Question 10 1AE01 - First Assistant Engineer

The Correct Answer is C **Explanation for Option C (saltwater feed heater):** Scale formation is primarily governed by temperature and concentration. In a multi-stage flash (MSF) evaporator system, the saltwater feed (brine) is heated to its highest temperature in the **saltwater feed heater** (often the brine heater). This heating process drives the precipitation of inverse solubility salts, such primarily calcium carbonate ($\text{CaCO}_3$) and magnesium hydroxide ($\text{Mg}(\text{OH})_2$), out of the solution. Since the solubility of these salts decreases significantly as the temperature rises, the highest rates of scale deposition occur where the water reaches its maximum temperature—which is in the tubes of the saltwater feed heater before the brine enters the flash chamber. **Why the other options are incorrect:** * **A) first-stage condenser:** The first-stage condenser operates at a relatively low temperature, condensing the vapor produced in the final flash stage. Although some cooling water flows through it, the temperature is far below the scaling threshold typical of the brine heater, and it is usually separating pure distillate from cooling water, not highly concentrated, hot brine. * **B) flash chamber:** The flash chamber is where the hot brine suddenly boils (flashes) due to reduced pressure. While the brine is at high temperature here, the surface area for deposition is much lower than in the small-diameter tubes of a heat exchanger. More importantly, the scaling reaction *occurs during the heating process* (in the feed heater) rather than during the flashing process itself. * **D) distillate cooler:** The distillate cooler uses either feed water or cooling water to cool the product (distillate). The distillate is essentially pure water and does not contain the dissolved solids necessary to form hard scale. Therefore, significant scaling is not a concern here.