What is the heating surface of a boiler characterized by?

The heating surface of a boiler is characterized by having water on one side and hot gases on the other side. This configuration allows for efficient heat transfer, as the hot gases transfer their energy to the water, causing it to heat up and eventually turn into steam. The design maximizes the surface area in contact with the hot gases, facilitating effective heat exchange.

When water is on one side, it absorbs the thermal energy from the hot gases, which can originate from the combustion of fuel. This process is essential in steam generation within a boiler, as the heating surface must be designed to maintain optimal temperature conditions for boiling water into steam.

The option regarding steam on one side and hot gases on the other describes situations that are not typical of the core heating surface definition, as the primary function of heating surfaces is to generate steam from water. Therefore, the correct choice underscores the relationship between the water being heated and the hot gases that provide the necessary thermal energy.