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Long life fan wanted

Discussion in 'Electronic Design' started by Allan Herriman, Jul 8, 2004.

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  1. Hi,

    I'm trying to find a *long life* axial fan to cool some mission
    critical electronics.

    Rough requirements:

    o At least 90k hours (= 1 decade) L10 life at 40 degrees Ta.

    o Can push at least 0.7 m^3/min against a 10Pa back pressure.
    (That's 25 CFM and 25 inwg if you like strange units.)

    o 12V

    o tachometer output

    o about 80 mm x 25 mm

    I'm currently using the Sanyo Denki 109L0812H402 which is great (100k
    hours life), except that it doesn't have the option for a tachometer
    wire. I need the tach so that I can know when a fan (in the bank of
    three) fails.

    Googling for long life fans just hits on a whole lot of "PC quality"

    Any ideas about manufacturers or part numbers?

  2. I read in that Allan Herriman <allan.herriman.hat
    > wrote (in <55kpe0plvkc4sch8mi5874al7pr7vfeo
    >) about 'Long life fan wanted', on Thu, 8 Jul 2004:
    Can you add a magnetic, optical or IR detector for fan rotation to the
    Sanyo fan, to substitute for a tacho?
  3. Ban

    Ban Guest

    If you put a self-heated PTC-pearl into the individual airflow, by
    monitoring the PTC current with a comparator you can reliably detect any
    flow threshold.
  4. Early detection of fan failure (in this case) involves comparison of
    the speed vs voltage characteristic compared with the same
    characteristic when new. I'd like to be able to detect changes of
    only a few percent.
    The ambient temperature may vary over a wide range, which makes any
    temperature based measurement (e.g. using a PTC-whatever) useless for
    my purposes.

    BTW, I wasn't asking for ideas on thermal management. I have a board
    that's waiting to have three three-wire fans plugged in; I want to
    know where to get a reliable three wire fan.

  5. Hi Allan,

    I am using Sunon KDE2408PTB1-6AF.318 , which is 24V, 80x25, 42.5cfm, has
    tacho output, also available for 12V(KDE1208.....), 0.23Inch H2O, 3.4W
    L10=124619h at 40°C.

    Best regards,

  6. Zak

    Zak Guest

    You could measure the current consumed and use the spikes to extract the
    speed. Some PC folks do this to get RPM monitoring out of a 2 wire fan.

    NKW Sepa or Japan Servo might have products too, but information is hard
    to find on the web. Probably needs a phone call.

  7. Joerg

    Joerg Guest

    Hi Allan,

    Failsafe seems to be one of your main concerns. Why not do it the old
    fashioned way, using a small metal strip on a switch in the airstream? A
    tach can fail to indicate plugging of the air path, at least to some
    extent. The ol' metal strip switch can be designed so that it does
    indicate that situation.

    Regards, Joerg
  8. N. Thornton

    N. Thornton Guest

    compare 2 thermistors, one in peak airflow, one not.

    I dont know either. In case you get stuck it might be poss to add
    another lil PCB to detect the fan frequency from its current waveform,
    and turn that into something that will feed your existing 3 wire PCB.
    I know its not what you want, just felt like saying it anyway... not
    sure why. Good luck.

    Regards, NT
  9. Bob Wilson

    Bob Wilson Guest

    What fails in almost all cases is the bearings. Assuming that the fan in
    question has ball bearings, you might consider bonding a small transducer such
    as the type if electret microphone used in cell phones to the frame of the fan.

    Ball bearings get increasingly noisy as they wear out. Monitoring the bearing
    noise may help predict approaching failure.

    Sleeve bearings used in el-cheapo fans will also become noisier,but it is not
    as pronounced.

  10. Hello Bob..... long time no see.

    I did a major replacement exercise on equipment
    fans a couple of years ago. In all cases it was
    the thrust bearing (or rubbishy thrust washer)
    that had failed, allowing the rotor to approach
    the stator, and start to mill away the stator's
    epoxy encapsulation.

    Oddly enough the rotor spun easily by hand, and it
    was impossible to push the rotor inwards enough to
    make it bind on the stator. But power a fan up, and
    the electromagnetic inward-pull on the rotor would
    immediately force it onto the stator.
  11. Bob Wilson

    Bob Wilson Guest

    Hi Tony,

    Yeah, I though it was about time to put in an appearance :)

    Your comment about the thrust washer implies that these fans were
    plain bearing types, since ball bearings don't require a separate
    thrust washer or bearing. My comments were directed to ball bearing
    fans, since they are more reliable in general, and the OP seemed to
    be interested in reliability.

    You're right that plain-thust-bearings (little more than washers)
    are the weak point of non-ball-bearing fans. This is because there
    is no hydrodynamic "wedge" of oil that can be built up to separate
    the rotating and non-rotating surfaces (as there is with a
    cylindrical plain bearing).

    Interestingly, several fan companies (e.g. panasonic, and others)
    have patented their versions of a specially-shaped plain thust
    bearing that tries to produce the above hydrodynamic wedge effect.
    The results claimed are that they have an equivilent life to ball
    bearing fans, and they are quieter and cheaper.

  12. Several solutions to this...

    For mission-critical, normal solution is Design Redundancy:
    o Serial Inline Fans -- 1 fails, other keeps running
    ---- if you put one as intake, it will experience cooler air, so last longer
    ---- Delta have static post-blade-foil airflow straighteners to reduce noise
    o Serial Inline Temp Controlled Fans -- 1 fails, other speeds up re temp
    ---- if both working, they run at a lower rpm, so last longer
    ---- ex - EBM-Papst 8412MV offer 3rd wire remote thermistor control
    --------- remote sensors allow themselves to be sited in exhaust (hot)
    --------- whilst the fans can be sited in serial intake for longer life (cooler)

    Whether you use a single/parallel or serial fan solution, you can...
    o Run fans at a lower rpm than spec - they will last longer
    ---- NMB have curve data for the effect of rpm as well as temp on L10 data
    o Ideal is to run fans at 70-80% of maximum - they will last longer
    ---- whilst maintaining the chosen fan under such in the sweet-spot of the P-Q curve
    o Run fans in air intake mode - they will last longer
    ---- ambient v exhaust temp delta is quite considerable in some applications
    ---- particularly where mission-critical involves a HVAC/plenum installation

    For mission-critical data systems I strongly prefer redundancy for 2 reasons:
    o Redundancy allows you to choose the downtime - not the machine
    ---- eg, scheduled to off-peak hours based on baseline user load
    o Servicing machines to replace fans involves humans & so human error
    ---- a notable %age of coloco downtime is caused by human error
    ---- either in servicing the affected machine, or in doing so off-lining others

    So redundancy like say RAID-1 is about availability:
    o L10 is just an expected figure based on an op-voltage/rpm & air temp
    ---- typically when 50% of the oil (by mass) in the bearing has oxidised
    o In a data Coloco HVAC-maintained environment, the data is useful
    ---- altho height of air intake from the floor affects intake-temp
    ---- along with HVAC plenum design/velocity & rack positioning
    o Deviate into non-HVAC environments, and mission-critical gets interesting
    ---- you get wider temperature swings & variability in user implementation
    ---- ignoring Management sticking the latest ABC report in the fluorinert tank :)

    Some of the high-availability Backbone routers use fans on both intake & exhaust,
    and will function with one operational - usually the exhaust fails before the intake.
    So if there is space, and cost isn't prohibitive, serial redundant fans works well.

    There are some 200k-hr fans out there, I think Sanyo has one and NMB.
    They may, however, not be commonly available - ie, OEM 50k-min order.

    An increasingly typical solution, is to outsource the problem to the PSU:
    o PSUs tend to have finite lives, relatively low MTBFs
    ---- depends on the design, cooling, and electrolytic capacitor ESR etc
    o Typical PSUs can have an MTBF similar to typical fans
    ---- and so making it a field-replaceable unit is common
    o Very high MTBF PSUs often uses redundancy to achieve the figures
    ---- and additionally to allow dual-power-feed for that redundancy

    However, if you are using a higher-grade industrial PSU (eg, 1M-hr MTBF)
    then the fans life becomes more important in relation to that unless PSU derated.

    For tacho signal outputs:
    o DC fans produce a switching noise which can be used to work out tacho
    ---- that is for fans who directly offer no tacho output
    o EBM-Papst fans with tacho have the code extension /2 on the end
    ---- eg, an 8412 NGML with tacho becomes 8412 NGML/2
    o NMB fans with tacho are generally to order only
    ---- they require the B noise level code to end in a 9, eg, B59
    ---- AND the A option code must also be 50-100, eg, B59-A100
    ---- so a B59-A10 despite the 9 would be a fan without tacho signal

    An alternative signal output is locked rotor:
    o This is a signal which goes high (typically) when the rotor stops (locked)
    o Such a condition would be when the fan is (well :) beyond service life
    o Typically Panaflo 1A use this method by default, tacho is by code 1BX
    ---- DEC Alpha, SUN & SGI use the 1A method
    ---- General PCs use the 1BX method - although there is some overlap

    Locked-Rotor can be cheaper to support re logic, although lesser if the
    mission-critical solution needs more involved reporting & notification as
    there are specific chip options to implement tacho fan monitoring/alarm.

    Contacting the major fan manufacturers with your requirements may get
    them to do the legwork for you in finding a solution to meet your needs.
    This is particularly true of NMB & EBM-Papst engineers.

    If the design of the fan cooling system isn't closed (and it should be done in
    the earliest stages re cost) then I still favour redundancy for mission-critical.
    If only, physical redundancy allows you to choose the downtime. You can
    pick a super-high 1.2M-hr MTBF Seagate drive, some still fail far earlier
    and it's that servicing downtime and inability to choose it that annoys/costs.

    Talking of which, I'm off to chase a low MTBF failure on a Maxtor :)

    Hope that helps, group helped me before so reciprocating.

    PS. Noisier/higher-flow fans are in production with tacho, hunt around.
    It's the very low noise ones which aren't commonly in production, usual
    reason since it costs an integrator 0p to make 1 fan go faster, but costs
    them quite a lot to use 2 fans re lost profit & perhaps low mkting benefit.
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