Civil Engineering Reference
In-Depth Information
Fig. 8.2
Heat transfer through a multilayer wall
The flow of heat is defined as follows:
t
i
t
o
Q
¼
A
R
th
internal and external surfaces with the same value (m
2
)
(ft
2
),
where
A
¼
W
W
ft
2
,
X
n
j
d
j
C
F
Btu
1
1
h
o
m
2
K
m
2
h
R
th
¼
h
i
þ
k
j
þ
¼
1
d
j
¼
thickness of the
j
layer (m) (ft),
inside temperature (K) (
C) (
F),
t
o
¼
t
i
¼
outside temperature (K) (
C) (
F),
K
¼
,
W
W
Btu
k
j
¼
thermal conductivity of the
j
layer
W
m
m
W
ft
2
C
ft
h
F
,
m
2
m
2
C
h
F
Btu
K
d
j
/
k
j
¼
thermal resistance of the
j
layer
¼
h
i
¼
heat transfer coefficient, radiation included, between the inside fluid and the
internal surface of the first layer,
,
¼
W
m
2
W
Btu
K
m
2
C
ft
2
h
F
h
o
¼
heat transfer coefficient, radiation included, between the external surface of
the last layer and the fluid outside
¼
W
m
2
W
Btu
K
m
2
C
ft
2
h
F
If one of the layers is made of insulating material, this layer has the highest
thermal resistance (
d
j
/
k
j
) and the heat flow is less than with a bare surface.
Table
8.4
shows heat losses from a composite wall. Notice that heat
transfer through the layers is reduced by insulation to roughly 10%.
