FAN PERFORMANCE
The first two sections of this guide contain information needed to select the right fan for the particular application. The information in this section is useful once the fan has been selected and installed on the job.
The fan curves and system resistance curves below will help to solve fan performance problems that may be encountered in a variety of applications.
Fan Dynamics
A fan is simply an air pump. The rate at which a fan can “pump” air depends on the pressure the fan must overcome. This principle also relates to water pumps. A water pump is able to deliver more water through a
2 in. diameter hose than a 1 in. diameter hose because the 1 in. hose creates more resistance to flow.
For a fan, every flow rate
At 0.25 in. Ps, this fan will deliver 1000 cfm. If the pressure increases, cfm decreases. If the pressure decreases, cfm will increase.
At 700 rpm, the operating point will slide along the fan curve as static pressure changes, but it will never lie off the curve. In order for a fan to perform at a point off the curve, the rpm must be changed.
The figure below illustrates how rpm affects the fan curve. Notice that the general shape of the curves are the same. Changing rpm simply moves the curve outward or inward.
Fan Curve |
| Varying Fan Curve | |||||||||||||||||||||||||
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System Dynamics
For a given flow rate (cfm), an air distribution system produces a resistance to airflow (Ps). This resistance is the sum of all static pressure losses as the air flows through the system. Resistance producing elements include ductwork, dampers, grills, coils, etc.
A fan is simply the device that creates the pressure differential to move air through the system.
The greater the pressure differential created by the fan, the greater the volume of air moved through the system. Again, this is the same principle that relates to water pumps. The main difference in our case is that the fan is pumping air.
Tests have established a relationship between cfm and Ps. This relationship is parabolic and takes the form of the following equation:
Ps = K x (cfm)2
Where K is the constant that reflects the “steepness” of the parabola. This equation literally states that Ps varies as the square of the cfm.
For example, whenever the cfm doubles, the Ps will increase 4 times. The figures on the next page graphically illustrate this concept.
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