1.Deaerator.
2.Low-Pressure Heater.
3.High-Pressure Heater.

Deaerator

Why Gases need to be Removed from Boiler feed Water Oxygen is the main cause of corrosion in hot well tanks, feed lines, feed pumps and boilers. If carbon-di-oxide is also present then the pH will be low, the water will tend to be acidic and the rate of corrosion will be increased. Typically the corrosion is of the pitting type where, although the metal loss may not be great, deep penetration can occur in a short period.

Elimination of the dissolved oxygen may be achieved by Mechanical or chemical methods but the former being economical.

The essential requirements to reduce corrosion are to maintain the feed water at a pH for not less than 8.5 to 9, the lowest level at which carbon-di-oxide is absent, and to remove all traces of oxygen. The return of condensate from the plant will have a significant impact on boiler feed water because less feed water treatment is required.

Water exposed to air can become saturated with oxygen and the concentration will vary with temperature, the higher the temperature the lower the oxygen content.

The first step in feed water treatment is to heat the water to drive off the oxygen. Typically a boiler feed tank should be operating at boiling point. This leaves oxygen content of around 7 ppb.

The Deaeration Principle

Using Henry's law of partial pressures, the principle behind deaeration can be explained as follows: The quantity of a gas dissolved in a given quantity of liquid is directly proportional to its partial pressure surrounding the liquid. Therefore, by reducing the partial pressure of the unwanted gasses in the surrounding atmosphere, the gasses are diminished. These partial pressures are reduced by spraying the liquid into a countercurrent flow of steam. The steam, which is free of non-condensable gasses, is the liquid's new atmosphere and Henry's law prevails. Using steam is advantageous in that the solubility of a gas in a liquid decreases with an increase in the temperature of that liquid. The liquid is sprayed in thin films in order to increase the surface area of the liquid in contact