The steam locomotive exhaust system consists of those parts of a steam locomotive which together discharge exhaust steam from the cylinders in order to increase the draught through the fire. It usually consists of the blastpipe (or first stage nozzle), smokebox, and chimney, although later designs also include second and third stage nozzles.
The primacy of discovery of the effect of directing the exhaust steam up the chimney as a means of providing draft through the fire is the matter of some controversy, Ahrons (1927) devoting significant attention to this matter. The exhaust from the cylinders on the first steam locomotive – built by Richard Trevithick – was directed up the chimney, and he noted its effect on increasing the draft through the fire at the time. At Wylam, Timothy Hackworth also employed a blastpipe on his earliest locomotives, but it is not clear whether this was an independent discovery or a copy of Trevithick's design. Shortly after Hackworth, George Stephenson also employed the same method but again it is not clear whether it was an independent discovery or a copy of a design from one of the other engineers.
The locomotives at the time employed either a single flue boiler or a single return flue, with the fire grate at one end of the flue. For boilers of this design the blast of a contracted orifice blastpipe was too strong, and would lift the fire. It was not until the development of the multi-tube boiler that the centrally positioned, contracted orifice blastpipe became standard. The combination of multi-tube boiler and steam blast are often cited as the principal reasons for the high performance of Rocket of 1829 at the Rainhill Trials.
Soon after the power of the steam blast was discovered it became apparent that a smokebox was needed beneath the chimney, to provide a space in which the exhaust gases emerging from the boiler tubes can mix with the steam. This had the added advantage of allowing access to collect the ash drawn through the fire tubes by the draught. The blastpipe, from which steam is emitted, was mounted directly beneath the chimney at the bottom of the smokebox.
The steam blast is largely self-regulating: an increase in the rate of steam consumption by the cylinders increases the blast, which increases the draught and hence the temperature of the fire.
Modern locomotives are also fitted with a blower, which is a device that releases steam directly into the smokebox for use when a greater draught is needed without a greater volume of steam passing through the cylinders. An example of such situation is when the regulator is closed suddenly, or the train passes through a tunnel. If a single line tunnel is poorly ventilated, a locomotive entering at high speed can cause a rapid compression of the air within the tunnel. This compressed air may enter the chimney with substantial force. This can be extremely dangerous if the firebox door is open at the time. For this reason the blower is often turned on in these situations, to counteract the compression effect.
The aim of exhaust system development is to obtain maximum smokebox vacuum with minimum back pressure on the pistons.
Little development of the basic principles of smokebox design took place until 1908, when the first comprehensive examination of steam-raising performance was carried out by W.F.M. Goss of Purdue University. These principles were adopted on the Great Western Railway by Churchward, and later developed by Samuel Ell in the 1950s using the GWR (then nationalised under BR) stationary testing plant. Ell was able to double the maximum steaming rate of the GWR Manor class by apparently minor alterations to the front end design, and more than doubled the rate for an LNER V2.
Andre Chapelon made a significant improvement with his Kylchap exhaust which incorporated a Kylälä spreader (second stage nozzle) and third stage cowl between the blastpipe (first stage nozzle) and chimney. This became popular at the end of the steam era (early-mid 20th century) and was used on the Nigel Gresley's Mallard which holds the official world speed record for steam locomotives. Other contemporary designs include the Giesl, and Lemaître exhausts which achieve the same aim by different means.
Further development was carried on by Chapelon's friend Livio Dante Porta, who developed the Kylpor, Lempor and Lemprex exhausts systems, and also developed sophisticated mathematical models to optimise their use for specific locomotives.
With the demise of commercial steam operations on mainline railways worldwide, there has been little funding for further development of steam locomotive technology, despite advances in materials technology and computer modelling techniques which might have enabled further improvements to efficiency.
Stephenson's Rocket is an early steam locomotive of 0-2-2 wheel arrangement. It was built for and won the Rainhill Trials of the Liverpool and Manchester Railway (L&MR), held in October 1829 to show that improved locomotives would be more efficient than stationary steam engines.
