Mechanical Engineering - Steam Boilers and Engines - Discussion

The heating surface is the area of the boiler where heat transfer occurs between the hot flue gases and the water. The difference comes down to the geometry of how these boilers are designed:

Design Configuration: In a water tube boiler, water flows inside the tubes while hot gases circulate them. In a fire tube boiler, the opposite occurs—hot gases are inside the tubes, and water surrounds them.

Surface Area Utilisation: Water tube boilers typically use a much larger number of smaller-diameter tubes. Because the external surface area of a tube is larger than its internal surface area (even for the same thickness), and because the compact arrangement allows for many more tubes in the same volume, the total available heating surface is significantly higher.

Evaporation Rate: This increased heating surface area is the primary reason why water tube boilers have a much higher rate of steam evaporation and can reach much higher pressures than fire tube boilers.

Why? Explain the reason.

Why & how? Explain.