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pressure on the system. This can be accomplished by diluting the surrounding
atmosphere with a scrubbing gas, thereby reducing the concentration of the dissolved
gas.
3.0 MECHANICS OF DEAERATION
A number of studies indicate that to inhibit corrosion in a steam system, the oxygen
content must be limited to a maximum level of .01 (.0075 cc/L). A true deaerator will
reduce the oxygen level to the .005 cc/liter level, and carbon dioxide to zero. A further
benefit of this process is simultaneous preheating of the feedwater. More rapid
removal of gases results when the liquid is sprayed in thin films and then violently
scrubbed by the incoming steam.
Based upon the previously stated laws, the modern deaerator evolved. This is
considered a two stage device, the preheater and the scrubber. A deaerator consists of
a pressure vessel in which water and steam are mixed with controlled velocities. This
raises the water temperature, thereby liberating the dissolved, non-condensable gases.
The effluent may then be considered free from corrosive gases.
Their removal by the deaerator protects the boiler, feed pumps, and the entire
feedwater system from the damaging effects of corrosion. This is accomplished by
reducing their concentration to an insignificant level. It is essential that the deaerator
first heat at the feedwater to a temperature corresponding to the operating steam
pressure, and then vigorously boil and scrub the heated water to fresh steam. This will
carry any traces of oxygen or carbon dioxide to the liquid surface. The partial pressures
of the oxygen and carbon dioxide in the steam atmosphere will be maintained as low as
possible, particularly at the point where the steam separates from the deaerated water.
Non-condensable gases must be evacuated from the deaerator at a rate equal to their
liberation. A vent condenser is utilized to concentrate the non-condensable as they
leave the vessel and avoid unnecessary steam venting.
4.0 OPERATION
Incoming water first enters the deaerator, thru spray valves, into a steam atmosphere in
the first stage preheater section. There, the water is heated to within 2 degrees of
steam temperature, removing virtually all of the oxygen and free carbon dioxide. This is
accomplished by spraying the water through self-adjusting spray valves, designed to
produce a uniform, thin, controlled film under all load conditions. These efficient valves
assure a constant temperature and uniform gas removal.
From the first stage section, the pretreated water, containing minute traces of dissolved
gases, flows into the second stage, or deaerator section. This section consists of either a
scrubber or, in tray type units, tray assemblies. Here the water is brought into direct
contact with an abundance of fresh, gas free steam. The steam enters this stage and is
mixed with the pretreated water. Very little steam is condensed here, as the incoming
preheated water has a temperature approaching that of the steam. The water leaving
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