12 February 2019

Deaerator Design and Operation

Function

A deaerator preheats boiler feedwater and removes dissolved gases, especially oxygen and carbon dioxide. These gases are undesirable because they cause corrosion and increase treatment chemical requirements. 

Operation

Deaerators operate based on the reduced solubility of dissolved gases as temperature increases. For example, the solubility of oxygen decreases from about five ppm at 150oF to about two ppm at 190oF. Virtually all the free carbon dioxide gas is removed by increasing the temperature to greater than 170oF.

Although the solubility of oxygen is theoretically zero at the boiling point, near complete removal is not possible without vigorous mechanical scrubbing. In a typical deaerator, this is accomplished by spraying the incoming water into a pressurized steam atmosphere, where it is heated to within a few degrees of the saturation temperature of the steam. Most of the non-condensable gases (mainly oxygen and free carbon dioxide) are released from the water to the steam and purged from the system by venting. The remaining traces are scrubbed from the water as it is broken up into fine droplets or films using a series of trays or spray nozzles. With good steam/water contact, the deaerator outlet temperature should be raised to within 1-4oF of the saturated steam temperature at the deaerator pressure. 

Types of Deaerators

Deaerating equipment is classified as either a deaerator or a deaerating heater. The distinction is based on performance ratings. Deaerators are capable of removing dissolved oxygen (DO) to 0.007 ppm (7 ppb) or less. Deaerating heaters are designed to reduce DO to below 0.04 ppm (40 ppb), which is less efficient than a true deaerator.

Design and Configuration

There are three common mechanical designs: spray-scrubber, spray-tray, and atomizing.

  • Spray-scrubber - water is sprayed into a steam atmosphere then falls into a baffled scrubber section, where it is scrubbed and heated
  • Spray-tray (or tray) - water is sprayed into a steam atmosphere then cascades onto a series of trays, where additional steam/water contact is made and heated
  • Atomizing - water is sprayed into a steam atmosphere created by atomizing jets, then is heated and falls to the storage section

                                    Spray Scrubber                                                                Spray-tray (Tray) Scrubber

The storage and deaerating sections can be separate or combined in a single tank. Single tank designs typically use a separate storage compartment to mix makeup and condensate before pumping to the deaerator. If the surge tank is not made of stainless steel, it may be necessary to feed oxygen scavenger to this section to effectively control corrosion. This practice can increase oxygen scavenger usage and costs significantly. The preferred configuration is to have cold makeup and hot condensate enter the deaerating section via separate lines, mixing in the deaerating chamber. Sometimes, makeup and condensate are mixed prior to the deaerator, with the combined feedwater being fed to the deaerating section. The release of oxygen when the hot condensate mixes with the cold makeup water can cause pitting and rapid failure of the downstream piping unless stainless steel is used. 

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