On a D-type boiler, what are the types of tubes it should have?

The D-type boiler is characterized by its unique design that incorporates both horizontal and vertical tubes, which facilitate efficient steam generation and water circulation. The horizontal tubes are typically used for the evaporation of water and the generation of steam, while the vertical tubes, which connect to the steam drum, allow for the appropriate circulation of water and steam, optimizing the heat transfer process.

This dual-tube arrangement enhances the boiler’s operational efficiency by ensuring that the water and steam can rise and flow effectively through the system. Thus, the inclusion of both horizontal and vertical tubes is crucial in maintaining the boiler's functionality and ensuring adequate steam production and response to varying loads. The other options suggest limitations on tube orientation that do not align with the operational design of D-type boilers, which thrive on the synergy provided by having both types.