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  1. #1
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    Jul 2014
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    Chiller BTU Station

    Finally took the time to take photos of a chiller "BTU Station" that I made last year.
    Typically you use the dP from the chiller supply & return gauges and determine the GPM from the factory chiller pressure-drop flow curve ('reflection' curve). Then you calculate Btu/hr from the dT (typically get the temps from the Chiller display panel or gauges if present).

    The only problem with this approach is that you only get spot readings - sometimes that's ok.
    My BTU station allows you to trend the data. Works pretty good. Avoid soldering your own 1/8" jacks - use the Onset gear (it's a 'jungle' out there ;-). See attached PFD.

    (I also posted this on the DYI section).
    Attached Images Attached Images

  2. #2
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    Jan 2013
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    Have you 'cross referenced' this with a calibrated flow meter, inline or ultrasonic to see how accurate it is?

  3. #3
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    Jul 2014
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    Thread Starter
    This is not a laboratory, however the BTU station has lab-like qualities behind it.

    The total btu/hr accuracy is dependent upon 3 components:
    1) Temp. Sensors. The accuracy of the Onset immersion temp. sensors is +/- 0.5F (~1% for the chiller temps). For measuring the delta temps., the sensor matching important. I tested the temp. difference between the (2) Onser temp. sensors: 0.05F (~0.5% for typical chiller dTs).

    2) Pressure Transducers. Onset offers Ashcroft pressure transducers to work with their data logger (ie, U12). Accuracy is rated at 1%. For 'lab' proof, I created the Christmas tree to also use my digital pressure gauge (0.5% accuracy) - I had it calibrated. The tree is also useful to have the analog pressure for the HVAC staff - often let's them know how inaccurate their gauges are (I've seen 8-12psi errors). Don't use analog pressures garages to determine the flow rate.

    3) Evaporator Pressure Drop Curve.
    The evaporator bundle is a 'head-loss' meter. Typical devices In this family are venturi, pitot tube, insertion tube and special valves. The evaporator bundle is an impact-measuring type. that uses friction or velocity head as a measurement of flow. Very repeatable.

    To get accurate psi & GPM numbers I scanned the reflection curve and used Photoshop to lay down grid lines (see pg. 4). This is probably the weakest part in terms of 'Lab' accuracy. For improved accuracy request the Performance Data ('Data Sheet', 'Job Info', etc) form the chiller manufacturer. Avoid using 'Test Data'. Also, verify the actual capacity with the manuf; sometimes the actual capacity does not match the nameplate (ie, modifications).

  4. #4
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    Only problem is Trane will no longer share evap info with anyone, it was hard enough to get when I was an employee there.
    Federal Reserve, stealing your kids futures since 1913

    UA290

  5. #5
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    Jul 2014
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    Thread Starter
    Like I said, you need to get your hands on that evap. pressure-drop curve first before you do anything - if not, "BTU Station" or just manual paper lookup just won't fly to get chiller gpm data. If getting the evap. curve is bupkis, the next options are: 1) trend the chiller pump pressure drop and use the manuf. pump curve, 2) trend the CH pump amps to calculate the running HP; knowing the nameplate HP and design GPM you can calculate the gpm with the pump laws (note these methods assume that the pump impeller is sized correctly and is in good condition - good argument in trying the evap. barrel dP method first). At the bottom of the barrel (no pun intended ;-), there's trending the pressure drop across the triple duty valve and using it's curve.

    Disney-world: there's a calibrated flow meter in the evap. line and the BAS-DDC is capable of trending it (w/o a fee?) .

    If you pay for a $$$ chiller, you should be given the Performance Data - IMHO.

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