How to use the deaerator dissolved oxygen calculator
Oxygen dissolved in boiler feedwater is the main driver of pitting in economisers, drums and feed piping. A deaerator heats the water to the saturation temperature of its operating pressure, which drives oxygen solubility close to zero. This tool works out the oxygen load that makeup water brings in, the removal efficiency you are achieving and the scavenger needed for what is left.
Saturated dissolved oxygen in the makeup is interpolated from the standard freshwater solubility table at atmospheric pressure; water at 68 °F holds about 9.09 mg/L. Returned condensate is already deaerated, so the average inlet concentration is the saturated value times the makeup fraction. Saturation temperature is derived from the operating pressure with the Antoine equation, and feedwater running cooler than that means the unit is not deaerating properly, so check the steam supply and the vent first.
Tray and spray deaerators are normally guaranteed to 0.007 mg/L, or 7 ppb, at the outlet. The residual is handled chemically, and the stoichiometric demand is 7.88 mg of sodium sulfite or 1.00 mg of hydrazine per mg of oxygen. Actual feed rates also carry a residual in the boiler water, so set them with your water treatment supplier.
Frequently asked questions
Almost all of the oxygen load arrives with fresh makeup. Returned condensate has already been deaerated and carries very little oxygen, so it is treated as zero when the inlet concentration is averaged.
No. The stripped oxygen and carbon dioxide have to leave through the vent. Throttling it lets non-condensable gases build up and outlet dissolved oxygen climbs very quickly.