FWG : Seawater into fresh water

Maritime transport plays a critical role in the global economy. Over 80% of the volume of international trade in goods is carried by sea. We have better understood how important this trade is, and the key role of seafarers, especially in the COVID-19 pandemic and the war in Ukraine we have been experiencing in recent years. So, how do the ships, which are gigantic floating factories, get the water needed for the operations and the personnel living in them while they continue their voyage for weeks through oceans? Do you think the seafarers drink sea water, or do they carry out all their needs with the bottled water they get from the ports? Let’s look at the answer to this together.

Sea water poses a risk for humans in long-term use in terms of the minerals it contains, causing dehydration and serious complications. For this reason, seafarers have another option besides consuming bottled drinking water taken from the ports on the ships, the miraculous machine that turns seawater into fresh water: the Fresh Water Generator!

Yes, you heard right, it is possible to obtain fresh water by separating the minerals contained in sea water. But there are places where this water is separated according to its intended use. If fresh water is to be consumed as drinking water, it has to go through a few additional processes on board. Before we talk about how, let’s look at other fresh water areas on ships that are used for non-potable purposes and why we can’t simply use sea water there.

The most important reason why we produce fresh water on board is that fresh water does not cause corrosion as much as sea water. Ships, as factories going in the ocean, run all their systems, from electricity to compressed air, with self-sufficient huge machines. These machines also need to be cooled like every machine. Fresh water plays an important role in this regard, reducing the effects of corrosion while providing cooling of machines such as the compressor producing compressed air, the generator producing electricity, and the main engine giving the thrust to turn the propeller. However, it is worth mentioning that the fresh water that cools the main engine is circulated between the expansion tank and the fresh water generator in a closed circuit by adding certain chemicals and also referred to as jacket cooling water. We will use this information later. Generators and compressors also use this closed circuit water. Moreover, the main substance of the steam to be produced by the steam generating boilers is fresh water, which has undergone chemical processes and is referred to as boiler feed water.

So how does the fresh water generator work, which turns sea water into fresh water? In short, its working principle is to produce a lowpressure area inside by an ejector and boil sea water at low temperatures, then making evaporated steam pass through a filter for separating salt and particulates and converting this steam into fresh water in a condenser. Let’s go into some details.

First of all, sea water enters the ship’s engine room by passing through filters called sea chests at the bottom of the ship. The ejector pump sucks this water and pumps it to the evaporator part of the fresh water generator by passing it through the ejector, which acts as a vacuum reducer. The evaporator is a heat exchanger where two different water meet with tubes or plates according to the type. Here, our low-temperature water is sea water, while the high-temperature water is the jacket cooling water that cools the main engine we mentioned before. The main engine is cooled with water at about 80 degrees Celsius in order not to be exposed to thermal stresses, and the waste heat energy of this water is used in the fresh water generator, so the energy is used more efficiently. Thanks to the ejector, which decreases the evaporator chamber’s pressure below the atmospheric pressure, 4050 degrees Celsius of sea water can boil with the jacket cooling water of 80 degrees Celsius!

The boiling water vapor passes through the filter called demister, leaves its carryover salts on the filter, and enters the other heat exchanger called the condenser. Since the sea water passing through here is at a lower temperature than the water vapor, it condenses the steam to become liquid, and thus our fresh water is obtained. This water is stored in tanks through pumps and is ready to be used for non-potable purposes.

On the other hand, the water to be used as drinking water, passes through the mineralizer to gain enough minerals and ultraviolet filter to kill bacteria and finally goes to the accommodation area, by being ready to be consumed by seafarers.

Hakan Yağcı

Sources

UNCTAD: Review of Maritime Transport 2022

Ozonlabs: Deniz Suyu İçmenin Zararları

Marine Engineering Online: Fresh Water Generator on Ships

Marine Insight: Fresh Water Generator

Explained

Sasebo Shipyard Ross Sea: Piping Diagram in Engine Room

Photo: Annie Spratt | Unsplash