In a water tube boiler, what is the function of downcomer tubes?

In a water tube boiler, downcomer tubes play a crucial role in the circulation of water within the system. Their primary function is to transport water from the steam drum back to the lower parts of the boiler. This is essential for the effective operation of the boiler because it ensures that water is returned to the heating surfaces where it can be heated and converted to steam.

As water is heated in the boiler, it rises to the steam drum, and the downcomer tubes allow the cooler water to descend back down, maintaining a continuous flow of water through the boiler. This circulation is vital for efficient heat transfer and helps prevent overheating and damage to the boiler components.

In contrast, the other functions listed in the alternatives—carrying steam, directing hot gases, or discharging exhaust—are associated with other components of the boiler system, such as steam pipes, combustion chambers, or flue gas systems. Hence, the specific task of downcomer tubes is accurately captured by their role in transporting water.