Reading a Thermal Performance Graph
0
100
Mounting Surface Temp
Rise Above Ambient—°C
80
60
40
20
0
0
200
Air Velocity—Feet Per Minute
400
600
800
16
12
8
4
0
1
2
3
4
Heat Dissipated—Watts
5
Thermal Resistance From MTG
Surface to Ambient—°C/Watt
1000
20
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.
EXAMPLE:
The output air volume
of a fan is given as 80 CFM. The output area
is 6 inches by 6 inches or 36 in
2
or 25 ft
2
.
To find velocity:
GRAPH A
0
100
Mounting Surface Temp
Rise Above Ambient—°C
80
60
40
20
0
0
1
2
3
Heat Dissipated—Watts
4
5
200
GRAPH B
Air Velocity—Feet Per Minute
400
600
800
1000
20
16
12
8
4
0
Thermal Resistance From MTG
Surface to Ambient—°C/Watt
Velocity (LFM) =
Volume (CFM)
area (ft
2
)
Velocity = 80 = 320
0.25
Velocity is 320 LFM, which at 80%,
derates to 256 LFM.
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).
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.
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.
Velocity
(LFM)* = Volume
(CFM)**
area (ft
2
)
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.
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
* Linear feet per minute
** Cubic feet per minute
AMERICA
EUROPE
www.aavidthermalloy.com
USA
Tel: +1 (603) 224-9988 email: info@aavid.com
Italy
Tel: +39 051 764011 email: sales.it@aavid.com
United Kingdom
Tel: +44 1793 401400 email: sales.uk@aavid.com
ASIA
Singapore
Tel: +65 6362 8388 email: sales@aavid.com.sg
Taiwan
Tel: +886(2) 2698-9888 email: sales@aavid.com.tw
11
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.
579802
CONVERTING VOLUME
TO VELOCITY
AAVID [ AAVID THERMALLOY, LLC ]
