138 posts
TimePosted 18/11/2010 07:29:59
lalbatros says

Re: Fan law

Hello Hussaini,

When I say volume, I really mean volume.

You could compare a fan to a bucket elevator.
Filling the buckets with sulfur hexafluorid (an heavy gas) or with wool or water or cement or steel, won't change the volume (m³) of the buckets. This will only change the amount of matter being elevated (Nm³ or moles, g, kg, tons).

FLS experts might not always be interrested in volumes but rather in gas quantities. The quantity of gas is directly related to the (kiln) production while the volume also changes with temperature. In this case the experts may need to convert measured volumes to quantities of matter using the density.

However, they can also be interrested in volumes of gas. Specially, when fan capacity is considered!

The fan laws are:

power varies like

RPM³ * Rho

pressure varies like

RPM² * Rho

mass flow varies like

RPM  * Rho

volume flow varies like


where Rho means the density.
Therefore also, constant power and constant RPM are not the same thing. 

What are the laws for bucket elevators?


I don't know what a  PG Test is.
I assume this is not a pregancy test!


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84 posts
TimePosted 18/11/2010 10:15:23
Kumar says

Re: Fan law

Dear lalbatros,

                         PG Test means Performance Guarantee Test.Whenever a new plant is installed or modification is carried out in the existing unit the machinery supplier assures the client of certain output and before handing over the plant to the client this test is carried out.


79 posts
TimePosted 19/11/2010 09:17:35
chari says

Re: Fan law

Dear Mr Lalbatros


what you mentioned is the change is figure with change in RPM of a fan. which is the fan law.

The law again states the change in figure if RPM remains constant and density of the gases change due to either temperture or dust load or altitude.

Air Volume 1 / Air Volume 2 = 1

Static pressure 1 / Static pressure 2 = density 1 / density 2

Brake horse power 1 / Brake horse power 2 = density 1 /density 2

Efficiency 1 / Efficiency 2 = 1

Noise level 1 / Noise level 2 = 1

( This is from Fan handbook by Frank P. Bleier )



41 posts
TimePosted 19/11/2010 13:03:48

Re: Fan law

Hello Michel Yes you Assumed it Right it's Not Pregnancy Test !! As said by DOGRA it's Performance Guarantee Test...

BS-848 (British Standard) is widely used in cement industry for Testing the performance of fans... Here is excerpt of BS-848 to calculate volumetric flow rate for Preheater Heater Exhaust gases

                          CASE - 1     CASE - 2

Temp        oC          292            320

Pressure   mmWG   500            500

Density   kg/m3       0.657         0.625

?P        mmWG        30               30

K                            0.83           0.83

DIA         m             3.65            3.65

Area      m2          10.46           10.46

Velocity   m/s      24.85             25.46

Vol'Flowratem3/sec 259.9          266.3

Where ?P is Dynamic pressure, K is pitot tube constant , g is accelaration due to gravity = 9.80665 m/s2

Density = (Absolute Pressure in Pa * Mol.Wt of gas)/(8314.4 * T in oK)

Velocity = K x SQRT(2 x g x Density)/Density

Volumetric flow rate = Velocity x Area

As it is very clear from above calculations, in case-2 All parameters being constant except Temperature which is increased By 28 oC there was decrease in density and Volumetric Flow rate was increased by 6.4 m3/sec....?

Mr.Chari I hold a copy of Fan Handbook, Selection, Application and Design, By Bleier, on Page 5.21 it reads as CFM1/CFM2 = 1 Which supports michel's explanation with bucket elevator and directly contradicts with BS-848 Ain't it?

Prove this wrong !!