Chapter hvac engineering Fundamentals: Part 1 Introduction

106
Chapter Five
Housing for
drive belt
Struts support bearing
and shaft housing
Figure 5.14
Axial-flow fan, cutaway view.
Figure 5.15
Typical normalized curve for an axial-flow fan.
Design Procedures: Part 3
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Design Procedures: Part 3
107
Figure 5.16
Typical normalized curve for a propeller fan.
overloading. In this connection, note that centrifugal fans are
nonoverloading—there is some horsepower rating which will never be
exceeded at a given speed.
Axial fan performance is governed by the blade shape and pitch, the
ratio of hub diameter to tip diameter, and the number of blades. Large
hub-to-tip ratios (0.6 to 0.8) relate to lower flow rates and higher pres-
sures than small hub-to-tip ratios (0.4 to 0.5). Airfoil blades are most
efficient. The fan volume can be varied by changing the speed. The
more common method of volume control is to use variable-pitch blades,
which allow changes in pressure and flow performance characteristics.