Look up and Interpolate Enthalpy Values for Water Using Steam Tables

This article gives a step by step list of instructions for correctly and accurately using steam tables. This article uses an example problem to help show a technique for approaching the steam tables and how to interpolate values if necessary.

Problem Statement: Find the change in enthalpy needed to heat saturated liquid water at .1 MPa and 40o C to superheated steam at .3 MPa and 190o C.

Things You'll Need

  • Steam Tables (Most likely in the back of your textbook)
    • This article uses Introduction To Chemical Engineering Thermodynamics
  • Calculator

Steps

  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. 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)
  3. Locate the temperature 40o C in the left hand column of the table.
  4. Follow it across the table to the enthalpy columns. 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
  5. Now locate the Superheated Steam table.
  6. Identify the values relative to our final pressure (0.3 MPa)
  7. Locate our final temperature (190o C)
  8. Recognize that 190 is not listed in the temperature column, therefore we must interpolate. 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_hi-H_low)/(T_hi-T_low )*(T_final-T_low ) ]+H_low For our example problem, Tfinal = 190o C
  9. Locate the temperature values just above and below 190o C, Thi and Tlow. In this case they are 200o C and 150o C.
  10. Now find the corresponding enthalpy values for 150o C and 200o C, Hhi and Hlow.
  11. Follow the equation above to find the interpolated enthalpy value at 190o C. [(2865.9-2761.2)/(200-150)*(190-150) ]+2761.2 H190 = 2844.96 kJ/kg
  12. Subtract our initial enthalpy value at 40o C ( 167.53 kJ/kg) from our final enthalpy value at 190o C (2844.96 kJ/kg), to find the change in enthalpy needed to heat water from its liquid phase to superheated steam. The answer is given below.

∆H = 2844.96 kJ/kg – 167.53 kJ/kg = 2677.43 kJ/kg

Warnings

  • These instructions show how to solve an example problem using the steam tables which gives a general sense of how to approach the steam tables for other problems. However, this article does not show all of the methods or solutions possible while using the steam tables.

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