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  1. #1
    Join Date
    Jul 2008
    Posts
    31

    split system capacity vs outside temp

    How do the ratings of split system air conditioning systems vary from the published values of total and sensible capacity (which are generally published at ratings of 85 to 105 degrees F) if the outdoor air temperature is only 70degrees F?

    I need to ask this question, because I have never designed a cooling system for an environment where the outside summer design temperture is only 70 degrees.

  2. #2
    Join Date
    Sep 2008
    Location
    Delaware
    Posts
    243
    Where, pray tell is it only 70 degrees? It gets hotter than that in Alaska. Around here we go for economizer when it gets down to 70. Your humidity can be a big issue and what conditions are you going for?

  3. #3
    Join Date
    Jul 2008
    Posts
    31
    Quote Originally Posted by Chiller Guy View Post
    Where, pray tell is it only 70 degrees? It gets hotter than that in Alaska. Around here we go for economizer when it gets down to 70. Your humidity can be a big issue and what conditions are you going for?
    Eureka, California. The main cooling load is the equipment load in the building. 64 percent of the total cooling load is equipment and lighting load.

  4. #4
    Join Date
    Feb 2008
    Posts
    24
    Design OA temp for Eureka California should be closer to 95 F

  5. #5
    Join Date
    Jul 2008
    Posts
    31
    Look in ASHRAE. It is 70, and the site I am designing for is less than 0.25 miles from the coast.

  6. #6
    Join Date
    Jul 2008
    Posts
    31
    No one has answered the original question yet.

  7. #7
    Join Date
    Feb 2008
    Posts
    24
    Draper1,

    Capacities will generally increase under lower than ARI rated conditions.
    How much depends on the manufacturer.

  8. #8
    Join Date
    Jul 2008
    Posts
    31
    Quote Originally Posted by ToeMoss View Post
    Design OA temp for Eureka California should be closer to 95 F
    The all time record after 92 years of record keeping is 87 degrees:
    http://www.ncdc.noaa.gov/oa/climate/...xtrememax.html

  9. #9
    Join Date
    Feb 2008
    Posts
    24
    I stand corrected.

  10. #10
    Join Date
    Sep 2008
    Location
    Santa Barbara, CA
    Posts
    7
    What you can do, as a designer, is check the refrigeration effect on a Mollier chart for the refrigerant you're using. Make one path at 85F and another at 70F condensing, and use the percentage increase that you calculate.
    You should check with the manufacturer's literature that it is rated for 70F - it probably is, but if you had a fixed orifice metering device, you may find the system at 0 superheat and returning liquid to the compressor.

  11. #11
    Join Date
    Mar 2004
    Location
    Austin, Texas
    Posts
    1,959
    If you want to do it the hard way - the ARI web site has some equations in one of the online publications that are used to predict performance at different conditions.

    If you want a somewhat easier way, plot the performance data you have and extend that line to the point you need (graphical extrapolation).

    If you want a much easier way - get a copy of the equipment manufacturer's selection software or have them run it for you.

    If you want a much easier way - install economizers and don't use A/C.

    My inlaws are from Eureka. I need to go back there sometime in the summer. But for some reason, I'm always too busy at home during the summer.
    If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?

  12. #12
    Join Date
    Jul 2008
    Posts
    31
    I contacted Trane and had the rep just run the program they have.

  13. #13
    Here in Canada we have our OA set for 55F and MA around 52F. As long as you keep your Delta T at 20F and your TD at 30F your system is running fine. Just make sure your super heat is in line via TX or subcooling Via orfice. Needless to say you would have to put a winter kit in for anything lower to stop the migration of refrigerant, but that's more for your IT rooms.

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