Chimney (locomotive)

The chimney (smokestack or stack in American and Canadian English) is the part of a steam locomotive through which smoke leaves the boiler. Steam locomotive exhaust systems typically vent cylinder exhaust through the chimney to enhance draught through the boiler. Chimneys are designed to carry exhaust steam and smoke clear of the driver's line of sight while remaining short enough to clear overhead structures. Some chimneys included features to avoid dispersing sparks.

The chimney was usually located above the smokebox at the leading end of the locomotive, furthest away from the driver's cab and firebox. The earliest locomotive chimneys were typically tall enough to sustain temperature-induced density difference draught through a fire-tube boiler while the locomotive was stationary; but following the example of Richard Trevithick's first locomotive in 1804, most designs diverted steam cylinder exhaust upward through the chimney to create a vacuum in the smokebox and accelerate airflow through the firebox while the locomotive was in motion.

Locomotives with high chimneys and low footplates had the additional advantage of keeping smoke and condensing steam above the engine driver's field of vision. Grade limitations of railways through hilly terrain required tunnels and overhead bridges imposing a loading gauge limiting the height of chimneys. Increasing the velocity of steam exhaust tended to both accelerate airflow through the firebox and lift the smoke higher above the end of the chimney. By the 1830s steam exhaust was directed through a contracted nozzle called a blastpipe to achieve desired velocity through the chimney. Pressure drop through the blastpipe nozzle was subtracted from the boiler pressure available to the steam pistons. Robert Stephenson estimated some locomotives lost half their power through blastpipe back pressure.

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