Question 30 QMED01 - Junior Engineer

The Correct Answer is D **Explanation for Option D (Temperature differential between the steam and feedwater):** Heat transfer processes, such as those occurring in a feedwater heater (which is a type of heat exchanger), are fundamentally governed by the temperature difference ($\Delta T$) between the hot fluid (steam) and the cold fluid (feedwater). The general equation for convective heat transfer rate ($Q$) is often expressed by Newton's Law of Cooling, and more comprehensively by the equation for a heat exchanger: $$Q = U A \Delta T_{LMTD}$$ Where: * $Q$ is the rate of heat transfer. * $U$ is the overall heat transfer coefficient (related to materials and fouling). * $A$ is the heat transfer area. * $\Delta T_{LMTD}$ is the Log Mean Temperature Difference, which is a specific measure of the *temperature differential* between the two fluids across the heater. Since $U$ and $A$ are usually fixed (or change very slowly due to fouling), the rate of heat transfer ($Q$) is **directly and most significantly proportional** to the temperature differential ($\Delta T$). A larger temperature difference provides a stronger driving force for heat flow, thus increasing the rate of heat transfer. **Explanation for Why Other Options are Incorrect:** * **A) pH of the feedwater:** The pH relates to the acidity or alkalinity of the water. While very high or very low pH can lead to corrosion or scaling (fouling) over long periods, thereby affecting the overall heat transfer coefficient ($U$), it does not directly or immediately govern the *rate* of heat transfer under normal operating conditions. Its effect is secondary and slow-acting. * **B) density of the feedwater:** Density is a thermodynamic property of the fluid. While changes in density affect specific heat capacity and mass flow rate slightly, its variation under normal operating pressures and temperatures is generally minimal compared to the impact of the temperature differential. It is a minor factor in the calculation of the convective heat transfer coefficient, not the primary driver of the heat transfer rate. * **C) speed of the main feed pump:** The speed of the main feed pump dictates the flow rate (mass flow rate) of the feedwater through the heater. While flow rate is an important factor (increasing flow rate increases the heat transfer coefficient $h$ and total heat transferred $Q$), the rate of heat transfer is fundamentally limited and dominated by the temperature difference between the two streams. If the temperature differential were zero, no heat would transfer regardless of flow rate. Thus, $\Delta T$ remains the most critical governing factor.