The Cyclic Devices

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The Cyclic Devices

• P M V Subbarao
• Professor
• Mechanical Engineering Department

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Cyclic Device A single system or a group of
thermodynamic systems, which are responsible for
execution of thermodynamic cycle by a pure
substance.
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Steam Power Plant A Heat Engine
Wout
Qout
Qin
Win
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Turbojet Engine A Heat Engine
Qin
Qout
Wout
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Reciprocating IC Engine A Heat Engine
Qout
Qin
Wout
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Greatness of Heat Engines

• Balance Sheet for A Heat Engine
• All heat inputs consume resources Total Input
  Heat Qin (Cost incurred).
• All heat outputs are just mere a loss.
• Net work Out put is positive and this is the
  final benefit Wnet (Benefit Achieved)
• Performance Benefit to cost ratio (in energy
  units).

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Thermodynamic Model for a General Heat Engine
First law Wnet Qin - Qout
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Cost Benefit Analysis
If Qin Wnet, then the efficiency of A Heat
Engine will be 100
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Air Conditioner
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Vapour Compression Refrigeration System A Heat
Pump
Win
Qout
Qin
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Greatness of A Heat Pump

• Heat Pump or Refrigerator
• Input Win (Cost incurred)
• Out put Qin or Qout (Benefit Achieved)
• Performance Benefit to cost ratio.

While using as a Refrigerator
While using as a heat pump
If Win 0, then COP ?.
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Thermodynamic Model for a General Heat Pump
HTR (Sink)
Qout
First law Win Qin - Qout
Win
HP
Qin
LTR (Source)
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Kelvin Planks postulate

• It is impossible to construct a heat engine
  which produces no effect other than the
  extraction of heat from a single source and the
  production of an equivalent amount of work
• On the other side
• It is possible to construct a heat engine which
  produces no effect other than the extraction of
  work from a single source and the production of
  an equivalent amount of heat

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The Impossible Engine
HTR (Source)
Qin
First law Wnet Qin
Wnet
Qout 0
LTR (Sink)
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Clausius postulate

• The Clausius statement It is impossible to
  construct a heat pump produces no effect other
  than the transfer of heat from a cooler body to a
  hotter body.
• However,
• It is possible to construct a heat pump produces
  no effect other than the transfer of heat from a
  hotter body to a colder body.

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The Impossible Heat Pump Perpetual Motion
Machine II Heat Pump Model
HTR (Sink)
QH
First law QH QL
?
QL
LTR (Source)
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Discussion of Second Law

• It is impossible to construct a heat engine with
  100 efficiency.
• It is impossible to construct a heat pump or
  refrigerator with infinite COP.
• Both are negative statements.
• They cannot be proved.
• They will remain correct till they are
  disproved.
• Violation of Kelvin Planks statement leads to
  violation of Clasius statement and vice versa.
• Can a heat engine be reversed to work as heat
  pump or refrigerator?
• If yes, what will be the COP of this reversed
  engine?
• Can a reversed engine perform same as forward
  engine between same reservoirs?
• What can really affect the performance of a
  reversible engine?

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Violation of any one Statement lead to violation
of the other automatically!
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Perpetual Heat Pump Model
HTR (Source)
QH
First law QH QL
Let us assume this is possible
?
QL
LTR (Sink)
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Perpetual Heat Pump A General Heat Engine
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Compound Heat Engine
HTR
QHE QHP QLP
HTR
QHP
QHE
Wnet
?
E
QLE QLP-Wnet
QLP
QLE
LTR (Sink)
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Perpetual Motion Machine II Compound Engine
HTR (Sink)
QHE QLP
QHP
QHE
First law QHP QLP
Wnet
?
E
QLP
QLE QLP-Wnet
QLP - QLEWnet
LTR (Source)
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Perpetual Motion Machine II Compound Engine
QHE QHP
QHP
QHE
Wnet
?
E
QLP
QLE QLP-Wnet
QLCQLP - QLEWnet
LTR (Source)