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Steam Tables and IAPWS-IF97: Reading Saturated and Superheated Properties

Published July 3, 2026 · by Industrial Unit Converter Editorial Team

Steam Tables and IAPWS-IF97: Reading Saturated and Superheated Properties

A process engineer at a Gulf Coast chemical plant is building a heat balance for a 5 MW extraction turbine. He needs enthalpy and entropy at four states: boiler exit at 100 psig saturated, turbine inlet at 100 psig and 500 degF, an intermediate stage at 100 psig and 85% quality, and condensate at 100 psia and 200 degF. He opens the IAPWS-IF97 tables and a Mollier diagram. The arithmetic is the same as it was in 1969 when Keenan, Keyes, Hill, and Moore published the international steam tables.

The IAPWS Industrial Formulation 1997 (current 2012 release) is the international standard. The Steam Quality Calculator uses IAPWS-IF97 region 4 saturation values. The dryness fraction is in Steam Quality Explained, the distribution side is in the Steam System Design guide, and efficiency in Boiler Efficiency: Direct vs Indirect Methods.

The IAPWS-IF97 region map (1 through 5)

The IAPWS-IF97 divides the water and steam phase diagram into five regions, each with its own fundamental equations.

  • Region 1: Saturated and compressed (subcooled) liquid. Industrial process feedwater and condensate sit in Region 1.
  • Region 2: Superheated vapor at low to moderate pressure (below about 100 MPa). Most utility boiler exits and turbine inlets live in Region 2.
  • Region 3: The critical region (T near 647.1 K, P near 22.064 MPa, or 705.4 degF and 3,208 psia). The most difficult region to model. Supercritical boilers and high-pressure research.
  • Region 4: The saturation line. Both h_f and h_g live here.
  • Region 5: Superheated vapor at high temperature (T above 800 degC, 1,470 degF). Added in 2012. Rare in industrial plants.

For most industrial work (P below 250 psig, T below 500 degF), only Regions 1, 2, and 4 matter. Region 3 matters for supercritical utility boilers (P above 3,208 psia). Region 5 is for advanced power cycles.

Saturated steam table: the working reference

The table below is the IAPWS-IF97 region 4 formulation, rounded to four significant figures for field use.

P (psig) P (psia) T_sat (degF) h_f (BTU/lb) h_g (BTU/lb) h_fg (BTU/lb) s_f (BTU/(lb-degR)) s_g (BTU/(lb-degR))
0 14.7 212.00 180.17 1150.5 970.3 0.3121 1.7567
50 64.7 281.03 250.09 1174.0 923.9 0.4112 1.6584
100 114.7 338.06 298.61 1187.4 888.8 0.4740 1.6027
150 164.7 365.99 330.55 1194.0 863.4 0.5147 1.5701
200 214.7 387.94 355.36 1197.0 841.6 0.5437 1.5463
250 264.7 406.13 376.07 1198.4 822.3 0.5676 1.5273
300 314.7 421.83 393.99 1198.6 804.6 0.5879 1.5112
400 414.7 448.43 424.13 1196.0 771.9 0.6213 1.4844
500 514.7 470.51 449.40 1191.4 742.0 0.6487 1.4619
600 614.7 489.49 471.60 1185.0 713.4 0.6719 1.4418
900 914.7 532.11 526.60 1163.0 636.4 0.7260 1.3914
1500 1514.7 596.39 619.70 1108.6 488.9 0.8081 1.3077
2400 2414.7 706.11 752.00 970.3 218.3 0.9113 1.1972

These values match the ASME Steam Tables (2014) and the Keenan, Keyes, Hill, and Moore compilation to four significant figures. The 2,400 psig row is subcritical (the critical point is at 3,208 psia and 705.4 degF), but the latent heat has collapsed to 218 BTU/lb, only 23% of the 970 BTU/lb available at 0 psig. The 250 psig row is the one to memorize: h_g peaks there at 1,198 to 1,199 BTU/lb, and most industrial process steam runs between 100 and 250 psig for the best combination of temperature, latent heat, and piping cost.

