Part 1: Controlling Condensate and Feedwater Dissolved Oxygen and Air Inleakage at the Source
By Robert D. Bartholomew, P.E., and Gary H. Roberts, P.E.
Air is comprised of 20.9% by volume oxygen (23.1% by weight) and ~0.04% by volume of carbon dioxide. Air is the ultimate source of all dissolved oxygen contamination of the condensate and feedwater. Deionized water in equilibrium with air can have ~8 to 14 parts per million (ppm) (~8,000 to 14,000 parts per billion [ppb]) as O2 of dissolved oxygen (DO), depending on the temperature and based on oxygen solubility. Carbon dioxide has a higher solubility and it undergoes equilibrium reactions with water. Therefore, even though carbon dioxide is a small proportion of air, substantial amounts can be absorbed in water. Based on experience, cation conductivities due to carbon dioxide absorption from the air can reach ~0.8-1.0 ﾵS/cm in demineralized makeup water. These conductivities theoretically correspond to about 0.50-0.75 ppm as CO2 of total inorganic carbon or about 0.41 to 0.64 ppm as CO2 of free carbon dioxide, which is in equilibrium with bicarbonate.
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