The Importance of Understanding Fan Curves
There are many factors one has to look for before buying an industrial fan. Choosing the right one for your workplace is not exactly the most straightforward thing to do, since there are several performance indicators that you have to look into before choosing the right kind of high-quality fan you need for your desired purpose.
While this may sound like a lot of work, it doesn’t have to be a daunting task. One just has to put effort into learning what exactly goes into the fan’s operations (you can also always ask your manufacturer, should you need help in understanding anything).
In this article, we’ll discuss one performance indicator you need to look at when gauging a fan’s efficiency – fan curves. Although it is a technical topic, learning about them will pay off greatly, as it will help you make a more informed decision when you select your industrial fan. Additionally, understanding fan curves will equip you for instance that requires trouble-shooting or any necessary alterations after the fan itself is installed.
Background about fan curves
But first, what are fan curves exactly, anyway?
In layman’s terms, a fan curve is a graphical representation of exactly what a fan can do. It is a measure of fan performance, which is determined by relationships (‘fan laws’). The three primary parameters whose relationships we look at are: the Static Pressure, Air Flow Volume (CFM), and Brake Horsepower (BHP).
The resulting curve from examining these relationships is what we use to determine the conditions that affect a fan’s functionality, and at what points the fan can operate most efficiently. Below is a typical graph for fan performance curves:
How to read fan curves
In order to read these performance curves, we need to first understand the basic parts of the graph. The first thing we need to know is that it has three axes, as shown in the image above. These three axes allow us to plot particular points in the graph, which create our performance curves:
- Horizontal Bottom Axis = Air Flow Volume (CFM)
- Vertical Left Axis = Static Pressure (SP)
- Vertical Right Axis = Brake Horsepower (BHP)
Figure 2: Fan Performance Curve Graph
Let us briefly go over the terminology we must familiarize ourselves with:
- Air Flow Volume (CFM) – Simply put, it is how much air a fan moves
- Static Pressure (SP) – The resistance of the fan pressure has to blow against to move air in the desired direction
- Brake Horsepower (BHP) – The minimum amount of power needed to operate the fan
The Static Pressure Curve (CFM vs. SP Curve)
The first curve we have to illustrate on the graph is the SP curve, which shows us the relationship of the fan’s static pressure (SP) capabilities in comparison to the fan’s air volume flow rate (CFM) for a specific fan speed.
Figure 3: Static Pressure Curve
When we plot a series of static pressure points versus specific flow rates at a given test speed, we get the SP curve, as seen in Figure 3. This is used to determine the fan’s pressure capability at any airflow volume.
The Brake Horsepower Curve (CFM vs. BHP Curve)
After plotting the SP curve, we can determine the BHP curve, which shows the correlation between the fan’s air volume flow rate (CFM) and its brake horsepower (BHP).
Figure 4: Brake Horsepower Curve
Such as with the SP curve, we are able to illustrate this curve (as seen in Figure 4) when we plot a series of brake horsepower points against specific air flow volume. It is important to determine an accurate BHP to know how much power is needed for a fan to operate at a specific point.
Determining Motor Horsepower
Once you’ve chosen your desired air flow volume and fan pressure, and upon drawing your SP and BHP curves, we can add the system line. This line is a parabolic curve, which is basically the sum of all possible CFM and SP combinations that can exist for the fan.
The system line allows us to then determine the operating point, which is the condition where the fan and system are in a stable equilibrium. It indicates what the fan’s performance will be when it is applied to your system and added to your location. To locate this point, simply draw a vertical line at the point where the SP curve and system line intersect to where it crosses the BHP curve, as illustrated in Figure 5:
Figure 5: System Line and the Operating Point
The next thing we need to do is to locate the intersection between the operating line and BHP curve, from which we draw a horizontal line to the right. This information will help you and your manufacturer find the motor horsepower you need to have for your desired pressure and CFM at a certain speed.
How are fan curves measured?
We now understand the basics about fan curves, but how or where can we get all of this information, anyway?
Industrial fan manufacturers perform actual tests where they examine the static pressure value of the fan for the different air flow volume to plot the fan curve. These are usually conducted in a test rig, in which air flow volume and air pressure can be adjusted and measured. Simply speak to your manufacturer if you require the information needed to draft your fan curves, or if you would like to get more information on the tests that they conduct.
Choosing the best fan for you
Industrial fans vary – not just by type, but by their size, the noise they produce, their airflow, and the horsepower they require. It is important for you to get the fan that is most ideal for your system and workplace, which is why it is a must to understand the concepts behind what allow them to operate.
Understanding fan curves won’t just help you select the right equipment; it will also help you ensure that it will always run efficiently even after it is installed – and even when you have to make adjustments.