The factors involved in fan performance evaluation and efficient system operation are as follows:
(1)System characteristics :
The system resistance is the sum of static pressure losses in the system.
It varies with the square of the volume of air flowing through the system.
Long narrow ducts with many bends and twists will require more energy to pull the air through them.
Thus, the system resistance increases substantially as the volume of air flowing through the system increases.
2) Fan characteristics :
The fan curve is a performance curve for the particular fan under a specific set of conditions.
Typically a curve will be developed for a given set of conditions usually including : fan volume, system static pressure, fan speed, and brake horsepower required to drive the fan under the stated conditions.
The intersection of the system curve and the static pressure curve defines the operating point. When the system resistance changes, the operating point also changes.
Once the operating point is fixed, the power required could be found by following a vertical line that passes through the operating point to an intersection with the power (BHP) curve.
A horizontal line drawn through the intersection with the power curve will lead to the required power on the right vertical axis.
(3) System characteristics and fan curves :
In any fan system, the resistance to air flow (pressure) increases when the flow of air is increased.
The pressure required by a system over a range of flows can be determined and a “system performance cur can be developed
This system curve can then be plotted on the fan curve to show the fan’s actual operating point at ”A” where the two curves intersect.
This operating point is at air flow Delivered against pressure P A fan operates along a performance given by the manufacturer for a particular fan speed.
The fan performance chart shows performance curves for a series of fan speeds.
At fan speed N, the fan will operate along the N performance curve as shown in Fig given below.
(4) Fan laws:
The fans operate under a predictable set of laws concerning speed, power and pressure.
A change in speed (RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM.
