The function of the deaerating heater is to remove dissolved non-condensable gases and to heat boiler feed water. A deaerating heater consists of a pressure vessel in which water and steam are mixed in a controlled manner. When this occurs, water temperature rises, and all non-condensable dissolved gases are liberated and removed and the effluent water may be considered corrosion free from an oxygen or carbon dioxide standpoint. Free air or other non-condensable gases should be vented prior permitting the fluid to enter the deaerator.
A deaerating heater is the watch dog of boiler plant as it protects the feed pumps, piping, boiler and any other piece of equipment that is in the boiler feed and return cycle from the effects of corrosive gases, i.e. oxygen and carbon dioxide, to a level where they are no longer a corrosion factor.
Principles of Deaeration
There is physical law which states that the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. Another law states, the solubility of a gas in a liquid decreases with an increase in temperature of the liquid. Experience has shown that more rapid and more complete removal of non-condensable gases from a liquid is obtained when the liquid is vigorously boiled or scrubbed by condensable or carrier gas bubbles.
Therefore, essentially the deaerating heater must first heat the feed water to as high a temperature as possible, i.e. to the temperature corresponding to the steam pressure, It must vigorously boil and scrub the heater water with fresh steam, which can carry to the liquid surface any traces of oxygen and carbon dioxide.
The partial pressure of the oxygen and carbon dioxide in the steam atmosphere must be maintained as low as possible, particularly at the point where the deaerated water separates from the steam. Non-condensable gases must be continually withdrawn from the heater at the rate at which they are being liberated.