Reversing Fan Direction

[Lmandrake]

If you do that you will basically get one square wave 12v signal per revolution. I don't think the standard tach signal is 12v and I am pretty sure that most tach outputs for fans are two square waves per revolution.

[jafb2000]

o Correct re 12V
---- it will generally blow the fan tacho monitoring circuitry
---- which can be by smoke, and may affect the chipset

o You can have 1 pulse per revolution on some fans
---- however far more usually it is 2

Note some motherboards have an option to set a divider
for the tacho pulses. It depends on how much the BIOS
writer abstracted from assumed to user-configurable.

[Starman97]

Umm... why dont you just flip the fan over if you want to reverse the airflow?

[deepfried]

Hi this is my first post =')

I was searching for this topic on google and found this thread, I hope its ok to raise this thread after so long. Reason I post is because I would like to figure out if it is at all possible to reverse the direction of airflow through a PC fan. I have 2 power supply fans I have recovered from junk computers, and I would like to experiment with counter rotating fans. My idea is similar to the way a jet turbine works, having 2 fans counter rotating but the direction of airflow being the same.
To my disapoiment I have realized that simply reversing the polarity on the fans will not work. So I hope someone can give me a hint as to what might work. Thanks for taking the time to read.

[DrMemory]

Yeah, your totally crazy. The hall effect switch is used to sense the position of the rotor so the electronic control circuit knows when to turn the windings on/off at the right time to keep the motor running at the right speed, and to sense the speed of the rotor. It supplies one of the functions of the commutator of a DC brush motor. The other function of the commutator, turning the windings on and off is supplied by the electronic conrtol IC. Attempting to bypass the hall effect switch and supply the windings directly will not work unless you want to manually turn the voltage to the windings on and off at the right time. Of course this would drastically reduce the speed at which the rotor turns. There may be a pin on the electronic control IC to reverse the direction. That would depend on the make and manufacturer of the IC.

[deepfried]

would reversing the polarity of the magnet inside the fan hub work? Just an Idea, I would try it but I only have the 2 fans and im afraid i would ruin one needlessly in the process. Thanks for replying, I know this is kinda silly. Just something to keep my mind busy for the time being.

[jafb2000]

You don't need counter-rotating blades to do what you want.

When you put 2 fans serially in-line (intake-2 to exhaust-1), you
generally effectively boost static pressure but not additively airflow.
o Technically you can occasionally boost cfm beyond that of a single fan
o More usually, you boost static pressure and so realise closer to free air cfm

For example, your enclosure has a static resistance 5Pa and you fit a single
fan of 25cfm in free-air, and you find your in-enclosure free air is 18cfm. If you
fit another 25cfm fan serially in-line, your in-enclosure free air is 24cfm.

This doesn't come at a cost however - noise :-)
o Additionally you need to vary the separation between 2 inline fans
o At some separation you will minimise noise for the given benefits

In reality this method is only used for redundant fans:
o You use tier-1 fans, NMB, Panaflo or EBM-Papst, some Nidec
o You fit them serially inline for redundancy
o Reason is not so much re risk of failure - but risk of service personnel
---- it allows YOU to choose when to replace the fan, not when the server does
---- it reduces the risk of disrupting other rack servers (common coloco downtime cause)

Many fans produce air in a Tunnel Vortex (eg, Panaflo) others more Radial.
The ideal fan type for making it work is Tunnel Vortex where noise is not so
critical, or fans which use an airflow straightener on the outlet - eg some Delta.

In general, you will suffer more turbulence noise, as the airflow impacting on
the leading edge of the foil will be at an angle it wasn't designed for. Thus the
blade will suffer more stall, moving it down the P-Q (Pressure-Airflow) efficiency
curve, with the resulting increase in noise from that inefficient blade turbulence.

Axial fans produce very little static pressure, but high airflow:
o Typical fan produces 0.1-0.2" elevation in a H2O (water) manometer
o Human beings can produce 16"+
Radial fans produce lots of static pressure, but low airflow:
o Typical fan produces 1-4" elevation in a H2O (water) manometer
o So whilst the cfm is less, *more* cfm is realised in high resistance applications

The only real use for radial fans (blowers) on PCs is for extremely high resistance
heatsinks comprising a very long, very dense, very deep, skived copper method.
Even here, axial fans can perform better where they are designed for such, an
example is the 40x28mm Sanyo high-velocity axial flow fan - and 40dB(A) too.

1U rack cases will be moving to a dual-motor, concentric shaft fan eventually.
It will give the cfm of an axial fan, but the static pressure of a radial fan - with
the result you get >25cfm at 1.2" of H2O for dense 1U applications, and 56dB(A).

Better to engineer a design to utilise the airflow in m/sec over a large surface
area, at least for lower noise, than rely on a pressure-orientated solution (noisy).
--
Dorothy Bradbury
www.stores.ebay.co.uk/panaflofan for quiet Panaflo fans & other items
www.dorothybradbury.co.uk (free delivery)

[deepfried]

Wow nice reply. I appricate your time and value your knowledge. I may try 2 fans serially in-line, I'm sure it would be a much more effective configuration than my idea. Your knowledge on this subject dwarfs mine, while I understood the majority of what you said some of it was lost to me. My experiance with electronics and airflow are all on a hobby level, as I have no formal education in either subjects. Again thanks for your reply, I really appricate your time.

[jafb2000]

My pleasure.

As always:
o Change 1 variable at a time
o Measure temperatures after it

That way you can best establish cause & effect.

If you can, always vary fan distance from a heatsink or such high airflow
resistance object - over a certain range of distances there will be little
variation in temperature, but considerable variation in noise.
--
DB.