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The formula for the air flow in elec­tronic cooling appli­ca­tions

The elec­tronic compo­nents in servers and data centers get hot – and this heat needs to be removed. A rough calcu­la­tion of the neces­sary cooling air flow produced by fans or other equip­ment is just the first step in ensuring optimum cooling.


Tobias Sieger, Team Leader Fluid Mechanics (Photo| ebm-papst)

An engineer’s life would be so much easier if perfect energy conver­sion were possible. However, our world is full of losses. Servers, network archi­tec­ture and solar inverters get hot. Their power dissi­pa­tion (Pv) is converted into heat output of equal size, which raises the temper­a­ture of the air supplied for cooling (∆T). If the compo­nents are just cooled with air, for example, it is only neces­sary to know the specific thermal capacity of the air (cp) and its density (ϱ) in order to calcu­late the air flow (V̇) the fans must produce to ensure that the air is kept within the maximum permis­sible heating limits.

However, as a rule, this calcu­la­tion is just a first approx­i­ma­tion that is followed by more complex modeling and thermal simu­la­tion. Along­side the specific geom­etry of the instal­la­tion site, factors include the heat transfer resis­tance of all the mate­rials involved, such as the compo­nents, assem­blies, the inte­rior air, hous­ings and the exte­rior air. In the case of solids, consis­tent simu­la­tions are rela­tively easy to achieve if the mass and heat transfer resis­tance of the indi­vidual mate­rials — such as copper and plas­tics — are known.

ebm-papst in data­cen­ters

From a struc­tural point of view, no two data centers are alike. That’s why with ebm‑papst there is not just one, but a variety of good solu­tions.

However, it can be tricky to deter­mine the resis­tance of compo­nent contacts, i.e., at the tran­si­tion between the indi­vidual compo­nents of an assembly. Factors such as surface rough­ness, air bubbles trapped in the adhe­sive, or gaps play a role here. However, the biggest chal­lenge in thermal insu­la­tion is always the transfer of heat from the surfaces of ­solids to the air and vice versa. In specific construc­tion situ­a­tions, many surfaces are often arranged consec­u­tively. Exact simu­la­tions are impor­tant because a differ­ence in temper­a­ture of just one or two Kelvin can have a signif­i­cant impact on the service life of elec­tronic compo­nents. 

CHT (conju­gate heat transfer) flow simu­la­tion is used to deter­mine the distri­b­u­tion of the heat transfer coef­fi­cients on the surface of a compact fan drive system. (Graphic | ebm-papst)

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