TITLE.PM5

74 ENGINEERING THERMODYNAMICS

Dharm

\M-therm/th3-1.p65

Integrating both sides, we get

(^) s
s
1
2
z^ ds = T
T
1
2
z^ cpw^
dT
T
s 2 – s 1 = cpw loge
T
T
2
1 ...(3.13)
If 0°C is taken as datum, then entropy of water per kg at any temperature T above this
datum will be
sf = cpw loge 273 T ...(3.14)

3.14. Entropy of Evaporation

The change of entropy (ds) is given by,

ds =dQT

or s 2 – s 1 = QT, where Q is the heat absorbed.

When water is evaporated to steam completely the heat absorbed is the latent heat and this

heat goes into water without showing any rise of temperature.

Then Q = hfg

and sevap. =

h

T

fg

s

...(3.15)

However, in case of wet steam with dryness fraction x the evaporation will be partial and

heat absorbed will be xhfg per kg of steam. The change of entropy will be

xh

T

fg

s

3.15. Entropy of Wet Steam

The total entropy of wet steam is the sum of entropy of water (sf) and entropy of evaporation
(sfg).

In other words, swet = sf +

xh

T

fg
s ...(3.16)
where swet = Total entropy of wet steam,
sf = Entropy of water, and
xh
T

fg

s

= Entropy of evaporation.

If steam is dry and saturated, i.e., x = 1, then

sg = sf +

h

T

fg

s

...(3.17)

3.16. Entropy of Superheated Steam

Let 1 kg of dry saturated steam at Ts (saturation temperature of steam) be heated to Tsup. If

specific heat at constant pressure is cps, then change of entropy during superheating at constant

pressure p