What is the key difference between drum-type boiler and once-through boiler?

• A. Drum-type uses no steam separation; once-through uses drum.
• B. Once-through stores water in a drum and separates steam.
• C. Both operate identically except for feedwater heater placement.
• D. Drum-type boilers store water in a steam drum with circulating water and steam separation; once-through boilers produce steam directly in a continuous flow without a drum.

Answer: (D)

The key difference is how steam is generated and whether a steam-water separation vessel is used. In a drum-type boiler, there is a steam drum that collects the steam after it separates from the circulating water. Water circulates through the boiler tubes, then into the drum where the steam is separated from the remaining water, and the drum also provides storage and level control. The generated steam is drawn off from the drum after this separation.

In a once-through boiler, there is no steam drum or storage vessel. Feedwater is pumped through the boiler in a continuous flow, heated as it passes, and exits as steam in one pass (or a few passes) without any separating drum in the vessel. This means there’s no separate phase separation inside a drum; steam quality is controlled by the flow and heat input in real time.

So the main concept is the presence of a steam drum with water–steam separation and storage in drum-type boilers versus the drumless, continuous-flow operation of once-through boilers. The other statements don’t fit because they mischaracterize the role of the drum, the presence of storage, or the fundamental flow arrangement.