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Thread: Calculating Target Subcooling

  1. #21
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    Quote Originally Posted by pacnw View Post
    Can I disagree somewhat with this?

    The mfrs that I have dealt with provide a SC value they wish to see for optimum performance/safety.

    Installer then must adjust charge AFTER installation to achieve this value. You'll add for any value under the target and the remove for any value over target.

    Mrs Jones has a 5' line set, factory charge usually is good for 15', so you remove charge to achieve SC.

    Mrs Smith has an 80' run, you add charge to achieve the SC.
    Hi pacnw. Sorry to bust your bubble but there is no disagreement. I agree w/ you..lol.

    Is what you said about adjusting the SC for diff lineset installs/length's in the unit
    paperwork anywhere?

  2. #22
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    Quote Originally Posted by TechmanTerry View Post
    Hi pacnw. Sorry to bust your bubble but there is no disagreement. I agree w/ you..lol.

    Is what you said about adjusting the SC for diff lineset installs/length's in the unit
    paperwork anywhere?
    You are NOT adjusting the subcooling numbers, you are adjusting the amount of refrigerant added or removed. For Carrier it is .6oz per foot of 3/8" liquid line, from 15' line set length.
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  3. #23
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    Quote Originally Posted by pacnw View Post
    You are NOT adjusting the subcooling numbers, you are adjusting the amount of refrigerant added or removed. For Carrier it is .6oz per foot of 3/8" liquid line, from 15' line set length.
    Wait a minute. So a unit w/ a 5' horizontal lineset has the same SC requirement as a lineset that is 10' vertical and 5' horizontal (total 15')?

  4. #24
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    Quote Originally Posted by pacnw View Post
    You are NOT adjusting the subcooling numbers, you are adjusting the amount of refrigerant added or removed. For Carrier it is .6oz per foot of 3/8" liquid line, from 15' line set length.
    The amount of r22/r410a being removed/added is how you adjust the units SC #s. If "a" system requires 12*SC and the system has only 6* SC, more freon is added and that raises the amount of SC,keep adding juice until you get the magic # of 12*SC.

    I have some notes from a Carrier Class, and we were told that each pass of the CU has 2-3-4* of SC in it.

  5. #25
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    My advice is worth as much as you paid for. If you have a long line set you have to add enough refrigerant to fill the line and should not actually bear on sub-cooling temp. Not having an actual chart mounted on inside of panel I would take a step back and look at the total system. If one does not look at the entire system you end up with part failure due to system problems. Why attempt to charge to i.e. 20* sub-cooling when low/ high side pressures are already high at say 15* sub-cooling. Stop and look at the low side as that is where customer satisfaction will be.

  6. #26
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    Quote Originally Posted by stumpdigger View Post
    My advice is worth as much as you paid for. If you have a long line set you have to add enough refrigerant to fill the line and should not actually bear on sub-cooling temp. Not having an actual chart mounted on inside of panel I would take a step back and look at the total system. If one does not look at the entire system you end up with part failure due to system problems. Why attempt to charge to i.e. 20* sub-cooling when low/ high side pressures are already high at say 15* sub-cooling. Stop and look at the low side as that is where customer satisfaction will be.
    KindaSorta, a res system, 3.5T,22/410, 3/8 "OD copper LL, 90*F ambient,w/ a 20' riser. How much SC is lost going up that hill? That is a "must know" # in order to Troubleshoot an AC system. It is also a "must know"# in saying YES to the Q of, "DO YOU HAVE A FULL LL AT THE METERING DEVICE"????


    Installing a pressure access fitting at the AHU would help a lot.

  7. #27
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    Quote Originally Posted by TechmanTerry View Post
    KindaSorta, a res system, 3.5T,22/410, 3/8 "OD copper LL, 90*F ambient,w/ a 20' riser. How much SC is lost going up that hill? That is a "must know" # in order to Troubleshoot an AC system. It is also a "must know"# in saying YES to the Q of, "DO YOU HAVE A FULL LL AT THE METERING DEVICE"????


    Installing a pressure access fitting at the AHU would help a lot.
    I have some different thoughts on that: Think of an old refrigerator with the condenser mounted on the back like a "grille", no forced air, the six foot long cap tube was mostly Cu pipe with very little refrigerant inside. Like a small animal it has a huge amount of surface area causing it to cool thus increasing sub-cooling. It does not want to flash off due to heat but rather condenses further as it gives up heat. Sure anyone would see bubbles in the sight glass during startup but after ten to fifteen minutes the juice is where it belongs and your riser like a cap tube will give off heat as your sub-cooling temp is higher than ambient. Please do the industry a favor and install a refrigeration ball valve in front of the low side so we can pump the charge into the condenser instead of spending all day taking the charge out. (they come with access ports).

  8. #28
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    Quote Originally Posted by stumpdigger View Post
    My advice is worth as much as you paid for. If you have a long line set you have to add enough refrigerant to fill the line and should not actually bear on sub-cooling temp. Not having an actual chart mounted on inside of panel I would take a step back and look at the total system. If one does not look at the entire system you end up with part failure due to system problems. Why attempt to charge to i.e. 20* sub-cooling when low/ high side pressures are already high at say 15* sub-cooling. Stop and look at the low side as that is where customer satisfaction will be.
    Under all normal conditions then going for 20* SC ,then the system press would also be normal. So,no problem.

