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6.4. Reversible and
Irreversible Processes
1
2
Reversible Processes
 The second law of thermodynamics state that no
heat engine can have an efficiency of 100%.
 Then one may ask, what is the highest efficiency
that a heat engine can possibly have.
 Before we answer this question, we need to
define an idealized process first, which is called
the reversible process.
 The processes discussed earlier occurred in a
certain direction. They can not reverse
themselves irreversible processes.
3
Reversible Processes
 A reversible process is defined as a process that
can be reversed without leaving any trace on
either system or surroundings.
 This is possible if the net of heat and net work
exchange between the system and the
surrounding is zero for the combined process
(original and reverse).
Quasi-
equilibrium
expansion or
compression
of a gas
4
 Reversible processes actually do not occur
in nature.
 They are simply idealization of actual
processes.
 Reversible processes can never be
achieved.
 You may be wondering, then, why we are
bothering with such fictitious processes:
1. Easy to analyze
2. Serve as idealized model
5
 Engineers are interested in reversible processes
because:
when Reversible processes are approximated
instead of the Actual ones
1. Work-producing devices such as car engine and
gas or steam turbine deliver the most work, and
2. Work-consuming devices such as compressors,
fan, and pumps consume the least work.
6
 Reversible processes can be viewed as
theoretical limits for the corresponding not
reversible ones.
 We may never be able to have a reversible
process, but we may certainly approach it.
 The more closely we approximate a reversible
process, the more work delivered by a work-
producing device or the less work required by
a work-consuming device.
 Processes that are not reversible are called
Irreversible processes.
7
 Reversible processes
 Ideal processes
 Irreversible processes
 Actual processes
8
6.5. Carnot Cycle
9
 Cycles that are composed of reversible
processes will give the maximum net
work and consumes the minimum work.
 One of these cycles is the
Carnot Cycle.
 Named for French engineer Nicolas Sadi
Carnot (1769-1832)
 It is composed of four processes as
follows:
10
Process 1-2: A reversible
isothermal expansion
 The gas is allowed
to expand
isothermally by
receiving heat ( QH)
from a hot
reservoir.
11
Process 2-3: A reversible adiabatic
expansion
The cylinder now is
insulated and the gas
is allowed to expand
adiabatically and thus
doing work on the
surrounding.
The gas temperature
decreases from TH to
TL.
12
Process 3-4: A reversible
isothermal compression
The insulation is
removed and the
gas is compressed
isothermally by
rejecting heat (QL)
to a cold reservoir.
13
Process 4-1: A reversible
adiabatic compression
 The cylinder is
insulated again
and the gas is
compressed
adiabatically to
state 1, raising its
temperature from
TL to TH
14
Net work done by Carnot cycle is the
area enclosed by all process
The Carnot
cycle is the
most efficient
cycle
operation
between two
specified
temperatures
limits.
15
Carnot cycle can be executed in many
different ways
16
Reversed Carnot Cycle
Process 2-3: The gas
expands isothermally at
TL while receiving QL
from the cold reservoir.
Process 3-4: The gas is
compressed
adiabatically raising its
temperature to TH.
Process 4-1: The gas is
compressed
isothermally by
rejecting QH to the hot
reservoir.
Process 1-2: The gas expands adiabatically (throttling
valve) reducing its temp from TH to TL.
17
Reversed Carnot Cycle
18
Carnot principles
1. No heat engine is more
efficient than a reversible
one operating between
the same two reservoirs.
2. The thermal efficiencies
of all reversible heat
engines operating
between the same two
reservoirs are the same.
Low temperature reservoir at TL
19
The Thermodynamic Temperature Scale
A temperature scale that is
independent of the properties of the
substances that are used to measure
temperature is called a
thermodynamic temperature scale.
That is the Kelvin scale, and the
temperatures on this scale are called
absolute temperatures.
L
H
revL
H
T
T
Q
Q
=





