Civil Engineering Reference
In-Depth Information
Table 8.2
Total
emissivity average
values (
Temperature range
C
F
Surface
ε
)
e
Aluminum
Commercial sheet
200-600
400-1,100
0.09
Heavily oxidized
100-550
200-1,000
0.3
Iron
Steel polished
40-250
100-500
0.085
Steel oxidized
250
500
0.8
Brick
Red
40
100
0.93
White refractory
1,100
2,000
0.29
Concrete
40
100
0.94
Glass
40
100
0.94
The abovementioned relationship is valid if some basic assumptions are
respected such as transparent medium, surface with particular spectrum emission
behavior, etc. For pipelines and tanks in industrial applications, these hypotheses
are widely accepted and the relationship can be used with a proper choice of
parameters.
Convection
: heat is transferred from a solid surface at one temperature to an
adjacent fluid, liquid or gas, at another temperature, when motion occurs. The
thermal power transfer from the system with a flat
A
surface is expressed as follows:
Q
¼
h
A
ð
t
s
t
f
Þ
ðÞ
where
t
s
and
t
f
are the surface and fluid temperatures and
h
is an empirical
parameter, depending on operating conditions and surface geometry, called the
heat transfer coefficient. The SI unit is W/m
2
K; the English unit commonly used
ft
2
F).
In most cases, heat is transferred by a combination of the abovementioned basic
modes through composite or multilayer systems, and the evaluation is generally
made by means of empirical parameters and linearization of the heat transfer
relationships. A simplified approach based on a monodimensional scheme is gen-
erally introduced in industrial applications. As a rule, radiation is included in the
heat transfer coefficient in a wide range of temperature values.
Values of
h
for different operating situations with surface exposed to the air are
given in Table
8.3
.
Values of
h
for fluid flowing in pipelines depend on many factors (temperature,
speed, state of pipe, etc.) and must be calculated by using special formulae. They
are greater than those for air and they range between hundreds and thousands
W/m
2
is: (Btu/h
