HOME

TheInfoList



OR:

A double chimney (or double stack, double smokestack in American English) is a form of
chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are t ...
for a
steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...
, where the conventional single opening is duplicated, together with the blastpipe beneath it. Although the internal openings form two circles, the outside appearance is as a single elongated oval.


Purpose

The classic exhaust design for a steam locomotive began with Hackworth's invention of the blastpipe, placed centrally within a tall chimney. Victorian developments reduced the chimney's height, such that natural draught was no longer significant. The standard design was then a circular drumhead
smokebox A smokebox is one of the major basic parts of a steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is ...
, with a single blastpipe nozzle leading into a chimney with a flared petticoat pipe beneath it. From the work of theorists such as W.F.M. Goss of
Purdue University Purdue University is a public land-grant research university in West Lafayette, Indiana, and the flagship campus of the Purdue University system. The university was founded in 1869 after Lafayette businessman John Purdue donated land and ...
, and later S.O. Ell of Swindon, guidelines were developed at each locomotive works, describing how these were to be proportioned. It was recognised both that a particular diameter of chimney and blastpipe would be needed for the steam-raising capacity of each boiler, and also that the conical taper from blastpipe to chimney could not be made too steep. As boilers became more powerful, not only did the chimney diameter need to become greater, but also the minimum height for the chimney was becoming longer – just as the increasing size of boilers restricted the clearance height available within the
loading gauge A loading gauge is a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. Their purpose is to ensure that rail vehicles can pass safely through tunnels and under bridges, and ke ...
. A chimney height of at least 24 inches was considered the minimum workable. By the 1930s, it was increasingly difficult to provide such a height and other solutions were sought. A solution to this limit was to adopt a double chimney. 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.


Kylchap blastpipes

A simultaneous development was the Kylchap blastpipe, combining the Kylälä spreader by Finnish engineer Kyösti Kylälä, and a further flue choke tube added by the French engineer André Chapelon. This split the blastpipe area into four smaller nozzles, and the vertical draught induction across three stacked venturis. Although the total blastpipe area remained constant, their perimeter, and thus the area for mixing with the exhaust gases, was doubled. The additional petticoats also improved the effectiveness of the blast in inducing a draught. 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.


Disadvantages

The first 50 of the Ivatt class 4MT 2-6-0 were built with double chimneys. These performed poorly however, and were noted as poor steamers. 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. When revised with a single chimney and improved gasflow in the smokebox, their steaming rate was raised from 9,000 lb/hour with a double chimney to 17,000 lb/hour with a single chimney, even though this was still below the theoretical limit, restricted by firegrate size, of 19,000 lb/hour. 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. The solution to this was to fit small ''Witte''-type
smoke deflectors Smoke deflectors, sometimes called "blinkers" in the UK because of their strong resemblance to the blinkers used on horses, and "elephant ears" in US railway slang, are vertical plates attached to each side of the smokebox at the front of a ste ...
of the German pattern.


Notable installations

Many double chimney installations, at least in the UK, were performed as experimental conversions in the 1930s, rather than as new builds.


LNER A4

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 . When Gresley designed his P2 class as successor to his A3s, he took this French work into account and also used a double chimney with Kylchap blastpipes. Two P2s were built initially, 2001 ''Cock O' The North'' and then 2002 ''Earl Marischal'', both in 1934. Also following French practice, 2001 was built with poppet valves, and for comparison 2002 kept the conventional piston valves. 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 10,000 (ten thousand) is the natural number following 9,999 and preceding 10,001. Name Many languages have a specific word for this number: in Ancient Greek it is (the etymological root of the word myriad in English), in Aramaic , in Hebrew ...
. 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. Both locomotives were considered successful, but 2002 had the edge for efficiency, put down to the smaller volumes within the valve chest. When the further members of the P2 class were built, they followed 2002 with piston valves and the extra smoke deflectors. The first of the A4 class reverted to a single chimney and a conventional blastpipe. They had the greatest attention paid to their gas flow generally, both the inlet and exhaust sides. With the P2s, there had been a tendency for an ''excess'' of draught, when working hard at a long cut-off, enough to lift the fire. To avoid this, the A4s used a 'jumper top' on their blastpipe, a loose ring which rose under the influence of a strong blast jet, increasing the effective nozzle diameter and so reducing the drawing effect of the blast. This device could not be applied to either a double chimney, nor to a Kylala blastpipe, but it is not clear if that was the only reason for the simplified single blastpipe. 4468 ''Mallard'' the 28th of the A4s was built with a double chimney and Kylchap blastpipes in 1938. This was considered successful and so the final three of the class, built a few months later, were also built with them. The entire class was refitted similarly in the 1950s, together with some of the A3s. Peppercorn's A2 pacifics were built, post-war, with similar double Kylchap blastpipes.


