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.
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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.
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.
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