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  1. #1
    Join Date
    Jun 2008
    Location
    NC
    Posts
    84

    Mechanical air flow switch or current transducer for fan proof?

    For the last 30yrs or so here at our faciility they have used differential air flow switches to prove air flow/fan running. As i am going through and replacing the old equipment,sensors and controls to our new Tridium/bacnet network I am wondering if sticking with the differential air flow switches are worth sticking with. I have never used the CT's for fan proof before and would just like your opinions on whether or not it is worth it to use them over the differential switches.

    At the same time i would probably throw one on the compressor at the same time and trend them both for energy usage.

  2. #2
    Join Date
    Apr 2007
    Location
    Amarillo by mornin'
    Posts
    859
    From what I've seen so far airflow switches last longer. CT's are probably one of most replaced end devices I replace from day to day.

  3. #3
    Join Date
    Mar 2012
    Location
    North Carolina
    Posts
    15
    Ct's can be hard to adjust with fans that run at lower speeds a lot, on a vfd. I installed a new drive years ago and proofed the motor with a ct. I thought I had adjusted it pretty good. When we did the first pm after that, we discovered that the belts were gone and the ct was still showing proof. I say, if the wire is already there, use a diff switch. I've never seen many problems with the ones we use.

  4. #4
    Join Date
    Jul 2008
    Posts
    1,481
    Quote Originally Posted by khumphreys View Post
    ... we discovered that the belts were gone and the ct was still showing proof. I say, if the wire is already there, use a diff switch. .
    ...ditto
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  5. #5
    Join Date
    Jun 2004
    Location
    Eugene, Oregon
    Posts
    1,018
    If differential pressure is high enough for a DP switch its a more proof positive way to prove a fan is operating. I've had problems with small fans using DP switches because of very low differential pressures. In those cases a CT would be better as long as it is not a critical fan.
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
    "Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
    http://www.campbellmechanical.com

  6. #6
    Join Date
    Jan 2013
    Location
    My own reality
    Posts
    27
    A DP that is piped out on both sides will far out last a CT. Furthermore a CT that isn't properly adjusted, some cant be, will not reflect a broken belt or other malfunction that allows the motor to spin but not the fan.

  7. #7
    Join Date
    Oct 2003
    Location
    Minnesota
    Posts
    1,382
    Quote Originally Posted by xarralu View Post
    From what I've seen so far airflow switches last longer. CT's are probably one of most replaced end devices I replace from day to day.
    Hmmm, interesting. Care to mention which make of CT's you see fail the most?

    I only ask because I myself don't see many of them fail. OTOH, I don't see many mechanical air flow switches fail, either. In either case I don't see much for failures until they're in the >15 year age range.

    On one recent project, a controls upgrade project, we replaced a bunch of CT's (>100) that had been in place since the mid 1990's. They were still working. It was just part of the upgrade contract that called for replacing all sensors that caused us to replace them.

    I'm not contesting your experience, I'm just wondering if you might be seeing some particular brand I might want to avoid.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  8. #8
    Join Date
    Nov 2009
    Location
    Tennessee
    Posts
    76
    I prefer the dp over the ct. I have seen the DP's fail but mostly in units that didn't receive proper maintenance and were very dusty. I think they also help if adjusted correctly to send notification that there is an "issue" with airflow like a coil that's froze, failed fan, dirty filter to name a few situations.

  9. #9
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,115
    I like DP both for proving and alarming to dirty filters and coils.

    With that said we end up putting in Veries Hawkeye H940 CT's because they come with built in relay at a very low cost and can be adjusted to show for a broken belt. Per unit it is not a huge price adjustment but on large installs(8 or more AHU) the cost of wiring conduit parts and time can make a decent difference as it only requires a 2 conduit wire instead of 4 and reduces the cost of conduit to a DP.
    If you're too "open" minded, your brains will fall out.
    Artificial intelligence is no match for natural stupidity.

  10. #10
    Join Date
    Feb 2013
    Location
    Central Pennsylvania
    Posts
    63
    Leave existing, working DP's in place. This a digital, or binary signal. Best for flow proof, especially with a VFD.

    Current switches (binary) marketed to work with VFD's are a PITA, IMHO. Please don't call a current switch a CT, you are insulting the CTs.

    Current Transducers (4-20ma analog) designed for low frequency use will provide reliable values, but you will need to profile the system and set-up your software as needed. Remember to install the CT between the drive and the motor. Electricians call these donuts. If you don't go with the cheapest available, they will probably last a very long time.

    Be careful with Variable Frequency Drives, they work by reducing the frequency (and voltage) available at the motor, and a lot of sensors are expecting 60Hz. If you put a sensor before the drive, you will read loads from the electronics, cooling fans, relays, etc.

    With higher-end drives, I would simply connect to the 4-20ma output of the drive that provides motor current - this will take into account all three phases, and you don't need to buy the CT

    For trending energy usage a VFD may even know what the KW load is. Your results may vary.

    Peace
    .ja.

  11. #11
    Join Date
    Feb 2013
    Location
    Central Pennsylvania
    Posts
    63
    Quote Originally Posted by XcelTech View Post
    I like DP both for proving and alarming to dirty filters and coils.

    With that said we end up putting in Veries Hawkeye H940 CT's because they come with built in relay at a very low cost and can be adjusted to show for a broken belt. Per unit it is not a huge price adjustment but on large installs(8 or more AHU) the cost of wiring conduit parts and time can make a decent difference as it only requires a 2 conduit wire instead of 4 and reduces the cost of conduit to a DP.
    Just wanted to say that this type does work well enough for fans without VFDs. right price range.

  12. #12
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,115
    I am not dogging you I am curious that I might learn something.

    I am trying to understand why you would put a CT on anything with a VFD?
    I can't think of a VFD or PLC I have encountered that couldn't do everything a CT could only better.
    If you're too "open" minded, your brains will fall out.
    Artificial intelligence is no match for natural stupidity.

  13. #13
    Join Date
    Feb 2013
    Location
    Central Pennsylvania
    Posts
    63
    'Cause I lost that battle to politics. We mostly have ABB ACH-550s, and they all have it. But, to standardize, since there are old Grahams out there, and tiny new japanese drives that come in on packaged units, we settled on the
    H720.

    I am now trying to figure out how to get reliable status from motors with ECMs.

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