Lesson 3 - Steam Theory SAQ
This lesson allows you to test yourself on the material covered in the Steam Theory lesson and determine the level to which you have met the learning outcomes.
Suggested Response - Q2
** Using the Mollier diagram in the figure below, answer the following questions.
a) What is the enthalpy of dry saturated steam at 200°C?
b) What is the pressure of the steam mentioned in question a)?
c) The steam is condensed isobarically to a wetness fraction of 0.7, what is the enthalpy now?
d) What is the temperature of the steam from question c)?
Figure: Mollier diagram for water
Response:
Firstly, remember that Mollier diagrams are in absolute pressure.
a) The enthalpy of dry saturated steam at 200°C is found by following the 200°C dark blue temperature line to the saturated vapour line of the phase envelope. Where these two lines intersect, is the point of interest. From that point, go straight down to find the enthalpy to be around 2,750 kJ kg-1 .
b) The pressure of the 200°C dry saturated steam is found by looking at the y-axis. Refer back to the same point from part a), and you will find that the pressure is at about 1.75 MPa a.
c) A wetness fraction of 0.7 means the steam is 30% vapour, so the quality is 30%. As the steam is condensed isobarically (at constant pressure), then we can simply follow the 200°C line to the green 30% quality line to find the point that represents the new state. If we go straight down from this point, then we will find the enthalpy to be at around 1,400 kJ kg-1.
d) If the condensation occurs at constant pressure, then the temperature remains the same at 200°C.