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17-61
Critical Radius of Insulation
17-83C In a cylindrical pipe or a spherical shell, the additional insulation increases the conduction resistance of insulation,
17-84C For a cylindrical pipe, the critical radius of insulation is defined as
hkrcr /
. On windy days, the external
17-87C It will decrease.
17-88E An electrical wire is covered with 0.02-in thick plastic insulation. It is to be determined if the plastic insulation on the
wire will increase or decrease heat transfer from the wire.
Assumptions 1 Heat transfer from the wire is steady since there is no indication of any change with time. 2 Heat transfer is
F.Btu/h.ft 5.2
2
h
Since the outer radius of the wire with insulation is smaller than critical radius of insulation, plastic insulation will increase
heat transfer from the wire.
17-63
17-91 Prob. 17-90 is reconsidered. The rate of heat transfer from the ball as a function of the plastic insulation
thickness is to be plotted.
Analysis The problem is solved using EES, and the solution is given below.
"GIVEN"
D_1=0.005 [m]
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.
17-92C Fins should be attached to the outside since the heat transfer coefficient inside the tube will be higher due to forced
17-93C Increasing the rate of heat transfer from a surface by increasing the heat transfer surface area.
17-94C The fin efficiency is defined as the ratio of actual heat transfer rate from the fin to the ideal heat transfer rate from the
17-96C Fins enhance heat transfer from a surface by increasing heat transfer surface area for convection heat transfer.
17-97C Effectiveness of a single fin is the ratio of the heat transfer rate from the entire exposed surface of the fin to the heat
17-100C If the fin is too long, the temperature of the fin tip will approach the surrounding temperature and we can neglect
17-65
17-103C The thicker fin has higher efficiency; the thinner one has higher effectiveness.
17-104C The fin with the lower heat transfer coefficient has the higher efficiency and the higher effectiveness.
17-105 A fin is attached to a surface. The percent error in the rate of heat transfer from the fin when the infinitely long fin
assumption is used instead of the adiabatic fin tip assumption is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The temperature along the fins varies in one direction only (normal to the
plate). 3 The heat transfer coefficient is constant and uniform over the entire fin surface. 4 The thermal properties of the fins
are constant. 5 The heat transfer coefficient accounts for the effect of radiation from the fins.
1-
2
2
m 116.7
4/m) 004.0()C W/m.237(
m) 004.0(C). W/m12(
c
kA
hp
m
17-67
17-108E Prob. 17-107E is reconsidered. The effects of the thermal conductivity of the spoon material and the length of
its extension in the air on the temperature difference across the exposed surface of the spoon handle are to be investigated.
Analysis The problem is solved using EES, and the solution is given below.
"GIVEN"
17-68
L
[in]
T
[F]
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
122.4
123.4
124
124.3
124.6
124.7
124.8
124.9
124.9
125
125
125
125
125
125
122.5
123
123.5
124
124.5
125
125.5
DT [F]
17-69
17-109 A DC motor draws electrical power and delivers mechanical power to rotate a stainless steel shaft. The surface
temperature of the motor housing is to be determined.
Assumptions 1 Heat conduction is steady and one-dimensional. 2 Thermal properties are constant. 3 Heat transfer by
radiation is negligible. 4 The surface temperature of the motor housing is uniform. 5 The base temperature of the shaft is
equal to the surface temperature of the motor housing.
17-71
17-111 Two cast iron steam pipes are connected to each other through two 1-cm thick flanges exposed to cold ambient air.
The average outer surface temperature of the pipe, the fin efficiency, the rate of heat transfer from the flanges, and the
equivalent pipe length of the flange for heat transfer are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The temperature along the flanges (fins) varies in one direction only
17-72
17-112 Circular aluminum fins are to be attached to the tubes of a heating system. The increase in heat transfer from the tubes
per unit length as a result of adding fins is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The heat transfer coefficient is constant and uniform over the entire fin
surfaces. 3 Thermal conductivity is constant. 4 Heat transfer by radiation is negligible.
17-73
17-113 A circuit board houses 80 logic chips on one side, dissipating 0.04 W each through the back side of the board to the
surrounding medium. The temperatures on the two sides of the circuit board are to be determined for the cases of no fins and
864 aluminum pin fins on the back surface.
W2.3 W)04.0(80 Q
C/W 15741.1
)m 0216.0(C). W/m40(
11
C/W 00463.0
)m 0216.0(C) W/m.30(
m 003.0
22
conv
2
board
hA
R
kA
L
R
2 cm
Rboard
RAluminum
Rconv
Repoxy
T2
17-75
17-115 Prob. 17-114 is reconsidered. The effect of the center-to center distance of the fins on the rate of heat transfer
from the surface and the overall effectiveness of the fins is to be investigated.
D=0.0025 [m]
k=237 [W/m-C]
S=0.6 [cm]
T_infinity=30 [C]
h=35 [W/m^2-C]
A_surface=1*1 [m^2]
"ANALYSIS"
