Rotary Positive Displacement Blowers (P-D Blowers) generate different noise characteristics depending upon the speed at which they are turning. As a blower’s speed increases the blower begins to generate high frequency noise in addition to the low frequency noise normally associated with P-D Blowers. The speed at which this occurs is called Transition Speed.
The rpm at which this transition occurs is not the same for different sized blowers. To be able to compare one size blower to another the industry uses Pitch Line Velocity. This is the speed of the pitch line (where the gears intersect) of the timing gear in a blower. After numerous experiments with different sized blowers the industry has settled upon the following pitch line velocities as the transition speeds.
For the blower intake transition speed is 3300 fpm pitch line velocity.
For the blower discharge transition speed is 2700 fpm pitch line velocity.
To calculate the pitch line velocity the formula is: GD * .262 * rpm.
GD = the gear diameter of the blower
Rpm = the rpm of the blower.
To help the user Stoddard Silencers’ catalogues – shown on this web site – have charts which show the rpm at which transition occurs for different gear size blowers. They also show the silencers recommended for blowers operating both above and below transition speed.
It should be noted that these speeds (and corresponding rpm’s) are not immutable. Due to manufacturing tolerances in the blower and sheaves and the tolerance in the speed of the blower motor, transition can occur plus or minus five percent (5%) of the calculated value. Stoddard Silencers recommends that if the customer’s blower is operating close to transition speed choose the silencer best suited for above transition speed blowers.
As an additional note, the overall noise level of a blower package is directly proportional to the pitch line velocity of the blower.