{{Short description|Heat-exchanging element in the firebox of some steam boilers}} thumb|A Nicholson syphon, before installation in the firebox [[File:Leader boiler under construction.jpg|thumb|Thermic syphons in the boiler of Bulleid's Leader class. They are particularly visible in the Leader boiler, as the firebox is dry-walled, rather than water-jacketed.]] [[File:Baureihe498.1.png|thumb|Czechoslovak State Railways 498.1 featured thermic siphons in the firebox]] '''Thermic siphons''' (alt. '''thermic syphons''') are heat-exchanging elements in the firebox or combustion chamber of some steam boiler and steam locomotive designs. As they are directly exposed to the radiant heat of combustion, they have a high evaporative capacity relative to their size. By arranging them near-vertically, they also have good water circulation by means of the thermosyphon effect.
==History== The concept of a self-circulating thermic syphon began with stationary boilers and relatively simple Galloway tubes. They reached their peak in steam locomotive boilers, where the complexity of a syphon was justified by the need for a compact and lightweight means of increasing boiler capacity. One of the best-known forms for locomotives was invented by the English locomotive engineer {{nowrap|John L. Nicholson}} who received a US patent.<ref>{{cite patent|country=US|number=1679051|title=Thermic Siphon for Locomotives}}</ref><ref>[http://www.athra.asn.au/library/Thermic_Syphons.pdf Thermic siphons]</ref><ref name="How Steam Locomotives Really Work, 44">{{cite book |last1=Semmens|first1=PWB|last2=Goldfinch|first2=AJ|year=2000|title=How Steam Locomotives Really Work|publisher=Oxford University Press|isbn=978-0-19-860782-3|pages=44}}</ref> The Nicholson form combined a complex shape that provided more heating area in a given space than did the earlier tubes and funnels, yet was simple to make, being folded from a single sheet of steel.
==Flued boilers== thumb|Galloway tubes in a Lancashire boiler The first high-pressure boilers were a large drum with a central flue, such as the Cornish and Lancashire boilers. Simple tubes were inserted across this flue.<ref >{{Book-Hills-Power from Steam|pages=134–135}}</ref>
==See also== *Thermosiphon
==References== {{Reflist}}
==External links== {{Commonscat|Thermic syphons}} *[https://www.bluebell-railway.co.uk/bluebell/locos/vt/news/news_01a26_1.html Photo of lower tube-sections of thermic siphons on Battle of Britain class no. 34059 ''Sir Archibald Sinclair'']
{{Boilers}}
Category:Boilers Category:Locomotive parts