In an early shell-type boiler, what part served as its heating surface?

In an early shell-type boiler, the heating surface is defined as the area through which heat is transferred from the combustion gases to the water. The lower part of the shell is the correct choice because that is where the combustion gases make direct contact with the water, facilitating efficient heat transfer. This area is crucial for generating steam as it allows the heat to raise the temperature of the water effectively.

The focus on only the lower part of the shell indicates that this boiler design utilized the gravitational flow of water, which was a common practice. The lower part would absorb the maximum amount of heat generated by the burner flames traveling through the shell, thereby enhancing the overall efficiency of steam production.

Other options include surfaces that may not directly contribute to the primary heating process as effectively as the lower part of the shell does. For example, options that reference additional areas or components may incorporate surfaces that do not receive the same direct heat transfer or may not be primarily designed for heating. Therefore, concentrating on the lower part specifically highlights the most critical area for heat exchange in the dynamics of the early shell-type boiler.