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Suitability of refrigerants used on board cargo ships
Choice of refrigerants:
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. Mechanical refrigeration makes possible the control of the pressure and
therefore, the temperature at which a refrigerant boils (within the limits of
critical pressure and temperature). The closed circuit ensures repeated use of
the same refrigerant with little or no loss to atmosphere.
any liquid can be used as a refrigerant if its pressure/temperature relationship is
suitable for the conditions. Although no perfect refrigerant is known, there are
certain factors which determine a refrigerant's desirability for a particular duty
and the one selected should possess as many as possible of the following
1. Moderate condensing pressure, obviating the need for heavily constructed
compressors, condensers and high pressure piping.
2. High critical temperature, as it is impossible to condense at a temperature
above the critical, no matter how much the pressure is increased.
3. Low specific heat of the liquid. This is desirable as throttling at the
expansion valve causes liquid refrigerant to be cooled at the expense of
4. High latent heat of vapourization, so that less refrigerant may be circulated
to perform a given duty.
5. The refrigerant should be non-corrosive to all materials used in the
construction of the refrigerating machinery and systems.
6. It should be stable chemically.
7. It should be non-flammable and non-explosive.
8. World wide availability, low cost and ease of handling are desirable.
9. The problem of oil return to the compressor crankcase is simplified when
an oil-miscible refrigerant is used, by the admixture of the oil and the
refrigerant in the system. With non-miscible refrigerants, once oil has
passed to the system, its return to the crankcase can only be effected with
10. The current concern with depletion of the ozone layer has resulted in a
new requirement that refrigerants should be environmentally friendly.
Strong pressure exerted to phase out CFCs and in particular R12, resulted
in the Montreal Protocol adopted in 1987 after ratification by 70
countries and additional conventions seeking to phase out these gases.
Finally the refrigerant should preferably be non-toxic, have satisfactory heat
transfer characteristics, and leakages should be easy to detect either by odour
or by the use of suitable indicators.
It is not proposed to list or deal with all known refrigerants, only those likely
to be encountered on board. These refrigerants are referred to by their trade names
, chemecal names or their internationally recognized numbers.
Use of ammonia
The ammonia used for refrigeration systems based on the use of a compressor,
condenser, expansion valve and an evaporator is dry (anhydrous)
in that there is no water in solution with it. It has the chemical formula NH3 but
as a refrigerant, it is coded with the number R717.
The good qualities of
ammonia as a refrigerant have been offset by its toxicity, flammability and
pungent odour, so that carbon dioxide and then CFCs (which replaced CO2)
were used at sea in preference. Now that R12 is to be phased out in the short
term and R22 at a later date, ammonia is being considered as a replacement,
because despite its local harmful effects and disadvantages, it is ozone friendly.
The upper and lower explosive limits for pure ammonia in air, are 27% and
16% by volume, respectively. With oil contamination, the latter may reduce to
4%. Long-term exposure to ammonia should be restricted to the current
threshold limit value (TLV).
Exposure to higher concentrations of 1500 ppm
will result in damage to body tissue and death may result at 2500 ppm.
However, ammonia leaks are instantly detected at less than 10 ppm
concentration by the pungent odour and this is a safety feature. Very few
people can endure ammonia when its concentration exceeds the TLV. In the
liquid form, ammonia causes chemical and frost burns.
Corrosion of brass, bronze and similar alloys , occurs in ammonia systems if there is any
water present. These materials are being avoided with steels being used instead.
Ammonia is highly soluble in water with which it forms ammonium
hydroxide, a weak base. About 1300 volumes of ammonia can be dissolved in I
volume of water at low temperature. However it is easily expelled by boiling.
This action makes the vapour absorption refrigerator possible. Refrigerators, of
this type do not require a compressor for operation, only a heat source.
Use of R744 CO2 as a refrigerant
When carbon dioxide (CO2) is used as a refrigerant the working pressures are
high, being about 70 bar at the compressor discharge and 20 bar at the
compressor suction. The machinery and system must therefore be of
substantial construction. The critical temperature is low (31°C) and this causes
problems in areas with high sea-water temperature. It also has a low coefficient
of performance. Poor miscibility with oil required that a drain system be
provided to remove oil continuously from the evaporator.
The gas is not explosive or flammable but a leak is potentially dangerous
because it can displace air and asphyxiate. It is also toxic. The liquid is stored in
steel bottles at high pressure, ideally in a cool space. A temperature rise will
cause a pressure rise in the bottles which is relieved by the rupturing of a safety
disc and release of gas.
Refrigeration system components Centrifugal,reciprocating, or screw compressors. Modern Shell and tube type condensers.Various refrigerant types, refrigerant flow control valves & Ancillary fittings etc.....
Modern refrigerants for cargo ships
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.....
Choice of refrigerants
Theoretically, almost any liquid can be used as a refrigerant if its pressure/temperature relationship is suitable for the conditions. Although no perfect refrigerant is known, there are certain factors which determine a refrigerant's desirability for a particular duty and the one selected should .....
Cargo refrigeration procedure Refrigerated cargo vessels usually require a system which provides for various spaces to be cooled to different temperatures. The arrangements adopted can be considered in three parts: the central primary refrigerating plant, the brine circulating system, and the air circulating system for cooling the cargo in the hold.....
How to troubleshoot refrigeration system faults
It is important that gas losses are minimized in systems. Gas used in this system is both expensive and a danger to health. The system is therefore to be kept in an absolutely gas-tight condition. On no account should the refrigerant be blown off to atmosphere. Before carrying out repairs the refrigerant should be pumped down to the liquid receiver or the condenser. The remaining gas should be vented off and the area well ventilated.....
Safety precautions for refrigeration plant and refrigerated compartments Adequate information should be available on each vessel, laying down the operation and maintenance safeguards of the refrigeration plant, the particular properties of the refrigerant and the precautions for its safe handling.....
Reefer container Individual containers with their own refrigeration plant are connected to the 440 or 220 a.c. sockets provided on deck. Systems designed for the cooling of refrigerated containers employ trunkings arranged so that containers stowed in stacks between built-in guide rails, can be connected to the suction and delivery air ducts of the ship's
refrigeration plant by bellows pieces operated pneumatically......
Marine machineries - Useful tags
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