All about Refrigeration system on board the ship

Construction and Working of Ships Refrigeration plant

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.

Credits: depositphotos.com

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

What are the Safety Devices on the Refrigeration System of a Ship?

A reefer system is the back bone of ships carrying refrigerated cargo. A malfunction of any of the components of the system can lead to degradation and wastage of perishable and cold storage cargo including provision for a ship. It is therefore important to maintain and run the refrigeration plant properly to avoid any breakdowns.

In this article we will learn about the safety feature of the refrigeration plant. Before that, you must read the construction and working of the ship’s refrigeration plant.

To have an efficient and hassle free operation of a ship’s refrigeration plant and its components, a good maintenance schedule and safety systems are adopted.

A safety system includes alarm, cut offs, and trips which safeguards the machinery and its parts from getting damage.

The main safeties adopted for refrigeration plants are

• 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.

How to Charge Refrigeration Plant on Ships?

The main use of refrigeration plants on ships is to keep cargo and food provisions at low temperature to prevent them from spoiling. The reefer plant is a combination of various machines such as compressor, valves, heat exchangers etc., which are connected to each other through pipes and joints.

While in operation, the refrigerant used in the refer plant gets consumed or is reduced in quantity because of leakage in the system. Reduction in quantity of refrigerant may lead to troubles in the plant such as-

• Short Cycling of Compressor

• Too low suction pressure

• Difficult to maintain temperature of rooms and holds

• Reduction in the efficiency of the plant

When the above mentioned problems occur, it indicates that the plant has to be charged with the refrigerant.

There are two methods for charging reefer plants: Liquid charging and Gas charging.

Now a day’s gas charging is preferred over liquid charging because it is more safe and simple.

Gas Charging of Refrigeration Plant:

For gas charging, a special T piece valve block with mounted pressure gauge is provided to combine three connectors inter-connecting:

-Vacuum pump

-Charging Cylinder

-Charging Point

Following steps are to be taken for charging gas into the reefer plant:

1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.

2. The discharge of the vacuum pump is to be connected in the empty recovery bottle

3. First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block

4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block

5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system

6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut

7. Now open the main valves in the charging cylinder and charging point of the reefer system

8. Do not overfill the system. Make sure the receiver has 5 % space for expansion

Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the atmosphere.

Note* :Gas bottle is kept on weighing scale for measuring the amount of charged supplied to the system.

More links related to refrigiration and its components:

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