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  1. #1
    Join Date
    Dec 2007
    Posts
    23

    OA strategy if compressor trips off on LP

    The preview of the building is it is based on condernser system with AHU on each floor.

    I got a comment from contractor as follows;
    ''I am having to stop the O/A earlier so that I can have sufficient cooling (Compressor) to counteract the solar load on the NE side + other higher load areas.
    There is one floor that I have been monitoring with this problem and finding that I cannot allow the O/A to run with the compressor as it then causes the Suction Pressure to drop too much and will cause the compressor to trip it’s LP control/flood back and damage valves etc.''

    Please advise me if there are any chances of getting OA , because as of now , we are locking OA despite OAT being 9-12 degree C.

    Would appreciate if anyone can give some more insights on it.

  2. #2
    Join Date
    Mar 2008
    Location
    Indy
    Posts
    355
    Tune your discharge air PID loops on the air handlers.
    P.S. I'm a lazy American, What is 9-12 Deg C?

  3. #3
    Join Date
    Dec 2007
    Posts
    23
    I mean keeping OA damper 100% open till 19 degree OAT while the first compressor that particular floor AHU kicks in at 15.4 degree C

  4. #4
    Join Date
    Dec 2007
    Posts
    23
    Quote Originally Posted by Heavyevans View Post
    Tune your discharge air PID loops on the air handlers.
    P.S. I'm a lazy American, What is 9-12 Deg C?
    Thanks can you eleborate more on it?

  5. #5
    Thats 48 to 66 for us yanks.

    Sounds like you just dont have a load to me. Returns open? Why can't the economizer keep up at least until 55 outside. Mixing box to small? To small OA intakes and the unit starves in econ? Better window coverings or stat location?

  6. #6
    Join Date
    Mar 2008
    Location
    Indy
    Posts
    355
    Quote Originally Posted by bennyk View Post
    Thanks can you eleborate more on it?
    Pressure and temperature are proportional to each other. If the AHU's are fully functional and operating as designed, and the LP switches are opening, then your discharge air is getting too cold.

  7. #7
    Join Date
    Feb 2007
    Location
    Wilmington, NC
    Posts
    18
    You need a enthalpy sensor. It will lock the compressor out below 70* F and 50% you can adjust the temp down lower. 70 is the max. Also I'd check the fresh air intake for proper and the mixing box like someone else said.

  8. #8
    Join Date
    Feb 2008
    Posts
    225
    Like others: Adjust you DA pid loop to maintain your setpoint if
    you trying to maintain discharge temp.

    In my opinion you SHOULD NOT use a DX cooling unit to maintain discharge.

    I would try to maintain return / space Temp then have the mechanical
    contractor check the charge in the unit because it should not
    drop below 38 degrees.

  9. #9
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,707
    Do the condensing units have a low ambient kit on it . If so units should be able to run normally at low ambient conditions without problem .

  10. #10
    Join Date
    Sep 2006
    Location
    Rhode Island
    Posts
    244
    If your outside air is below 55 deg f why do you need cooling? It seems to me that you are missing an opportunity for free cooling by closing your outside damper at that temperature. Do your economy damper controls have a mixed air sensor? If so why not just increase the mixed air temperature setting to a little higher temperature instead of closing the OSA damper?

    I think someone else already mentioned it..... You might want to ask your contractor if he checked the refrigerant charge. It may possibly be a little low.

  11. #11
    Join Date
    Dec 2007
    Posts
    23
    Quote Originally Posted by shifferbrains View Post
    If your outside air is below 55 deg f why do you need cooling? It seems to me that you are missing an opportunity for free cooling by closing your outside damper at that temperature. Do your economy damper controls have a mixed air sensor? If so why not just increase the mixed air temperature setting to a little higher temperature instead of closing the OSA damper?
    Thanks
    Thats exactly what I wanted to do... Look at the my contractor comments which I am still struggling to understand.

  12. #12
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281
    Talk to your mechanical and advise them to install "Hot Gas Bypass" at the minimum and the prementioned low abient if needed, unloading if ya can as well helps if you can and/or they can add?

    Controls can only fix so much.

  13. #13
    Join Date
    Sep 2006
    Location
    Rhode Island
    Posts
    244
    If your outside air is below 55 deg f why do you need cooling? It seems to me that you are missing an opportunity for free cooling by closing your outside damper at that temperature. Do your economy damper controls have a mixed air sensor? If so why not just increase the mixed air temperature setting to a little higher temperature instead of closing the OSA damper?

    I think someone else already mentioned it..... You might want to ask your contractor if he checked the refrigerant charge. It may possibly be a little low.

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