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  1. #1
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    Question about sub-cooling



    Dear Friends,

    Which sentences below about sub-cooling are true ?

    1 – At condenser with receiver tank we have not sub-cooling because we have mixture of "liquid + gas" at saturation area inside of receiver tank.

    2 – At condenser with receiver tank we have sub-cooling but we can't increase sub-cooling

    3 – At condenser without receiver tanks by increasing refrigerant we can increase sub-cooling (I saw this sentence at SWEP book)

    Which sentence is correct ?

    Sincerely yours.



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    Re: Question about sub-cooling

    Are we doing your examination questions now?
    Brian - Newton Abbot, Devon, UK
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    Re: Question about sub-cooling

    Quote Originally Posted by Brian_UK View Post
    Are we doing your examination questions now?
    Dear Brian_UK,
    Really it is not examination, Trust me. In each source I see a different meaning, It made me confused, I want to know finally which one is true about meaning of sub-cooling.

    Sorry if I bothered you.

    Sincerely yours.

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    Re: Question about sub-cooling

    No bothering, no worries.
    Brian - Newton Abbot, Devon, UK
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    Re: Question about sub-cooling

    1. This is disputed between engineers and can definitely vary depending on piping configurations
    2. partly correct and incorrect, if you started charging a system the sub-cooling would increase as you add more refrigerant, until the receiver starts holding liquid, then it will stay constant until the receiver is full and starts filling up the condenser with liquid, at this time the sub-cooling will continue to increase at you add more refrigerant.
    3. with no receiver, sub-cooling is varied by the refrigerant charge alone.

    hope this helps

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    Cool Re: Question about sub-cooling

    Quote Originally Posted by A.Mortezania View Post
    Dear Friends,

    Which sentences below about sub-cooling are true ?

    1 At condenser with receiver tank we have not sub-cooling because we have mixture of "liquid + gas" at saturation area inside of receiver tank.

    2 At condenser with receiver tank we have sub-cooling but we can't increase sub-cooling

    3 At condenser without receiver tanks by increasing refrigerant we can increase sub-cooling (I saw this sentence at SWEP book)

    Which sentence is correct ?

    Sincerely yours.
    In theory, if vapour was present above the liquid, then the liquid would at the saturation point and therefore sub cooling cannot be present. BUT.... For the sub cooled liquid to come up to boiling or saturation point, heat must exchanged and this takes time. The shorter the time the liquid is held in the receiver, the more sub cooling will be retained. Also the smaller the surface area of the liquid, the less the heat exchange, as a result vertical recievers often retain more sub cooling than horizontal ones.

    On the question regarding adding further sub cooling to the liquid once it leaves the condenser this can be achieved by virtue of the height of the liquid column as this can result in an increase in liquid pressure but only if the condenser is significantly ABOVE the point of expansion.

    In essence don't ever use a receiver unless you have to ...

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    Re: Question about sub-cooling

    Quote Originally Posted by A.Mortezania View Post
    Dear Friends,

    Which sentences below about sub-cooling are true ?

    1 – At condenser with receiver tank we have not sub-cooling because we have mixture of "liquid + gas" at saturation area inside of receiver tank.

    2 – At condenser with receiver tank we have sub-cooling but we can't increase sub-cooling

    3 – At condenser without receiver tanks by increasing refrigerant we can increase sub-cooling (I saw this sentence at SWEP book)

    Which sentence is correct ?

    Sincerely yours.
    Hi all,

    After following this forum and articles for several months, I finally decided to join. And the main reason is, this topic is fascinating me for years and couldn’t find a satisfactory explanation.

    I’ll give you my point of view:
    Subcooling is defined as the saturated temperature of the condensing pressure minus liquid temperature exiting the condenser. That means, in the receiver enters subcooled liquid, which expands there, thus existing both saturated liquid and vapour in it. The pressure at the receiver should be that of the saturated subcooled temperature.
    Since it is almost mandatory to build an equalizing pipe from discharge to the receiver, liquid can be subcooled as long as the outside conditions allow for it (e.g. wet bulb temperature, coil surface, etc.), which means, none of the sentences is correct.

    What do you think?

