How to Determine Over-Amperage on Pulley Size Change?

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The discussion focuses on determining over-amperage issues related to a VPB-808 fan setup, where the user is struggling to achieve the desired airflow of 1300 CFM at 7" SP. Current configurations show the motor drawing 2.8A while only producing 950 CFM, raising concerns about pulley sizes and potential slippage. Suggestions include measuring actual static pressure and fan performance, as well as considering larger pulleys or a higher horsepower motor if necessary. The conversation emphasizes the importance of accurate measurements to diagnose issues effectively, as the existing calculations may not reflect the true performance of the system. Ultimately, adjustments to the fan setup and thorough testing are recommended to ensure optimal operation.
  • #31
Haotranphotomask said:
We have a tool that requires 1300 CFM of exhaust that was previously tied to another exhaust system. They were running into problem with that system, so they decided to move that tool's exhaust trunk to the fans (the fans in this thread, one of them is a spare) that are abandoned in place. The system that was previously tied to these fans are no longer in operation and the tie-in trunk was capped for the tie in of the new tool.

The tool itself has about 11 exhaust lines, all tied into an 8" trunk. This trunk has about 200 ft of piping and 12 elbows. Then it's expanded to a 12" trunk (about 20 ft with 4 elbows). Then it's reduced to the inlet of the fans.
The discharge side is pretty much just a straight 15ft 12" line.

Consider that the poor fan is working in a sucking-from-many-points situation, rather than pushing air, which is its best performance configuration and design assumption.

It can only create certain lower-than-atmospheric pressure in its inlet; the rest of the moving-air-work is done by the atmosphere pushing air through many inlets, duct branches and changes of direction and toward that unique low pressure zone.

Multi-brach exhaust duct systems need to be tested and balanced, as well as carefully sealed from undesired leaks.
Otherwise, your piece of equipment could be “feeling” only a fraction of the exhaust efforts of your motor and fan.

With enough upstream restriction, the fan could be operating to a minimum flow (like having a close upstream damper), way off the desired operational point.

Please, also see:
https://www.spiralmfg.com/designing-efficient-dust-collection-system/

You may be able to find good guidance in ASHRAE manuals, if available.
 
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  • #32
Likely not possible, but... move the fan(s) as close to the machine as possible so they push the air out thru that 220 feet of plumbing. (Better yet, put the fan directly above, on the roof?)

The present configuration is like trying to drink a thick milkshake thru a 6ft. straw with a dozen bends and kinks in it; definitely non-productive.
 
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  • #33
Tom.G said:
Likely not possible, but... move the fan(s) as close to the machine as possible so they push the air out thru that 220 feet of plumbing. (Better yet, put the fan directly above, on the roof?)

The present configuration is like trying to drink a thick milkshake thru a 6ft. straw with a dozen bends and kinks in it; definitely non-productive.
That configuration is nearly impossible, as how far the fan was built from the tool.

erobz said:
The fan or the motor would be the issue in that case.

And actually, I was basing that off of measured 7 inWC @ 950 CFM in the present system. So it can't be reversed like that.
The actual static pressure measured is ~5.5 inWC @ 950 CFM. So using the equation you provided, we may need an increased motor for 10 inWC @ 1300 CFM.
 
  • #34
Haotranphotomask said:
The actual static pressure measured is ~5.5 inWC @ 950 CFM. So using the equation you provided, we may need an increased motor for 10 inWC @ 1300 CFM.
Can you determine if that operating point is on your fan curve @ 2668 RPM?
 
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  • #35
erobz said:
Can you determine if that operating point is on your fan curve @ 2668 RPM?
Fan Curve.JPG

The current fan curve I have is for the old system. The manufacturer has not been quite responsive about providing new fan curves.
 
  • #37
erobz said:
https://vironintl.com/wp-content/uploads/2016/02/Viron-VPB-Series-Push-Exhaust-Blower.pdf

Use this link (That is at a different RPM than what you are claiming to be running).

According to these tables something doesn't seem correct to me, but double check.
I have not measured the actual RPM of the motor. We just ordered a manometer for this purpose. Hopefully we will receive it soon.

There is one thing I have not considered. Do I have to factor in the motor efficiency? The nameplate has a motor efficiency of 89.5%. Does that mean that the motor is not running at 1760 RPM even at max frequency, and is actually running at 1575 RPM?
 
  • #38
Haotranphotomask said:
I have not measured the actual RPM of the motor. We just ordered a manometer for this purpose. Hopefully we will receive it soon.

