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Calculate cfm capacity for a duct fan

Calculate cfm capacity for a duct fan

(OP)
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.

Thanks for any help.

Sid
Replies continue below

RE: Calculate cfm capacity for a duct fan

First, please use full words initially as not everyone uses the same abbreviations - RTO?

Trust no one and any flow figure is only at a specified differential pressure / inches / mm of water. You need the fan curve before you can do anything.

You have the wrong fan curve and can't do anything with one that has that different a power ratio.

Is this for the same actual fan? same design and diameter or what?

Can you post what you've been given as we're working in the dark here.

What make (new YOrk BLower?), model, power, size etc? What does the nameplate say?
what speed does it run at?
Can you make the fan run faster? You could then use the affinity laws, but first yu need to know what the unit is dong now.

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

RE: Calculate cfm capacity for a duct fan

You will need to use a tachometer to determine the RPM of the fan at its current operating point. Drill a hole in the duct and use a pitot tube with a magnehelic gauge to determine the velocity. The magnehelic will give you the velocity pressure (Total pressure - static pressure) in inches of water column (in. w.c.) or velocity directly depending on the gauge. You can convert velocity pressure with fpm = 4005*sqrt(in. w.c.). With this information, you will know the flow rate. You now have enough information to select a fan curve based on the measured operating RPM and see where along the curve the fan is operating based on the flow rate. If you would like to change the fan speed, look up fan affinity laws.

RE: Calculate cfm capacity for a duct fan

(OP)
My apologies.

RTO is Regenerative Thermal Oxidizer. We use it to burn off volatiles in our exhaust/process stream so we can vent to atmosphere.

I have attached the fan curve as well as the print for the fan. The current motor is at 150hp and runs at 1790 rpm at 60Hz. Full load amps is 175. Currently, when VFD is at 30Hz, I am pulling 95amps.

Same exact fan being used. I have several cfm readings and static pressures recorded along several different trunks of duct leading to the main 36" Diameter duct going to the RTO fan outside the plant.

The VFD changes speed based on a Delta P across the RTO. The larger the pressure drop, the faster the fan spins. Basically, 20HZ or so at startup up or running idle then ramps up to 30Hz at max drop across the RTO.

Whats on the nameplate of the of the New York Blower fan gives you the fan print attached.

I hope this helps you help me.

thank you so much guys.

Sid

RE: Calculate cfm capacity for a duct fan

(OP)
I have the rpm of the fan at load from a ratio calculation and sheaves based on the print. At 30Hz, mtr is spinning at 895 rpm ish and the fan is at 1173 rpm.
I have measured all air velocities/cfm of all trunks contributing to the main 36" diameter trunk. I have also measured the cfm through the 36" duct going to the exhaust fan. I currently have roughly 9500 cfm going to the exhaust fan and thats at 30Hz.

My goal is to remove as much unnecessary air/cfm going to the exhaust fan so I can put another piece of equipment on the RTO with the current fan. I have been eliminating hoods and other sources of air to reduce the 9500 cfm giving me more fan capacity. I just do not know if the existing fan really is at a 15000 cfm of capacity.

I hope this makes sense of what I'm trying to do.

Sid

RE: Calculate cfm capacity for a duct fan

As you change the ductwork, you are altering the static pressure. Once you have settled on the overall duct layout of the system, calculate the static pressure of the system at your desired flow rate and see if your fan is adequate.

RE: Calculate cfm capacity for a duct fan

The drawing thoughis showing two things - it seems to be designed to work at 1100 ft altitude, but more importantly at between 380 to 430F, hence the density of the air is a lot lower and hence the power requirement is a lot lower.

The 250hp is for ambient temp air.

What temp is your air at and what altitude are yu at?
What is the differential head across the fan - that's the crucial missing bit of information here

Forget the 15000 figure - it's just getting in the way.

It is clear though that you have some spare capacity in the system . However I don't think you'll get to 60htz as the power generally goes close to ratio of speed ^3.

So to max out your motor, you're looking at a speed increase of your fan up to about 1450 rpm. That would generally give you a flow increase of about 2000 cfm.

