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Thread: Subcooling

  1. #1
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    Subcooling



    Dear friends,
    I have simple question about subcooling I can not understand.

    In sources I see that condensers splitting in 3 section. De superheater/Condenser/Subcooler.
    It means above of air-cooled condenser refrigerant de-superheates, at middle condenses and at bottom subcools.

    Now we have R22 and weather is 45C and we have big condenser with Delta T = 5C. it means condensing temperature would be 45+5=50C and condenser pressure would be 320 PSI.

    Liquid subcooling means that liquid temperature at outlet of condenser must be lower than 50C. For example if outlet temperature of liquid at condenser/receiver be 44C it means we have 50-44 = 6C subcooling.

    Even we use very big air-cooled condenser twice we can not make liquid line temperature same as weather. It means also with very big condenser when we touch liquid line it is a little warmer than weather temperature (because condenser has delta T).

    Now what is a meaning of subcooling here ??? Even by increasing condenser surface we can not change liquid temperature lower than weather temperature !

    Sincerely yours.



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    Re: Subcooling

    Bigger condenser will reduce delta T.

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    Re: Subcooling

    Quote Originally Posted by A.Mortezania View Post
    Now what is a meaning of subcooling here ??? Even by increasing condenser surface we can not change liquid temperature lower than weather temperature
    Subcooling is measured using the High side pressure. This will be higher than the ambient temperature.
    Only water cooled condensers can go below ambient.
    Last edited by NH3LVR; 21-12-2017 at 07:58 PM. Reason: Did not use quotes correctly

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    Re: Subcooling

    Quote Originally Posted by Segei View Post
    Bigger condenser will reduce delta T.
    Dear Segei,
    Yes you are right, With bigger condenser we decrease delta T. I accept this. But even you increase condenser 10 times twice you could not have liquid line cooler than environment temp (You can not reach to delta T =0) .
    Also in this situation when you touch liquid line it is little warmer than around air.

    Sincerely yours.

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    Re: Subcooling

    Quote Originally Posted by NH3LVR View Post
    Subcooling is measured using the High side pressure. This will be higher than the ambient temperature.
    Only water cooled condensers can go below ambient.
    Dear My fiend,
    OK. Let me check water cooled condenser.

    Suppose that inlet water temp to condenser is 30C and if we have delta T = 5C then our condensing temp Tc would be 35C and discharge pressure with R22 would be 220 PSIg.
    Twice if we have for example 6C liquid subcooling, It means liquid line must be 35-6=29C !
    If you increase heat rejection area on condenser twice you can not reach to liquid line 29C or even 30C.

    We could have subcooling if for example you enter liquid line in extra heat-exchanger and cool it more by expansion valve or any other cool sources.
    But I can not understand how can we have subcool liquid at condenser when we can not have delta T = 0 or Delta T < 0 !

    Sincerely yours.

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    Re: Subcooling

    I was running out to an appointment and in a hurry yesterday. I was not clear when I said water cooled, I meant to say evaporator condenser.
    See this link for a explanation of how you can go below ambient.
    https://en.wikipedia.org/wiki/Evaporative_cooler
    Although a properly install Evap will not sub cool substantially if it is free draining to a receiver.

    As to the air cooled condenser sub cooling, the easiest way to understand this is to put your measuring gear on an operating condenser with a sub cooling section. It will become apparent very quickly as I explained in a my previous post.

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    Re: Subcooling

    As I can see you talk only about design conditions (air 45C or water 30C). If you have air 0C you will get a lot of subcooling. A refrigeration plant never run at design conditions it merely cross them at times.

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    Re: Subcooling

    Quote Originally Posted by Segei View Post
    As I can see you talk only about design conditions (air 45C or water 30C). If you have air 0C you will get a lot of subcooling. A refrigeration plant never run at design conditions it merely cross them at times.
    Dear Segei,
    Yes I agree with you. I say simple example for you. Suppose that at desert we use air-cooled condenser and for example discharge pressure with R22 is 300 PSI and outlet of condenser is 100% saturation liquid but warm.
    Wen you splash water on surface of condenser fins, you decrease discharge pressure, Then you will also decrease dew point of refrigerant on air-cooled condenser and also liquid line also would be cooler, You are right.
    By splashing water on fins also you reduce dew point on condenser, Dew point and pressure of discharge both of them decrease and twice where is our sub-cooling ?

