Correct Answer - Option 2 : Priming difficulties in feed pumps
The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level and improve a plant's thermal efficiency by raising the water temperature.
In addition, deaerators provide feedwater storage and proper suction conditions for boiler feedwater pumps.
Vacuum deaeration has been used successfully in water distribution systems.
However, pressure deaeration (with steam as the purge gas) is normally used to prepare boiler feedwater. Steam is chosen as the purge gas for several reasons:
- It is readily available
- It heats the water and reduces the solubility of oxygen
- It does not contaminate the water
Only a small quantity of steam must be vented because most of the steam used to scrub the water is condensed and becomes a part of the deaerated water
In order to deaerate the boiler feedwater, water is sprayed into a steam atmosphere.
This heats the water to within a few degrees of the temperature of the saturated steam.
Because the solubility of oxygen in water is very low under these conditions, 97 to 98% of the oxygen in the incoming water is released to the steam and is purged from the system by venting.
Although the remaining oxygen is not soluble under equilibrium conditions, it is not readily released to the steam.
Therefore, water leaving the heating section of the deaerator must be scrubbed vigorously with steam to maximize removal.
