Condensing steam locomotive

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Mersey Railway locomotive Cecil Raikes, showing the prominent exhaust pipes leading back to the water tanks

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.

Thermodynamics

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 rather it decreases. 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.

Purpose

There are two usual reasons for fitting condensing equipment - reducing exhaust emissions and increasing range.

Reduced exhaust emissions

Underground railways

Metropolitan Railway A Class Note the large valves in the steam return pipes, switching between condensing and non-condensing modes.

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.[1] 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[1] (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.

Roadside tramways

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.

Increased range

South African Class 25 Note the extremely large tender, with side louvres to cool the condensers
South African Class 25

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.

Exhaust draught

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.[2] 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.

Air condenser

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).

Anderson system

The Anderson condensing system [3] 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

With tender air condensers

See also

References

  1. ↑ 1.0 1.1 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Roosen 1961, p. 244
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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