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Join Date: May 2007
Posts: 15

# Calculating CFM for an Exhaust Fan

07/10/2007 7:17 AM

Hi, Can anyone help me how to calculate cfm of exhaust fan with undernoted parameters.

Motor 2 HP, Dia of fan 24", 1400 rpm, 50hz/220v.

Thks.

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Anonymous Poster
#1

### Re: Calculate CFM of exhaust fan

07/10/2007 7:19 AM

The shaft work required from any fluid mover equals the pressure rise times the volumetric flowrate, all in compatible units.

Guru

Join Date: Jul 2007
Location: Minnesota
Posts: 529
#2

### Re: Calculate CFM of exhaust fan

07/10/2007 7:46 AM

Have you checked the manufacturers catalog? Often they have charts or formula's

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Anonymous Poster
#17

### Re: Calculate CFM of exhaust fan

11/08/2008 7:25 AM

i don't check the manufacturs cataloues for cfm.please give the full details.

Anonymous Poster
#22

### Re: Calculate CFM of exhaust fan

07/26/2009 11:28 AM

how to calculate the cfm of the ext fan

Anonymous Poster
#24

### Re: Calculate CFM of exhaust fan

02/03/2010 4:14 AM

Blower Details

HP 5

Pressure 20 inch

Off Topic (Score 5)
Anonymous Poster
#21

### Re: Calculate CFM of exhaust fan

07/16/2009 3:47 AM

hi i want the calculation of CFM for Hot Air Blower motor capacity - 160 Hp RPM - 1440 regarding the salt refinery for 75 tons per hour (TPH)

Power-User

Join Date: Jun 2007
Location: Birmingham UK
Posts: 253
#3

### Re: Calculating CFM for an Exhaust Fan

07/10/2007 8:45 AM

What kind of fan is it axial/centrifugal/simple blade. What if anything does the fan manufacture's data say?

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Anonymous Poster
#4

### Re: Calculating CFM for an Exhaust Fan

07/10/2007 12:49 PM

Hi,

Calculation of CFM as per your data is a manufacturer design engineer's field. there are many factors to be consider. engineering furmula's have been modified with so many options to arrive due to many features are added.

I dont think you are on this field, are you?

If anybody have given you the specs of the fan without knowing the CFM, try this method and compare the specs from the manufacturers equipment data that are vaillable in the net or to the dealers nearest you.

CFM=volume of space in cubic feet / rate of air change in minutes.

there is a standard table for "air change in minutes for adequate ventilation" that correspond to the type of building.

once you get the CFM requirements. ask to the exhaust fan dealer and you will get the total specs of the equipment.

regards

Power-User

Join Date: Feb 2007
Posts: 104
#5

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 11:14 AM

I work in the HVAC industry, and I work with this data everyday. Everyone has told you this, but they are right. You need to know more then just the HP and that it is 24". Blade type is very important as well as the manufacturer. If you can give me a model number and a manufacturer I can get you the fan data. If you only know the blade type then there is a general formula that will give you an approximate CFM with any given HP. But it is very general. Each blade type is different for every manufacturer. What you really need is the fan curve for this blade type and configuration. The fan curve will give you the CFM at any given Static pressure.

If you can't find any more info on this fan then I need to know at least what they of fan it is. Even the application would help.

Commentator

Join Date: Mar 2007
Posts: 58
#11

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 4:43 PM

Air capacity CFM of any fan tightly depends on the ductwork system resistance.

Usually a fan is selected for designed system, not system for a fan.

In spite of your "work in the HVAC industry" you did not realize that static pressure in not proper basis to select a fan. Fan must be selected on basis of total resistance of system i.e. fan total pressure.

HTTP://mysite.verizon.net/restkgfn/airpolcontrol

Any questions apcco@verizon.net

Anonymous Poster
#14

### Re: Calculating CFM for an Exhaust Fan

09/29/2007 9:16 AM

Anonymous Poster
#20

### Re: Calculating CFM for an Exhaust Fan

03/04/2009 3:22 PM

i need to know how to calculate an exhuast fan ... ive tried everything but i dont really know how to do it. I was wondering if you can help me out my manufacture is a baldor industrial motor with an rpm of 1725 the catalog number is rl1319a but i cant find anything of the cfm

Anonymous Poster
#6

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 11:48 AM

Turn it on, find our the speed of air as simple as a vane type speed of air marker.

Once you have the speed of air find out the diameter of the fan find out the area multiply it by the speed of air and units should be converted to cfm.

Luis Carlos Iglesias jimco421@yahoo.com

Thanks till next time

Anonymous Poster
#7

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 11:49 AM

The air flow depends on the geometry of the fan (blade quantity, style, aerodynamic characteristics, etc) - it is not simple.

However, you can estimate the air volume from the catalog of the mfr of your fan or one that looks similar.

Or you can measure the discharge velocity and use the continuity equation to estimate the flow: (velocity) x (Area) = flow [fpm x sq ft = cfm].

Associate

Join Date: Dec 2005
Posts: 54
#8

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 12:21 PM

The size and length of your exhaust duct, if any will will affect your backpressure, and thus, your CFM's.

