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  1. #1
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    Confused Subcooling- proper measurement

    I have learned a great deal from this site, and have gotten great information re a Trane Voyager 20 ton RTU that i inherited- ( more on that in another thread).
    my question is subcooling, and how to measure TOTAL sub cooling in terms of system performance and charging a system.
    I was taught the "beer can cold" method and also 60/ 225 is normal gauge readings. plan on being more of an engineer when looking at these systems, mostly air cooled R-22 split and package units with a TXV.
    I have read several threads on here and the opinions vary somewhat as to how to properly measure TOTAL subcooling.
    The most popular form of measuring subcooling that i have see in terms of consistancy is discharge pressure at discharge service valve, convert to a temperature, take a temprature reading at the liquid line close to the expansion valve, subtract the two, and the resulting difference is subcooling.
    As understand it, the preferred method is taking a liquid line pressure downstream of the condenser, and a temp reading by the expansion valve for proper subcooling, but that is not always feasiable.
    What is the most effective method to measure subcooling in terms of proper system performance?

    Thanks guys

    Mikeymoe

  2. #2
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    The MOST effective way is to take pressure and temperature at the same point.

    Other methods may work but will be less accurate due to pressure drops.


    On to verifying charge...

    Throw that 60/225 crap off the roof. It's bogus.

    Most manufacturers will publish charging data that should be followed.


    Also, take the time if you haven't already to submit an application for pro membership. We talk about this stuff in substantially more detail in the 'locked' forums.

  3. #3
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    Yes- if you're running 60 psi on a newer system the evap is too cold or underfed. Most of your newer equipment will run around 45° evap temp, ideally. Without a chart I'm front of me I think that's around 75-80 psi for R-22. But the manufacturer's data is always the way to go.

  4. #4
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    Subcooling in my personal opinion should be measured as the liquid leaves the condenser. It can give you a good idea of condenser performance if you have enough data, AND lets you know that you have enough to overcome changes in the pt relationship caused by pressure drop in the liquid line between the condenser and tx valve. I am sure others may disagree, and that fine, but in my view, subcooling's primary directive is to make sure you dont flash before you get to the evap, but then again I am still learning this stuff and am willing to be taught more.

  5. #5
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    Quote Originally Posted by flange View Post
    Subcooling in my personal opinion should be measured as the liquid leaves the condenser. Easiest to do usually. It can give you a good idea of condenser performance if you have enough data, AND lets you know that you have enough to overcome changes in the pt relationship caused by pressure drop in the liquid line between the condenser and tx valve. (Usually, but not always...kinked lines, TXV inlet screens, unseen driers, debris in the liquid line, sabotage... I am sure others may disagree, and that fine, but in my view, subcooling's primary directive is to make sure you dont flash before you get to the evap, (Exactly right.) but then again I am still learning this stuff and am willing to be taught more.
    Pay attention to subcooling on every job regardless of charging method.
    "I have never let my schooling interfere with my education."
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  6. #6
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    best way to get charge correct is to weigh it in, but on a txv system ,subcooling using a manufactures charging chart that takes into account the ambient operating temps is ideal. yes subcooling " should " indicate a solid colum of liquid to txv but so many other things can be learned from this data,( over charge under charge plugged drier dirty coil, even helped find a fan blade that had the wrong pitch) proper temps and pressures ( the only place in the system where a pt chart applies is at the point of total cond, evap saturation." change of state".) can tell the whole story of how the system is operating. dont forget the basics though IE volts and amps. good luck to all and be safe out there

  7. #7
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    We do work for several manufacturers of equipment, and so far, havent found one who can accurately calulate proper charge for their systems. In fact, most have no clue how to "properly charge" their own equipment, and will give you ranges for superheat and subcooling. This is for larger scale systems say fifty tons and up. As an example, we have two *************** systems right now. splits with ten feet of tubing between them. calculated charge has been put in, then another twenty pounds. With one compressor running in cold ambients they do okay, when it warms up and second compressor comes on, suction pressure plummets and trips freeze protection on coil. since it might hit fifty five tomorrow, we may get closer, but that will probably put us thirty pounds over factory calcs. just saying. this is commonplace for us. With one manufacturer in particular, we know anything they sell is thirty percent light...minimum.

  8. #8
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    subcooling's primary directive is NOT to overcome pressure drops in the liquid line.

    subcooling's primary directive IS to feed the txv with a solid column of liquid refrigerant. how much subcooling is necessary is determined by the sizing of the txv...if the txv was designed with 5F subcooling, then 5F is how much you have to have to achieve full txv capacity. if it was designed with 15F subcooling to achieve txv capacity, then 15F is how much subcooling you need.

    subcooling's secondary directive is to overcome liquid line pressure drops and changes in ambient temperature, condenser loading, compressor capacity changes, condenser fan speeds, evaporator loading, etc.

    the best place to take a subcooling reading depends on what you are going to do with the numbers you get. if you are looking at condenser performance then you need to measure it there. if you are trying to determine why a txv is not feeding then you need to measure the subcooling right before the txv.

    there are lots of reasons and places to measure subcooling.
    How to make the perfect "Half-Hitch" knot or any other boyscout knot in 3 easy steps...

