Forced air-cooling for packaged electronic enclosures can be achieved by either evacuation (with fan on exhaust side) or pressurizing the enclosure with a fan on the intake side. This choice should be made early in the design process. Although both theoretically use the same volume of air to dissipate the heat, they have different effects on placement of components within the enclosure.
When using an exhaust fan, the air distribution inside the cabinet is flexible and heat from the fan itself is not dissipated into the cabinet. Evacuation has the disadvantage of reducing the pressure within the enclosure, so airborne dust is drawn in through all the vents and cracks in the enclosure. Filtering of the fan on the exhaust side is extremely difficult. A further benefit is that the enclosure is slightly pressurized so that dust is not drawn into the enclosure from the surrounding environment.
The disadvantage of intake fans is that filters must be changed frequently to eliminate dust accumulation. A clogged filter can severely restrict airflow, causing elevated temperatures in an enclosure that may be more of a problem than the dust itself. Another disadvantage of a fan that pressurizes the system is that air dissipated by the fan motor can slightly warm the incoming air. This can reduce the air's cooling effect. Components that have the most critical cooling requirements should be placed closest to the air inlets. High temperature components should be placed closest to the air outlets.
If exclusion of dust is required, it is better to use a fan that pulls air into the enclosure. In this configuration, a filter at the fan inlet can remove dust from the incoming air.
Air that is drawn into the fan flows in a continuous, non-turbulent movement called laminar flow, which allows for a uniformly distributed airflow velocity in the enclosure. This is important in eliminating stagnant air and hot spots. Air exhausted from the fan is turbulent. Heat dissipation in a turbulent airflow can be up to twice that of a laminar flow with the same volumetric flow rate, except that the turbulent airflow region near a fan exhaust is normally limited.
Developing a well-defined airflow path through the whole enclosure is essential to minimizing airflow waste. Vents should be at least 50 percent larger than the fan openings themselves. Care must also be taken to eliminate air re-circulation in a fan, as over ninety percent of the airflow can be lost. Baffles may be used to eliminate re-circulation of the same air since an airflow path will always take the path of least resistance.
Subassemblies and components within the enclosure should be positioned to direct the airflow to places that require cooling. Component placement should always be considered in order to take advantage of natural convection; for example, placing warm components above cool components. Avoid placing large components so that they shield smaller components from the flow of air. Use baffles, where necessary, to direct the airflow to critical hot spots.
Smaller systems usually use axial cooling fans, where airflow is perpendicular to the fan blades. The airflow required to dissipate the heat generated can either be obtained by calculation or from a graph. This airflow requirement will depend on the heat generated within the enclosure and the maximum temperature rise permitted. When estimating the power dissipated within a system, use a worst-case estimate for a fully loaded system to allow for the possibility of future changes and additions of heat generating subsystems.
In many applications, using an intake fan rather than an exhaust fan can double or triple the life of the fan. The heated air passing over an exhaust fan stresses the fan's bearings much more than the 25°C air flowing over an intake fan.
