# Understanding Fan Curves

What is a Fan Curve?
It is a characteristics curve, which shows the relationship between static pressure and the air flow rate of the fan. The fan manufacturers perform actual test to find out the static pressure value of the fan for the different air flow rates to plot the fan curve.

Please note from the fan curve that, as the flow rate increases the static pressure decreases, means at higher flow rate the fan will be able to withstand lower system resistance (or back pressure).

How to Select a Fan

• After you posses the fan curve from the supplier, you have to know or create one more curve that is system curve.
• The intersection point of the fan curve and the system curve will give you the operating point of the fan or you can tell your fan supplier what is the static pressure and flow rate you are looking for.
• The fan supplier will then select a suitable sized motor to the fan.

Conclusion

The article has explained the fan curves basics, but drawing correct system curve is not a easy task and selecting the fan’s operating point depends largely on preparing correct system curve. I will write something next on how to draw a system curve.

Image Courtesy

Bureau of Energy Efficiency

www.energyefficiencyasia.org

## 5 thoughts on “Understanding Fan Curves”

1. Jon says:

Dear Suve,
Thank you for your great article.I am not sure to well understanding the differences between fan curve and system curve.Why in system curve the pressure increases when the flow increases.On the other hand,in fan curve,as the flow increases,the pressure decreases.
Looking forward to hearing from you at the earliest.
Best Regards
Jon

thank you sir

3. MechGuru says:

Well, let’s start with the fan, The input power for a fan can roughly be calculated using the following equation: input Power (KW) =(flow rate X pressure)/fan efficiency. You see for a fixed amount of input power for a fan, the pressure will decrease with the increase of flow rate. Now, for a system (lets say a air tight compartment), as flow of air increases i.e., the more and more air are pushed inside the compartment, the pressure will keep increasing. Hope it clears.

4. Tebissa says:

In a cooling fan drive system for a thermal power plant, the air flow and air pressure have the
following performance curve relationship. H 1.03N 2  0.56NQ  0.59Q2, where N is
rotational speed of the fan, Q stand for flow rate, H stand for air pressure with all units in per
unit. Base of variables are 1800r/min for speed, 1000m3/min for flow rate and 4243N/m2 for air
pressure. Efficiency of fan is given by   0.5 0.3Q, Q being in per unit. The fan head curve
can be expressed as H  Q2 with damper fully opened. If the dapmer opening level is
represented by k, then the curve can be modified to H  kQ2. Operating point of the fan is
located at the intersection of the performance curve and the system head curve. The operating
point can be controlled either by controlling the damper opening level or the speed of the motor
driving the fan. If the required air flow for a year is assumed as follws depending on the load of
the power plant, answer the following questions.
50%, 4000 hours, 30% for 2000 hours, 20% for 2000 hours
a. Select an electric machine to drive the fan from following choices. 100Hp, 125Hp,
150Hp, 200Hp, 250Hp, 300Hp
b. Calculate total electric energy (KWH) to drive the fan during the year for the following
two cases. Assume 95% efficiency for electric machine regardless of load.
i. Damper opening level

5. Tebissa says:

In a cooling fan drive system for a thermal power plant, the air flow and air pressure have the
following performance curve relationship. H 1.03N 2  0.56NQ  0.59Q2, where N is
rotational speed of the fan, Q stand for flow rate, H stand for air pressure with all units in per
unit. Base of variables are 1800r/min for speed, 1000m3/min for flow rate and 4243N/m2 for air
pressure. Efficiency of fan is given by   0.5 0.3Q, Q being in per unit. The fan head curve
can be expressed as H  Q2 with damper fully opened. If the dapmer opening level is
represented by k, then the curve can be modified to H  kQ2. Operating point of the fan is
located at the intersection of the performance curve and the system head curve. The operating
point can be controlled either by controlling the damper opening level or the speed of the motor
driving the fan. If the required air flow for a year is assumed as follws depending on the load of
the power plant, answer the following questions.
50%, 4000 hours, 30% for 2000 hours, 20% for 2000 hours
a. Select an electric machine to drive the fan from following choices. 100Hp, 125Hp,
150Hp, 200Hp, 250Hp, 300Hp
b. Calculate total electric energy (KWH) to drive the fan during the year for the following
two cases. Assume 95% efficiency for electric machine regardless of load.
i. Damper opening level.
Speed of electric machine controlled using inverter whose efficiency is assumed
to be constant at 95%.
iii. If the rate for electric city energy is 0.4 Birr/kWh what is the cost saving by
electric motor speed control.
c. Discuss with your friends the advantages of controlling the motor speed than damper
opening level control.
d. Discuss also why the flow rate is less than 100%
how can i solve this problem?help me please