How to Determine Over-Amperage on Pulley Size Change?

[erobz]

Why have a reducer and expander back to back? Did the white painted expander come with the fan and/or did this used to be connected to an 8" duct system? You could be losing a significant amount of static pressure through that set of transitions right at the fan. See:
https://www.achrnews.com/articles/96369-minimizing-system-effect

Yeah, I was going to have them back calculate the SP at the inlet.

 

[erobz]

Another thing: When you are only running 1 fan. The other fan is completely isolated from the system via closing off its discharge dampers?

 

[Haotranphotomask]

What is the measured SP at the inlet port relative to atmosphere?

About -5.5" WC at the inlet and 0.5" WC at the discharge.

Why have a reducer and expander back to back? Did the white painted expander come with the fan and/or did this used to be connected to an 8" duct system? You could be losing a significant amount of static pressure through that set of transitions right at the fan. See:
https://www.achrnews.com/articles/96369-minimizing-system-effect

I am not sure I can answer that question. The fan design was like this when I started this project and the OG engineer left without much details.

Another thing: When you are only running 1 fan. The other fan is completely isolated from the system via closing off its discharge dampers?

Yes, the damper is closed when one fan is running.

 

[erobz]

As @russ_watters pointed out the inlet conditions are not ideal. You should get new inlet bells. I'm going to estimate the SP at the actual inlet and get back to you.

 

[jrmichler]

The RPM on motor pulley: 1920 RPM

Standard four pole induction motors run 1750 to 1760 RPM at full load, and 1798 RPM at no load when running on 60 Hz power. In order for the motor to run 1920 RPM at full load, it would need to be on a VFD set at about 65 Hz. If the motor is driven by a VFD, check the operating frequency. It will be shown on a display on the VFD cabinet. If the motor is driven by 60 Hz power, then the measured RPM is wrong. And so is the measured blower RPM.

 

[erobz]

About -5.5" WC at the inlet and 0.5" WC at the discharge.

I am not sure I can answer that question. The fan design was like this when I started this project and the OG engineer left without much details.

Yes, the damper is closed when one fan is running.

It's hard to say for certain (I don't have access to the ASHREA duct fitting database https://www.ashrae.org/technical-resources/bookstore/duct-fitting-database). But from what I could piece together on the net; working backwards from the point of measurement to the inlet is probably at most another 0.5"WC through those transitions, flange, flexible coupling. So maybe 6.5 "WC. So that isn't saving it...It still appears to be performing off its curve.

Check the RPM's again. As @jrmichler pointed out its not running at the rated RPM, for a 4 pole induction motor. If you think its running at 1920 RPM, and it is running at 1760 RPM, that that means the fan is turning at about 2500 RPM. and if you follow that over at 1077 cfm, we expect 6.5-7" WC.

Otherwise, is it possible the fan was ever cut \trimmed?

And you have verified similar measurements for SP on both fans, running independently?

 

[Lnewqban]

@Haotranphotomask
The location of the green circle in the picture is not appropriate for accurate measurement of the outlet static pressure.

Please, see:
https://www.systemair.com/fileadmin...echnical_Handbook_EN_2019-07_E2029_web_no.pdf

 

[erobz]

@Haotranphotomask
The location of the green circle in the picture is not appropriate for accurate measurement of the outlet static pressure.

Please, see:
https://www.systemair.com/fileadmin...echnical_Handbook_EN_2019-07_E2029_web_no.pdfView attachment 318467

They are measuring at the red circles. The green circles are a mechanical damper on the discharge.

 

[Lnewqban]

They are measuring at the red circles. The green circles are a mechanical damper on the discharge.

Thank you very much for the correction.

That makes the situation even worse!
The air turbulence inside any duct (not a pipe in this case) is greatly increased by any backdraft damper.

The damper of the fan in stand-by automatically closes itself under the discharge pressure of the working fan.

 

[erobz]

Thank you very much for the correction.

That makes the situation even worse!
The air turbulence inside any duct (not a pipe in this case) is greatly increased by any backdraft damper.

The damper of the fan in stand-by automatically closes itself under the discharge pressure of the working fan.

Yeah, I suspect we could add another 0.5"WC from the open damper. Maybe bringing the total to 7" WC. There would still be a discrepancy. However, if something turns out to be fishy with the measured motor rpm as @jrmichler expects, then everything seems to fall into place - and the fan is operating as expected...which would be good because we could then end the free of charge remote trouble shooting!

 

[erobz]

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.

 

[Lnewqban]

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

 

[erobz]

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.

 

[Dullard]

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?