Civil Engineering Reference
In-Depth Information
applications, temperatures range between 523 K (250
C; 452
F) and 623 K
(350
C; 662
F).
Typical values of velocity are 20-40 m/s for steam and 2-4m/s for water.
The volumetric flow rate can be calculated by multiplying the fluid
velocity (m/s) by the pipe section (m
2
). Mass flow rate is equal to volume
for steam density values.
8.3
Basic Principles of Pipeline Losses and Insulation
Insulation of any hot or cold system plays an important role in energy
saving. Insulation can reduce by at least 90 % the unwanted heat trans-
fer, that is distribution losses occurring with a bare surface.
To better understand the economic criteria for the thickness of pipeline and
insulation, the basic concepts of heat transfer are briefly reviewed below.
The same concepts can be applied to any problem concerning heat transfer and
thermal energy balance.
Thermal energy is transferred in three main modes: conduction, radiation, and
convection.
Conduction
: heat is transferred through both fluids (gas and liquid) and solids when
the two sides of a volume of these materials are at different temperatures. The
thermal power transfer is proportional to the temperature gradient through a
proportionality factor
k
, called thermal conductivity, and the surface.
Thermal conductivity (or
k
-value in insulation technology) is the measure of
the capability of a material to transmit heat. Substances with high values of
thermal conductivity, such as copper, are good thermal conductors, and those
with low conductivity, such as polystyrene foam or cork, are good insulators.
Thermal conductivity is expressed as the quantity of heat that will be conducted
through unit area of a layer of material of unit thickness with unity difference of
temperature between the faces in unit time. The Si unit is W/m
K; the English unit
F).
The expression of the thermal conduction power transfer
Q
through a surface
A
(Fourier's Law) is as follows:
commonly used is: (Btu/h
ft
Q
¼
k
A
Δ
t
x
ðÞ
ð
=
Þ
W
, Btu
h
t
x
is the temperature gradient between the two surfaces (K/m;
C/m;
F/ft).
The sign of
Q
, minus in accordance with the laws of thermodynamics,
where
Δ
is
omitted here.
