Cooling performance with 2 fans?

I've got a piece of lab test equipment that's about 30 years old. The cooling fan is very loud and I'm looking for ways to quiet it down.

I measure the fan's noise at 53dB (using A-weighted dB meter at 1m on-axis), although the fan mfr only rates it at 39dB. I attribute the discrepancy to "marketing reality distortion" or "wishful engineering".

The fan has no obstructions, front or rear, in the near airflow path. It does have an input screen, but removing this doesn't change the measured noise.

The design information I have found attributes the majority of self-generated noise of a free-standing fan to the air turbulence caused by the motor support struts.

The fan has these specs, according to the mfr's web site:

115v, 75Vmin; 60hz; 0.180a; 14.5w; 2650rpm; 90.0cfm; 0.22in h2o / 55.0Pa; 39dB.

I've looked around for a fan pushing that much air and quickly realized that a quieter one probably can't be found. So I'm looking at other options.

From the information I can find I understand that using 2 fans in series increases static pressure; in parallel, increases CFM. If use 2 slower fans stacked together in series, will I be able to obtain the same cfm while running at less-than full speed? It looks like static pressure won't be a problem with 2 fans. (There is no way of adding fans in parallel without major enclosure hacking which is not an option.)

This is where I've been getting my info from:

Any suggestions would be greatly appreciated.

Thanks,

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DaveC
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DaveC
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On Sat, 17 Mar 2007 10:11:18 -0700, DaveC Gave us:

How about age of the bearings and crud on the fan blades?

Reply to
MassiveProng

Any chance of using ducting to move the fans to somewhere else? What I am thinking of is the plastic wire-reinforced concertina tubing supplied with many air conditioners/ bathroom exhaust fans/tumble driers/etc.

A short length of ducting to an acoustically damped box with a suitable fan may be a solution.

Otherwise, if there is room in the kit, an internal heat exchanger with an external low or zero noise radiator can be very effective. The sort of thing used for many PCs is what I had in mind. That may elinminate the need for an external fan on the kit. Car heater matrixes are an excellent source of small heat exchangers/radiators.

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Sue
Reply to
Palindrome

Excellent suggestions, Sue, as usual.

Hadn't thought of this. Definitely one possibility, since the equipment is "planted" more or less permanently in that location on the bench.

I walked through Fry's the other day and looked briefly at PC moboards. Plumbing! I hadn't looked closely at any since PIII days. What a surprise!

No one source of heat in this box that I could focus a heat exchanger at; it's a huge old rack-mount HP siggen stuffed to the hilt with analog discretes. I suppose just getting cooler air inside via plumbing of some kind means that the air would not have to be moved around as fast as it is now. Hmm...

Thanks!

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DaveC
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Reply to
DaveC

Possible no need to focus that much - if you use a PC kit, just bolt the "processor" water cooling block to a conventional finned & fanned processor heatsink and put it in the equipment case. That will cool the air circulating inside the box and possibly mean that you don't need to pump in external air at all. You may want to add an over temperature alarm/cutout as you may not notice if the cooling is working or not.

Car scrapyards are a good source - newer cars have quick release plumbing and the interior heater and turbo inter-cooler heat exchangers are just about the right size, YMMV.

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Sue
Reply to
Palindrome

But the external unit will need to push air, of course, to get rid of the transferred heat. Unless I put the cooling lines through the wall, or such, I'm back where I started from: "wossssssssh" (c:=

Reply to
DaveC

On Sat, 17 Mar 2007 17:28:46 GMT, Palindrome Gave us:

He could even switch to DC fans at that point, and use a thermally controlled version that only spools up fast when needed.

Place the thermal monitor in the exit stream, OR on a critical component/sink.

Good call, BTW.

Reply to
MassiveProng

If you put the microphone right in the airflow it is likely that the flow of air over the microphone caused the microphone to perceive more "sound" than actually existed. Have you ever heard a TV reporter standing in a windy place. Even with a windscreen on the microphone these effects can easily produce the level you observe. Special methods have been developed by the computer industry to measure these small fans.

The common solution is to use two slower fans in parallel. Each would provide half the cfm needed, but running at the slower speed they typically produce much less sound and also as another benefit typically use much less electricity. Considering your space constraint, the other common option is a fan with more agressive, more carefully designed blades that can turn more slowly but still move the same amount of air with less noise. I am not sure if such are available for small applications such as yours. On large systems variable speed drives or belt drives can be used to adjust speed.

Reply to
Noral Stewart

that's *one meter*. At 1m, that'd have to be one *huge* fan to generate wind noise over the mic...

But not the fan industry. All the specs I've seen show 1m, on-axis, as the accepted method by fan manufacturers.

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DaveC
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Reply to
DaveC

By far th simplest is to slow down the RPM by whatever means is convenient. Judging by the CFM, it's not very big. Unfortunately, the only option is to apply a lower voltabe by means of a variac or step-down transormer...

My approximate experience is that by halving a fan RPM, the noise emission reduces by about 20 dB. Adding a second similar fan, the sound increase by 3 dB for a net gain of 17 dB.

If the airflow that results from half speed is sufficient, you have a soultion.

If the room is big enough and you are far enough away, the sound level might be that low.

The vast majority of noise from such small fans is blade turbulence noise with a low level tone at the blade passage frequency.

The problem stems from ecomomy and space. It's much more possible to sell many more small fans (running at high RPM) that also will fit many more places, than it is to sell large fans that turn slowly (thus being quieter). So guess what's built and stocked for sale!

Find a way to operate the fan at reduced RPM.

Angelo Campanella.

Reply to
Angelo Campanella

On Sun, 18 Mar 2007 22:43:21 GMT, Angelo Campanella Gave us:

AC fans are typically synchronous and slow very little by reduced voltage.

