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  1. #14
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    Jan 2019
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    Why has nobody mentioned a "fixed orifice plate". +/- 5% accuracy if installed properly.

  2. #15
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    Feb 2016
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    Louisburg Kansas
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    Deux 1,
    The discussion is primarily about how to measure flow on existing systems. The fixed plate orifice would have to be installed and only has a turn down ratio of 10 to 1. If I was going to do that I would install a magnetic flow meter that has a turn down ratio of 30 to 1 and an accuracy of better than 1%. The mag meter also requires a lot less undisturbed flow than the orifice plate. The mag meter also costs a lot more money.
    No man can be both ignorant and free.
    Thomas Jefferson

  3. #16
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    Jul 2014
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    10:1 turn down for an FPO? That's pretty good - maybe there's a brand out there. I'm heard that FPOs typically have a turn down between 3:1 to 5:1. A mag flow meter has a great turn down, but maybe not needed to cost-effectively monitor chiller flow. Typical chillers operate above 40%. Ok, lets say above 30%. For a 1000 Ton chiller @ 2.4gpm/ton the max flow rate is 2,400 gpm (10F design drop). 30% of 2,400 gpm = 720gpm. The turn down ratio = 2400/720 = 3.3:1. In my IMHO, I wouldn't use an FPO, especially if you have to modify the piping. Just slap in a pair of reliable pressure transducers to trend the pressure drop acorss the chiller evap barrel. Of course to get GPMs, you'll need the chiller factory pressure drop -gpm data.

  4. #17
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    Feb 2016
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    you are on target tridiumtech. The point someone made about fouled tubes is also true but if you use the pump, chiller pressure drop and triple duty valve for comparison you can tell if the tubes are fouled. Mag meters are very expensive and cost prohibitive in most cases but can read to 0.5% accuracy with ideal flow and around 2% with very little undisturbed flow. If you want to add the economical best choice I think I would go with the ventura meter. It requires only about 5/2 pipe diameters up and down stream for good accuracy and cost moderately more than an orifice flange.
    When I did balance work I had to take readings for flow meter calibration and you would be surprised at how many that had to track variable flow couldn't be set up for full range accurate operation. That is one reason I like chiller pressure drop.
    No man can be both ignorant and free.
    Thomas Jefferson

  5. #18
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    Jul 2014
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    Thread Starter
    Determining flow from the pressure drop on the pump assumes that the impeller is correct or there's little wear.
    When doing chiller system performance analysis the president of the energy company I work for likes to do all three:
    1) CH evap barrel, 2) drop on pump and 3) drop on the triple-duty. And we also do a 4th: monitor the pump amps and
    use the pump laws + the design flow rate (again assumes proper impeller). It's time consuming, especially doing several
    spot readings during the day (rather than using trending loggers). The benefit is that you can look the client 'in the eye' with
    your flow #'s. When you tell the client the flow is low due to the installed pumps didn't match the design prints - emotions can
    run 'high'. I've seen plant techs get pretty heated when they're told the real condition of their plant and why its not meeting the design capacity. System performance and energy efficiency go hand-in-hand.

  6. #19
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    Feb 2016
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    The pump impeller can be verified by running a shut off head test. You need a pump curve to match the results with. It takes some experience working with pumps to determine if the impeller is ok because in 20 years of testing water systems I only saw two pumps that followed the chiller exactly. I don't know about you guys but I found the triple duty valve to be the least reliable but close enough for comparison.
    The pump power is a good point. Since I did TAB I had to do a full work up and always had that information. I found that application of the pump laws was a lot more reliable than application of the fan laws because pressure readings on water systems is a lot more accurate than air systems. The occasional unstable system though drove me crazy.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #20
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    Jul 2014
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    I've only done a few shut off head tests, or I should say, been around when they were being done. Maybe a bit of 'black art', but hands-on experience is very important. At a pharmaceutical plant job, a NEBB co-worker was troubleshooting why a new pump, at lower elevation, could get enough gpm up to an AHU. So he started the pump to get it up to full flow. Then he slowly closed the discharge valve until it was fully closed, then read the supply and discharge pressure. The pressure readings were compared to the max. pressure found on the pump curve. We found out that the new pump was too small. Ah, sure, you do the same test to see if you impeller is worn (or sized incorrectly).

    I went to the "Engineering Toolbox" and found this equation: head (ft) = [(d x n)/1840]^2. d = impeller outside diameter (inches), n = wheel velocity (rpm). Do you ever get that 'technical' and use this formula? Thank for the reply Wayne.

  8. #21
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    I always compared the actual shut off head to the pump curve. Good condition accurately sized impellers almost always test within 5 feet of the shut-off head shown on the curve.
    This conversation has been fun thanks.
    No man can be both ignorant and free.
    Thomas Jefferson

  9. #22
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    Jul 2014
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    Sounds good. But back to that comment about 'fouled' tubes. How much fouling does it take to really 'wreck' a dP drop across the barrel (ie, >5%). Now I've seen an AHU coiled fouled to about 50% flow restriction - but that unit was put in 1968 (Boeing Plant, WA) and the HVAC crew put in a new coil in 2007 - almost 40 years. What that says it that PM isn't done on AHU coils, but they sure do on Chillers, or they should. In my HON days I'd see the mechanical PM guys do a tube clean pretty regularly. And if you watch the chemistry, the fouling should be minimum concern. So maybe that's a good question to ask the plant staff if you going look at the CH barrel dP - when's the last time you clean yar tubes? Controls PM is good, but mechanical PM is a must - those guys are great. If the plant runs shoddy in "Hand" mode, controls don't mean much.

  10. #23
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    Jul 2014
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    Thread Starter
    S

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