    Sub-cooling means when condensing pressure be constant then you have liquid line cooler than environment.

    At your example when air temperature is 0C, discharge pressure would not be 300PSI, at 0C air temperature with delta T = 10, Discharge pressure would be 105 PSI and dew point is 10C. Twice when you touch liquid line it is a little warmer than 0C !

    I know we can use extra heat-exchanger at liquid line and name it subcooler for sub-cooling liquid, My question is it : at air-cooled condenser can we have subcool liquid at outlet of condenser or we can not ?

    (Sorry my mother tongue is not English, I do my best to explain clear to understand better. I apologize if for every item I explain more than need to transmit my mean better)

    Sincerely yours.

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    Re: Subcooling

    Quote Originally Posted by A.Mortezania View Post
    Sub-cooling means when condensing pressure be constant then you have liquid line cooler than environment.
    .
    Incorrect.

    The term subcooling refers to a liquid existing at a temperature below its normal boiling point.
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    Re: Subcooling

    Look at condensation process. In condenser refrigerant gas becomes liquid. This liquid have saturated temperature. Outlet or bottom of the condenser is flooded. In this flooded area liquid refrigerant will be cooled below saturated temperature. Subcooling is temperature difference between saturated temperature and outlet temperature. Outlet or bottom of evaporative condensers are not flooded and outlet liquid has saturated temperature. R22 condensing pressure is 300 psig. Saturated temperature is 55C. If you cool liquid refrigerant to 50C, subcooling will be 55-50=5C. At air temperature of 0C you can get condensing pressure of 300 psig if condenser operate at reduced capacity. Assume that air condenser has 20 fans but you run only 1 located at outlet of the condenser. Condensing pressure will be 300 psig but you will be able to get subcooling 30-40C.

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    Re: Subcooling

    Quote Originally Posted by Segei View Post
    Look at condensation process. In condenser refrigerant gas becomes liquid. This liquid have saturated temperature. Outlet or bottom of the condenser is flooded. In this flooded area liquid refrigerant will be cooled below saturated temperature. Subcooling is temperature difference between saturated temperature and outlet temperature. Outlet or bottom of evaporative condensers are not flooded and outlet liquid has saturated temperature. R22 condensing pressure is 300 psig. Saturated temperature is 55C. If you cool liquid refrigerant to 50C, subcooling will be 55-50=5C. At air temperature of 0C you can get condensing pressure of 300 psig if condenser operate at reduced capacity. Assume that air condenser has 20 fans but you run only 1 located at outlet of the condenser. Condensing pressure will be 300 psig but you will be able to get subcooling 30-40C.
    Dear Segei,
    Ok. I understood your mean.
    I have a question regarding your saying, Please make it clear if we turn off fans during low ambient temperature it is important that we turn off fans at inlet of discharge pipe or at outlet of condenser ?
    If it is possible make it clear for me.

    Sincerely yours.

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    Re: Subcooling

    I've understood the theory that the outlet fan should stay on to cool refrigerant and cycle inlet to ensure hot gas is present there to help force liquid out instead of it just sitting there.
    That seems logical but I've seen freezer systems with fans working opposite to this with no grief and twin fan AC systems will keep the top fan (condenser inlet) on so the air is also being blown over the inverter board and cycle the lower fan if required.
    I guess in practice you'd be unlucky to have a problem if the outlet fan was switched.
    Cheers and happy christmas!
    Andy.

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    Re: Subcooling

    Quote Originally Posted by A.Mortezania View Post
    Dear Segei,
    Ok. I understood your mean.
    I have a question regarding your saying, Please make it clear if we turn off fans during low ambient temperature it is important that we turn off fans at inlet of discharge pipe or at outlet of condenser ?
    If it is possible make it clear for me.

    Sincerely yours.
    You can have another fan sequence and system will work. However, I think that subcooling will be greater if outlet fan cycle last. Typically, I work with industrial refrigeration and with evaporative condensers. To get more information about this issue, I suggest you to find posts of Glenn Moore. Recently, he has written very good posts about air condensers.

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