A Scotch marine boiler is a fire-tube system: hot gas travels through tubes surrounded by water. Cylindrical furnaces sit low in the shell, feed combustion chambers, and route smoke aft before it returns through small tubes toward the funnel. Steam collects above the tubes, while rods and the tube structure brace the pressure-loaded shell. Separate combustion chambers reduce the risk that exhaust from one furnace will reverse into another and injure stokers. The illustrated four-furnace arrangement was less common than versions with two or three furnaces. Titanic's installation included 29 Scotch-type boilers, with 24 double-ended and five smaller single-ended units. Replacement Scotch boilers were still available when the video was recorded, reflecting the design's long use in merchant shipping.
Notable Point · 47:17-50:03
Follow the fire-tube cycle inside Sackville's boiler
The Scotch boiler's returning furnace gases heated water around the tubes, while separate combustion chambers reduced the risk of blowback toward stokers.