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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