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  1. #1
    Join Date
    May 2011
    Location
    CA
    Posts
    123

    Pressure-Enthalpy charts

    I've spent a lot of time reading over the last 2 days learning how to use a Pressure-Enthalpy chart. I'm kinda proud to say I finally understand it (mostly). Then, at the end of the chapter which deals with pressure-enthalpy charts in my textbook, the following statement is written: "These charts and tables are not normally used in the field for troubleshooting but are for engineers to use to design equipment. They help the technician understand the refrigerants and the refrigerant cycle".

    Would seasoned refrig. mechanics agree with this statement? How much do you use the pressure-enthalpy chart?

  2. #2
    Join Date
    May 2003
    Location
    Utah
    Posts
    735
    Is it used in the real world? Probably not too much.

    Could it be of assistance? Absolutely. How much better would a technician be at troublehooting with a thorough understanding of what is going on in the system at any given point, and how various system conditions will affect performance and efficiency. An understanding of the PE chart can best accomplish that.

    Here's a good overview of some real world scenarios that are given great clarity with the use of the PE chart.

    http://sporlanonline.com/5-200.pdf

  3. #3
    Join Date
    Mar 2012
    Location
    Maine
    Posts
    16
    I have never had to use a pressure enthalpy chart. I guess I have yet to be in a situation where I could use it.

  4. #4
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Bunny,

    Thanks for the link to the Sporlan doc....great stuff!

  5. #5
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,069
    I cannot say I have ever physically drawn out a Pressure-Enthalpy chart in the field.

    But I have plenty of times visually drawn one in my head to help understand the situation.

    Then sorta plan out the cause and effect of what I'm about to do.

    And how that will effect the graph in my mind.

  6. #6
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Pressure-Enthalpy charts are very useful as an educational tool. Do engineers used them? No, assuming they're adept at using a PC. NIST REFPROP http://www.nist.gov/srd/nist23.cfm can generate refrigerant thermodynamic and thermophysical data quite easily for any refrigerant of interest.
    If the only tool you have is a hammer, you tend to see every problem as a nail. – Abraham Maslow

  7. #7
    Join Date
    Feb 2010
    Location
    NE Alabama
    Posts
    301
    Not sure what a pressure/enthalpy chart is. I use the data I record in my mind from the readings i record and the actions of the system I am working on, and use different procedures to undersand the system whether it be Hvac, refrigeration (walkin freezers/coolers,reachin/undercounter freezers/coolers or chillers, ice machines) with the controlling factor being the refrigerant involved. I get there; turn it off; let the pressures equalize while checking for cleanliness of coils and operation of any/all components; then turn the system on and watch the gauges, and amp draw, and temps of evap coils to come to a decision in my mind about which way to jump. Unscientific but for me reliable.

  8. #8
    Join Date
    Apr 2012
    Location
    Los Angeles
    Posts
    19
    I just saw one for the first time...very confusing!

  9. #9
    Join Date
    Jun 2003
    Location
    Virginia
    Posts
    376
    I used to use them in the field a long long time ago. They do help you understand what's happening in the system while you're learning the trade. I found them especially useful on compound compression systems and cascade systems.. Also found them useful on systems that were critically sized...

    But not so much anymore. Most techs come to a point in their career when they understand the finer points of the refer cycle that they don't really need them anymore.

    One point to make is that your plot on the chart is different than they teach you in school. You need to plot according to pressure drop throughout the system...

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