Question 40

The Correct Answer is A ### Why Option A is Correct **A) Combustion gases flow through the tubes.** This statement accurately defines a fire-tube boiler. In this design, the hot combustion gases (the products of combustion from the furnace or firebox) are channeled through a series of tubes that are submerged in water. Heat is transferred from the hot gas inside the tubes, through the tube walls, to the water surrounding them, thus creating steam. ### Why Other Options Are Incorrect **B) Flames impinge on the tubes.** While the tubes carry very hot gases, direct, sustained flame impingement usually occurs in the furnace or firebox, which is the chamber *before* the gas enters the narrow heat exchange tubes. In a fire-tube boiler, the tubes are submerged in water, which prevents severe overheating and limits the opportunity for direct, high-intensity flame impingement on the main tube bank. **C) Combustion occurs in the tubes.** Combustion (the burning of fuel) occurs in the furnace or firebox (a large, dedicated chamber). The tubes carry the *products* of combustion (the hot gases) after the combustion process is largely complete. The tubes are too narrow and numerous to be the primary combustion chamber. **D) Water flows through the tubes.** This configuration describes a **water-tube boiler**. In a water-tube boiler, water is pumped through the tubes while the hot combustion gases surround the outside of the tubes. This is the exact opposite of the fire-tube design.