A condensing steam locomotive is a type of locomotive designed to recover exhaust steam, either in order to improve range between taking on boiler water, or to reduce emission of steam inside enclosed spaces. The apparatus takes the exhaust steam that would normally be used to produce a draft for the firebox, and routes it through a heat exchanger, into the boiler water tanks. Installations vary depending on the purpose, design and the type of locomotive to which it is fitted. It 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.
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 due to a reduction of airflow to the firebox that heats the steam boiler. In fact it may reduce it considerably. Condensing the steam from a high volume gas to a low volume liquid causes a significant pressure drop at the exhaust, which usually would add additional power in most steam engines. Whilst more power is potentially available by expanding down to a vacuum, the power output is actually greatly reduced compared to a conventional steam locomotive on account of the lower air flow through the firebox, as there is now no waste steam to eject into the firebox exhaust in order to pull more air into the firebox air intake. In order to produce similar power, air to the firebox must be provided by a steam driven or mechanically driven fan. This often cancels out any improvement in efficiency.
The temperature of the exhaust steam is greater than typical stationary or ship-based steam plant of similar power due to having fewer waste recovery stages, as ships often have a compound steam engine with an additional low pressure stage or even a low speed turbine. Waste heat on modern steam plants is often recovered using heat exchangers. However, condensing locomotives do not have this benefit due to the waste heat being expelled to the surrounding air and not being recovered, and therefore none of the energy in the waste steam is recovered to do mechanical work. In many conditions the temperature gradient is often much worse due to using air instead of having an abundant source of cooling water as naval or stationary steam power plants have. The Anderson condensing system significantly reduces these losses by only partially cooling the waste steam before compressing it into condensate, then pumping the high temperature condensate back into the boiler in order to recover the unused waste heat. This greatly reduces energy waste.
Because of the relatively high temperature in a locomotive condenser and the rejection of the heat to the air, 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, and having to pump the condensate back into the boiler is likely to reduce the power output over what was achievable from simply venting to atmosphere.
These restrictions do not apply to marine or stationary steam engines due to not having size or weight restrictions. Ships often had massive waste steam recovery systems, such as the 400 ton waste steam turbine used to recover very low 6psi waste steam on the Titanic and its sister ships. This is several times the weight of an entire locomotive, and so is clearly not feasible as a form of waste steam recovery for locomotives.
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. 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.