Deaeration in Boilers

Deaeration refers to the process of removing dissolved gases such as oxygen and carbondioxide from the water in a boiler. Dissolved Oxygen in Water causes corrosion by the formation of rust on the

surfaces of the boiler and the piping (rust). Carbondioxide which is dissolved in the water forms carbonic acid which also causes corrosion.

Classification of Deaerators in Boilers

Hence, it is essential that these two gases are removed from water. 

Deaerators can be classified into 

Mechanical Deaerators

These Deaerators separate the gases by a mix of high temperature and mechanical action Chemical Deaerators 

Chemical Deaerators

Chemical deaerators work by passing the water through chemicals which absorb the oxygen and the carbondioxide. 

Vacuum Deaerators

Vacuum Deaerators or Membrane Contractors work by passing the water through hollow fibres. The water is made to pass on the outside of the hollow fibre. A vacuum is created on the inside. The gases pass through the membrane on to the inside and drawn into the vacuum pump. 

Vacuum Deaeration is a method of removing dissolved gases from water. Removing dissolved gases from water is necessary as they can cause corrosion.

Working of Vacuum Deaerators

The principle on which Vacuum Deaerators are based is called Henry's Law. 

Henry's Law states that the gas solubility in a solution reduces as the partial pressure of the gas above the solution decreases. The Deaerator consists of a tower with baffles. The tower is made of galvanized or reinforced steel. Water is drawn to the top of the tower and made to fall through the baffles. The falling water is in the form of thin films. 

This creates a large contact area between the water and the air. A vacuum pump creates a vacuum in the inside of the tower. This lowers the air pressure on the inside. The dissolved gases in the water are drawn to the vacuum and are removed. In some vacuum deaerators the water is increased in lower the solubility. The amount of vacuum to be created depends on the temperature of the water. Vacuum deaerators are highly efficient and can deliver water with dissolved oxygen less than 5 ppb (parts of billion). 

Chemical deaeration is the use of Chemicals to remove the dissolved gases, usually oxygen. Chemical deaeration is usually used after Mechanical deaeration. Even after mechanical deaeration, all the oxygen will not be removed. 

 A chemical known as an oxygen scavenger is used. This ensures that all the oxygen has been removed. A common oxygen scavenger is Sodium Sulphite. Sodium sulphite reacts with the trace amounts of oxygen. Sodium sulphite, however, cannot be used at high pressure as it can decompose to acidic gases which can increase corrosion. 

Another oxygen scavenger is hydrazine. Hydrazine reacts with oxygen and produces volatile compounds which do not dissolve. Hydrazine also does not cause corrosion. However, the downside is that it is a carcinogen (cancer causing substance) and thus has to be used very carefully. It may be banned in the future. 

Oxygen Attack refers to the corrosive action of dissolved action on the boiler.  Dissolved oxygen causes pitting on the boiler surface.  Oxygen enters the boiler through the feed water.  Though, the deaerators remove a large amount of oxygen, the oxygen that remains can cause corrosion. 
When the feed water is heated, the oxygen becomes even more aggressive resulting in severe corrosion.

If the water contains ammonia, this results in corrosion of components containing copper and copper alloys such as bearings. 

Corrosion also results in deposits on the heat transfer surfaces which affect efficiency. 

Corrosion caused by oxygen is usually localized.  Oxygen Corrosion can also be extensive. 
Oxygen Attack is not monitored and prevented can result in failure of the boiler components. 
Heating the feed water reduces its solubility and reduces the dissolved oxygen.  Mechanical deaerators can further reduce the dissolved oxygen level.  Finally, chemical deaerators such as sodium sulphite can scavenge the remaining oxygen ions.