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A r macell - Technical Ar ticles
Issue 3
Key terms in low-temperature insulation:
by Dipl.-Ing. Hubert Helms, Armacell GmbH
Convection
Part 3:
THE PARAMETER
“HEAT TRANSFER
COEFFICIENT”
In the second part of this series, the enormous influence of the relative humidity on
the insulation thickness needed to prevent
condensation was explained. The heat
transfer coefficient has a similarly significant
influence. The term “heat transfer” means
the transfer of heat between a fluid and a
solid wall (e.g. a wall of a pipe or vessel). A
distinction is made between the inner heat
transfer – i.e. the transfer of heat between
vessel or pipe medium and the pipe or vessel wall – and the outer heat transfer – i.e.
the transfer of heat between the vessel/pipe
wall or its insulation material and the ambient medium (Figure 1).
Figure 1: Heat transfer coefficient
Thermal radiation
Convection makes a considerable contribution towards improving the heat transfer coefficient. The faster the ambient air
flows,the more heat is transported. In practice, it is therefore essential to ensure that
pipes and ducts do not lie too close to each
other or at an insufficient distance from
walls and other installations.
Apart from the difficulties of installing insulation material professionally if this is the
case, there is also the danger of a build-up
zone being created.
In this area, the circulation of air (convection) which is needed for a sufficiently high
surface temperature is stopped, i.e. in such
build-up zones the heat transfer coefficient
is lower because the contribution of convection decreases (Figure 2). As a result
of this, the risk of condensation forming
increases significantly.
=
k
+
For the calculation of the insulation thickness needed to prevent condensation, the
influence of the inner heat transfer is negligibly small and will not, therefore, be taken
into account in the following.
Ar m ace l l
The proportionality factor α, in our case α
outside (αa), is also known as the heat transfer coefficient and has the unit W/(m² . K). It
depends on the type of flowing medium, the
flow speed, the character of the wall surface
(rough or smooth, shiny or dark) and further
parameters. The heat transfer coefficient usually consists of heat transfer through convection and heat transfer through radiation.
2 - Absorption:
The radiation which strikes the surface of
a body with a lower temperature is transformed into heat.
The measure for the emissive power of
a material is the emission coefficient З .
The measure for the absorptive power is
the absorption coefficient a. The emissive
power of a body of a certain colour is
exactly as great as its absorptive power.
A vessel which is completely black has
the greatest absorptive or emissive power.
Table 1 shows the emission and absorption
coefficients of some surfaces of insulation
systems. As can be seen from the table,
it is to a large extent the nature of the
surface of the insulation material or its
jacket – apart from the influence of other
shining bodies – which determines the
contribution of radiation αS to the heat
transfer coefficient.
s
When heat is transferred, the heat flow is
proportional to the wall surface and the
difference in temperature at the wall of the
object.
1- Emission:
On the surface of a body with a high temperature heat is transformed into radiated
energy.
Dark-coloured bodies emit more radiated
energy than light-coloured ones; on the
other hand, dark-coloured bodies also
absorb more thermal energy than lightcoloured ones
α α α
a
Thermal radiation is a type of heat transfer
where the heat is transferred by electromagnetic waves. The transfer of energy
through radiation is not restricted to one
transfer medium. Unlike thermal conduction or convection (heat flow), thermal
radiation can also spread in a vacuum. In
the case of thermal radiation, the mechanism of heat transfer consists of two subprocesses:
Figure 2: Build-up zones stop convective heat transfer .
In DIN 4140, therefore, a distance of 100
mm between the insulated pipes and from
the pipes to the wall or ceiling is required.
In the case of vessels, apparatus etc. the
distance should even be 1000 mm.
An insulation material on the basis of synthetic rubber absorbs considerably more
thermal energy than, for example, an aluminium foil.
This has an extremely positive effect on the
insulation thickness required to prevent
condensation, i.e. the higher the absorptive
power is, the smaller the insulation thickness becomes.
In the next issue:
Part 4 - Water vapour transmission
Armacell-Technical Articles | March 2006 | Page 1
Issue 3: The parameter “Heat transfer coefficient”
From the explanations given above it has
become clear that the heat transfer coefficient is influenced by many factors which
cannot, as a rule, be determined exactly
and clearly. However, it is important to
define a value for the heat transfer coefficient which is as close to reality as possible.
Formulae for approximate calculations of
the heat transfer coefficient can be found
in the appropriate standards.
By way of simplification: where the conditions as far as space is concerned are
normal, the following assured empirical
values can be expected for the �a-value for
installations insulated with AF/Armaflex,
for example (Figure 3):
With this article on the heat transfer coefficient, the last parameter which has an
influence when determining the insulation
thickness needed to prevent condensation
has been presented. As has already been
emphasized in the previous articles, the
parameters should be researched as realistically as possible before insulation work
begins, to avoid unpleasant surprises later.
Apart from preventing surface condensation, a reliable insulation system must also
ensure that water does not collect within
the insulation material. This is caused by
processes of water vapour transmission
which will be the subject in the next issue.
Surface / vertical radiation
ε=a
Aluminium foil, shiny
Aluminium, oxidized
Steel, galvanized, shiny
Steel, galvanized, dusty
stainless austenitic steel
Alu-zinc, smoothly polished
Paint-coated sheet metal
Cellular glass
Synthetic rubber
Plastic jacket
0,05
0,13
0,26
0,44
0,15
0,16
0,90
0,90
0,90
0,90
Table 1: Emission and absoption coefficients
of surfaces of insulation systems.
- unpainted black or painted with protective coating Armafinish 99: 9 W/(m²· K)
- with galvanized steel sheet jacket:
7 W/(m² · K)
- with aluminium or stainless steel sheet
jacket: 5 W/(m² · K)
Ar m ace ll
Figure 3: Typical values for heat transfer
coefficients for AF/Armaflex
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In the next issue:
Part 4 - Water vapour transmission
Armacell-Technical Articles | March 2006 | Page 2