138 posts
TimePosted 19/11/2010 21:55:06
lalbatros says

### Re: Fan law

Hello Hussaini,

This BS-848 example is easy to understand if you notice that the pressure has been kept constant. To keep the pressure constant, it has been necessary to change the rotation speed of the fan.

To analyse this, let us again write the fan laws:

 volume: V2 = V1   w2 / w1 pressure: p2  = p1    (w2 / w1)2   (r2 / r1) power: W2 = W1   (w2 / w1)3   (r2 / r1)

Therefore, we can also write:

 pressure: p2  = p1    (V2 / V1)2   (r2 / r1) power W2 = W1   (V2 / V1)3   (r2 / r1)

and therefore:

 V2 = V1   (r2 / r1) -1/2  (p2 / p1) 1/2

In the BS-848 example, this gives numerically:

 V2 = 259.9 (0.625 / 0.657)-1/2 (1)-1/2  V2 = 266.47 m3/s

This value is very close to the result obtained in case 2 (within 0.06%), in good agreement with the fans laws.

In the same way, you can easily calculate by how much the speed and the power has changed.

Michel

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79 posts
TimePosted 21/11/2010 04:07:17
chari says

### Re: Fan law

dear Mr. Hussaini,

I think the data of case 1 and case 2 are given for the same system and fan duct and same resistance circuit.

let us go from back side , the volume is high in case 2  , the velocity is also high in case 2  than the pressure of the system will also increase as square of velocity. where as the pressure mentioned is not changed at all (500 mmWG in both cases)

if you think in other way, if pressure (system resistance)is not changed than how the velocity is changed and volume changed.

I hope you may check the readings again because it contradicts geometry of fan and RPM.

chari