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  1. #14
    Join Date
    Jan 2011
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    1,190
    this is for evaporator AND condenser water flows. the tonnage calculation doesn't care whether it is evaporator or condenser and it doesn't know the difference anyway.
    Perhaps what we have here is a matter of semantics. When a performance test is done at the factory the paperwork doesn't show that the condenser was producing more tonnage than the evaporator. You work for JCI. If you have attended one of their centrifugal classes the two formulas are presented. When one is on a job trying to diagnose a problem the two different divisors (24 for evaporator and 30 for condenser) are used. How else is one to determine if the condenser and evaporator are doing the same amount of work? Using 24 as the constant for both sides will only lead to confusion. We know the condenser has to do more "work" to overcome the heat of compression. If one uses the same constant for both, then they are going to have to do the math to determine if the condenser is rejecting the same heat as the evaporator, hence the reason for the constant of "30" for the condenser. The math is already done.

    Every reference I have found talks about the two different constants. Here's oneand another. ARI standard 550/590-98 says the same. But I guess in the end, we're all going to believe what we want to believe.

  2. #15
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,440
    Quote Originally Posted by KnewYork View Post
    ...When one is on a job trying to diagnose a problem the two different divisors (24 for evaporator and 30 for condenser) are used. How else is one to determine if the condenser and evaporator are doing the same amount of work?...
    the condenser and the evaporator NEVER do the same amount of work. the condenser always has to do more work. if it isn't doing more work than the evaporator, then there is a major problem and heat is being transferred in the wrong direction or heat is building up in another area.



    Quote Originally Posted by KnewYork View Post
    ...We know the condenser has to do more "work" to overcome the heat of compression. If one uses the same constant for both, then they are going to have to do the math to determine if the condenser is rejecting the same heat as the evaporator, hence the reason for the constant of "30" for the condenser. The math is already done...
    i think the math is pretty easy to do....condenser work minus evaporator work = 'other heat loads' and when you use the proper formulas (that i presented) you can use them at all possible loads and under every condition...not just brand new and not just at full load and not just chillers. the formulas that i presented are energy loads that the water gained or lost no matter where they came from.

    and the math is NOT already done using those constants because you are assuming way more than you are probably measuring including ambient heat gain (or loss), fouling, etc.
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  3. #16
    Join Date
    Oct 2011
    Location
    La.
    Posts
    282
    Carrier model 19dk61126AF

    1.When conducting a performance evaluation should I have the heat recovery pump running regardless of outdoor conditions?

    ans.According to the design sheet, your condenser is rated at 212 tons. I would run both as this is what the unit was designed for and add both of the rejected heats to come up with total heat rejected.

    2.Should the condenser tonnage equal the Evap. tonnage plus the energy required to power the motor converted to BTU?

    ans.Who cares where the heat comes from, it is a combination of all the factors stated above. Who cares about 25 or 30. Just worry about that it exists unless you are an engineer.

    3.Notice in the picture of the design specs it does not list the condenser rated tonnage, why is that?

    ans.Carrier is lazy! You just need to do some math and it is there. BTUH = GPM x T x 500 (for water)

    Good luck,

  4. #17
    Join Date
    Mar 2011
    Location
    Malaysia
    Posts
    32
    Interesting thread.

    I agree with using (GPM X Delta T)/24 for both condenser and chilled water side (for DT design of 10 Deg F). Fully hermetic chiller will mean all electrical input has to be computed into the heat load as the kW is converted into rotational energy which is transferred to the refrigerant. Semi hermetic - some assumption on how much motor heat and inefficiency is thrown out to the surroundings and how much is into the system. External drive type will mean 90% of energy (assuming the motor eff is 90%) into the motor is into the system.

    The use of condenser tons calculation using (GPM X Delta T)/30 is a shortcut which I feel has not been explained clearly to people operating chillers. I had the same argument just last week, (discussion) where the techs working in a semiconductor factory insisted I was wrong until I showed them that technically they are same but not accurate.

    As for the test, it is a called a heat balancing test - but heat balancing test at site is used to check if measurement are accurate or not - i.e. is your flowmeter and temperature sensors accurate - if there is a small variation (+-5%) between condenser and chilled water load, it means measurements are accurate enough. If there is a large difference, something is not right in the measurements or some other heat load rejection has been missed - i.e. heat recovery chillers where you need to add both the cooling tower heat rejection and the heat captured for recycling into the equation.
    I have seen some "smart" designs that assumed all heat rejected can be recycled, thus during low cooling loads, the system becomes imbalanced.

    Chiller performance check will now depend on:

    Power consumed by the chiller for cooling energy generated (kW/ton) with flow as per design values and Condenser water temperature into the chiller. Here you will have to get performance curves from the manufacturer for different Condenser In temp and different loading as the design specs will assume a fix CW in temperature when in reality your CW in can vary. You have to monitor the chiller performance of a period of time using flowmeters and accurate thermistors (constantly checking heat balance) and plot the data against the curves for different CW in temperature - a 3 D charting tool will be an excellent way to do. Or else take out data for a range of CW in temperature within the design range and plot it against the design value.

    With the above you will know your chiller performance.

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