The Wairakei Geothermal system contains a mixture of pressurised hot water and steam of varying proportions. However, only steam can be used to drive steam turbines so all the water is removed to prevent damage to the turbines. Steam without water droplets is called ‘dry steam‘.
The Three States of Water.
The compound H2O can occur in three states
- Ice – when in a solid state
- Water – when in a liquid state
- Steam – when in a colourless and odourless gaseous state.
(Note : the white clouds seen around boiling water are tiny water droplets, called water vapour, not steam).
Like all elements and compounds the temperature at which H2O changes from one state to another depends on the pressure and its purity.
Water freezes (or melts) at 0°C but only when it is:
- Pure water.
- Dissolved salts in water lower the melting point and raise the boiling point of water.
- At sea level.
- The pressure at sea level is called ‘one atmosphere’ or ‘one bar’. Higher pressures raise the boiling point and lower pressures decrease the boiling point.
- Weather changes alone can alter the boiling point of water at sea level, by 3 degrees
- On top of Mt Everest the reduced air pressure means water boils at 68°C.
- The altitude of Wairakei is 370m above sea level, which means water boils at 98.7°C
Underground pressures may be so high that at 374°C water remains a liquid. Above 374°C however water is indistinguishable from steam.
What really is temperature?
The temperature of water can be measured by using a thermometer, but the temperature reading is only a guide to the average speed of water molecules. The higher the temperature the faster the molecules are moving.
- In a 20°C room the average speed of the air molecules is about 330 metres per second. In a household freezer this average speed drops to about 300 metres per second. However because the measurement is of the average speed – some molecules are moving faster and some slower.
- In 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. This explains how water evaporates.
Dry steam has all of its water molecules in the gas state.
Dry Steam (ie without water droplets) is important in thermal power generation because even the tiniest water droplets can damage turbine blades.
Flashed steam is steam that has been created by a pressure drop rather than a temperature rise. Flashed steam is usually made by allowing hot water to escape into a lower pressure vessel. The Flashed steam can then be used to drive turbines and generators.
Temperature and pressure determine the boiling point of a liquid.The need for a boiler to have a pressure gauge is illustrated in the animation below. (Note: you may need to refresh/reload the page to see the animation working).