Results 1 to 4 of 4
  1. #1



    Under for your interest, the topic, Troubleshooting The
    Refrigerent System With Superheat & Subcooling.
    It ststes under critical temperture differences.
    The low side superheat should be between 20 and 30 de-
    grees. A/C manuals i have looked at show adjusting super
    heat on fixed orfice systems down to 5 degrees. What am
    i missing here?

    Thanks for any help.

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,346
    Originally posted by pipe threader

    Under for your interest, the topic, Troubleshooting The
    Refrigerent System With Superheat & Subcooling.
    It ststes under critical temperture differences.
    The low side superheat should be between 20 and 30 de-
    grees. A/C manuals i have looked at show adjusting super
    heat on fixed orfice systems down to 5 degrees. What am
    i missing here?
    Fixed orifice superheat will vary with mass flow rate and interior heat load. For example, let's say you have a hot day at 102 degrees, and an interior that the system is able to maintain comfortably at 74 degrees. Since the outdoor ambient is high, the condenser will be running a higher head pressure. This places more pressure on the liquid column feeding the fixed orifice, resulting in a greater pressure differential across the orifice and into the evaporator.

    Result? Greater mass flow rate, resulting in more refrigerant entering the evaporator, which gives you a lower superheat. Reduced airflow via dirty filters or air circulation restrictions will only aggravate the condition.

    Take a look at Jim Wheeler's charging chart for fixed orifice systems. If you plot out the superheat using his numbers, you can see that with variation in indoor and outdoor conditions, the superheat can range from near zero to pushing 40 degrees. It opened my eyes when I did this, only confirming what I already believed about the fuzziness of fixed restrictor metering devices.
    • 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.

  3. #3

    Wanting to learn




    Thanks Shophound

  4. #4
    I like shophounds answer. Also for a better understanding of superheat and subcooling check out the thread An Explanation on Superheat & Subcooling, it's also in the For Your Interest section. The best way to charge a system is by using the manufactors charts.

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