The more you know the better.
Understanding a fan curve gives you the advantage. If you want to select the fan that best suits your needs and your requirements, you’re going to need to be able to read a fan curve.
What Is a Fan Curve?
A fan curve is a way to graphically understand the performance of a particular fan. With this data, you are able to decide which fan works best for your application and make a more informed decision about the purchase you are making.
A fan performance curve is essentially a graphical representation of exactly what a fan can do.
Fan curves illustrate a few key points — including:
- CFM (cubic feet per minute)
- RPM (revolutions per minute)
- SP (static pressure)
- BHP (brake horsepower)
Understanding how these measurements work means that you need to understand where they sit on the graph.
Horizontal Bottom Axis: Air Volume Flow Rate (SCFM or m3/sec.)
Vertical Left Axis: Static Pressure (inches water gauge, pascals, or mm water gauge)
Vertical Right Axis: Brake horsepower (BHP or KW)
Ok, so now that we understand where certain measurements lie on the graph, it’s time to dive into exactly how to read them.
When you’re looking at a fan curve, what you’re actually seeing is two performance curves related to the same fan. This relationship will paint a picture of a fan’s performance and give you a better picture of what type of fan works best for your application.
Once you get to know all of the acronyms and understand exactly how these measurements work, you’ll be better equipped to make the right decision.
CFM vs. SP Curves
The SP or Static Pressure Curve — shows a very specific relationship. This type of fan performance curve shows the static pressure capabilities of a fan compared with the fan’s air volume flow rate for a specific fan speed.
CFM vs. BHP Curves
Like we mentioned, once you get to know the acronyms these fan names start to make sense. This particular curve illustrates the correlation between a fan’s air volume flow rate and its brake horsepower
Choosing Your CFM
Select your desired CFM and get to work. Draw a line vertically upwards to intersect the CFM and SP curves.
Choosing Your SP
You’ll want to draw a horizontal line through your vertical left axis at the fan pressure you want, right where it intersects the vertical line you just drew. If the intersection of these two lines is not directly on the curve — you have a little more work to do.
You have to calculate and draw a System Curve and go back…redraw the fan curve at a different RPM.
Choose Your Motor Horsepower
To select your desired motor horsepower, draw a line vertically up from the intersection of the System Curve and the SF vs. SP curve — all the way to where it crosses the CFM vs. BHP curve for the chosen fan. This information helps you find the BHP you will need to have for the desired pressure and CFM at a certain fan speed.
There You Have It
It seems pretty complicated but once you see it and understand how the measurements correlate, it becomes much simpler. Choosing the right fan for the right application is essential. Don’t get stuck not knowing. Learning all you can about fan performance curves puts the power in your hands.
