When designing forced-air cooling, the fan should be sized to the cross-sectional area of the power supply within the enclosure to ensure the necessary velocity of air passes over the surface of the components. In general, air should be directed along the long axis of the power supply. However, a further consideration is the layout of any internal heat sinks. The fins of the largest of the heat sinks should run parallel to the direction of airflow. Also, airflow will be restricted by obstructions and there are often many large and tall components mounted inside power supplies that will impede airflow. To prevent pressure from building up and to reduce the efficiency of the fan, the exit port for the air should be at least 50% greater in area than the entry port. It can make sense for the fan to be sized larger than necessary as a larger fan can deliver a greater amount of cooling air but at a lower speed than a smaller fan. By running slower, the fan will run more quietly, which can be crucial in applications where acoustic noise is a factor but conduction or natural convection does not provide sufficient cooling. Establishing the airflow (Q) that a fan must provide will depend on the temperature difference (ΔT)  between the ambient temperature and the maximum operating temperature of the supply, and the total power (W) that must be dissipated (which may need to include not just the power lost due to the supply’s inefficiency but also heat dissipated within the electronics of the system if the fan is being used to cool the entire equipment).