The Long and Short of Centrifugal Fan Troubleshooting

How to Troubleshoot Centrifugal Fan Air Volume, Pressure, and Power Consumption

Centrifugal fan troubleshooting is definitely a science. But to application engineers like us, there’s also an art to it. The trick is knowing what to measure and the system effects that can influence those measurements. And it all depends on the specific application. The really short answer to the question of how to troubleshoot a centrifugal fan is to call an application engineer at the fan manufacturer or your manufacturer’s rep. They’ll know exactly where to look. But if you want to take a peek under the hood yourself, here’s a quick overview of where to start, what to measure, what to watch for, and ways to adjust.

Where to Start with Centrifugal Fan Troubleshooting

To troubleshoot a centrifugal fan, start with the ideal: the fan curve provided by the fan manufacturer. Assuming you have the correct speed of the fan and the density of the gas moving through it, that curve will tell you what you would expect to see in terms of static pressure, volume, and horsepower consumption under perfect circumstances.

Perfect circumstances in an industrial fan application mean you don’t have crazy ductwork requiring modification at the inlet and outlet, allowing plenty of room (five times the duct diameter) for the air to move straight in and out without sharp turns or impediments. It’s rare to see perfect circumstances in real-life applications, so there’s almost always something to watch for.

What to Measure When Troubleshooting a Fan

With the fan performance curve in hand, the next step is to take some measurements. The two easiest things to measure when troubleshooting are: static pressure differential and amp draw.

Amp Draw

Amp draw never lies. It’s exactly how much power is being consumed by the motor to drive the fan.

Static Pressure Differential

Outlet static pressure minus inlet static pressure, measured by putting a pressure tap near the outlet and the inlet. This allows you to measure pressure between the outlet and discharge(exhaust) and static pressure from the tap to the very intake way before the fan.

In an ideal scenario, the differential pressure intersects the horsepower curve at the exact point where it matches the amp draw you’re getting off the motor. If those are in alignment, you can be confident that the volume going through your fan is based on where that static pressure point was on your static pressure curve.

Centrifugal Fan Troubleshooting: What to Watch For and Ways to Adjust

If you’re not getting enough volume, pressure, or amp draw and your centrifugal fan troubleshooting readings don’t match your fan curve, the culprit is system effects resulting from adjustments required to operate in your specific application versus the ideal scenario described above. You need to change the system to eliminate system design issues.

Fan Speed

One way to adjust the system to overcome the system effects is to increase the speed of the fan if the amperage is available and you’re on a Variable Frequency Drive (VFD). If you just want to max your performance all the time, and you have the fan connected to a VFD, you can program the fan to operate at maximum full load amperage, provided it’s operating within the maximum safe speed rating of the fan design.

Dead Pressure

The other big system effect is dead pressure. Dead pressure is extra static pressure in the system pulling performance to a different point on the curve. Several things can create dead pressure, including:

  • Elbows in front of the inlet of the fan requiring an inlet box
  • Outlets turning the wrong way
  • Pick-up points that are not transitioned properly
  • Dirty filters that need to be changed
  • Dampers that are closed that need to be opened

Ultimately, we advise that you call your fan manufacturer or their representative to troubleshoot your options.

Troubleshooting Example from Video

For example, an outlet pressure reading of 13 inches wg and an intake reading of negative seven inches wg would indicate 20 inches of static pressure. Now you take your amp draw off your motor and you get that you’re doing that at 100 HP. But when you look at your fan curve, what you see is that 20 inches of static pressure looks like it should be taking 150 horsepower from your motor. So why the discrepancy? What’s the problem?

The damper and inlet box are creating system effects, but the port above the box isn’t picking up the effect of the inlet box adding about an inch wg in static pressure. If you adjust for that and go back to the curve, it shows that you should be doing at least 130 horsepower, so why is your motor still only reading 100 HP? Now you need to look at the outlet damper, set (for example) at 70% open because when you first installed it, that was the specified set point. This is fabricating static pressure too, so your fan actually sees four inches of static pressure in the damper.

As you look back at your fan curve, you now have 20 inches wg of differential pressure, plus an inch of dead pressure on your inlet box, plus four inches of dead pressure on your outlet damper. Add all those numbers together and you have about 25 inches of static pressure. Go back to your fan curve. Now does it make more sense? As you go down from 25 inches wg instead of 20, you’re probably intersecting a point on your horsepower curve that’s very close to 100 HP, which aligns with what you read when you looked at the amp measurement that you took off your motor legs.

Hear it from an Application Engineer

Senior Application Engineer Chet White demonstrates centrifugal fan troubleshooting in this ten-minute video.

When you need to specify a centrifugal fan or blower for your industrial process application, or if you need centrifugal fan or blower maintenance, repair or retrofit, reach out and connect with one of our application engineers to discuss the details of your project.

Related Content

For more information on troubleshooting and application system effects mentioned in this article, check out these additional pages:

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