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Example of the correction factor for a heatsinkR

th

in forced cooling, compared

with convection cooling

The R

th

of the air cooler when changing from convection cooling to forced cooling can

be described as:

convection

,th

forced ,th

Rf

R

⋅ =

With regard to the power P or operating point of the fan in forced cooling, the air speed

and the air pressure or back pressure, the same applies as for the considerations about

a liquid cooler, whichwill be covered inmore detail in thenext section.

In heatsinks with liquid cooling, the thermal energy is dissipated into the fluid that is

flowing through. In most cases, this is water or a mixture of water and glycol, and may

also contain additives. An important criterion in the design of a liquid cooler is the flow

velocity of the cooling medium and the pressure drop in the total cooling circuit.

Pressure dropmeans the difference in pressure that the coolingmedium has to produce

in order to flow in a cooling circuit. The causes of pressure drop are determined by the

frictional and/or gravitational forces that act on the cooling medium, so the specific

pressure drops caused by inlet connecting sockets, bends, etc. in the system have to be

added together to give an overall view. If themanufacturer of the heatsink and the other

components being used has not provided any information in the form of a pressure drop

characteristic as a function of the flow velocity, so as to set the pressure drop for the

entire system, the systemmust bemeasured.

The relationship between the pressure drop and the flow velocity is expressed by an

exponential function. The higher the pressure drop, the greater the resulting flow

velocity

). To achieve the required flow velocity, a pump with the appropriate

pressure rating must be used. If the pressure as a function of the flow velocity of the

pump has been provided

), the operating point can be determined by

superimposing the diagram for the pressure drop as a function of the flow velocity,

which lies at the intersection of the two graphs

).

Several basic points should benoted to reduce the pressuredrop in the system:

•

The cross-section of the ducts for the cooling medium should be as large as

possible.

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