Air cooler fans
Fans may be either centrifugal or of the propeller type; the air circulation
systems being based on a pressure requirement of about 50 mm W.G. (water
gauge). All of the electrical energy of the fan motors is dissipated in the form of
heat and has to be removed by the refrigerating plant. Fan output should be
variable so that it can be reduced as heat load diminishes. There was no
problem with d.c. motors but with a.c. either the motors are two speed, or each
cooler has a number of fixed speed fans which can be switched off individually
to suit the load. In the latter case, provision must be made to blank off the
stopped fans to prevent air loss.
The capacity of the fans is determined by the number of air changes per hour
required in the cargo chambers, and this is influenced by the maximum
calculated heat load. In a system using air coolers and fans, all the heat load
must be carried away by the circulating air and the difference between delivery
and suction air temperatures is directly proportional to the weight of air being
circulated. Since the temperature difference is limited by the allowable
temperature spread in the cargo chambers and maximum temperature spread in
the cargo chambers and maximum load can be estimated, the selection of
suitable fans is straightforward.
In most installations the number of air changes required per hour, based on
an empty chamber, varies between 40 for dead cargoes such as frozen meat or
fish and 80 for fruit cargoes, such as bananas which evolve heat freely.
It is essential to measure and log the temperature of refrigerated cargo to
ensure that the correct conditions are maintained and also to provide a record
should there be complaints from a shipper. Mercury or spirit thermometers
suspended from screwed plugs in vertical steel tubes with perforations hung in
the cold chamber have been replaced by remote reading devices.
Electrical resistance and electronic self-balancing thermometers use the
principle of the Wheatstone bridge. The former rely on a galvanometer to
indicate a balance. In the latter the unbalanced current causes an electric motor
to adjust the resistance. All necessary cargo temperature readings are obtained
on modern reefers and container ships on a data logger which makes an
The temperatures and pressures relating to refrigerant gas and liquid,
cooling water, brine and the ambient are also required. Most of these are
obtained from direct reading instruments.
Carbon dioxide measurement
Carbon dioxide concentration in the cargo chamber is important when fruit or
chilled beef is carried. The electrical CO2 indicator operates on
the principle that CO2 is a better heat conductor than air. A sample of air with
CO2 content, is passed over platinum resistance wires carrying a constant
heating current. Between the sample chambers CO2 is absorbed to give a
differential reading. The wire temperature is less when CO2 content is higher.
The temperature difference is detected on a Wheatstone bridge circuit through
a suitably calibrated milliammeter which gives a direct CO2 reading.
This very necessary operation presents no difficulty when the cooling medium
is brine. All that is required is a brine heater with brine pump and
circuits to circulate hot brine through the coolers.
In direct expansion systems, defrosting can be effected by separate electric
heaters installed in the evaporator grids or by providing a
means of bypassing the condenser so that hot gas from the compressor
circulates the evaporator directly.
Heat leakage and insulation
The total load on a cargo refrigerating plant is the sum of:
- surface heat leakage from the sea and surrounding air;
- deck and bulkhead edge leakage from the same sources;
- heat leakage from surroundings into system pipes;
- heat equivalent of fan and some brine pump power;
- cooling of cargo not precooled at loading;
- respiratory heat of live cargoes;
- heat introduced by air refreshment of live cargoes.
The load arising from 1, 2 and 3 can be much reduced by the efficient use of
insulation. A number of materials are used for this including slab cork, glass and
mineral wools, expanded plastics, aluminium foil and polyurethane. The latter,
although generally most costly, is the best insulator, having the lowest
coefficient of conductivity, with the further advantages of being impervious to
air leaks and almost impervious to the passage of vapour, when the material is
foamed in situ,
Materials which contain CFCs should not be specified. Some rigid urethane
foams (polyisocyanates and polyurethanes) and expanded polystyrene or
phenolics may contain CFCs. These materials are used for their low thermal
conductivity, high resistance to the passage of vapour, good mechanical
properties and ease of construction. They can be produced so as to be free of
CFCs but with higher thermal conductivity.
All of the materials mentioned have to be enclosed by linings for protection
and the prevention of air leakage. The design and construction of the linings
makes a greater contribution to the efficiency of an installation.
The heat balance test which replaced an earlier unsatisfactory version, was
introduced by the major Classification Societies in 1947. In this trial,
temperatures in the refrigerated spaces are reduced to a specified figure and
then after a lapse of time sufficient to remove all residual heat from the
insulation and structure, the spaces are maintained at constant temperature for
at least six hours by varying the compressor output. During this period all
temperatures and pressures, speeds and electrical consumption of compressors,
fans and pumps are carefully logged and the compressors' output is noted from
From this information it is possible to compare the efficiency of the
insulation with the theoretical estimate made during the design stage and also
to decide whether or not the installation can maintain these temperatures in
maximum tropical sea and ambient conditions. Obtaining the theoretical
estimate entails taking each external surface of the individual chambers
separately and considering all factors affecting the heat leakage. These factors
include the pitch, depth and width of face of all beams, frames and stiffeners
buried in the insulation, the type of grounds securing the linings, the presence
of which have their effect in reducing the effective depth of the insulation.
Hatches, access doors, bilge limbers, air and sounding pipes also have their
effect on heat leakage and must come into consideration. It should be noted
that in these calculations the laboratory value of the insulation is generally
increased by about 25% to allow for deficiencies in fitting.
It has been found that the overall co-efficient of heat leakage in well
insulated installations can vary between 0.454 W/m2/°C for 'tweendecks in
small lightly framed ships and 0.920 W/m2/°C for fully refrigerated moderate
sized ships having deep frames with reverse angles. Where there are also
buried air ducts, the effective depth of the insulation may reduce to little more
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.....
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
Marine diesel engines
||Steam generating plant
||Air conditioning system
||Emergency power supply
||Exhaust gas heat exchangers
||Feed extraction pump
|| Four stroke engines
|| Fuel injector
|| Fuel oil system
|| Fuel oil treatment
Lub oil filters
MAN B&W engine
Oily water separator
Overspeed protection devices
Piston & piston rings
Sewage treatment plant
Starting air system
Two stroke engines
Drydocking & major repairs
Deck machineries & cargo gears
|| Control and instrumentation
||Engine room safety