The most precise method for calculating heating/cooling system for cabinet is to use dedicated software for cabinet thermal calculation. ProClima software is available on schneider web.
Besides that, it is also possible to calculation required fan flow manually, though certain inaccuracy will be caused by not-knowing the amount of heat dissipated by cabinet walls.
Manual calculation can be done based on following known (input) data:
- Total heat produced inside cabinet (W) - Ptotal
- Max ambient temperature (of cooling air) (deg.C) - Tomax
- Max allowed temp inside cabinet (deg.C)  - Timax
- Parameters of cooling air (specific thermal capacity c, density ro)

Min air flow is calculated as :

Qmin=Ptotal /(ro*c*(Timax-Tomax))

Where Qmin is in m3/s
Ptotal in kW
ro in kg/m3
c in kJ/(kg.K)
Tomax, Timax in degrees celsius

To get value of flow in m3/h, multiply result by 3600.
Qmin (m3/h) = 3600 * Qmin (m3/s)

In case of air:
ro=1.1kg/m3 (at 40 degrees Celsius)
c= aprox 1kJ/kgK

Example:
Devices inside cabinet produce total heat losses 560W (Ptotal=0.56kW)
Max operational temperature for devices inside cabinet is 50 degrees celsius (Timax=50 C)
Temperature of cooling air (ro=1.1kg/m^3, c=1kJ/KgK) never exceeds 25 degrees celsius (Tomax=25 C)

Min amount of air is then:

Q [m3/s] =0.56 kW /(1.1 kg/m3 * 1kJ/kgK * (50-25)K) = 0.56kW / 27.5 kJ/m3 = 0.020 m3/s = 20 liters/s
Q [m3/h] = 3600 * Q [m3/s] = 3600 * 0.020 = 73 m3/h

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