Page 112 - Root Cause Failure Analysis

Fans, Blowers, and Fluidizers 103

Fan Laws
The mathematical relationships referred to asfan laws can be useful when applied to
fans operating in a fixed system or to geometrically similar fans. However, caution
should be exercised when using these relationships. They apply only to identical fans
and applications. The basic laws are
Volume in cubic feet per minute (cfm) varies directly with the rotating
speed (rem).
Static pressure varies with the rotating speed squared (rpm’).
Brake horsepower (BHP) varies with the speed cubed (rpm3).

The fan-performance curves shown in Figures 8-3 and 84 show the performance of
fans of the same type but designed for different volumetric flow rates, operating in the
same duct system handling air at the same density.

Curve #1 is for a fan designed to handle 10,OOO cfm in a duct system whose calcu-
lated system resistance is determined to be 1 in. water (gauge). This fan will operate
at the point where the fan pressure (SP) curve intersects the system resistance curve
(TSH). This intersection point is called the point ofrating. The fan will operate at this

Figure 8-3 Fan peflormance curve #I (Higgins and Mobley 1995).