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Specific Speed

The equation, q=wCpΔt provides the first step in the selection of an air mover for a cooling application by estimating the required amount of airflow. Of equal importance, however, is the need to quantify the resistance that this airflow generates in the equipment, i.e. the flow resistance characteristic of the system.
Calculations of the system resistance provide only rough approximations in most cases. Tests taken from a rough mock-up of the equipment, with particular attention to potential high-resistance elements such as small openings, sharp turns in the air direction, filters and otherwise densely packed portions of the system, is the recommended preliminary approach. Rotron personnel and facilities are always available to the customer to aid in this and subsequent air mover selection, test and evaluation work. When the preliminary flow and pressure values are defined, the specific speed of the air mover can be calculated. Specific speed is a dimensionless geometry-defining parameter which is the speed at which an air mover will deliver one unit of volume against one unit of pressure rise. It is expressed as: 

Where    Ns = specific speed 
  N = operating speed in RPM
  Q = airflow in cubic feet per minute (CFM)
  P = the operating point static pressure in inches of H2O

The figure above displays the specific speed ranges for the various types of Custom Division air moving devices. Note that the calculation of specific speed requires that the approximate speed of the air mover be known.