Page 5 of 5 FirstFirst 12345
Results 53 to 58 of 58
  1. #53
    Join Date
    Jun 2012
    Posts
    37
    Quote Originally Posted by Bseliger View Post
    If im staring at a sight glass by the condensing unit and the box is lets say, 30 feet down the way and 20 feet vertical. How is it that clearing the glass 50 feet away ensures a solid column of liquid to the metering device?
    Your temp of your refrigerant will be warmer than the ambient air so unless you are sending your liquid line a very very long ways through a space that is warmer than the ambient air it will actually pick up more sub cooling before it gets to your metering device. Worst case scenario, you don't pick up any more sub cooling from your sight glass to your metering device.

  2. #54
    Join Date
    Jun 2012
    Posts
    37
    Quote Originally Posted by jpsmith1cm View Post
    We start knowing nothing, then, we gain a small amount of knowledge and all too many think that they then know it all.

    Once we realize that we'll never know enough, then we're on the path to true learning.
    I agree, I meet way too many people that think they have it all figured out. I like this site and learn from it as well. No one likes a smart***, you all can call me out if I ever do.

  3. #55
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,946
    Quote Originally Posted by Bseliger View Post
    If im staring at a sight glass by the condensing unit and the box is lets say, 30 feet down the way and 20 feet vertical. How is it that clearing the glass 50 feet away ensures a solid column of liquid to the metering device?
    While Doug has a point, it's hard to hear the TXV whistle from the rooftop while you're adding gas...

  4. #56
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,487
    Quote Originally Posted by Bseliger View Post
    If im staring at a sight glass by the condensing unit and the box is lets say, 30 feet down the way and 20 feet vertical. How is it that clearing the glass 50 feet away ensures a solid column of liquid to the metering device?
    Quote Originally Posted by Flip14 View Post
    Your temp of your refrigerant will be warmer than the ambient air so unless you are sending your liquid line a very very long ways through a space that is warmer than the ambient air it will actually pick up more sub cooling before it gets to your metering device. Worst case scenario, you don't pick up any more sub cooling from your sight glass to your metering device.
    Bseliger brings up a good point with his example question. Any time you have a piping run with a significant vertical rise it's necessary to consider the effect of the static lift in addition to that of the pressure drop due to the equivalent length of the run.

    The total pressure loss...ie, the drop due to the length plus the loss due to static lift...affects the available, or apparent subcooling at the TXV.

    The example has a run of 50 lineal feet, but to figure the pressure drop due to the run you must go by the total equivalent length (TEL), which includes all elbows, fittings and valves. There are some very good resources available to determine these values, so for this example let's say the TEL is 70 ft and the expected pressure drop equals 5 psi.

    Then there's the pressure loss due to lift. There are charts for this too, but to explain how it's done. The density of R22 for example, is 71 lb/cu ft. If you envision a cu ft foot of liquid as a cube, divide it up into columns 1" x 1" x 1 ft high. The weight of this column will give you the pressure exerted by any column per foot of height. For R22, 71 lb/cu ft divided by 144 sq in is about .5 psi per foot, so a 20 foot lift would be .5 x 20 = 10 psi.

    If you look at your PT chart for R22 in the normal condensing range of 110-120F SCT you'll see that for every 10 psig the saturation temperature changes about 3F. This 3F also equals the loss of apparent subcooling due to the 20 lift.

    Now we can apply this same number to the 5 psi pressure drop we determined earlier...ie, 5 psi = 1.5F. Adding these together we can now see that we can expect a total loss of subcooling of 4.5F. Considering many refrigeration system only produce about 5F of subcooling, we might want to consider how to insure we get that solid column of liquid to the TXV.

    One way is to add a suction liquid heat exchanger. They're relatively inexpensive and quite effective. Another way, especially if you have much higher losses would be to use a DX subcooler.

  5. #57
    Join Date
    Jan 2007
    Location
    Vancouver Island, B.C.
    Posts
    2,485
    Quote Originally Posted by jpsmith1cm View Post
    While Doug has a point, it's hard to hear the TXV whistle from the rooftop while you're adding gas...
    Agreed JP......so I usually kidnapped anyone to help me......
    For the first 5 years in the trade my old partner would not let me put a SG in a system......overnight vacuum to 'dry' it and charge to a 'quiet' TXV and you never overcharged even a 1/4 hp system then. The .55/.45 psi/ft was not an issue and dynamic losses were also accounted for. With the ternary blends this is even more applicable today.....FWIW
    "The quality you deserve is not expensive---it's priceless"

  6. #58
    Join Date
    Sep 2002
    Posts
    13,551

    Well; it's like this -

    I think that the argument over where there is subcooling and where there isn't subcooling is BS to me.

    So first I want to see good subcooling in the liquid line: 8-10-12

    Proper subcooling to the expansion device ensure that it will be able to control SSH well.

    After that comes suction superheat. SSH numbers are BS without proper SC.

    For SSH I want to see 16-18-20 at the compressor. More for higher temps - less for lower temps.

    PHM
    ------




    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, 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
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

Page 5 of 5 FirstFirst 12345

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event