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  1. #1
    Join Date
    Aug 2007
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    72

    Best Way to Measure Chiller Load?

    200-ton rotary chiller, there are a few ways to estimate load on the machine:

    Heat balance, evaporator side
    Heat balance, condenser side
    % of full load amps

    Doing heat balance on both sides, usually yields two fairly different values. This is probably due to instrumentation limitations which result in imprecise measurement of fluid flow rates. Usually flow through the evap or condenser is estimated based on dp.

    Usually the heat balance load values don't agree too well with % of full load amps, either. Not sure which is the most indicative of actual operating tonnage of the machine.

    Any suggestions?

  2. #2
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    Dec 2010
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    Down by the river
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    500 x gpm x TD = BTU/H

  3. #3
    Join Date
    Aug 2007
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    72
    Quote Originally Posted by Southern Mech View Post
    500 x gpm x TD = BTU/H
    Yes, I know. The problem is getting a decent measurement of gpm.

  4. #4
    Join Date
    Sep 2008
    Location
    A land down under
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    If the heat balance result does not balance (within 5%) it is an indication that one or more of the readings taken is in error.

    % of full load amps is relative to the compressor work/lift (delta P) - the compressor can be at RLA without the chiller producing design tons. You don't purchase a 200 amp chiller but you do size the mains wiring off RLA ((MCA = 1.25 *(Primary RLA) + (4000/motor voltage))

    To answer your question IMHO Accurately measure the evaporator performance...





    • Measure the actual evaporator pressure drop and determine the actual evaporator water flow.
    • Determine the evaporator capacity.
    • Tons = delta T(F) x gpm / 24 or KWr = delta T (C) x l/s / 0.239
    Last edited by Screwit; 07-28-2011 at 04:29 PM. Reason: correction in post
    Necessity is the mother of invention

  5. #5
    Join Date
    Sep 2007
    Location
    Bay Area California
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    321
    as you mentioned...
    pressure drop thru the barrols against flow chart from the manufacture...
    or get your pump design data and get gpm off your pump curve...

  6. #6
    Join Date
    Jun 2008
    Location
    Windsor, Ont.
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    458
    Explain what you mean by "Heat Balance". The amount of heat rejected in the condenser is always going to exceed the amount of heat gained in the evaporator due to heat of compression and motor heat. The best way I know to calculate a chillers performance is with known evaporator flow and temperature difference across evaporator. GPM x 500 x TD(temperature difference) = BTUH, divide that by 12000 to get your tonnage.

  7. #7
    Join Date
    Jun 2008
    Location
    Windsor, Ont.
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    458
    I guess the question was answered while I was posing a response.

  8. #8
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
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    136
    Get an accurate GPM of the evaporator, buy comparing actual DP to design DP. Then do the math. Condenser tonnage will always be higher than evaporator tonnage due to superheat, enthalpy, and all that stuff. Depending on the head of the unit a screw machine will do considerable more tonnage than the rated tons. A 200 ton chiller may do 250 tons on a pull down, with condenser water below design. FLA percentage does not always follow the tonnage. The KW/Ton changes through out the compressor gas curve.

  9. #9
    Join Date
    Aug 2007
    Posts
    72
    Thanks to all who replied.

    I need to upgrade the pressure instrumentation on the chilled water (evap) side, to get a decent dp measurement.

  10. #10
    Join Date
    Dec 2010
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    Down by the river
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    Quote Originally Posted by PowerPlay View Post
    Thanks to all who replied.

    I need to upgrade the pressure instrumentation on the chilled water (evap) side, to get a decent dp measurement.
    gotta have acurate gpm measurments, I keep a set of nice gauges for checking flow, I never trust the ones that are kept inline for ''real'' conditions

  11. #11
    Join Date
    Jun 2010
    Location
    Tampa, FL
    Posts
    101
    Usually pressure gauges are sufficient for field work. Better quality and larger size give better results. I had an analog dp meter that read out in inches wc that was much more accurate than gauges. Now days a transducer of the proper range should yield the best results for dps other than a factory test stand flow meter. Factory submittal data should be available for set up and checking of machine performance. Part load performance data is usually also available.

  12. #12
    Join Date
    Sep 2008
    Location
    A land down under
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    306

    Flow Measurement and Heat Balance

    The water pressure drops across the evaporator and condenser must be measured and be confirmed to be very close to or at the design values. For better accuracy use a single pressure gauge connected with a manifold.

    If the measured pressure drops are very close to or at design, then the water flow rate (gpm/lps) can also be assumed to be very close to or at design. The design flow rates can then be used in the formula to determine capacity.

    If the measured pressure drops are not very close to design values, then the actual water flow rates must be determined using other methods. These may include the use of ultrasonic flow measuring devices, or by using pressure drop/gpm curves or pump curves.

    The total heat balance is a method for verifying the accuracy of measurement in a chillers performance evaluation if the result is within 5%.
    Heat Balance % = Evap Tons + Motor Tons (Motor Tons = kW / .2844)- Cond Tons x 100/Cond Tons
    Necessity is the mother of invention

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