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  1. #1
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    Capacity Control Pressure Settings



    Hi Guys!

    I am looking for some advice on how pressure switches should be set. I am working on a system with a freezer room and cold room. Freezer set point -20 and cold room set 4 degrees. The compressor has capacity control (50/100%). The gas is R404a (30kg).

    The suction pressure is always around 1 Bar until it reaches temperature and pumps down.

    LP switch is set for 1 bar with a differential of 1.
    The capacity control switch is set for 2.5 bar with a differential of 0.5.

    Can anyone tell me if these are the correct settings? I am relatively new to the trade. My understanding was that if its running around 1 bar it should be on @ 50%. Then if the suction pressure was to rise (for example) to 3 bar it would kick in to 100% capacity again to bring it back down to 1 bar?

    The system always runs at 100% capacity no matter what, i don't think this is right?

    Any information appreciated, Thank you.



  2. #2
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    Re: Capacity Control Pressure Settings

    KegSalad,
    If you have app for pressure temperature or chart, convert pressures to saturated temperature for that refrigerant.
    The pressure/temperature should be 5-10 deg C lower than your room temperature.
    Mid any lower then is to cold & probably unload compressor.
    Prove that unloaded actually works & behaves by unloading when pressure lowers & loads when pressure increases.
    Try all that & see what you come up with.

  3. #3
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    Re: Capacity Control Pressure Settings

    .

    The issue you have is the 2 temps in the 2 cold rooms.

    The freezer will need an evaporation pressure of 1 - 1.5 barg and the fridge should be at 3 - 4 barg. Do you have evaporator pressure regulation fitted to the fridge side of system?

    It's not good for the fridge to run with the same pressure as the freezer. Then because you have 50% - 100% capacity control you basically have all or nothing. If it is running continually it is because the load is there and the demand is there.

    Does it have evaporator pressure contol?

    Rob
    .
    Last edited by Rob White; 09-03-2019 at 10:23 AM.
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  4. #4
    Join Date
    Sep 2016
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    Re: Capacity Control Pressure Settings

    Hi there,

    Thanks for the replies.

    Yeah there is an EPR valve fitted to the cold room and this is set to 2 bar?

    I just thought when running at 1 bar suction the compressor should only be running @ 50% load.
    Then when the suction pressure rises to 2.5 bar it will cut in @ 100% load to bring it back down.

    Is what i'm thinking right? Or am i on the wrong track here?

    Thanks in advance for any info.

  5. #5
    Join Date
    Dec 2007
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    australia
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    Re: Capacity Control Pressure Settings

    Kegsalad,
    Check my earlier post & see what you make of it.
    I'm not being critical, but trying to show a way to analyze system.
    There is a lot of questions, but that's how you work thing out.

    I'm assuming you are a refrigeration apprentice.

    This small system seems simple, but nothing is simple when you start asking a few questions.

    1/ If unloader set at 2.5 bar with .5 bar differential & you are running at 1 bar then how do you now unloader is working at all, check it out.

    2/ From settings you give, correct settings or not ,it should be unloaded, does it work?

    3/ Do rooms get down to temperature at any point, as that's when unloaders would or could work, as well as when freezer on defrost.
    4/ are evaporators clear of ice, are all fans working.
    5/ does refrigerant charge seem ok, full sight glass.
    6/ what is discharge pressure/temperature & ambient temperature
    7/ chiller back pressure regulator set at 2 bar = -20 deg is to low for 4 deg room, it should be about 4 bar =-5 deg C
    8/ what are the room temperatures
    9/ is there actually anything wrong, or are you observing the unloaders only?

    Alway convert pressure to temperature, otherwise it means very little.

    R404a
    1 Bar = -30 deg C
    2 Bar = - 20 deg C
    2,5 Bar = -15.7 deg C
    LP switch set 1 Bar cut in -30 deg cut out 0 Bar = -45.6 deg C

    From this unloader does not necessarily have to be unloaded, but close, as 1 bar = -30 deg C, which is 10 deg below freezer room set point. If it did unload then it would have to load again at -25 deg to maintain -20 deg room temperature. Sometimes if you do that it might unload, then load again straight away because the load of the system is there, but not enough for fully loaded & to much for unloaded
    It may only unload if on defrost, or one of the rooms is cycled off on temperature.

    At the moment unloader does not appear to be working if scale on controller is to be believed.

    If either not down to temps then fully loaded assuming they have a liquid line solenoid for each.
    Room temperature thermostats also need to be checked to see if set at correct temperature & differential.
    Last edited by RANGER1; 10-03-2019 at 08:55 PM.

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