An early shell-type boiler had ___________ as its heating surface.

The heating surface of an early shell-type boiler predominantly consisted of the lower portion of the shell. This design focuses on maximizing the area available for the transfer of heat from the combustion gases to the water, while the upper part of the shell is less effective for heating purposes because steam forms at the top, and the heated water needs to be near the source of heat for efficient conversion to steam. By utilizing only the lower part of the shell, these boilers could effectively heat water, leading to steam generation while minimizing the amount of heat lost through the upper shell that does not contribute to the heating process. This design was pivotal in early boiler technology, intending to optimize efficiency and functionality.