Tuesday, January 7, 2020

Reefer system on ships

Main Components of Refrigeration plants

Any refrigeration unit works with different components inline to each other in series. The main components are:
Compressor: Reciprocating single or two stage compressor is commonly used for compressing and supplying the refrigerant to the system.
• Condenser: Shell and tube type condenser is used to cool down the refrigerant in the system.
Receiver: The cooled refrigerant is supplied to the receiver, which is also used to drain out the refrigerant from the system for maintenance purpose.
Drier: The drier connected in the system consists of silica gel to remove any moisture from the refrigerant.
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.
Expansion valve: An Expansion valve regulates the refrigerants to maintain the correct hold or room temperature.
Evaporator unit: The evaporator unit act as a heat exchanger to cool down the hold or room area by transferring heat to the refrigerant.
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 manually 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.

1. Reefer compressor safety- 

Low Pressure or LP cut off: This is a compressor safety which cut off the compressor in the event of pressure drop in the suction line. The pressure of the suction line is continuously sensed by the control unit and when it goes below the set value, which means the room is properly cooled, the LP cut out will auto trip the compressor. When the pressure rises, indicating there is flow of refrigerant in the line due to increase in room temperature, the LP switch will start the compressor.
High pressure or HP cut out: As the name suggests, the high pressure cut out activates and trips the compressor when the discharge side pressure increases above the limit value. The HP cut out is not auto reset and has to be done manually. The reason behind it is to manually attend the fault which is leading to rise in pressure, else this situation can lead to overloading of compressor parts and may damage the same.
Oil differential cut out: This safety is again for compressor as it is the only machinery in the circuit having rotational parts which requires continuous lubrication. In the event of low supply or no supply of lube oil to the bearing, the differential pressure will increase and activates a trip signal to safeguard the bearing and crankshaft.
Relief valves: Relief valves are fitted in discharge side of compressor and will lift and safeguard the compressor in the event of over pressure. One relief valve is also fitted in the condenser refrigerant line to avoid damage to the condenser if there is high pressure in the discharge line.
Solenoid valves: Master solenoid valve is fitted in the common or main line after the condenser discharge. It closes when compressor stops or trips to avoid over flow of refrigerant in to evaporator. All holds or rooms are fitted with individual solenoid valve which control the flow of refrigerant to that room.
Oil heater: Oil heater is provided for the compressor crank case oil and prevents compressor from getting excessively cold which may effect the lubrication of the parts.
• Bursting disc in between suction and discharge- nickel with thickness 0.05mm. – pg-344 mc-G
• Fridge rooms high temp alarm
• Cooling water low press alarm
• Motor overload
• Safety spring valve in discharge valve which allow the complete lift of discharge valve when liquid enters in at suction and thus it shortcycle the refrigerant
• Belt driven – (he will cross question any safety valve on head of comp in belt driven- NO)Relief valve on condenser
• Unloaders 

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