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Componenets required for a refrigeration plant on board cargo ships
Refrigeration is a process in which the temperature of a space or its contents is reduced to below that of their surroundings. Heat is a form of energy and is indestructible and it is not a substance and can be dealt with only from its effect on substances. When two substances are brought into thermal contact, the heat starts to flow one into the other until they are both at the same temperature, at which no more heat can flow. At this point, where no more heat can be extracted known as absolute zero.
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.
Example in melting ice produces refrigeration under easily controlled conditions and with a minimum expenditure of the refrigerant. It because large quantities of latent heat are absorbed by ice in melting. Similarly a liquid in changing its state to vapour absorbs large quantities at latent heat.
The vapour pressure corresponding to its freezing temperature and it critical temperature that above which the vapour cannot be liquefied no matter what pressure is applied (regardless of pressure) to it and its known as saturated temperature and the vapour known as dry saturated vapour.
If however, dry saturated vapour is not in contact with its liquid, it will rise in temperature and becomes a superheated gas.
However, if it is cooled with no vapour present, it becomes sub-cooled liquid.
Thus both superheated gas and sub-cooled liquid of any refrigerant have specific heats and the heat absorbed during evaporation considered as latent heat (when no further change in temperature) and the temperature at which a liquid evaporates depends directly on the pressure existing at its surface.
There are three main physical law on refrigeration principle which depends on:-
i) Any refrigerant in evaporating takes in heat forms it surrounding means latent heat of evaporation (no further change in temperature)
ii) The temperature at which a refrigerant evaporates depends on the pressure existing at the inter-face between the vapour and the liquid.
iii) Any vapour or gas can be condensed to a liquid by suitably compressing and cooling it latent heat of condensation.
A refrigeration plant on board employs below components.
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.
Reciprocating compressors cover the whole spectrum of refrigeration
requirements at sea, from air conditioning to low temperature cargo
installations. They are normally of a compact design and may be of an
in-line, V or W configuration.
The construction arrangement can be seen and the
principle of operation is similar in many respects to an air compressor,
For low-temperature applications the machine may be arranged as a
two-stage compressor and some machines are made so that they can be
changed from single to two stage, depending on cargo requirements. As
the crankcase is subject to refrigerant pressure, the drive shaft seal is
required to prevent a flow of refrigerant out of the compressor or
ingress of air. In semi-hermetic or hermetic machines this problem is
obviated as the motor and compressor are in one casing.
Screw compressors have replaced reciprocating compressors in large
installations for two reasons. Firstly, fewer and more compact machines
are used; secondly, a reduced number of working parts results in greater
reliability with reduced maintenance requirements.
There are two types
of screw compressor; one employs two rotors side by side and the other,
which is a more modern development, is a single rotor with two star
wheels, one on either side. As the star wheels compress the gas in
opposite directions, the thrust on this type of rotor is balanced. The principle of operation for both
types is similar to a screw-type positive displacement pump.
To achieve a seal between the rotors, oil is injected into the
compressor: to prevent this being carried into the system, the oil
separator is larger and more complex than the normal delivery oil
separator associated with a reciprocating compressor.
some of the heat of compression is transferred to the oil, a larger oi!
cooler has to be fitted, which may be either water or refrigerant cooled.
Since a.c. motor driven compressors are usually single speed, some
form of cylinder unloading gear is necessary to reduce the compressor
capacity. This unloading gear usually comprises a means of holding the
suction valves open.
Condensers are generally water cooled, as mentioned previously, and
are of the shell and tube type. A typical modern unit is shown in Figure
in which it will be seen that the refrigerant passes over the tubes and
the cooling water is passed through the tubes. In the case of sea water
cooled condensers it is usual to have a two-pass arrangement through
Where condensers are of 3 m and over in length between tube plates it
is quite usual to have a double refrigerant liquid outlet so that the
refrigerant drains away easily when the vessel is pitching or rolling.
Read more on marine condenser troubleshoot guideline
Evaporators fall into two categories: refrigerant to air and refrigerant to
secondary refrigerant types.
The most simple of the refrigerant to air type is in the form of a bank
of tubes with an extended surface of gills or fins. In these the refrigerant
is expanded in the tubes while the air is passed over the fins by
This type of unit will be found in the domestic cold
stores in which the fan and coil unit are one, and a larger version in
direct expansion cargo or air conditioning systems where the fan or fans
may be remote.
A more elaborate design is used for secondary refrigerant cooling
which takes the form of a shell and tube vessel. Such a type is illustrated
in Figure and employs direct expansion. In this case the refrigerant
passes through the tubes and the secondary refrigerant is passed over
the tube bank. The refrigerant is sprayed into the tubes so as to ensure
an even distribution through all the tubes.
Any oil present is not sprayed
and drains away. In this type of evaporator two features are employed to
improve heat transfer efficiency. On the refrigerant side there is a
centre tube with a spiral fin fitted around it (as illustrated) or the insert
may be in the form of an aluminium star which has a spiral twist on it.
Also, baffles are arranged on the brine side to deflect the brine across
the tube bank.
Refrigerant flow control valves
It is usual to have a solenoid valve in the liquid line prior to the
expansion valve or regulator. This shuts or opens as determined by the
thermostat in the space or the secondary refrigerant being cooled. It
may also be used to shut off various circuits in a cooler when the
machine is operating on part-load conditions.
Fig:Thermostatic expansion valve or regulatoror
The expansion valve/regulator is a more complex piece of equipment
which meters the flow of refrigerant from the high-pressure to the
low-pressure side of the system. This may be of the thermostatic type, as
shown in Figure .
The bulb senses the temperature of the refrigerant
at the outlet from the evaporator and opens or closes the valve
accordingly. The design of the valve is critical and is related to the
pressure difference between the delivery and expansion side. Therefore,
it is essential that the delivery pressure is maintained at or near the
maximum design pressure.
Thus, if the vessel is operating in cold sea
water temperatures it is necessary to re-circulate the cooling water to
maintain the correct delivery pressure from the condenser. If this is not
done, the valve will 'hunt' and refrigerant liquid may be returned to the
Delivery oil separators are essential for screw compressors, but for other
systems, depending on the design criteria and length of pipe run, they
may or may not be fitted.
Refrigerant driers are essential with the Freon gases to remove water
from the system, otherwise freezing of the water can take place in the
A liquid receiver may be fitted for two reasons. Firstly, to give a
sufficient reserve of refrigerant in the system to cater for various
operating conditions (this is known as a back-up receiver). Secondly, for
storage of the refrigerant where it is required to pump over, i.e. store,
the charge for maintenance purposes. In very small systems this pump
over can sometimes be achieved in the condenser.
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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