Elcon HiD 2000 Series - Cabinet Heat-Dissipation

4-5
4.10 Cabinet Heat-Dissipation
For integration into cabinets, or other enclosures that restrict free air movement, the expected temperature rise
within the enclosure should be calculated. Normal practice is to design for a 10 to 20 °C ambient temperature rise
inside a cabinet. The cabinet should be provided with top and bottom louvers to provide convection air flow. The
use of cabinet fans is recommended for high module density installations.
Effects of operating temperature
The failure rate probability of any electronic component has an exponential relation to the increase of its operating
temperature (Arrhenius theory). For instance, a change from 25 to 50 °C (77 to 122 °F) can cause a failure rate
ten times higher, with consequential downgrading of components and of calibration stability. The operating
temperature of electronic circuits should be as low as possible by placing installations in a cool or conditioned
environment.
Cooling of control cabinets
The installation of electronic equipment in a control cabinet determines an adverse increase in its operating
temperature proportional to the dissipated power. It is important that optimum power dissipation in the cabinet as
well as the elimination of the produced heat be provided, so as to minimise the increase above the ambient
temperature. An obvious measure consists of installing the equipment which is mostly affected by temperature in
the bottom portion of the cabinet, where temperature can be 15-20 °C (27-36 °F) lower than in the upper part.
The most common panel cooling methods, in increasing order of effectiveness, are:
a) Natural convection
b) Forced ventilation
c) Air conditioning
a
1
) Natural convection in closed panels:
It is the least effective and should used only when the dissipated power is moderate and when the system operates
in a dusty or aggressive (harsh) environment. The temperature rise and dissipated power for a given temperature
rise are expressed by the following formulas:
∆t
P
S K
=
⋅
and
P
t
S
K
=
⋅
⋅
∆
∆t (°C) = temperature rise
P (W) = dissipated power
S (m
2
) = heat exchanging surface
K (W/m
2
°C) = thermal conductivity coefficient (K = 5.5 for painted steel sheets)
As an example suppose, a typical 600 x 2000 x 600 mm cabinet installed with the four sides free (heat exchanging
surface = 5.2 m
2
) and considering a temperature rise ∆t = 10 °C (18 °F), the possible power dissipation in the
cabinet is: P = 10 x 5.2 x 5.5 = 286 W
a
2
) Natural convection in open panels:
In this case, in addition to the heat exchange with panel sides, heat is removed by cool air flowing through the
equipment. Inlet and outlet ports in the lower and upper ends of the cabinet are required and the air flow path must
be kept free from obstacles. For a properly engineered cabinet like the one considered in A1, the improvement
can reach 100%.