Fans, Centrifugal

Besides being used in process plants, centrifugal fans are common in building heating and air-conditioning systems. Vendors for these systems should be consulted for required options

Types of Fans

Three types of centrifugal fans are available: (1) forward curved blade fans, (2) radial blade fans, and (3) backward curved blade fans. The characteristics of the forward curved blade fan make it the most appropriate type of cooling tower service. By virtue of the direction and velocity of the air leaving the fan wheel, the fan can be equipped with a comparatively small size housing, which is desirable from a structural standpoint. Also, because the required velocity is generated at a comparatively low speed, forward curved blade fans tend to operate quieter than other centrifugal types. Note the relatively small rpm and head values. The envelopes on the head versus volume curves each indicate a separate fan model is available. Centrifugal fans are usually of sheet metal construction, with the most popular protective coating being hot-dip galvanization. Damper mechanisms are also available to facilitate capacity control of the cooling tower.

Fan

All propeller-type fans operate in accordance with common laws. For a given fan
and cooling tower system, the following is true:
1. The capacity (cfm) varies directly as the speed (rpm) ratio, and directly as the
pitch angle of the blades relative to the plane of rotation.
2. The static pressure (hs) varies as the square of the capacity ratio.
3. The fan horsepower varies as the cube of the capacity ratio.
4. At constant cfm, the fan horsepower and static pressure vary directly with the
air density.
If, for example, the capacity (cfm) of a given fan were decreased by 50 percent
(either by a reduction to half of design rpm, or by a reduction in blade pitch angle
at constant speed), the capacity ratio would be 0.5. Concurrently, the static pressure
would become 25 percent of before, and the fan horsepower would become 12.5
percent of before. These characteristics afford unique opportunities to combine cold
water temperature control with significant energy savings.