  9. #29
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    Refer to vendor information when available for TXV's. Typically they list 8-16 df. Split systems, especially with long or higher elevation evaporators may require more than mfg spec. This is where it may be a balancing act between condenser approach/dsch pressure and maintaining a liquid seal feeding the txv. R-22 & R410a typically need 4-6 df of subcooling to maintain a liquid seal at the txv and keep steady ssh.

    Never trust just sc or sh. Do a full log sheet and cross reference all readings for correct ranges. Charges are more critical on microchannel coil units and some heatpumps have critical charges due to indoor & outdoor capacity match-ups.

  10. #30
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    ehsx, what is "df"?

  11. #31
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    df= F?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  12. #32
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    Quote Originally Posted by TechmanTerry View Post
    Wait a minute. So a unit w/ a 5' horizontal lineset has the same SC requirement as a lineset that is 10' vertical and 5' horizontal (total 15')?
    Why not?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  13. #33
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    Quote Originally Posted by pacnw View Post
    Why not?
    There are two diff pressure losses that affect the freon that concern us.

    How do we chose the proper size LL? We look up the proper LL size based on an allowable "friction pressure drop" per 100'. And that is based on a 1* loss(suction line is based on a 2* loss)

    The 2nd factor is height. Or also called a riser. For now allow me some leeway. For each foot of LL riser there is a approximately a 1/2 lb pressure loss(drop). a 10' riser has a 5 lb press drop. A 20' riser has a 10lb press drop(loss). So isn't a press drop = to a temperature drop? That is why SubCooling is needed. SC is needed to offset that temp loss to keep the liquid in the LL a liquid. When the liquid in the LL gets to ambient temp the liquid starts to boil(flash gas) and that liquid/vapor screws up the metering device,the evap coil,the evap SH, the SH at the comp, the oil return, the discharge SH,& the discharge line temp.

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  15. #34
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    Degrees Fahrenheit. Easier than finding symbols...

  16. #35
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    Quote Originally Posted by ehsx View Post
    Degrees Fahrenheit. Easier than finding symbols...
    But that leads to DF what? SC? SH?

  17. #36
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    Subcooling figures are written on the outside of a lot of condensing units.If the sub cooling amount is not written on the unit or in the literature I use 10-12 degrees.The sub cooling number is usually plus or minus 2 degrees.If I have a normal line set length.(Up to 35 feet) I charge to nameplate Subcooling.If i have a longer line set,I add 2 degrees(Go to 12 degrees SC) That seems to work pretty well.If you really want to be exact,Put a sightless at the furnace or air handler.Whenthe glass clears,Check the sub cooling .Write that number on the outdoor unit for future reference.I have charged as high as 15 degrees Subcooling with a long run in the attic, using a sight glass.Before adding the sight glass,I thought I was fully charged at 12 degrees SC.Sight glass at the furnace/air handler is a good service tool, and cheap enough.

  18. #37
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    Quote Originally Posted by Gleng View Post
    Subcooling figures are written on the outside of a lot of condensing units.If the sub cooling amount is not written on the unit or in the literature I use 10-12 degrees.The sub cooling number is usually plus or minus 2 degrees.If I have a normal line set length.(Up to 35 feet) I charge to nameplate Subcooling.If i have a longer line set,I add 2 degrees(Go to 12 degrees SC) That seems to work pretty well.If you really want to be exact,Put a sightless at the furnace or air handler.Whenthe glass clears,Check the sub cooling .Write that number on the outdoor unit for future reference.I have charged as high as 15 degrees Subcooling with a long run in the attic, using a sight glass.Before adding the sight glass,I thought I was fully charged at 12 degrees SC.Sight glass at the furnace/air handler is a good service tool, and cheap enough.
    Holy Crap. Now thats a good straight forward,normal way of looking at things. That exactly what I have done,and will continue to do.

  19. #38
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    Quote Originally Posted by ehsx View Post
    Degrees Fahrenheit. Easier than finding symbols...
    Please just use the * symbol, It's in the right place: " evap pressure is at 68.5# 40*F going into coil with 10* super-heat" at condenser.

  20. #39
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    Quote Originally Posted by TechmanTerry View Post
    There are two diff pressure losses that affect the freon that concern us.

    How do we chose the proper size LL? We look up the proper LL size based on an allowable "friction pressure drop" per 100'. And that is based on a 1* loss(suction line is based on a 2* loss)

    The 2nd factor is height. Or also called a riser. For now allow me some leeway. For each foot of LL riser there is a approximately a 1/2 lb pressure loss(drop). a 10' riser has a 5 lb press drop. A 20' riser has a 10lb press drop(loss). So isn't a press drop = to a temperature drop? That is why SubCooling is needed. SC is needed to offset that temp loss to keep the liquid in the LL a liquid. When the liquid in the LL gets to ambient temp the liquid starts to boil(flash gas) and that liquid/vapor screws up the metering device,the evap coil,the evap SH, the SH at the comp, the oil return, the discharge SH,& the discharge line temp.
    once had a boss that would not believe he would have pressure drop on a water hose going 200' up the side of a coker.

  21. #40
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    Quote Originally Posted by stumpdigger View Post
    once had a boss that would not believe he would have pressure drop on a water hose going 200' up the side of a coker.
    Anyone do Hydronic Heating?

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