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  1. #53
    Join Date
    Dec 2008
    Location
    Dixiana, AL
    Posts
    2,609
    Quote Originally Posted by RC20 View Post
    I would not touch a VFD driven device unless the mfg discuses what they have done to stop the bearing arching in their literature. While some will argue, my experience is that all new VFDs destroys bearings, its just a matter of how soon.
    I guess I'll have to argue. I've got vfd's on chillers, pumps, fans, and pretty much anything else you can think of, and have yet to see a bearing failure due to problems caused by the vfd. Granted, I haven't seen everything, and if I said I'd never see one, that's about like saying that any motor installed with a vfd is going to have a bearing failure due to the drive. Pretty dogmatic statement for a subject that large.

  2. #54
    Quote Originally Posted by Randy S. View Post
    Sand filters make very good incubators.

    What are you running for a biocide?
    Nothing. Job is "plans and specs". Does not call for any chemical in cooling towers. When I approched eng. about this, he said owner does not want any chemical in there tower.

  3. #55
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,822
    The owner just displayed his gross ignorance of the most basic principle of tower operation.

    That having been said, he deserves the host of costly, but easily avoidable, problems coming with the chillers and the tower.
    God Bless our Veterans

    God Bless the USA

  4. #56
    Join Date
    Jul 2009
    Location
    Wa
    Posts
    115
    My Company installed a multistack chiller to replace a 30 year old Dunham Bush screw machine. The tower is open and it has been a complete nightmare to keep strainers clean. I have removed the strainers in two years probably 20 times, we had a lakos seperator installed in the loop which has been a complete waste of the customer money and doesn't work. We have had several engineers look at flows for the lakos and upsized the condenser pumps to insure proper gpm for lakos to filter. The bottom line is the lakos filter is a wall ornament at best, if you don't put the multistack on a closed loop it will be a constant maintenance nightmare period!!

  5. #57
    Join Date
    Mar 2008
    Location
    NY NY
    Posts
    181
    Multi Stack as well as all the other modular chillers is a victim of the 20 micron strainer. The strainer installed to protect the minute passages through the brazed plate heat exchanger requires frequent attention.

    Multi Stack does offer an automatic blow down option, which helps in keeping the strainer clean. All being said it is fibers smaller than 20 micron which create or problems. The fibers will staple onto the strainer, accumulate thus reducing the free area over a period of time to zero.

    I have two facilities with no filtration on the condenser water with the blow down option installed. I have installed dual set point differential pressure switches on the condensers. The low differential set point is for low flow lockout and the high differential initiates an alarm indicating restricted flow. The high set point is 2 ft head below the maximum allowable pressure drop. Still cleaning strainers but they are cleaned before the unit becomes a problem.

    The solution as mentioned in this thread is to decouple the tower from the heat exchanger with a frame plate. Then your problem is solved.

  6. #58
    Join Date
    Mar 2008
    Posts
    64
    Quote Originally Posted by klove View Post
    I guess I'll have to argue. I've got vfd's on chillers, pumps, fans, and pretty much anything else you can think of, and have yet to see a bearing failure due to problems caused by the vfd. Granted, I haven't seen everything, and if I said I'd never see one, that's about like saying that any motor installed with a vfd is going to have a bearing failure due to the drive. Pretty dogmatic statement for a subject that large.
    And I have had all units that VFDs are on have bearing failures.

    I will accept one mans failure is a problem another man has not found yet, or realized the underlying cause.

    I have not let one run to destruction, so by that definition I have not had failures. I have had all of them (3 different mfgs) create bearing damage (noise) that I changed out before they became a failure (though I will contend when it becomes noisy it has failed)

    Read this one all the way through.

    http://www.plantservices.com/article...22.html?page=1

    I have been able to confirm the issue in two supporting ways to the noise.

    1. The grease is severely deteriorated in a short period of time due the arching (read SKFs literature discussing Ceramic Hybrid Bearings on that subject-though I found the that out before I got their confirming literature)

    2. The grease smell like an electric train (or slot car)

    Keep in mind I had an old drive that had no issues on the motor till the drive died and was replaced by a modern one, within 3 months the bearing noise as atrocious (belt removed). This stopped when I put the Ceramic Hybrid Bearings on it (as have all the others).

    One older drive would start to manifest bearing noise in 8 months or so.

  7. #59
    Join Date
    Jul 2007
    Location
    The Hot South
    Posts
    1,299
    Are the motors that are failing rated for Inverter Duty? I've worked on hundreds of drives with no motor bearing failures unless the wrong grease is being mixed with the EM grease that in the motor from the factory.

    Quote Originally Posted by RC20 View Post
    And I have had all units that VFDs are on have bearing failures.

    I will accept one mans failure is a problem another man has not found yet, or realized the underlying cause.

