978-0073398198 Chapter 14 Part 8

subject Type Homework Help
subject Pages 10
subject Words 5216
subject Authors Afshin Ghajar, Yunus Cengel

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14-137
14-181 Liquid toluene evaporates into air from the open-top of a cylindrical container. The concentration of toluene at a
certain location is to be determined.
Pa 1333)kPa 325,101(
mmHg 760
mmHg 10
0, ==
A
P
/sm 1063.7
27325
2734.6
)/sm 10084.0( 26
5.1
24 =
+
+
=
AB
D
Pa 3.904
1333325,101
K) 273K)(6.4/kmolmPa m)(8314 (0.010
,
3
=
+
LA
P
Then the concentration of toluene is determined to be
3
+
3
,
K) 273K)(6.4/kmolmPa (8314
Pa 3.904
TR
P
u
LA
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14-138
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14-139
14-183E The top section of a solar pond is maintained at a constant temperature. The rates of heat loss from the top surface
of the pond by radiation, natural convection, and evaporation are to be determined.
Assumptions 1 The low mass flux conditions exist so that the Chilton-Colburn analogy between heat and mass transfer is
applicable since the mass fraction of vapor in the air is low (about 2 percent for saturated air at 80F). 2 Both air and water
/sft10167.0
/sft10288.2
7298.0Pr
FftBtu/h01469.0
23
24
=
=
=
=
k
A-1E). The emissivity of water is 0.95 (Table A-18). The
( )
/sft1068.2/sm1049.2
atm1
K2.297
1087.11087.1 2425
072.2
10
072.2
10
air-OH2
===== P
T
DDAB
Qevap
Qrad
Qconv
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14-140
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14-141
14-184E The top section of a solar pond is maintained at a constant temperature. The rates of heat loss from the top surface
of the pond by radiation, natural convection, and evaporation are to be determined.
Assumptions 1 The low mass flux conditions exist so that the Chilton-Colburn analogy between heat and mass transfer is
applicable since the mass fraction of vapor in the air is low (about 2 percent for saturated air at 90F). 2 Both air and water
/sft 10697.1
/hft 10328.2
7290.0Pr
FftBtu/h01481.0
24
24
=
=
=
=
k
0.95 (Table A-18). The mass diffusivity of water vapor
Qevap
Qrad
Qconv
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14-142
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14-143
14-185E A swimmer extends his wet arms into the windy air outside. The rate at which water evaporates from both arms and
the corresponding rate of heat transfer by evaporation are to be determined.
Assumptions 1 The low mass flux model and thus the analogy between heat and mass transfer is applicable since the mass
( )
/sft102.53=/sm1035.2
atm1
K9.288
1087.11087.1 2425
072.2
10
072.2
10
air-OH2
==== P
T
DDAB
7-35, the result is
 
125
000,282
180,46
1
)628.0/4.0(1
)628.0()180,46(62.0
3.0=
000,282
Re
1
)Sc/4.0(1
ScRe62.0
3.0Sh
5/4
8/5
4/1
3/2
3/15.0
5/4
8/5
4/1
3/2
3/15.0
=
+
+
+
+
+
+=
ft/s0.1265
ft3/12
/s)ft1053.2)(125(
Sh 24
mass =
==
D
D
hAB
The air at the water surface is saturated, and thus the vapor pressure at the surface is simply the saturation pressure of water at
the surface temperature (0.5073 psia at 80F). The vapor pressure of air far from the water surface is determined from
lbm/h 1.97=lbm/s 1047.5
4
=
and
Btu/s 0.573=== Btu/lbm) 8lbm/s)(104 1047.5( 4
evap fgvhmQ
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14-144
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14-145
14-187 Benzene-free air flows in a tube whose inner surface is coated with pure benzene. The average mass transfer
coefficient, the molar concentration of benzene in the outlet air, and the evaporation rate of benzene are to be determined.
Assumptions 1 The low mass flux model and thus the analogy between heat and mass transfer is applicable. 2 The flow is
fully developed.
Properties Because of low mass flux conditions, we can use dry air properties for the mixture at the specified temperature of
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14-146
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14-147
Fundamentals of Engineering (FE) Exam Problems
14-189 The basic equation describing the diffusion of one medium through another stationary medium is
CCd
)/(
CCd
)/(
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14-148
14-192 Nitrogen gas at high pressure and 298 K is contained in a 2-m 2-m 2-m cubical container made of natural rubber
whose walls are 3 cm thick. The concentration of nitrogen in the rubber at the inner and outer surfaces are 0.067 kg/m3 and
0.009 kg/m3, respectively. The diffusion coefficient of nitrogen through rubber is 1.510-10 m2/s. The mass flow rate of
nitrogen by diffusion through the cubical container is
L=0.03 [m]
rho_A_1=0.067 [kg/m^3]
14-193 A recent attempt to circumnavigate the world in a balloon used a helium filled balloon whose volume was 7240 m3
and surface area was 1800 m2. The skin of this balloon is 2 mm thick and is made of a material whose helium diffusion
Ci=0.2 [kmol/m^3]
L=0.002 [m]
M=4 [kg/kmol]
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14-149
14-194 Carbon at 1273 K is contained in a 7-cm-inner-diameter cylinder made of iron whose thickness is 1.2 mm. The
concentration of carbon in the iron at the inner surface is 0.5 kg/m3 and the concentration of carbon in the iron at the outer
surface is negligible. The diffusion coefficient of carbon through iron is 310-11 m2/s. The mass flow rate carbon by diffusion
through the cylinder shell per unit length of the cylinder is
14-195 The surface of an iron component is to be hardened by carbon. The diffusion coefficient of carbon in iron at 1000ºC is
given to be 310-11 m2/s. If the penetration depth of carbon in iron is desired to be 1.0 mm, the hardening process must take at
least
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14-150
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14-151
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14-152
14-200 Air flows through a wet pipe at 298 K and 1 atm, and the diffusion coefficient of water vapor in air is 2.510-5 m2/s.
If the heat transfer coefficient is determined to be 80 W/m2ºC, the mass transfer coefficient is
(a) 0.0022 m/s (b) 0.036 m/s (c) 0.074 m/s (d) 0.092 m/s (e) 0.13 m/s
rho=1.184 [kg/m^3]

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