Page 179 - Introduction to Marine Engineering

Refrigeration, air conditioning and ventilation 165

example it will boil off or evaporate at a low temperature and reasonable
pressure and it will condense at a temperature near normal sea water
temperature at a reasonable pressure. The refrigerant must also be free
from toxic, explosive, flammable and corrosive properties where
possible. Some refrigerants have critical temperatures above which the
refrigerant gas will not condense. This was one of the disadvantages of
carbon dioxide, which was used for many years on ships. Ships operating
in areas with very high sea-water temperatures had difficulty in
liquefying the carbon dioxide without some additional sub-cooling
system. A further disadvantage of carbon dioxide was the very high
pressure at which the system operated, resulting in large and heavy
machinery,
Between the carbon dioxide era and the present refrigerants, methyl
chloride and ammonia were used. Due to its explosive properties,
methyl chloride is now banned for shipboard use. Ammonia is still
employed, but requires special ventilation.
The modern refrigerants are fluorinated hydrocarbon compounds of
various formulae, with the exception of Refrigerant 502, which is an
azeotropic (fixed boiling point) mixture of Refrigerant 22 and
Refrigerant 115. These are usually refered to as Treons' with a number
related to their particular formula.
Refrigerant 11 is a very low-pressure refrigerant which requires a
large circulation for a particular cooling effect. It has particular
advantages when used in air conditioning units, since it will have a low
power consumption.
Refrigerant 12 was one of the first fluorinated hydrocarbon
refrigerants, as these numbered substances are known, to become
readily and cheaply available. A disadvantage is that evaporator
pressures are below atmospheric and any system leaks draw in air and
moisture.
Refrigerant 22 is now probably the most common refrigerant. It
provides a considerable range of low-temperature operation before the
evaporator pressure drops below atmospheric conditions. There is also a
space saving as the compressor displacement is about 60% of that
required for Refrigerant 12.
Refrigerant 502's particular advantages are that the displacement
required is similar to that of Refrigerant 22. Gas delivery temperatures
from the compressor are greatly reduced, and therefore there is less
likely to be a break-up of the lubricating oil and stressing of the delivery
valves.
All the above refrigerants are non-corrosive and may be used in
hermetic or semi-hermetic compressor units. Refrigerant 502, however,
does have less effect on the lacquers and elastomers employed in
compressors and motors. At present Refrigerant 502 is still an expensive
gas and not readily available worldwide.