Serial Fans Push Harder?

I once mounted an IN fan and an OUT fan on a sealed box that contained an instrument. I tested for more output by removing the IN fan and monitoring the out fan. My test was simply streamers on the output fan, but it convinced me more air flowed with two fans. As I recall these were 4.5 in fans.

Mikek

I thought it was the opposite - axial fans are better at blowing rather than sucking.

Eg for my fume extraction I have an inline axial fan, I have mounted it close to the fume input to maximise flow.

Of course there is more flow. But is there as much as mounting them side by side?

NT

FWIW, My IN/OUT fans were on top of the box about 6 inches apart. Mikek

If it's for cooling, the series arrangement is redundant and the parallel one isn't. In a parallel array, a lot of the air from the good fan will blow backwards out of the dead one.

Cheers

Phil Hobbs

Den torsdag den 17. juli 2014 10.13.53 UTC+2 skrev snipped-for-privacy@gmail.com:

you'll have to look at the datasheet to see how back pressure etc. affects performance

you can get dual motor fans

-Lasse

some pc boxes have several input fans to make sure cold air gets blown onto the hot bits, and a single output fan to get the air out the right way, that also keeps the inside at a positive pressure so dust doesn't get sucked in through the cracks

assuming all the fans work ..

-Lasse

I think that for such an arraingement to be efficient, the fans should be dissimilar. Just like a cascaded steam engine uses graduated piston sizes, and the impellers in a turbine have different pitches, one fan shouls "load up" the other in a way.

Take a vacuu cleaner and plug up the hose. The motor RPMs goes upbecause of the vacuum created at the air inlet. This partial vacuum lowers the load on the impellers.

So to get more air, the idead would be to load up as much air at the intake side as possible.

I can't say for sure that the pitches if the fan blades should be varied from front to back of such a compound system, but it does seem to make sense.

Another example would be a multi-stage compressor, or lab quality vacuum pump. Each stage is not necessarily identical.

no. mostly it's because the blower is doing less work when it's not pumping air. block the outlet and the motor will also speed up.

In general, my insight was that fans like to operate in "free air." So I'd think that they have a non-linear flow vs. resistance curve - if you block a fan just a little, then the flow goes way down. So if you put 2 in series, then each fan is closer to the ideal, and you get closer to 'free air' for both.

In general, I am intrigued with low-flow, quiet room filters which have large areas and are driven by small fans or blowers. Advantage is too pull Radon daughters or viruses out of the air.

Anybody got a low-cost Geiger counter design that works?

Look up pump curves

NT

if your purpose is radon remediation, there are other considerations besides just airflow. Such as,,,, you don't want to de-pressurize the living area. A good strategy is to de-pressurize under the slab and pressurize the living area.

Are you able to detect radon with a geiger counter? I have an old 60's CD geiger counter (which you can get on ebay) and have not found it very effective for measuring radon, but in theroy it should be able to detect it's daughters.

Mark

I've done it, but the outflow of a fan is turbulent, and the result when that hits the input of the second fan, is noise. It got more loud than anything got cool.

I remember someone trying that at a company where I worked, and what was accomplished was a really weird oscillation without enhanced cooling.

Where it works best is using an inlet fan to suck thru a filter, then an exhaust fan for the box. ...Jim Thompson

A vacuum cleaner fan has stacked blades off one motor. More force. I once used three fans stacked, to pump air through a long pipe. Must have been at least 30 foot. Was for an incubator to be used in a MRI, eggs !

Greg

In a turbine you have two fans on one shaft in a cascade to produce more pressure. So to compress the stream further you need a higher pitch on the second blade. Fans as input and output on a box aren't the same. They may move the same air, but once the input air stream enters the box it is no longer a stream. The output fans then can use the same pitch to move the air out of the box. The output stream is not much compressed at all since it is blowing into a low pressure space. The output fan just lowers the pressure inside the box allowing both fans to move more air than one alone. As someone mentioned, these fans are very sensitive to back pressure.

This is pretty much right for axial fans although I'm not sure why you call that non-linear. If you look at the specs for one of these axial fans used for electronics cooling you will find they are in fact pretty linear. They do however have a steep curve that drops off quickly with pressure, stalling at just an inch of water or less.

Yes, Charles Wenzel (techlib) has been a good source of designs in that area. The pin diode detector has small area, but it's nice to be solid state.

As far as Radon detection, I mispoke, as I'm interested in the daughters, The reason is physiological: They get stuck in the lungs and cause cancer via short range radiation. Radon itself is fairly harmless, since it diffuses throughout the body.

jb