Timothy Hackworth was an English steam locomotive engineer who lived in Shildon, County Durham, England and was the first locomotive superintendent of the Stockton and Darlington Railway.
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 to heat water in the locomotive's boiler to the point where it becomes gaseous and its volume increases 1,700 times. Functionally, it is a steam engine on wheels.
Sans Pareil is a steam locomotive built by Timothy Hackworth which took part in the 1829 Rainhill Trials on the Liverpool and Manchester Railway, held to select a builder of locomotives. The name is French and means 'peerless' or 'without equal'.
The Kylchap steam locomotive exhaust system was designed and patented by French steam engineer André Chapelon, using a second-stage nozzle designed by the Finnish engineer Kyösti Kylälä and known as the Kylälä spreader; thus the name KylChap for this design.
Main components found on a typical steam locomotive include:
An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic pump with no moving parts except a valve to control inlet flow.
A fire-tube boiler is a type of boiler invented in 1828 by Mark Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.
A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are three types of superheaters: radiant, convection, and separately fired. A superheater can vary in size from a few tens of feet to several hundred feet.
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 exhausted to the atmosphere through the chimney. Early locomotives had no smokebox and relied on a long chimney to provide natural draught for the fire but smokeboxes were soon included in the design for two specific reasons. Firstly and most importantly, the blast of exhaust steam from the cylinders, when directed upwards through an airtight smokebox with an appropriate design of exhaust nozzle, effectively draws hot gases through the boiler tubes and flues and, consequently, fresh combustion air into the firebox. Secondly, the smokebox provides a convenient collection point for ash and cinders ("char") drawn through the boiler tubes, which can be easily cleaned out at the end of a working day. Without a smokebox, all char must pass up the chimney or it will collect in the tubes and flues themselves, gradually blocking them.
The Lempor ejector is a steam locomotive exhaust system developed by noted Argentine locomotive engineer Livio Dante Porta. The ejector's name is a portmanteau of the names of Porta and Belgian locomotive engineer Maurice Lemaître. The Lempor ejector follows the principles of the de Laval nozzle.
The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.
A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure but, at pressures above this, it is more usual to speak of a steam generator.
A shell or flued boiler is an early and relatively simple form of boiler used to make steam, usually for the purpose of driving a steam engine. The design marked a transitional stage in boiler development, between the early haystack boilers and the later multi-tube fire-tube boilers. A flued boiler is characterized by a large cylindrical boiler shell forming a tank of water, traversed by one or more large flues containing the furnace. These boilers appeared around the start of the 19th century and some forms remain in service today. Although mostly used for static steam plants, some were used in early steam vehicles, railway locomotives and ships.
Kyösti Kylälä was a Finnish railroad engineer and self-taught inventor. In 1919 he patented in the UK an 'Improved means for increasing the draught in steam boilers, especially on locomotives.'
Boilers for generating steam or hot water have been designed in countless shapes, sizes and configurations. An extensive terminology has evolved to describe their common features. This glossary provides definitions for these terms.
The Sentinel boiler was a design of vertical boiler, fitted to the numerous steam wagons built by the Sentinel Waggon Works.
The chimney is the part of a steam locomotive through which smoke leaves the boiler. As well, steam locomotive exhaust systems typically vent cylinder steam exhaust through the chimney, to enhance the draught through the boiler. Chimneys are designed to carry the exhaust steam and smoke clear of the driver's line of sight while remaining short enough to clear overhead structures. Some chimneys included apparatus to suppress the dispersal of sparks.
The South African Railways Class 10B 4-6-2 of 1910 was a steam locomotive from the pre-Union era in Transvaal.
A double chimney is a form of chimney for a steam locomotive, where the conventional single opening is duplicated, together with the blastpipe beneath it. Although the internal openings form two circles, the outside appearance usually forms a single elongated oval.