cyclesreversibleForThe second Carnot principle state
that the thermal efficiencies of all
reversible heat engines operating
between the same two reservoirs
are the same.
ηth, rev = f (TH,TL)
20
Efficiency of a Carnot Engine
 For a reversible cycle the amount of heat
transferred is proportional to the temperature
of the reservoir.
H
L
rev
Q
Q
−=1η
H
L
T
T
−=1
Only true for the
reversible case
21
COP of a Reversible Heat Pump and a
Reversible Refrigerator
HL
revHP
QQ
COP
−
=
1
1
,
HL TT−
=
1
1
1
1
,
−
=
LH
revR
QQ
COP
1
1
−
=
LH TT
Only true
for the
reversible
case
22
How do Reversible Carnot Heat Engine
compare with real engines?





>
=
<
engineheatimpossible
engineheatreversible
engineheatleirreversib
rev,th
rev,th
rev,th
th
η
η
η
η
thermal
η thη≡





>
=
<
engineheatimpossible
engineheatreversible
engineheatleirreversib
rev,th
rev,th
rev,th
th
η
η
η
η
23





>
=
<
orrefrigeratimpossibleCOP
orrefrigeratreversibleCOP
orrefrigeratleirreversibCOP
COP
rev,R
rev,R
rev,R
R
1
1
−
=
L
H
R
Q
Q
COP
1
1
−
=
L
H
rev,R
T
T
,COP
COP of Carnot Refrigerator
How do Carnot Refrigerator compare with
real Refrigerator?
COP of Refrigerator
24
COP of Carnot Heat PumpCOP of real Heat Pump
H
L
HP
Q
Q
COP
−
=
1
1
H
L
rev,HP
T
T
COP
−
=
1
1





>
=
<
PumpHeatimpossibleCOP
PumpHeatreversibleCOP
PumpHeatleirreversibCOP
COP
rev,HP
rev,HP
rev,HP
HP
How do Carnot Heat Pump compare with
real one?
25
How to increase the efficiency of a real heat
engine?
H
L
rev,th
H
L
th
T
T
Q
Q
−=⇒
−=
1
1
η
η
1- Increase TH but you are limited with melting
temperature of the engine material.
2- Decrease TL but you are limited with your
environment.
26
Example (5-8): Heating a House
by a Carnot Heat Pump
A heat pump is to be used to heat
a house during the winter, as
shown in the figure at right. The
house is to be maintained at 21oC
at all times. The house is
estimated to be losing heat at a
rate of 135,000 kJ/h when the
outside temperature drops to -
5oC. Determine the minimum
power required to drive this heat
pump.
Sol:
27
Example (1)
28
Example (2)

