A simplified heat pipe system and its thermal equivalent are shown below to explain the area of thermal resistance of a heat pipe system.

The total thermal resistance of the heat pipe (evaporator surface to the condenser) is given by manufacturer.  Additional thermal resistance is encountered in the interface to the heat pipe and within the heat source and heat sink.




EXAMPLE: We want to determine the temperature drop at a given watt. Heat source dissipating 60 watts through a ¼ inch water fill heat pipe to air cooled fins with 0.5w/in2/°c above ambient.

We assume:

1-Ambient Temperature: 30°c

2-Horizontal orientation of the heat pipe system

3-Heat pipe input on source: 1 inch

4-Heat pipe input on sink: 1.5 inch

5-Interface material thickness 0.001 inch and 0.05°c/w/in2

Thermal resistance interface (Ri1) = 0.05/∏x 0.25 x 1 = 0.064°c/w

Thermal resistance heat pipe (Rhp) = 0.35°c/w given by manufacturer

Thermal resistance interface (Ri2) = 0.05/∏x 0.25 x 1.5 = 0.042°c/w

Total resistance (Rt) = 0.064 + 0.35 + 0.042 = 0.456°c/w

Temperature change from input heat source to output heat sink:

                              ΔT1(°c) = 60 w x 0.456 = 27.4°c

An additional temperature drop, normally the most significant one, will be found between the heat sink and the ambient air. This drop (ΔT2) must be added to ΔT1 to obtain the source temperature.

ΔT2 = 60w/0.5 x ∏ x 1.5 x o.25 = 102°c + 30°c (ambient) = 132°c

The heat source temperature: ΔT1 + ΔT2 = 137.72°c

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