Presentation on theme: "Entropy changes of pure substances: retrieving data from property tables. Tds relations relate properties of any pure substance exactly to entropy change."— Presentation transcript:
Entropy changes of pure substances: retrieving data from property tables. Tds relations relate properties of any pure substance exactly to entropy change. But it is not convenient to use them. Instead tabulated data for s are used, e.g. – Table B.1. for water. –Table B.5 for refrigerant R134a The tabulated value s actually means the entropy change from chosen reference state which is assigned a value 0 (e.g. entropy of saturated liquid water is assigned a value 0 kJ/kg/K) at the triple point of water where t= C and P=0.61 kPa) In the superheated vapor region and compressed liquid region (Table B.1.4) s can be calculated given P and T. In the saturated liquid region: s=s f +xs fg When no compressed liquid data is available the saturated liquid table (e.g. Table B.1) can be used to get an approximate value
Horizontal line segment entropy change during an isenthalpic process (application: throttling) Vertical line segment enthalpy change during an isentropic process (application: turbine). Constant pressure lines are inclined straight lines in the two-phase region, since temperature is also constant. In the superheated region, isotherms lines become more and more horizontal as pressure is reduced (h=h(T) for an ideal gas.) h-s plot or the Mollier diagram for a pure substance.
An actual h-s chart (Mollier diagram) for water
T-s and h-s plots: checklist Show critical point, SLL and SVL on T-s plot. Show constant P lines on T-s plot. Show constant v lines on T-s plot. Show constant P lines on h-s plot. Inside the vapor dome, why straight lines; why inclined; why diverging from one another. Show critical point, SLL, SVL on h-s plot.