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  1. #1
    Join Date
    Oct 2007
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    Richmond, VA
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    362

    superheat and subcooling readings

    So, Its come up in a debate at the shop as to what are the proper superheat and subcooling numbers for your typical walk in cooler applicationsand typical walk in freezer application. So, I'm wondering what you guys think about it. I'm trying to promote a discussion here and am very curious to see your responses. TIA

  2. #2
    Join Date
    Oct 2011
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    Chicagoland Area
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    4,587
    Typical...LOL
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  3. #3
    Join Date
    Jun 2012
    Posts
    37
    When I'm looking at my readings my superheat is always more important than my sub cooling. This is because sub cooling can fluctuate according to ambient conditions. Copeland recommends 20 degrees superheat 6" off the compressor. I always shoot for that. I think there is a lot that needs to be considered with these readings, such as line set length, line set location, is the suction line insulated propperly, ect. Usually walk-in's have a sight glass and I always try to clear the sight glass.

  4. #4
    Join Date
    Oct 2007
    Location
    Richmond, VA
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    362
    2sac, come on, give me some real input. I'm looking for your opinion. I'm just trying to generate some discussion on the subject.

    Flip14, I agree that superheat is important, but I think it is also important to have a certain amount of subcooling to ensure a solid column of liquid reaches the metering device. I too, try to clear the sightglass once I am close to setpoint.

  5. #5
    Join Date
    Sep 2008
    Location
    Western PA
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    25,567
    Quote Originally Posted by CommtechinVA View Post
    Flip14, I agree that superheat is important, but I think it is also important to have a certain amount of subcooling to ensure a solid column of liquid reaches the metering device. I too, try to clear the sightglass once I am close to setpoint.
    I've done commercial refer for a long time and haven't measured subcooling much more than a small handful of times.

    Once you install a receiver on the system, you're still worried about a solid column of liquid, but the way that you ensure it changes from subcooling measurement to a sightglass.

    So, I guess to answer your questions, subcooling "is what it is" and superheat should to be as low as it can safely run without damaging the compressor.

  6. #6
    Join Date
    Jun 2012
    Posts
    37
    Most walk ins have txv's so even if your sub cooling goes down, say its 110 degrees outside, your txv's should compensate for that. It's job is to maintain a superheat. In my experience I've found that a sight glass clears at around 10 degrees sub cooling. I remember looking at a sub cooling chart on sporlan's web site. I'll try to find it again and post it.

  7. #7
    Join Date
    Feb 2008
    Location
    California
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    2,066
    Quote Originally Posted by CommtechinVA View Post
    So, Its come up in a debate at the shop as to what are the proper superheat and subcooling numbers for your typical walk in cooler applicationsand typical walk in freezer application.
    So what did your shop guys come up with?

    What is the proper subcooling value?

    What is the proper superheat value?

  8. #8
    Join Date
    Apr 2011
    Location
    michigan
    Posts
    172
    I was taught that you check the charge on a system based on the type of metering device, if its a fixed orfice or cap tube you charge to superheat at the compressor. But the "number" is not necessairly always the same. Trane and carrier have a slide rule type of chart to help like on residential split systems or roof top units, the chart is for a specific refrigerant, you line up the ambient temp with the wet bulb space temp and it will give you a range for the superheat, for example on a split residential type application if the ambient is 55, the system would be correctly charged at say 20 superheat where the same system at 80 ambient would require about 8-10 superheat because of the lower head pressure, same with a walk in cooler or freezer, although all of the walk in coolers and freezers I have been on have been a txv system in which case you would charge to subcooling because you want to first make sure the txv has a full column of liquid supplying the txv, some just charge till the glass is clear and dont check subcooling but depending on the box temp you could charge till the glass is clear if the box was hot and you may be over charged. I like to charge to clear the glass but also check the subcooling, I charge to 10-12 subcooling using liquid pressure/temp. going from discharge pressure sometimes I will go with a little more subcooling but usually not more than 12-15 subcooling. After I am sure the txv has a full column of liquid I will then check the superheat to be sure the txv is adjusted properly and to be sure the compressor is getting enough cooling to protect the compressor, on a txv system I usually go between 8-12 superheat, the magic number everyone trys to get is 10 but if I am 8-12 usually that is pretty close.

  9. #9
    Join Date
    Feb 2008
    Location
    California
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    2,066
    that all sounds like A/C work.

    That process wont fly in refrigeration land.

  10. #10
    Join Date
    Oct 2007
    Location
    Richmond, VA
    Posts
    362
    Quote Originally Posted by Phase Loss View Post
    So what did your shop guys come up with?

    What is the proper subcooling value?

    What is the proper superheat value?
    One guy said subcooling on walk in refrigerator should be 8 degrees.
    A tech asked him and then asked me. I told him it wasn't all that important because you have a receiver. He was bothered by the fact that he got two different answers from the field supervisors. I told him to clear the sightglass and be sure to have some subcooling. As for superheat, I told him 20 degrees at the compressor. I know this can vary but it seems to be a good starting point. This situation is what prompted me to ask you guys what your thoughts are.

  11. #11
    I believe in OEM specs but a good rule of thumb is 10-12 superheat at evap, and 20 at comp.,
    And subcooling 10-18.
    I honestly havent found it any diff. from application to application. Heard it with compression ratio.. evap temp.. and the diff between comfort a/c and refrigeration but nothing with subcooling and superheat.

    Keep me posted if there are any finds out there,,, guaranteed.

  12. #12
    Join Date
    May 2011
    Location
    Spring City, Pennsylvania, United States
    Posts
    190
    10-12 degrees sh at evaps for cooler or freezer is very high imo. Freezer I usually set around 6. And as said before if system has a reciever I don't worry about sc.

  13. #13
    Join Date
    Sep 2008
    Location
    Western PA
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    25,567
    Quote Originally Posted by Toddbrewster View Post
    I was taught that you check the charge on a system based on the type of metering device, if its a fixed orfice or cap tube you charge to superheat at the compressor. But the "number" is not necessairly always the same. Trane and carrier have a slide rule type of chart to help like on residential split systems or roof top units, the chart is for a specific refrigerant, you line up the ambient temp with the wet bulb space temp and it will give you a range for the superheat, for example on a split residential type application if the ambient is 55, the system would be correctly charged at say 20 superheat where the same system at 80 ambient would require about 8-10 superheat because of the lower head pressure, same with a walk in cooler or freezer, although all of the walk in coolers and freezers I have been on have been a txv system in which case you would charge to subcooling because you want to first make sure the txv has a full column of liquid supplying the txv, some just charge till the glass is clear and dont check subcooling but depending on the box temp you could charge till the glass is clear if the box was hot and you may be over charged. I like to charge to clear the glass but also check the subcooling, I charge to 10-12 subcooling using liquid pressure/temp. going from discharge pressure sometimes I will go with a little more subcooling but usually not more than 12-15 subcooling. After I am sure the txv has a full column of liquid I will then check the superheat to be sure the txv is adjusted properly and to be sure the compressor is getting enough cooling to protect the compressor, on a txv system I usually go between 8-12 superheat, the magic number everyone trys to get is 10 but if I am 8-12 usually that is pretty close.
    That's all well and good in AC land.

    In refrigeration, that isn't going to work out real well for you.

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