The removal of dissolved gases from boiler feedwater is an essential process
in a steam system. The presence of dissolved oxygen in feedwater causes rapid
localized corrosion in boiler tubes. Carbon dioxide will dissolve in water,
resulting in low pH levels and the production of corrosive carbonic acid. Low
pH levels in feedwater causes severe acid attack throughout the boiler system.
While dissolved gases and low pH levels in the feedwater can be controlled or
removed by the addition of chemicals, it is more economical and thermally efficient
to remove these gases mechanically. This mechanical process is known as deaeration
and will increase the life of a steam system dramatically.
Deaeration is based on two scientific principles. The first principle can be
described by Henry's Law. Henry's Law asserts that gas solubility in a solution
decreases as the gas partial pressure above the solution decreases. The second
scientific principle that governs deaeration is the relationship between gas
solubility and temperature. Easily explained, gas solubility in a solution decreases
as the temperature of the solution rises and approaches saturation temperature.
A deaerator utilizes both of these natural processes to remove dissolved oxygen,
carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater
is sprayed in thin films into a steam atmosphere allowing it to become quickly
heated to saturation. Spraying feedwater in thin films increases the surface
area of the liquid in contact with the steam, which, in turn, provides more
rapid oxygen removal and lower gas concentrations. This process reduces the
solubility of all dissolved gases and removes it from the feedwater. The liberated
gases are then vented from the deaerator.
With these principles in mind, Sterling Deaerator Company employs a two-stage
system of heating and deaerating feedwater. This system reduces dissolved oxygen
concentration to less than 0.005 cc/liter (7 ppb), and completely eliminates
the carbon dioxide concentration.