Condensing Steam Locomotive - Thermodynamics

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. 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.