    1. Remove your meter leads from the meter and very carefully return them to their storage case or bag.
    2. Wait 2 seconds
    3. Very carefully remove your meter leads from their storage place and enjoy your new knot!

  9. #9
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    Ah, semantics. Without proper subcooling, liquid will flash prior to getting to the tx valve in many systems, especially poorly designed systems, thus proper subcooling is required to overcome the pressure drop. It is used for efficiency gains as well, especially in refrigeration and more specifically in cascade systems.
    I dont remember ever seeing a submittal that gave you a required subcooling for a tx valve by the way, and to be honest, WTF would it matter to the tx valve, which only looks at suction line temp? The tx valve does, however need a given differential pressure across it to work properly, thus the need to limit subcooling in low ambient operation.

  10. #10
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    Quote Originally Posted by flange View Post
    ...I dont remember ever seeing a submittal that gave you a required subcooling for a tx valve by the way, and to be honest, WTF would it matter to the tx valve, which only looks at suction line temp?...
    i have not seen any submittals stating that before either, however, i have sized many expansion valves. to size a txv you need to know:

    1. Saturated Evaporator Pressure (usually listed in temperature)
    2. TXV pressure differential (liquid line pressure minus saturated evaporator pressure plus orifice/nozzle pressure drop)
    3. Entering liquid line TEMPERATURE

    so, as you can see, the subcooling IS used in sizing txv's. as for the txv, it looks at temperature AND pressure (most do anyway, won't get into all of the other types of expansion valves right now).


    Quote Originally Posted by flange View Post
    ...The tx valve does, however need a given differential pressure across it to work properly...
    yup!
    How to make the perfect "Half-Hitch" knot or any other boyscout knot in 3 easy steps...

    1. Remove your meter leads from the meter and very carefully return them to their storage case or bag.
    2. Wait 2 seconds
    3. Very carefully remove your meter leads from their storage place and enjoy your new knot!

  11. #11
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    that sounds like fluid inertia

    What is the head pressure while all this is going on?

    BTW: How is the head pressure being controlled?

    Is head pressure direct acting or is there receiver pressurizing on the liquid?

    And what is the application?

    PHM
    ------




    Quote Originally Posted by flange View Post
    We do work for several manufacturers of equipment, and so far, havent found one who can accurately calulate proper charge for their systems. In fact, most have no clue how to "properly charge" their own equipment, and will give you ranges for superheat and subcooling. This is for larger scale systems say fifty tons and up. As an example, we have two *************** systems right now. splits with ten feet of tubing between them. calculated charge has been put in, then another twenty pounds. With one compressor running in cold ambients they do okay, when it warms up and second compressor comes on, suction pressure plummets and trips freeze protection on coil. since it might hit fifty five tomorrow, we may get closer, but that will probably put us thirty pounds over factory calcs. just saying. this is commonplace for us. With one manufacturer in particular, we know anything they sell is thirty percent light...minimum.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
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    andy schoen could probably answer the liquid temperature question in 3 words , but keep in mind that specific txv would be designed for specific refrigerant ,and application, so liquid temp in 90% of appliction with assumed condensing pressures, temperature relates directly to subcooling or liquid temp supplied , so in order to compensate for minimum press diff , different orifices would be requd as example regards stan
    Keep it simple to keep it cool!

  13. #13
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    Thanks for all of the responses guys.
    the reason for my initoal question is simply to determine if i have a solid column of liquid to the TXV on the system in question and if a unit is cooling properly based on the engineering that went into designing the unit.
    I was taught this in trade school, then when you start working in the field, fist guy (s) you work with use the 60/225 rule. Bear in mind that i do understand that there are many many variables that affect suction and head pressure. i threw the 60/225 out there to illustrate a point.
    IMHO, i believe manufaturers can measure CFM and static in a lab condition. we all know that once the unit leaves the factory, the lab mesurements can be trown out the window, and its up to us to deliver the performance from the unit.
    Anywho, i believe its in our best interest to determine if a unit is delivering the cooling required based on design and accurate measurements, i.e subcooling, superheat, CFM, etc.
    i am taking a new approach to he way i have been doing things, and trying to work smarter in diagnosing a unit.

    I have learned a lot by reviewing the threads and posts on this site, especially with a Trane Voyager that i picked up on a PM
    Last edited by Mikeymoe; 03-03-2011 at 12:03 PM. Reason: delete angry emoticon

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