Driving fans faster by extra air flow

[Cicero]

Will there be any adverse consequences of driving fans faster than their design speed by supplying extra air flow from a larger fan?

The real situation concerns one of my computers. It is running with the sides removed. It has the usual CPU fan and and an 80mm system fan extracting at the back. I think that the effect of this fan is minimal at the moment with the sides removed.

I have an ordinary domestic fan directed at one side of the open case (the more open side - not the back of the motherboard). When this fan is blowing air into the computer the speeds of the CPU fan and the system fan increase by about 5% (roughly 2500 / 2600 rpm). This is obviously the effect of the larger external fan driving the two internal fans as the speeds consistently increase / decrease as the external fan is switched on / off.

Can the fans be damaged, or are they likely to go up in smoke in this situation?

Tia.

Cic.

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[Ian Stirling]

Double the RPMs for long periods - or do this with large fans - or those with AC motors, and you might get problems. For PC fans and such as samll mount, it's utterly not an issue.

[The Natural Philosopher]

Yes, but its not needed with sides removed.. I spent many many years designing electronics, and the slow buildup of heat inside a closed box

- or even one with vents in it - is staggering when you first come across it. Only an extractor fan stabilizes the interior of a CLOSED box to ambient..but with the sides off, convection will suffice.

I am not sure this is really necessary, unless you are above 40C ambient..the chips won;t want to go above 100C case much..but the localised fans will probably do this anyway.

I can't see any issues with small fans being speeded up.

[Colin Wilson]

I think that depends on the design of the case - IIRC someone on one of the computer groups (Dorothy Bradbury, who incidentally sells the rather nice Panaflow fans) described some systems as having a ducted flow, and with the side removed it can actually restrict the airflow / cooling effect.

If Dorothy reads this and i`ve attributed it wrongly, sorry - but I got a plug in for your Panaflows anyway :-)

(if it wasn`t Dorothy, I think it must have been someone who used to lurk on the LinuxEmporium forums, and did work on nuclear subs (!))

[Cicero]

======================== Many thanks for all replies. It's good to know that there's no danger of damage or a call to the local Fire Brigade.

The possibility of overheating in this weather is a bit of a worry although my temperatures (Sempron 2.4 - 27 degrees case and 50 degrees CPU) are not excessive. My main concern is to ensure that these temperatures don't escalate out of control - hence the external fan. It seems that the fan isn't a good idea unless it's switched on before the computer is switched on because I got a very sharp fall in temperatures when the fan was suddenly switched on after startup. This could be almost as harmful as a bit of overheating so I think better case ventilation is the answer.

Cic.

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[Colin Wilson]

I don`t think i`ve had a CPU yet that ran at less than 62 degrees off- load !

[meow2222]

Fraid this isnt so, several puters rely on the controlled forced airpath for cooling of some parts, and opening the case stops the flow in some areas and can result in overheating. Many P1 CPUs were unfanned and worked like this, but there are also other chips on a modern mobo which sometimes require this forced cooling. If you want to run a case open you need to check the temp of the various chips, check nothing gets too hot.

I dont understand why youre concerned about overheating with such low temps. You could add another 100C to that before any damage occurs.

a nonissue.

NT

[<me9]

I've only had one that anyway aproached that. I ditched that box a while back. The current one is running at 53C with the room at 26C.

[The Natural Philosopher]

If its true, then yes, but I have yet to see a machine made like that.

Proper forced air cooling is nearly always BLOWN not SUCKED.

The way the cooling fan on a PC CPU BLOWS.

[The Natural Philosopher]

Your chips will not burn out till over 100C case. They will stop functioning correctly at somewhere around 60-80C and -15C.

I have had SPARCS go completely weird..memory faults, crashing programs with random code execution..and become quite normal once a seized CPU fan was replaced.

The temperatures in many machine rooms are well in excess of what we are seeing right now. Around 45 degrees in one I had control of before we installed aircon..the machines took it.

[Colin Wilson]

IIRC boxes made by Sun were like this...

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Cicero" saying something like:

Two months ago I fitted a 120mm fan to the side casing of my PC, blowing in towards the CPU fan, so has exactly the effect you describe (the CPU fan tacho is indicating that it's running slightly faster than before).

No problems so far, and temps are at least 10C lower when working hard and 20C lower at normal load.

[meow2222]

many compaqs too

NT

[marvelus]

Mine sucks and blows. A 6 incher on the front blowing over the HDs and a couple of sucking 3 inchers on the back, one of which is in the PSU.

[Richard Cole]

Currently CPU at 45C, room at 28C

(That's a 120mm fan on the CPU + 2 smaller vans sucking out of the back of the case + PSU fan).

Richard Web pages:

- caravanning, - personal web site and I loves the domain name for email.