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    Re: Question about sub-cooling

    .

    Subcooling always fasinate's me, or more specifically different people's interpretation of it.

    It is simple really but it always causes debate.

    Liquid is subcooled if it is lower in temperature than the saturation temperature.
    So tap water in a glass at 20degs C is subcooled by 80degs C (K if being pedantic).

    Liquid in a receiver is a complicated calculation because of the expansion required to fill the space inside the receiver.

    Subcooled liquid leaves the condenser and subcooled liquid leaves the receiver. What the liquid does inside the receiver is what causes the confusion, the liquid expands to fill the space so technically the condition of the Refrigerant inside the receiver is saturated but actually the liquid at the bottom is still subcooled and can be measured, the vapour above the liquid is superheated.

    That is only while the system is running because the moment the system shuts down, the refrigerant starts to saturate fully.

    The state of the liquid inside the receiver (while the system is running) always causes argument.



    Rob

    .
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    Re: Question about sub-cooling

    Quote Originally Posted by Rob White View Post
    .

    Liquid is subcooled if it is lower in temperature than the saturation temperature.
    So tap water in a glass at 20degs C is subcooled by 80degs C (K if being pedantic).

    .
    Sorry Rob, but I can't agree with you on this. Saturation temperature of water in contact with air, as tap water in a glass, is the wet bulb temperature in such conditions. So, in a room at 25 C, 80% HR, wet bulb temperature is 22,4 C and thus, subcooling is about 2,4 C.

    Quote Originally Posted by Rob White View Post
    .

    Subcooled liquid leaves the condenser and subcooled liquid leaves the receiver. What the liquid does inside the receiver is what causes the confusion, the liquid expands to fill the space so technically the condition of the Refrigerant inside the receiver is saturated but actually the liquid at the bottom is still subcooled and can be measured, the vapour above the liquid is superheated.

    .
    The pressure in the receiver is lower than the one exiting the condenser, because of the expansion in the receiver itself. So there is saturated vapour and saturated liquid in the receiver, only the conditions have changed and, regarding the condensing pressure, they appear as superheated vapour and subcooled liquid.

    Regards

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    Re: Question about sub-cooling

    Quote Originally Posted by cool-noob View Post
    Sorry Rob, but I can't agree with you on this. Saturation temperature of water in contact with air, as tap water in a glass, is the wet bulb temperature in such conditions. So, in a room at 25 C, 80% HR, wet bulb temperature is 22,4 C and thus, subcooling is about 2,4 C.



    Regards
    See there you go, my point exactly

    Valid points and if I'm honest not ones that I had thought of,
    The glass one. I understand your receiver explanation but I don't
    ever remember the glass of water being explained to me like that.

    How bl**dy dare you spoil my analogy of tap water and subcooling.



    Good post though and now you have me thinking.

    Regards

    Rob

    .
    Last edited by Rob White; 25-04-2017 at 07:06 PM.
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    Re: Question about sub-cooling

    Quote Originally Posted by cool-noob View Post
    Sorry Rob, but I can't agree with you on this. Saturation temperature of water in contact with air, as tap water in a glass, is the wet bulb temperature in such conditions. So, in a room at 25 C, 80% HR, wet bulb temperature is 22,4 C and thus, subcooling is about 2,4 C.



    The pressure in the receiver is lower than the one exiting the condenser, because of the expansion in the receiver itself. So there is saturated vapour and saturated liquid in the receiver, only the conditions have changed and, regarding the condensing pressure, they appear as superheated vapour and subcooled liquid.

    Regards
    Right you've done it now, yeah, you've done it now.
    Yeah you got me thinking and you won't like me when you get me thinking.

    You're wrong, yeah you've picked the wrong fella to pick an argument with,
    yeah, you are wrong, what ya gonna say about that eh??



    Only kidding and being serious,you got me a thinking and I
    don't do a thinking



    So, I totally understand both your analogies but I think
    I have found a flaw in your thought process.

    Water in a glass at room temperature my be a few degrees
    lower due to wetbulb difference and RH, but
    how would that explain the same water in a saucepan being
    heated to 80 degsC?

