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  1. #1
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    Abnormal sub-cooling - lack of refrigerant



    Hi!
    I need your help to understand something about subcooling...
    In particular i don't uderstand why a low subcooling (less than 4C), in some case, takes place
    when there's a lack of refrigerant.
    In my opinion, i belive that a lack of refrigerant should cause the increase of subcooling, bucause the energy provided from the air (system condensed in air) is moved to less quantity of refrigerant.
    I think it is the same situationt that occurs when i cool by outside a flow water in a pipe. If i decrease the flow, the temperature of the water decrease (DT).

    TX!



  2. #2
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    Re: Abnormal sub-cooling - lack of refrigerant

    .

    Don't confuse the definition.

    Increase subcooling = the liquid is colder than the condensing temp
    decrease subcooling = the liquid is nearer to the condensing temp.

    Sub cooling for free can only ever achieve about 4 to 5 degs of subcooling
    because the heat transfer from the liquid to the ambient air around it
    has its limits (namely the ambient temp itself)

    If you deliberately design subcooling and include something to actually
    subcool the liquid, you have to refrigerate the liquid line and then you can
    add any amount of subcooling (but at a cost).

    If a system is short of refrigerant then you tend to have no subcooling
    and that is why you see bubbles in the sight glass.

    But because of the way blended (zeotropic) refrigerants react you can
    still have bubbles in liquid even though the system has some subcooling.

    So remember the basics subcooling is reverse to superheat.

    The more the subcooling the colder the liquid is below condensing temperature.

    Regards

    Rob

    .

  3. #3
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    Re: Abnormal sub-cooling - lack of refrigerant

    Ambient subcooling is based on the Saturated cond temp and outdoor air. Obviously you can not get liquid colder than ambient air temp. The reason there is low subcooling when low on refrigerant is because there is not enough refrigerant to flood the condenser and allow the refrigerant to be subcooled. As you increase the amount of refrigerant in the system there is more refrigerant in the condenser which will raise subcooling because there is more liquid in condenser(flooded) and it can be subcooled. Also subcooling is increased because as you add refrigerant your SCT will get higher.

  4. #4
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    Re: Abnormal sub-cooling - lack of refrigerant

    Quote Originally Posted by Coorsman777 View Post
    Ambient subcooling is based on the Saturated cond temp and outdoor air. Obviously you can not get liquid colder than ambient air temp. The reason there is low subcooling when low on refrigerant is because there is not enough refrigerant to flood the condenser and allow the refrigerant to be subcooled. As you increase the amount of refrigerant in the system there is more refrigerant in the condenser which will raise subcooling because there is more liquid in condenser(flooded) and it can be subcooled. Also subcooling is increased because as you add refrigerant your SCT will get higher.
    Sorry but I'm continuing to don't understan.
    If you sustain that in the condenser there's a low quantity of fluid refrigerant, in parity of energy moved, the temperature of the fluid refrigerant should be lover (increasing the S/C). Just for example if you heat with the same fire two different quantity of water, the lover quantity will reach the higher temperature because :

    the heating exchange equation is :

    Q=m*cp*DT

    A parity amount of heating, it causes a different DT depending on the quantity m ( of course for the same fluid - cp= constant).
    In the same time if decrease the flow of water in a refrigerant coil (air/eater), in parity of air flow, will occur that the DT of the water will increased!

    Just a question... Do you retain that a lack of refrigerant cause, inside the chilling circuit, a decreasing of mass flow?I think yes
    I do not think that there is a variation of the volume flow.

  5. #5
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    Re: Abnormal sub-cooling - lack of refrigerant

    Both pots of water will get to the same temp which will be determined by the heat of the fire. The lower quantity will get there faster.

    The refrigerant will only get as cold as the ambient air will allow. As you add refrigerant your SCT goes up and ambient stays the same therefore your subcooling goes up.

    Subcooling is you liquid refrigerant leaving temp subtracted from your SCT. If you have a and ambient of 70F a liquid temp of say 71F and a SCT of 75F you have 4F subcooling. You add refrigerant and raise the SCT to 80F with more refrigerant in the condenser and have liquid temp of 73F you now have 7F subcooling.

    I guess what I am trying to say is with less refrigerant you can have the same condenser outlet temp with less subcooling.

  6. #6
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    Re: Abnormal sub-cooling - lack of refrigerant

    arien,
    Unless you have a level of "liquid" in the condenser & liquid reciever you cannot subcool it.

    If it is short of refrigerant its not a true & correct cycle so calculations mean nothing.

    Tell us why you think you need subcooling!

  7. #7
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    Re: Abnormal sub-cooling - lack of refrigerant

    Under sized lquid lines will create sub cooling and short of refrigerant indicators at expansion valve.

  8. #8
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    Re: Abnormal sub-cooling - lack of refrigerant

    Quote Originally Posted by RANGER1 View Post
    Tell us why you think you need subcooling!
    subcooling is necessary to goes up the efficiency of the refrigerator!

    Is possibile that, because of is present a lack of refrigerant, the flow spedd inside the pipe of condenser is lover and consequently the exchande decrease. If decrease the exchange in the same way i don't have a high value of subcooling.

  9. #9
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    Re: Abnormal sub-cooling - lack of refrigerant

    Lack of refrigerant does not change the (volumetric) size of your compressor.

    If you had a whole lot extra air cooled condenser, the best it could do is lower the condensing pressure to just above ambient temperature. Depending on how a condenser is "drained" an excess of condenser will not by itself create subcooling....Subcooling can only be created in the condenser if there is reason for liquid refrigerant to reside in the condenser; and even then the temperature of the liquid can only just approach ambient.

    So if you are running short charged, you will not have any excess refrigerant to reside in the condenser; so subcooling will decrease. Also running short charged tends to feed some extent of excess vapor to the evaporator, which means its internal surface is not wetted, which means its heat transfer is not good, and again depending on control, the evaporator will never make load; and probably will run at low suction.

    All of which says the circuit's performance will not be good and the compressor will run a long period of time to make the target temps or conditions: or a high proportion of time; so your COP is p-poor.

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