What is the term for the heat supplied by the fuel to the steam in a boiler?

The term that describes the heat supplied by the fuel to the steam in a boiler is boiler efficiency. This concept relates specifically to how effectively the boiler converts the energy from the fuel into usable heat energy for producing steam. Boiler efficiency is calculated by comparing the useful energy output, which is the steam produced, to the energy input from the fuel.

When considering the other terms: heat transfer refers to the movement of thermal energy from one physical system to another, which occurs during the operation of a boiler but does not specifically refer to the heat supplied by the fuel. Thermal conductivity is a property of materials that measures their ability to conduct heat, but it does not directly relate to the fuel or the steam in the boiler context. Heat exchange describes the process of heat transfer between two or more bodies or systems, which may happen in a boiler but again does not specifically pinpoint the heat supplied by the fuel.

Thus, the correct answer is focused on the efficiency aspect, highlighting how well the boiler utilizes the heat from the fuel to generate steam.