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Thread: Calculating Target Subcooling

  1. #1
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    Calculating Target Subcooling

    So calculating target superheat is the way to go with charging a fixed metering device. Cool.

    But with TXVs I always hear to charge according to subcooling... and then its normally an awkward silence from me... because Im waiting to hear how Im supposed to know how may degrees of subcooling. Not all manufacturers (most that Ive seen) place this info on the nameplate.

    So how is this done? Is the subcooling ≈ superheat?


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    It's definitely not the same thing, if that's what you mean. Typically the manufacturer will give a subcooling value. It varies based mostly based on the condenser, but there are other variables. The ones I'm most familiar with are Carrier, which will give you a single value to shoot for (which if you were to plot it on a chart, would simply be a straight line), or with the larger equipment, a charging chart to calculate it, which (if you think about what you are measuring) is a varying amount of subcool depending on the conditions that exist while you're trying to charge it.

    Some Lennox residential units I remember required you to match the condenser and evaporator in a chart, and pick the proper value based on that. That was pretty critical for their heat pumps, to avoid high pressure trips in heating mode that wouldn't have been an issue in cooling mode. The reason I remember that was a series of calls to a new neighborhood where the startup guy had just charged every unit to 10 subcooling during the summer. They worked fine until house after house began calling after the first cold snap. I looked through the literature and found the the required subcooling for those particular equipment matchups was only 2.

    Its not always on the dataplate, but it should be in the literature somewhere, you know, in the bag of paperwork you're supposed to read at installation. If you really cannot find it, shoot for 10, and then adjust accordingly.


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  4. #3
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    We use these.
    There is a app but it's clunky last time I tried it
    Attached Images Attached Images  

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    Quote Originally Posted by joeysmith View Post
    We use these.
    There is a app but it's clunky last time I tried it
    That calculator is helpful for superheat charging because it's one of those ones that takes into account the indoor humidity.

    I'm not sure how that thing helps for subcooling anymore than using the PT chart on your gauge face. That's really all that calculator is. Notice how it says "Locate the Mfr's required subcooling on the dataplate, if missing, use 8 to 10" Sounds like what I already said.


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    Quote Originally Posted by CHAINIK View Post
    That calculator is helpful for superheat charging because it's one of those ones that takes into account the indoor humidity.

    I'm not sure how that thing helps for subcooling anymore than using the PT chart on your gauge face. That's really all that calculator is. Notice how it says "Locate the Mfr's required subcooling on the dataplate, if missing, use 8 to 10" Sounds like what I already said.
    Ha! Guess you're rite.
    Shows how much I actually use the subcool side of it I guess.

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    from an older thread with a subcool chart shown..its a basline of what is typically used..
    https://hvac-talk.com/vbb/showthread...bcooling/page2

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    if you want to see how another chart is used....

    https://cheapstery.com/review/superh...ditioner-92478

  10. #8
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    There is a big difference between "knowing"what your SC is on a given system.

    And then, How do you "know" what the SC IS SUPPOSED TO BE on a given system?

    What info do you need to "know" to get the exact amount of SC "required" on Mrs.Smith home?

    What Brands put SC# on their Nametags? IDK.

    Once you "know"that this AC system needs 5*F SC, who labels the system as such? Or 15*F SC?

    Simple Q's.What is the purpose of SC? What benefits are there from having SC?

    The SC is diff on a system w/ a 80' horizontal lineset run vs a system w/ a 60' vertical run w/20'horizontal lineset.

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    Quote Originally Posted by TechmanTerry View Post
    There is a big difference between "knowing"what your SC is on a given system.

    And then, How do you "know" what the SC IS SUPPOSED TO BE on a given system?

    What info do you need to "know" to get the exact amount of SC "required" on Mrs.Smith home?

    What Brands put SC# on their Nametags? IDK.

    Once you "know"that this AC system needs 5*F SC, who labels the system as such? Or 15*F SC?

    Simple Q's.What is the purpose of SC? What benefits are there from having SC?

    The SC is diff on a system w/ a 80' horizontal lineset run vs a system w/ a 60' vertical run w/20'horizontal lineset.
    Isn't this OPs original question? Is there a way to calculate required subcooling if the target is not provided by the manufacturer? Are you asking as well, or are you saying is there a method?


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    Quote Originally Posted by CHAINIK View Post
    Isn't this OPs original question? Is there a way to calculate required subcooling if the target is not provided by the manufacturer? Are you asking as well, or are you saying is there a method?
    I agree, where's the answer?

    The Mfrg that do put the so-called"target SC" on their nametags do it to a "cover all" application. The Mfrg knows the lineset limitations they prescribe for their equipment, so under the "worst" situation (highest vertical separation,typically) they know that in order to provide a full boat of freon to the metering device a 15-20*F SC is required.

    Now that same size split AC, on Mrs.Jones house has a 5' line set. That system would still be ok w/ the same 15-20*F SC as shown on the nametag. But Mrs.Jones AC could work properly w/ just 5*F SC.

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    Quote Originally Posted by Sabr7 View Post
    So calculating target superheat is the way to go with charging a fixed metering device. Cool.

    But with TXV’s I always hear to charge according to subcooling... and then it’s normally an awkward silence from me... because I’m waiting to hear how I’m supposed to know how may degrees of subcooling. Not all manufacturers (most that I’ve seen) place this info on the nameplate.

    So how is this done? Is the subcooling ≈ superheat?


