Deaerator is basically used to eliminate air and other gases from the liquid. For example, from feed water being used in steam generating boilers. The oxygen which is dissolved in the feed water is very dangerous for the boiler as it may cause corrosion in the boiler by reacting with metal of the walls of the boiler.
In Power Plants where boiler is working for production of steam from boiler feed water. Deaerator resolves the purpose of removal of undesirable dissolved gases and dissolved oxygen from the boiler feed water before entering boilers. Deaerator are designed in such a way that the dissolved oxygen content in the outlet water is about 7 ppb by wt.%.
Deaerator works on the Henry’s law and inverse solubility of water.
- Inverse Solubility of water.
When the temperature of water is increased, the dissolved oxygen content in the water is decreases. Therefore, the water temperature is increased by adding steam in Deaerator, the dissolved gas solubility is decreased, and the gases are removed from water
- Henry’s law
The solubility of the gas in a liquid is directly proportional to the partial pressure. Therefore, if we decrease the partial pressure of the dissolved gas by adding steam in Deaerator, its solubility decreases, and the gas is removed from water.
- Types of Deaerator
- Spray type Deaerator:
In Spray type Deaerator the Pre-deaeration of feed water sprayed in the steam space and the feed water is contacted with the steam deaeration happens. The pre-deaeration is attained by the Stork spraying device. With due operating environment the sprayer assures heating of the condensate to saturation temperature and a very large area for mass transfer. The solubility of oxygen in water at saturation conditions is practically zero, while oxygen transfers from the water droplets to the surrounding steam. As the steam condensates on the water, the concentration of oxygen in the direct vicinity of the sprayer is increased making it possible to vent a small amount of steam with high oxygen concentration.
- Tray type Deaerator:
A tray type Deaerator with a vertical domed Deaerator section pointed abve a horizontal boiler feed water storage cylinder. Feed water walks into the vertical deaeration section above the perforated trays and flows downward through the perforations. The low-pressure deaeration steam enters below the perforated trays and flows upward through the perforations and had a good contact with the feed water. The steam strips the dissolved gas from the boiler feed water and exits via the vent at the top of the domed section. The Deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sprayer pipe in the bottom of the vessel, provided to keep the stored boiler feed water warm
- Vacuum type Deaerator
It contain three main parts, 1).vacuum Deaerator,2).vacuum pump unit and 3).boiler feed water pump. Deaeration tank is made by galvanized or SS. There is inside bottom inside the tank, under which a reservoir for Deaerated water is mounted. Filters are on the top of the bottom. A vacuum pump/liquid ring pump is connected with the Deaerator for sucking the progressive dissolved gasses. A negative pressure is maintained in the Deaerator system to increase the liberation of dissolved gasses from the boiler feed water.
- Construction of Deaerator:
Deaerator is always constructed on height from the pump to maintain optimum pressure before suction and the heater after the Deaerator are known as low pressure heaters.