Course No Course Title Credits

as the ratio of the overall temperature difference to the sum of the thermal
resistances:
T1
−T4
T1
−T
q=
=
1 ∆x
A
1
+ +
∑R
h A K A h A
∴
4
i
A
The overall heat transfer by combined conduction and convection is
frequently expressed in terms of an overall heat-transfer coefficient U, defined
by the relation
q=
U A∆
T overall
o
where ∆ T overall
= T 1
−T
4 and U is
U
1
∆x
1
A
= + + W/m
hi
K
A
h
2 .K
o
value as
The overall heat-transfer coefficient is also related to the R
U =
1
R value
A more important application is heat transfer from a fluid outside a
cylinder, through a metal wall, to a fluid inside the tube, as often occurs in
heat exchangers.
boundaries.
Figure 5.3 Resistance analogy for hollow cylinder with convection