Remaining materials on Entropy and second law

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.

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