Refrigerated cargo, initilly used to be carried in one of the compartments of a general cargo ship. The compartment, invariably would be just forward orjust aft of the engineroom.
Different Refrigerated Cargoes
Frozen Cargoes: These cargoes are carried in frozen state and is also in deep frozen prior to loading. The temperature maintained is about -8 to -120C. This temperature keeps the growth of bacteria down. Deep frozen cargoes are not living, and do not respire or produce heat, so the task of the refrigeration system is much simpler.
Chilled Cargoes:. These cargoes are carried at temperatures around -20 to +60C. In case of chilled cargoes, to maintain the right temperature is very important. The precise temperature control is more important than frozen cargoes. Examples of cargoes in this category are: cheese, eggs & fresh vegetables.
Air Cooled Cargoes: These cargoes are carried at a temperature from +20 to 120C. Generally these types of cargoes are fruits.
Properties of Refrigerated Cargoes
- Rapid deterioration if proper temperatures are not maintained during loading, voyage and discharging.
- Susceptible to tainting and moisture contact damage.
- Effected by presence of CO2.
Carriage temperatures of certain cargoes
Frozen lamb, mutton, beef, pork – 8oC to – 10oC
Packed in cases and carried at – 10oC to – 12oC
Fish < -120C
Cheese +50C to 7oC
Apple +10C to 20C
Pears -10C to 00
Grapes & peaches -1oC to 2oC
Oranges and lemons +20C to 6oC
Vegetables Above zero
Onions and potatoes Ventilation
Construction of Reefer Space
Insulation must be provided to enclose the refrigerated compartments effectively. Acrylic foam, cork, glass reinforced plastic, etc may be used for insulating purposes. Glass fiber has many advantages that are desired. Thus, it is light, vermin proof and fire resistant, and does not absorb moisture.
On the decks and tank top, the insulation must be appropriate to take load. Thickness would depend on the type of material used and the temperature to be maintained in the compartment. Insulating material is placed between hull plating and the GI sheets or aluminium alloy using metal screws. The frames are used to provide support.
An air space of about 50 mm is maintained between tank top and the insulation above.
Suitable insulated doors are provided to cold rooms. These are 25 to 30 cm thick doors of GI sheets covering the inside material of steel frames wooden supports and insulation. Brine traps are provided in drains from the tween decks and insulated holds. The brine in the trap forms an effective seal against ingress of warm air. It does not freeze, thus, helps in removal of water from the compartment.
A reefer ship may be of two types:
1. Conventional vessels with normal hatches and cranes, derricks, etc. Hatches must be closed if it rains.
2. Vessels with side doors. Ramps, conveyors, etc shorten the turn around time considerably. During rain the cargo work may continue.
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; and
- to preserve perishable foodstuffs during transportation.
Ships refrigeration plant can be for:
- the small one for provisions; or
- large one for reefer vessels.
Refrigeration engineer or Chief engineer as the case may be is responsible for:
- Domestic ref. plant.
- Cargo ref. plants
- Air conditioning plants
- Ventilation and heating plants
- Cargo refrigerated containers
Components of a Refrigeration System
The four main components of a refrigeration system working on the vapour compression cycle are:
- The compressor
- the condenser
- the expansion valve
- the evaporator.
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.
- Promote 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.
- Transfer latent heat received from evaporator to the cooling medium.
- Allow the liquid refrigerant (still at pressure) to go 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) causing it to boil at the low temperature of the evaporator.
- Control the flow of refrigerant to the evaporator thermostatically.
Evaporator : The function of the evaporator is to:
- Cool the air in the fridge space.
- Cause the refrigerant to receive latent heat and evaporate.
- Through a fan circulate the air around it.
Three desirable properties of a refrigerant are:
- Low boiling point
- low condensing pressure
- high specific enthalpy of vaporisation. (This reduces the quantity of refrigerant in circulation and lower machine speeds, sizes, etc).
Details of refrigeration cycle
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 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.
- 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 reaches a control valve alongside an evaporator. This regulating valve meters the flow of liquid refrigerant into the evaporator, which is at a lower pressure.
- The liquid refrigerant, boils and evaporates cooling the air.
- The gas is slightly superheated as 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.
