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HVAC Airflow - Big Mistake??

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Saint Jerome

Saint Jerome

Student
Dec 18, 2023
1
Hi all...

I have a business provide soundproof meeting pods for numerous applications, and we have been trying to overcome an issue with airflow in the pods for some time. The pods have 750RPM fans which generate an airflow which is not quite adequate and has input (supply and return doesn't really apply) ducts in the ceiling with air being pushed into the pod. The walls have internal vents for output air (baffled within the walls) to exit outside top.

We thought it made good sense to ramp up the fans to 900RPM which generate 50% more airflow (and not a lot more noise) and this appeared to work OK.

However, over time, these 900RPM fans are all failing, and the actual airflow is NOT much greater than the older 750RPM fans. In fact, we measure airflow under the ceiling input vents and this increases dramatically (30%) when the door is open, but drops back when door is closed.

I suspect we are seeing an impact from output vents being too small, but this is causing back pressure on the fans, ultimately killing them!

Worse still, it might be that the additional input airflow is causing some sort of interference (turbulence) which is actively reducing airflow with the 900RPM fans to LESS than the original 750RPM fans.

I know this is light on with the technical aspects, but can anyone comment on this situation?
 
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A fan develops both flow (volume) and pressure which are related to speed and power of the machine driving the fan itself. In ramping up the speed from 750 to 900, you have increased speed by 20%, with the intent being you're providing more flow. Great!

But now you're requiring (1.2)^2 the power to produce that flow. If you don't have the power, you won't have the pressure. And without the pressure, you won't have the flow you thought you were going to get. In essence - you're on a whole new fan curve. The hint that you're needing more power? The fan motors are all trying to run at 120% of nameplate just to keep up - and the thermal stress is killing the insulation. (Note - it is also possible that operation at 120% speed is causing mechanical issues with either motor rotor construction, bearing life, or fan hub/blades.)

Limit the pressure drop through the circuit as much as possible. This means ensuring the pressure drop in the "exit" portion is kept low (wall vent cross-section, any ducting between wall vent and eventual exit as short and straight as possible, and eventual exit as large as possible). Keep any losses in the "enter" portion low (large cross-section, straight as possible, etc.). As you noted, with the door open you see more flow - which means the backpressure is higher than expected. Note that backpressure is a function of the room volume, the route (and restrictions) from inlet ceiling duct to outlet wall duct, and pressure drop in duct "runs".

Converting energy to motion for more than half a century
 
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