Thermodynamics phase change question

[nate9519]

My thermo teacher was talking about 2 phase rankine cycles and he said the once water boils at atmospheric pressure its temperature can not get higher than 212 no matter how much heat you add. He said the only way you can raise the temperature of steam past 212 F is the raise the pressure. Why is this the case?

 

[BvU]

At that temperature and pressure there is equilibrium between the two phases. Heat added increases the vapour fraction, heat removed decreases it.
'No matter how much heat' is true in case of an infinite supply. In reality, when all the water is evaporated and still more heat is addded, the temperature will increase.

A higher pressure corresponds to a higher vapour liquid equilibrium temperature (a higher boiling temperature).

 

[SteamKing]

My thermo teacher was talking about 2 phase rankine cycles and he said the once water boils at atmospheric pressure its temperature can not get higher than 212 no matter how much heat you add. He said the only way you can raise the temperature of steam past 212 F is the raise the pressure. Why is this the case?

Your thermo teacher needs to pick up a set of steam tables and look carefully inside.

At one atmosphere of pressure, the equilibrium vapor temperature for saturated vapor is indeed 212 °F (or 100 °C), and this temperature changes with pressure. However, if pressure is held constant and heat is added to the saturated vapor, there will no longer be any saturated liquid remaining, as all of that is converted to vapor. The vapor can continue to absorb heat and its temperature will increase above the saturation temperature. This substance is then called superheated vapor and is used quite frequently in Rankine cycles. In fact, turbines operate most efficiently when using superheated vapor and can tolerate steam having only a small percentage of moisture content when it is fully expanded, just before it is exhausted to the condenser.

http://www.engineersedge.com/thermodynamics/saturated_superheated_vapors.htm

Another important fact to know about superheated vapor is that in this state, there is no longer any connection between pressure and temperature, unlike there is along the saturation line, which separates the superheated region from the saturated region of the vapor phase.

Below is a Mollier diagram, a plot of enthalpy versus entropy for water. The saturation line is indicated. The superheated vapor region is above and to the upper right of the saturation line on this diagram.

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