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Anonymous Poster

Calculate Air flow

06/26/2007 9:27 AM

How can I calculate the air flow produced by a blower in a pipeline if I only have blower power, rpm,dimensions and pipe diameter.

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Active Contributor

Join Date: Apr 2007
Posts: 16
#1

Re: Calculate Air flow

06/26/2007 9:58 AM

Q=flow(m3/s)

Q=Speed*Area

Area pipe=(pi*d2)/4

Speed=Volumen blower/(area blower*RPM)

be careful with units.

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Member

Join Date: Mar 2007
Location: INDIA
Posts: 8
#2
In reply to #1

Re: Calculate Air flow

06/27/2007 12:25 AM

please clarify how you have calculated speed of blower?

Thanks and regards,

Vishal

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Anonymous Poster
#8
In reply to #2

Re: Calculate Air flow

06/27/2007 2:08 PM

It is volumen of air in blower more exactly.

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Anonymous Poster
#3
In reply to #1

Re: Calculate Air flow

06/27/2007 12:35 AM

my next question is how to calculate the volumen blower & area blower?

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Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
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#4

Re: Calculate Air flow

06/27/2007 4:27 AM

You haven't mentioned blower delivery pressure. This should be fairly simple to find, by fitting a gauge or from system characteristics, eg if air is blown into a tank below water level you can work it out from depth above injection point, plus a bit for pipe friction loss.

You have blower power - you don't give details but you need blower shaft input power (motor nameplate power doesn't tell you much). Can find this from motor current and motor efficiency. Allow a few % for V-drive if there is one.

You can then back-calculate flow from the usual power formula. Need to know or assume blower efficiency η. Depends on type of blower but eg Roots type has efficiency 60 - 65% depending on size, bigger machine higher eff. In any consistent set of units

Power = P1*Q1/n/η*((P2/P1)n-1)

where P1 and P2 are absolute pressures in and out, Q1 is inlet flow (actual conditions) and polytropic exponent n = 0.23.

All this depends a bit on system size and pressure, but you say blower, which implies discharge pressure ~ 0.1 - 1 barg and above is OK for that.

Codey

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Associate

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#5

Re: Calculate Air flow

06/27/2007 11:27 AM

You also could use the BHP definition, but will need suction and discharge pressure:

BHP = (Q*S*TDH)/82

Where,

BHP: Break Horse Power

Q: Flow (ft3/s)

S: Relative density

TDH: Total Dynamic Head (ft)

Note that the operation principle is the same used in pumps calculation. Keep in mind that the BHP definition includes efficiency concept.

BSChE.

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Guru

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#7
In reply to #5

Re: Calculate Air flow

06/27/2007 12:16 PM

Hello Jolsac - not sure about your formula. I make it

HP = Q(ft3/s)*density(lb/ft3)*TDH(ft)/550 and putting density = 62.5*S (water is 62.5 lb/ft3) gives HP = Q*S*TDH/8.8. But this is the fluid HP, BHP is the shaft power into the pump = fluid HP/pump efficiency. Taking pump efficiency say 65% (they vary quite a bit) gives BHP = Q*S*TDH/5.72.

If you meant BHP = (Q*S*TDH)/8.2, works back to eff 8.2/8.8 = 93%, more like motor eff than pump.

Also these formulas are applicable to liquids, but except for very low pressure rise aren't accurate for compressible fluids.

Cheers....Codey

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Associate

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#9
In reply to #7

Re: Calculate Air flow

06/27/2007 2:47 PM

Hi Codey.

It seems you are right, I have been checking the numbers and when using 550ftlbf/s (hp), the formula gives 8.8. Let me go further into the document and find out where comes the 82 from (sounds like a dot is missing).

Thanks.

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Anonymous Poster
#6

Re: Calculate Air flow

06/27/2007 11:30 AM

Buy a damn pitot tube and manometer

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Anonymous Poster
#10

Re: Calculate Air flow

06/28/2007 12:11 AM

Go to the EBM Ziehl website - you can download the calc's and their product line chateristics there will give you all the answers, but a pitot tube & manometer is the best in practical determination

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Power-User

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#11

Re: Calculate Air flow

06/28/2007 8:54 AM

Added into any calculation would have to be the efficiency of the blower. It's not going to be 100%.

The standard formula for a fan in I-P units (cu-ft per minute, total pressure across the fan in inches H2O, and brake horsepower)

bhp = (cfm * fan-pressure)/(6356 * efficiency)

so cfm = bhp * 6356 * efficiency/fan-pressure

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Guru

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#12
In reply to #11

Re: Calculate Air flow

06/28/2007 11:48 AM

Hello Howetwo - as you say, your formula is for a fan, which has pressure rise typically a few inches water, depending on type. But the original post refers to a blower, which usually means ΔP a few metres water (up to 10m for Roots type).

The fan formula is not applicable for higher ΔP due to compressibility, which reduces the power for given inlet flow and ΔP. Please see #4 for formula. This reduces to the fan formula when ΔP << P1 (the n drops out).

According to my calc, for P1 = 1 bara and ΔP = 0.1 bar (=40" water, ~ max for fans), fan formula gives power high by factor 1.038. Of course it gets worse for higher ΔP.

Cheers....Codey

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Anonymous Poster
#13

Re: Calculate Air flow

07/29/2007 6:20 PM

Take A traverse air flow reading (measured in FPM) and calculate Sq. footage of pipe diameter (PIE R Square) Divide by 144 (Sq Ft). and Multipily FPM X Sq.Ft. of pipe to get CFM.

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Power-User

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#14
In reply to #13

Re: Calculate Air flow

11/04/2007 12:41 AM

Now that was a sensible answer....

All those other formulas have variables that are being guessed at so they would skew the answer to uselessness.

I've compared the traverse air flow readings along with flowhood measurements and compared the results against manufacturer's listed airflow calculation depending on the current draw of their fans and found that their method is not too accurate. Nothing is as good as actual air-flow readings and use above formula for CFM.

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