Question 29

The Correct Answer is D The DC heater (Deaerating Condenser or Deaerator Heater) is a critical component in a steam power plant's feedwater system. **Explanation of why option D ("all of the above") is correct:** The primary function of the DC heater is threefold, covering options A, B, and C: 1. **(A) Heat Feedwater:** The DC heater uses steam (usually extracted bleed steam) to directly contact and heat the incoming cold condensate (feedwater). Heating the water improves efficiency and prepares it for the boiler. 2. **(B) Store Feedwater:** The DC heater typically includes a large storage tank (often called the DC storage tank) beneath the deaerating section. This tank holds a reserve supply of heated, deaerated feedwater, ensuring a steady flow to the boiler feed pumps, especially during transient load changes. 3. **(C) Remove Air (Oxygen and other non-condensable gases) from Feedwater:** This is arguably the most crucial function. By heating the water to saturation temperature (the boiling point corresponding to the pressure inside the vessel) and spraying it through a steam atmosphere, the dissolved corrosive gases, primarily oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$), are separated from the water and vented out of the system. This prevents catastrophic corrosion in the boiler, superheaters, and turbines. Since the DC heater performs heating, storage, and deaeration, the option covering all three functions is correct. **Brief explanation of why the other options (A, B, and C) are individually incorrect:** While options A, B, and C describe vital functions of the DC heater, none of them fully describe its complete role. * **A) Heat feedwater:** This is true, but it ignores the equally important functions of deaeration and storage. * **B) Store feedwater:** This is true, but storage is a secondary function; the primary thermodynamic functions are heating and deaeration. * **C) Remove air from feedwater:** This is the most critical function (deaeration), but it ignores the necessary heating and storage capabilities inherent in the unit's design.