How to Look up and Interpolate Enthalpy Values for Water Using Steam Tables

Step-by-Step Guide

1. 1

   Step 1: Define what we are trying to find and label the given values.

   Enthalpy change is described as ∆H = Hfinal
   - Hinitial.
   
   Tinitial = 40o C , Tfinal = 190o C , Pinitial = .1 MPa , Pfinal = .3 MPa
2. 2

   Step 2: Find the Steam Tables in the back of your textbook that have values for saturated water.

   Usually labeled Saturation Temperature and Saturation Pressure. (First page may look like the image below) ,, Labeled HL, HVap or HV.
   
   We just know that our fluid is initially a liquid, so we will use the liquid enthalpy value, HL, as our initial enthalpy value.
   
   HL =
   167.53 kJ/kg ,,,, Interpolating values gives us a best guess, when the desired temperature or pressure is between two available values.
   
   Interpolating follows the formula, HD (desired enthalpy) = +H_low For our example problem, Tfinal = 190o C , In this case they are 200o C and 150o C. ,,, The answer is given below.
3. 3

   Step 3: Locate the temperature 40o C in the left hand column of the table.

4. 4

   Step 4: Follow it across the table to the enthalpy columns.

5. 5

   Step 5: Now locate the Superheated Steam table.

6. 6

   Step 6: Identify the values relative to our final pressure (0.3 MPa)

7. 7

   Step 7: Locate our final temperature (190o C)

8. 8

   Step 8: Recognize that 190 is not listed in the temperature column

9. 9

   Step 9: therefore we must interpolate.

10. 10

    Step 10: Locate the temperature values just above and below 190o C

11. 11

    Step 11: Thi and Tlow.

12. 12

    Step 12: Now find the corresponding enthalpy values for 150o C and 200o C

13. 13

    Step 13: Hhi and Hlow.

14. 14

    Step 14: Follow the equation above to find the interpolated enthalpy value at 190o C. +2761.2 H190 = 2844.96 kJ/kg

15. 15

    Step 15: Subtract our initial enthalpy value at 40o C ( 167.53 kJ/kg) from our final enthalpy value at 190o C (2844.96 kJ/kg)

16. 16

    Step 16: to find the change in enthalpy needed to heat water from its liquid phase to superheated steam.

Detailed Guide

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