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Member

Join Date: Nov 2008
Posts: 6

# Calculating CFM for a Blower

11/05/2008 4:51 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?

This discussion was "closed" on 11/08/2008 4:00 PM. No new comments are allowed.

Guru

Join Date: Oct 2006
Location: India, 200 Km. North of Delhi.
Posts: 1377
#1

### Re: Calculating CFM for a Blower (URGENT)

11/05/2008 11:59 AM

To get the value of free airflow you need to measure air velocity at any one side of fan with a right size of tunnel, and AtLeast should have some flow straightener to make flow laminar.

Better is to get a copy of AMCA108 and follow it.

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Guru

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#2

### Re: Calculating CFM for a Blower

11/05/2008 10:08 PM

Not a Blower man but see if this helps you:

Flow Vs Speed

V2/V1 = S2/S1

Example:

Blower 1750 rpm(s1) delivers 1000 cfm(v1). How many cfm (v2) will it deliver if speed increased to 3500 rpm(s2) ?

V2 = v1 x s2/s1 = 1000 x 3500/1750= 2000 cfm

Pressure Vs Speed

P2/P1 = (s2/s1)2

Horsepower Vs speed

Hp2/Hp1 = (s2/s1)2

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#3

### Re: Calculating CFM for a Blower

11/06/2008 7:08 AM

Best bet is to get the fan curve from the manufacturer of the unit. Get/calculate the static pressure difference and follow the curve.

Guru

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Posts: 4633
#4

### Re: Calculating CFM for a Blower

11/06/2008 8:19 AM

Hello katherina:

is the 'duct' or pipe the same size as your fan? Which is 28"?

And is your mptor speed variable?

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Location: Los Angeles, California
Posts: 35
#5

### Re: Calculating CFM for a Blower

11/07/2008 1:25 AM

Katherina, Here is a partial answer: (1) If motor RPM is 1470, motor pulley is 250mm diameter, and fan pulley is 200mm diameter then fan RPM is 1838 RPM. (2) The blower wheel diameter of 710mm is about 28 inches. (3) To completely describe a blower wheel you must state: diameter, width, openings (One end or both end), and type. (4) I asume "BC" stands for backward curve type, which means it is fairly standard. (5) Know that blower wheels are only part of a system. Presumably the system has both inlet and outlet ductwork. The ultimate performance will be tied to the pressure restrictions of the ductwork.

After you know the fan width (In inches), go to www.forestryforum.com/members/donp/blowercalc.htm and play around. Look for "The Math Behind the Calculator" on the same page. Any calculation with one or more missing parameters will be inherently flawed, However, Good Luck!

Member

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Posts: 6
#6

### Re: Calculating CFM for a Blower

11/07/2008 2:36 AM

the thing is that we fabricate the blower, fan blades, etc all ourselves. so i don't have the fan performance. i don't even have the air velocity produced by the blower. plus, I don't have the equipment to measure the air velocity, etc.

if i wanted to consider the fan blades area and no. of fan blades, also the fan blades curve, does anyone know what is the formulae or calculation that i should do, together with the above information that I have post earlier, in order to obtain the air velocity and CFM?

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#7

### Re: Calculating CFM for a Blower

11/07/2008 4:12 AM

Hi Katherina,

How did you do dynamic balancing of the impeller? I hope you are aware of the dangers of running a rotating part without balancing.

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sisira
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#8

### Re: Calculating CFM for a Blower

11/07/2008 5:44 AM

we have a balancing machine. we did balancing 1st, then we install it. dont worry bout that...

anyone help me with this calculation. i need to know the air flow rate

Guru

Join Date: Oct 2006
Location: India, 200 Km. North of Delhi.
Posts: 1377
#9

### Re: Calculating CFM for a Blower

11/07/2008 9:55 AM

Hi.

1 Do you already have a device for velocity measurements, please specify what kind of is it?

2 Do you want set a production test facility or just want to check one blower?

3 Do want to check airflow only at zero pressure or want to check at at various point?

4 have you made any duct already? if yes what is the size?

