The Cyclic Devices

P M V SubbaraoProfessor

Mechanical Engineering Department

Create Resources Parallel to Nature…..

Cyclic Device: A single system or a group of

thermodynamic systems, which are responsible for

execution of thermodynamic cycle by a pure substance.

Steam Power Plant: A Heat Engine

Qin

Qout

Wout

Win

0WQ 011

i

i

i

i

WQ

11 jiii WQ

Turbojet Engine: A Heat Engine

Qin

Qout

Wout

Reciprocating IC Engine : A Heat Engine

QinQout

Wout

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).

11 iiii WQ

out

iiin

iiout

iiin

ii WWQQ 1111

Thermodynamic Model for a General Heat Engine

Wnet

First law:Wnet = Qin - Qout

Qin

High Temperature Reservoir (Source)

Qout

Low Temperature Reservoir (Sink)

HE

in

net

Q

WEfficiency ,

in

i

i

in Q

Q

Q

W

1

If Qin = Wnet, then the efficiency of A Heat Engine will be 100%

Cost Benefit Analysis

Cost Total

BenefitNet Ratiocost Benefit to

Air Conditioner

Vapour Compression Refrigeration System : A Heat Pump

Qout

Qin

Win

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.

in

outin

W

QQ or (COP) ePerformanc oft Coefficien

outin

in

in

in

QQ

Q

W

Q

ref

outin

out

in

out

QQ

Q

W

Q

hp

If Win = 0, then COP = .

While using as a Refrigerator :

While using as a heat pump :

Thermodynamic Model for a General Heat Pump

Qout

Qin

Win

First law:Win = Qin - Qout

HTR (Sink)

LTR (Source)

HP

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

The Impossible Engine

Qin

Qout = 0

Wnet

First law:Wnet = Qin

HTR (Source)

LTR (Sink)

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.

The Impossible Heat PumpPerpetual Motion Machine II : Heat Pump Model

QH

QL

First law|QH| = QL

HTR (Sink)

LTR (Source)

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?

Violation of any one Statement lead to violation of the other automatically!

Perpetual Heat Pump Model

QH

QL

First law|QH | = QL

HTR (Source)

LTR (Sink)

Let us assume this is possible

Perpetual Heat Pump & A General Heat Engine

QHP

QLP

HTR

LTR

QHE

QLE

EWnet

Compound Heat Engine

QHP

QLP

LTR (Sink)

QHE

E Wnet

QLE = QLP-Wnet

HTR

QLE

HTR

QHE = QHP = QLP

Perpetual Motion Machine II : Compound Engine

QHP

QLP

First lawQHP = QLP

QHE

E Wnet

QHE = QLP

QLE = QLP-Wnet

QLP - QLE=Wnet

LTR (Source)

HTR (Sink)

Perpetual Motion Machine II : Compound Engine

QHP

QLP

QHE

E Wnet

QHE = QHP

QLE = QLP-Wnet

QLC=QLP - QLE=Wnet

LTR (Source)