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Refrigeration of cargo spaces and storerooms

Refrigeration is a process in which the temperature of a space or its contents is reduced to below that of their surroundings. Refrigeration is used in the carriage of some liquefied gases and bulk chemicals , in air conditioning systems, to cool bulk CO2 for fire fighting systems and to preserve perishable foodstuffs during transport of foodstuff .

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Ships refrigeration plant may vary from the small domestic refrigerating unit for provisions to large plant for reefer vessels. The Chief Engineer is responsible for the correct temperatures being maintained, delegating the good operations and maintenance of the plant to the 2/E. Larger plants may have a Refrigeration Officer. Machinery under ship's engineer responsibility may include: All maintenance recommendations from the makers have to be carried out regularly and according to instructions, entered in the refrigeration maintenance log, together with the test of all cut outs, i.e. HP, LP, LO, HT, that have to be carried out at regular intervals, generally one month.

All adjustment must be made according to standard good practice and records of the same entered in the log.

Filter separators and driers should be regularly cleaned in order to have always the circuit moisture, dirty and oil free. When shutting down a plant all refrigerant gas must be pumped in the liquid receiver or condenser.

Four basic components of a refrigeration system The four main components of a refrigeration system working on the vapour compression cycle are:
Compressor :The function of the compressor in a refrigeration system is to raise the pressure of the vapourised refrigerant, causing its saturation temperature to rise so that it is higher than that of seawater or an air cooled condenser. The compressor also promotes circulation of the refrigerant by pumping it around the system.

Condenser :The function of the condenser is to liquefy the refrigerant and sub cool it to below the saturation temperature by circulating seawater or air. Latent heat originally from the evaporator is transferred to the cooling medium. The liquid refrigerant still at pressure produced by the compressor passes on to the expansion valve.

Expansion valve : The function of the expansion valve in a refrigeration system is to regulate the flow of refrigerant from the HP side of the system to the LP side of the system. The drop in pressure causes the saturation temperature of the refrigerant to fall so that it will boil at the low temperature of the evaporator. The expansion valve controls the flow of refrigerant to the evaporator thermostatically.

Evaporator : The function of the evaporator in the refrigeration system is to cool the air in the fridge space. It does this because the temperature of the refrigerant entering the evaporator is lower than that of the air in the space and this causes the refrigerant to receive latent heat and evaporate. The evaporator normally has a fan to circulate the air around it.

Three desirable properties of a refrigerant are:
  1. Low boiling point
  2. low condensing pressure
  3. high specific enthalpy of vaporisation. (This reduces the quantity of refrigerant in circulation and lower machine speeds, sizes, etc).
The effects of insufficient refrigerant in the system are a low reading on the LP pressure gauge and a lack of frost on the suction pipe.

The high pressure (HP) cut out is fitted on the discharge side of the compressor in a refrigeration system. This will shut down the compressor in the event of an over pressure and can only be manually reset.

Details of refrigeration cycle -How the system works ?

Refrigeration of cargo spaces and storerooms employs a system of components to remove heat from the space being cooled. This heat is transferred to another body at a lower temperature. The cooling of air for air conditioning entails a similar process.

The transfer of heat takes place in a simple system: firstly, in the evaporator where the lower temperature of the refrigerant cools the body of the space being cooled; and secondly, in the condenser where the refrigerant is cooled by air or water. The usual system employed for marine refrigeration plants is the vapour compression cycle as shown in diagram here.

Vapour compression cycle

Fig: Vapour compression cycle

The pressure of the refrigerant gas is increased in the compressor and it thereby becomes hot. This hot, high-pressure gas is passed through into a condenser. Depending on the particular application, the refrigerant gas will be cooled either by air or water, and because it is still at a high pressure it will condense. The liquid refrigerant is then distributed through a pipe network until it reaches a control valve alongside an evaporator where the cooling is required. This regulating valve meters the flow of liquid refrigerant into the evaporator, which is at a lower pressure. Air from the cooled space or air conditioning system is passed over the evaporator and boils off the liquid refrigerant, at the same time cooling the air.

The design of the system and evaporator should be such that all the liquid refrigerant is boiled off and the gas slightly superheated before it returns to the compressor at a low pressure to be recompressed.

Thus it will be seen that heat that is transferred from the air to the evaporator is then pumped round the system until it reaches the condenser where it is transferred or rejected to the ambient air or water.

It should be noted that where an air-cooled condenser is employed in very small plants, such as provision storerooms, adequate ventilation is required to help remove the heat being rejected by the condenser. Also, in the case of water-cooled condensers, fresh water or sea water may be employed. Fresh water is usual when a central fresh-water/sea-water heat exchanger is employed for all engine room requirements. Where this is the case, because of the higher cooling-water temperature to the condenser, delivery temperatures from condensers will be higher than that on a sea water cooling system.

Temperature Records

Temperatures of domestic refrigerated rooms have to be corrected daily by the 2nd Engineer or delegated Officer, passed to the Chief Engineer and to the Master. On larger plant suitable logs will be supplied in order to enter temperature of the cargo and all other relevant details.

Related Info:

  1. Refrigeration system components
  2. Centrifugal,reciprocating, or screw compressors. Modern Shell and tube type condensers.Various refrigerant types, refrigerant flow control valves & Ancillary fittings etc.....

  3. Modern refrigerants for cargo ships
  4. 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.....

  5. Choice of refrigerants
  6. 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 .....

  7. Cargo refrigeration procedure
  8. 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.....

  9. How to troubleshoot refrigeration system faults
  10. 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.....

  11. Safety precautions for refrigeration plant and refrigerated compartments
  12. 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.....

  13. Reefer container
  14. 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......

  15. Air conditioning
  16. Single-duct & twin-duct marine air conditioning system - An air conditioning system aims to provide a comfortable working environment regardless of outside conditions. Satisfactory air treatment must involve a relatively 'closed' system where the air is circulated and returned......

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Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump || Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor || Marine incinerator || Lub oil filters || MAN B&W engine || Marine condensers || Oily water separator || Overspeed protection devices || Piston & piston rings || Crankshaft deflection || Marine pumps || Various refrigerants || Sewage treatment plant || Propellers || Power Plants || Starting air system || Steam turbines || Steering gear || Sulzer engine || Turbine gearing || Turbochargers || Two stroke engines || UMS operations || Drydocking & major repairs || Critical machinery || Deck machineries & cargo gears || Control and instrumentation ||Fire protection ||Engine room safety ||

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