Condensing steam locomotive
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A condensing steam locomotive differs from the usual closed cycle condensing steam engine, in that the function of the condenser is primarily either to recover water, or to avoid excessive emissions to the atmosphere, rather than maintaining a vacuum to improve both efficiency and power. It takes the form of a series of pipes, valves and other ancillary equipment usually attached to an otherwise conventional steam locomotive. The apparatus takes the exhaust steam that would normally be lost up the chimney and routes it through a heat exchanger, into the normal water tanks. Installations vary depending on the purpose, design and the type of locomotive to which it is fitted.
Unlike the surface condenser often used on a steam turbine or marine steam engine, the condensing apparatus on a steam locomotive does not normally increase the power output. In fact it may reduce it considerably. Whilst more power is potentially available by expanding down to a vacuum, the corresponding low density (high specific volume) implies extremely bulky low pressure cylinders or a turbine would be needed to actually extract it. So with a more practical volume ratio the condenser pressure would be near atmospheric rather than at a more typical low pressure, and the temperature would be correspondingly higher. In exhausting hot steam to the condenser, the temperature gradient between the exhaust steam and the cooling water is greater, so that a smaller heat exchange surface area is needed than would be required for typical stationary or ship-based steam plant of similar power. However none of the energy in the hot steam is available to do mechanical work. Because of the relatively high temperature in a locomotive condenser, the potential improvement in thermal efficiency expected from including the condenser in the cycle is not usually realised within the space constraints of a typical locomotive. Indeed, losses due to viscous friction in the condenser piping are likely to reduce the power output over what was achievable from simply venting to atmosphere.
There are two usual reasons for fitting condensing equipment - reducing exhaust emissions and increasing range.
Reduced exhaust emissions
Originally developed for the Metropolitan Railway to allow their locomotives to work the tunnels of the London Underground. This system was devised by Daniel Gooch and developed by Beyer Peacock. Steam is diverted from the exhaust steam pipes into the water tanks via condensing pipes within the same tanks. The water in the tanks could quickly heat up near boiling point, reducing the condensing effect on the exhaust steam. It was not unknown for the tanks to be emptied and refilled with cold water on a regular basis. Ordinary injectors will not work with hot water (until hot-water injectors were developed) so condensing locomotives were usually fitted with axle-driven boiler feedwater pumps. When not working in tunnels, the steam was directed to the blast pipe and up the chimney in the usual way.
In Britain, locomotives working on roadside steam tramways were required by law to have condensers. Water tank condensers (as above) were sometimes used but air-condensers were more common. A steam tram engine usually had a full-length roof and this was surmounted by a nest of air-cooled copper tubes in which the exhaust steam was condensed. Kitson & Co. made many engines of this type. The system was satisfactory for tram engines (which were very low-powered) but would not have worked for larger railway locomotives.
Generally this was a more sophisticated installation that used forced air cooling to condense the exhaust steam. The system was intended to reduce the problems of getting enough water to steam locomotives running through desert and very arid areas.
A drawback of condensing the exhaust steam is that it is no longer available to draw the fire, by use of the blastpipe. The draught must thus be generated instead by a steam-driven fan. Where possible, this has been arranged to use exhaust steam, although in some cases live steam was required, with extra steam and thus fuel consumption.
Types of condenser
Steam locomotive condensers may be water-cooled or air-cooled.
Water tank condenser
Here, the exhaust steam is blown into cold water in the locomotive's water tanks. A non-return system must be fitted, to prevent water from the tanks being drawn into the cylinders when the steam is shut off. This system was mainly used for locomotives working in tunnels.
Here, the exhaust steam is blown into an air-cooled radiator, similar to that used for the cooling system of an internal combustion engine. This system was used on small tram engines (where the condenser was mounted on the roof) and on large tender engines (where the condenser was mounted in the tender).
The Anderson condensing system  uses an air-cooled condenser but the steam is only partially condensed to form an aerosol of water droplets in steam. This aerosol is then liquified by pressure, using a specially-designed boiler feed pump. A fuel saving of nearly 30% (compared with exhausting to the atmosphere) was claimed for the Anderson system but this seems paradoxical. One would expect a higher fuel consumption because of the power required to compress the aerosol.
Locomotives fitted with a condensing apparatus
With water tank condensers
- Caledonian Railway 0-4-4T
- Great Eastern Railway class G69 2-4-2T
- Great Eastern Railway class L77 0-6-2T
- Great Eastern Railway class M15 2-4-2T
- Great Northern Railway class J13 0-6-0T
- Great Northern Railway class L1 0-8-2T
- Great Northern Railway (later LNER) class N2 0-6-2T
- Great Western Railway Metropolitan Class 2-4-0T
- Great Western Railway 633 class 0-6-0T
- Great Western Railway 9700 class 0-6-0PT (a variation on the 5700 Class)
- London, Chatham and Dover Railway R class 0-4-4T
- LMS Fowler 2-6-2T
- Mersey Railway 0-6-4T No.5 Cecil Raikes (preserved at the Museum of Liverpool)
- Metropolitan Railway A Class 4-4-0T
- Metropolitan Railway B Class 4-4-0T
- Metropolitan Railway C Class 0-4-4T
- Metropolitan Railway D Class 2-4-0T
- Metropolitan Railway E Class 0-4-4T
- Metropolitan District Railway 4-4-0T
With tender air condensers
- Deutsche Reichsbahn class 52. Around 200 of these were built with condensing tenders, to reduce the visible exhaust plume and so avoid air attacks on the Eastern Front of World War II.
- Russian SO class. From 1936 some of these were built with P11 condensing tenders for use across deserts in Turkestan.
- South African Class 20 2-10-2
- South African Class 25 4-8-4
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- Semmens, P.W.B.; Goldfinch, A.J. (2003) . How Steam Locomotives Really Work. Oxford: Oxford University Press. p. 277. ISBN 978-0-19-860782-3.
- Roosen, Dr.-Ing. R. (17 March 1960). "Class "25" Condensing Locomotives on the South African Railways — Design and Operating Experiences". J. Inst. Locomotive Engineers 50:2 (274): 243–280. Paper Nº607.
- "The Holcroft-Anderson Recompression Locomotive". Aqpl43.dsl.pipex.com. 2008-04-01. Retrieved 2012-02-17.
- "National Museums Liverpool". Liverpoolmuseums.org.uk. Retrieved 2012-02-17.
- Roosen 1961, p. 244
- "9: Near East to Far East". The World's Railways and How They Work. Odhams. 1947. pp. 182–183.