Report

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=0.010 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=sf+xsfg • When no compressed liquid data is available the saturated liquid table (e.g. Table B.1) can be used to get an approximate value s @T , P s f @T h-s plot or the Mollier diagram for a pure substance. •Horizontal line segment T Tds dh vdP h T s P 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.) 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.