Question 44 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is C **Explanation for Option C (Correct Answer):** The illustration MO-0195 typically depicts a modern marine boiler designed for high pressure and high output, characteristic of machinery installed in vessels like harbor tugs (especially modern, powerful ones). 1. **Water-Tube Type:** Modern high-pressure marine boilers are almost exclusively of the water-tube type. In a water-tube boiler, the water flows inside the tubes while the hot combustion gases flow around the outside. This design allows for rapid steam generation, high operating pressures, greater safety (due to smaller diameter tubes), and faster response times, making it ideal for propulsion systems requiring quick power changes. 2. **Forced Circulation:** While many older, lower-load water-tube boilers use natural circulation, modern high-duty marine boilers (especially those designed for high pressure and rapid response) often utilize **forced circulation** (assisted circulation). Forced circulation uses pumps (circulating pumps) to maintain a positive, high-velocity flow of water through the tubes, ensuring even heating, preventing hot spots, and allowing the boiler to handle fluctuating loads more efficiently. This setup is common in D-type or similar water-tube arrangements used for propulsion. Therefore, the arrangement describing a high-duty marine boiler like the one typically shown in MO-0195 is the **water-tube type utilizing forced circulation.** **Explanation of Incorrect Options:** * **A) The boiler is of the fire-tube type and utilizes forced circulation:** Fire-tube boilers (where hot gases pass through tubes surrounded by water) are generally limited to lower pressures and outputs. While forced draft may be used for combustion, the internal water circulation is almost always natural, not forced. * **B) The boiler is of the water-tube type and utilizes natural circulation:** While natural circulation water-tube boilers (like A-type or older bent-tube designs) are common, the "forced circulation" classification (C) is more accurate for the highest-duty, high-pressure modern marine propulsion boilers typically represented in standard illustrations like MO-0195, especially given the context of a modern, powerful tug. * **D) The boiler is of the fire-tube type and utilizes natural circulation:** This describes older Scotch boilers or smaller auxiliary boilers. It is not characteristic of the high-performance main steam generation machinery typically depicted in diagrams for modern tug propulsion systems.