If you’ve ever stood next to a large centrifugal fan in a factory, warehouse, or HVAC system, you might have noticed how it behaves differently when the ductwork it’s connected to is open vs. blocked a little. As someone who’s been supplying centrifugal fans for over a decade, I get questions about this all the time: “Why does the fan draw less amperage when we close a damper?” or “Why won’t it move as much air as we calculated?” The answer ties straight to the fan’s performance curve—the single most important tool for designing, installing, and maintaining these units, and it’s not just a generic chart. It’s a window into how your fan will actually perform in your specific setup, not just a manufacturer’s sales sheet number. Centrifugal Fan
Let’s start with the basics to ground this. A centrifugal fan works by drawing air into its center, spinning that air with a bladed impeller at high speed, and throwing it outward against the casing, which channels the air into the ductwork. The two key variables here are airflow rate (measured in cubic feet per minute, or CFM, in the U.S.; cubic meters per hour, or m³/h, internationally) and total pressure (the resistance the fan has to overcome to move air through your system, measured in inches of water gauge, or inches WG, or Pascals). Every centrifugal fan has a unique relationship between these two, and that’s exactly what the performance curve maps out.
You can’t talk about a centrifugal fan’s performance curve without clarifying one non-negotiable: it’s tied directly to the fan’s operating speed and impeller size. Run the same fan at 1,200 RPM vs. 1,800 RPM, and the curve shifts completely. Make the impeller larger, and it will move more air at every pressure point. I see so many customers mix this up when they’re sizing a fan: they’ll grab a curve for a 10-inch impeller and assume it applies to a 12-inch, or a curve for 1,500 RPM and think it works for 1,000 RPM, and that’s how they end up with a fan that’s too big or too small for their needs.
Now, let’s break down the three core segments of a typical centrifugal fan performance curve, because it’s not just a single line. First, the far left end, which we call the “dead end” or zero airflow point. That’s when the outlet of the fan is completely blocked—no air moves at all, so CFM is zero, but pressure is at its highest. Think about closing the damper on your furnace and turning the fan on: the motor will draw more power here, because it’s working against no airflow, just compressing air inside the casing. I always warn new customers not to leave a fan at this zero airflow point for long; over time, that excess heat from the motor can burn out windings, especially in smaller, direct-drive fans.
Next, the middle section is the “operating range,” the part we use for almost all applications. This is where airflow increases as pressure decreases, and it’s the steep, almost linear part of the curve most people recognize. This is where your system’s design point comes in—your ductwork, filters, coils, and other components create a “system resistance curve” that crosses the fan’s performance curve at exactly the CFM and pressure your fan will actually deliver. If your system’s resistance curve crosses the fan’s performance curve in the middle range, you’re in good shape: the fan is operating efficiently, not working too hard or too soft. The peak efficiency point—where the fan uses the least power to move the most air—is usually right in the middle of this segment, and that’s where we always want to place your design point for longest fan life and lowest operating costs.
Then there’s the far right end of the curve, where airflow is at its maximum possible and pressure is very low. This happens when your ductwork is wide open, no extra resistance from filters, dampers, or elbows. At this point, the fan is moving as much air as its impeller speed and size can physically push, and it’s doing it with very little pressure. You might think this is ideal for maximizing airflow, but here’s the catch: on most centrifugal fans, this far right segment is unstable, sometimes called the “surge region” or “unstable operating zone.” Run the fan here for too long, and you can get pulsating airflow, increased noise, and even damage to the impeller or motor. I’ve had customers who tried to run a warehouse exhaust fan with no ductwork at all, just pulling air out of the wall, and within a few months the impeller was bent from those constant pulsations.
Now, let’s talk about the two other lines that almost always accompany the performance curve, because they’re just as important as the airflow vs. pressure line. The first is the brake horsepower (BHP) curve, which shows how much power the fan motor needs at every point on the curve. That’s the line that explains why closing a damper makes the fan draw less amperage: at the zero airflow end, BHP is actually higher than at peak efficiency, and it drops as you move into the middle range, only starting to rise again as you hit the right end of the curve. That’s a common point of confusion for maintenance teams, especially when they’re checking power usage to diagnose fan problems. The second is the efficiency curve, which lines up right with the middle segment of the airflow-pressure curve. Efficiency is usually measured as a percentage, and manufacturers test this in controlled lab conditions to make sure their curves are accurate—though I always tell customers that real-world efficiency will be a little lower than the published curve, because ductwork leaks, dirt buildup, and bends all add extra resistance that manufacturers don’t count in lab tests.