But everything is changing so your flow at a given pressure drop may change as well.

Best thing to let everyon see what is going on is sketch a flow diagram with sizes, temperatures, flow rates, pressures now and what you want ot change as words are not helping. We can only see what you tell us.

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

RE: Calculate cfm capacity for a duct fan

(OP)
Okay little inch. Thank you for your work. I have a screen shot of a whiteboard that I have a sketch on. I also have temps and flows for each pipe. Let me put something together. Give me a few please.

RE: Calculate cfm capacity for a duct fan

(OP)
Here is the screen shot of my white board.

Green is air flow direction. Purple SP is sample port of the approximate location of my pitot tube readings. Air temp at the SP4 is 100F. Temp at the very inlet of the main RTO exhaust fan outside is 224F. Thats basically the air thats coming out of the RTO after being treated at 1500F. Altitude is 1180' above sea level here in Hickory NC.

The red "MAG" is the magnehelic that controls the speed of the E. That E fan sends all the exhaust from process through the 36" duct out to the RTO main fan that then exhausts to atmosphere.

Unfortunately, I do not have the delta P across the fan. That area is wrapped with a bunch of sheet metal over a bunch of insulation.

I'll be back. I have to go over to the coater where this process is.

Thank you so much.

RE: Calculate cfm capacity for a duct fan

Still not quite sure what you're trying to do here, but there are a few factors that you need to allow for, if this fan is the one on the exhaust of the RTO?

1) Your inlet temp is 224F?, but the fan design seems to be around 400F. This makes a lot of difference to air density which is why your motor and fan will never get to more than about 40% of its rated speed if the DP across the fan doesn't suddenly get a lot less before the motor trips on excess amps.

2) So it looks to me pretty much like your fan is only good at 225F for about 11,000 CFM give or take. But the devil is in the detail.

3) As said above, it all depends on where the variable pressures are. Does this fan exhaust to the atmosphere via a fairly short duct?
If so then increasing flow / speed of the fan will lower the pressure on the outlet of the ROT to presumably allow for more flow.

does the controller control to maintain a set differential pressure across the RTO or?? Does the fan speed influnece the pressure drop or is that basically a crude flow rate measurement. I'm confused exactly what this fan is doing for you.

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

RE: Calculate cfm capacity for a duct fan

It sounds like proportional control for flow rate control. This is definitely more of a challenging problem than the initial problem statement led me to believe. What are the exhaust requirements for the additional equipment?

RE: Calculate cfm capacity for a duct fan

That's what I'm thinking as well.

Would be great to know the whole picture at the start, but suspect the fan is there to try and keep the whole system upstream the fan below atmospheric pressure?

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

RE: Calculate cfm capacity for a duct fan

(OP)
LittleInch;

1. Yes this fan in question is the one on the exhaust of the RTO. Yes the inlet temp at the RTO exhaust fan is 224F. I understand 400F being a less dense air volume than 224F. But I do not understand how/why you say this "This makes a lot of difference to air density which is why your motor and fan will never get to more than about 40% of its rated speed if the DP across the fan doesn't suddenly get a lot less before the motor trips on excess amps". Is it because the fan needs to work harder (greater fan load) at 224F thereby drawing more amps at the increased load? The DP across the fan getting less allows for more air to be pulled through lessening the load on the fan which lowers the amps?

2. I do not see where you get 11000 cfm at 224F. Can you scribble your calcs PLEASE. I need to learn or see your method.

3. Variable pressures upstream are in the 6, 8, and 12" ducts based on what product we run. The pressures and temps on the white board shot is our hottest running product. This is when the delta P across the RTO is at its highest and fan goes from 20Hz to 30Hz then pulling the 95amps. The exhaust from the RTO is an approximate 12' stack to atmosphere.

The Controller for the VFD seems to react to the delta P across the RTO. As the delta P goes up, so does the speed of the fan. It does not appear there is a setpoint in the program that the fan is trying to maintain.

I hope this makes sense and I apologize if I am not articulating this very well. This project was dropped in my lap and I am trying to learn and lend help/a solution to their new equipment coming in. Its either make the existing work with the new equipment coming, or purchase something new.

