Page 189 - Introduction to Marine Engineering

Refrigeration, air conditioning and ventilation 175

The chamber or zone must be airtight, and any leakage of gas is
replaced by injecting the required volume into the zone. A low oxygen
alarm and sampling points within the chamber protect the cargo from
suffocation, which would occur if the oxygen level was less than 0.5 per
cent by volume.
The CA chamber will not support human life and rules exist for the
use of CA, which ensure adequate safety precautions are taken prior to
entry. Locks and alarms are fitted to CA spaces, and if entry is required
complete aeration must take place. Ventilation outlets must be safely led
out into the atmosphere, well way from air conditioning units and
accommodation.

System faults
During operation a number of particular problems can occur which will
affect the plant performance.
An overcharge or excess refrigerant in the system will be seen as a
high condenser pressure. The refrigerant should be pumped to the
condenser and the excess released from there.
Air in the system will also show as a high condenser pressure. With the
condenser liquid outlet closed the refrigerant charge should be pumped
in and cooled. Releasing the purge valve will vent off the air which will
have collected above the refrigerant.
Under-charge will show as a low compressor pressure and large
bubbles in the liquid line sightglass. A leak test should then be carried
out over the system to determine the fault and enable its rectification. A
leak detector lamp for Freon refrigerants may be of the methylated
spirit type, but more commonly uses Calor gas (butane/propane). The
Freon is drawn into the flame and the flame will change colour, going
from green to blue depending on the concentration of the gas.
When charging the system with more gas the main liquid valve should
be closed and gas introduced before the regulating valve until the system
is correctly charged. (It is possible to charge on the outlet side of the
regulating valve and is quicker, but this requires a good amount of
experience to prevent liquid carrying over and damaging the
compressor.)
Moisture in the system may change to ice and close up the regulating
valve, resulting in a drop in pressure on the evaporator side and a rise in
pressure on the condenser side. The drier should be examined and the
drying chemicals will probably require replacing. A correctly operating
regulating valve will have frosting on the outlet side but not on the inlet
side.