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  1. #1
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    Basic Doubt - Evaporation Temperatures and Cooling Power



    Good day,

    I have some doubts that certainly are basic for almost all that usually post in this forum, but for me not being a technician is something that I would like to clarify or understand better.

    Usually when we need to choose a condensing unit, we normally have a table with a range of evaporating temperatures and the rated cooling power for each one of those, that by what I remember if the evaporation temperature is high also the cooling power is higher, this means that if I calculate that I will need 2.5KW of cooling power and I have a condensing unit rated to 2500W cooling power evaporating at +7.2C (+45F), taking into account that I only will need to maintain a tank full of water at 24C this should be the right choice? Or I should pick up one that is rated to 2500w at +0C for example?

    Bringing this to another reality If I have an air conditioner condensing unit with 2.5KW cooling power (9000BTUs) more or less, that I think normally use a compressor of medium high temperature refrigeration (today almost are R410a), that I always have heart to say that should be evaporating between 0C and 10 C, I should get more cooling power if I put it evaporating at +0C or at +5C for example?

    I hope not being confusing with my doubts.



    Thank you

    Ric



  2. #2
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    Re: Basic Doubt - Evaporation Temperatures and Cooling Power

    Correct
    +5C

  3. #3
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    Re: Basic Doubt - Evaporation Temperatures and Cooling Power

    Your AC rated for 2.5 kW is indeed given at 7C but it will give +/-1.25 kW at -5C.
    For your application, a DT of 10K is fine, so you can choose 5C or 7C for a water temperature of 24C, even 10C will work.
    But you have to select also your heat exchanger (water/refrigerant) for this conditions, water temperature in, flow, capacity,...
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  4. #4
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    Re: Basic Doubt - Evaporation Temperatures and Cooling Power

    Thank you both for your answers,

    Peter_1, you have said that a given rating for a typical air conditioner outdoor condenser it is calculated based on an evaporator temperature of 7C, if this it was linear it is supposed to give me 1.77KW at 0C, do you think that this also mean the capillary metering device installed under those units it is calculated from factory to work in that range of evaporating temperatures (more or less from 0C to 10C?)

    I have installed a machine here just for testing, the label says that has 2.6KW of cooling but I do not know at what testing conditions this is calculated (Under the Label says ISO 5151 if someone know those values or have the document that could send me, I will be great full), when installed I have increase the charge because the evaporator is at 10 meters distance from the condenser instead of the original 3 meters from factory.

    When run it evaporating temperature is 6C but the amp meter show 3.7A to 3.8A amps, I think that this go beyond the amp limit indicated under the label from the condenser, I say that I think because the machine only have the rated power limit mention in watts (Cooling: 821W and Heating 1120W) and for cooling those should be Amps = 821watts / 230volts = 3.56Amps.

    After this I have recovered some refrigerant until the compressor reached an evaporating temperature of 0C to 1C with an outdoor temperature of 21C and in those conditions the amp meter indicated 3.1Amps.

    I have run it before with multiple outdoor temperatures and with 38C outdoor temperature for example the evaporating stabilizes between 3C and 4C and the amp meter marks 3.3A.

    The problem is that by my calculation the condenser is loosing lots of performance even at low outdoor ambient temperatures, for example with an outdoor temperature of 21C I have calculated that the condenser is producing only 1.5KW to 1.6KW, should this be normal?

    Anybody knows some functions to calculate condenser looses based under outdoor temperatures?


    Thank you for all

    Ric

  5. #5
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    Re: Basic Doubt - Evaporation Temperatures and Cooling Power

    As evaporating temperature increases, evaporating pressure increases. As evaporating pressure increases, the density of the refrigerant vapor increases. As the density of the refrigerant vapor increases, more refrigerant is forced into the cylinder on each downstroke of the piston, therefore the capacity of the compressor is increased.

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