# Cooling Fan

The size, specification and location of a cooling fan will depend upon the amount of heat that has to be dissipated from the object that has to be cooled. Therefore, in selecting a fan the first task is to estimate the heat to be dissipated, because the overall system air temperature differential above the inlet ambient is directly proportional to the heat dissipated. It is then possible to estimate the amount of required cooling air. The basic heat transfer equation is:

where
= amount of heat transferred to the system [Watts]
Cp = specific heat of air [Jkg-1K-1]
= mass flow rate of air [kg s-1]
ΔT = desired air temperature differential [K]

There are a number of points that should be kept in mind with regards to the position of a cooling fan:
1. Locate components with highest heat dissipation near the enclosure air exits.
2. Size the enclosure air inlet and exit vents at least as large as the venturi opening of the fan used.
3. Allow enough free area for air to pass with velocity less than 7 meters/sec.
4. Avoid hot spots by spot cooling with a small fan.
5. Locate components with the most critical temperature sensitivity nearest to inlet air to provide the coolest air flow.
6. Blow air into cabinet to keep dust out, i.e. pressurize the cabinet.
7. Use the largest filter possible, to:
1. Increase dust capacity
2. Reduce pressure drop.