What Are Centrifugal Fans and Axial Fans, and Why Does the Difference Matter?
If you're looking at fans and blowers—specifically backward inclined blowers, backward curved blowers, or inline axial fans—you're not alone. These are the heavy lifters in most commercial and industrial ventilation systems. And the choice between axial and centrifugal can make or break your system's performance.
Let's cut through the jargon. Axial fans move air along the axis of the fan blades—think of a big ceiling fan. Centrifugal fans (which include backward inclined and backward curved blowers) pull air into the center and then throw it out at a 90° angle using a rotating impeller. The core difference? Pressure.
In my role coordinating emergency HVAC parts for manufacturing facilities, I've seen firsthand how picking the wrong fan type causes a cascade of problems. At one client's factory in March 2024, they had 36 hours to fix a ventilation bottleneck. The engineer on-site had spec'd an inline axial fan because they assumed it'd be cheaper and quieter. But the system needed higher static pressure to push air through a long duct run with filters. The axial fan couldn't handle it. We had to rush-ship a backward inclined blower—cost an extra $700 in expedited fees (on top of the $2,400 base cost). But we saved their production line. Missed deadline would've meant a $50,000 penalty clause with their client.
When Should You Choose an Inline Axial Duct Fan?
Axial duct fans—including inline axial fans—are your go-to when you need high volume airflow at low pressure. Think large spaces where the air path is relatively straight, and you don't need to overcome significant resistance from filters, coils, or long duct runs.
Good applications include:
- General ventilation of warehouses, factories, or large open areas
- Exhaust systems where the duct run is short and direct to the outside
- Make-up air systems that bring fresh air into a building
- Process cooling where you're moving large volumes of air over hot equipment
The big upside? Axial fans are usually cheaper and more compact than centrifugal blowers. They install inline with your ductwork, which saves space. And for straight-shot applications, they're incredibly efficient.
When Should You Choose a Backward Inclined or Backward Curved Blower?
This is where it gets interesting. Backward inclined blowers and backward curved blowers are types of centrifugal fans. They're designed to handle higher static pressure. This means they can push air through longer duct runs, against tighter filters, or into systems with multiple branches.
I had a project last year where a food processing plant was trying to use axial fans for a ventilation system with extensive ductwork and HEPA filters. The axial fans couldn't maintain sufficient flow when the filters loaded up (which happens over time, obviously). We ended up retrofitting with inline centrifugal fans—specifically backward inclined ones—and the pressure problem disappeared.
Backward curved blowers have a slight advantage: their blades are more efficient at higher speeds, which can mean lower energy consumption for the same job. But honestly, the difference is often marginal unless you're running them 24/7 at high load.
Key applications:
- HVAC systems with duct runs longer than 50-100 feet
- Dust collection or fume extraction where you need to overcome filter resistance
- Clean rooms requiring HEPA filtration (high static pressure)
- Exhaust systems serving multiple hoods or points (branch ductwork increases resistance)
How Do They Compare on Performance and Costs?
Here's a practical comparison. This was accurate as of Q4 2024; the market changes fast, so verify current pricing.
In-line axial fan (typical 10-inch, 2000 CFM, 2 speeds):
- Static pressure capability: 0.5 to 1.2 inches w.g. (water gauge)
- Approximate cost (unit only): $500 - $1,200
- Noise level: Can be louder at high speed; blade-pass frequency can be an issue
- Efficiency at high pressure: Drops significantly above 1.0 inches w.g.
Backward inclined/inline centrifugal blower (similar capacity):
- Static pressure capability: 1.5 to 4+ inches w.g.
- Approximate cost (unit only): $1,500 - $3,500
- Noise level: Generally quieter, especially at higher static pressures
- Efficiency at high pressure: Maintains efficiency well
So you can expect to pay roughly 2-3x more for a backward curved blower compared to an axial fan of similar airflow. But if your system needs that pressure capability, the axial fan simply won't get the job done.
What About 'Inline' Centrifugal Fans?
You'll also come across inline centrifugal fans. These merge the compact, duct-mounted form factor of an axial fan with the higher-pressure capability of a centrifugal blower. They use a backward inclined or backward curved impeller inside a cylindrical housing that bolts directly into your ductwork.
In my experience, an inline centrifugal fan is often the sweet spot for retrofit projects where space is tight. We used them at the food processing plant I mentioned earlier. They're more expensive than axial fans (typically 30-50% more) but way cheaper than trying to redesign the entire duct system to accommodate a traditional centrifugal blower in a scroll housing.
How Do You Make the Right Choice?
Here's a simplified decision tree I use:
- First, determine your required static pressure. This is the hardest part. You need to calculate the pressure drop through your longest duct run, filters, dampers, and any other components. If you don't have a fan selection program, an HVAC engineer can do this. Seriously, guess wrong here and your system won't work.
- Under 1.5 inches w.g. total static pressure? An axial fan might work fine. Check the manufacturer's performance curve at your target CFM.
- Over 1.5 inches w.g.? You need a centrifugal fan—backward inclined or backward curved. Don't waste time trying to make an axial fan work.
- Not sure? Go with a backward inclined blower. It's the safer bet.
I've never fully understood why some engineers try to save the upfront cost by spec'ing axial fans in borderline situations. My best guess is they underestimate the system resistance during design. If someone has insight, I'd love to hear it. But I've seen the aftermath—systems that underperform, are noisy as heck, and have to be retrofitted at greater cost. We lost a $30,000 contract in 2021 because our competitor's axial fan system didn't meet the performance spec (they'd promised lower price, but couldn't deliver). The client ended up paying more for centrifugal replacements. That's when we implemented a 'pressure verification before purchase' policy.
Honestly, I'm not sure why some manufacturers' fan curves don't seem to match real-world performance. There's definitely variability. That's why getting a submittal drawing with tested performance data is worth the wait.
