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 When a fan speed needs to be changed in order to change the airflow, the
motor pulley size is increased to increase the fan flow, and decreased to
decrease the fan flow. Because the motor pulley is usually smaller than the
fan pulley, it is less expensive to change the motor pulley than the fan
pulley. For a change in cfm, the new pulley setting (P d2) is calculated as
follows:
P d2 =
P dl x (cfm2/cfml)
original pitch diameter
where:
P dl =
P d2 =
new pitch diameter
c f m1 =
air quantity at the fan, original
c f m2 =
air quantity at the fan, new
FIGURE 2-15
V-Belt Pully Standard Groove Dimensions For Sheaves
Figure 2-16 shows a variable pitch sheave. The two halves of the sheave
are two different pieces, with one threaded onto the shaft of the other.
Depending how closely the halves are positioned to each other determines the
actual pitch diameter of the sheave. Variations of 30 percent from the
nominal pitch diameter are commonly available with this type of sheave. The
variable pitch sheave is only slightly more expensive than the fixed pitch
sheave in the smaller sizes. However, in the larger sizes, or for multiple
groove sheaves, the variable pitch sheave can be two or three times the
price of a fixed sheave.
2-13
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