Which type of tubes are NOT typically used in a D-type boiler?

The correct answer identifies that only vertical tubes are not typically used in a D-type boiler. D-type boilers are designed with a particular configuration where components, including the tubes, are arranged to optimize steam generation and heat transfer.

In a D-type boiler, the arrangement typically consists of both horizontal and curved tubes that facilitate effective circulation of water and steam. The horizontal tubes are positioned to allow for a large surface area for heat exchange, while curved tubes can help in maintaining the flow path and pressure throughout the system.

Vertical tubes, on the other hand, do not align with the operational design and flow mechanics preferred in a D-type boiler. Their orientation would not support the intended efficient operation of this type of boiler, which is centered around maximizing heat transfer and steam movement in a compact space.

Curved and horizontal configurations help in maintaining a balance of pressure and promoting the efficient movement of water and steam, fundamental in a D-type boiler’s design and function.