Deaeration principle
Today the deaerator is an integral part of the steam system. Corrosion in boiler cycles is mainly due to the presence of non-condensable gases, such as oxygen and carbon dioxide, or low pH values. While the pH is increased by adding chemicals, it is more economical to remove non-condensable gases mechanically. This mechanical process is known as deaeration, and its use significantly increases the service life of the steam system.
Using Henry’s partial pressure law, the principle of de-aeration can be explained as follows: the amount of gas dissolved in a given amount of liquid is directly proportional to its partial pressure surrounding the liquid. Therefore, by reducing the partial pressure of unwanted gases in the surrounding atmosphere, the gases decrease. These partial pressures are reduced by spraying liquid into a countercurrent stream of steam. The vapor, free from non-condensable gases, is a liquid of the new atmosphere, and the law of Heinrich prevails. The use of steam is advantageous in that the solubility of a gas in a liquid decreases with increasing temperature of that liquid. The liquid is sprayed into thin films to increase the surface area of the liquid in contact with the steam, which in turn provides for faster removal of oxygen and lower gas concentrations.
Given these principles, Deaerator uses a two-stage heating system and feedwater deaeration. This system reduces the oxygen concentration to less than 0.007 ppm. and completely eliminates the concentration of carbon dioxide when tested by the APHA method. Oxygen concentration testing should be conducted in accordance with the ASME Performance Test Code 12.3. Other testing methods can be used in case of mutual agreement between the parties.
Deaerator operation - first stage
The first stage of deaeration is shown in Figure I. The main element in our condensate condenser zone is a self-adjusting spray valve, which allows the incoming water to be removed, which must be deaerated, in the form of a thin-walled, hollow conical spray. As the steam flows countercurrently, contacting water occurs with contacting steam followed by latent heat transfer. When the falling water reaches the stack of the tray (tray deaerator) or the collecting basin (deaeration deaerator), its temperature is within 2 ° F (1ºC) of the flowing temperature of the saturated steam. At this point, most of the dissolved oxygen and free carbon dioxide was removed. Since almost all of the steam has been condensed, non-condensable gases and a small amount of “transport” steam escape through the ventilation duct.
Deaerator operation - second stage
Tray type deaerator
Partially deaerated water enters the tray at saturation temperature. The heated water flows into the chutes, zigzagging through the counter-current flow. This arrangement provides additional retention time to provide the ultimate oxygen band with the cleanest vapor. The two-stage tray deaeration method is the most reliable method for providing critical performance over the full load range.
Spray deaerator
Water from the collection basin flows down vertically downwards and into the scrubber section where it comes into contact with the incoming steam. Through holes with a careful size of steam and water, mix well, heat and remove the remaining gases from the water. The mixture moves to the upper part of the scrubber housing, and the steam is separated from the water and gases and continues to flow upward into the region area and the air condensation zone.
Water from the collection basin flows down vertically downwards and into the scrubber section where it comes into contact with the incoming steam. Through the holes with a carefully selected diameter of steam and water are strongly mixed, thus causing heating and flashing water when it moves the mixture to the upper part of the scrubber body. At this point, the steam is separated from the mixture and continues to flow into the condensate condensation zone or the first stage of our deaerator.