When you say "what is the fan doing for me", it allows us to exhaust to atmosphere the newly "scrubbed" air from the RTO. By permit of course and by removing the oily mist volatiles from our process air stream.

AgMech;

I do not know what "proportional control for flow rate control" is. Please explain sir. The new equipment has been spec'd by our equipment builder to need 6300 cfm.

Keeping the whole upstream system below atmospheric pressure? Unfortunately, this system was design in 1999 and put in in 2000. All the old guard is gone, companies have been bought and sold, I am left with what I have trying to figure out the controls ands what controls what.

Sid

RE: Calculate cfm capacity for a duct fan

Sid,

First off I've suddenly realised that one of the issue I think is getting in the way is the way you look at gas flow. When you have temperatures more than 10-20 F apart, the volume and density makes it difficult to compare like with like. Either you need to add pressure and temperature at each flow number or convert to standard conditions or use mass.

So e.g. your 8500 CFM at 100F is not going to be the same CFM at the entry to your blower at 225F, but a bigger number ( approx 10,400 CFM)

Q Is it because the fan needs to work harder (greater fan load) at 224F thereby drawing more amps at the increased load?
Yes compared to the data written on the drawing you sent in, which used approx 400 F. This makes the same ft^3 weigh approx 25% more.

The DP across the fan getting less allows for more air to be pulled through lessening the load on the fan which lowers the amps?
I might have got this wrong to be honest, but in general less DP means less work so less power.

Q I do not see where you get 11000 cfm at 224F. Can you scribble your calcs PLEASE. I need to learn or see your method.
Ok, I need to revise a bit now I've allowed for temperature changes. I'm also using the fan affinity principle which tends to get a bit inaccurate when you have large changes, but this says flow is proportional to relative change of speed, head is prop to change^2 and power is prop to change ^3
But here goes:
Your current fan is now doing 10,400 CFM @ 225F and drawing 95 amps versus 175 A full load at an RPM of 1173 vs full speed of 2200.
Now in affinity laws, your power should be 1/8 of the max ( 1173/2200) ^3 =0.15 so 27A. Now add in for the higher density at 225F vs 400F x 1.25 = 34A. Now this is a LOOONG way from your 96A so this is one reason why the affinity laws don't work very well when you get big differences in speed or size.
But if you reverse this and say 96A is correct due to inefficiencies and other fixed loads, then the reverse is:

The max power increase you can get is a multiplier in speed of 175A/95A = 1.85. The cube root to get to a multiplier of speed is about 1.23. so an RPM increase up to 1173 * 1.23 = ~ 1450. The same increase applies to the flow rate so your max power flowrate is 10,400 * 1.23 = 12,800 CFM @ 225F.

Now that could easily be a higher number due to better efficiency of the blower at higher RPM.

Now this is all a bit inaccurate because you don't have the fan curve at different speeds. See if the blower company can give you the fan curve at different speeds equating to your frequency of 20,30,40, 50 htz etc.

Now the 15,000 cfm case MIGHT be that somewhere the figure is stated as max flowrate at standard conditions (60F, sea level) for your 150hp motor. It's certainly about that number.

You need to get the best data you can from the vendor, but if not try doing a series of monitoring or tests on the blower to get htz, fan speed, flowrate somewhere, temp going into the blower.

But if you're adding another 5,500 CFM @ 100F into the RTO then to me your fan looks to be close to or maybe over the max power limit. There is a big discrepancy though between what the fan data sheet says it can do which is about 23000 cfm @ 225F once you correct for higher density at lower temperature.
So my back of a fag packet numbers are giving you a flow @ 225F of somewhere between 13,000 and 23,000 cfm (!) Reality is probably somewhere in between as at that lower RPM, I can only imagine that the efficiency is really quite low, but that's the data you need from the blower vendor. Note on your fan curve, the power curve starts at about 70hp (!!). So there seem to be a lot of fixed losses somewhere and hence extra flow doesn't take the same amount of power per 1,000 ACFM.

