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Understanding how to use steam tables

  1. Feb 6, 2016 #1
    1. The problem statement, all variables and given/known data
    I have a table for 5 sets of data. Each set includes data from one ore more of the following properties - temperature, pressure, energy, and steam quality. I have to look at steam tables for water to find the missing values. For the data set in question I'm given T = 75 C, and P = 500 KPa. I need to find the energy and steam quality, and from steam quality I get number of phases present.

    2. Relevant equations
    for finding x or energy if calculation is required - u = x*(u liquid) + ((1-x)*(u gas))
    just rearrange the above to find x if needed

    3. The attempt at a solution
    One of the data sets gives me a temperature and pressure of 75 C and 500 KPa, nothing else. I looked at water's saturated steam tables for this temperature and pressure. I found that P>Saturated Pressure and T<Saturated Temperature (at these given values), so I'm assuming that I'm actually looking at a compressed liquid. I then looked at the steam tables for compressed liquid, but I can't find a data set for 75 C and 500 KPa. The temperature range includes 75 C, but all the pressures are in the multiple MPa's. Can someone help me clarify how to properly approach these tables? I'm a bit confused here. Any and all help is appreciated, thank you.
     
  2. jcsd
  3. Feb 6, 2016 #2

    SteamKing

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    What is 500 KPa expressed in units of MPa? This should be a simple conversion. What do the prefixes K and M stand for in the SI system?
     
  4. Feb 6, 2016 #3

    Yes, 500 KPa is .5 MPa unless I'm mistaken. I kept the conversions in mind. On the compressed liquid table the lowest pressure available is 5 MPa.
     
  5. Feb 6, 2016 #4

    SteamKing

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    Then get another table.

    You can get tables from the NIST web site which cover a greater range of pressures for compressed liquid.

    http://www.nist.gov/srd/upload/NISTIR5078-Tab3.pdf
     
  6. Feb 7, 2016 #5
    The internal energy can't be varying much with pressure for a compressed liquid because water is nearly incompressible. So just use the saturated value.
     
  7. Feb 8, 2016 #6

    jim hardy

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    SteamKing's link seems to exactly answer the question, i see an entry for 0.5mPa and 75C
    but the number is strange to me because i'm used to ASME 1968 tables in BTU's and Fahrenheit... and that table doesn't say..........
    314.4 ? I suppose it's an SI value for something ? Looks about right to be kJ per kg , around 135 BTU/lb ,
    per http://www.endmemo.com/sconvert/kj_kgbtu_lb.php
     
  8. Feb 8, 2016 #7
    Yes. My steam tables give 313.9 kj per kg at saturation pressure
     
  9. Feb 8, 2016 #8

    jim hardy

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    Thanks Chestermiller ! Makes sense now.

    With Imperial steam tables enthalpy of compressed water is almost intuitive, ~a BTU/lb per degF above 32.
    75C is on Fahrenheit scale 135 degrees above freezing, so around 135BTU/lb . And as you said pressure has a very minor effect.

    old jim
     
    Last edited: Feb 11, 2016
  10. Feb 11, 2016 #9
    Thank you everyone for your help! I appreciate it.
     
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