7-1
Solutions Manual
for
Fundamentals of Thermal Fluid Sciences
5th Edition
Yunus A. Çengel, John M. Cimbala, Robert H. Turner
McGraw-Hill, 2017
Chapter 7
THE SECOND LAW OF THERMODYNAMICS
PROPRIETARY AND CONFIDENTIAL
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you are a student using this Manual, you are using it without permission.
7-7C Heat engines are cyclic devices that receive heat from a source, convert some of it to work, and reject the rest to a
sink.
7-11C (a) No, (b) Yes. According to the second law, no heat engine can have and efficiency of 100%.
work.
7-3
7-15 The rates of heat supply and heat rejection of a power plant are given. The power output and the thermal efficiency of
this power plant are to be determined.
Assumptions 1 The plant operates steadily. 2 Heat losses from the working fluid at the pipes and other components are
taken into consideration.
7-4
7-18 The power output and thermal efficiency of a heat engine are given. The rate of heat input is to be determined.
Assumptions 1 The plant operates steadily. 2 Heat losses from the working
7-5
7-21 The heat rejection and thermal efficiency of a heat engine are given. The heat input to the engine is to be determined.
Assumptions 1 The plant operates steadily. 2 Heat losses from the working
fluid at the pipes and other components are negligible.
Analysis According to the definition of the thermal efficiency as applied to
1th
7-22 The power output and fuel consumption rate of a power plant are given. The thermal efficiency is to be determined.
Assumptions The plant operates steadily.
MW 500
H
Q
7-23 The power output and fuel consumption rate of a car engine are given. The thermal efficiency of the engine is to be
determined.
Engine
25.6% 0.256
kW 215.1
kW 55
outnet,
th
H
Q
W
sink
Furnace
7-6
7-24 The United States produces about 51 percent of its electricity from coal at a conversion efficiency of about 34 percent.
The amount of heat rejected by the coal-fired power plants per year is to be determined.
Analysis Noting that the conversion efficiency is 34%, the amount of heat rejected by the coal plants per year is
kWh 103.646 12
kWh 10878.1
34.0
kWh 10878.1
12
12
coal
th
coal
out
coalout
coal
in
coal
th
W
W
Q
WQ
W
Q
W
7-25E The power output and thermal efficiency of a solar pond power plant are given. The rate of solar energy collection is
to be determined.
Assumptions The plant operates steadily.
sink
7-26 A coal-burning power plant produces 300 MW of power. The amount of coal consumed during a one-day period and
the rate of air flowing through the furnace are to be determined.
MW 5.937
32.0
MW 300
th
outnet,
in
W
Q
MJ 101.8s) 360024(MJ/s) 5.937( 7
inin tQQ
kg/s 401.8 kg/s) 48.33(fuel) air/kg kg 12(AF)( coalair mm
sink
HE
Coal
Furnace
1.8781012 kWh
ηth = 34%
out
Q
Source
7-10
7-43 A refrigerator is used to keep a food department at a specified temperature. The heat gain to the food department and
the heat rejection in the condenser are given. The power input and the COP are to be determined.
Assumptions The refrigerator operates steadily.
Analysis The power input is determined from
kW 0.417
kJ/h 3600
kW 1
kJ/h) 1500(
kJ/h 150033004800
in LH QQW
The COP is
2.2 kJ/h 1500
kJ/h 3300
COP
in
W
QL
7-44 The COP and the power consumption of a refrigerator are given. The time it will take to cool 5 watermelons is to be
determined.
Assumptions 1 The refrigerator operates steadily. 2 The heat gain of the refrigerator through its walls, door, etc. is
negligible. 3 The watermelons are the only items in the refrigerator to be cooled.
12C
30C
R
H
Q
L
Q
3300 kJ/h
4800 kJ/h
in
W
7-11
7-45E The rate of heat supply and the COP of a heat pump are given. The power consumption and the rate of heat
absorption from the outside air are to be determined.
PROPRIETARY MATERIAL. © 2017 McGraw-Hill Education. Limited distribution permitted only to teachers and educators for course preparation. If
you are a student using this Manual, you are using it without permission.
fast enough, forming a low pressure region behind the piston. The lower pressure that pushes the piston produces less work.
devices that operate on reversible processes deliver the most work, and work consuming devices that operate on reversible
processes consume the least work.
The Carnot Cycle and Carnot’s Principle
isothermal compression, and reversible adiabatic compression.
Carnot cycle operating between the same temperature limits.
7-16
Carnot Heat Engines
supplied to the working fluid of a heat engine, the higher the thermal efficiency.
7-70 Two pairs of thermal energy reservoirs are to be compared from a work-production perspective.
Assumptions The heat engine operates steadily.
Analysis For the maximum production of work, a heat engine operating between the energy
reservoirs would have to be completely reversible. Then, for the first pair of reservoirs
K 325
L
T
0.560 K 625
K 275
11
maxth,
H
L
T
T
7-71E The sink temperature of a Carnot heat engine, the rate of heat rejection, and the thermal efficiency are given. The
power output of the engine and the source temperature are to be determined.
Assumptions The Carnot heat engine operates steadily.
Analysis (a) The rate of heat input to this heat engine is determined from the definition of thermal efficiency,
Btu/min 800
LQ
Q
Btu/min 800
rev
L
Q
QH
TH
TL
7-18
7-75 Problem 7-74 is reconsidered. The effects of the temperatures of the heat source and the heat sink on the power
produced and the cycle thermal efficiency as the source temperature varies from 300°C to 1000°C and the sink temperature
varies from 0°C to 50°C are to be studied. The power produced and the cycle efficiency against the source temperature for
sink temperatures of 0°C, 25°C, and 50°C are to be plotted.
7-19
7-76E The claim of an inventor about the operation of a heat engine is to be evaluated.
Assumptions The heat engine operates steadily.
920 R
7-20
7-79 The source and sink temperatures of a geothermal power plant are given. The maximum thermal efficiency is to be
determined.
Assumptions The power plant operates steadily.
Analysis The highest thermal efficiency a heat engine operating between two
specified temperature limits can have is the Carnot efficiency, which is determined
K 273140
H
T
Carnot Refrigerators and Heat Pumps
conditioner. The smaller the difference between the temperature limits a refrigerator operates on, the higher is the COP.
Therefore, an air-conditioner should have a higher COP.
by the refrigerator. In reality, the work consumed by the refrigerator will always be greater than the additional work
produced, resulting in a decrease in the thermal efficiency of the power plant.
consumed by the heat pump. In reality, the work consumed by the heat pump will always be greater than the additional
work produced, resulting in a decrease in the thermal efficiency of the power plant.
20C
140C