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22-99
,
TFUAQCFlms
22-102
22-121 A single-pass cross-flow heat exchanger uses hot air (mixed) to heat water (unmixed), and the required surface area
of the heat exchanger is to be determined.
Assumptions 1 Steady operating condition exists. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
is negligible. 3 Fluid properties are constant. 4 Changes in the kinetic and potential energies of fluid streams are negligible.
0.92
4.2
3080
100220
26.0
30220
3080
12
21
11
12
F
tt
TT
R
tT
tt
P
(Fig. 22-19d)
W10275.6K )3080)(KJ/kg 4183)(kg/s 3()( 5
in ,out , ccpccTTcmQ
The log mean temperature difference for the counter-flow arrangement is
)30100()80220(
21
TT
22-104
22-123 Oil in an engine is being cooled by air in a cross-flow heat exchanger, where both fluids are unmixed; with a
specified correction factor, the outlet temperatures of the oil and air are to be determined.
22-106
22-125 Water is heated by geothermal water in a double-pipe counter-flow heat exchanger. The mass flow rate of the
geothermal water and the outlet temperatures of both fluids are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic
22-108
22-127 A water-to-water heat exchanger is proposed to preheat the incoming cold water by the drained hot water in a plant to
save energy. The heat transfer rating of the heat exchanger and the amount of money this heat exchanger will save are to be
determined.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
kW 18.43=kJ/s) 6.25)(72.0(
max QQ
Then amount of fuel and money saved will be
ar therms/ye1677
kJ 500,105
therm1
78.0
kJ/year 1038.1
efficiency Furnace
savedEnergy
saved Fuel
8
Money saved = (fuel saved)(the price of fuel)
= (1677 therms/year)($1.00/therm)
= $1677/year
22-110
22-129 Air is to be heated by hot oil in a cross-flow heat exchanger with both fluids unmixed. The effectiveness of the heat
exchanger, the mass flow rate of the cold fluid, and the rate of heat transfer are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic
and potential energies of fluid streams are negligible. 4 The overall heat transfer coefficient is constant and uniform.
ccpccc
ccphhh
mmcmC
mmcmC
1.006=C)kJ/kg. (1.006
1.075=C)kJ/kg. (2.155.0
Therefore,
cc mCC
006.1
min
Air
Oil
80C
22-111
22-130 The inlet conditions of hot and cold fluid streams in a heat exchanger are given. The outlet temperatures of both
streams are to be determined using LMTD and the effectiveness-NTU methods.
22-113
22-131 . . . 22-134 Design and Essay Problems
22-134 A counter flow double-pipe heat exchanger is used for cooling a liquid stream by a coolant. The rate of heat transfer
and the outlet temperatures of both fluids are to be determined. Also, a replacement proposal is to be analyzed.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
