Double chimney
The classic exhaust design for a steam locomotive began with Hackworth's invention of the blastpipe, placed centrally within a tall chimney.
The standard design was then a circular drumhead smokebox, with a single blastpipe nozzle leading into a chimney with a flared petticoat pipe beneath it.
This allowed adequate cross-section area for airflow, whilst reducing the diameter of each and thus the minimum height needed for an acceptably gentle taper.
This split the blastpipe area into four smaller nozzles, and the vertical draught induction across three stacked venturis.
Although there is no reason why one approach, either the double chimney or the Kylchap blastpipe, depends on the other, interest in both was generally simultaneous and so both were often installed together.
Work on the static test plant at Rugby discovered that there was both no advantage to the double chimney and also that it had been poorly designed initially.
[4] A minor disadvantage could be a 'softer' exhaust blast for the purpose of lifting the external smoke clear of the driver's vision.
When the LNER A3 class were fitted with double chimneys in the late 1950s, they suffered problems with smoke obscuring the view from the cab.
Nigel Gresley, the CME of the LNER, was a keen follower of French locomotive practice, particularly the work of André Chapelon and the Nord 'Superpacifics' of Marc de Caso [fr].
When Gresley designed his P2 class as successor to his A3s,[iv] he took this French work into account and also used a double chimney with Kylchap blastpipes.
To avoid problems with smoke obscuring the driver's vision, both were built with wedged tops to their smokebox and wing plates to the upper sides of it, as had been used for 10000.
With the sharper exhaust of the poppet valve-equipped 2001, this was successful and smoke was projected upwards, clear of the cab windows.
2002 had a softer exhaust though and gave trouble, until it was rebuilt with additional smoke deflectors, spaced about 18 inches parallel to the existing wing plates.
[13] These modifications were part of a series of experiments by George Ivatt to improve the already excellent Stanier-designed Black 5.
These engines were considered as hungrier for coal than usual as the audibly sharp exhaust bark had an effect on the firebox draught.
The mechanical drive design was considered successful and was copied for the few later Caprotti engines built for the BR Standard classes.
All the Caprotti engines kept their double chimneys and it was noticeable that their still-staccato exhaust had no trouble in lifting smoke clear of the cab.
[18] At the same time, there was also a proposal to test a Giesl ejector on the 9F[vii] Possibly as a comparison for this, 92178 was built with a double chimney.
The double chimney was so successful[20] that it was adopted as standard for all 9F built from 92183 onwards,[viii] including the three fitted with a mechanical stoker.
The Caprotti company had recommended the use of a Kylchap blastpipe, to counter the adverse effects of this, but this had been ignored in favour of a Swindon-designed conventional double chimney, based on their experience with the King class.
It had a short service life, as a combination of both the rapid withdrawal of steam traction on British Rail and also its poor performance and a reluctance to expend effort in solving this.
During this restoration, the boiler draughting and ashpan air supply were both examined and found to be unexpectedly restrictive.
The operational locomotives of the modern Union Pacific Steam Program, UP 844, UP 4014 and the now retired UP 3985 all retained their double chimneys.
Double stacks were a common feature on several of the PRR's duplex locomotive classes and other experimental designs in the late steam era.
Pennsylvania Railroad class S2 used a unique quadruple stack system as part of its experimental steam turbine design.
Pennsylvania Railroad 5550 a new build based on the T1 class is currently underway being built by the T1 Steam Locomotive Trust.
