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Chapter 1
Basics of Heat Transfer
187E
A 200ft long section of a steam pipe passes through an open space at a specified temperature. The
rate of heat loss from the steam pipe and the annual cost of this energy lost are to be determined.
Assumptions
1
Steady operating conditions exist.
2
Heat transfer by radiation is disregarded.
3
The
convection heat transfer coefficient is constant and uniform over the surface.
Analysis
(
a
) The rate of heat loss from the steam pipe is
D =4 in
280
°
F
L=200 ft
Q
Air,50
°
F
2
ft
4
.
209
ft)
200
(
ft)
12
/
4
(
=
=
=
ππ
DL
A
s
Btu/h
289,000
=
F
)
50
280
)(
ft
4
.
209
(
F)
.
Btu/h.ft
6
(
)
(
2
2
pipe
°
−
°
=
−
=
air
s
s
T
T
hA
Q
&
(
b
) The amount of heat loss per year is
QQ
t
==
×
=
×
&
(,
.
Δ
289 000
2532 10
9
Btu / h)(365 24 h / yr)
Btu / yr
The amount of gas consumption per year in the furnace that has an efficiency of 86% is
therms/yr
438
,
29
Btu
100,000
therm
1
86
.
0
Btu/yr
10
532
.
2
Loss
Energy
Annual
9
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
=
Then the annual cost of the energy lost becomes
Energy cost
Annual energy loss)(Unit cost of energy)
= (29,438 therms / yr)
therm)
=
=
(
($0.
/
58
$17,074 / yr
188
A 4m diameter spherical tank filled with liquid nitrogen at 1 atm and 196
°
C is exposed to
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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, Energy, Mass, Heat

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