Freshwater is one of the most critical resources on a ship. Due to the limited storage capacity of a ship, it becomes unviable to carry sufficient drinking and domestic water for long voyages. Hence, ships utilize Fresh Water Generators (FWG) to produce drinkable and usable water from seawater. The most common types of FWG are Shell and Tube FWG, which is simple in design, reliable and requires low maintenance.
Shell and Tube Fresh Water Generator is a condensation type desalination system that produces fresh water from seawater using the waste heat from the main engine jacket cooling water.
The evaporation process takes place under a vacuum so that seawater boils at a temperature lower than the normal boiling point around 45-60°C.
Main Components
The components of Shell and Tube FWG are:
The Shell and Tube Fresh Water Generator works in a series of simple steps:
Seawater is drawn into the system by the ejector. Part of this water is used for cooling in the condenser, while the remaining portion enters the evaporator.
An air ejector or vacuum pump creates a vacuum inside the generator. This reduces the boiling point of seawater allowing evaporation at relatively low temperatures.
Hot jacket cooling water from the main engine flows through the tubes of the evaporator. The surrounding seawater absorbs this heat and begins to evaporate.
The generated steam passes through a demister, which removes salt particles and water droplets. This ensures that only clean water vapour reaches the condenser.
Cold seawater flowing through the condenser tubes cools the water vapour converting it into fresh water.
The distilled water is collected in a chamber and pumped to the ship’s fresh water tank. Before storage, the salinity is checked by a salinometer to ensure the water meets acceptable quality standards.
Some key benefits include:
Despite its reliability, certain issues may affect the system’s performance:
To ensure efficient operation:
In conclusion, Shell and Tube Fresh Water Generator is one of the most reliable systems used to provide freshwater in the maritime industry. It provides a sustainable way of obtaining fresh water from a plentiful supply of seawater while also saving fuel. This system achieves significant improvement in energy efficiency by using the main engine’s jacket waste heat to evaporate seawater stored in the evaporator.
Moreover, the system operates under a vacuum to allow the evaporation process to take place effectively at relatively low temperatures.
Hence, with proper maintenance and inspection, FWG can be a crucial component of a ship’s operation.
Operating under vacuum lowers the boiling point of seawater, allowing evaporation at low temperatures using waste heat.
The demister removes salt droplets from water vapour before condensation, ensuring high quality fresh water.
The system generally uses the main engine jacket cooling water as its heat source.
It continuously monitors the salt content of the produced water and diverts water with excessive salinity to drain if necessary.
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