Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

substituting forLglass,kglass,Lair,kair, we have

Note the units ofR-value. TheR-value for the double-pane glass window in U.S. Customary

units is

As you can see from the results of Examples 11.6 and 11.8, ordinary glass windows do not

offer much resistance to heat flow. To increase theR-value of windows, some manufacturers

make windows that use triple glass panes and fill the spacing between the glass panes with Argon

gas. See Example 11.11 for a sample calculation for total thermal resistance of a typical exte-

rior frame wall of a house consisting of siding, sheathing, insulation material, and gypsum wall-

board (drywall).

Convection

Convectionheat transfer occurs when a fluid (a gas or a liquid) in motion comes into contact

with a solid surface whose temperature differs from the moving fluid. For example, on a hot

summer day when you sit in front of a fan to cool down, the heat transfer rate that occurs from

your warm body to the cooler moving air is by convection. Or when you are cooling a hot

Rtotal0.4 a

m

(^2) #
K
W
b°
1 W
3.4123
Btu
h
¢°
5
9
(^) °R
K
¢a
3.28 ft
1 m
b
2
0.7
°R#ft
2
Btu
h
Rtotal
0.008 1 m 2
1.4 a
W
m#K
b

0.01 1 m 2
0.025 a
W
m#K
b

0.008 1 m 2
1.4 a
W
m#K
b
0.4 a
m
(^2) #
K
W
b
322 Chapter 11 Temperature and Temperature-Related Parameters
■Figure 11.10 The double-pane glass window of Example 11.8.
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