Reading a Thermal Performance Graph
The performance graphs you will see in this
catalog (see graph 579802) are actually a
composite of two separate graphs which
have been combined to save space. The small
arrows on each curve indicate to which axis
the curve corresponds. Thermal graphs are
published assuming the device to be cooled
is properly mounted and the heat sink is in
its recommended mounting position.
CONVERTING VOLUME
TO VELOCITY
579802
Air Velocity—Feet Per Minute
0
200
400
600
800
1000
Although most fans are normally rated and
compared at their free air delivery at zero
back pressure, this is rarely the case in most
applications. For accuracy, the volume of
output must be derated 60%–80% for
the anticipation of back pressure.
100
80
20
16
12
8
60
40
4
0
20
0
0
1
2
3
4
5
Heat Dissipated—Watts
EXAMPLE: The output air volume
of a fan is given as 80 CFM. The output area
is 6 inches by 6 inches or 36 in2 or 25 ft2.
To find velocity:
GRAPH A
GRAPH B
Air Velocity—Feet Per Minute
400
600 800
0
200
1000
Volume (CFM)
Velocity (LFM) =
20
16
12
8
100
80
area (ft2)
80
60
Velocity =
= 320
0.25
40
4
0
20
0
Velocity is 320 LFM, which at 80%,
derates to 256 LFM.
0
1
2
3
4
5
Heat Dissipated—Watts
GRAPH B is used to show heat sink per-
formance when used in a forced convec-
tion environment (i.e. with forced air flow
through the heat sink). This graph has its
origin in the top right hand corner with
the horizontal axis representing air velocity
over the heat sink LFM* and the vertical
axis representing the thermal resistance of
the heat sink (°C/W). Air velocity is calculat-
ed by dividing the output volumetric flow
rate of the fan by the cross-sectional area
of the outflow air passage.
GRAPH A is used to show heat sink perform-
ance when used in a natural convection envi-
ronment (i.e. without forced air). This graph
starts in the lower left hand corner with the
horizontal axis representing the heat dissipa-
tion (watts) and the vertical left hand axis
representing the rise in heat sink mounting
surface temperature above ambient (°C). By
knowing the power to be dissipated, the
temperature rise of the mounting surface
can be predicted. Thermal resistance in natu-
ral convection is determined by dividing this
temperature rise by the power input (°C/W).
DESIGN ASSISTANCE
Aavid Thermalloy can assist in the design
of heat sinks for both forced and natural
convection applications. Contact us for help
with your next thermal challenge. For more
information, visit our web site at:
www.aavidthermalloy.com
Velocity (LFM)* = Volume (CFM)**
area (ft2)
EXAMPLE A: Aavid Thermalloy part number
579802 is to be used to dissipate 3 watts of
power in natural convection. Because we are
dealing with natural convection, we refer to
graph “A”. Knowing that 3 watts are to be dis-
sipated, follow the grid line to the curve and
find that at 3 watts there is a temperature
rise of 75°C. To get the thermal resistance,
divide the temperature rise by the power
dissipated, which yields 25°C/W.
EXAMPLE B: For the same application
we add a fan which blows air over the heat
sink at a velocity of 400 LFM.
The addition of a fan indicates the use of
forced convection and therefore we refer
to graph “B”. This resistance of 9.50°C/W is
then multiplied by the power to be dissi-
pated, 3 watts. This yields a temperature
rise of 28.5°C.
* Linear feet per minute
** Cubic feet per minute
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11
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