Chapter 3
Steady Heat Conduction
331
An exposed hot surface of an industrial natural gas furnace is to be insulated to reduce the heat loss
through that section of the wall by 90 percent. The thickness of the insulation that needs to be used is to be
determined. Also, the length of time it will take for the insulation to pay for itself from the energy it saves
will be determined.
Assumptions
1
Heat transfer through the wall is steady and onedimensional.
2
Thermal conductivities are
constant.
3
The furnace operates continuously.
4
The given heat transfer coefficients accounts for the
radiation effects.
Properties
The thermal conductivity of the expanded perlite insulation is given to be
k
= 0.052 W/m
⋅
°C.
Analysis
The rate of heat transfer without insulation is
Insulation
R
o
T
∞
R
insulation
T
s
L
A
==
(2
3
m)(1.5 m)
m
2
&
(
)
(
.
(
)(80
)
Qh
A
T T
s
=−
=
°
−
°
=
∞
10
3
30
1500
W/m
C) m
C
W
22
In order to reduce heat loss by 90%, the new heat transfer rate and
thermal resistance must be
&
.
&
&
()
.
Q
Q
T
R
R
T
Q
total
total
=×
=
=⎯
→
⎯=
=
−°
=°
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This note was uploaded on 01/14/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Mass, Heat

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