TITLE.PM5

PROPERTIES OF PURE SUBSTANCES 75

Dharm

\M-therm/th3-1.p65

= cps loge

T

Ts

F sup

HG

I

KJ.

Total entropy of superheated steam above the freezing point of water.

ssup= Entropy of dry saturated steam + change of entropy during

superheating

= sf +

h

T

fg

s

+ cps loge

T

Ts

F sup

HG

I

KJ = sg + cps loge^

T

Ts

F sup

HG

I

KJ ...(3.18)

3.17. Enthalpy-Entropy (h-s) Chart or Mollier Diagram

Dr. Mollier, in 1904, conceived the idea of plotting total heat against entropy, and his dia-
gram is more widely used than any other entropy diagram, since the work done on vapour cycles
can be scaled from this diagram directly as a length ; whereas on T-s diagram it is represented by
an area.
A sketch of the h-s chart is shown in Fig. 3.11.

310° C

220° C

2 T^5

T 4

T 3

T 2

T 1 Saturation

line

Line of constant

temperature

P 2

p 1

x 1

x 2

3

x = 0.8

p = 0.1

p = 0.02 bar

Constant drynessLines of

p=1bar

p=5bar

p = 50 barp = 10 bar

p 4

p 3

x = 0.9

p=100bar

Lines of constant

pressure

p = 0.01 bar

(^2805) 1000 bar
Enthalpy, h kJ/kg
2000
Critical
point
Entropy, s kJ/kg K
bar
x
Fig. 3.11. Enthalpy-entropy (h-s) chart.
— Lines of constant pressure are indicated by p 1 , p 2 etc., lines of constant temperature by
T 1 , T 2 , etc.
— Any two independent properties which appear on the chart are sufficient to define the
state (e.g., p 1 and x 1 define state 1 and h can be read off the vertical axis).
— In the superheat region, pressure and temperature can define the state (e.g., p 3 and T 4
define the state 2, and h 2 can be read off).