Results 1 to 5 of 5
  1. #1
    Join Date
    Jan 2008
    Location
    India
    Age
    36
    Posts
    8
    Rep Power
    0

    negative enthalpy???



    i was just doing some calculations in some numerical when i found out that some enthalpy and entropy values for ammonia and almost all for R744 are negative.
    How can the enthalpy be negative?? i m all confused now!!



  2. #2
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: negative enthalpy???

    Don't worry about negative values. They simply reflect the reference temperature used for the refrigerant. Look on the tables or PH diagram and find the temperature where the enthalpy goes negative. That's your reference temperature for that set of data.

    ASHRAE typically uses -40C (or F). IIR and others usually use different values. Always use the same data for your calculations. If you use one table for one value and another table for a different value you can quickly have an incorrect answer.

    Enthalpy difference is hg - hf, so if the values are negative you still get the delta enthalpy.
    If all else fails, ask for help.


  3. #3
    Join Date
    Jan 2008
    Location
    India
    Age
    36
    Posts
    8
    Rep Power
    0

    Re: negative enthalpy???

    i was actually optimizing the heat exchanger temperature in a cascaded system between Ammonia(HT cycle) and CO2(LT cycle) and got those values as negative. now when i calculate the system COP, the negative sign messes everything up.
    anyways, can you explain the concept of reference temperature?? i am kinda confused a lil more about it, dont we already use the temperature as a reference, so why use another particular temperature for a reference, or is it a reference for something else that i am missing

  4. #4
    Join Date
    Aug 2005
    Location
    USA
    Posts
    5,326
    Rep Power
    25

    Re: negative enthalpy???

    If you look at a pressure-enthalpy diagram on the "x" axis (enthalpy) for zero enthalpy, then read directly above this to where the enthalpy line crosses the temperature lines you will see the reference temperature used in the data.

    For instance, in one Mollier diagram I have here for ammonia it shows the reference temperature is set to -40°F. At that point the enthalpy listed is zero.

    Another point to be careful of is what the minus sign may actually indicate. In some textbooks a negative sign is simply a derivation showing heat removed or work extracted from a process. While a positive result shows heating or work input.
    If all else fails, ask for help.


  5. #5
    Join Date
    Jan 2008
    Location
    India
    Age
    36
    Posts
    8
    Rep Power
    0

    Re: negative enthalpy???

    agreed,
    harder i tried, harder it became

Similar Threads

  1. pressure enthalpy chart for R22
    By marc5180 in forum Training
    Replies: 11
    Last Post: 10-03-2009, 06:22 AM
  2. enthalpy wheel
    By Lc_shi in forum New Technologies
    Replies: 1
    Last Post: 21-10-2006, 02:26 AM
  3. How to calculate enthalpy?
    By thing in forum Refrigerants
    Replies: 5
    Last Post: 29-10-2005, 03:10 PM
  4. negative pressure ward for hospital
    By watana in forum Air Conditioning
    Replies: 5
    Last Post: 13-03-2005, 11:44 PM
  5. Positive or negative?
    By acman in forum Technical Discussions
    Replies: 12
    Last Post: 22-05-2002, 04:13 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •