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Cooling performance with 2 fans?

Discussion in 'Electronic Equipment' started by DaveC, Mar 17, 2007.

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  1. DaveC

    DaveC Guest

    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;

    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.

  2. MassiveProng

    MassiveProng Guest

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

    Palindrome Guest

    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.
  4. DaveC

    DaveC Guest

    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.

  5. Palindrome

    Palindrome Guest

    DaveC wrote:
    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.
  6. DaveC

    DaveC Guest

    Possible no need to focus that much - if you use a PC kit, just bolt the
    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:= <- hair on end emoticon

    Definitely a cool (c: possibility, but I think the concertina tubing-and-box
    solution looks simplest. But I will keep thinking on the heat-exchanger
    solution; one of those geek things that one turns over in one's mind in the
    idle moments...

  7. MassiveProng

    MassiveProng Guest

    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

    Good call, BTW.
  8. 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.
  9. DaveC

    DaveC Guest

    I measure the fan's noise at 53dB (using A-weighted dB meter at
    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.
  10. 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
    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.
  11. MassiveProng

    MassiveProng Guest

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

    DaveC Guest

    Find a way to operate the fan at reduced RPM.

    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.


  13. Lionel

    Lionel Guest

    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.
  14. ari

    ari Guest

    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 (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:)



    PS To my understanding most research done by this kind of fan
    manufacturers in last almost 10 years has been concentrating on motor
  15. 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
  16. Jon Elson

    Jon Elson Guest

    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
    (in general) but you indicate no major obstructions.

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

  17. Guest

    Guest Guest

    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

    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 !!!
  18. "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
  19. 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.
    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
  20. daestrom

    daestrom Guest

    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.

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