Reply to
MassiveProng

Too risky, I think. This is a waveform generator and is sensitive to heat, I suspect (haven't yet used it to the point where I need it to be really stable for a period of time...).

I plan to use a larger fan in a remote location (3-feet away) placed in a box and connected to the generator with concertina hose. The large fan will turn at reduced speed pushing the same cfm.

All that remains is a simple way to calibrate my new fan's speed so it is pushing the same cfm.

Ideas?

Thanks,

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DaveC
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Reply to
DaveC

The obvious solution is to control the fan speed with a temperature sensing circuit of some sort, preferably designed to default to full-speed if it fails. Something as simple as a forward biased diode, an opamp or comparator, a trimpot & a transistor, mounted in the hottest part of the case would be enough. Calibrate with a thermometer & the pot. Easy.

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Reply to
Lionel

Not sure how much effort you wanna put into this, but if you wanna go deeper than rules of thumb (lowering the fan speed or using temperature control etc.), you might wanna take a look at

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(app. 1 Mb). Hope, it explains the problem statement and issues you are dealing with.

You can skip the first half of the article since it deals with Low noise design and machinery acoustics and put your effort on the case. To my knowledge those are almost the only result dealing with comparison of single and parallel fan operation in this kind of application. Please, correct if i'm wrong.

And about two fans in series. It depends on how you do it, but basically you should be able to degrease K. The benefit depends on what happens to airflow velocity components (swirl?).

BTW, any comments, public or private, on the writing are welcome. But let's not get into a dB discussion (this is for acoustics group), please:)

BR,

ari

PS To my understanding most research done by this kind of fan manufacturers in last almost 10 years has been concentrating on motor development.

Reply to
ari

Not necessarily. It is a matter of the flow velocity. Even small fans that do not produce much cfm can produce velocity. A velocity of 1500 ft/min or

25 ft/sec will produce about 53 dBA reading on a microphone with a 90 mm windscreen even if the actual sound heard from the fan is much lower. If there is no windscreen on the microphone, the indicated level will be much higher, so even a lower velocity could produce the 53 dBA. I do not believe small computer fans would typically produce 1500 ft/min, but if someone put a microphone without a windscreen in the flow, I could conceive some microphones giving 53 dBA.

I do not know what fan sizes or manufacturers you are referring to. Large ventilating fan sound is measured using reverberant rooms to establish the sound power emitted rather than a level at a distance.

This document illustrates the use of what they call a "Maling Box" after the inventor George Maling of IBM. The box is also sometimes called an "INCE Plenum" as it was standardized by the Institute of Noise Control Engineering.

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Reply to
Noral Stewart

Two fans in series will do worse than one of the same fan, except where the static pressure is very high. Lower speed fans stall at lower static pressure (in general) but you indicate no major obstructions.

One way I do this and avoid enclosure hacking is to hang the replacement fans outboard in some manner. Tangential blowers can be very quiet and can deliver as much static pressure as a boxer fan. But, their shape is very different.

Jon

Reply to
Jon Elson

i like the extenal sucker arrangement using ducting, allows to place motive fan elsewhere, even directed to a sound damping box.

perhaps even consider reverse flow of the air stream? if possible may allow the unit itself to 'absorbe' the sound energy within the mass of the unit. may be tricky though.

fan blades with little or no struts and obstruction always are quieter, no turbulennce or like.

newly designed fans, the ones actually 'designed' to reduce turbulent noise may be of help. requires u to do lotsa research on largly unavailable data though.

consider reduced air flow, but running closer to heat margin of the unit, tricky too. lower the room thermostat?

punch hole thru back wall, send the noise into somebody elses space!! ;-))

HP step sig gen i have sounds like a locomotive, i wish they had used variable rate fans!

or, get new equipment!! hound the boss, wife or sponsor, pitch it that the 'new' unit will save power, less space, last longer !!!

such,

tubing-and-box

the

Reply to
<hapticz

"I'm glad you asked".

I've been studying and fighting fan (ALL sizes) noise for years. The "fifth power" rule applies; fan noise increases to the fifth power of the RPM. Personal tests show a 20 dB rise for a doubling of fan RPM. If you add a second fan alongside to still get 90 CFM (wow; big fan for electronics these days), then run two side by side, but at half RPM.

AC fans are difficult to slow down; variable speed drive (VSD) is done for big building HVAC fans by fabricating lower ac frequencies. But you can try a dropping resistor, or connecting two ac fans in series. With DC fans (plenty out there these days, especially in the "Muffin Fan" series), you can easily run them slow, since the permanent magnet DC motors run exactly proportional to applied voltage.

So half rpm gives 20 dB less noise. Placing two half speeders side by side gives 3 dB more noise; net gain is 17 dB. Not too shabby...

Good Luck

Angelo Campanella

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Reply to
Angelo Campanella

Then you need to mount a thermistor mounted on the critical component, either the frequency determining component (solid state oscillator amplifier, or on the frequency determining capacitor or resistor), or the amplitude amplifier if that is the critical parameter. This will serve to monitor airflow.

stable

A small personal table mounted fan with a blade diameter of 8" to 12" may serve. A cardboard box lined with fiberglass or open cell foam may serve as the "enclosure". A smooth walled aluminum duct will have the least flow resistance. The duct diameter should be large, say 4", as used for a clothes dryer exhaust.

The thermistor mentioned above should do.

Angelo Campanella

Reply to
Angelo Campanella

This seems like a good idea for several reasons. It serves to aid in air flow 'calibration', but it can also monitor performance over time, looking for degradation. Hoses, fans and filters are subject to dirt that degrade performance. A simple monitoring circuit with alarm set-points would be ideal here.

daestrom

Reply to
daestrom

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