Superheating means to heat something above its boiling point
without the formation of bubbles of vapor (without it actually
boiling). It can be prevented by placing something into what you
are trying to cook to diffuse its energy.
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The amount of energy transferred from the steam is a function of
the temperature difference between the input and output of the
turbine. Also, superheating the steam ensures that there are no
water molecules that can damage the turbine blades.