flow rate and static pressure

[macmet]

I am curious to know whether the power of a fan can be determined knowing the flow and static pressure. I have tried calculating it but always come up below the value given in our supplier's specs.

I do not understand why power = density * gravity * head * flow rate does not lead to apparent correct result.

Can anyone help me?

Regards

 

[walkes]

Check out this link for energy approximation tips:

http://www.kele.com/archive/solutions/Eats.pdf

Fan H.P.= CFM x SP/(6355 x efficiency)

 

[macmet]

Thanks walkes.

I'm doing this to set up a spreadsheet to save sometime in the future. Can you tell me what a typical fan efficiency would be?

Cheers

 

[lilliput1]

Typical fan eff = 0.65
but small fans can go down as low as 0.40

 

[cme]

I've never seen a fan hit its curve .....

 

[DRCommisioningEng]

it is rare to see a fan hit its curve but it does happen occationally, usually the reason why a fan doesn't hit its curve is because it is hard to get accurate suction and discharge pressures at the fans.

 

[friartuck]

Air Power=pressure x vol flow

N/m2 (Or pascals) x m3/sec = watts

This does not include for fan efficiency which has already been suggested. It varies not only between fan types but for each fan. The efficiency will vary depending on where the fan is operating.

With regards to the fan 'hitting its curve'.

A fan is tested in 'ideal' situations, i.e. a nice straight inlet and outlet condition (For an inline fan).

If you change the inlet and outlet conditions, i.e. you do something silly like install it in a real system with a bend on the inlet and a transformation on the outlet, you will actually change the fan curve. ASHRAE I think calls this the system effect (Or is it the fan effect)...either way, the system will affect the fan in such a way that it no longer behaves the same.

So when a commissioning engineer tries to measure the fan duty and operating point, it is common to see that the fan is not operating on its curve. This only hapens when you have a goos system design/installation with straight inlets and outlets.

Friar Tuck of Sherwood