LMS Jubilee

Five members of the
LMS Jubilee class The London Midland and Scottish Railway (LMS) Jubilee Class is a class of steam locomotive designed for main line passenger work. 191 locomotives were built between 1934 and 1936. They were built concurrently with the similar looking LMS Stanie ...
were experimentally fitted with double chimneys at different times. The first was 5684 ''Jutland'', a double chimney with Kylchap petticoats in 1937. This improved both the steaming capacity and also reduced coal consumption, although it was removed after a year owing to problems with the excessive draught causing spark-throwing from the chimney and a build-up of excess smokebox ash. 5742 ''Connaught'' and 5553 ''Canada'' were then fitted with plain double chimneys in 1940, which was removed from ''Canada'' after a short time, but which ''Connaught'' carried until 1955. As part of experiments at Rugby test plant, 45722 ''Defence'' was fitted with a double chimney from 1956 to 1957. In 1961 a double exhaust was fitted to 45596 ''Bahamas'' which carried it through withdrawal and into preservation. Two further engines, 5735 ''Comet'' and 5736 ''Phoenix'' were rebuilt with a 2A taper boiler and double chimney in 1942. They were to have been a prototype for the rebuilding of the entire class but, in the end, the only Jubilees so to be treated. All the Royal Scot class were rebuilt along similar lines as were many of the Fowler Patriot locos.


LMS Black 5

LMS 'Black 5' 4767 was completed on the last day of the LMS, 31 December 1947. It was unique amongst the 842-strong class in that it featured outside
Stephenson link motion The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for various kinds of steam engines. It is named after Robert Stephenson but was invented by his employees. ...
in addition to other experimental features; a double chimney, Timken
roller bearing In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...
s throughout and electric lighting. These modifications were part of a series of experiments by George Ivatt to improve the already excellent Stanier-designed Black 5. The double chimney was removed in 1953, owing to the already-good performance of the standard chimney and problems with the softer blast of the double chimney not clearing smoke away from the cab so well. 4765 and 4766 had also trialled a similar double chimney, but with the standard Walschaerts valvegear. Some of the first BR-built Black 5s were built with
Caprotti valvegear The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920s by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear. While basi ...
. A batch of twenty, 44738-44757, were built, of which the last three also had double chimneys. The boilers of the Caprotti engines were raised by 2 inches, further reducing the clearance for the chimney height. The Caprotti valveboxes were arranged with the inlet valves on the outside, fed by large and prominent steam pipes, and the exhaust valves on the inside. The large clearance volume which was unavoidable within the valve chest of the Caprotti's
poppet valve A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...
s had a similar effect to a long
lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...
on the valve setting. In contrast to the Stephenson engine, this led to these engines performing well at speed but poorly for climbing. Although the intention for trialling the Caprotti valve gear had been to reduce maintenance, coal consumption was still important. These engines were considered as hungrier for coal than usual as the audibly sharp exhaust bark had an effect on the firebox draught. The double chimney examples though had a softer bark and so consumed less. A second batch of two more Caprotti-fitted engines was tried in 1951, 44686 and 44687. These were of a different design, with an improved mechanical drive to the camboxes, and both with double chimneys. To reduce the 'dead-space' volume within the valve chest, the exhaust valves were reduced in size to now be the same as the inlets. The mechanical drive design was considered successful and was copied for the few later Caprotti engines built for the BR Standard classes. The behaviour of good high-speed performance but a lack of power with low cutoffs was the same as the other engines though. 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.


Great Western King class

In September 1955, GWR King class 6015 ''King Richard III'' was fitted with a double chimney for trials. These were successful and so the whole class was refitted with them. Some of the Castle class were similarly refitted. Tests with a dynamometer car in hauling the '' Cornish Riviera'' in 1956 did not show an evident increase in performance, but did show an 8% improvement in the efficiency of coal consumption. Water consumption remained constant, indicating that this was an improvement in combustion and heat transfer, rather than the reduction in engine exhaust back pressure, indicated by some other tests.


