8. Steam Turbines

In its first twenty years, the steam turbine reduced the coal consumption of electric generating stations by 75 percent. Part of this efficiency is due to turbines suiting the high speed and the constant load of electrical generators. Today the steam turbine still plays an important role in generating electricity worldwide.
fitting new blades
Fitting new blades to the central shaft of Mixed Pressure turbine 11. This end of the shaft will finally carry 7 stages of blading - Image: Heurisko Ltd. Panorama.
turbine from above
An aerial view of the shaft of Mixed Pressure steam turbine 11 which is being rebladed. Notice the 8 stages on the left and 7 on the right. The person gives an idea of the scale as does the lathe which occupies the whole floor - Image: Heurisko Ltd.
single blade
A single blade from the low-pressure steam turbine that drives Generator 9. Others are stacked behind - Image: Heurisko Ltd. Panorama.

The Science of Steam Turbines

The earliest use of a steam turbine in a power station was in 1888 in the North of England. It is believed that Charles Parsons sketched the original design of this turbine on the back of an envelope. It took $300 000 of research and 25 years for engineers to improve its efficiency by just 2%, an indication of Parson's genius.

All of the turbine designs prior to Parsons' invention were inefficient, and he realised that this was because the steam was expanded in a single step.

"I therefore decided to split up the fall in pressure of the steam into small fractional expansions over a large number of turbines in series, so that the velocity of the steam nowhere should be great" - Charles Parsons.

Within Parsons' turbine each succeeding turbine blade was made larger. This enlargement was done in three ways by

  1. increasing the height of each blade
  2. increasing the diameter of the succeeding drums
  3. altering the angles and openings between the blades, twisted along their length and changed shape. Before that, they were constant profile.

Between each set of blades are diaphragms, these have blades that twist the opposite direction to the rotor blades. Steam expands and speeds up through the diaphragms then the turbine blades extract heat and work from the steam by slowing down the steam.

These principles are still part of modern steam turbine design.

Heat to Kinetic Energy

This energy conversion occurs in the turbines of the Wairakei power station. Each of the 11 turbines is

  • made up of a central shaft with blades around the outside
  • made efficient by careful design of the blade shape to capture the energy of the expanding steam
  • designed to operate with a specific pressure of steam
  • designed to spin at a precise speed.