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

Visit airprofan.com/ - Call 715-365-3267 - Email [e-mail]
Troubleshoot the application, not the ideal. Be sure to factor in system effects, and don’t be fooled by isolated readings. We’ll explain.
VISIT OUR WEBSITE:
airprofan.com/
SUBSCRIBE TO OUR CHANNEL FOR OTHER VIDEO TIPS & HELPFUL INFO:
/ @airprofan
SUBSCRIBE TO OUR EMAIL LIST:
airprofan.com/...
LET’S CONNECT!
Email: [e-mail]
LinkedIn: / airpro-fan-and-blower
Full Transcript:
This is a real-life dust collector application. If you need to troubleshoot a fan like this, you need to check 3 things called out on the manufacturer-provided curve: static pressure, volume, and horsepower consumption.
In an ideal scenario, you’d have at least 5 times the duct diameter of duct straight into the inlet and off the outlet. That’s rare, but let’s start there. The 2 easiest things to measure when troubleshooting are static pressure differential and amp draw. Amp draw never lies. It’s exactly how much power is being consumed by the motor to drive the fan. In an ideal scenario, the differential pressure (outlet minus inlet static pressure) intersects the horsepower curve at the exact point where it matches the amp draw you’re getting off the motor. If those line up, 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.
Measure the differential 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. The differential between them should line up on your fan curve with the actual amp draw on your motor.
In the ideal situation, the curve is accurate in terms of the speed of your fan and the density of the gas moving through it. Now all of that has to apply to your actual scenario, If you have the speed or density wrong, your curve won’t show you exactly what’s happening. The application behind me is not ideal, it’s reality.
There’s an outlet damper here. That’s not ideal, but it is a great way to cut back on the volume if it’s too much for what you actually need. It also adds to your static pressure. Even if it’s wide open, the fins are in the air’s path so it’s going to impede the flow, creating static pressure.
On the intake side, there’s an inlet box, and if you watch our inlet box video, you’ll know this is the best way to do a really bad thing. You don’t want a 90-degree intake right before the inlet of the fan - it’s bad for your fan’s overall efficiency, and further adds to static pressure.
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 damper. Let’s say you set your damper at 70% open because when you first installed it, that was the specified set point. This is fabricating static pressure, so your fan actually sees 4 inches of static pressure in the damper.
As you look back at your fan curve, you now have 20 inches 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 from25 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.
That’s how you assess/troubleshoot a real fan application. This is just one example. There are many more things that could affect your static pressure readout. I see applications all the time where an outlet comes up and instantly goes the opposite direction from the way the air is coming off the fan wheel. In an application like this, that’s terrible. It’s dead pressure for the fan, but you might not measure it if your outlet pressure readout is further downstream.
Consider how that manifests itself back on your manufacturer’s fan curve as you’re troubleshooting real applications like this one.
#IndustrialFans #CentrifugalFans #Manufacturing