How to Determine Over-Amperage on Pulley Size Change?

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It appears as though I underestimated the head loss from the damper.

https://www.engineeringtoolbox.com/hvac-damper-pressure-loss-d_1873.html

In a 12 inch duct @ 1050 CFM, that's a damper face velocity ##\approx 3000 ~ \rm{\frac{ft}{min}}##.

According to the chart that's close to 1.5"WC loss for the damper. That would give us a total of ##\approx 6 + 0.5+1.5 = 8## "WC.

If your motor rpm is correct ( it is run off a VFD ), then it indicates the fan is performing as expected.

It's possible someone decided to overspeed the motor a bit to make up for missing the design spec (or underestimating it).

If this turns out to be the case, (probably) the most economical route is to get a larger motor and change gearing to match the desired rpm as opposed to a system redesign. You'll have to check the ROR for each option.

$$ H(1300) \approx \frac{8}{1050^2}1300^2 = 12.3~\rm{WC}$$

I'd give that a 10% safety factor...so spec the motor hp/rpm about 13" WC @ 1300 CFM.
 
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Haotranphotomask said:
I was able to measure the rotation of the fan.

Motor pulley size: 5.25”
Fan Pulley size: 3.75”
Center-to-Center: 13.5”
The RPM on motor pulley: 1920 RPM
The RPM on fan pulley: 2727 RPM
The static pressure is ~6”WC, but we are only getting 1050 CFM of flow through with one fan. I was able to get ~1200CFM if I have two fans on.

Would it be because the 8" trunk is too restrictive for 1300CFM of flow? What if I increase the fan size/resheave the belt? Would the 8" trunk still be limiting how much flow I could get through?
Yes, 8-inch round metal is not enough for that volume.
Maybe noise level should be considered if you follow the route of more input power.

Could you install additional ducts in parallel with that unfortunate 8-inch?

Time for some duct system calculations.
Please, see:
https://www.engineeringtoolbox.com/sizing-ducts-d_207.html
 
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Lnewqban said:
Yes, 8-inch round metal is not enough for that volume.
Maybe noise level should be considered if you follow the route of more input power.

Could you install additional ducts in parallel with that unfortunate 8-inch?

Time for some duct system calculations.
Please, see:
https://www.engineeringtoolbox.com/sizing-ducts-d_207.html
To me it looks like PVC (or worse CPVC) duct. There could be a sizable upfront cost with material/installation considerations if that is the case.

I think 2 motors up front cost, and annualized 2hp of additional electricity are going to win out

But as you say. Noise may dictate the system redesign expense.

In an industrial setting, I never calculated noise levels (not saying that one shouldn't - thinking back I did make a loud system once). For the most part If it was loud (it was just like everything else in the plant), you wear ear defenders\plugs! The tolerance of noise probably varies plant to plant and how much ambient noise already exist.
 
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I may have missed it (I looked). How are you measuring your actual CFM? I ask because:

1. Your blower dP is about where it should be
2. Your inlet absolute pressure is reasonable for the described plumbing at the described flow

Any chance that you're performing as expected and just have a bad flow measurement?