When selecting AC Fans or DC Fans for cooling a power supply system, you’ll want to check the CFM (Cubic Feet per Minute) rating. Fans are rated in CFM based on the expected free-flow of the air coming from them. This CFM rating is based on no back pressure. That means no obstructions to airflow, i.e. a bare fan.
In general real-world applications, airflow obstructions may come from fan finger guards, small exhaust opening sizes, and larger components such as heat sinks. To ensure minimal back pressure in your application, try to ensure the exit ports for your fans in the enclosure are about fifty percent larger than the minimum entry port. In most applications, you may find other heat loads and components that can obstruct the airflow path, therefore it is better to select a fan with a higher CFM rating than your calculations might otherwise call for. Remember: there’s no such thing as too much airflow when it comes to cooling, higher CFM is always an upgrade.
Another good way is to use a larger fan with slower speed. Larger fans running at a slower speed can deliver the same airflow as a smaller fan running at a higher speed. The larger fan running at a slower speed will have a higher MTBF rating and should be quieter in the application. Fan speed control circuits are typically used to control the amount of airflow from the device.
If there are heat sinks in the system, you should always direct the airflow in the same direction as the spaces between the fins of the heat sink. The airflow should never be directed perpendicular to the fins.