Roxul Industrial Insulation Process - Manual (Page 129)

1.2 Insulation of piping
The rate of radiation depends on factors such as
the material of the cladding (emission ratio
ε ), the
surface temperature and the orientation of the
object in relation to other components.
The calculation procedures are explained in the
VDI 2055 and DIN EN 12241 standards. A detailed
description will not be given at this point.
Use the following procedure to obtain an
approximate estimate of the external surface
coefficients of heat transfer
α
a
. It applies in
respect of the following boundary conditions:
Applicable only for free convection
∆ϑ ϑ ϑ = − ≤
O L
K 60
d m
a
0 5 ,
The following applies for horizontal pipes:
a
= A +
α ϑ 0,05⋅Δ
W
m K 2 ⋅
The following applies for vertical pipes and walls:
a
=B+
α ϑ 0,09⋅Δ
W
m K 2 ⋅
O
is the surface temperature of the cladding
L
is the ambient temperature
The values for A and B have been compiled for a
number of materials and surfaces in the table
shown below.
Surface A B
Aluminum, rolled 2,5 2,7
Aluminum, oxidized 3,1 3,3
Galvanized sheet, bright 4,0 4,2
Galvanized sheet, tarnished 5,3 5,5
Austenitic steel 3,2 3,4
Alu-Zinc – sheet 3,4 3,6
Non-metallic surface 8,5 8,7
Supplementary values Δλ Thermal
bridges
In addition to the insulation thickness, the total
heat loss from insulated objects depends on
thermal bridges, which have a negative impact on
the insulation system. A distinction is drawn
between thermal bridges caused by the insulation
and thermal bridges caused by the installation.
Thermal bridges caused by the insulation system
include support constructions and spacers,
whereas thermal bridges caused by the installation
include pipe hangings and supports, flanges and
brackets.
Allowances are made for these thermal bridges in
the form of supplementary factors that are
multiplied by the surface coefficients of heat
transfer.
Table 3 of the VDI 2055 includes relevant
supple men tary values for thermal bridges caused
by the insulation.
Coming Soon: the thermo-technical engineering
calculation program "ROCKASSIST" can be used to
calculate heat losses from objects whilst allowing
for thermal bridges. Please contact the ROXUL
®
Technical Services Team for assistance.
2.3 Bases for thermal calculations
2.3.2 Heat transfer (European basis and terms)
126