3-172
3-173
3-221 A 1–cm-diameter, 30-cm-long fin made of aluminum (k = 237 W/mºC) is attached to a surface at 80ºC. The surface is
exposed to ambient air at 22ºC with a heat transfer coefficient of 11 W/m2ºC. If the fin can be assumed to be very long, its
efficiency is
(a) 0.60 (b) 0.67 (c) 0.72 (d) 0.77 (e) 0.88
3-222 A hot surface at 80C in air at 20C is to be cooled by attaching 10-cm-long and 1-cm-diameter cylindrical fins. The
combined heat transfer coefficient is 30 W/m2C, and heat transfer from the fin tip is negligible. If the fin efficiency is 0.75,
the rate of heat loss from 100 fins is
(a) 325 W (b) 707 W (c) 566 W (d) 424 W (e) 754 W
3-174
3-175
3-225 Two finned surfaces with long fins are identical, except that the convection heat transfer coefficient for the first finned
surface is twice that of the second one. What statement below is accurate for the efficiency and effectiveness of the first
finned surface relative to the second one?
(a) higher efficiency and higher effectiveness
(b) higher efficiency but lower effectiveness
(c) lower efficiency but higher effectiveness
(d) lower efficiency and lower effectiveness
(e) equal efficiency and equal effectiveness
3-226 A 20-cm-diameter hot sphere at 120C is buried in the ground with a thermal conductivity of 1.2 W/mC. The
distance between the center of the sphere and the ground surface is 0.8 m, and the ground surface temperature is 15C. The
rate of heat loss from the sphere is
(a) 169 W (b) 20 W (c) 217 W (d) 312 W (e) 1.8 W
3-176
3-227 A 25-cm-diameter, 2.4-m-long vertical cylinder containing ice at 0ºC is buried right under the ground. The cylinder is
thin-shelled and is made of a high thermal conductivity material. The surface temperature and the thermal conductivity of the
ground are 18ºC and 0.85 W/mºC, respectively. The rate of heat transfer to the cylinder is
(a) 37.2 W (b) 63.2 W (c) 158 W (d) 480 W (e) 1210 W
3-228 Hot water (c = 4.179 kJ/kgK) flows through a 80 m long PVC (k = 0.092 W/mK) pipe whose inner diameter is 2 cm
and outer diameter is 2.5 cm at a rate of 1 kg/s, entering at 40°C. If the entire interior surface of this pipe is maintained at
35oC and the entire exterior surface at 20oC, the outlet temperature of water is
(a) 35oC (b) 36oC (c) 37oC (d) 38oC (e) 39°C
3-177
3-229 The walls of a food storage facility are made of a 2–cm-thick layer of wood (k = 0.1 W/mK) in contact with a 5-cm–
thick layer of polyurethane foam (k = 0.03 W/mK). If the temperature of the surface of the wood is –10oC and the
temperature of the surface of the polyurethane foam is 20oC, the temperature of the surface where the two layers are in
contact is
(a) -7oC (b) -2oC (c) 3oC (d) 8oC (e) 11°C
3-230 The equivalent thermal resistance for the thermal circuit shown here is
(a)
3423230112 RRRRR BA ++
(b)
34
2323
2323
0112 R
RR
RR
RR
BA
BA +
+
+
(c)
342323
2323
0112
0112 1
RRR
RR
RR
RR
BA
BA +
+
+
+
(d)
(e) None of them
2323
0112 R
RR
RR
BA +
3-178
3-231 The 700 m2 ceiling of a building has a thermal resistance of 0.52 m2K/W. The rate at which heat is lost through this
ceiling on a cold winter day when the ambient temperature is –10oC and the interior is at 20oC is
(a) 23.1kW (b) 40.4kW (c) 55.6kW (d) 68.1kW (e) 88.6kW
3-232 A 1 m-inner diameter liquid oxygen storage tank at a hospital keeps the liquid oxygen at 90 K. This tank consists of a
0.5-cm thick aluminum (k = 170 W/mK) shell whose exterior is covered with a 10–cm-thick layer of insulation (k = 0.02
W/mK). The insulation is exposed to the ambient air at 20oC and the heat transfer coefficient on the exterior side of the
insulation is 5 W/m2K. The rate at which the liquid oxygen gains heat is
(a) 141 W (b) 176 W (c) 181 W (d) 201 W (e) 221 W
3-179
3-180
3-235 Computer memory chips are mounted on a finned metallic mount to protect them from overheating. A 512 MB
memory chip dissipates 5 W of heat to air at 25oC. If the temperature of this chip is not exceed 60oC, the overall heat transfer
coefficient – area product of the finned metal mount must be at least
(a) 0.14 W/oC (b) 0.20 W/oC (c) 0.32 W/oC (d) 0.48 W/oC (e) 0.76 W/oC
3-236 In the United States, building insulation is specified by the R-value (thermal resistance in hft2oF/Btu units). A home
owner decides to save on the cost of heating the home by adding additional insulation in the attic. If the total R–value is
increased from 15 to 25, the home owner can expect the heat loss through the ceiling to be reduced by
(a) 25% (b) 40% (c) 50% (d) 60% (e) 75%
3-181
3-237 A triangular shaped fin on a motorcycle engine is 0.5-cm thick at its base and 3-cm long (normal distance between the
base and the tip of the triangle), and is made of aluminum (k = 150 W/mK). This fin is exposed to air with a convective heat
transfer coefficient of 30 W/m2K acting on its surfaces. The efficiency of the fin is 75 percent. If the fin base temperature is
130oC and the air temperature is 25oC, the heat transfer from this fin per unit width is
(a) 32 W/m (b) 57 W/m (c) 102 W/m (d) 124 W/m (e) 142 W/m
3-238 A plane brick wall (k = 0.7 W/mK) is 10 cm thick. The thermal resistance of this wall per unit of wall area is
(a) 0.143 m2K/W (b) 0.250 m2K/W (c) 0.327 m2K/W (d) 0.448 m2K/W (e) 0.524 m2K/W
3-239 ··· 3-245 Design and Essay Problems
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