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  1. #14
    Join Date
    Apr 2007
    Location
    Kaufman county, Texas
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    9,978
    Quote Originally Posted by ryan1088 View Post
    I've seen this on chillers with subcoolers on them. Seems like on those you get about 12 degrees before the thing clears. The first time I saw it I actually had 2 thermometers on there just to make sure I wasn't crazy.
    My thinking is we are not reading an actual sub-cooling, more sort of an average of the stuff bouncing around in there. Some of it is sub-cooled, some of it is superheated, it is all shooting through the line so fast it does not have time to make a perfect mix/equilibrium.

    Theoretically, 2 degrees subcooling would be a comfortable slight overcharge for a TXV to provide 100% liquid to the TXV, but in actual field operating conditions you will need 18 on a typical RTU. Below 16, bubbles.
    "You boys are really making this thing harder than it has to be". Me

    "Who ARE you people? And WHAT are you doing in my SWAMP!?" Shrek

    Service calls submitted after 3PM will be posted the next business day.

    I give free estimates [Wild Ass Guesses] over the phone.

    "I am sorry for interrupting, please continue with your quarreling" Some chick on TV

  2. #15
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,594
    Possibly. But if you notice a lot of the chiller/rtu manufacturers recommend to charging to 16-20 degrees of subcooling on most things? Coincidence?

  3. #16
    Join Date
    Apr 2007
    Location
    Kaufman county, Texas
    Posts
    9,978
    Quote Originally Posted by ryan1088 View Post
    Possibly. But if you notice a lot of the chiller/rtu manufacturers recommend to charging to 16-20 degrees of subcooling on most things? Coincidence?
    I generally shoot for 18 SC. I think 16 is too low, over 18 I am watching my head pressure reaction pretty close but I have some carrier units with chart asking for 24 or so, does nothing but raise head pressure,... screw the chart. But I am talking RTUs, I have seen a 2nd story attic residential unit with condenser in alley that had to have 60 degrees subcooling, measured there in the alley, to get liquid to the TXV in the attic. I did not have a sightglass in the attic, no need, I could hear the vapor whooshing through the TXV. My service manager at the time was pi55ed, impressed, but still pi55ed.
    "You boys are really making this thing harder than it has to be". Me

    "Who ARE you people? And WHAT are you doing in my SWAMP!?" Shrek

    Service calls submitted after 3PM will be posted the next business day.

    I give free estimates [Wild Ass Guesses] over the phone.

    "I am sorry for interrupting, please continue with your quarreling" Some chick on TV

  4. #17
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    449
    There are many circumstances to consider when properly charging a system as everyone knows. The reasons for proper refrigerant charge are important and many factors need to be considered for a properly designed and operating system.

    A common design for air conditioning & heat pump:
    Evaporator Temperature 40° to 50° F
    Suggested Superheat: 8° to 12° F

    The TXV should be set to maintain proper superheat for the evaporator, not for the compressor. TXV’s usually require the sub cooled method for charging. Liquid sub cooling is required so that you have only liquid entering the expansion valve with no bubbles present. However the position of the TXV will change based on the load condition and if not consistent the TXV will hunt trying to maintain the super heat set point. The design conditions need to be established first for any system such as tonnage, proper air flow, sensible and latent loads along with refrigerant piping. There will be times when the system has been pushed beyond it's design criteria and this will need to be considered.

    The basics as most know are as follows:
    Liquid line saturation temperature minus actual liquid line temperature is called sub cooling. As the liquid travels through the condenser, its temperature begins to drop below the temperature at which it condensed. (I.e. R-22 @ 250 Lbs = 117°) When the liquid is at a temperature that is cooler than the condensing temperature, the liquid enters a state called sub cooled. When a refrigerant liquid is sub cooled, the refrigerant is pure liquid.

    Note: The liquid can be no cooler than the temperature surrounding the condensing coil. If the refrigerant liquid leaves the condenser coil at a temperature that is colder than the air passing through the coil, there is a pressure drop in the coil which is causing the temperature of the refrigerant to fall. For example if a drier were installed and it became plugged the temperature at its outlet would be cold.

    The refrigerant level will vary by how much refrigerant charge is in the system. At high charge levels the sub cooling will be high and at low charge levels the sub cooling will be lower.

    Importance of sub cooling: The liquid that exits the condenser coil must travel down the liquid line to the evaporator metering device. As the pressure keeps dropping due to friction loss between the refrigerant and piping. As the liquid pressure falls, the corresponding saturation temperature also drops.

    This reduces the temperature difference between the saturation temperature and the liquid line temperature. In other words, the sub cooling at the condenser coil outlet is greater than it is 50 Ft down the liquid line in the case of a split system.

    Flash Gas: When the pressure loss in the liquid line is great, the liquid may reach a pressure where the saturation temperature for the pressure equals the temperature of the liquid. At that point, vapor will form along the liquid. This is called flash gas.

