Chapter 15
Cooling of Electronic Equipment
Air Cooling: Natural Convection and Radiation
1571C
As the student watches the movie, the temperature of the electronic components in the VCR will
keep increasing because of the blocked air passages. The VCR eventually may overheat and fail.
1572C
There is no natural convection in space because of the absence of gravity (and because of the
absence of a medium outside). However, it can be cooled by radiation since radiation does not need a
medium.
1573C
The openings on the side surfaces of a TV, VCR or other electronic enclosures provide passage
ways for the cold air to enter and warm air to leave. If a TV or VCR is enclosed in a cabinet with no free
space around, and if there is no other cooling process involved, the temperature of device will keep rising
due to the heat generation in device, which may cause the device to fail eventually.
1574C
The magnitude of radiation, in general, is comparable to the magnitude of natural convection.
Therefore, radiation heat transfer should be always considered in the analysis of natural convection cooled
electronic equipment.
1575C
The effect of atmospheric pressure to heat transfer coefficient can be written as
h
h
P
conv P atm
conv
atm
,
,
.
)
2
(W / m
C
1
where P is the air pressure in atmosphere. Therefore, the greater
the air pressure, the greater the heat transfer coefficient. The best and the worst orientation for heat
transfer from a square surface are vertical and horizontal, respectively, since the former maximizes and
the latter minimizes natural convection.
1576C
The view factor from surface 1 to surface 2 is the fraction of radiation which leaves surface 1 and
strikes surface 2 directly. The magnitude of radiation heat transfer between two surfaces is proportional to
the view factor. The larger the view factor, the larger the radiation exchange between the two surfaces.
1577C
Emissivity of a surface is the ratio of the radiation emitted by a surface at a specified temperature
to the radiation emitted by a blackbody (which is the maximum amount) at the same temperature. The
magnitude of radiation heat transfer between a surfaces and it surrounding surfaces is proportional to the
emissivity. The larger the emissivity, the larger the radiation heat exchange between the two surfaces.
1578C
For most effective natural convection cooling of a PCB array, the PCB should be placed vertically
to take advantage of natural convection currents which tend to rise naturally, and to minimize trapped air
pockets. Placing the PCBs too close to each other tends to choke the flow because of the increased
resistance. Therefore, the PCBs should be placed far from each other for effective heat transfer (A distance
of about 2 cm between the PCBs turns out to be adequate for effective natural convection cooling.)
1579C
Radiation heat transfer from the components on the PCBs in an enclosure is negligible since the
view of the components is largely blocked by other heat generating components at about the same
temperature, and hot components face other hot surfaces instead of cooler surfaces.
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 Spring '06
 Rajadas

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