Question 43 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is C **Explanation for Option C (3):** The illustration figure labeled **3** typically represents a **La Mont boiler** or a similar design of a forced-circulation steam generator. In a forced-circulation boiler, a pump is used to actively push (force) the water/steam mixture through the evaporator tubes, overcoming the natural density difference (which drives natural circulation). The La Mont design is a classic example where a centrifugal pump forces the feedwater through small-diameter boiler tubes, making it suitable for high-pressure applications and compact installations like those found on articulated tug-barge (ATB) units where space is often limited. **Explanation for Incorrect Options:** * **Option A (1):** Figure 1 usually depicts a **fire-tube boiler** (or a simple shell boiler). This type involves hot combustion gases passing through tubes submerged in water. It is a natural circulation type and is typically used for lower pressures and smaller capacities, not the high-pressure forced circulation typically required for modern powerful marine propulsion plants. * **Option B (2):** Figure 2 often represents a **straight-tube water-tube boiler** or another variant of a natural circulation water-tube boiler (e.g., a Babcock & Wilcox design). In this design, circulation relies on the difference in density between the hot water/steam mixture in the risers and the cooler water in the downcomers. This is a natural circulation system, not a forced-circulation system. * **Option D (4):** Figure 4 commonly illustrates a **once-through boiler** (like a Benson boiler or Sulzer Monotube). While these are technically forced-circulation (as the feed pump forces the water through the entire system), they do not feature the typical separate steam drum and internal circulation pumps found in the generalized forced-circulation type like La Mont (Figure 3), which often still uses a drum, albeit with pumped circulation assistance. In the context of standard marine engineering illustrations differentiating fundamental types, Figure 3 is the clearest representative of a classic forced-circulation system.