Question

Give 2 examples of how to determine a thermodynamic properties (H, S, G, etc) of a substance by using the thermodynamic diagrams (PH Diagram, TS Diagram and Mollier Diagram).

Give 2 examples of how to determine a thermodynamic properties (H, S, G, etc) of a substance by using the tables of thermodynamic properties.

Each example must be different from each other

Answer 1

• Thermodynamic properties like pressure, temperature, enthalpy, entropy and volume of a substance are represented using a single diagram called thermodynamic diagram.
• The most common diagrams are Temperature-Entropy diagram(T-S), pressure-Enthalpy(P-H) diagram and Enthalpy-Entropy diagram (Mollier's diagram).
• These diagrams are drawn based on data of water but these can be applied to any substance.
• example 1: Using a P-H diagram determine the enthalpy of refrigerant R12 when the evaporator temperature is -20 ⁰C and 1.5bar pressure. The superheated temperature difference is 10⁰C.
• First, draw the indicated pressure line of 1.5 bar(0.15MPa) on P-H diagram of R12.
• determine the temperature as -20+10= -10 ⁰C. and plot the line(below the 0 lines). extend the right side to the pressure line. Mark the point as 1.
• The enthalpy value corresponding to point 1 is the value of enthalpy at that temperature.
• example 2:
• Superheated steam at 400⁰C and 1bar pressure. find enthalpy and entropy.
• Use molliers diagram. identify the 400⁰C isotherm line and 1bar isobar line. the intersection point is named as A.
• To find enthalpy, draw a straight horizontal line towards the left diagonal enthalpy lines. the values at the intersection give the value of enthalpy.
• Similarly from point A draw a vertical line towards the bottom entropy line. the intersection point gives the value of entropy at A.

Thermodynamic tables:

• Thermodynamic properties are tabulated so that data can be obtained more accurately than from diagrams.
• The values are given so close and for the values in between them need to be interpolated linearly.
• These are of two types. They are Saturated steam tables and superheated steam tables.
• Steam tables are used for obtaining values accurately almost for any substance.
• Example 1:
• Find liquid enthalpy value and saturated vapor entropy value at 227⁰C
• T H(kJ/kg) S(kJ/kg⁰C)
  226 971.5 6.239
  228 980.9 6.2249
• use linear interpolation (y-y₁)=(y₂-y₁)(x-x₁)/(x₂-x₁)
• then the value of enthalpy is H=976.5kJ/kg
• the value of entropy is S=6.23195kJ/kg⁰C
• Example 2
• At 10kPa and 348.15⁰C find enthalpy and entropy.
• using the superheated steam table at the given temperature and pressure the enthalpy is 2640kJ/kg and entropy is 8.3168kJ/kg⁰C.

Note: please refer to the respective charts.