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Ch 6b 2nd law

  • 2. 2 Reversible Processes  The second law of thermodynamics state that no heat engine can have an efficiency of 100%.  Then one may ask, what is the highest efficiency that a heat engine can possibly have.  Before we answer this question, we need to define an idealized process first, which is called the reversible process.  The processes discussed earlier occurred in a certain direction. They can not reverse themselves irreversible processes.
  • 3. 3 Reversible Processes  A reversible process is defined as a process that can be reversed without leaving any trace on either system or surroundings.  This is possible if the net of heat and net work exchange between the system and the surrounding is zero for the combined process (original and reverse). Quasi- equilibrium expansion or compression of a gas
  • 4. 4  Reversible processes actually do not occur in nature.  They are simply idealization of actual processes.  Reversible processes can never be achieved.  You may be wondering, then, why we are bothering with such fictitious processes: 1. Easy to analyze 2. Serve as idealized model
  • 5. 5  Engineers are interested in reversible processes because: when Reversible processes are approximated instead of the Actual ones 1. Work-producing devices such as car engine and gas or steam turbine deliver the most work, and 2. Work-consuming devices such as compressors, fan, and pumps consume the least work.
  • 6. 6  Reversible processes can be viewed as theoretical limits for the corresponding not reversible ones.  We may never be able to have a reversible process, but we may certainly approach it.  The more closely we approximate a reversible process, the more work delivered by a work- producing device or the less work required by a work-consuming device.  Processes that are not reversible are called Irreversible processes.
  • 7. 7  Reversible processes  Ideal processes  Irreversible processes  Actual processes
  • 9. 9  Cycles that are composed of reversible processes will give the maximum net work and consumes the minimum work.  One of these cycles is the Carnot Cycle.  Named for French engineer Nicolas Sadi Carnot (1769-1832)  It is composed of four processes as follows:
  • 10. 10 Process 1-2: A reversible isothermal expansion  The gas is allowed to expand isothermally by receiving heat ( QH) from a hot reservoir.
  • 11. 11 Process 2-3: A reversible adiabatic expansion The cylinder now is insulated and the gas is allowed to expand adiabatically and thus doing work on the surrounding. The gas temperature decreases from TH to TL.
  • 12. 12 Process 3-4: A reversible isothermal compression The insulation is removed and the gas is compressed isothermally by rejecting heat (QL) to a cold reservoir.
  • 13. 13 Process 4-1: A reversible adiabatic compression  The cylinder is insulated again and the gas is compressed adiabatically to state 1, raising its temperature from TL to TH
  • 14. 14 Net work done by Carnot cycle is the area enclosed by all process The Carnot cycle is the most efficient cycle operation between two specified temperatures limits.
  • 15. 15 Carnot cycle can be executed in many different ways
  • 16. 16 Reversed Carnot Cycle Process 2-3: The gas expands isothermally at TL while receiving QL from the cold reservoir. Process 3-4: The gas is compressed adiabatically raising its temperature to TH. Process 4-1: The gas is compressed isothermally by rejecting QH to the hot reservoir. Process 1-2: The gas expands adiabatically (throttling valve) reducing its temp from TH to TL.
  • 18. 18 Carnot principles 1. No heat engine is more efficient than a reversible one operating between the same two reservoirs. 2. The thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same. Low temperature reservoir at TL
  • 19. 19 The Thermodynamic Temperature Scale A temperature scale that is independent of the properties of the substances that are used to measure temperature is called a thermodynamic temperature scale. That is the Kelvin scale, and the temperatures on this scale are called absolute temperatures. L H revL H T T Q Q =      cyclesreversibleForThe second Carnot principle state that the thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same. ηth, rev = f (TH,TL)
  • 20. 20 Efficiency of a Carnot Engine  For a reversible cycle the amount of heat transferred is proportional to the temperature of the reservoir. H L rev Q Q −=1η H L T T −=1 Only true for the reversible case
  • 21. 21 COP of a Reversible Heat Pump and a Reversible Refrigerator HL revHP QQ COP − = 1 1 , HL TT− = 1 1 1 1 , − = LH revR QQ COP 1 1 − = LH TT Only true for the reversible case
  • 22. 22 How do Reversible Carnot Heat Engine compare with real engines?      > = < engineheatimpossible engineheatreversible engineheatleirreversib rev,th rev,th rev,th th η η η η thermal η thη≡      > = < engineheatimpossible engineheatreversible engineheatleirreversib rev,th rev,th rev,th th η η η η
  • 24. 24 COP of Carnot Heat PumpCOP of real Heat Pump H L HP Q Q COP − = 1 1 H L rev,HP T T COP − = 1 1      > = < PumpHeatimpossibleCOP PumpHeatreversibleCOP PumpHeatleirreversibCOP COP rev,HP rev,HP rev,HP HP How do Carnot Heat Pump compare with real one?
  • 25. 25 How to increase the efficiency of a real heat engine? H L rev,th H L th T T Q Q −=⇒ −= 1 1 η η 1- Increase TH but you are limited with melting temperature of the engine material. 2- Decrease TL but you are limited with your environment.
  • 26. 26 Example (5-8): Heating a House by a Carnot Heat Pump A heat pump is to be used to heat a house during the winter, as shown in the figure at right. The house is to be maintained at 21oC at all times. The house is estimated to be losing heat at a rate of 135,000 kJ/h when the outside temperature drops to - 5oC. Determine the minimum power required to drive this heat pump. Sol: