Problem 3.8: A deaerator heats boiler feedwater and vents a

From Chapter 3 Problem Set. Level 3: System Integration.

Question

A deaerator heats boiler feedwater and vents a small amount of steam to atmosphere. Using Henry’s Law and the solubility behavior described in this chapter, explain why both the heating and the venting reduce dissolved oxygen. Then explain why the same equipment does nothing to remove dissolved calcium.

Answer

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Oxygen is held by physical equilibrium, not hydration. Heating lowers its solubility, so the water can hold less; venting steam lowers the partial pressure of oxygen above the water. Both actions push the equilibrium toward the gas phase. Calcium is not governed by equilibrium with the atmosphere. It was dismantled from a solid and is held in solution by hydration shells, so no amount of heating or venting will drive it out as a gas.