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Modern refrigerants for cargo ships

The refrigerant employed in the compressor, condenser and evaporator systems : In all refrigeration, heat is collected in a suitable fluid and this fluid is removed from the space substance being cooled carrying the heat with it. Such fluid known as refrigerant. All refrigerants using on board might be sub-divided into primary and secondary refrigerants.

Primary refrigerants

This is the refrigerant employed in the compressor, condenser and evaporator system and certain properties are essential requirements. For 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.

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.

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