The need for forced-air cooling should be determined at an early stage
in system design. It is important that the design plans for good airflow
to heat-generating components and also allows adequate space and power
for the cooling fan.
The first stage in designing a forced-air cooling system is to estimate
the required airflow. This depends on the heat generated within the enclosure
and the maximum temperature rise permitted. The power dissipation figure
used should be a worst-case estimate for a fully loaded system.
(Please place the mouse cursor over each item to show a diagram)
Formula
Graph
Impedance
Operating Point
Multiple Fan Use
Formula
The airflow required can be obtained either by calculation or from a graph.
The equation for calculation is:
or
Q1
:
Airflow required in m3/min
Q2
:
Airflow required in Cubic Feet /min
H
:
Heat dissipated in watts
ΔT
:
Temperature rise above inlet temp o C
Graph
In the following graph, the vertical axis represents the heat to be removed and the horizontal
axis represents the airflow; both axes are logarithmic.
Power Dissipation vs. Airflow
for Various Temperature Increases
The sloping lines define the temperature rise in o C. To use the graph, find the sloping line that represents the permitted temperature rise.
Then, find the point on this line that corresponds to the heat to be removed.
The horizontal position of this point shows the airflow required.
System
Impedance
Obstructions in the airflow path cause static pressure within the enclosure.
To achieve maximum airflow, obstructions should be minimized. However,
obstructions in the form of baffles may be necessary to direct the airflow
over the components that need cooling.
Typical Relationship between Airflow and Static pressure for an Axial Cooling
Fan
The left figure
shows the nonlinear relationship between airflow and
static pressure for a typical fan.
System Operating Point
The left figure and the figure below shows the nonlinear relationship between airflow and static pressure for
a typical fan. The performance of a fan in a specific application is determined
by the intersection of the System Impedance Curve and the Fan Characteristic
Curve. The System Impedance Curve is a property inherent to an individual electronics
enclosure. This curve can easily be generated experimentally, by testing
the enclosure pressure at various airflow rates.
Typical Relationship between Airflow and Static pressure for an Axial Cooling
Fan
Intersection of Fan and Enclosure Curves
Multiple Fan Use
The following figures show the performance characteristics for parallel and series operation of two identical fans.
The parallel operation
The series operation
"An additional fan in parallel to the first increases airflow in a low static pressure situation. An
additional fan in series increases the airflow in a high static-pressure enclosure."
Formula
or
