7/24/2019 CHE 325 [Module 3]
1/53
MODULE THREE: SECOND LAWOF THERMODYNAMICS
Objectives
1. To introduce second law ofthermodynamics
2. To consider corollaries of the second
law of thermodynamics3. To consider applications of the
second law of thermodynamics
7/24/2019 CHE 325 [Module 3]
2/53
Introdction
It is ! "!tter o# ever$d!$e%&erience t'!t t'ere is ! de(nitedirection #or spontaneous
processes
Spontaneous processes arenatural
occurrence processes with thetendency
of attaining equilibrium
7/24/2019 CHE 325 [Module 3]
3/53
%!"& es o &on !neos*rocesses
An hot object at temperature Ti placed in an
environment of temperature To get cools to aTemperature T and eventually has a temperatureTo.
n conformity with the conservation of energy
principle! internal energy lost by the object e"ualsthe internal energy gained by the environment.
Inverse process would not ta#e placespontaneously
i.e nternal energy of the
environment would not decrease
spontaneously! while the object
warmed from TotoTi
7/24/2019 CHE 325 [Module 3]
4/53
E%!"&)es o# S&ont!neos *rocesses
Air having high pressure $iin a closed
container would %ow spontaneously tothe lower pressure $oof the surrounding!once the container is opened.
&low of air ceases once the air pressuree"uals the pressure of the surroundings.
Inverseprocess would not ta#e place
spontaneouslyi.e air would not %ow spontaneously
from the
surrounding at $ointo the
7/24/2019 CHE 325 [Module 3]
5/53
%!"& es o &on !neos*rocesses
A mass suspended by a cable at
elevation 'iwould fall when released. (hen it comes to rest! initial potential
energy lost by the mass e"uals internal
energy gained by both the mass and thesurroundings. Inverseprocess would not ta#e placespontaneously
i.e the mass would not returnspontaneously to its initial elevation! while its internal
energy or that
7/24/2019 CHE 325 [Module 3]
6/53
E%!"&)es o# S&ont!neos*rocesses
n each case described above! initialcondition of the system could berestored! but not in a spontaneous
process.
Auxiliary devices would be re"uired.
These devices would re"uired energyinput to perform! resulting into
permanentchange in the condition of
7/24/2019 CHE 325 [Module 3]
7/53
Introdction Spontaneous processes can proceed only in
a particular direction. The rst law ofthermodynamics gives no information
about direction; it states only that whenone form of energy is converted into
another, identical quantities of energy areinvolved regardless of the feasibility of the
process.
First law fails to address both the directionof the process and the extent of change ofenergy from one form to another !ence,
another general principle is required
7/24/2019 CHE 325 [Module 3]
8/53
Introdction
"an one form of energy becompletely converted to anotherform# n practice! when energy istransferred from one form to
another! there is often adegradation of the supplied energyinto a less useful form.
)econd Law of Thermodynamicsaddresses some of these
di*culties
7/24/2019 CHE 325 [Module 3]
9/53
Co""on)$ Used Ter"s Heat (thermal) reservoir is a su*ciently
large system in stable e"uilibrium to whichand from which +nite amounts of heat can betransferred without any change in itstemperature
A high temperature heat reservoir from whichheat is transferred is sometimes called a
heat source $eg furnace%.
A low temperature heat reservoir
from which heat is transferred is
sometimes called a heat sin& $eg
atmosphere%.
7/24/2019 CHE 325 [Module 3]
10/53
Co""on)$ Used Ter"s
Work reservoir is a su*ciently largesystem in stable e"uilibrium to which andfrom which +nite amounts of wor# can betransferred adiabatically without any
change in its pressure
Thermodynamic cycle: A system hascompleted a thermodynamic cycle when the
system undergoes a series of processes andthen returns to its original state! so that theproperties of the system at the end of thecycle are the same as at its beginning.
7/24/2019 CHE 325 [Module 3]
11/53
Co""on)$ Used Ter"s A reversib)e &rocessis one in which both the
system and its environment can be returned toexactly the states they were in before theprocess occurred.
He!t En+ine: A device or machine thatproduce wor# from heat in a cyclic process. ,.g.)team power plant in which the wor#ing %uid-2/0 periodically returns to its original state.
A thermodynamic system operating in athermodynamic cycle to which net heat istransferred and from which net wor# is delivered.
7/24/2019 CHE 325 [Module 3]
12/53
Co""on)$ Used Ter"s
'or&ing substance of the engine isthe material within the engine thatactually does the wor#.
,xamples include steam in steampower plant and the gasolineairmixture in an automobile engine.
7/24/2019 CHE 325 [Module 3]
13/53
St!te"ent , o apparatuscan operate insuch a way that
its only eect -insystem andsurroundings0 isto convert heatabsorbed by asystemcompletely into
wor& done by
St!te"ent -
o process ispossible solely inthe transfer ofheat from onetemperature
level to a higherone
St!te"ents o# t'e Second L!. o#r"od$n!"ics
7/24/2019 CHE 325 [Module 3]
14/53
St!te"ent , c!n be e%&ressed in!not'er #or" !s:
St!te"ent ,!t is impossible by a cyclic process
to convert the heat absorbed by
a system completelyinto wor#done by the system
7/24/2019 CHE 325 [Module 3]
15/53
C)!sis s!$s 4t will arouse other changeswhile the heat is transferred from
the low temperature object to thehigh temperature one5
6elvin says4t will arouse other changes while
the heat from the single thermalsource is ta#en out and is totally
changed into wor#.5
)econd Law of Thermodynamics
7/24/2019 CHE 325 [Module 3]
16/53
)econd Law of Thermodynamics
/e)vin0*)!nc1 St!te"ent: t is impossible for any device that
operates on a cycle to receive heatfrom a single reservoir and
produce a net amount of wor#. o heat engine can have a
thermal e*ciency of 1778.
