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529802B02500G Datasheet(PDF) 11 Page - Aavid, Thermal Division of Boyd Corporation |
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529802B02500G Datasheet(HTML) 11 Page - Aavid, Thermal Division of Boyd Corporation |
11 / 116 page 11 EUROPE ASIA Italy Tel: +39 051 764011 email: sales.it@aavid.com United Kingdom Tel: +44 1793 401400 email: sales.uk@aavid.com Singapore Tel: +65 6362 8388 email: sales@aavid.com.sg Taiwan Tel: +886(2) 2698-9888 email: sales@aavid.com.tw AMERICA USA Tel: +1 (603) 224-9988 email: info@aavid.com www.aavidthermalloy.com 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. 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. 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. CONVERTING VOLUME TO VELOCITY 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 in2 or 25 ft2. To find velocity: 80 0.25 Velocity is 320 LFM, which at 80%, derates to 256 LFM. 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 Heat Dissipated—Watts 0 20 40 60 80 100 01 2 3 4 5 Air Velocity—Feet Per Minute 20 16 12 4 8 0 0 400 200 600 800 1000 GRAPH B Velocity (LFM) = Velocity = = 320 Air Velocity—Feet Per Minute Heat Dissipated—Watts 20 0 20 40 60 80 100 01 2 3 4 5 16 12 4 8 0 0 400 200 600 800 1000 GRAPH A 579802 Reading a Thermal Performance Graph Velocity (LFM)* = Volume (CFM)** area (ft2) * Linear feet per minute ** Cubic feet per minute Volume (CFM) area (ft2) |
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