320 Chapter 11 Temperature and Temperature-Related Parameters
or
(11.15)
We will discuss Ohm’s law in more detail in Chapter 12. Comparing Equation (11.13) to Equa-
tion (11.15), note that the heat flow is analogous to electric current, the temperature difference
to voltage, and the thermal resistance to electrical resistance.
Now turning our attention back to thermal resistance and Equation (11.13), we realize
that the thermal resistance for a unit area of a material is defined as(11.16)
Rhas the units of C / W (K / W) or Fh / Btu (Rh / Btu). When Equation
(11.16) is expressed per unit area of the material, it is referred to as theR-value or theR-factor.(11.17)
whereRhas the units of
or
Note that neitherRnorRis dimensionless and sometimes theR-values are expressed per unit
thickness. TheR-value orR-factor of a material provides a measure of resistance to heat flow:
The higher the value, the more resistance to heat flow the material offers. Finally, when the
materials used for insulation purposes consist of various components, the totalR-value of the
composite material is the sum of resistance offered by the various components.ft
(^2) #
°F
Btu
h
°
ft
(^2) #
°R
Btu
h
¢
m
(^2) #
°C
W
a
m
(^2) #
K
W
b
R
L
k
R¿
L
kA
I
V
Re
L
T 2
T 1
T 1 T 2
L
kA
k, A
■Figure 11.9
A slab of material and its thermal resistance.
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