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

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

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)

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

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

Now locate the Superheated Steam table.

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

Locate the temperature values just above and below 190o C, Thi and Tlow. In this case they are 200o C and 150o C.

Now find the corresponding enthalpy values for 150o C and 200o C, Hhi and Hlow.

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