TITLE.PM5

96 ENGINEERING THERMODYNAMICS

Dharm
\M-therm/th3-2.p65

Highlights

1. A pure substance is a system which is (i) homogeneous in composition, (ii) homogeneous in chemical
   aggregation, (iii) invariable in chemical aggregation.


2. The triple point is merely the point of intersection of sublimation and vapourisation curves. It must be
   understood that only on p-T diagram is the triple point represented by a point. On p-V diagram it is a line,
   and on a U-V diagram it is a triangle.


3. Steam as a vapour does not obey laws of perfect gases unless and until it is highly in super dry condition.


4. Dryness fraction is the ratio of the mass of actual dry steam to the mass of steam containing it.

i.e., Dryness fraction =

m

mm

s

sw+

where, ms = Mass of dry steam contained in steam considered

mw = Mass of water particles in suspension in the steam considered.

5. Superheated steam behaves like a gas and therefore, it follows gas laws. The law for adiabatic expansion is
   pv1.3 = C.


6. External work of evaporation = p(vg – vf)
   Internal latent heat = h
   pv
   fg J
   − g
   Internal energy of steam, u :
   (i) For wet steam : uh
   pxv
   J
   =−..g

(ii) For superheated steam : uh

pv

= sup− J

.sup. (J = 1, in SI Units)

7. Entropy of water when heated from temperature T 1 to T 2 ;

s 2 – s 1 = cpw loge

T

T

2

1

If 0°C is taken as the datum then entropy of water at any temperature T, above this datum will be

sf = cpw loge 273 T

Entropy of evaporation :

s

h

evap T

fg

s

.= ...(when water is evaporated to steam completely)

=xhTfg

s ...(when water is evaporated partially and dryness fraction of steam is x)

Entropy of steam :

swet = sf +

xh

T

fg

s ...[wet steam (x < 1)]

ss

h

g f T

fg

s

=+ ...[Dry and saturated steam (x = 1)]

ssf hTfg c TT

sup s pses

=+ +log sup ...(Superheated steam)