MARINE FRESH WATER GENERATOR

Methods employed to produce fresh water from sea water is mainly as given below: -

• Evaporation of sea water and condensation of vapor being produced.
• Reverse Osmosis 

The most commonly used method is the evaporation method as it is more feasible than the reverse osmosis.

Different types of equipment used for the generation of fresh water from sea water is as follows: -

1. Single and double effect shell type fresh water generator
2. Vacuum type fresh water generator
3. Single and double flash type fresh water generator

The Vacuum type fresh water generator is used on board the ship which is usually manufactured by Alfa-Laval or by Atlas. 

Problem of scale formation: - 

As we all know that scale formation is a major problem in machines which uses sea water as it contains various salts which as corrosive properties thereby damaging the machines.

Proportions of salts in sea water is as follows: -

• Sodium Chloride- 80%
• Magnesium Chloride-10%
• Magnesium Sulphate- 6%
• Calcium Sulphate- 4%   

Major types of scales found in evaporator is as follows: -

1. Calcium Carbonate - Soft scale
2. Magnesium hydroxide - Hard scale
3. Calcium sulphate - Hard scale

These scales are generated mainly due to temperature and density of evaporation.

If the Temperature is less than 80 degree Celsius, calcium carbonate scale is formed and if it exceeds above 80 degree Celsius, magnesium hydroxide scale is formed.

Usual density of evaporator content should be 80,000 ppm, if it exceeds above 96,000 ppm calcium sulphate scale formation takes place.

Method of controlling scale formation: -

• Low pressure evaporator plant - operates below 80 degree Celsius
• Magnetic treatment - alters the charge of salt
• Flexing element - cracks of the scale
• Chemical treatment - Organic polyelectrolyte, Polyphosphate, Ferric chloride 

Low pressure vacuum evaporator is generally used on board the ship as it is more economical and produces soft scales which can be removed very easily.

A complete overview of a fresh water generator used on board a ship is explained as below: -

Low Pressure vacuum evaporator (Alfa-Laval): -

Pic: Plate type FWG

Parts of a fresh water generator: -

• Pressure vessel containing both evaporator and condenser
• Aluminum bronze tube or plate type heater (evaporator)
• Titanium plate type condenser
• Knitted Monel wire demister
• Sea water ejector pump
• Air/brine educator
• Fresh water pump
• Salinometer

Operation: -

• The sea water pump supplies the feed into the condenser unit and then is filled into the evaporator unit.
• The sea water is filled inside the tubes in the condenser unit first and further it is filled in the evaporator unit outside of the tubes and overflows as brine.
• A required vacuum is created inside the chamber using the air educator, a pressure of 0.06 MPa is maintained inside the pressure vessel.
• Brine educator is used to remove the brine from the pressure vessel which overflows from the evaporator unit.
• Due to the generation of vacuum, low pressure generated tends to reduce the boiling point of water to 70-80 degree Celsius.
• Main engine jacket cooling water at a temperature of 80 degree Celsius is introduced into the evaporator which evaporates the sea water present.
• The non-evaporated sea water is discharged over board as brine.
• The vapor being generated passes through the demister which ensures that no droplets of sea water is carried over to the condenser along with the vapors.
• The generated vapors then pass through the condenser and gets condensed and gets collected in the outlet duct.
• A distillate pump is operated to pump the fresh water produced into the fresh water storage tank.
• Salinometer is provided which measures the salt content in the distillate.
• If salinity of water is more it controls a solenoid valve which will divert the flow to bilge or it is returned to evaporator for re-processing.  

Pic: Shell and tube type FWG

The Heating medium used for evaporating the sea water is: -

• Hot steam from boiler, generally in steam ships or conventional oil tankers.
• Hot fresh water from jacket cooling water.

Advantages of using a low pressure vacuum evaporator are as follows: -

1. Control in the type of scale formation due to low pressure.
2. Uses heat energy from main engine cooling water outlet.
3. Due to High temperature differences for lower pressures, heat transfer is improved.

A brief overview of Salinometer: -

• It measures the salinity of fresh water produced by the fresh water generator.
• The salinity is measured in "parts per million" ppm.
• It consists of an electrode unit which measures the conductiveness and this value is calibrated to a certain value of ppm.
• It gives audible and visible warning when salinity increases above a specified value.
• Maximum salinity of a fresh water should be near about 10 ppm.
• According to requirement salinity of fresh water should be 2-2.5 ppm.