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Airspeed of cooling fan and input electrical power amount

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Andy

Sat Oct 25 2014, 03:45AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi
I would like to use a fan for cooling and was wondering if theres a way to work out the airspeed aprox of it, and the input electrical power needed for that speed.

Cheers
Andy

Sulaiman

Sat Oct 25 2014, 06:21AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Most axial fans are specified by
Supply voltage
Supply current OR Wattage
Airflow, usually in c.f.m. (cubic feet per minute)
and airspeed=cfm/sq.ft. (area of fan)

e.g.

Andy

Sun Oct 26 2014, 12:03AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thanks Sulaiman
Wasn't thinking, just thought about fans you buy at the hardware store.

Andy

Fri Nov 07 2014, 11:23PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

I'm trying to cool down a LED grown light system I'm making but need to workout some air pressure calculation.

On mouser and element14 they have 28-31/m3/min fans, and was wondering what would the pressure be or more the foumla if they went into a small area, but with a opening the other end of the area at 1 atm.

If for eg the fans workout at 0.47m3/sec and that went into 0.016m3 area, would the pressure increase 28 times or would it go up alittle and the velocity increase aswell.

Still alittle lost at the variables I need on the fans.
Side note can a air compress be run without a storage tank?

Cheers all

Erlend^SE

Sat Nov 08 2014, 12:08AM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Water cooling?

Sulaiman

Sat Nov 08 2014, 01:03AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Have a look at 'static pressure' in a datasheet for an axial fan .... it's miniscule
axial fans are fairly efficient at moving volumes of air at small pressure differences.
Tangential fans / blowers can generate a higher pressure.

BigBad

Sat Nov 08 2014, 03:41AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Yes, axial fans can generate decent pressure... if you put a whole lot of them in a row like jet engines do.

Like 20 axial fans spaced along a tube. It's not really practical. Jet engines only do it because they need a small frontal cross-sectional area.

If you overload the fans, the wings that are the blades will stall out. That's why you need so many; individually they're wimpy.

But pressure is not a lot of use. Fluid cooling is mostly about the fluid speed; because high speed gives a thinner 'boundary layer'. Thin boundary layers allow the heat to escape from the LED. Roughly, halve the boundary layer thickness=twice the heat loss. Inside the boundary layer, the air isn't moving, so it acts as an insulator; that's why it's so important.

If you want to know whether the pressure will go up appreciably; calculate the airspeed from conservation of mass. If the airspeed is significantly less than the speed of sound, then it won't.

Andy

Sat Nov 08 2014, 04:10AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thank you you three

Water cooling is out, as the design circuit board/LED/heat transfer to copper wires like a hedgehog going out the back of the PCB eg are already worked out, saying that its still in the design stages :?

I had a look at blowers, but the psi is still only about "1" which wouldn't help unless multi units could be used to add up.

The fans in series might be doable, but why do you say that if the speed is less then the speed of sound then it won't work?

The pressure will get changed to speed hopefully near the wires, theres planning to be 9 pcbs 300mm*300mm with 50 LEDs each, and a op-amp mosfet current source, with 3 heat sinks for 3 mosfets, run at around 2.2*50-2.7*50 volts in series.

Cheers all

BigBad

Sat Nov 08 2014, 04:26PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

No, I'm saying you don't want pressure you want airspeed.

Airspeed thins the boundary layer around your LEDs.

To calculate the airspeed divide the area the air is moving through into the volume of air the fan delivers.

Basically if you've got a fan that is 0.01 m^2 area and it delivers 0.05 m^3 per second then that's an airspeed of 0.05/0.01 = 5m/s.

Ash Small

Sat Nov 08 2014, 07:26PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Put a fan on the inlet and one on the outlet, you'll move a lot more air this way. If you need more airflow, use more fans, more inlets and more outlets. This is generally the best way to move air. placing them is series will move more air, but not as much as in parallel.

EDIT: Please see my comment below.

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