"24V DC Cooling Fan" fan than revolutions have an important role in the selection of the fan, the more convenient especially for a wide range of centrifugal fan performance curve dimensionless selection. Than the number of revolutions of the fan with the concept of specific speed pumps, but also the same meaning than the number of revolutions in the application. Currently, the fan model establishment of specific speed is according to formula (4-21) calculated results and the provisions of the unit. "24V DC Cooling Fan" fan than ns is the number of revolutions of the wheel in terms of a single, multi-stage (stages of i) and double suction blower fan, which are i ns specific speed stage fan? N? P20 ?? ?? i? (4-22) n double suction fan ns? one of p202 (4-23) than the number of revolutions is the basic performance parameters "24V DC Cooling Fan" fan. For the performance of the previous discussion also applies to parameters specific speed. In addition, ns specific speed and also used to calculate the size of the unit concerned, following discussions on these issues separately.