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Zahed Sheikh | October 2006

Calculating Liquid Loop Resistance

a simple liquid cooling loop has the following components:

a liquid cold plate

  • a pump
  • a radiator (liquid-to-air heat exchanger)

the liquid cold plate absorbs the heat from the device. the liquid that flows through the cold plate heats up and is pumped to the radiator section where it exchanges heat with the ambient. the cooled liquid is then returned to the cold plate as shown in the diagram below.

in choosing a cooling loop, you should try to design a loop with the lowest possible thermal resistance for the whole loop instead of focusing on optimizing a single component.

the total resistance for the loop is defined as:

where ts is the maximum surface temperature of the cold plate and ta is the ambient temperature.

the temperature difference across the cold plate is expressed as:

the cold plate surface temperature can be calculated from the following equation:

where t1 is the liquid temperature entering the cold plate.

the thermal resistance of the radiator is related to its inlet and ambient temperatures through

the total resistance can then be written as:

dividing both sides by q, will yield the equation for the loop total resistance

where rconv is defined as:


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