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# Blowers Performance Curve

By Author: Oleg Tchetchel
Total Articles: 25

While multi-rating tables are convenient, fan performance curves offer additional information such as - how much reserve pressure head exists between the design pressure and the peak static pressure, the maximum power the fan might draw, and the efficiency of operation.

Fan curves are based on laboratory test data and are sometimes referred to as "test curves". A typical test curve will often define the performance parameters for a specific design and size of fan, operating at a given speed, moving a gas of a given density. Inspection of this graph will show that it is actually composed of four separate curves:

• Static Pressure vs. Volume Curve: This plot is the one often referred to as the "fan curve" or "characteristic curve" because it defines all the possible pressure-volume combinations the fan is capable of producing given stated conditions (i.e. rpm and gas density). Note that this curve has two regions - one marked by dashed lines and the other by a solid line. Fans must be selected so that the design point is located on a solid portion of the curve, preferably in an area of high operating efficiency. Operation on the dashed portion of the curve should be avoided as it is a zone of potentially unstable performance. For this reason it is wise to allow some reserve between the peak static pressure and the design pressure to compensate for a higher resistance to flow than anticipated by the design calculation.

• Static Efficiency vs. Volume Curve: In most instances it is desirable to have a fan perform as close to its peak efficiency as possible. The static efficiency vs. volume curve illustrates the efficiency of fan performance at a glance.

• Power vs. Volume Curve: This plot illustrates the power draw of the fan for any point on the characteristic curve.

• System Curve: The system curve defines the volume flow rate versus pressure characteristics of the system in which a fan will be installed.

For most applications, the volume flow rate to pressure relationship is governed by the following equation, often called the "duct law":

Once the system designer has determined the system pressure loss (P) for one flow rate (CFM), it is very easy to calculate the corresponding pressure loss for any other flow rate using this "law". The system curve is not included on the performance curve when it is issued from the fan manufacturer and its determination is left to the system designer.

Oleg Tchetchel
Industrial Ventilating Designer
airknife@cogeco.ca
Northern Fan Co.
http://www.northernindustrialsupplycompany.com/burlington/request7.html
http://www.northernindustrialsupplycompany.com/burlington/request2.html

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