.

Controlled atmosphere
Refrigerated cargo ships are making increased use of Controlled
Atmosphere (CA), a technique which increases the storage life of fruit
and vegetables. Oxygen and carbon dioxide levels, and relative humidity,
are independently controlled to within close tolerances within a
particular CA zone. This slows down the ripening of fruit and vegetables
during their transportation. In a CA zone oxygen levels may be as low
as from 1 to 12 per cent, carbon dioxide from 0 to 25 per cent and
relative humidity is kept within 40 to 90 per cent.