Page 110 - Root Cause Failure Analysis

Fans, Blowers, and Fluidizers 101

performance
A fan is inherently a constant-volume machine. It operates at the same volumetric
flow rate (Le., cubic feet per minute, or cfm) when operating in a fixed system at a
constant speed, regardless of changes in air density. However, the pressure developed
and the horsepower required vary directly with the air density.

The following factors affect centrifugal-fan performance: brake horsepower, fan
capacity, fan rating, outlet velocity, static efficiency, static pressure, tip speed,
mechanical efficiency, total pressure, velocity pressure, natural frequency, and suction
(inlet) conditions. Some of these factors are used in the mathematical relationships
referred to as fan laws, which are discussed later in the chapter.

Brake Horsepower
Brake horsepower (BHP) is the power input required by the fan shaft to produce the
required volumetric flow rate (cfm) and pressure.

Fan Capacit?,
The fan capacity (FC) is the volume of air moved per minute by the fan (cfm). NOTE
The density of air is 0.075 pounds per cubic foot at atmospheric pressure and 68°F.

Fan Rating
The fan rating predicts the fan’s performance at one operating condition, which
includes the fan size, speed, capacity, pressure, and horsepower.

Outlet Velocity
The outlet velocity (OV, feet per minute) is the number of cubic feet of gas moved by
the fan per minute divided by the inside area of the fan outlet, or discharge area, in
square feet.

Static Eficiency
Static efficiency (SE) is not the true mechanical efficiency, but is convenient to use in
comparing fans. This is calculated by the following equation:

0.000157 x FC x SP
Static Efficiency (SE) =
BHP
Static Pressure
Static pressure (SP) generated by the fan can exist whether the air is in motion or
trapped in a confined space. SP is always expressed in inches of water (gauge).

lip Speed
The tip speed (TS) is the peripheral speed of the fan wheel in feet per minute (fpm).

Tip Speed = Rotor Diameter x IC x Rotations per Minute (rpm)