Class notes Chemical Engineering Thermodynamics (CH126P)

Lesson 4.1 : Heat Engines 4 Heat Pumps
Carnot Heat Engines and Heat Pumps Simple Steam Powerplant
Heat Engine Working Fluid : water
-

device that operates in a cycle consisting of four (4)
reversible steps ① Boiler: Isothermal
-
end result : heat transfer from a high temperature -


Expansion
:
reservoir to a low -


temperature reservoir along with water is evapora
the of work 4 transformed
generation
steam

steps :

D Adiabatic compression ② Turbine: Isentropic Expansion
2) Absorption of heat → Isothermal Expansion :
steam expansion drives a shaft generatin
,


3) Adiabatic Expansion ( generation of work) shaft work
4) Release of heat → Isothermal compression ③ Condenser : Isothermal compression
wate
: steam is condensed into
liquid
Heat Engine ( compression)
generation of work
coming from heat energy
→




one
step
= no work = irreversible
process ④ Pump: Isentropic Compression
i.
water from condenser to boiler
TH carry
T, =
high-temperature reservoir
QH
Qt ,
=
heat supplied
Heat Wont = work done Heat Engine : thermal
Efficiency nth
W out ,

Engine Q, =
heat rejected
I low dictates the % energy converted
to useful
temperature reservoir Mtn of
- -
- -




Qi
work
T
,

What you get W
Mtn =

for
=

what you paid QH

Energy Balance: Qa = Wt Qc

Q Qc
Mtn = =
I -




¥c

Heat Pumps and Refrigerators
Heat Pumps opposite -

of heat engines
-

same steps but in reverse
-
end result: heat transfer from low temp
reservoir to high temp reservoir
i.
only possible w/ work input
Isothermal -


constant temperature
Isentropic -

constant
entropy Heat Pumps 4 Refrigerators
-

same working principle but diff outputs
.

,Heat Pump : QH (space is hotter than the outside) Carnot Efficiency
Refrigerator : Qc ( space is colder than the outside )
Carnot Efficiency : heat engine
Basic Working Principle of heat pumps & refrigerators -

defined in terms of the temperatures of its reservoi
TH t Te
Refrigerant fluid he =
't To
= I -




¥ H

① Compressor : Isentropic
compression -

maximum possible efficiency in this case is higher
② Condenser : Isothermal for larger temperature differences between reservoirs
compression steam
power plants efficiency of work extraction from
-
:

③ Turbine : Isentropic Expansion heat is increased if the temperature difference between
④ Evaporator: Isothermal the hot and cold sinks is increased
Expansion
for more efficient powerplant:
COP : coefficient of Performance , B a) increase It b) decrease To
thermal
efficiency
-




t
no
= T (TH Tc) -




What get
Bnp =
you =

what you paid for
QH
=

QH -

Qc


Bret =
What you get
you paid for
=
# a




=
whiff
QH -

Qc
Actual of steam power plants :
Mtn of majority L 35%

he would be higher .




Carnot cycle
ideal processes of the heat engine and heat pump Carnot Efficiency : heat t refrigerators
pumps
-




-




ideality of the process is apparent w/ the two -
likewise defined in terms of the temperatures of the ho

isentropic processes in the cycle and cold reservoirs
-

Nicolas Leonard Sadi Carnot "


theoretical construct bcs of 2nd law of thermo
Bc 'np=
I purely that To
.




cycle gives the Max efficiency for extraction of Bo ref =
To
work from heat ( heat engines)
,



TH -


To
-



cycle that the Max efficiency for conversion
gives
of work to spontaneous heat transfer (heat
non
pumps
-

in contrast to heat engines the Carnot efficiency of ,



and refrigerators ) heat pumps d refrigerators increases as the temperatu
difference between the reservoirs decreases
Carnot Cycle : heat engines to air conditioners have
-


refrigerators higher energy
Reverse Carnot Cycle : heat
pumps
and
refrigerators efficiencies for inside temperatures that are closer
to the outside temperature

T cop 1B = better process = t (TH Tc) -

, ③

Given: powerplant ( heat engine)

350°C
QH f- 0.55%
tr

→ W
Req 'd: Ty to have

↳ Qc M -0.35
17=304
Examples:
① Mc
tf
0.55
=L -

0.5135 A- Nc -
-
0.282
,



Given: refrigerator if
Req'd :
Carnot COP n=0 -35 ,
no - 0.35 =
0.6364

TH =
33°C
Bc ,ref=
T *
TH -
Tc t -




Mc -_¥T → TH =
= 833.7k

W 293.15K 560.6°C
TH
-

-




-7306.15-293.1554 -11


Qc
B. ref -_
22.55
"
To = 20°C -
key Takeaways :

energy bat WtQ- QH 1) The Carnot 4- step theoretical
:
Bret = = cycle is a

W=Qµ -
Qc thermodynamic cycle that generates work from an


input of heat with the rejection of waste heat .




②
2) The Reverse Carnot cycle consists of the same 4
Steps but in the opposite direction wk uses an
Given: Carnot heat engine Req 'd : Q' c.
W input of work to accomplish a non spontaneous flow -




of heat from low to high temperature reservoirs
TH 5258
.




n I
=


e -

-




t Q' it 3) The Carnot efficiency the maximum possible efficien
'



is
Qa -

-
250kW
of heat for specified set of heatsink
W
iv. NO ,
conversion a

→
temperatures The Reverse Carnot efficiency is the
IIT
.




UH maximum possible COP for refrigerators and heat
IQ
-




,

←
"
PUMPS .




50-1273.15
=L
1=502 5251-273.15
- -




4) The efficiency of a real cycle CANNOT exceed the
W Moin 0.5951
=

Carnot efficiency That would be a violation
-
-
.
.




0.59511250kW) Q'H=WtQc of the second law of
W
=
thermodynamics .




-11
- 149kW Qc=Qa
'
-
in

-
Qc -
- 101kW "