    To prevent this condition from occurring, it is important to charge the system so there is enough sub-cooling to overcome the pressure loss that occurs in liquid lines. The amount of sub cooling will be determined by liquid line length, liquid lift, fittings, liquid line size and overall condenser size.

    This is not a complete scenario as there is much more to consider such as total super heat which will also indicate proper charge and if set correctly will help reduce flood back to the compressor in a part load condition. This subject is long and there are many opinions here which make for an interesting discussion.

    ___________________________________________
    Trouble shooting is not part of the repair……understand the symptom’s and you will find a solution.

  5. #18
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,008
    Good post alcomech.

    I would like to add that a liquid even 3 deg over ambient usually signals abnormality.

    It would have to be 16seer or better to get within 5.

    Also a plugged drier could (but not would) have a large temp drop.

    Also like to add use caution while charging a heat pump above 10-12 sc to obtain solid column.

  6. #19
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    449
    Thanks SBKold, you are absolutely correct as there are so many different design issues to consider. But i get your point, thanks for the input.

  7. #20
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,008
    Also to you sir I love liquid refrigerant diagnostic discussions

  8. #21
    Join Date
    Apr 2007
    Location
    Kaufman county, Texas
    Posts
    9,978
    Quote Originally Posted by alcomech View Post
    There are many circumstances to consider when properly charging a system as everyone knows. The reasons for proper refrigerant charge are important and many factors need to be considered for a properly designed and operating system.

    A common design for air conditioning & heat pump:
    Evaporator Temperature 40° to 50° F
    Suggested Superheat: 8° to 12° F

    The TXV should be set to maintain proper superheat for the evaporator, not for the compressor. TXV’s usually require the sub cooled method for charging. Liquid sub cooling is required so that you have only liquid entering the expansion valve with no bubbles present. However the position of the TXV will change based on the load condition and if not consistent the TXV will hunt trying to maintain the super heat set point. The design conditions need to be established first for any system such as tonnage, proper air flow, sensible and latent loads along with refrigerant piping. There will be times when the system has been pushed beyond it's design criteria and this will need to be considered.

    The basics as most know are as follows:
    Liquid line saturation temperature minus actual liquid line temperature is called sub cooling. As the liquid travels through the condenser, its temperature begins to drop below the temperature at which it condensed. (I.e. R-22 @ 250 Lbs = 117°) When the liquid is at a temperature that is cooler than the condensing temperature, the liquid enters a state called sub cooled. When a refrigerant liquid is sub cooled, the refrigerant is pure liquid.

    Note: The liquid can be no cooler than the temperature surrounding the condensing coil. If the refrigerant liquid leaves the condenser coil at a temperature that is colder than the air passing through the coil, there is a pressure drop in the coil which is causing the temperature of the refrigerant to fall. For example if a drier were installed and it became plugged the temperature at its outlet would be cold.

    The refrigerant level will vary by how much refrigerant charge is in the system. At high charge levels the sub cooling will be high and at low charge levels the sub cooling will be lower.

    Importance of sub cooling: The liquid that exits the condenser coil must travel down the liquid line to the evaporator metering device. As the pressure keeps dropping due to friction loss between the refrigerant and piping. As the liquid pressure falls, the corresponding saturation temperature also drops.

    This reduces the temperature difference between the saturation temperature and the liquid line temperature. In other words, the sub cooling at the condenser coil outlet is greater than it is 50 Ft down the liquid line in the case of a split system.

    Flash Gas: When the pressure loss in the liquid line is great, the liquid may reach a pressure where the saturation temperature for the pressure equals the temperature of the liquid. At that point, vapor will form along the liquid. This is called flash gas.

    To prevent this condition from occurring, it is important to charge the system so there is enough sub-cooling to overcome the pressure loss that occurs in liquid lines. The amount of sub cooling will be determined by liquid line length, liquid lift, fittings, liquid line size and overall condenser size.

    This is not a complete scenario as there is much more to consider such as total super heat which will also indicate proper charge and if set correctly will help reduce flood back to the compressor in a part load condition. This subject is long and there are many opinions here which make for an interesting discussion.

    ___________________________________________
    Trouble shooting is not part of the repair……understand the symptom’s and you will find a solution.
    Yes, and it is important to note here for the casual reader, we are trying to measure the quality of the refrigerant at one point in the system,... As inaccurate as that becomes, you cannot check pressure on the discharge and the temperature on the liquid-line, due to pressure drop. Same goes for checking pressure and temp across a filter-drier. The place where one has to check pressure MUST be where the temp is also checked for the most accurate reading.

    Still, it is common to have 10 degrees subcooling reading and bubbles in the sight-glass. There are many factors to consider, custom application to each jobsite.
    "You boys are really making this thing harder than it has to be". Me

    "Who ARE you people? And WHAT are you doing in my SWAMP!?" Shrek

    Service calls submitted after 3PM will be posted the next business day.

    I give free estimates [Wild Ass Guesses] over the phone.

    "I am sorry for interrupting, please continue with your quarreling" Some chick on TV

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