Describe the difference between fire-tube and water-tube boilers, including where each type is typically used.

• A. In a fire-tube boiler, hot combustion gases pass through tubes surrounded by water; fire-tube boilers are common for smaller capacities.
• B. In a fire-tube boiler, water flows inside tubes heated by combustion gases; fire-tube boilers are common for smaller capacities.
• C. In a water-tube boiler, hot combustion gases pass through tubes surrounded by water; water-tube boilers are used for higher pressures and larger capacities.
• D. In a water-tube boiler, water flows inside tubes heated by combustion gases; water-tube boilers are used for higher pressures and larger capacities.

Answer: (A)

The fundamental idea being tested is how the hot combustion gases and the water/steam are arranged inside the boiler, and how that arrangement affects what the boiler is best suited for. In a fire-tube boiler, hot combustion gases travel through tubes that are surrounded by water. The heat from the gases transfers through the tube walls into the surrounding water, turning it into steam. This setup is simple and robust, with a large water volume and relatively low operating pressures, which makes it economical and reliable for smaller capacities. That’s why fire-tube designs are commonly used for smaller steam generation needs.

For contrast, water-tube boilers place water inside the tubes and have the hot combustion gases pass around the outside of the tubes. This configuration supports much higher pressures and larger steam outputs, which is why water-tube boilers are favored in power plants and large industrial facilities. The option describing the fire-tube arrangement and its use captures this key distinction and typical application.