The Deaeration Principle
Today a deaerator is an essential part of a steam system. Corrosion in boiler cycles is caused mainly by the presence of non-condensable gasses such as oxygen and carbon dioxide, or by a low pH value. While the pH is raised by the addition of chemicals, it is more economical to remove non-condensable gasses mechanically. This mechanical process is known as Deaeration and its employment increases the life of a steam system dramatically.
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 with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations.
With these principles in mind, Deaerator employs a two-stage system of heating and deaerating feedwater. This system reduces oxygen concentration to less than 0.007 ppm, and completely eliminates the carbon dioxide concentration when tested by the APHA method. Testing for oxygen concentration shall be done in accordance with ASME Performance Test Code 12.3. Other methods of testing may be used if mutually agreed upon by the parties involved.
Deaerator Operation – Stage One
The first stage of deaeration is shown in Figure I. The prime element in our vent condenser zone is the self-adjusting spray valve that allows incoming water, which is to be deaerated, to discharge as a thin-walled, hollow cone spray. Because steam flows countercurrent, intimate water to steam contact occurs with consequent latent heat transfer. As the falling water reaches the tray stack (tray type deaerator), or the collection basin (spray type deaerator), its temperature is within 2°F (1ºC) of the counter-flowing saturated steam temperature. Most of the dissolved oxygen and free carbon dioxide have been removed at this point. Since nearly all of the steam has been condensed, the non-condensable gasses and the small amount of “transport” steam exits through the vent piping.
Deaerator Operation – Stage Two
Tray Type Deaerator
The partially deaerated water enters the tray stack at saturation temperature. The heated water flows down over the trays, zigzagging through counter flow steam. This arrangement provides additional retention time to allow a final oxygen strip by the purest steam. The two-stage tray deaeration technique is the most reliable method for meeting critical performance over a complete load range.
Spray Type Deaerator
Water from the collection basin flows down the vertical down comer and into the scrubber section where it comes in contact with upcoming steam. Through carefully sized orifices, the steam and water violently mix, heating and removing the remaining gasses from the water. The mixture moves to the top of the scrubber housing and there the steam separates from the water and gasses and continues to flow up into the spray area and the vent condensing zone.
Water from the collection basin flows down the vertical down comer and into the scrubber section where it comes in contact with upcoming steam. Through carefully sized orifices, the steam and water violently mix, thus setting off heating and flashing of the water as it propels the mixture to the top of the scrubber housing. At this point steam separates from the mixture and continues to flow up into the vent condensing zone, or Stage One of our deaerator.