There is one thing I have not considered. Do I have to factor in the motor efficiency? The nameplate has a motor efficiency of 89.5%. Does that mean that the motor is not running at 1760 RPM even at max frequency, and is actually running at 1575 RPM?
You have to figure out what rpm you are running at. Purchase a hand tachometer. The rpm of an asynchronous AC motor only barely changes under normal loading ( the torque speed curve is very steep - minimal slip in that region translates to wide range of loading capacity ). It should be pretty much running at nameplate rpm.
 
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  • #39
Haotranphotomask said:
There is one thing I have not considered. Do I have to factor in the motor efficiency? The nameplate has a motor efficiency of 89.5%.

The nameplate motor efficiency is just the ratio of shaft output power to electrical input power at its rated load.
 
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  • #40
Haotranphotomask said:
I have not measured the actual RPM of the motor. We just ordered a manometer for this purpose. Hopefully we will receive it soon.
Could it be a tachometer instead?

Remember, it is the actual rpm value of the fan what matters for evaluating its performance.
The belt could always be slipping some, specially at high torque.
 
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  • #41
Lnewqban said:
Could it be a tachometer instead?
Yeah it is the tachometer I was talking about.

Lnewqban said:
Remember, it is the actual rpm value of the fan what matters for evaluating its performance.
The belt could always be slipping some, specially at high torque.
Thank you for this information! I was not focusing on the fan RPM.
 
  • #42
Lnewqban said:
Could it be a tachometer instead?

Remember, it is the actual rpm value of the fan what matters for evaluating its performance.
The belt could always be slipping some, specially at high torque.
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?
 
  • #43
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?
Your fan is turning 2727 RPM, and you are measuring 6" WC across fan inlet to outlet, and your fan is putting out 1050 CFM ( as opposed to the curves ##\approx 1700 ~\rm{CFM}##), then you have other issues, because the fan is not running on its "curve".

Are you sure you are measuring the static pressure correctly?
 
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  • #44
erobz said:
Your fan is turning 2727 RPM, and you are measuring 6" WC across fan inlet to outlet, and you fan is putting out 1050 CFM, then you have other issues, because the fan is not running on its "curve".
Yeah when I got the measurements, I was very confused. I am thinking it is due to the 8" trunk being restrictive. I cannot figure out what the problem could potentially be.
 
  • #45
Haotranphotomask said:
Yeah when I got the measurements, I was very confused. I am thinking it is due to the 8" trunk being restrictive. I cannot figure out what the problem could potentially be.

Are you sure you are measuring the static pressure correctly? Where have you taken the measurements. A quick diagram of the fan and surrounding ductwork would be helpful.
 
  • #46
erobz said:
Are you sure you are measuring the static pressure correctly? Where have you taken the measurements. A quick diagram of the fan and surrounding ductwork would be helpful.
Fan.JPG
IMG_0387.JPG

I am measuring the SP at the red circles. The green circle is a backdraft damper. I don't think the pressure drop through that damper is significant.

There is about 20ft of 12" piping on the inlet. But the remaining 8" piping totals up to 200 ft. The inlet ports consist of 1 ft of 3"-5" pipes.
 
  • #47
Haotranphotomask said:
View attachment 318429View attachment 318428
I am measuring the SP at the red circles. The green circle is a backdraft damper. I don't think the pressure drop through that damper is significant.

There is about 20ft of 12" piping on the inlet. But the remaining 8" piping totals up to 200 ft. The inlet ports consist of 1 ft of 3"-5" pipes.
I'm confused. The inlet has 200 ft of piping. The discharge appears to have 20 ft of 12" piping?
 
  • #48
erobz said:
I'm confused. The inlet has 200 ft of piping. The discharge appears to have 20 ft of 12" piping?
Sorry for the confusion. The inlet consists of 200 ft of 8" piping, then expanding tee to 20 ft of 12" piping, and then reduce to 8" to tie into the inlet of the fans. The discharge is another 20 ft of 12" piping.
 
  • #49
Haotranphotomask said:
Sorry for the confusion. The inlet consists of 200 ft of 8" piping, then expanding tee to 20 ft of 12" piping, and then reduce to 8" to tie into the inlet of the fans. The discharge is another 20 ft of 12" piping.
What is the measured SP at the inlet port relative to atmosphere?
 
  • #51
russ_watters said:
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.
 
  • #52
Another thing: When you are only running 1 fan. The other fan is completely isolated from the system via closing off its discharge dampers?
 
  • #53
erobz said:
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.
russ_watters said:
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.
erobz said:
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.
 
  • #54
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.
 
  • #55
Haotranphotomask said:
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.
 
  • #56
Haotranphotomask said:
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?
 
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  • #59
erobz said:
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.
 
  • #60
Lnewqban said:
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!
 

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