Results 1 to 6 of 6
  1. #1
    Join Date
    Oct 2005
    Location
    Ohio
    Posts
    41
    can anyone explain why absolute (14.7)has to be addedto suction & discharge pressure to find the compression ratio .I was asked this question by my boss & did not know the reason why.

  2. #2
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    It is a function of math that requires you to change to absolute pressure in order for the ratio to be meaningful. Some teachers claim that it is to keep the calculations out of a vacuum. That is incorrect!


  3. #3
    Join Date
    Oct 2004
    Posts
    31
    Originally posted by rix870
    can anyone explain why absolute (14.7)has to be addedto suction & discharge pressure to find the compression ratio .I was asked this question by my boss & did not know the reason why.
    rix,

    At sea level with an ambient temperature of 70 degrees F with dry air , Barometric Pressure exerted on everything, (including the refrigerant in your circuit) is 14.696 pounds per square inch absolute. (Correct your boss on his use of 14.7 if you dare!)Barometric Pressure is the weight of air pushing down on the earth.

    I'm not sure any more what the formula is for calculating the compression ratio....if you know it please let me know.

  4. #4
    because we can not read atmospheric pressure (14,7 psI) with our gauge set...read zero when not connected to a pressurized system......you have to add it!!!!
    Don't interrupt me while i'm talking to myself

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,290
    I'm not sure any more what the formula is for calculating the compression ratio....if you know it please let me know.
    (psig discharge + 14.7) divided by (psig suction + 14.7) = compression ratio
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    Feb 2005
    Posts
    344

    Its all relative

    0 psig is relative to atmospheric pressure which will read 0 psig when the pressure guage is open to the air.

    When we use absolute pressure, which is relative to an absolute vacuum, then 0 psi is indeed zero. Even if a vacuum is part of the calculation the value will still be a positive value.

    In theory a perfect vacuum is impossible to attain so any presence of an atmosphere will be a positive value.

    Since compression ratio is Discharge/Suction and division by zero is undefined we must keep the suction pressure a positive interger even if it is in a vacuum.

    Best regards...

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