Fan capacity Calcs 2

[mateno]

How can i calculate cfm of air exhaust from any fan, i know that fan diameter and rpm have something to do with it but waths the right formula for making this calculation?

Thanks

Tomas Juanz

 

[willard3]

Fan Laws

The Fan Laws are the basic proportional relationships between fan speed, flow, pressure, and power. They are most useful for determining the impact of extrapolating from a known fan performance to a desired performance.

The most common change made to a fan is that of altering its rotational speed. For a given speed change percentage:

1. Flow change is directly proportional.
2. Pressure changes by the square of the proportion.
3. Power changes by the cube of the proportion.

A detailed explanation follows:
The Fan Laws are frequently used to estimate air moving device requirements in both of the above situations. Additionally the Fan Laws can be used to calculate the performances of air movers of different sizes and speeds, assuming proportionality regarding linear and angular dimensions, fluid velocities, and internal fluid forces. The basic relations for air movers are:

CFMa = CFMb X (SIZEa / SIZEb)3 X ( RPMa / RPMb) (eq.1)

PRESSa = PRESSb X (SIZEa / SIZEb)2 X ( RPMa / RPMb)2 X (pa / pb) (eq. 2)

Hpa = HPb X ( SIZEa / SIZEb )5 X ( RPMa / RPMb)3 X (pa / pb) (eq. 3)

where the a and b subscripts denote the fan under consideration and a previously tested fan, respectively.

Note that the cfm, size and speed per eq. 1 are independent of density, and that horsepower per eq. 3 is simply the product of flow and pressure per equations 1 and 2.

In all of the Fan Law equations, the fan or fans are assumed to have the same efficiencies at the various operating points under consideration.

 

[MintJulep]

The fan laws will not answer the question asked however.

You need the performance curve of the blower in question.

 

[ChrisConley]

The reason you need the fan curve is because there are many more factors that influence fan performance than just wheel diameter and rpm: static pressure, style of fan (forward curved, airfoil, etc.) blade design, fan design.

 

[Golestan]

If you want to start at the beginning, you need to go to the turbomachniary handbooks and fluid mechanics texts to be able to find the air flow produced by any fan. Then of course, the rpm and other factors will come into play.

 

[lilliput1]

Is there balancing data for the installation? Can you measure airflow using pitot tube traverse?