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    subcooling is not a fixed number... ambient outdoor temps effect subcooling.

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    Now, do a good size Commercial AC job. Then you have to know how to figure out the "needed" SC for a given job. Back then/when a "Nomograph"was required for figuring out SC.

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    Quote Originally Posted by TechmanTerry View Post
    Now, do a good size Commercial AC job. Then you have to know how to figure out the "needed" SC for a given job. Back then/when a "Nomograph"was required for figuring out SC.
    a few ice cold tall boys tell me how much sc is needed lol

  18. #14
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    Finding target sh...fun stuff.

    Around 20 years ago there was a guy on usenet i think he went by cbhvac then and he cracked the math behind the latent loading on the evap etc. for the target superheat. Wb x 3 -80 - amb /2 so to speak. I brpught that formula written properly over here to htalk for additional discussions. To date i havent found a superheat chart or slide rule that it doesnt hit within the tolernces.

    Subcooling is quite a bit different. In order to feed correctly the tev needs solid liquid. That would fall into the 1 degree sc the tev can feed correctly, however, 1 degree sc doesnt assure 100% liquid. Everyone is taught you can't have sc liquid and vapor at the same time and that is true when the fluid is not moving. Inside a vapor compression refrigeration system it is moving pretty fast actually and vapor can be present in the subcooled liquid stream. Under very specific conditions it can be present at very high subcooled levels such as 30 and 40 degrees. From a practicle perspective, at 10 degrees you are going to be 100%liquid to the tev.

    If you look at a mollier diagram and plot out the cycle, you get a graphic that can help explain it. Since sc is a specific heat change it doesnt take a lot of energy to gain and since the pressure dictates the load on the crankshaft of the compressor that gain is nearly free.

    Since the change of state at the tev is near adibatic it is a very effective improvement. Instead of being 70/30 liquid vapor mix, by subcooling you can get a greater quality of liquid into the 3vap. Say an 80/20 mix so net refrigeration effect goes up without significant increased power requirements. If you can get 2 degrees more sc out of the same shaft pressures, you gain about 1% net ref effect.

    The sc curve may be a lot harder to plot as the condenser performance varies with far more factors and the actual balence point floats with load and ambient.

    Tweaking the sc is more of an efficiency andjustment so to speak.

  19. #15
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    i use the manufacturer charts...its easy reading..

  20. #16
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    sometimes

    Quote Originally Posted by Unlimited1 View Post
    i use the manufacturer charts...its easy reading..
    That often doesn't exist, especially with build up type systems.

    Resi and roof top units you usallydo have the best refrence right on the unit.

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    Quote Originally Posted by idontgetit View Post
    That often doesn't exist, especially with build up type systems.

    Resi and roof top units you usallydo have the best refrence right on the unit.
    If everything was easy, I would be retired! Lol The charts are not really that different with manufacturers. It gets me in the Zone where a good 10-20 degree split Is needed between RA and SA..

    It seems all engineers ultimately design for that spec..I see more airflow problems (Wrong fan speed) (Collapsed duct/flex) ( fire damper closed) (clogged filters) when the head pressure looks to be correct but the lowside is off..

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  23. #18
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    It's been a good long while for me but I never heard of using subcooling for a fixed orifice system. A TXV sure, stack up some liquid and the valve will regulate superheat for you. Subcooling numbers will definitely change over the range of outdoor temp which is why it's nice to do at a median temp. A fixed orifice is not that much different than a cap tube when it comes to operation, more of a calculated upgrade. These cardboard slide rulers are nice for superheat as they take wet bulb temperature into account for total load. Mine is from Carrier circa 1986 and it still works well on just about anything. I would suggest picking one up and charging a fixed orifice that way (and note your subcooling and temps for yourself). My slide rule is two sided and like the other the subcool section requires the input you seek. Oddly under the TXV calculator there is a "proper airflow range" calculator based on indoor air wet and dry bulb with proper leaving air temps. As you use it, it changes the required subcooling (by chance). Now I dont know or remember if you are supposed to use them together but??? COPYRIGHT 1986 Carrier Corporation GT24-01 020-434 good luck.

  24. #19
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    SubCooling? hAY,HEY. is that the same thing as being the Air Conditioning repair guy onboard a US. Navy Submarine? That would be me and a few others.

    SubCooling? Would that be something to do w/ insuring that only liquid freon enters the TXV? That means NO VAPOR,as in liquid only. Does that also mean that if there was a SGMI at the AHU then you could "see" that SC does in fact exist at the TXV, or not? Does that also mean that if there was an "Electronic Ultrasonic Sight Glass" instrument at the AHU then you could "see" & "hear" that this elusive SC is in fact alive and well and proper at the TXV?

    I have seen and heard activity at the TXV while at the same time that the SGMI at the CU showed full, THOUSANDS of times.

  25. #20
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    Quote Originally Posted by TechmanTerry View Post
    The Mfrg that do put the so-called"target SC" on their nametags do it to a "cover all" application. The Mfrg knows the lineset limitations they prescribe for their equipment, so under the "worst" situation (highest vertical separation,typically) they know that in order to provide a full boat of freon to the metering device a 15-20*F SC is required.

    Now that same size split AC, on Mrs.Jones house has a 5' line set. That system would still be ok w/ the same 15-20*F SC as shown on the nametag. But Mrs.Jones AC could work properly w/ just 5*F SC.
    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.
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