British Railways Standard classes

The BR Standards had been designed from the outset with their draughting based on the earlier work of S.O. Ell. Despite this, some of the classes steamed poorly, notably the Class 4 4-6-0. In 1957, 75029 was successfully fitted with a double chimney, leading to its adoption across many of the class. At the same time, there was also a proposal to test a Giesl ejector on the 9F Possibly as a comparison for this, 92178 was built with a double chimney. The double chimney was so successful that it was adopted as standard for all 9F built from 92183 onwards, including the three fitted with a
mechanical stoker A mechanical stoker is a mechanical system that feeds solid fuel like coal, coke or anthracite into the furnace of a steam boiler. They are common on steam locomotives after 1900 and are also used on ships and power stations. Known now as a spr ...
.


''Duke of Gloucester''

The BR Standard class 8 ''Duke of Gloucester'' was the last of the BR Standard classes to be designed, as the need to build any more of such large express passenger locomotives had initially been rejected. This led to it ignoring one of the design principles of the Standard classes and rather than two cylinders with outside
Walschaerts valvegear The Walschaerts valve gear is a type of valve gear used to regulate the flow of steam to the pistons in steam locomotives, invented by Belgian railway engineer Egide Walschaerts in 1844. The gear is sometimes named without the final "s", since ...
it instead used three cylinders and
Caprotti valvegear The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920s by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear. While basi ...
, with cam-actuated poppet valves. The Caprotti's valves opened fully more rapidly than piston valves, giving a sharp exhaust bite. 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. Given the sharpness of the blast, these Swindon principles had produced a relatively small blastpipe, with a double chimney of similarly small proportions. In service, ''Duke of Gloucester'' performed infamously poorly, with a reputation for poor steaming. It had a short service life, as a combination of both the rapid withdrawal of steam traction on
British Rail British Railways (BR), which from 1965 traded as British Rail, was a state-owned company that operated most of the overground rail transport in Great Britain from 1948 to 1997. It was formed from the nationalisation of the Big Four (British ra ...
and also its poor performance and a reluctance to expend effort in solving this. ''Duke of Gloucester'' was rescued from Barry scrapyard and an extensive restoration was required. During this restoration, the boiler draughting and ashpan air supply were both examined and found to be unexpectedly restrictive. These were assumed to be the root causes of the poor performance, which was borne out by the improved performance after both were remedied. Although the choke size of the chimneys was in proportion to the blastpipe, their overall size was a fraction of comparably sized boilers on the Merchant Navy pacifics and the A2 pacifics. Restoration involved constructing a pair of Kylchap blastpipes and chimney flue chokes. As well as improving draughting, these blastpipes also reduced back pressure on the cylinders, further improving efficiency.


United States


Union Pacific Railroad

Several late steam era designs on the
Union Pacific Railroad The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pac ...
, particularly those designed during Otto Jabelmann's time as the railroad's chief mechanical officer; used double smokestacks. The FEF-3, the third and final iteration of the Union Pacific FEF Series were all built new with double stacks. Like their British contemporaries, the double stacks caused issues in creating massive amounts of smoke which could block the engineer's view, as such the FEF-3's were soon equipped with elephant ear style smoke deflectors. Smoke deflectors would later be placed on all the other FEF locomotives, including those without double chimneys. One of the railroad's FEF-2s, No. 831, was fitted with an experimental triple chimney. Other Union Pacific designs to get double stacks were the late-era
articulated locomotive An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independent of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to neg ...
s built for the railroad, including the final group of 1942 built Union Pacific Challenger locomotives and the entire run of
Union Pacific Big Boy The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962. The 25 Big ...
locomotives. While some Challenger and Big Boy locomotives were equipped with smoke deflectors, smoke was not as problematic on them as it was on the FEF class due to their much longer boilers. A handful of the double chimney equipped Challengers were diverted to the
Denver and Rio Grande Western Railroad The Denver & Rio Grande Western Railroad , often shortened to ''Rio Grande'', D&RG or D&RGW, formerly the Denver & Rio Grande Railroad, was an American Class I railroad company. The railroad started as a narrow-gauge line running south from ...
, from which they would later serve on the Clinchfield Railroad. 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.


