Fan cooled enclosure

The temperature rise within the enclosure is directly proportional to the thermal resistance of the enclose and the total power dissipated within the enclosure. The thermal resistance is a function of the shape and size of the enclosure, and also to the amount of exposed surface area. Given the dimensions of the enclosure, and the environment within which it is mounted, it is possible to approximate the enclosure thermal resistance for a sealed enclosure.
The addition of ventilation grills will reduce the thermal resistance, but not by a significant amount. (Typically less than 10%)
If the thermal resistance of the enclosure needs to be reduced significantly, the only way to accurately control the temperature rise is by the addition of forced ventilation fans. With forced ventilation, the amount of airflow required for a given temperature rise is proportional to the total power dissipated.
When fans are used, adequate open area for air input and air output must be provided. If the open area equals the size of the fan, the velocity of the air flow will be equal to the velocity of the air through the fan. If the open area is too small, the airflow velocity will increase, the pressure across the fan will increase and the airflow will reduce.
Inlet and exhaust ports can provide a much higher resistance to air flow, and thereby restriction in ventilation than is generally appreciated. Wherever possible, it is preferable to keep the smallest dimension of the open area at no less than 10 mm. If the smallest dimension is reduced below this figure, the boundary effect around the edges of the opening will reduce the effective open area. A good rule is to try for twice the area of the fans in open area for the inlet and exhaust ports with the minimum dimension of the opening at 10mm or greater.
If the smallest dimension is halved, then allow for double the open area.
A further reduction in the minimum opening dimension should be compensated for by and increase in the open area. Failure to do this will result in reduced air flow and increased temperature rise. One solution where very fine mesh is employed is to double the fan capacity. Air filters can severely restrict the air flow, but will often be accompanied by flow/pressure curves.
Where these curves are available, they can be superimposed on to the fan curves and the actual expected flow can be easily predicted. NB Small box fans very quickly lose airflow when a constrictive filter is applied.

Typical Air flow figures for small fans:



Size (mm)
Speed
(cfm)

119 x 25
High
80

119 x 38
Low
50

119 x 38
Medium
75

119 x 38
High
100

172 x 51
High
280




The required airflow can be calculated from the total power dissipated in the enclosure and the maximum allowable temperature rise. Likewise, the temperature rise can be calculated for known Power dissipation in the enclosure and airflow.