326 Chapter 11 Temperature and Temperature-Related Parameters
material, gypsum wallboard (drywall), glass, and so on — and convection losses through the wall
surfaces exposed to the indoor warm air and the outdoor cold air. The total resistance to heat
flow is the sum of resistances offered by each component in the path of heat flow. For a plane
wall we can write:The total resistance to heat flow is given by
The equivalent thermal resistance circuit for this problem is shown in Figure 11.12.
q
1 TinsideToutside 2 A
gR
170 20211502
14.43
520
Btu
h
0.170.811.3211.00.450.6814.43 h#ft
(^2) #
°F/Btu
(^) aRR 1 R 2 R 3 R 4 R 5 R 6
q
TinsideToutside
gR ¿
1 TinsideToutside 2 A
gR
To = 20°F Ti = 70°F
Outside film
resistance
Inside film
Siding Sheathing Insulation resistance
batt
Gypsum
board
■Figure 11.12 The equivalent thermal resistance for Example 11.11.
When performing heating load analysis to select a furnace to heat a building, it is com-
mon to calculate the heat loss through walls, roofs, windows, and doors of a building from
qUA T. In this equation, Urepresents the overall heat transfer coefficient, or simply
the U-factor for a wall, roof, window, or a door. The U-factor is the reciprocal of total ther-
mal resistance and has the units of. For the above example problem, the U-factor for
the wall is equal to:. Using this Uvalue, the heat loss
through the wall is then calculated from
.
Windchill Factor
As all of you know, the rate of heat transfer from your body to the surrounding air increases
on a cold, windy day. Simply stated, you lose more body heat on a cold, windy day than you
do on a calm, cold day. By now you understand the difference between natural and forced
convection heat transfer. The windchill index is intended to account for the combined effect
of wind speed and the air temperature. Thus, it is supposed to account for the additional
520
Btu
1 150ft h
2
T 1 0.0693 2170 202 °F
Btu
qUA (^) h#ft (^2) #°F 2
U
1
©R
1
14.430.0693
Btu
h#ft^2 #°F
Btu
h#ft^2 #°F
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