Which tubes do not generate steam during high firing rates?

Downcomers are specifically designed to transport feedwater from the steam drum to the water wall in a boiler system. Their primary function is to ensure a continuous flow of water to the evaporator sections of the boiler where steam generation occurs. During high firing rates, downcomers do not participate in the steam generation process, as their role is to bring water into the boiler rather than facilitating the production of steam.

In contrast, evaporator tubes, superheater tubes, and generating tubes are all involved in the steam production process. Evaporator tubes are where the bulk of the steam generation takes place as water is heated and converted into steam. Superheater tubes work to increase the temperature of the steam produced in the evaporator, and generating tubes are also integral to the steam generation system. Thus, during periods of high firing rates where steam is being produced rapidly, these tubes are actively involved in generating steam, unlike the downcomers.