Question 41 QMED01 - Junior Engineer

The Correct Answer is B. ### Why Option B ("gas temperature") is Correct: The primary function of a waste heat boiler (WHB) is to recover thermal energy from the hot exhaust gases of a main engine (usually a diesel engine on a ship) to produce steam. The maximum pressure (and corresponding temperature) that can be developed within the boiler is fundamentally limited by the **heat input**. The heat input available to the boiler is directly proportional to the temperature of the exhaust gas entering the boiler. Higher exhaust gas temperatures lead to: 1. A higher rate of heat transfer across the boiler tubes. 2. A greater potential for generating steam at higher enthalpy (temperature and pressure). If the exhaust gas temperature is too low, the boiler may only be able to generate saturated steam at a low pressure. If the exhaust gas temperature is very high, the boiler can generate higher-pressure superheated steam, constrained only by the design limits (material strength) of the boiler itself. Therefore, the *maximum attainable pressure* is determined by the highest possible heat source temperature, which is the main engine exhaust gas temperature. ### Why Other Options Are Incorrect: * **A) gas composition:** While the composition (e.g., presence of sulfur, water vapor) affects the dew point, corrosion potential, and specific heat capacity of the gases, these factors have only a minor, secondary influence on the bulk heat transfer rate and the maximum achievable steam pressure compared to the absolute gas temperature. * **C) pressure:** The exhaust gas pressure leaving the engine is slightly above ambient (atmospheric) pressure, usually only 0.2 to 0.4 bar gauge. This relatively low static pressure is required to push the gases through the boiler and funnel. This low pressure is negligible in the thermodynamic equation governing heat transfer and the resultant steam pressure inside the boiler tubes (which can be 8 to 18 bar). The steam pressure is determined by the heat *temperature*, not the gas *pressure*. * **D) timing:** Engine timing (injection and valve timing) primarily dictates when combustion occurs relative to the piston position. While timing *influences* the exhaust gas temperature (a retarded timing generally increases exhaust temperature), timing itself is an operational parameter of the engine, not the direct determinant property of the fluid (the exhaust gas) that sets the boiler’s pressure limit. The immediate determinant property is the temperature (B).