    By my thinking 80 is 20 lower than 100 and in a normal
    atmosphere of 1 bar water should boil at 100 degsC, so
    if that is the case the water will be subcooled by 20 degs.

    How can the wet bulb analogy answer that?

    I think the difference is in a live, working system and
    a stagnant dormant system???

    A glass at room temperature can be compared to wet bulb
    and RH, but in a working system the rules change?

    The same in a receiver, a stood receiver is saturated but
    a working system is different. Yes there will be subtle
    differences in the pressures between inlet and outlet
    pressure but not enough to truly saturate the liquid?

    More likely, the liquid actually stratisfies in temperature
    bands and liquid is subcooled to the bottom, saturated in
    the middle and slightly superheated to the top.

    The working systems pressure is kept the same because
    The compressor is pumping against the restriction of
    the metering device.

    There, yeah, there you go, you've done it now, got me all
    worked up and thinking...................



    Told you it causes debate

    Rob

    .
    Last edited by Rob White; 25-04-2017 at 08:49 PM.
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    Re: Question about sub-cooling

    A couple of points.
    When you are measuring sub cooling you are likely to be measuring incorrectly, you measure the surface temp, which is a combination of liquid temp and ambient temp. Therefore sub cooling seems larger than it actually is.
    Under a steady state condition you are not expanding the liquid into the receiver, you are taking the liquid volume out and replacing it directly.
    Do we get sub cooling out of a free flowing condensor. The argument is that on the cond tube walls a film is produced which is cooled below saturation (with little or no turbulance), then falls into the receiver. During the falling time does it absorb energy from the vapour that it falls through, reducing the sub cooling. When in the reciever, we enter the stratification and liquid head levels.

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    Re: Question about sub-cooling

    Quote Originally Posted by mad fridgie View Post
    During the falling time does it absorb energy from the vapour that it falls through, reducing the sub cooling.
    Could the subcooling increase during the fall if only some of the liquid is lost from absorbing energy from the vapour?
    Last edited by seanf; 26-04-2017 at 10:01 PM.

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    Re: Question about sub-cooling

    Quote Originally Posted by seanf View Post
    Could the subcooling increase during the fall if only some of the liquid is lost from absorbing energy from the vapour?
    If energy was absorbed from the vapour, either the sub cooled liquid would rise in temperature or if the liquid is at saturation point a small proportion would convert to vapour, without a temperature change.

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    Re: Question about sub-cooling

    Some links to previous posts on the subject

    http://www.refrigeration-engineer.co...ght=subcooling

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    Re: Question about sub-cooling

    Quote Originally Posted by mad fridgie View Post
    If energy was absorbed from the vapour, either the sub cooled liquid would rise in temperature or if the liquid is at saturation point a small proportion would convert to vapour, without a temperature change.
    If the liquid is at saturation and absorbing energy from the vapour and the liquid below the vapor liquid interface would you not end up with the liquid below being cooled?
    Last edited by seanf; 27-04-2017 at 10:37 PM.

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    Re: Question about sub-cooling

    Quote Originally Posted by seanf View Post
    If the liquid is at saturation and absorbing energy from the vapour and the liquid below the vapor liquid interface would you not end up with the liquid below being cooled?
    I think you are looking at 2 possible different points,
    1; changes in pressure (like after an expansion valve), where it uses the heat of itself to boil itself, reducing the temperature.
    2; Absorption, simalar to when you wet your finger and becomes colder, and is not near saturation.

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    Re: Question about sub-cooling

    Quote Originally Posted by mad fridgie View Post
    I think you are looking at 2 possible different points,
    1; changes in pressure (like after an expansion valve), where it uses the heat of itself to boil itself, reducing the temperature.
    2; Absorption, simalar to when you wet your finger and becomes colder, and is not near saturation.
    Thats me just asked a daft question, and probably onto another.

    If the liquid and vapour is at saturation there would be no heat transfer. But would you get an energy transfer if the vapour and liquid is trying to get there states into equilbrium? So energy added to the liquid surface from the vapour and no cooling below because of no energy being transfered from the liquid below.
    Last edited by seanf; 28-04-2017 at 11:25 PM.

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