Estimation of fan flowrate 2

[mohtogh]

Hello,
Can we estimate flowrate of a fan with its kilowatt and/or diameter and/or "water volume of it" ?
Thanks

 

[TangoCleveland]

By "water volume of it", might you mean the inches water column? This is a pressure term. You could try checking catalogs for a comparably sized fan (same power, diameter, and pressure). I've done velocity traverses across fan inlets to estimate flowrate as well.

 

[ChasBean1]

mohtogh -

I found this, but haven't confirmed its reliability:

Fan HP = (cfm x S.P.) / (6355 x fan efficiency)

There are 0.7457 KW per HP.
The static pressure (S.P.) is the total across the fan, in inches of water.

This may help, as it relates pressure with KW. (I'm assuming you meant pressure in inches of water when you mention water volume?) Diameter unfortunately doesn't figure into this.

 

[mohtogh]

Thanks friends,
1. What is the range of efficiency for fans?
2. What is the range of S.P for fans ? (I think it has a limit,if exceeding that, it named compressor no fan)
3. I know "inch of water" as a unit of pressure , But my idea is another . From a non-technical point of view,however a "small" fan can not produce 1000000 CFM !Thus CFM relate to "overall volume of fan " .Can you correlate this ?
Thanks

 

[ChasBean1]

mohtogh -

1. 30-60%, including motor losses (probably a good variability here, with some higher than this range).

2. 0.1 to 12 in. w.c. typical, from what I've seen.

3. Might be able to correlate between similar fan types. For example, I don't think water volume of a fan and flow will be ratioable between a backward inclined centrifugal fan and a vane axial fan. But I think there would be a correlation (with some data scatter) between flow and impeller volume for a given set of centrifugal inline fans, and flow and impeller volume for a given set of tube axial fans.

Will any of this help? -CB

 

[mohtogh]

Thank you very much chasBean1 .
In your formula , efficiency is in percent (e.g. 30% ) or in fraction (e.g. 0.3 ) ?

 

[km1997]

You can pretty much scale fan performance using the basic fan law:
flow rate (CFM) ~ diameter^3 * RPM
pressure (in water) ~ diameter^2 * RPM^2
power (hp or kw) ~ diameter^5 * RPM^3


Would like to know how to scale the above with fan thickness, given axial fans?

KM

 

[denniskb]

Go to my website and download FanCalc.xls and it will provide the answer you want.

(Unfortunately the downloads have stopped due to a server problem but should be up again soon) Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[sacem1]

There is an answer about fans turning into compressors relating to max static presure, but there are centrifugal blowers in between and not to mention if you consider a turbine a type of fan and then the pressures would go way high.

I would say that the difference between fans and blowers with compressors is in that the latter are positive displacement units, even if they are screw type, and if enough backpressure is encountered the motor propelling the compressor would be stalled as they can not proceed forward with the compression.
Blowers will also increase their power use if enough back pressure is found but usually will not stall, even if they could burn out the motor by excesive power demand if not correctly protected.
Fans by the other hand simply stop moving air when they find a backpressure that exceeds its capacity.

 

[Tunalover]

What are you trying to cool?

Are you simply trying to determine the fan's unrestricted flow rate? If so, then why not ask the manufacturer? If, like many in the electronics industry, you tend not to believe the manufacturer-published unrestricted fan flowrate curves (static pressure on y-axis and flowrate on x-axis), you can send them out to test labs that can do the test for about $500/fan in less than a week.

If the fan is part of a system that you are trying to cool then the actual fan flowrate will always be less than the unrestricted fan flowrate. I hate to be a "wet blanket", but I'm afraid that to determine the "operating point" of your design you will have to determine the flow impedance curve (static pressure on y-axis and flowrate on x-axis) for the space you're cooling (enclosure, room, etc.). This can get involved! However, once you have this curve, then you can superimpose it on the fan's unrestricted flowrate curve. Where the two curves intersect is the operating point which tells you the actual flowrate and pressure. Usually you can get a decent system impedance curve using a spreadsheet program and common loss coefficients but CFD (Computational Fluid Dynamics) codes are best (e.g. FLOTHERM, FLUENT, ICEPAK). A second option is to build a prototype and send it out for testing. Again, there are test labs that can do this kind of thing. This provides you with what I choose to call Vdot1=flowrate at system operating point.

Once you have a Vdot1, compare it with the one calculated using:

Vdot2=Qdot/[rho*Cp*(Tout-Tin)]

where
Vdot2=the flowrate required to maintain Tout,

Qdot=internal heat dissipation (energy/unit time),

rho=the air density evaluated at Tbar=(Tout+Tin)/2,

Tout=the outlet temperature which usually=the maximum allowable internal air temperature,

Tin=the inlet air temperature, and

Cp=the specific heat of air evaluated at Tbar.

If Vdot1>Vdot2 then the system is adequately designed from the standpoint of heat rejection.

Sorry for a complicated answer but to do it right you really have to go through this stuff.

If you need referrals for test labs, I'll give them to you but will not vouch for them. Also, one good text with examples on determining system impedance curves is David S. Steinberg's "Thermal Analysis of Electronic Equipment", any edition.

I hope this helps!

Tunalover