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Siemens D 21.4 · 2017
Tools and configuration
Configuration notes
Heat dissipation
13
■
Configuration
(continued)
Control cabinet without forced ventilation
A control cabinet without forced ventilation conducts the heat
loss
P
v
generated in the interior to the surrounding air (external
temperature
T
a
) through the surface. For the heat flow,
the
following applies in the steady state:
k
is the heat transfer coefficient,
A
is the effective cooling surface
of the control cabinet, and
Δ
ϕ
is the temperature difference
between the internal cabinet temperature and the external
temperature
Δ
ϕ
=
T
c
-
T
a
The transfer of heat through the walls of the control cabinet is
determined by the heat transfer of the interior air to the cabinet
wall, heat conduction within the cabinet wall and heat transfer
from the cabinet wall to the external air. The heat transfer is to be
calculated by the heat transfer coefficient
α
, and heat conduc-
tion by the heat conductivity
λ
and the thickness
d
of the cabinet
wall. The resulting equation for the possible heat loss
P
v
is:
P
v
= [1/(1/
α
i
+
d
/
λ
+ 1/
α
a
)] ×
A
×
Δ
ϕ
=
k
×
A
×
Δ
ϕ
P
v
=
k
×
A
×
Δ
ϕ
Typical values for the heat transfer coefficient
k
in the case of
control cabinets with walls of painted stainless steel which are
up to 2 mm (0.08 in) thick:
The calculating procedures of IEC 60890 (VDE 0660 Part 507)
can be used for determining the ambient temperature
T
c
in the
interior of the control cabinet. All heat sources in the control
cabinet must be taken into account in the calculation, e.g.
Line Modules, Motor Modules, power supplies, filters, reactors.
It is important to determine the effective cooling surface depen-
dent on the method of setting up the control cabinet. The
standard can also be used for control cabinets with ventilation
openings (natural convection).
The estimated temperature
T
c
and the temperature distribution in
the control cabinet should be checked with measurements for
every application since local hotspots can form, e.g. in close
proximity to a source of heat or a hotspot.
Control cabinet with air conditioner
The control cabinet emits heat via its surface and the air
conditioner.
Manufacturers provide information on the design of the air
conditioner, e.g. Rittal:
www.rittal.de/produkte/system-klimatisierung/index.asp
Q
v
= k × A ×
Q
= P
k
value
Stationary (non circulating) air in the
control cabinet and stationary (non
circulating) external air
α
i
=
α
a
= 6 W/(m
2
× K)
approx. 3 W/(m
2
× K)
Circulating air in the control cabinet
and non-circulating external air
α
i
= 40 W/(m
2
× K);
α
a
= 6 W/(m
2
× K)
approx. 5.2 W/(m
2
× K)
P
c
T
c
T
a
T
a
T
i
a
G_D211_EN_00229
Height
Temperature distribution
Transfer of heat
Heat
conduction
Heat
transfer
Heat
transfer
v
© Siemens AG 2017