Deaerator is one of the open type contact feed water heater used in boiler water treatment. It is the device that is widely used for the removal of oxygen and other dissolved gases from feed water supply to boiler.
The presence of dissolved gases like oxygen and carbon-dioxide in water makes the feed water corrosive, as they react with metal piping and other metallic equipments forming iron oxides (rust) where as dissolved carbon-dioxide combines with water to form carbonic acid that causes further corrosion.
Deaeration is based upon ‘’DALTON and HENRY’’ which states that quantity of gas that dissolves in a liquid decreases as the temperature of the liquid increases and the quantity of dissolved gas becomes zero when temperature of liquid reaches the boiling point or saturation temperature i.e all the dissolved gases will be liberated, the above said process is called mechanical Deaeration.
Complete oxygen removal from condensate cannot be attained by mechanical Deaeration, so in order to last traces of oxygen and other dissolved gases chemical Deaeration is required i.e by adding oxygen scavengers.
Sodium sulphite (Na2SO3) or Hydrazine (N2H4) is injected in suitable calculated doses into feed water (i.e Deaerator outlet) at the suction of the boiler feed pump
Working Principle of Deaerating Heater
The main purpose of Deaeration is for removal of unwanted dissolved gases and dissolved oxygen from the boiler feed water before entering into boiler. Most Deaerators are designed in such a way that dissolved oxygen content in Deaerator outlet must be within 5-10 ppb by wt%.
The incoming condensate to the Deaerator is sprayed from the top of the Deaerator (inside) so as to expose large surface area and bled steam from turbine is fed from the bottom as shown in figure. The steam flows upward through the falling condensate, heating it to the saturation temperature causing liberation of dissolved gases from the feed water to the boiler.
Released dissolved gases along with some watervapours which is again condensed back in vent condenser and gases are vented out. The heated and deaerated condensate along with condensed steam gets into Deaerator feed storage tank from where feed water suction to BFP (boiler feed pump) exists.
The above diagram shows the operating principle of Deaerating heater or Deaerator.