I think in essence what your controller is doing is trying to maintain a fixed pressure going into the RTO from the ducts. Hence as flow increases, DP across the RTO increases and then the exhaust fan increases speed to create a lower pressure in the fan inlet as the outlet pressure is fixed (atmospheric minus a small amount for losses in your exit duct).

So in summary, Try getting the fan curves, even at sea level and 70F for different fan speeds. Then you can determine the reduction in power for the lower density of air at your operating temperature (it's about a 70% reduction in power for 70F to 225F) for the same actual flowrate.

Does this make more sense and others please pick holes in my assumptions here....

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

RE: Calculate cfm capacity for a duct fan

Quote (LittleInch)

Now this is all a bit inaccurate because you don't have the fan curve at different speeds. See if the blower company can give you the fan curve at different speeds equating to your frequency of 20,30,40, 50 htz etc.

Accurate fan curves are important to see what you have, so do as LittleInch says. I also agree with LittleInch's latest post.

My advice on how I would approach this: I think of these problems in a conservation of mass sort of way. As Little has alluded to, temperature is affecting volume, but you aren't losing or gaining mass. This is assuming a system with no leaks, but this assumption is perfectly fine. Work backwards from all hoods. What volume do you need at each hood? This will drive everything, just like a load calculation for power. At each junction, sum the total flow from the contributing branches until you finally have a sum of flow at the fan in question. This is the bare minimum of what the fan must handle. Calculate the static based on the ductwork length, fittings, etc. (The Industrial Ventilation Handbook is a good guide). Using your required cfm, static, and fan curve at 50 hz, see if the fan is capable. If it is, see if your motor is capable. If the fan or motor are inadequate, go shopping.

I hope this helps.

Sidebar: If you need a new fan, select a fan on the descending side of the efficiency curve. In the real world, the fan will actually operate more efficiently.

RE: Calculate cfm capacity for a duct fan

(OP)
Good morning LittleInch sir,

My apologies for not getting back sooner as I saw your reply over the weekend, but didn't want to try to digest looking at my phone. We had a team mate here at work with a family emergency and our schedules all have been turned upside down. I am just now getting the chance to look at your help.

First, thank you again for your help and taking the time. I really appreciate this. After speaking to our controls team, they did tell me that the RTO fan, the main fan outside is controlled by a PID loop. What he said was, when the fan has no load, is less than 20% on VFD, or the process is idle, keep the speed at 20%. If the load on the fan is greater than 40% on the VFD, do not let the VFD go higher than 40%. So basically, if the pressure is low enough that the fan wants to run below 20%, the VFD keeps it at 20%. If the pressure makes the fan want to run higher than 40%, the VFD keeps it at 40% and will not let it go higher. Any pressure in between 20 and 40%, the VFD allows the fan to float between 20 and 40%. I hope this makes sense.

I had a rep from New York Blower out yesterday and he said to help tailor the fan curve that I gave you guys, he really needs the static pressure at the inlet to the fan, just like you guys said. So I am going to work on that.

I also have a meeting here with DURR systems in about 90 minutes to try to get some understanding about the current fan and the controls. They are the group that installed this RTO/control system back in 2000 when the company was called REECO. DURR bought them out.

I wanted to reply back to you guys, but honestly, I have to digest your reply and look at your calcs. Thank you again for doing this.

I will write back once I speak with DURR today and hopefully shed more light.

Have a nice morning.

Sid

RE: Calculate cfm capacity for a duct fan

(OP)
Working on it Ag. Thank you sir. He told me yesterday he can give me all the curves I want, but they need a static pressure on the inlet of the fan. I have cfm going to the fan and amps at certain Hz, but no pressure where they need it.

RE: Calculate cfm capacity for a duct fan

Good luck, Sid. You are on the right track.

RE: Calculate cfm capacity for a duct fan

(OP)
Thanks Ag. The vote of confidence definitely helps.

I'll let you know.

RE: Calculate cfm capacity for a duct fan

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.

RE: Calculate cfm capacity for a duct fan

(OP)
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

RE: Calculate cfm capacity for a duct fan

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.

RE: Calculate cfm capacity for a duct fan

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.

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