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Scale Formation in Freshwater Generators

Updated on July 23, 2013

Any machinery or system working with seawater as a medium, prone to severe contamination and need frequent maintenance. This is applicable for seawater coolers, pipe lines, filters, etc. But the condition worsens while higher temperatures and pressures. This happens in freshwater generators or evaporators (Equipment used on board ships for generating freshwater by evaporating seawater and condensing it). Scale formations can be effectively reduced to an acceptable limit or even can be eliminated by using a variety of techniques.

Alfa Laval Freshwater Generator with Plate Type Evaporator and Condeser
Alfa Laval Freshwater Generator with Plate Type Evaporator and Condeser

Scale Forming Salts in Seawater

Seawater contains various salts which undergo chemical reactions with each other under favourable temperatures and pressures. The salts are present in the following approximate proportions.

Salt
Quantity (Approx.)
Sodium Chloride
80%
Magnesium Chloride
10%
Magnesium Sulphate
6%
Calcium Sulphate
4%
Freshwater Generator with Chemical Dosing Tank for Preventing Scale Formation
Freshwater Generator with Chemical Dosing Tank for Preventing Scale Formation

Scale Formation in Boilers

To prevent scale formation in boilers following techniques are possible:

  1. Keep a reserve of chemicals in the boiler, which will combine with any potential scale forming salts entering the boiler and effectively neutralize them.
  2. Pass all feed water through units in the feed system, which will convert scale forming salts into a harmless form.
  3. Ensure that scale forming salts do not enter the boiler in any significant quantity, by using an effective distillation system.
  4. Use a combination of all three methods, which is the course often followed in water tube boiler installations.

Scale Formation in Freshwater Generators

Scale forms at the evaporator side of the freshwater generator, since seawater is heated up in evaporator. Condenser plates usually contain only soft mud which can be easily cleaned up.

Three types of scales found in fresh water generators or evaporators:

Calcium Carbonate, CaCO3 (Soft scale)

Magnesium Hydroxide, Mg(OH)2 (Hard scale)

Calcium Sulphate, CaSO4 (Hard Scale)

Freshwater Generator Condenser Side Plates
Freshwater Generator Condenser Side Plates

Calcium carbonate and Magnesium hydroxide scale formation mainly depends on the temperature to which seawater is heated. On the other hand calcium sulphate scale forms as the density of the evaporator contents increase.

The following chemical reactions takes place when sea water is heated:

Ca(HCO3)2 ------> Ca + 2HCO3

then

2HCO3 ------> CO3 + H2O + CO2

If heated up to approximately 80 Degree Celsius, then

CO3 + H2O ------> HCO3 + OH

then

Mg + 2OH ------> Mg(OH)2

How to Prevent Scale Formation in Evaporators

From the above chemical reactions we conclude that:

  1. If sea water temperature in the freshwater generator is below 80 Degree Celsius, calcium carbonate scale forms and if heated above 80 Degree Celsius, magnesium hydroxide scale predominates.
  2. If the density of seawater inside (high concentration brine formed by evaporation of water vapour from it) is greater than 96000 ppm, it leads to the formation of calcium sulphate scales. These scales can be prevented by keeping the brine density about 80000 ppm.

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    • Taranwanderer profile image

      Taranwanderer 2 years ago

      Interesting - useful for even a novice like myself.

    • jabelufiroz profile image
      Author

      Firoz 4 years ago from India

      Thank you Martie.

    • martie01 profile image

      Eric Martin 4 years ago from CLEVELAND, OH

      Nice and informative hub on scale formation reduction and prevention.