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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 clear overhead structures. Some chimneys included features to avoid dispersing sparks.
== Function ==
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.〔White, p.111〕
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.〔White, p.112〕
As boiler design improved heat transfer efficiency, blast pipe diameters increased to reduce back pressure; and blastpipes became shorter to discharge below the chimney rather than within it. Ross Winans placed conical "petticoat pipes" above blastpipes about 1848〔 to form the convergent portion of a venturi tube with the chimney forming the divergent portion.〔Sears & Zemansky, p.315〕 Improved understanding of compressible flow encouraged more sophisticated blastpipe and venturi chimney designs. George Jackson Churchward, working at Swindon on the Great Western Railway, formulated a simple equation for calculating the ideal dimensions for chimneys which worked well for the early years of the 20th century, but soon become outdated as engine powers increased. André Chapelon in France continued to work on chimney dimensions, and studied them in conjunction with blastpipe dimensions as a complete Steam locomotive exhaust system, such as his famous Kylchap system as fitted to many locomotive classes worldwide. Even after the end of commercial steam in most of the developed world, the Argentinean engineer Livio Dante Porta continued to work on developing steam locomotive exhaust systems including refining equations to give better chimney dimensions.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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