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  1. #1
    Join Date
    Oct 2005
    Location
    BC
    Posts
    29

    Heat Recovery and Airside Economizer

    We're in the process of putting together a proposal to retrofit 3 rooftop mounted 35 ton DX AHUs with economizers. The system will be converted to operate with a new air cooled heat recovery chiller and chilled water coils will replace the DX coils in the AHUs.

    This is a VAV with HW reheat system, so the recovered heat will be used throughout much of the year in place of or as a supplement to the boiler. Now what I'm wondering is if in lower ambients there's any point at which it would make sense to shut down the mechanical cooling in favor of free cooling (but no heat recovery).

    I did some looking online and found some mention of heat recovery and economizer coordination but no specifics of how this would be implemented.

    Anyone have experience with this?

    Let me know if more detail is needed regarding the existing and proposed system arrangement.

  2. #2
    Join Date
    Aug 2009
    Location
    Kansas City
    Posts
    20
    Are you talking about a water cooled heat recovery chiller that uses condenser water to supplement the reheat system while simultaneously creating chilled water?

    Running the mechanical cooling while the ambient temp is low enough to provide full cooling to the building with the economizers is a waste of energy and violates every energy code that I've seen (may not be relevant in your state or city).

    The heat recovery chiller uses electricity to supplement the reheat. If you have electric boilers, it would be more efficient to let them pick up the reheat load during economizer hours. If you have fossil fuel fired boilers, it is more cost effective to run the boilers with gas or oil vs electricity to supplement the reheat system.

  3. #3
    Join Date
    Oct 2005
    Location
    BC
    Posts
    29
    Thanks for the input brewidus.

    Yes, this is an air cooled chiller (Carrier30RB) capable of full heat recovery with its auxillary coaxial condenser. So the hot water reheat loop would be piped into this auxillary condenser which receives hot gas whenever reheat loop is below setpoint.

  4. #4
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,711
    Quote Originally Posted by refer madness View Post
    Thanks for the input brewidus.

    Yes, this is an air cooled chiller (Carrier30RB) capable of full heat recovery with its auxillary coaxial condenser. So the hot water reheat loop would be piped into this auxillary condenser which receives hot gas whenever reheat loop is below setpoint.
    So there is a main boiler? What kind of building is this?
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  5. #5
    Join Date
    Oct 2005
    Location
    BC
    Posts
    29
    Quote Originally Posted by Dallas Duster View Post
    So there is a main boiler? What kind of building is this?
    Yes, there's a 70% efficiency boiler (1800MBH) with 3 way mixing valve which resets HW loop temp off of OAT . It's a 3 story office building. Right now the boiler supplies 100% of the heating requirements of the building at VAV unit HW reheat coils throughout the building.

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