Page 180 - Introduction to Marine Engineering

166 Refrigeration, air conditioning and ventilation

Environmentally friendly refrigerants
Most modern refrigerants are chlorofluorocarbons (CFCs) which have a
damaging effect on the ozone layer, in addition to accumulating in the
atmosphere and causing global warming. The Montreal Protocol, signed
in 1987 and reviewed in 1990 and 1992, regulates the manufacture and
use of CFC gases. The phasing-out of existing CFCs is likely to form part
of a new Annex to MARPOL 73/78 and alternatives are being sought for
shipboard use.
Refrigerant 12 or CFC 12 can readily be replaced by HFC 134A, which
causes zero ozone depletion and alternatives to CFC11, HCFC 22 and
R502 are being developed. Other choices include the use of carbon
dioxide or ammonia, both previously used as refrigerants but potentially
dangerous if incorrectly handled. Lloyds Register has issued Guidance
Notes for Marine Ammonia Plant as a supplement to the Rules, owing to
the resurgence of interest in this refrigerant.

Secondary refrigerants
Both large air conditioning and cargo cooling systems may employ a
secondary refrigerant. In this case the primary refrigerant evaporator
will be circulated with the secondary refrigerant, which is then passed to
the space to be cooled. Secondary refrigerants are employed where the
installation is large and complex to avoid the circulation of expensive
primary refrigerants in large quantities. These primary refrigerants can
be very searching, that is they can escape through minute clearances, so
it is essential to keep the number of possible leakage points to a
minimum.
In the case of air conditioning plants, fresh water is the normal
secondary refrigerant, which may or may not have a glycol solution
added. The more common secondary refrigerant on large cargo
installations is a calcium chloride brine to which is added inhibitors to
prevent corrosion.

System components

Compressors
There are three types of compressor in use at sea: centrifugal,
reciprocating, and screw.
Centrifugal compressors are used with Refrigerants 11 or 12 and are
limited in their application to large air conditioning installations. They
are similar in appearance to horizontal centrifugal pumps and may have
one or more stages.