Converting Sea Water Into Drinkable Water In A Tanker

When on a voyage, tankers utilize water that is purified throughout journey by in-house fresh water making plants. As a matter of fact, distilled or fresh water is utilized in tankers as an engine cooling medium. In the past, the purification process was done through evaporators that used steam from the main diesel cooling water as a heating medium to evaporate the sea water. Unfortunately, these evaporators were not as efficient as the types that are used today.

Types of evaporators utilized abroad tankers

There is a wide array of different kinds of efficient evaporators in use to produce fresh water on tankers today. These include:

• Multi-stage flash evaporators

These kinds of evaporators utilize a multi-stage process that has two main components; the flash drum and the sea water heater. However, what should be noted is that these are two separate units. Depending on the main propulsion unit, seawater can be heated using the main engine cooling and heating system or steam. The heated water is then pumped into the flash drum, which has multiple sections, but all at a lower pressure than that of the water heater.

Most of the hot seawater is flashed through the first section, before going on through the remaining sections, flashing as it moves through them. Steam then rises up the flash drum through a demister. Upon making contact with the condenser tubesis, it is condensed and then pumped through a salinometer to the boiler feed or fresh water tanks. Should the salt content in the distillate rise to an unacceptable level, the salinometer’s alarm goes off, resulting to the distillate being diverted to bilges.

• Tube and coil evaporators

This system consists of a steel container that has a series of heating pipes near the bottom of another container that is fed by hot water or steam from the main engine. A tube condenser that is cooled by seawater is installed near the top of steel container. But how does it work? A vacuum is drawn in the container by air ejectors that are operated by pressurized sea water or steam.

Seawater is fed into the evaporator covering the heating pipes. Heat is then supplied to the pipes, and this combined with vacuum conditions starts to boil the seawater to produce steam. This steam then rises up through a demister into the tube condenser where it is evaporated to form distilled water. This water is then collected and pumped through the salinometer to the storage tanks.

• Osmosis equipments

Seawater can also be converted into fresh water through the process of reverse osmosis. Osmosis is a natural process that occurs as a result of osmotic pressure occurring between two substances that are separated by a semi-permeable membrane. When two substances of different concentrations are separated, the solvent from the less concreted solution simply flows into the higher concentrated solution, with the membrane blocking the solids. In the ship’s engine room, reverse osmosis takes place in a pressure container that contains a tank holding a quantity of freshwater and seawater separated by a semi-permeable membrane. In a natural osmosis process, the freshwater would flow into the seawater. When pressure is applied to the seawater side, this process is duly reversed. This results to the seawater flowing into the freshwater side and the solids being stopped by the membrane.

All in all, freshwater is required in tankers for a wide range of purposes. During most oil tanker fit outs, we have to consider the use of freshwater as it is required for cleaning purposes, and also for cooling the systems. Without freshwater, the entire tanker fitting processes can never be effectively accomplished.