4. Dry Steam, Wet Steam, Water and Ice

The Wairakei Geothermal system contains a mixture of hot water and steam. However only steam can be used to drive steam turbines. All the hot water must be removed to stop the turbines being damaged. Steam without water droplets is called 'dry steam'.
Not steam but rather clouds of white water vapour from a silencer - Image: Heurisko Ltd.
It is water vapour that is the real problem - Image: Heurisko Ltd.
The sign illustrates one problem in a geothermal area - Image: Heurisko Ltd.

The Three States of Water

H2O can occur in three states

  • When in a solid state it is called ice
  • When in a liquid state it is called water
  • When in a gas state it is called steam.

Steam is invisible and has no smell. The white clouds seen around boiling water are not steam but tiny water droplets, called water vapour.

Water freezes (or melts) at 0°C but only when it is

  • Pure water
    Dissolved salts in water lower its melting point and raise its boiling point.
  • At sea level
    The pressure at sea level is called 'one atmosphere' or 'one bar'. Higher pressures raise the boiling point and lower pressures drop the boiling point.

For example

  1. Weather changes can alter the boiling point of water by 3°C , if you stay in the same place
  2. On top of Mt Everest water boils at 68°C.
  3. Wairakei is 370m above sea level, which means water boils at 98.7°C.
  4. Under the ground, pressures may be so high that water remains a liquid at 374°C.

What is temperature?

The temperature can be measured with a thermometer, Temperature tells us the speed of the molecules. The higher the temperature the faster the molecules are moving.

For example
In a 20°C room the speed of the air molecules is about 330 metres per second. In a household freezer this speed drops to about 300 metres per second. However some molecules are moving faster and some slower.

For example
Within a glass of water some molecules are moving slow enough to freeze while others are moving fast enough to boil. As some boil, they leave the glass and the water level drops. People usually say, “The water has evaporated ".

Dry Steam

In Dry steam all of the water molecules in the gas state.

Dry Steam is important in steam turbines because even the tiniest water droplets can damage turbine blades.

Flashed Steam

Flashed steam is steam that has been created by a drop in pressure rather than a temperature rise. Flashed steam is made by allowing hot water to escape to a lower pressure. The Flashed steam can be used to drive steam turbines.