Most of us see steam only when we boil the kettle, but we can learn something even from this. We observe that because it is able to lift the kettle lid, steam can do work. Because we can scald our hands with it, we know that it can transfer heat energy.

Also, if we look closely at the steam coming out of the spout, we can see water droplets, or a water mist, within it, indicating it is what we call ‘wet steam’.

All boilers, of whatever size, are doing the same thing as the kettle, i.e. boiling water. For low grade applications such as heating, we may need only hot water.

For turbines we are interested in much higher temperatures and pressures to produce steam of the right quality. We have already said that the steam produced by a kettle is wet steam.

This sort of steam is made up of water droplets and ‘pure steam’, i.e. steam which does not have water droplets. If the steam was taken away from the water from which it is produced, and more heat energy added, some of the droplets would change into pure steam, and the steam would be drier.

Eventually all the water droplets would have changed state and the steam would be dry.

The steam produced from the boiling water is at the same temperature as the water. This is called the saturation temperature, ts .

The steam cannot rise above this temperature until all the water droplets have disappeared, because all the heat energy supplied is used to change the state of the water droplets, i.e. latent heat of vaporization.

A soon as the steam has dried, and if more heat energy is supplied, the temperature of the steam will increase to produce superheated steam, i.e. steam above the saturation temperature of the water from which it was produced.

Using tables of steam properties – steam tables – we can find the energy of the steam. This energy is available to do work in a turbine or to be transferred for heating purposes.

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