Superheated steam and the h_g peak

The h_g peak matters because it controls where the largest enthalpy drop happens in a steam turbine. Below 100 psig, h_g rises with pressure. Above 300 psig, h_g falls with pressure. The rule of thumb: place the largest pressure drop where the latent heat is still useful. Above 1,000 psig, the latent heat is gone and you are running on sensible heat, which means a multi-stage reheat unit.

A worked check. At 100 psig saturated, h_g = 1,187.4 BTU/lb. At 250 psig saturated, h_g = 1,198.4 BTU/lb. At 600 psig saturated, h_g = 1,185.0 BTU/lb. The 250 psig value is the peak. A 250 psig throttle gives 11 BTU/lb more energy per pound than a 100 psig throttle, and 13.4 BTU/lb more than a 600 psig throttle. Across 100,000 lb/hr, that is 1.1 to 1.3 MMBTU/hr of design difference. Lower s_g means a more ordered vapor at the same pressure drop, which means more work per pound extracted. At 100 psig saturated, s_g = 1.6027 BTU/(lb-degR). At 250 psig saturated, s_g = 1.5273. At 600 psig saturated, s_g = 1.4418. Below about 100 psia exhaust pressure, the expansion lands in the wet region. The last-stage quality limit is in Steam Quality Explained.

The Mollier diagram

A Mollier diagram plots specific enthalpy (h) on the x-axis and specific entropy (s) on the y-axis. It shows the saturation dome, isobars sloping downward to the right, isotherms (nearly vertical in the superheat region), and constant-quality lines through the wet region.

For Rankine cycle analysis, plot the four key states (pump inlet, boiler exit, turbine exit, condenser exit) and trace the cycle. The vertical drop from boiler exit to turbine exit (constant s, isentropic expansion) is the ideal turbine work. The vertical rise from pump inlet to boiler exit (constant P) is the pump work.

The Mollier diagram is most useful for explaining why the turbine staging is the way it is, and for checking whether a proposed operating point actually sits inside the cycle. For numerical work, IAPWS-IF97 software is faster and more accurate.

Worked example: four states in a Rankine cycle

The four states below are the points a heat balance needs for a 5 MW extraction turbine at 100 psig throttle. The values come from IAPWS-IF97 region 4 (States 1 and 3), region 2 (State 2), and region 1 (State 4).

State 1: Saturated steam at 100 psig (boiler exit).

  • P = 114.7 psia, T = 338.06 degF
  • h = 1,187.4 BTU/lb, s = 1.6027 BTU/(lb-degR)

State 2: Superheated steam at 100 psig, 500 degF (turbine inlet with superheat).

  • P = 114.7 psia, T = 500 degF
  • h = 1,283.0 BTU/lb, s = 1.7067 BTU/(lb-degR)
  • The 162 degF of superheat added 95.6 BTU/lb of enthalpy.

State 3: Wet steam at 100 psig, quality 0.85 (intermediate stage).

  • P = 114.7 psia, T = 338.06 degF
  • h = 298.6 + 0.85 * 888.8 = 1,054.0 BTU/lb
  • s = 0.4740 + 0.85 * 1.1287 = 1.4334 BTU/(lb-degR)
  • where s_fg = 1.6027 - 0.4740 = 1.1287 BTU/(lb-degR)

State 4: Subcooled water at 100 psia, 200 degF (condensate).

  • h_f at 200 degF = 168.1 BTU/lb, v = 0.01634 ft^3/lb, P_sat = 11.5 psia
  • h = 168.1 - 0.01634 * (100 - 11.5) / 5.404 = 168.1 - 0.27 = 167.8 BTU/lb
  • The pressure correction is small for liquids (about 0.3 BTU/lb).

