Class notes Chemical Engineering Thermodynamics (CH126P)
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Course
CH126P (CH126P)
Institution
Mapua University
This document provides notes on the different processes as well as gas cycles in the study of chemical engineering thermodynamics. This document also provides insights into the thermodynamics of reacting systems. Examples and illustrations as well as sample problems are also provided to help in a b...
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 "
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