5 Can you spend good money on set up? or you just want a make shift arrangement?

6 do you have any standard available with you? and you don't know how to use it or you don't have any standard too?

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#10

### Re: Calculating CFM for a Blower

11/07/2008 11:06 AM

Hi rakesh,

I see you (only you) are moving in the right direction. Measuring velocity of air is the only way confirm air flow rate. Once again air flow rate (cfm) varies with the static pressure.

From what Katerina has told us I can only assume she has copied some existing design of a fan. If she has designed the fan herself she would not have asked the original question in the first place.

Sorry Katerina to be so blunt. when you ask questions in a forum like this you have to come out with the full story!

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#14

### Re: Calculating CFM for a Blower

11/07/2008 9:51 PM
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#15

### Re: Calculating CFM for a Blower

11/07/2008 10:00 PM

GA from me.

Velocity cfm meter also is called Anemometer. She also will need a Pitot tube as she will have to plot the fan curve. If she does not she will be working in the dark.

As Rakesh said if she wants to be precise she will have to build a test rig too.

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sisira
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#11

### Re: Calculating CFM for a Blower

11/07/2008 8:09 PM

Hi.

1 Do you already have a device for velocity measurements, please specify what kind of is it?

ans : no

2 Do you want set a production test facility or just want to check one blower?

ans : i just need to check one blower

3 Do want to check airflow only at zero pressure or want to check at at various point?

ans: i have no idea wat is dis.

i just want to know from my motor spec, the pulley and motor diamater, considering the fan diameter,its blades shapes, thickness and no. of blades it have. I want to know how much is the air flow rate produced. but, if i wanted to know the air flow rate produced, then i have to know the air velocity. right?

4 have you made any duct already? if yes what is the size?

nope. the duct is not done yet. i wonder if anyone knows the formulae and could provide me with one.

5 Can you spend good money on set up? or you just want a make shift arrangement?

i do not understand this ques. we do not want to spend any amount of money to measure its air flow rate.

6 do you have any standard available with you? and you don't know how to use it or you don't have any standard too?

no. we do not have any standard.

i need a formula on how to calculate with considering the fan, motor, etc. does anyone know that?

tq

Score 1 for Off Topic
Guru

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Posts: 4633
#12

### Re: Calculating CFM for a Blower

11/07/2008 8:25 PM

Hello katherina:

I need to know what the fan and duct will be used for. Is it just to warm the factory, or to circulate air, or to remove possible dangerous fumes from whatever you may be working on?

If it is for air circulation, can you tell me the size of the room or all the rooms total size please?

I will look again for the duct. The bigger the duct the less air-flow speed there will be you see Katherina.

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#13

### Re: Calculating CFM for a Blower

11/07/2008 9:50 PM

Hello katherina:

I need to know what the fan and duct will be used for. Is it just to warm the factory, or to circulate air, or to remove possible dangerous fumes from whatever you may be working on?

If it is for air circulation, can you tell me the size of the room or all the rooms total size please?

I will look again for the duct. The bigger the duct the less air-flow speed there will be you see Katherina.

it will be used to remove dangerous particle before it goes to the air. specifically the air will be transferred to a wet scrubber

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#16

### Re: Calculating CFM for a Blower

11/07/2008 10:57 PM

Hello katherina:

You say you will have a water scrubber the air passes through?

What kind of dust will be taken out of the air. And is it a private pass time or a business this will be used for?

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Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=\$=|O|=\$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]
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#17

### Re: Calculating CFM for a Blower

11/08/2008 12:46 AM

yes, industrial usage.

for dust particle such as sand

Guru

Join Date: Jun 2008
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Posts: 4633
#18

### Re: Calculating CFM for a Blower

11/08/2008 5:56 AM

Hello katherina:

have you read my Private Mail about the problem you are having?

Just a thought but which Country are you going to run this Fan in?

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Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=\$=|O|=\$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]