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Heat Transfer

Heat Transfer & Thermal Conductivity Are Not Linearly Related

Heat transfer has three modes: conduction; convection; and radiation.  Only the conduction mode is dependent on material conductivity.  Heat often moves through a part faster than it can be removed from its surface.  Thus, excess conductivity is often wasted.

 
Thermally conductive plastics can transfer heat like metals and ceramics in many applications.

 

This curve demonstrates the non-linear relationship between heat transfer and material thermal conductivity.
The curve is generic.  It applies to any application having both conduction and convection components to the total heat transfer.  [Radiation is typically small and is ignored in this calculation.]
The shape of the curve is the same regardless of the application.  The quantitative values on the axes are not shown  because they depend on the power, part size and convective cooling conditions.  They become fixed for any given application and set of conditions. 
Its obvious from the shape of the curve that heat transfer depends on material thermal conductivity but there is also a point, a knee in the curve, where increasing thermal conductivity produces negligible improvement in the heat transfer.
Model of Heat Transfer (1 dimensional) across a flat plate
Its easy to show the non-linear relationship between heat transfer and material thermal conductivity using this simple example or model [right].
One can image a block of material with a thickness of 1/2 inch.  5 Watts of power are input on one side of the sample and the heat is carried away on the opposing side with a fan.  The heat transfer is represented by the sum of the delta Ts (or temperature differentials) and can be calculated as a function of the material thermal conductivity [as seen in the next example graph below].

 

 

 

 

 

Thermal conductivity, K (W/mK)

Thickness, l = 0.5 inch
Convective heat transfer coefficient, h = 50 W/m2K
Cross sectional area, A = 1.5 in X 1.5 in
Power, q = 5 watts
Ambient temperature, Ta = 25C

Model of Heat Transfer (1 dimensional) across a flat plate

 

For this particular example or set of conditions the heat transfer is maximized for any material having a thermal conductivity above about 5 W/mK. 
The threshold where increasing thermal conductivity does not significantly improve heat transfer depends entirely on the specific application conditions.  It depends on the input power, the size and shape of the part, and the convective heat transfer conditions.
In this application, a thermally conductive plastic could transfer heat as well as any metal.  With a different set of conditions, even copper or diamond might not be sufficiently conductive to maximize the heat transfer. << Previous page          Continue to next>>
 

See Also

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Thermally Conductive Plastics For Resistors & Switches

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Thermally Conductive Plastics For Temperature Sensors

Thermal sensors can be directly over-molded or encapsulated with thermally conductive plastics to maximize heat transfer to the sensing device while providing protection from the operating environment. >>More

CoolPoly Selection Tool

Not sure which material best suits your application?  Try our new CoolPoly selection tool.

FAQs

Answers to the most commonly asked questions regarding thermally conductive polymers.

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