The refrigeration plants on merchant vessels play a vital part in carrying refrigerated cargo and provisions for the crew on board. In reefer ships, the temperature of the perishable or temperature sensitive cargo such as food, chemical, or liquefied gas, is controlled by the refrigeration plant of the ship. The same plant or a smaller unit can be used for maintaining the temperature of different provision rooms carrying food stuffs for crew members.
The main purpose of ship’s refrigeration plant is to avoid any damage to the cargo or perishable material so that it is transported in good and healthy condition. Refrigeration prevents growth of micro-organisms, oxidation, fermentation and drying out of cargo etc.
Main Components of Refrigeration plants
Any refrigeration unit works with different components inline to each other in series. The main components are:
1. Compressor: Reciprocating single or two stage compressor is commonly used for compressing and supplying the refrigerant to the system.
2. Condenser: Shell and tube type condenser is used to cool down the refrigerant in the system.
3. Receiver: The cooled refrigerant is supplied to the receiver, which is also used to drain out the refrigerant from the system for maintenance purpose.
4. Drier: The drier connected in the system consists of silica gel to remove any moisture from the refrigerant
5. Solenoids: Different solenoid valves are used to control the flow of refrigerant into the hold or room. Master solenoid is provided in the main line and other solenoid is present in all individual cargo hold or rooms.
6. Expansion valve: An Expansion valve regulates the refrigerants to maintain the correct hold or room temperature.
7. Evaporator unit: The evaporator unit act as a heat exchanger to cool down the hold or room area by transferring heat to the refrigerant.
8. Control unit: The control unit consist of different safety and operating circuits for safe operation of the refer plant.
Working of Ship’s Refrigeration Plant
The compressor acting as a circulation pump for refrigerant has two safety cut-outs- Low pressure (LP) and High Pressure (HP) cut outs. When the pressure on the suction side drops below the set valve, the control unit stops the compressor and when the pressure on the discharge side shoots up, the compressor trips.
LP or low pressure cut out is controlled automatically i.e. when the suction pressure drops, the compressor stops and when the suction pressure rises again, the control system starts the compressor. HP or high pressure cut out is provided with manual re-set.
The hot compressed liquid is passed to a receiver through a condenser to cool it down. The receiver can be used to collect the refrigerant when any major repair work has to be performed.
The master solenoid is fitted after the receiver, which is controlled by the control unit. In case of sudden stoppage of compressor, the master solenoid also closes, avoiding the flooding of evaporator with refrigerant liquid.
The room or hold solenoid and thermostatic valve regulate the flow of the refrigerant in to the room to maintain the temperature of the room. For this, the expansion valve is controlled by a diaphragm movement due to the pressure variation which is operated by the bulb sensor filled with expandable fluid fitted at the evaporator outlet.
The thermostatic expansion valve supplies the correct amount of refrigerants to evaporators where the refrigerants takes up the heat from the room and boils off into vapours resulting in temperature drop for that room.
This is how temperature is maintained in the refrigeration plant of the ship.
You might also like to read – Guidelines on Quality of Refrigerant Used on Ships
References: Marine Aux Machinery by H.D.McGeorge