1. Semester
3
Engineering
Thermodynamics
Second Law of
Thermodynamics
Crated by :- Manthan Kanani
1
Babaria Institute
Of Technology
Mechanical
Engineering

2. 2nd Law Of thermodynamic
2
ENGINEERING THERMODYNAMICS
Introduction
 A process must satisfy the first law in order to occur.
 Satisfying the first law alone does not ensure that the process will take
place.
 Second law is useful:
 provide means for predicting the direction of processes,
 establishing conditions for equilibrium,
 determining the best theoretical performance of cycles, engines a
nd other devices.

3. ENGINEERING THERMODYNAMICS
2nd Law Of Thermodynamic
3
A cup of hot coffee does n
ot get hotter in a cooler ro
om.
Transferring heat to a wire w
ill not generate electricity.
Transferring hea
t to a paddle wh
eel will not caus
e it to rotate.
These processes cannot occur
even though they are not in vi
olation of the first law.

4. 2nd Law Of Thermodynamic
4
ENGINEERING THERMODYNAMICS
Second Law of Thermodynamics
Kelvin-Planck statement
 No heat engine can have a th
ermal efficiency 100 percent.
 As for a power plant to opera
te, the working fluid must ex
change heat with the environ
ment as well as the furnace.

5. 2nd Law Of Thermodynamic
5
ENGINEERING THERMODYNAMICS
Heat Engines
 Work can easily be converted to other forms of
energy, but?
 Heat engine differ considerably from one anot
her, but all can be characterized :
o they receive heat from a high-temperature sour
ce
o they convert part of this heat to work
o they reject the remaining waste heat to a low-
temperature sink atmosphere
o they operate on a cycle

6. 2nd Law Of Thermodynamic
6
ENGINEERING THERMODYNAMICS
The work-producing d
evice that best fit into
the definition of a hea
t engine is the steam p
ower plant, which is a
n external combustion
engine.

7. 2nd Law Of Thermodynamic
7
ENGINEERING THERMODYNAMICS
Thermal Efficiency
 Represent the magnitude of the energy wasted in order to comp
lete the cycle.
 A measure of the performance that is called the thermal efficie
ncy.
 Can be expressed in terms of the desired output and the requir
ed input
th
DesiredResult
RequiredInput
 For a heat engine the desired result is the net work done an
d the input is the heat supplied to make the cycle operate.

8. 2nd Law Of Thermodynamic
8
ENGINEERING THERMODYNAMICS
The thermal efficiency is always less than 1 or less than 100 per
cent.
th
net out
in
W
Q

,
W W W
Q Q
net out out in
in net
,  

where

9. 2nd Law Of Thermodynamic
9
ENGINEERING THERMODYNAMICS
 Applying the first law to the cyclic heat engine
Q W U
W Q
W Q Q
net in net out
net out net in
net out in out
, ,
, ,
,
 

 

 The cycle thermal efficiency may be written as
 th
n e t o u t
in
in o u t
in
o u t
in
W
Q
Q Q
Q
Q
Q



 
,
1

10. 2nd Law Of Thermodynamic
10
ENGINEERING THERMODYNAMICS
 A thermodynamic temperature scale related to the heat transfers betw
een a reversible device and the high and low-temperature reservoirs b
y
Q
Q
T
T
L
H
L
H

 The heat engine that operates on the reversible Carnot cycle is calle
d the Carnot Heat Engine in which its efficiency is
threv
L
H
T
T
,  1

11. 2nd Law Of Thermodynamic
11
ENGINEERING THERMODYNAMICS
Heat Pumps and Refrigerators
 A device that transfers heat from a low temperature medi
um to a high temperature one is the heat pump.
 Refrigerator operates exactly like heat pump except that t
he desired output is the amount of heat removed out of th
e system
 The index of performance of a heat pumps or refrigerator
s are expressed in terms of the coefficient of performance.

12. 2nd Law Of Thermodynamic
12
ENGINEERING THERMODYNAMICS

13. 2nd Law Of Thermodynamic
13
ENGINEERING THERMODYNAMICS
COP
Q
W
Q
QQ
HP
H
netin
H
H L
 
,
COP
Q
W
R
L
net in

,

14. 2nd Law Of Thermodynamic
14
ENGINEERING THERMODYNAMICS
Carnot Cycle
Process Description
1-2 Reversible isothermal heat addition at high temp
erature
2-3 Reversible adiabatic expansion from high temper
ature to low temperature
3-4 Reversible isothermal heat rejection at low temp
erature
4-1 Reversible adiabatic compression from low temp
erature to high temperature

15. 2nd Law Of Thermodynamic
15
ENGINEERING THERMODYNAMICS
Execution of Carnot cycle in a piston cylinder device

16. 2nd Law Of Thermodynamic
16
ENGINEERING THERMODYNAMICS

17. 2nd Law Of Thermodynamic
17
ENGINEERING THERMODYNAMICS
 The thermal efficiencies of actual and reversible heat engines operatin
g between the same temperature limits compare as follows
 The coefficients of performance of actual and reversible refrigerators
operating between the same temperature limits compare as follows

18. 2nd Law Of Thermodynamic
18
ENGINEERING THERMODYNAMICS
Entropy
 The 2nd law states that process occur in a certain direction, not in
any direction.
 It often leads to the definition of a new property called entropy, w
hich is a quantitative measure of disorder for a system.
 Entropy can also be explained as a measure of the unavailability of
heat to perform work in a cycle.
 This relates to the 2nd law since the 2nd law predicts that not all he
at provided to a cycle can be transformed into an equal amount of w
ork, some heat rejection must take place.

19. 2nd Law Of Thermodynamic
19
ENGINEERING THERMODYNAMICS
Entropy Change
 The entropy change during a reversible process is defined as
 For a reversible, adiabatic process
dS
S S


0
2 1
 The reversible, adiabatic process is called an isentropic process.

20. 2nd Law Of Thermodynamic
20
ENGINEERING THERMODYNAMICS
Entropy Change and Isentropic Processes
The entropy-change and isentropic relations for a process can be su
mmarized as follows:
i. Pure substances:
Any process: Δs = s2 – s1 (kJ/kgK)
Isentropic process: s2 = s1
ii. Incompressible substances (liquids and solids):
Any process: s2 – s1 = cav T2/T1 (kJ/kg
Isentropic process: T2 = T1

21. 2nd Law Of Thermodynamic
21
ENGINEERING THERMODYNAMICS
iii. Ideal gases:
a) constant specific heats (approximate treatment):
s s C
T
T
R
v
v
vav2 1
2
1
2
1
 , ln ln
2 2
2 1 ,
1 1
ln lnpav
T P
s s C R
T P
 
for isentropic process
2 1
1 2.
k
s const
P v
P v
   
   
   
for all process

22. 2nd Law Of Thermodynamic
22
ENGINEERING THERMODYNAMICS
Isentropic Efficiency for Turbine

23. 2nd Law Of Thermodynamic
23
ENGINEERING THERMODYNAMICS
Isentropic Efficiency for Compressor

24. 2nd Law Of Thermodynamic
24
ENGINEERING THERMODYNAMICS
THANKS FOR YOUR AT
TENTION