What distinguishes a flue from a tube?

The correct choice highlights that a flue is characterized by having an internal diameter of 4 inches or more. In the context of stationary engineering, understanding the distinction between a flue and a tube is important because it relates to their functions and applications in systems such as boilers and heat exchangers.

Flues are specifically designed for the passage of exhaust gases from combustion processes, and they typically need to accommodate larger volumes of gas, which is why the diameter specification is set at 4 inches or more. This larger size is crucial for efficient ventilation and to prevent back pressure that could affect the operation of a heating system.

In contrast, tubes, which may have a smaller internal diameter, can serve various purposes, including serving as conduits for liquids or gases, not strictly limited to steam transfer or water. Therefore, understanding these size distinctions is critical for ensuring proper system design and compliance with safety regulations.