


Enclosure VentilationEnclosures housing Electrical and Electronic equipment often need ventilation. The amount of ventilation is not always sufficient for the power dissipated within the enclosure and consequently the temperature rise with in the enclosure is often excessive resulting in premature failure or incorrect operation of equipment housed within the 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%) 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:
The "Electrical Calculations" software provides functions for calculating the thermal resistance and temperature rise for non fan cooled enclosures based on the enclosure installation, height, width and depth, and the total power dissipated within the enclosure. The air flow and temperature rise for fan cooled enclosures can also be calculated. Download "Electrical Calculations" 

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