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What should I do if didn't find the given pressure?

  1. Mar 24, 2015 #1
    I want to know what should I do if I don't find the given pressure in a problem in the super heated tables ?
    I have P=1.1MPa and I want to get the " h " but I don't find this pressure in the tables
     
  2. jcsd
  3. Mar 24, 2015 #2
    Linearly interpolate.

    Chet
     
  4. Mar 24, 2015 #3
    Isn't there another way ? My teacher told me another way to do it that takes less time in the exam , but I can't remeber it
     
  5. Mar 24, 2015 #4
    How long does it take to linearly interpolate?

    Chet
     
  6. Mar 24, 2015 #5
    I don't know exactly but if there is a faster way , I should go with it
     
  7. Mar 24, 2015 #6
    I can't think of a faster way. I bet I could do the linear interpolation in less than a minute. Interpolation is the standard way of working with tables.

    Chet
     
  8. Mar 24, 2015 #7
    Could you tell me how to do the interpolation for this : I have P=1.1MPa T=250°C and I want to get "h"
    Sorry I am not familiar with this type of solution so I need your help :)
     
  9. Mar 24, 2015 #8
    At what values of the pressure does the table give values of h? (on either side of 1.1 MPa) What are those values of h at 250 C?

    Chet
     
  10. Mar 24, 2015 #9
    I can't understand . what I know that it is at the superheated vapour Tables
     
  11. Mar 24, 2015 #10
    Write down some of the numbers from your table. For example in the steam tables I have,

    P = 10 bars, T = 240 C, h = 2920
    P = 10 bars, T = 280 C, h = 3008
    P = 15 bars, T = 240 C, h = 2899
    P = 15 bars, T = 280 C, h = 2993

    Your turn.

    Chet
     
  12. Mar 24, 2015 #11
    That's the question. Which table ? I don't have a table for the pressure 1.1MPa . I have for 1.0 MPa and for 1.5 MPa
     
  13. Mar 24, 2015 #12
    I'm acutely aware of that. What does your table give for h at 1.0 MPa and 1.5 MPa? After you provide those values, I will show you how to get the value at 1.1 MPa.

    Chet
     
  14. Mar 24, 2015 #13

    russ_watters

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    Staff: Mentor

    Amr719, it doesn't seem like you know what it means to "interpolate". Did you look that up after Chestermiller said it is what is needed?
     
  15. Mar 25, 2015 #14
    Shocking, huh? Don't they teach interpolation in high school algebra any more?

    Chet
     
  16. Mar 25, 2015 #15

    SteamKing

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    Staff Emeritus
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    Homework Helper

    The problem is, "interpolate" is too big to fit on a calculator key. :oops:
     
  17. Mar 28, 2015 #16
    No I know it but like I said before I'm not familiar with it because my teacher doesn't use this method. That's all
     
  18. Mar 28, 2015 #17
    Table for 1.0MPa : Table for 1.5MPa:
    T=240,h=2920.4 T=240,h=2899.3
    T=280,h=3008.2 T=280,h=2991.7
     
  19. Mar 28, 2015 #18
    And, what, you never noticed that these are the same values I gave you from my table in post #10?

    I am going to show you how to get the value of h at 250 C and 1.0 MPa. Then you are going to show me how you apply the same interpolation approach to get the value of h at 250 C and 1.5 MPa.

    $$h(250 C,1 MPa) = 2920.4 + \frac{(250 - 240)}{(280-240)}(3008.2-2920.4)=2942.4$$

    Now I want you to apply this same algorithm to get the value of h at 250 C and 1.5 MPa. Do you think you can do that?

    Please don't tell me at this point that you need to have the value of h at 1.1 MPa. I know that. There will be another step after you complete this step.

    Chet
     
  20. Mar 28, 2015 #19

    h(250C,1.5MPa)=2899.3+((250-240)/(280-240))*(2991.7-2899.3)=2922.4
     
  21. Mar 28, 2015 #20
    Looks good. Now, do you think you can take these results for h at 250 C for 1.0 MPa and 1.5 MPa and, by using this same kind of interpolation algorithm for pressure, find the value of h at 250 C and 1.1 MPa?

    Chet
     
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