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  1. #1
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    What type of chiller is this?

    I am fairly need to the world of HVAC.....still trying to learn as much as I can.

    Today, while at work I was talking to a guy who used to be a central plant operator at a 56 story office building.

    He told me about their chilled water system...I know I am green but I have never heard of a system like this.

    He said that the system "is a 10 degree split" and that "the chiller does not run constantly but rather cycles on and off"

    First, I have no idea what a "10 degree split" means...

    Secondly, I had no idea there was such a thing as a chiller that cycles on and off....I am not sure how that would even work? Is there a name for this type of chiller?

    The only chillers I have ever seen--- run constantly---- either cooling the building directly or....... charging a thermal storage system during at night during off peak hours.

    Thanks

  2. #2
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    The "10 split" means that the whole system is designed around a 10 ∆T temp rise. At full flow through the chilled water coils, you'll have a 10 difference.
    As for the chiller cycling, not sure there. Even old recip chillers had the capability of loading and unloading. The only time it would cycle is if the building load was less than the lowest load the chiller could run down to. At that point, there would be a temperature offset to keep it from turning on and off rapidly. For ballpark numbers, say you have a chilled water setpoint of 45. If the chiller hits 40, it would shut off. When the chilled water rises to 50, it would start again. Hope that helps.
    -----Stop, step back, relax and have another go at it.-----

  3. #3
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    Thread Starter
    He also told me that the chillers heated the building in the winter using something called "Hot-gas bypass"....Does anyone know how this works? Thanks

  4. #4
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    Quote Originally Posted by CharlesNeal23 View Post
    He also told me that the chillers heated the building in the winter using something called "Hot-gas bypass"....Does anyone know how this works? Thanks
    That statement is incorrect. Hot gas bypass is internal to the chiller and only "false loads" the chiller during low loads to keep it from cycling off. All it does it dump hot gas from the condenser into the evaporator.
    If it heats, then there must be a separate condenser on a closed loop. This is usually called heat recovery.
    -----Stop, step back, relax and have another go at it.-----

  5. #5
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    Quote Originally Posted by BennyD View Post
    That statement is incorrect. Hot gas bypass is internal to the chiller and only "false loads" the chiller during low loads to keep it from cycling off. All it does it dump hot gas from the condenser into the evaporator.
    If it heats, then there must be a separate condenser on a closed loop. This is usually called heat recovery.
    Yes, he did say it is a heat recovery system.
    However I am still confused how "hot gas bypass'' is used for heat recovery? He wasn't able to expain it so I understood....

    I did a search on the web, but apparently this a rare design.....Does anyone have any further reading on how this might work--I'd love to see a schematic etc.....I am very curious.

    Thanks

  6. #6
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    These are not rare! Anyway, the biggest reason for hot gas bypass in heat recovery is that the heat needs to come from the chilled water loop. If there isn't much there (winter), the chilled water temp will keep dropping and the chiller will shut off. The hot gas bypass will keep the chiller running longer before shutting off. A unit should not remain in hot gas bypass for too long, I try to keep it to a half hour, then let the unit cycle off.
    On another note, hot gas bypass and heat recovery are 2 separate entities that compliment each other. Search each one independently. You can have one without the other or both, it depends on the design of the system.
    -----Stop, step back, relax and have another go at it.-----

  7. #7
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    To expand on what Benny said in post #2, if the building has a chiller plant with multiple chillers and it is operated by a Building Automation System (this a very common setup), then as load varies one of the chillers could be automatically started and stopped as the day goes on. For example, a plant with two 500 ton chillers. A building that requires 700 tons during the heat of the day, but only 300 tons during the cool night. It will need both at some times, and only one at others, to satisfy the building. Even then, with a typical office building, and a properly set up BAS, that should result in only one start and one stop per day.

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  9. #8
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    Quote Originally Posted by Nuclrchiller View Post
    To expand on what Benny said in post #2, if the building has a chiller plant with multiple chillers and it is operated by a Building Automation System (this a very common setup), then as load varies one of the chillers could be automatically started and stopped as the day goes on. For example, a plant with two 500 ton chillers. A building that requires 700 tons during the heat of the day, but only 300 tons during the cool night. It will need both at some times, and only one at others, to satisfy the building. Even then, with a typical office building, and a properly set up BAS, that should result in only one start and one stop per day.
    Well said!
    -----Stop, step back, relax and have another go at it.-----

  10. #9
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    Think of the basic refrigeration cycle. The evaporator is made to get cold and so it absorbs heat. The condenser is made to get hot and so rejects heat.

    In the case of a water cooled chiller the condenser rejects the heat into water. The water is then usually cooled by a cooling tower or dumped down a drain.

    A condenser on such a chiller can be used to instead supply that heated water as a way to heat a conditioned space - rather than putting it outside somewhere.

    "Hot Gas Bypass" really means: "bypassing the condenser". Hot discharge line vapor is put into the evaporator - typically between the TXV and the evaporator inlet. Because the hot gas did not have it's heat removed in the condenser - it serves to supply that heat to the evaporator. This is used to keep the machine running even though there is not enough external heat being supplied to the evaporator.

    See what I mean?

    PHM
    ------------




    Quote Originally Posted by CharlesNeal23 View Post
    Yes, he did say it is a heat recovery system.
    However I am still confused how "hot gas bypass'' is used for heat recovery? He wasn't able to expain it so I understood....

    I did a search on the web, but apparently this a rare design.....Does anyone have any further reading on how this might work--I'd love to see a schematic etc.....I am very curious.

    Thanks
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  11. #10
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    Do you know what kind and/or brand of chillers these are? I'm guessing a centrifugal based off the size of the building.
    -----Stop, step back, relax and have another go at it.-----

  12. #11
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    Rule of thumb is the larger the motor, the less it likes being cycled often. When started, it takes a finite time for the motor whether hermetic or air cooled to remove the heat from inrush or locked rotor to be removed from the windings. Speaking as retired Carrier rep since 1976.

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