Reading A Fan Curve [How to Read]

It has become increasingly difficult to Reading A Fan Curve.  In light of software that allows fans to be selected based on inputs, choosing the best fan has become as simple as entering a set of operating conditions and letting the software decide which fan to use.  Suppose the ventilation system changes years after a fan is installed, changing the original operating conditions? What will be done then? A fan curve can be useful to determine the fan performance curve in the new conditions.

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What is a Fan Curve? [fan curve explained]

axial fanA fan curve is a graph that displays how well a fan performs.  Typically, fan curves show the relationship between volume and static pressure as well as volume and brake horsepower.  These relationships are not linear as the name implies.  I will discuss how to read fan curves later on.

In order to understand a fan curve, it is essential to recognize that it represents the fan’s performance as influenced by three parameters: blade pitch angle, speed and diameter.  Changing any one of those parameters will change the fan performance as well as the curves showing the relationship between volume, static pressure, and brake horsepower.

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On a fan curve, three key terms are used.  Fan curves show the amount of air produced by the fan on their horizontal axis.  The most common unit of fan volume measurement in the US is cubic feet per minute.

The vertical axis of the fan curve will show the static pressure and brake horsepower.  As we discussed in our article on Fan Pressure Losses, most US manufacturers incorporate velocity losses and dynamic pressure losses into their fan curves.  The static pressure on the fan curve is defined as the sum of static pressure losses for the ventilation system components.  This pressure is measured in water gauges or inches. w.g.

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According to automotive industry terminology, brake horsepower (or Bhp) is the amount of force required to slow down a motor. An air flow curve indicates how much horsepower is needed to produce a particular volume of air.  Depending on how much horsepower is required, the fan motor must be sized appropriately.

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Reading A Fan Curve

Having reviewed the definition and terminology of a fan curve, let’s now explore how to read one.  An example fan would be a 30″ supply fan powered by a 2 hp motor spinning at 1750 rpm with a 15-degree blade pitch angle.

  The system was chosen to provide 10,000 CFM at 0.5 in. w.g. Our software produces the following fan curves: Volume vs. Static Pressure and Volume vs. Brake Horsepower. The red circle on the curve represents the point where the fan is operating at its current operating level within the ventilation system.  In addition to the curves above, our software can also provide curves for fans whose diameter and speed are the same but whose blade pitch angles are 10, 20, 25, 30, 35, and 40 degrees.  With these fan curves, we will be able to improve our example.

For the sake of argument, let’s imagine that the customer wants to add a filter section after operating the supply fan for a while.  The filter section has a static pressure loss of 0.5 in. w.g. ; meaning that the new static pressure loss of the ventilation system is 1.0 in. w.g.

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The new operating point is determined by locating 1.0 in. w.g. in the curve of Volume versus Static Pressure.  Follow that curve until it intersects with our supply fan curve.  Here is where we need to operate.

In order to determine the air flow at the operating point, we move straight down until the horizontal Volume axis intersects the operating point.  At 1.0 in. We will now only be able to produce 7,500 CFM from our supply fan.

The new operating point on Volume vs. Brake Horsepower curve will be found when we intersect our fan curve.  From the right, we can see that 7,500 CFM are produced at 1.0 im with the required Bhp. W.g. is below 2.  If the customer is satisfied with 7,500 CFM of air flow, then there would be no need to change the motor on the fan.

If a customer wants to maintain 10,000 CFM of air flow, is that possible?  What modification do we need to make to the fan to achieve this?   Finding 10,000 CFM on the Volume axis can be found by examining the Volume vs. Static Pressure curve.  Go straight up to where the curve intersects 1.0 in. w.g. line.  Taking a 20 degree pitch angle and looking at our new operating point, it is on the curve for the fan.

By finding 10,000 CFM on the Volume vs. Brake Horsepower curve and moving straight up until our curve intersects that of the 20-degree blade pitch angle fan, we understand the brake horsepower required for a 30″ fan running at 1750 RPM and 20 degree blade pitch angle. Moving to the left from the new operating point, we find ourselves intersecting the Bhp axis at about 2.6.

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Based on the curves obtained from the fan, we anticipate the customer can maintain 10,000 CFM without totally replacing the fan with a 20-degree propeller blade pitch angle and a 3-horsepower motor.

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Non-linear Relationship

Volume and static pressure are not linearly related, as are Volume and brake horsepower.  Ventilation system static pressure impacts air flow from a fan more as it increases.  Because some of the fan curves have stall ranges, we do not show stall ranges in our selection software.  Within the stall range, the same amount of static pressure could result in two or more volume points.

We don’t show the full fan curve because our selection software doesn’t allow a user to select a stall range.  The full cutoff curves for fan blades with pitch angles between 20 and 40, as displayed by our selection software, would look as follows:

When making a selection from a fan curve, it is necessary to identify the stall range and to select within it.

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We hope that this example has demonstrated the importance of the skill set, as well as how to read fan curves.  In making selections and solving problems, our ventilation experts are well versed in fan curves.  Our ventilation experts are available to assist you with reading a fan curve or to answer any questions you may have about fan curves.

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