Preparation of Spaces to Receive Cargo
The refrigerated cargo requires very careful preparation of the space. Cleanliness is of great importance I respect of foodstuffs. The space must be free of odours and micro-organisms. The brine pipes, insulation, bins, gratings, air ducts, etc need particular attention. Disinfectant fluid may be used to prevent formation of mould. Deodorizing agents may be used for air and surface. The lingering smell, which is absorbed by the insulation may require the removal and renewal of the respective gratings, etc. Fans should be run in both directions to clear smells and dust, etc., from the air trunking.
Cleaning at ambient temperature
It is desirable that holds are allowed to warm to ambient temperature before cleaning begins. The odours of residue from previous cargo are then more readily detectable and are not masked by the residue remaining in frozen state.
Bilges and scuppers
Bilges must be clean, tested and U-bend vapour traps sealed with brine to prevent cross taint between compartments. Brine traps are provided to seal the drain pipe from tween decks to bilges in order to allow only one drain passage from tween deck to bilges and stop any back flow of gases/ foul odours from bilges to cargo space. These should be checked and topped up to prevent cold air from entering the bilges and freezing them or odour from reaching the refrigerated compartment
Sensors and indicators
Thermometers, gas sampling points, fire detection and extinguishing equipment should be carefully checked and inspected.
Pipes and joints
Inspection should be made of any pipes passing through the space, particularly their joints, for signs of leakage.
Insulation and gratings
Timber dunnage though common, nowadays, reefer ships are fitted with large gratings bolted to the deck and folding `tween deck hatch covers. The gratings can be of timber or alloyed aluminium construction. Plastic gratings have been tried now. The gratings are of sufficient strength to support a forklift truck with a loaded pallet.
Closing arrangements (weather and `tween deck covers) and access hatch plugs should also receive careful examination.
The space can then be cooled down to slightly below the carrying temperature and held there for at least 24 hours to ensure that all the residual heat is removed from insulation and other fittings within the space. The air temperature will quickly rise when the compartment is opened for loading and every opportunity should be taken to run the fans during breaks in the loading operation.
Advantages of precooling:
- temp fluctuations reduce during loading;
- carriage temp is achieved ASAP; and
- dunnage is cooled.
Ventilation after cooling
- After cooling, or throughout in the absence of cooling, the rate of fresh air ventilation for fresh produce should be specified.
Loading, handling and carriage
General and particular carriage requirement must be known to the ship’s staff. Colour of fruits, characteristics, origin, age, etc would be useful.
Condition as received
- Fruit should not be ripe. Thus, upon peeling banana skin it should give fiber thread.
- Softened carcasses must be rejected. Blood-stains indicate that a carcass has partially thawed since initial freezing and should be considered with suspicion.
- Cargo received for carriage should be pre-cooled to the carrying temperature.
Maintaining the temperature
- Hold duly cleaned and deodorized must be maintained at suitable temperature for the commodity.
- Close monitoring and maintaining specified temperature during loading, transportation and discharging very important.
- It may be necessary to close a compartment for some time and then re open it to maintain required temperature. Working shoes should be covered with appropriate material, when in reefer space.
- The possible rise in the temperature of the surface during transit should be attended. Chilled cargoes are of less concern than the cooled cargoes, but great care must be taken during receiving and stowage due their sensitive nature.
- 1Frosting of grid pipes can reduce cooling due the formation of insulating layer and give water upon melting. Cooling must be adjusted accordingly.
- Compartments not in use should be kept closed.
Correct stowage is important
- To carry different commodities in a single cargo space could be complex. Different temperature requirement, compatibility, tainting, etc must be considered.
- Cargo should not be dragged or thrown. It should be handled with proper slings.
- Thick paper should be pasted over joints to prevent air leak.
- The airflow and securing must be considered while stowing cargoes.
- Dunnage should be used efficiently: to cause required separation from metal; form appropriate ventilation channel; support frazil cargo; and protect ducts, etc.
- Fruit should not be stowed with any other cargo that might be tainted.
- Individual commodities should be kept separated, even though they may require the similar temperature and humidity.
Ventilation and odour
- Control of CO2 concentration in the compartment with ventilation is essential for certain commodities
- Fruit and vegetables respire, taking in oxygen and producing carbon dioxide. This produces heat. The temperature change should be controlled accordingly.
All fruits and vegetables need ventilation even if carried at ambient temperatures.
- Even a pleasant smell trespassing into some food product where any foreign smell may cause a loss of taste. When dealing with living cargo adequate amount of fresh air must be introduced. Also, the expelled air is not drawn into a space where it might cause contamination.
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