RMS Titanic - Turbine

Above, photo of Britannic's turbine under assembly.
The rings of blades can be clearly seen.

Unlike a reciprocating steam engine, a turbine can be designed to produce power from the continued expansion of steam all the way down into the high vacuum range that exists within the condensers. The turbine driving the center screw of the Titanic was designed to utilize the steam exhausted from the reciprocating engines at a pressure of 9 PSIA, expanding this steam all the way down to a terminal pressure of 1 PSIA, at which point, the steam – now virtually a warm fog - was exhausted into the main condensers.

A steam turbine is designed to operate something like a windmill. Using this analogy and modifying it somewhat will help to understand how a turbine works -- imagine a windmill with hundreds of very small blades, rather than 6 or 8 large blades. Then, assemble many rings of such blades - like connecting multiple windmills front-to-back on the same shaft – and place it inside a large cylinder, or housing, and you have the basic concept of a turbine engine.

Although it used steam at a pressure of only 9 PSIA, the turbine engine was designed to produce a considerable amount of power - 16,000 horsepower at 165 RPMs. Unlike the reciprocating engines, the turbine engine could not be run in reverse and so a means for the steam to bypass the turbine engines had to be incorporated into the system. When the reciprocating engines were running in reverse, a pair of large changeover valves routed the steam from the reciprocating engines directly to the condensers, bypassing the turbine entirely.