Calculating CFM from a fan

[jmorgan]

Any tips? I know the size, not sure of the speed but its around 30mph with an anonomuyterthingamybob in the air flow. Suppose there is a lot more to it than that. No info with the fan BTW.

Need to know where I stand with it. That is bin it or carry on with it in sorting a slight cooling problem I am slowly fixing.

Cheers

[cptsideways]

Work out the area of the fan, = sq ft x ft per sec from the speed. Should = CFM.

However not as simple as that, as its down to where the air has to go & where it comes from too.

[GreenV8S]

Measuring the air speed is the hardest part. If you then convert that from miles-per-hour to feet-per-minute and multiply by the fan area in square feet you have got the cfm value you wanted. It will change depending how much obstruction there is though.

If you have cooling problems, getting enough air flow is obviously important (and getting the hot air out of the engine bay is almost as important as blowing cold air over the rad), but it is far more important that the air you blow though the rad is cold fresh air. If you reduce the air flow by 50% you only reduce the cooling a little bit. If you replace 50% of the cold air with hot air you get 50% less cooling. If you see what I mean!

[jmorgan]

cptsideways said:

Work out the area of the fan, = sq ft x ft per sec from the speed. Should = CFM. However not as simple as that, as its down to where the air has to go & where it comes from too.

Ah right,. Cheers. Have to close a few gaps as well.

Get the calculator out after.

Edited to add, Sorry Peter, missed that in replying to Declan. Info noted and will be used. Cheers.

>> Edited by jmorgan on Wednesday 7th July 20:47

[jmorgan]

Getting around to the maths now. I get 1440 cfm. Think my sums are wrong. Thats for an 11 inch.

So 30mph converts to 2640fpm then pi r sq gives 28.2 inches the further sums get 1440 cfm. And I have 2 of them.


Bugger.

[Le TVR]

Not quite so simple as the flux velocity of the air blown is not constant across the diameter of the fan. The 30mph is probably the peak reading?

Typical 11 inch fan (Tripac etc) drawing around 9 Amps would be 900 - 1000 cfm. But this figure could drop drastically with pressure, ie any obstruction such as radiator etc..... Axial fans like these lose cfm rapidly as the intake is obstructed.

[KDNGRIFF]

Check out the Kenlowe.com website. They have all the calcs for their own different sizes of fans.

[jmorgan]

Flow was taken at the peak point. One thing I did notice was that there ias a lot of dead area behind the fan. Airflow seems at its best away from the center by further than I would have expected and at an angle. It would seem that the last 2" of the fan actually dooing something. Not thought about it a lot before but there is an awful lot of the fan area that appears to be in the way so to speak.


As for Kenlowe, I have looked at their site, cheers. As these are new I wanted to see if I could stay with them. 170 squids and all that down the drain.


Live and learn.

[Mr2Mike]

jmorgan said:

Flow was taken at the peak point. One thing I did notice was that there ias a lot of dead area behind the fan. Airflow seems at its best away from the center by further than I would have expected and at an angle. It would seem that the last 2" of the fan actually dooing something.

That is typical for an axial fan. Obviously the tips of the blades are moving much faster than the center, and there will be no air flow at all at the hub. The blade pitch is usualy coarser toward to middle to compenstate to some extent. An axial fan also centrifuges the air, so a lot of it is thrown out at an angle. This is why an axial fan is far more efficient if it is ducted.