Net turbine work per pound is (h_2 - h_3) at 100% isentropic efficiency = 229.0 BTU/lb. Pump work is about 0.3 BTU/lb. Heat input is h_2 - h_4 = 1,115.2 BTU/lb. The Rankine efficiency on this 100 psig throttle is 229.0 / 1,115.2 = 20.5%. For a real 5 MW unit, plus turbine mechanical losses (3-5%), generator losses (2-3%), and moisture loss, the net Rankine efficiency is 15-17% at 100 psig. Raising the throttle to 600 psig with reheat and a regenerative feedwater cycle gets that to 30-35%. The full efficiency chain is in the Boiler Efficiency: Direct vs Indirect Methods guide.

Common lookup mistakes

The five errors below are the ones a plant engineer is most likely to make on a first-pass heat balance. Dollar numbers reference a 50,000 lb/hr, 150 psig natural gas firetube at $8/MMBTU, where 1% efficiency is $50,000 per year.

1. Using the saturated table for superheated steam. The 100 psig saturated h_g of 1,187.4 BTU/lb is wrong for 100 psig at 500 degF (the right number is 1,283.0 BTU/lb). The 95.6 BTU/lb error, applied across 100,000 lb/hr, is a 9.5 MMBTU/hr design error. Use the superheat table (region 2) once T is above T_sat.

2. Mixing US and SI units in the same calculation. US tables are in BTU/lb, degF, psia. SI tables are in kJ/kg, degC, MPa. A 1.0 kJ/kg number is 0.43 BTU/lb, not the same number. Pick one basis and convert all inputs before the lookup.

3. Confusing h_fg (latent heat) with h_g (saturated vapor enthalpy). h_fg is h_g minus h_f. h_g is the total enthalpy of saturated vapor relative to the reference state (h = 0 at 32 degF saturated liquid). Using h_fg in place of h_g in the wet-steam quality formula gives quality values above 1.0 for any steam drier than 50%.

4. Using gauge pressure (psig) in a table that expects absolute (psia). A 100 psig gauge reading is 114.7 psia absolute. Looking up 100 in an absolute table gives h_f and h_g for 100 psia (saturation at 327.8 degF), not for 100 psig (saturation at 338.06 degF). The 10 degF shift gives an enthalpy error of about 10 BTU/lb. Add 14.7 to every gauge reading before the lookup. At 5,000 ft elevation, atmospheric pressure is about 12.2 psia, not 14.7.

5. Looking up the wrong region. Subcooled liquid (region 1) and saturated liquid (region 4 boundary at the same T) have different h values because the region 1 value includes a pressure correction. At 200 degF and 100 psia, h_f (region 4) = 168.1 BTU/lb, h (region 1) = 167.8 BTU/lb. Use region 1 for compressed liquid, region 4 only for the saturated boundary.

A bonus error: linear interpolation between table rows is off by 0.1-0.5% in enthalpy and entropy. For a 0.5% error on a $4 million per year fuel bill, that is $20,000 per year of design drift. Use IAPWS-IF97 software for any value that is not exactly on a table row.

Modern software for steam properties

Printed tables are still in use at plants built before 2000, but most new work uses software.

Software Cost Best for Source
CoolProp 6.x (Bell et al. 2014) Free, open source General engineering, Python/MATLAB/C++ http://www.coolprop.org
NIST REFPROP 10.0 License fee Research, high-accuracy https://www.nist.gov/srd/refprop
Spirax Sarco Steam Tools Free, web Plant engineering https://www.spiraxsarco.com/resources-and-tools/steam-tables
Engineering Toolbox Free, web Quick lookups https://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html
iapws (Python) Free, open source Direct IAPWS-IF97 calls PyPI
Spirax Sarco mobile app Free Field reference iOS / Android

For most engineering work, CoolProp is the recommended default. It implements IAPWS-IF97 directly, runs in Python and MATLAB, and is fast enough for cycle calculations. REFPROP is the gold standard for research. The values on this site, in the Steam Quality Calculator and the Boiler Efficiency Calculator, use IAPWS-IF97 region 4 for saturated states and region 2 for superheated states, matching this table to four significant figures.