Pennsylvania Railroad

Double stacks were a common feature on several of the PRR's
duplex locomotive A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive. The concept was first used in France in 1 ...
classes and other experimental designs in the late steam era.
Pennsylvania Railroad class S1 The PRR S1 class steam locomotive (nicknamed "The Big Engine") was a single experimental duplex locomotive of the Pennsylvania Railroad. It was designed to demonstrate the advantages of duplex drives espoused by Baldwin Chief Engineer Ralph P. Jo ...
a lone prototype 6-4-4-6 used a double stack as well as the
Pennsylvania Railroad class Q2 The Pennsylvania Railroad's class Q2 comprised one prototype and twenty-five production duplex steam locomotives of 4-4-6-4 wheel arrangement. They were the largest non-articulated locomotives ever built and the most powerful locomotives ever sta ...
a
4-4-6-4 A 4-4-6-4, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is one that has four leading wheels followed by four coupled driving wheels, a second set of six coupled driving wheels and four trailing wheel ...
duplexs which had a total of 26 locomotives built. The
Pennsylvania Railroad class T1 The Pennsylvania Railroad (PRR) class T1 duplex-drive 4-4-4-4 steam locomotives, introduced in 1942 with two prototypes and later in 1945-1946 with 50 production examples, were the last steam locomotives built for the PRR and arguably its most c ...
a duplex class with 52 locomotives produced also carried double stacks. Pennsylvania Railroad class S2 used a unique quadruple stack system as part of its experimental steam turbine design. This one off engine introduced in 1944 was scrapped by 1952. Pennsylvania Railroad 5550 a new build based on the T1 class is currently underway being built by the T1 Steam Locomotive Trust. As of 2021, the engine's boiler is being constructed and when complete will contain a double stack like the original locomotives.


Southern Pacific Railroad

Several of the Southern Pacific Railroad's larger engines such as the Southern Pacific class AC-9 and the Southern Pacific class AC-12 all used double stack systems. These were also equipped with "stack splitters" a design feature going back to Southern Pacific's single stack Mallet's that reduced the speed of the exhaust to prevent damaging the roofs of snow sheds. Several of these engines were built with a Cab forward
4-8-8-2 Under the Whyte notation for the classification of steam locomotives, a 4-8-8-2 is a locomotive with four leading wheels, two sets of eight driving wheels, and a two-wheel trailing truck. Other equivalent classifications are: UIC classification ...
design, placing the locomotive cab on the forward part of the engine to prevent asphyxiation during long tunnel and snow shed segments on the Southern Pacific. Southern Pacific 4294 the only survivor of Southern Pacific's cab forward locomotives is preserved in Sacramento, California at the California State Railroad Museum.


South America


L.D.Porta's ''Argentina''

Livio Dante Porta's experimental meter gauge locomotive ''Argentina'' was a 1948 rebuild of a former
4-6-2 Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, six powered and coupled driving wheels on three axles and two trailing wheels on one axle. The loco ...
into a
4-8-0 Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, usually in a leading truck or bogie, eight powered and coupled driving wheels on four axles and no traili ...
. The locomotive incorporated a double chimney in addition to other improvements intended to improve steam flow and fuel consumption. Porta's exhaust designs ultimately evolved into the Lempor ejector. The experimental locomotive was a success, and other locomotives would be modified to include Porta's upgrades although not many received double chimneys like ''Argentina'' had. In the 1970s Porta designed a large locomotive to fit the North American Plate C loading gauge that was based on ''Argentina's'' design, including double chimneys. That locomotive was never built. ''Argentina'' was preserved in the Mate de Luna station in San Miguel de Tucuman, its current status is unknown with some reports suggesting it has since been scrapped.


South Africa


South African Class 26 4-8-4 The South African Railways Class 26 4-8-4 of 1981, popularly known as the ''Red Devil'', is a 4-8-4 steam locomotive which was rebuilt from a Class 25NC locomotive by mechanical engineer David Wardale from England while in the employ of the Sou ...

Following on L.D. Porta's design methodology, the South African Class 26 4-8-4 ''L.D. Porta'' (better known via its nickname ''Red Devil'') was equipped with a double chimney system outfitted with Lempor ejectors along with a Gas Producer Combustion System to improve efficiency.


Notes


References


Sources

* * * * * * * *


Further reading

* * * * {{Cite book , title=The Fire Burns Much Better ... , publisher=Camden Miniature Steam Services , first=J. J. G. , last=Koopmans , year=2014 , isbn=978-1909358058 Steam locomotive exhaust systems