thermodynamic process to find the enthalpy.

1. BY
INDRAKUMAR R PADWANI
BE MECHANICAL MBA MARKETING PGDCA
LECTURER IN MECHANICAL ENGG DEPT
GOVERNMENT POLYTECHNIC GODHRA
GUJARAT GUJARAT

2.  A thermodynamic process is the transformation of a system from
an initial state to final state
 This transformation is accompanied by changes in the pressure ,
volume and temperature of the system
 THERMODYNAMIC PROCESSES ARE REPRESENTED BY PRESSURE-
VOLUME LINES
 THERE ARE FOUR TYPES OF CYCLIC PROCESSES
 ISOCHORIC PROCESS
 ISOBARIC PROCESS
 ISOTHERMAL PROCESS
 ADIABATIC PROCESS

3.  P-V graph showing an different processes
 1. ISOTHERMAL 2.ISOCHORIC 3. ISOBARIC 4. ADIABATIC
p
v
1
4
3
2

4.  In an isochoric process the volume is kept
constant.(i.e. ∆V=0)
THIS IMPLIES THAT THE WORK DONE BY THE GAS IS ZERO (i.E, W=0)
The change in internal energy during an isocharic process is equal to the
heat supplied to the system.( I.E.∆U=q)
The p-v graph of an isocharic process is a vertical line.
P=pressure
Q=heat supplied
T=temperature
∆U=change in internal energy
V=volume
W=workdone
p
v
1
4
3
2

5.  ISOBARIC PROCESS
 In a isobaric process the pressure is kept constant (i.e ∆P=0).The
work done by the gas is given by P∆V.
 The change in internal energy during an isobaric process is given
by ∆U=Q-p∆V.
 The p-v graph of an isobaric process is a horizontal line
p
v
1
4
3
2

6.  ISOTHERMAL PROCESS
 In an isothermal process, the temperature is kept constant(i.e.∆T=0).However there
may be heat exchanged between the system and its surrounding.
 The p-v graph of an isothermal process is a curve
p
v
1
4
3
2

7.  ADIABATIC PROCESS
 In an adiabatic process there is no transfer of heat between
the system and surrounding s(i.e.Q=0)However the
temperature may not be constant
 The change in internal energy during an adiabatic process is
equal to the work done on the system….(i.e.∆U=W)
p
v
1
4
3
2

8. PROCESS CHARACTERISTICS
ISOCHORIC ∆V=0,W=O,∆U=Q
ISOBARIC ∆P=0 ; W=P ; ∆ ; ∆U=Q=−P∆V
ISOTHERMAL ∆T=0
ADIABATIC Q=0 ; ∆U=W

9. PROPERTY SYMBOL UNITS
PRESSURE P Pa
TEMPERATURE T K
VOLUME V m
DENSITY Þ Kg/m
GAS CONSTANT R J/K
ENTHALPHY H J
ENTROPY S J/K
INTERNAL ENERGY U J
HEAT CAPACITY U J/K
THERMAL
CONDUCTIVITY
k W/(m.k)
Surface Tension N/m
v
3

10.  Before going to the mollier chart it is important to the study
of various properties of steam :
 SATURATION TEMPERATURE (T )
 At given pressure the temperature at which water boils is
known as saturation temperature.
 SUPERHEATED TEMPERATURE (T )
 At a given pressure temperature of steam which is higher
than its saturation temperature is known as superheated
temperature.
 DEGREE OF SUPERHEAT (T - T )
 For a given pressure the difference between superheated
temperature of steam and its saturation temperature is
known as the degree of superheat
sat
sup
sup sat

11.  Dryness fraction of steam (x ) :- Dryness fraction of wet
steam is the ratio of mass of dry steam contained to the total
mass of the wet steam. Suppose M kg of wet steam contains
Mg kg of actual dry steam and Mf kg of water particles within
it so
DRYNESS FRACTION x= mass of dry steam
total mass of wet steam
x = Mg
Mg+Mf
 Latent heat of vaporization (h ): It is the amount of heat
absorbed to evaporate 1 kg of water at its boiling point or
saturation temperature without change of temperature. It is
denoted by h and its value depends upon the pressure.
fg
fg

12.  Enthalpy or total heat of steam(h ):-It is the amount of heat
absorbed by water from freezing point to saturation
temperature plus heat absorbed during evaporation
 Enthalpy or total heat of steam=Sensible heat + Latent heat
 It is denoted by h .The expression for the enthalpy of wet
steam, dry
 Steam and superheated steam is given by
1.Wet steam : The enthalpy of Wet steam is given by H = h +
x.h
2.Dry steam : The enthalpy of dry steam
H= h + xh
For dry steam x=1
H=h + h
g
g
fg
f
f fg
fgf

13.  Superheated steam : The enthalpy of superheated steam is
given by
H = h +h +Cp(t -t )
=h + Cp(t - t )
sup f fg sup sat
g sup sat
•Where Cp= specific heat of water at constant pressure is usually taken as 4.2

14.  IT IS GRAPHICAL REPRESENTATION OF STEAM TABLES IN
WHICH THE ENTHALPHY (h) IS PLOTTED ALONG THE
ORDINATE AND THE ENTROPY(S) ALONG ABSCISSIA.
 THE MOLLIER DIAGRAM HAS FOLLOWING LINES
 1. DRYNESS FRACTION LINE
 2.CONSTANT VOLUME LINE.
 3.CONSTANT PRESSURE LINE
 4.ISOTHERMAL LINES .
 The Mollier diagram is useful when analyzing the
performance of adiabatic steady flow processes, such as flow
in nozzles , diffusers , turbines and compressors.

15.  Calculate the enthalpy of 1 kg of steam at a pressure of 10
bar
A. DRYNESS FRACTION = 0.75
B. STEAM DRY AND SATURATED
C. STEAM IS SUPERHEATED TO 230
USING 1. MOLLIER CHART
2.STEAM TABLE
 NOW FROM STEAM TABLE
 P=10 bar At pressure 10 bar tsat=179.9
 M= 1kg hf = 762.6
 X=0.75 hfg =2013.6
 tsup =230

16. A. H= M(hf +x.hfg)
=1(762.6+0.75(2013.6)
=1(762.6+ 1510.2)
=2272.8 KJ/kg…..
B. H= M(hf+hfg)
=2776.2 KJ/kg
C. superheated steam
H = M(Hg) + CP(tsup –tsat)
=2776.2+2.0 (230-179.9)
=2776.2+2-0(50.1)
=2776.2+100.2
=2876.4 KJ/kg
.

17. •a
•b
•c
MOLLIER CHART

18.  From the mollier diagram chart
 We have to calculate enthalpy
 See the mollier diagram
 Mollier diagram contains pressure line, dryness fraction line and
superheated temperature.
A. In this example pressure is 10 bar and dryness fraction 0.75
 See the pressure line 10 bar line and dryness fraction line 0.75
in mollier diagram at which they intersects that gives us
ENTHALPY
A. BY MOLLIER DIAGRAM ENTHALPY H IS 2275 KJ/kg

19. B. When x = 1 and Pressure 10 bar line intersect in
the Mollier diagram that gives enthalpy.
H = 2776.2 KJ/kg
C. Now, when pressure is 10 bar and T = 230 C in
Mollier diagram that intersect that give enthalpy
2881.41 KJ/kg
sup