Seafarers sailing out on the deep seas are surrounded by sea water but not a single drop of it is fit for drinking. How do they source fresh water?
In the days of the past, seafarers sailing on ships powered by the winds and ocean currents had two options. They either stored fresh water in tanks before departing from the port or collected rainwater and used it for domestic purposes. Both options required exercising a very tight control on its use, and hence sailors rationed the precious commodity strictly.
However, in the modern world where ships are multi-fold bigger and powered by diesel engines, the source of fresh water is entirely different.
Even if the ship can lift fresh water at ports if required, this water is only a backup as this is expensive and large quantities are required.
Water on modern ships is required not only for human and domestic use, but also, and more importantly, for producing steam in the boilers for heating, and cooling the machinery.
Fresh water is economically produced from the surrounding sea water using the process of distillation. All of us are familiar with the experiment which we can recollect from our school days, when saltwater was boiled in a flask and the rising vapour was collected and condensed in a different location, thus separating the salt from the water.
A similar principle is applied; while saltwater in the school lab had to be heated to 100°C to get it boiling, on an ocean-going ship, the water is made to boil off (flash off) at much lower temperatures of around 52°C in a vacuum chamber.
The temperature of the incoming sea water is typically in the range of 2-30°C, depending on the ocean region and season in which the ship is sailing. Regardless, its temperature still needs to be raised to over 50°C for it to flash off in the vacuum chamber.
How does this temperature increase take place?
The heat generated by the main engine is used to heat up the sea water from its ambient temperature to above 50°C. The thus heated seawater is then admitted to the vacuum chamber where it flashes off leaving a salt rich solution called brine, which is removed.
The flash off is directed to another chamber where the vapour is cooled by the same, colder incoming sea water. The vapour condenses into trays and is then led into storage tanks. The fresh water produced in this manner has salt content as low as 10 ppm (parts per million or mg/kg) and can be used for both machinery and domestic use.
On an average, about 20-30 tons of fresh water can be produced depending on the size of the Fresh Water Generator, which is sufficient for a crew of 25 persons, engines and the boiler.
On cruise liners and passenger ships however, where the number of persons onboard are several hundred and the ships are propelled by multiple engines, a much larger quantity of fresh water is required. These ships use special reverse osmosis systems which produce very large quantities of freshwater – over 250 tons/unit/day – from seawater .
The principle of reverse osmosis is very different from the distillation process described above. Seawater is pumped under high pressure of 50 bar through special membrane tubes. These tubes also separate the salt; they are permeable only to fresh water which they produce with a salt content of around 200 ppm. This water is still satisfactory for human consumption and domestic use.
Salt content in the freshwater produced from seawater is critical especially when water is used in machinery or boilers as it can corrode or form hard scales on the surfaces of the equipment, leading to failure. The fresh water produced is short of minerals and nutrients required for human consumption, and hence is passed through mineralizers and sterilizers before being consumed or put to use.
Only distillation can remove the salt to very minute levels. It enables us to not only use the waste heat of the main engine but also produce large quantities of water every day for all forms of use onboard ocean-going ships. Indeed, this process is a great example of how humanity’s advances in science and technology have paved the way forward in making the seemingly impossible possible.
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