Converting Salty to Fresh Water-Fresh Water Generator

  • Suman Paul 
  • 9 min read

Water on board a vessel is used in almost every operation. The only source is the water from the sea. The sea water cannot be used directly as they are highly corrosive that could easily wear down the various metal parts of the vessel. People have discovered and come to know that the fresh water is less corrosive in nature than the sea water. However, fresh water is not in ample supply, unlike sea water. So, we need to produce it using special equipment called Fresh Water Generator. In the following study we have discussed the various aspects of a Fresh Water Generator.

Fresh Water Generator
Image Credit: WARTSILA

Introduction

Fresh Water on board a ship has many applications such as in cooling systems, in cargo hold cleaning etc. The most significant and vital role of fresh water is in the domestic uses as in washing clothes, drinking and bathing. The ships going on long voyages, into the oceans, uses Fresh Water Generator to produce their own fresh water from the seawater which is present in abundance.

How do sailors get fresh water?

The ships going on coastal voyages can refill there fresh water tanks once the port arrives or they can buy bottled water bottles along with the provinces. However, many ship owners choose to cut the extra cost of buying the water bottles or refilling the tanks which eventually increase the weight of the ship as well. To cope with such problems, the mariners use Fresh Water Generator to produce fresh water on board a ship, during the voyage, for various uses.

What is the principle of Fresh Water Generator?

The Fresh Water Generator or the FWG either uses the principle of distillation or the concept behind reverse osmosis.

  • Distillation is the process of boiling to separate mixtures’ constituents which boil at different temperatures. In our case, seawater, is heated at the boiling temperature. As a result, the salt is left behind and the water boils off which then condensed to obtain pure water.
  • Reverse osmosis refers to the process of water purification using a semi-permeable membrane through which unwanted large particles cannot pass. Thus, freshwater is obtained.

NOTE: Most of the modern ships use the distillation process as it is more efficient, productive and reliable.

How does a marine Fresh Water Generator work?

Fresh Water generators are nothing but heat exchangers, wherein, sea water is purified and converted into fresh water with the help of distillation process. The sea water is a mixture of fresh water, salt and minerals. The sea water is evaporated to steam and is condensed to form fresh water. The residue left behind mainly consists of salts which has to be cleaned frequently. The basic components of a Fresh Water Generator are:

  • Air tight casing
  • Ejector pump
  • Eductor nozzle
  • Distillate pump
  • Salinometer

Air Tight Casing: It comprises the evaporator and the condenser of the fresh water generator.

Fresh Water Generator
Shows the evaporator and condenser of the Fresh Water Generator.

Ejector Pump: The ejector pump is used to draw the sea water from the main sea water line and deliver it to the eductor nozzle.

Eductor Nozzle: The eductor nozzle acts like a pump to create vacuum inside the fresh water generator with the use of the Venturi effect.

Fresh Water Generator
Fig. Shows the cross section of an eductor nozzle.

Distillate Pump: It is used to draw the fresh water (distillate) formed in the condenser to deliver it into the fresh water tanks.

Salinometer: The salinometer is a device which is used to measure the salinity of water in terms of parts per million (PPM).

Fresh Water Generator
Fig. Shows a salinometer.

The Fresh Water Generators are available in either Tube type design or the Plate type design. The working principle of both the Fresh Water Generators are one and the same.

In a Tube type fresh water generator, the main engine jacket water is flowing through the tube in the evaporator section, surrounded by sea water supplied by the pumps.  The sea water after receiving heat from the jacket water becomes hot and starts to boil. In due course of time, the sea water starts evaporating and the steam rises up into the condenser. In the condenser, the steam is made to pass through a wire-mesh baffle screen to eliminate the unwanted impurities. The heat of the steam is extracted in the condenser tubes where sea water is flowing through them and they absorb the heat and condenses the steam into liquid. The liquid receiver is used to collect the condensed steam or the condensate. The distillate or the condensate pump is used to draw the water and transfer it to the fresh water tanks. In doing so, the water is being tested and analysed by the salinometer. If the salt content is less than 10 parts per million then it is allowed to flow into the fresh water tank. Otherwise, it is diverted back into the evaporator.

Fresh Water Generator
Fig. Shows the working of the Fresh Water Generator when salinity is within limit(Tube type Fresh Water Generator).
Fresh Water Generator
Fig. Shows the working of the Fresh Water Generator when the salinity is more than 10 ppm(Tube type Fresh Water Generator).

How do we start the Fresh Water Generator on board a ship?

The Fresh Water Generator is used when the vessel is underway and away from shore. The reason behind it is that the water in deep sea in less contaminated. Further, the sea water near shore can carry mud or some unwanted particles.

To start the Fresh Water Generator the following is to be done:

  • The main engine rpm should be checked. If it is less than 50rpm then the main engine jacket water will not be hot enough to boil the sea water.
  • After closing the drain valve at the bottom of the Fresh Water Generator, open the suction valve so that the sea water is supplied to the generator for evaporation, cooling in the condenser and into the educator for creating vacuum.
  • Now we need to open the discharge valve so that the sea water is dumped into the sea after circulating inside the Fresh Water Generator.
  • After closing the vacuum valve, start the main sea water pump and observe the pressure of the sea water pump.
  • Now wait until 90% vacuum is build up and is reflected in the gauge present on the generator.
  • After vacuum is achieved open the feed water treatment valve which is used to prevent the scale formation which reduces the heat transfer rate.
  • Open the outlet for the hot jacket water followed by the inlet valve. The valves must not be opened at once but it should be done gradually.
  • In sometime we will notice that the vacuum is decreasing as steam is being generated inside the Fresh Water Generator.
  • Open the valve for the draining the fresh water and switch on the salinometer followed by distillate/condensate pump to draw the fresh water from the generator.
  • When the salinometer reading is below 10 ppm open the valve to allow the fresh water to flow into the fresh water tanks. Otherwise, redirect it towards the generator.

How to increase the production of fresh water generators?

The production of the fresh water in the Fresh Water Generator can be increased by:

  • Further lowering the generator pressure. The boiling point of water reduces with pressure. So, with less amount of energy we can produce more steam. Thus, production of fresh water will also increase.
  • Increasing the flow rate of the jacket water which would lead to more heat transfer and there will be a increase in the production of steam and fresh water.
  • Increasing the surface area of the evaporator tubes would also help in better heat transfer. So, the production of Fresh Water Generator will also increase.

Common problems and solutions in a Fresh Water Generator:

  • Formation of scales: The salts of the sea water form hard crust or scale type formation is seen very frequently. To cope with such problems, the feed water is treated with chemicals like caustic soda, sodium phosphate etc. so that the scale formation does not take place. A muddy sludge is being produced which settles at the bottom and can easily be drained out using valves.
  • Low rate of fresh water generation: This would imply that steam is not being produced efficiently. The flow rate or the temperature of the jacket water must be increased. The problem can also be resolved by increasing the flow rate of cooling sea water in the condenser.
  • Distillate is having high salinity value: This implies that the evaporation of the water is taking place at a very high rate. In such situations lowering the flow rate of the jacket water is useful. It might also happen as a result of the evaporation temperature being low. To cope with it, the pressure of the Fresh Water Generator can be increased. As a result, the boiling temperature would increase and hence the evaporation temperature will rise.

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