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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.

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.

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.

Also, because 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

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 the tubes.

condensers

Fig:Condensers

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

Evaporators fall into two categories: refrigerant to air and refrigerant to secondary refrigerant types.

EvaporatorFig:Evaporator

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 circulating fans.

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.


Thermostatic expansion valve or regulator

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 compressor suction.


Ancillary fittings

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 expansion valve.

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.



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