Results 1 to 11 of 11
  1. #1
    Join Date
    Apr 2008
    Posts
    155

    Sub cooling from discharge pressure?

    I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
    My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
    I've seen dischagre pressures range 15-35lbs above liquid.
    What the heck?

  2. #2
    Join Date
    Jun 2005
    Location
    Las Vegas NV
    Posts
    1,152
    Quote Originally Posted by mixsit View Post
    I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
    My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
    I've seen dischagre pressures range 15-35lbs above liquid.
    What the heck?
    He is incorrect. You want to measure the liquid line pressure/temp to get subcooling. The term "discharge pressure (compressor hot gas line)" is sometimes referred to as the liquid line pressure in error.

  3. #3
    Join Date
    Apr 2006
    Location
    mechanical room
    Posts
    1,255
    well, measuring discharge superheat could indicate issues and alot of chillers do measure it, but this guy basicly has no clue what he's talking about.

  4. #4
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    5,361
    Quote Originally Posted by mixsit View Post
    My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
    The discharge line is Super Heated Vapor, not saturated liquid.

    Stick with measuring subcooling on the liquid line (after the refrigerant leaves the condenser).

    We can only hope this is what he means, but he has a hard time explaining it.
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
    Join Date
    Jul 2005
    Posts
    5,351
    Quote Originally Posted by mixsit View Post
    I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
    My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
    I've seen dischagre pressures range 15-35lbs above liquid.
    What the heck?
    It's easy to get confused when using terms like SCT and SDT. What these are, as far as we're concerned, are the temperature corresponding to the pressure as read directly on our gauges. With that definition it follows that

    SC = SCT-LLT, or equivalently SC = SDT-LLT.

    It is taken for granted that, according to instructions, you've measured the pressure at the same point that you measured temperature, and that you've measured these either at the condenser outlet, reciever outlet, MD inlet, etc, depending upon where it is you desire to know the SC. HTH.

    But, in most cases you can use discharge pressure and LL temp on resi or light commercial units to calculate SC "close enough". This is right in the service manuals and many textbooks. More than a 5psi drop through the condenser would be excessive.

  6. #6
    Join Date
    Feb 2006
    Location
    USA
    Posts
    4,381
    That's why it irks me when manufacturers don't install service access on the liquid line in rtus . . .

    Sent from my HTC Sensation 4G using Tapatalk 2

  7. #7
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,304
    Educate him on the uses of Discharge Super Heat to trouble shoot a refrigerant issue.

  8. #8
    Join Date
    Apr 2008
    Posts
    155
    Quote Originally Posted by hvacrmedic View Post
    ...It is taken for granted that, according to instructions, you've measured the pressure at the same point that you measured temperature, and that you've measured these either at the condenser outlet, reciever outlet, MD inlet, etc, depending upon where it is you desire to know the SC. HTH.

    But, in most cases you can use discharge pressure and LL temp on resi or light commercial units to calculate SC "close enough". This is right in the service manuals and many textbooks. More than a 5psi drop through the condenser would be excessive.
    Indeed. This happened to be a 30 ton split and yes, I'll use discharge for 'close enough or go-no-go estimating. Where there's a bunch of units and you know the typical pressure difference or have a track record that would be a decent example in a 'close enough situation.

    Thanks all. Guess I just needed a cross check/re-affirm from that 'Everything you know is wrong' feeling!
    This is not measure/document sub cooling.

  9. #9
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,700
    Quote Originally Posted by hvac wiz 79 View Post
    That's why it irks me when manufacturers don't install service access on the liquid line in rtus . . .

    Sent from my HTC Sensation 4G using Tapatalk 2
    Must be a Trane.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

  10. #10
    Join Date
    Feb 2006
    Location
    USA
    Posts
    4,381
    Quote Originally Posted by Dallas Duster View Post
    Must be a Trane.
    Yep . . . . Then today i open a small precedent & find one on the liquid line. Maybe trane is getting better lol

    Sent from my HTC Sensation 4G using Tapatalk 2
    Attached Images Attached Images  

  11. #11
    Join Date
    Sep 2006
    Location
    River Forest IL
    Posts
    70

    Bear in mind-

    Carrier roof top units for example, won't give you a liquid line access port if the unit has a cap tube/piston feed. You'll be charging based on superheat anyway. It's nice to be able to check both, but not necessary in their accountant's mind-it saves them .09cents per unit/circuit by leaving out the access port.
    Gotta be tough when you're dumb!

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