Standards and best practices

  • IAPWS-IF97 (2012 release), Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. The international standard.
  • ASME Steam Tables (2014), companion tables to IAPWS-IF97 in US units.
  • ASME BPVC Section I (2023), Rules for Construction of Power Boilers.
  • Keenan, Keyes, Hill, and Moore (1969), Steam Tables: International Edition (Metric Units). Wiley. The original international edition, still in field use.
  • Bell, Wronski, Quoilin, and Lemort (2014), the open-source CoolProp paper. Industrial and Engineering Chemistry Research, 53(6), 2498-2508.

Practical application

For most industrial process work, the IAPWS-IF97 table is enough. A heat balance on a 5 MW extraction turbine, a process steam header audit, a sterilization cycle, a humidification system: all of these sit in regions 1, 2, and 4, and the four-state Rankine cycle is the right framework. For supercritical and ultrasupercritical utility work (region 3), IAPWS-IF97 software is the only practical option. Region 5 (above 800 degC) is rare in industrial plants; the practical applications are advanced power cycles and research. For research and regulatory work, the IAPWS-IF97 publication is the citation of record.

Frequently asked questions

What is the difference between IAPWS-IF97 and the older steam tables?

IAPWS-IF97 is the international standard from the International Association for the Properties of Water and Steam, current 2012 release. The older IFC-67 (1967) and IFC-84 (1984) formulations are superseded. The ASME Steam Tables (2014) and the Keenan-Keyes-Hill-Moore compilation are printed tables consistent with IAPWS-IF97 to four significant figures.

How many regions does IAPWS-IF97 have, and which one do I use?

Five. Region 1 is subcooled liquid, region 2 is superheated vapor, region 3 is the critical region, region 4 is the saturation line, and region 5 is superheated vapor above 800 degC. For industrial process work, use region 1 for liquid, region 2 for superheated vapor, and region 4 for saturated states.

What software should I use for steam properties?

CoolProp 6.x is the recommended default. Free, open source, runs in Python and MATLAB, implements IAPWS-IF97 directly. NIST REFPROP 10.0 is the gold standard for research. Spirax Sarco's web tools are good for plant lookups. The iapws Python package is a lightweight option.

Why does h_g peak at 250 to 300 psig?

h_g rises with pressure below about 250 psig because the higher pressure shifts the saturation temperature up and increases the vapor enthalpy. Above 300 psig, the increase in saturation temperature is offset by the collapse of the latent heat, and the total h_g falls. The peak is at 250 to 300 psig at 1,198 to 1,199 BTU/lb.

How do I look up the right h for a superheated state, and what is the Mollier diagram actually useful for?

Use the superheat table (region 2), not the saturated table. At 100 psig and 500 degF, the saturated h_g is 1,187.4 BTU/lb, and the superheated h is 1,283.0 BTU/lb. The 95.6 BTU/lb gap is the energy the superheater adds. The Mollier chart is for visualizing cycles. For a Rankine cycle, plotting the four states and tracing the cycle gives an immediate sense of the work output, the heat input, and the heat rejected. The Steam Quality Calculator detects superheat and reports the degree of superheat.

References and further reading

  • IAPWS (2012 release). Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. International Association for the Properties of Water and Steam.
  • ASME (2014). Steam Tables: Thermodynamic Properties of Water and Steam (Compact Edition). American Society of Mechanical Engineers.
  • Keenan, Keyes, Hill, and Moore (1969). Steam Tables: International Edition (Metric Units). John Wiley and Sons.
  • ASME (2023). Boiler and Pressure Vessel Code, Section I: Rules for Construction of Power Boilers. American Society of Mechanical Engineers.
  • Bell, I.H., Wronski, J., Quoilin, S., and Lemort, V. (2014). Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp. Industrial and Engineering Chemistry Research, 53(6), 2498-2508.
  • NIST (current). REFPROP 10.0 Reference Fluid Thermodynamic and Transport Properties Database. National Institute of Standards and Technology.
  • Spirax Sarco (current edition). The Steam and Condensate Loop Book. Spirax-Sarco Limited.

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