Power-User

Join Date: Nov 2005
Posts: 116
#9

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 1:05 PM

Any calculation you make will still be theoretical. I would stick some sort of anemometer or pitot tube into the flow path and obtain a direct reading. Many such tools are available depending on the accuracy you require. A pitot tube will be more work because you will have to take readings from the center to the wall of the duct in some increments (the more readings you take the more accurate your profile will be).

Just grab some mechanical vane anemometer (probably the most cost effective solution), or, if you do this work all the time, invest in a nice one with high accuracy.

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Active Contributor

Join Date: May 2007
Posts: 15
#10

### Re: Calculating CFM for a blower

07/11/2007 1:32 PM

Actually I want to calculate CFM of a blower which is 24" diameter. The blower has 3 normal blades. It is operated by a 2HP motor which rotates ar 1450 RPM.

thks.

Power-User

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Location: Bolingbrook Illinois, a southwest suburb of Chicago.
Posts: 367
#12

### Re: Calculating CFM for a blower

07/11/2007 5:46 PM

Could you at least give us the approximate "pitch" of the blades?

RichH

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Posts: 24
#13

### Re: Calculating CFM for an Exhaust Fan

07/11/2007 10:29 PM

Yes.

1. The efficency of the motor is typically 75 to 90 %. You may find more on the motor tag, my guess is the lower than the higher.

2. The efficiency of the fan may be 70% to 90 %, lower for simple sheet metal blades, higher for twisted blade or blade integral with the hub and small tip clearance. 60% if the fan is not ducted.

3. Hence, e.g. 2 hp x 0.75 x 0.6 = 0.9 HP is transfered to the air.

4. Energy balance: 0.9 x 735 (W per HP) = pi/4 x (Dia x 0.0254)^2*AirDensity/2 x Velocity^3

Hence Velocity = {(0.9 x 735)/[3.14/2 x (24 x 0.0254)^2*1.25/2]}^0.33

Volume = Velocity x 3.14/4 x (24 x 0.0254) in cubic meters

For cubic feet you multiply by 1/(0.305)^3 which is about 30.

Anonymous Poster
#15

### Re: Calculating CFM for an Exhaust Fan

06/19/2008 7:37 PM

HUH?!?!

Anonymous Poster
#16

### Re: Calculating CFM for an Exhaust Fan

11/05/2008 4:26 AM

does anyone know how to calculate a blower CFM, given:

Motor Spec:

motor RPM = 1470rpm

3 phase induction motor

Type Y 180L-4

415V, 40 Amps

Power = 30HP / 22kW

Frequency = 50Hz

diameter of motor pulley = 250mm

Blower spec :

diameter of fan pulley = 200mm

Fan diameter = 710mm

Thickness of blades = 3 mm

Type of fan = Centrifugal Fan (Type BC)

Efficiency = 85%.

Does anyone know how do i calculate the flow rate of the blower from the given motor @ fan RPM?

and if i want to know the air velocity produced at input and also at output?

Participant

Join Date: Feb 2009
Posts: 2
#18

### Re: Calculating CFM for an Exhaust Fan

02/27/2009 4:08 AM

Hi Everyone,

Am a new bee to this Feature. Am creating a simulator application for HVAC. I also need to calculate the CFM of exhaust fan. Is there any generalized formula to calculate the approximate air flow rate of exhaust fan given motor & fan specifications. If anyone know, please do tell me. It'll be a great help for my application. Thanks in advance..

Regards,

Vijipriya Rajamani

Off Topic (Score 5)
Participant

Join Date: Feb 2009
Posts: 2
#19

### Calculating CFM for an Exhaust Fan

02/27/2009 4:14 AM

Hi Everyone,

Am a new bee to this Feature. Am creating a simulator application for HVAC. I also need to calculate the CFM of exhaust fan. Is there any generalized formula to calculate the approximate air flow rate of exhaust fan given motor & fan specifications. If anyone know, please do tell me. It'll be a great help for my application. Thanks in advance..

Regards,

Vijipriya Rajamani

Anonymous Poster
#23

### Re: Calculating CFM for an Exhaust Fan

08/20/2009 11:46 AM

calculate air flow of blower and devided it by 1.699, you will get CFM

Anonymous Poster
#25

### Re: Calculating CFM for an Exhaust Fan

02/17/2010 5:40 PM

I understand you need the fan curve if you want to be remotely accurate and my exercise is pretty ridiculous unless you know what I was doing, but please remember I just need to be in the ball park:

Riddle me this: what is the collective HP of all HVAC fans in a building, given only their annual natural gas bill?

Given their gas bill and an assumption of annual heating hours, I get the CFM.

As if that wasn't enough, this is where you REALLY start to hate me, I want the total HVAC fan HP in the building. These are all manufacturing facilities so I think that you should be able to pick an average duct work pressure drop, blade type etc. in order to pick a rule of thumb HP per CFM, but what would it be? Most facilities are standard roof top MAUs so I feel like there should be a rule for this.

Any help?

Anonymous Poster
#26

### Re: Calculating CFM for an Exhaust Fan

04/01/2010 7:15 PM

there ought to be some way to take the area of the intake and the rpm of the fan to get a decent result. after all, assuming standard air density, there is going to be a limit to how much can get in at a certain speed, right?