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  1. #1
    Join Date
    Jun 2010
    Location
    Charlotte, NC
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    155

    Replacement chiller

    I need to replace an old chiller. It is designed to be an indoor water-cooled chiller. I am curious if it matters whether it is centrifugal or screw type.

    What are the advantages and disadvantages of each if any? The building as a fairly variable load.

    Thanks for any opinions and information.

  2. #2
    Join Date
    Jan 2008
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    Dallas ,Texas
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    What size chiller we talking about?

  3. #3
    Join Date
    Jun 2010
    Location
    Charlotte, NC
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    155
    Yes, I probably should have given that information.

    Between 250 and 300 tons.

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    That the lower end of sizing for centrifugals.

    My experience with Trane and York units at least is that the Centrifugals are more expensive up front, more expensive to rebuild and major repairs, but are more efficient. They don't unload quite as low as a screw machine 30-40%, so they can't swing as a screw machine. The screw machines seem to be a little more robust in terms of their ability ot operate with moisture, high approach, poor maintenance, incorrect charge etc. IF things get out of whack on a centrifugal, it can start surging and tear itself up in short order.

    Centrifugals are much quieter...especially compared to for example a York screw machine. Need earplugs around that puppy. The circulation pump is louder than the Trane centrifugals we have.

    That's my experience.


    IF you were going smaller, you could also look at scroll based machines. But they top out aroudn 130-150 tons I believe.

  5. #5
    Join Date
    Jan 2008
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    Dallas ,Texas
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    Like Moto said centrifugals are more efficient but a screw can be a bit more rugged as far as high condenser water is concerned . Most of the bigger screw compressors can be rebuilt just like a centrifugal. I would push the Carrier chiller first then a Trane but that's just my take on it.

  6. #6
    Join Date
    Jul 2012
    Location
    Western KY
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    1,221
    If the 250-300 ton is the top end of load and drops off from there the screw machines would make more sense IMO. I prefer the carrier water cooled screws myself but the Trane machines are good too.

  7. #7
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155
    Thanks everyone, for the great advice.

    It looks like the size will be 275 tons. It is for a 100,000 sf building in Georgia. That is what is there now, and I thought about reducing the size a little, since it never maxes out, but think I probably won't. That may be a little oversized, but looking at how the efficiency increases as you move to down 90% capacity and keeps on getting better down to 50%, I am not sure I want to go down to 250 tons, which might be a little undersized.

    But how low can I go before I have to worry about a modern design centrifugal surging? If I can get down to 20% or 30% of capacity safely, I imagine I will be able to go to economizer cooling at that point.

    I was surprised to see the Trane, using HCFC 123, seems to perform better than even magnetic bearing centrifugals. Are low pressure systems, or is HCFC, just that much better than HFC 134?

    Thanks again

  8. #8
    Join Date
    Jul 2007
    Location
    The Hot South
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    1,332
    Do you currently have 1 chiller for the building, or do you have multiple chillers on the same loop? If you only have 1, then I would go with 2 smaller chillers. That way, you have redundancy and can run more efficient at smaller loads. The up front costs will be higher but the first time the chiller goes down, you'll be wishing you had a backup. I would push Trane chillers first, then a Carrier IMHO.

  9. #9
    Join Date
    Dec 2007
    Location
    Soudern Kaliphonistan
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    281
    Quote Originally Posted by R123 View Post
    Do you currently have 1 chiller for the building, or do you have multiple chillers on the same loop? If you only have 1, then I would go with 2 smaller chillers. That way, you have redundancy and can run more efficient at smaller loads. The up front costs will be higher but the first time the chiller goes down, you'll be wishing you had a backup. I would push Trane chillers first, then a Carrier IMHO.
    14 years ago, we replaced a 250t centrifugal with (2) 120t screws. I cannot count the number of times I have thanked our good Lord above for providing me the courage to stand up to the bean counters when this project was on the drawing board.

    as stated, more up front cost.......well worth it from a maintenance and engineering perspective.....

    good luck with the project!

  10. #10
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
    Posts
    897
    give us a call at mann mechanical, mannmechanical.com and give us a chance to bid on it

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