Roxul Industrial Insulation Process User Manual
Page 128
°C
The following symbols are used in this calculation:
q
R
Heat flow density per m pipe
W/m
ϑ
M
Temperature of the medium in
°C
ϑ
L
Ambient temperature in
°C
d
1
External diameter of pipe
m
d
a
External diameter of insulated pipe
m
α
i
Surface coefficient of heat
transfer inside
W/(m² K)
α
a
Surface coefficient of heat
transfer outside
W/(m² K)
λ
1
…λ
n
Thermal conductivity of the individual
insulation layers
W/(m K)
k
Coefficient of thermal transmittance W/(m² K)
d
1
…d
n
Diameter of individual layers of
insulation
m
HINT
When performing thermo-technical calcula-
tions in insulation technology, the internal heat
transfer does not generally need to be
considered. This simplification is based on the
assumption that the medium is the same
temperature as the interior of the pipe. The
following terms may therefore be omitted from
the calculations shown above:
1
α
i
remove from the denominator in the equation
for the wall
1
d
i
i
⋅α
remove from the denominator in the
equation for pipe insulation
The surface temperatures
O
ϑ
can be calculated
as follows:
O
W
a
M
L
L
=
k
( - )+
ϑ
α
ϑ ϑ
ϑ
⋅
°C for walls
O
M
L
a
i
n
n
=
( - )
s
s
s
ϑ
ϑ ϑ
α
α
λ
λ
λ
α
⋅
+
+
+ +
+
1
1
1
1
2
2
....
aa
L
+
⎛
⎝⎜
⎞
⎠⎟
ϑ
°C
O
R
a
a
M
L
L
=
k
d
( - )+
ϑ
π
α
ϑ ϑ
ϑ
⋅ ⋅
⋅
°C for pipe insulation
products
O
a
a
i
i
=
d
d
d
d
ϑ
α
α
⋅ ⋅
⋅
+
1
2
1
ln
22
2
2
1
3
2
2
⋅
+
⋅
+ +
⋅
λ
λ
λ
ln
....
ln
d
d
d
d
a
n
nn
a
a
L
d
+
+
⋅
1
α
ϑ
M
L
( - )
ϑ ϑ
HINT
The internal heat transfer can once again be
disregarded (see hint in previous column).
The characteristic of emitting heat from a surface
(e.g. the external sheet cladding) into the
surrounding medium, which is usually air, is
described by means of the external surface
coefficient of heat transfer “α
a
”. The surface
coefficient of heat transfer is made up of the rate
of convection and radiation.
α
α
α
a
k
r
=
+
The following symbols used in this calculation:
α
k
the rate of convection
α
r
the rate of radiation
The rate of convection consists only of free
convection (air movement due solely to variations
in density as a result of temperature), forced
convection (blowers, wind) or of a mixture of free
and forced convection. The convection also
depends on the geometry of the building
component.
125