In a boiler, energy from the fuel is transfered to liquid water in order to create steam. At first, cold water gets warmer and receives energy in the form of “sensible heat”, right until the boiling point.
Once the boiling point is reached, the water’s temperature ceases to rise and stays the same until all the water is vaporized. The water goes from a liquid state to a vapour state et receives energy in the form of “latent heat of vaporization”. As long as there’s some liquid water left, the steam’s temperature is the same as the liquid water’s. Steam is then called saturated steam.
When all the water is vaporized, any subsequent addition of heat raises the steam’s temperature. Steam heated beyond the saturated steam level is called superheated steam.
Why is saturated steam generally preferable to superheated steam?
Industries normally use saturated steam for heating, cooking, drying or other procedures. Superheated steam is used almost exclusively for turbines. The various types of steam have different energy exchange capacities and this justifies their different uses.
Energy transfer capacity, also known as the heat transfer coefficient (U), is used to compare types of steam. Its value is determined by the number of watts that goes through per surface unit and per degree of temperature difference. The greater this value, the greater the heat transfer for a given situation.
Figure 1 shows the heat transfer capacity according to the source of steam used. We can see that the heat transfer capacity of saturated steam is much higher than that of water or superheated steam.
Superheated steam must cool down before condensing
Superheated steam only yields sensible heat in an exchanger. It must therefore cool down before heating another substance. Superheated steam stuck to a surface cools down while yielding energy to the exchanger. However, superheated steam farther away from the surface cannot easily cool down and yield its energy, because superheated steam is an insulator (a bad conductor of heat), as all gases.
Superheated steam is a thermal insulator
Superheated steam has the same U coefficient as air, which is used in all good insulators, such as mineral wool or styrofoam.
Because it conducts heat badly, superheated steam has a poor heat transfer capacity, even though it is hotter than saturated steam and contains more energy. The fact that heat conduction is low in superheated steam induces a temperature profile between the steam and the heating surface. Figure 2 shows this profile.
Heat transfer is far superior when steam is saturated
Since the temperature of saturated steam is uniform, no temperature profile can appear between steam and heating surface.
In a heat exchanger, saturated steam is not in direct contact with the heating surface since the condensation of steam induces a thin layer of condensate (condensate film) on the heating surface.
We have seen previously that the energy exchange capacity of liquid water is lower than that of saturated steam. The condensate film formed on the heating surface is attracted by gravity and flows toward the bottom of the exchanger. This film’s thickness is relatively even over the entire heating surface of the appliance, unless the heating surface is too long vertically. The flow equalizes the condensate film’s temperature and prevents the formation of a temperature profile. This maintains the heat transfer between steam and liquid to a maximum.
Figure 3 shows a temperature profile when saturated steam is used. The difference with the previous diagram is that there is now a layer of condensate on the heating surface and steam temperature is constant.
Superheated steam is more efficient in a turbine
In a turbine, steam must be dry. Water droplets are not allowed, because they could break the vanes. At the high rotation speed of the vanes in a turbine, the centrifugal force applied on small drops of water can become very important, unbalance the vanes and cause them to break. That is the first reason why superheated steam is better in a turbine.
In a turbine, it’s the amount of work supplied by steam that matters. The quantity of work that steam supplies depends on its energy and superheated steam contains more energy than saturated steam. Superheated steam pushes on the vanes of a turbine, instead of condensing and losing its capacity to yield its energy in the form of work. This is the second reason why superheated steam is more efficient in a turbine.