A lot of people ask me if all centrifugal fans have the same shape performance curve, and the answer is no—there are three main types, depending on the fan’s impeller design, which is called the “blade shape” and it changes everything. The most common are forward-curved blades, backward-inclined blades, and airfoil blades. Forward-curved blades have a very steep, high pressure curve at the low airflow end, dropping off quickly as airflow increases. These are the cheapest to manufacture, so they’re often used in small HVAC systems, but they have a narrow operating range and lower efficiency, which means higher power bills over time. Backward-inclined blades have a flatter curve, with peak efficiency right in the middle, and they handle higher pressures better than forward-curved fans, making them popular for industrial applications like dust collection or conveyor ventilation. Airfoil blades are the most efficient, with a very wide operating range—great for large warehouses or commercial buildings where airflow needs can change a lot. Their curve is the flattest, so pressure doesn’t drop as quickly as airflow increases, which means they can handle a wider variety of system setups without being too unstable.
I see mistakes with performance curves every day, mostly from customers who either don’t bother to look at the curve at all, or misinterpret it. One of the most common is oversizing the fan. A customer will look at the maximum airflow number on a curve and pick a fan that can move 10,000 CFM, when their actual system only needs 7,000 CFM. What happens then? The system’s resistance curve will cross the fan’s curve at a point way to the left of peak efficiency, so the fan is operating at low efficiency, wasting power, and the BHP is higher than it needs to be, leading to higher electricity bills. Another mistake is not accounting for system changes. If you add a new filter to your HVAC system, or run the ductwork through a longer, more complex path, your system resistance goes up, so your design point moves to the left on the curve. We always tell customers to build a 10% to 15% safety factor into their system resistance when they’re picking a fan, so they don’t end up with a unit that can’t deliver the airflow they need once their system is fully installed.
Another thing no one tells you about performance curves is how they change over time. A fan’s curve isn’t static—dust buildup on the impeller or casing will make the fan heavier and reduce airflow, shifting the curve down to the left. Bent impeller blades from impact, or worn bearings, will also change the curve, usually making the peak airflow lower and efficiency drop. That’s why regular maintenance isn’t just about fixing breakdowns; it’s about checking the performance curve annually to make sure your fan is operating as it should. I’ve worked with a food processing plant that noticed their exhaust fan wasn’t moving enough air, and when we tested the curve, we found it had dropped 20% from its original curve, all because of a thick layer of flour dust on the impeller. A quick cleaning brought the curve back to spec, and they saved thousands of dollars in electricity that month.
If you’re in the market for a centrifugal fan, don’t just look at price or maximum airflow. Ask to see the performance curve for the fan you’re considering, and make sure you match your system’s design point to the peak efficiency area of the curve. If you need help interpreting the curve, or sizing a fan for your specific application, our team has decades of experience working with all types of systems, from small commercial HVAC to large industrial ventilation. We can help you pick the right impeller size, speed, and blade design, and make sure your fan operates at peak efficiency for years to come. If you’re ready to discuss your requirements, get in touch with our team to start a conversation about what you need, and we’ll walk you through every step to find the right centrifugal fan for your operation.
BLDC Motor Fan References:
ASHRAE Handbook—HVAC Systems and Equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Fan Engineering Handbook. The Air Movement and Control Association International, Inc.
Industrial Ventilation: A Manual of Recommended Practice. American Conference of Governmental Industrial Hygienists.
Hangzhou Jiangnan Electric Motor Co., Ltd.
Hangzhou Jiangnan Electric Motor Co., Ltd. is one of the most reliable centrifugal fan manufacturers and suppliers in China, specialized in providing high quality customized service. We warmly welcome you to wholesale centrifugal fan for sale here from our factory. For price consultation, contact us.
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