    I have not let one run to destruction, so by that definition I have not had failures. I have had all of them (3 different mfgs) create bearing damage (noise) that I changed out before they became a failure (though I will contend when it becomes noisy it has failed)

    Read this one all the way through.

    http://www.plantservices.com/article...22.html?page=1

    I have been able to confirm the issue in two supporting ways to the noise.

    1. The grease is severely deteriorated in a short period of time due the arching (read SKFs literature discussing Ceramic Hybrid Bearings on that subject-though I found the that out before I got their confirming literature)

    2. The grease smell like an electric train (or slot car)

    Keep in mind I had an old drive that had no issues on the motor till the drive died and was replaced by a modern one, within 3 months the bearing noise as atrocious (belt removed). This stopped when I put the Ceramic Hybrid Bearings on it (as have all the others).

    One older drive would start to manifest bearing noise in 8 months or so.

  8. #60
    Join Date
    Jun 2007
    Location
    Here. I'm right here i tell you!
    Posts
    465
    Alot of drives made nowdays don't actually re-create a sine wave, Or at lease a true one. It is made up of a series of short pulses grouped together that their amplitude resembles a sine wave. Ceramic bearings are certainly a good solution to the problem. I have had several blower motors experience bearing arcing. The square d altivar drives (I think they were the 66 model) were on all of them A Dv/Dt network may also help.

  9. #61
    Join Date
    Mar 2004
    Location
    midwest
    Posts
    954
    To agree with most posts and based on hands on experience you need good clean water to avoid issues. Closed loop is the best way to go if you decide to use a tower make sure you have a good sand filter system installed. Very maintenance intensive and make sure the owner knows this before investing in a new chiller!
    An astronaut stuck in space was asked by a reporter, "How do you feel?"
    "How would you feel," the astronaut replied, "if you were stuck here, on top of 20,000 parts each one supplied by the lowest bidder?"

    "do it right or do it twice"

  10. #62
    Join Date
    Mar 2008
    Posts
    64
    Quote Originally Posted by R123 View Post
    Are the motors that are failing rated for Inverter Duty? I've worked on hundreds of drives with no motor bearing failures unless the wrong grease is being mixed with the EM grease that in the motor from the factory.
    The reality is Inverter Duty does NOTHING to address this issue. So called Inverter Duty is an insulation specification, not the shaft current problem. Yes there is some winding stuff as well, but that does not deal with the rotor which is the heart of this issue.

    Call it the BIG LIE, or like the Prime Mortgage debacle where most of the parties involved are pretending there is not a problem. Admitting it would cause the primary culprits pits (drive mfgs) to have to make their VFDs high quality and more money , or motor mfgs flat say, if you are going to run a motor on a VFD, it is going to cost you another $1000 to put the right bearings and shaft brush on it.

    Its like a game of chicken, if you are the first one to say so, then your stuff gets written out of any specification because its going to cost. Its really an NEA or NEMA issue to get standards out, but that has not been addressed (though there is now reference to Level 3 VFDs that stop this-which seem to be a UPS quality unit).

    Read the last part of the first page and the second page of that report carefully. That’s where the current (bad pun intended) heart of the situation lies.

    You build up potential in the rotor because of the lousy sine wave they produce and the associated high switching speeds, lack of filtration, and one you have that, you get ball arching. There some numbers out there, but I recall its less than a volt to arch across the grease barrier (dielectric grease does not help as its not thick enough at the ball/race interface).

    There are a lot of half way fixes, but the ceramic hybrid bearings are part of a sure fix. Chuck Yung recommends the shaft grounding brush as well to ensure it does not build up enough potential to go someplace else (mounting those can be a problem).

    So far, belt driven fans seem ok, though the worry is a direct couple pump might create issues in the system due to the dissimilar voltage paths, eat up impellers etc.

    Grease incompatibility is NOT a factor. I use double sealed bearings on all my motors and all my motors are sealed bearings, and all the motors involved with the VFDs have been sealed bearing for some time (now SKF, but Fafnir before Timken bought them out and went cheap), and the VFDs eat those up quite happily.

  11. #63
    Quote Originally Posted by RC20 View Post
    I would not touch a VFD driven device unless the mfg discuses what they have done to stop the bearing arching in their literature. While some will argue, my experience is that all new VFDs destroys bearings, its just a matter of how soon.
    Turbocor compressors have magnetic bearings. Rotor shafts and impellers levitate during rotation and float on a magnetic cushion. I am currently working on start-up six tubocor compressors on three Mcquay chillers. This technology is very exciting.

  12. #64
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,822
    Wait till you see the WME line from McQuay/Daikin.

    500 tons on their own mag-lev compressor.
    God Bless our Veterans

    God Bless the USA

  13. #65
    Join Date
    Mar 2008
    Posts
    64
    That should prove to be interesting.

    Whole different dynamic in having a magnetic field between the two parts. A strong filed might not let current do that, but they have to be pretty close tolerance.

    Would it still be prone to aching, and would it make any difference if it did arch?

    I do not foresee that class equipment in my world ever, but who knows what’s around the next bend.

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