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  1. #1
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    Determining CH Flow

    One part of checking out the status of a chiller system is to review the chiller flows and primary vs. secondary flow (& decoupler flow).

    Often, CH systems do not have gpm meters (oh, so fun).

    Calculating water flows from field readings is part art and part science, which results in an estimate. Best to get a couple of 'votes' and take the average - which mean different ways to calculate flows (it's time consuming).

    I've seen & done three ways:
    1). Take pumps delta-P readings and using the pump curve.
    2) Take delta-P reading across the CH evap bundle and use the factory bundle curve. (see attached PDF).
    3) Take current readings of the pump motor. Use the motor nameplate data and do some 'back calc' math to get GPM.

    kVA = 1.73 x kV x Amps
    kW = kVA x PF
    Calc. HP = kW x Motor Eff.
    Actual GPM = (Calc. HP/nameplate HP)^1/3 x design gpm

    Appreciate any comments or other methods to get CH gpm. Even if the BMS graphics displays flow - do you trust it when checkin' out the water balancing?
    Attached Images Attached Images

  2. #2
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    The most reliable indication of water flow thru the chiller is with chiller pressure drop. Test the pump shut-off head and compare it to the pump curve. Measured shut-off head should be within 5 to 7 feet of that shown on the pump curve. Set the flow thru the chiller by chiller pressure drop and then measure pump delta P and compare with operating point on the pump curve. That gives you a pretty good baseline for primary flow. Secondary flow is not as cut and dried because the flow varies and efficiency varies with it. My preference is to measure pump shut-off head and then establish water flow as close as possible to design with pump delta P. Measure pump KW and compare against where it should be at design. Back that up with reading flow indicated by the triple duty valve and compare with pump test results. Compare measured temperatures to determine direction of flow thru the bridge and approximate the quantity. If the bridge is blind the difference in readings is about as close as you can get to bridge flow. It takes all of that as far as I'm concerned to get close enough to have confidence in the final results. If tested with low secondary flows the results in my opinion will be less than satisfactory.

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  4. #3
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    I also prefer chiller pressure drop.
    Just remember that tube cleanliness and tube obstructions can affect that. Usually only have to worry about that on open systems, but always best to keep it in mind.

  5. #4
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    Excellent point Nuclchiller,
    My response only addressed water flow measurement. In my opinion the best indication of how efficient the system is operating is the secondary return water temperature. Low return temperature indicates excess secondary water flow and wasted pumping horsepower. The closer that temperature is to design the better.

  6. #5
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    Thread Starter
    Great replies. Sometimes it's not 'learning', but confirming.
    One of seasoned "NEBB" guys did take readings at the Triple Duty Valve.

    So, what if CHWS has a field installed CHW flow meters that transmits back to the BAS?
    I've seen many - clamp on ultrasonic (Siemens Sitrans series); turbine; vortex (Onicon).
    Do you use it? Trust it? Like the saying goes, 'trust everybody, but cut the cards"

    If the plant has put in a Hartman or OE CH optimization 'program' - they use very accurate flow meters - they put them on each chiller and the on the main's. Why? Because the 'black box' needs to calculate accurate BTUs (flow & DT). Seems reasonable to use that flow data.

  7. #6
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    It takes a high dollar flow meter to track variable flow. If properly calibrated the cheaper ones are OK.

  8. #7
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    Thread Starter
    Ok. Given gold-plate or 'buick-type' flow sensor (& told there were 'solid') - would you rely on the flow data at the BMS graphics?

  9. #8
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    I would research the quality and accuracy of the flow meter including allowable turn down ratio, verify it had been sized correctly and accurately calibrated. If satisfied with all that I would trust the BAS because it basically regurgitates what it is told. When I did design and needed an accurate flow meter for variable flow I only trusted the magnetic type. That was a while back and there now may be others.

  10. #9
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    Thread Starter
    Well said. Really do appreciate the feedback. Thanks all!

  11. #10
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    As a chiller tech, I recall one customer who contracted with JCI to handle all their controls. Their plants all had flow meters that appeared to work very well. I couldn't begin to tell you what brand they were, but I do recall they had a local digital display that I almost always compared my pressure gauge measurements against. I ALWAYS measured with a gauge, but curiosity compelled me to look at the flow meter readings. I don't remember seeing appreciable differences.

    However, I have seen some that I didn't trust. Ironically, among the worst were early versions supplied by Trane, installed at the factory on their centrifugal chillers. I used to work for Trane. I typically set the chiller panel to ignore them. Every one I saw was part of an option package, where flow measurement wasn't even being used. This was back in the 90's, I have heard they use a much improved flow meter now.

  12. #11
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    Thread Starter

    Math Check

    Small update to that pump HP math. Forgot the 746W per HP.

    kVA = 1.73 x kV x Amps
    kW = kVA x PF
    Calc. HP = (kW x Motor Eff.)/(0.746kW/HP)
    Actual GPM = (Calc. HP/nameplate HP)^1/3 x design gpm

  13. #12
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    Record the model and the serial number of the chiller. Contact the manufacturer's sales representative. They may be able to provide you with the chiller selection information. This document will provide you the design pressure drop and design flow. If you measure the pressure drop accross the chiller's barrell, you can calculate the flow.

    Actual Flow = SQRT(Actual DP / Design DP) * Design Flow

  14. #13
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    Thread Starter
    Good reply - that's exactly what I had to do on a recent new 40T chiller install. There was a big 'bugaboo'
    regarding getting good flow values between the DDC tech (on a MetaSys/JCI system) and the tech setting up the Acronis flow meter.

    So I put in my Chiller BTU Station (see Energy Section - 'CH BTU Station'). Was suspect since it was low flow system (40T unit).
    The chiller was located in Monterey, CA. The OAT weather in late September was cold, so the chiller didn't run to ofter, when it did it was for a short duration. Cool weather meant low flow. Had to purchase Pete's Plugs adapters to interface the 1/4" NPT connectors on my pressure sensors.

    Tried to used the factory curves from the Aqua Snap manual - but it didn't work out. So used the "square-root' method as you pointed out. The trick was using the delta P value at Zero GPM. Real world is different that the 'paper world'. Got some reasonable flow results - wish I had warmer weather (another 'real world' situation). In the end we were basically satisfied that the Acronis/DDC flow setup was ok and could trust the gpm values reported on the MetaSys front-end. Having good flow #'s means good chiller BTU calculation. We also trended the chiller amps to get kW/Ton.

    See attached PDF.
    Attached Images Attached Images

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