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  1. #1
    Join Date
    Apr 2004
    Napoleon, Ohio
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    I am a student tech at my factory. I was trying to determine if a roof top condensor AC unit was low on a charge. (r22) I had my gauges attached and used my temp/pressure chart (outdoor ambient temp + 35degrees heat of comp.) and looked up the high side pressure. The actual high side pressure was fluctuating with the condensor fan operation rather frequently. Can you reference the high side for this situation or do you just rely on low side pressures? When calculating what the low side should be (for an AC unit in an office)what temperature do you use? I have been using 45 degrees (minus an addition 15 degrees for internal refridgerant temp) If I use 45 degrees and the air blowing into the cooled room is 55 degrees, does this mean there is a problem? I guess I just need some real world rules vs book theories. Thanks again for your time.

  2. #2
    Join Date
    Aug 2003
    Fort Worth, TX
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    A combination of real world time and book time is the best way to go.

    As it is, your approach to charging R22 systems needs tweaking. Pressures tell only part of the story. The rest is obtained by superheat, subcooling, proper airflow, clean coils, clean filters, unrestricted supply and return air ductwork, no significant ductwork leaks, properly operating TXV or fixed orifice metering device, no refrigerant circuit restrictions (including dirty filter-driers), etc.

    Superheat and subcooling (SH/SC) take prime emphasis. But first establish that all the other items I listed after SH/SC are all good. It is pointless to attempt charging an a/c system where one or more of those things are out of whack. You'll never get it right, and could contribute to reducing the life expectancy of the unit. As techs we're supposed to do the opposite (prolong equipment life).

    In your case with this unit, if you found the condenser fan motors cycling, it's possible your unit has a low ambient kit installed and is regulating head pressure by cycling the condenser fan motor. This is done when a unit must operate in cooling mode in below normal outdoor air temperatures, or the unit is equipped with some form of capacity control.
    That being so, often a manufacturer will specify that a system with a low ambient kit or capacity control system must be charged with all outdoor fans running continuously, and possibly the compressor cylinder banks should be fully loaded (capacity control systems).

    Even so, knowing the expected superheat/subcooling parameters for the unit you are working on will help you know if you're in the ballpark when the system is operating under normal conditions. The manufacturer will often post these parameters on the unit itself.

    TXV systems are charged by subcooling.

    Fixed orifice systems are charged by superheat.

    Use your pressures mainly as a reference to obtain saturated vapor temperatures and saturated condensing temperatures.

    Invest in a good set of digital thermometers with type k thermocouples. They will quickly make your life a lot easier.

    Building Physics Rule #1: Hot flows to cold.

    Building Physics Rule #2:
    Higher air pressure moves toward lower air pressure

    Building Physics Rule #3:
    Higher moisture concentration moves toward lower moisture concentration.

  3. #3
    Join Date
    May 2004
    Los Angeles, CA
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    Dear Shophound,

    Well said......

    Respectfully Submitted,
    John J. Dalton

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