ABSTRACT
Radiant heat transfer requires a line-of-sight to the flame. Only this heat transfer mechanism can oper ate in a vacuum. For example, the Earth receives essen tially all its heat from the sun through this mechanism. Convection requires the bulk movement of a hot fluid. In a boiler, hot combustion gases transfer heat to the outer tube wall via convection. Convection occurs naturally by means of buoyancy differences between hot and cool flu ids, termed natural convection, or by motive devices such as fans or blowers creating high velocities, termed forced convection. Heat transfers from the outer to inner tube wall by conduction-the predominant heat transfer mode through metals. Inside the tube, convection is the pre dominant mode of heat transfer to the inside fluid. A more complete discussion of heat transfer is given in Chapter 7.