&or a power plant to operate! thewor#ing %uid must exchange heatwith the environment as well asthe furnace.
t , i
7/24/2019 CHE 325 [Module 3]
17/53
eat ,ngine (or# can be easily converted completely to other forms
of energy
9onverting other forms of energy to wor# is not thateasy
(or# can be converted to heat directly and completely
eat can be converted to wor# directly! but not
completely 9onverting heat to wor# re"uires the use of a device
called a heat engine
eat engines come in many forms! pure heat engines
-steam power plants0 and semi heat engines -gas turbines0
,ach heat engine operates by using
a wor&ing substance $(uid%
i
7/24/2019 CHE 325 [Module 3]
18/53
eat ,ngines They receive heat from a high
temperature source -eg. &urnace0.
They convert part of this heat
to wor# -mostly through
rotating shaft0. They reject the waste heat to
lowtemperature sin#.
-eg. Atmosphere! :iver0. They operate on a cycle
$urposely built to convert heat
to wor#.
i
7/24/2019 CHE 325 [Module 3]
19/53
eat ,ngine;in < amount of heat
supplies to steam inboiler from hightemperature source-furnace0
;out < amount of heatrejected from steam incondenser to a lowtemperature sin#
(out < amount of wor#delivered by steam as itexpands in turbine(in < amount of wor#
re"uired to compresswater to boiler ressure
7/24/2019 CHE 325 [Module 3]
20/53
eat ,ngine ,*ciency
7/24/2019 CHE 325 [Module 3]
21/53
C)!ss Wor1
1. (hich of the following heat engineconditions
is most preferable>
a0 absorbed heat of 377 = and discarded?7 = of heat
b0 absorbed heat of 3?7 = and 377 = ofwor# done
c0 @77 = of wor# done and 177 = of heatdiscarded.
7/24/2019 CHE 325 [Module 3]
22/53
C)!ss Wor1
2. A Carnot engine receives 250 kJ/s of heat
from a heat-source reservour at 525 C and
rejects heat to a heat-sink reservour at 50 C.
What are the poer deve!oped and the heat
rejected"
7/24/2019 CHE 325 [Module 3]
23/53
)olution
,*ciency of 9arnot eat ,ngine!
< where
< ?7 C # $50 % 2&'( ) # '2' ) and< 2?2 C # $252 % 2&'( ) # 525 )
*o < < < 7.3@B
E2cienc$ o# C!rnot He!t En+ine3 4567898
7/24/2019 CHE 325 [Module 3]
24/53
E%ercises
1. Describe a process that would satisfythe conservation of energy principle!but does not actually occur in nature.
2. To increase the thermal e*ciency of areversible power cycle operatingbetween thermal reservours attemperatures Tand T9! would it bebetter to increase Tor decrease T9bye"ual amountsE
3. (hat is the thermal e*ciency of anheat en ine that absorbed 3B@ # and
7/24/2019 CHE 325 [Module 3]
25/53
E%ercises
@.
/btain and
R # i t d H t
7/24/2019 CHE 325 [Module 3]
26/53
Re#ri+er!tors !nd He!t*"&s
eat moves in nature from hightemperatures to lower temperatures! nodevices re"uired.
The reverse process! heat from lowtemperatures to high temperature! re"uires
special devices called refrigerators
or heat pumps
(or#ing %uid used in refrigeration cycle is called a refrigerantT'e objective o# re#ri+er!tor is to
re"ove 'e!t #ro" t'e re#ri+er!ted s&!ce
7/24/2019 CHE 325 [Module 3]
27/53
*"&s
The refrigerant enters the compressor as
vapour and compressed to condenser pressure t condenses as it %ow through the coils of the
condenser by rejecting heat to
the surrounding medium ts pressure and temp.
drop drastically as it
expands in capillary tube.
t evaporates in evapo
rator by absorbing heat
from the refrigerated space.
e r +er! ors !n e!
7/24/2019 CHE 325 [Module 3]
28/53
e r +er! ors !n e!*"&s
n household refrigerator! the freeGer
compartment -where heat is absorbed bythe refrigerant0 is the evaporator.
9oils behind the refrigerator -where heat
is dissipated to surroundings0 is thecondenser.
n a refrigerator,the interiorof the unit is the cold reservour!
while the warmer exterior
is the hot reservour.
7/24/2019 CHE 325 [Module 3]
29/53
*"&
n the refrigeration process! wor# 'is usedto remove heat )9from the cold reservoir
and de osit heat into the hot reservoir.
7/24/2019 CHE 325 [Module 3]
30/53
Coe2cient o# *er#or"!nce CO*;
"oe*cient of +erformance!
< a0 < 1?77 #=! < 1?7 #=.b0 < 1@77 #=! < 1J77 #=.
c0 < 1J77 #=! < @77 #=.
d0 < ?.
) l ti
7/24/2019 CHE 325 [Module 3]
40/53
)olution / '20 )
# 2+0 )
t is reversible! if
Top Related