Calculate cfm capacity for a duct fan

[cityjack]

Good morning,

I have been tasked with exhausting more cfm (adding new equipment) to an already installed industrial exhaust fan that is going to an RTO here at our plant.
I have been told that the capacity of the fan is 15000 ACFM. As I try to eliminate hoods and other not necessary air sources going to the fan to try to free up some capacity for the new equipment, it dawned on me.....how much do I trust "he said she said" tribal knowledge on the capacity of this fan.
So I reached out to New York Blower to get the fan curve on this fan. Fan curves do not make sense to me honestly or are just starting to. This fan curve given to me is for a 250hp mtr and we have a 150hp mtr installed that is running by a VFD. I need to somehow create a fan curve for the way I am running this fan now so I can see if 15000 is indeed the correct number or if its something else.

Please forgive my rooky self as I am trying to learn. I really do notr know how to go about this.

Thanks for any help.

Sid

 

[LittleInch]

City jack, No problem, shit happens.

It is a bit confusing though when you talk about htz in one response wrt output from the VFD, but % in another. % of what?

It sounds though like someone has realised that going to 60Htz at a lower air temperature will overload the fan motor.

From what I gather from your description, you have a relatively fixed outlet pressure (atmospheric minus a small friction loss in the duct, but a variable inlet pressure. Therefore the request from the vendor might not be feasible. Biggest thing he can do for you is to give you the curves and power at different speeds. Power he might only have at 70F, but if he could do it at your actual temp (225F) that would be even better so you can see what your max flow condition is.

As said before, to make sense of the different numbers floating around, you either need to calc forward and back to standard conditions or use mass (lbs, kg, whatever) and then convert back to volume where required at the temperature you have at that point.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cityjack]

My apologies Little. Its always Hz.

Yes, I have heard that the lower temp/higher density of air will fault the drive. You are correct, the inlet pressure to the fan varies based on the product we run.

I just asked DURR to give me the fan curve using my 150hp motor at the static pressure they designed it for when new assuming the ceramic bed would be pristine and unclogged. I will also now ask for different speeds/Hz. Thank you for this.

When you say calculate forward to standard conditions, are you saying scfm? Standard temp, elevation, and air density? DURR is telling me the fan performance numbers on the print I sent are ACFM, which makes sense given the different temps and elevations. But they are telling me that the 31,429 acfm is 15000 scfm at my pressures. Here is where I am getting lost.

Sid

 

[AgMechEngr]

It is sort of like choosing coordinates. Once you start with a condition, stick with it.

Yes, scfm is standard. ACFM=SCFM at standard conditions. 14.7 psia and 70 deg. F.

 

[LittleInch]

city jack / sid,

I think my calcs also showed that at the max power of the motor at your operating temp (225F) it was about a max of 40 htz so that makes sense.

when you're working with gases especially, there are two ways of looking at flow.

ACFM - Actual cubic feet/min is exactly what it says ( the actual volume) - but this number for the same mass of gas varies by temperature and pressure so you always need to quote the temperature and pressure that the acfm refers to. Just saying xxxx ACFM without sayin @ YY F and ZZ pressure is meaningless.

SCFM - standard cubic feet per minute - coverts acfm to a fixed volume flow at "standard" conditions. There is unfortunately no fixed definition, but commonly this is 101625 Pa (sea level) and 15C / 60F.

So in your case the 31,429 is acfm at 1,100ft altitude and approx 400F. So the density of the air changes from ~ 0.075(@70F and SL) to ~0.041 (400F and 1100ft altitude).

So when they calculate SCFM, 31,429 acfm turns into ~15,000 scfm. There's clearly some other factor other than density going on here (viscosity etc), but if that's what the vendor says then I can believe it.

Does that make sense?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.