Engineering guide
The H2O module calculates the properties of water and steam according to the industrial formulation IAPWS-IF97. If you need to calculate water and steam properties — density, specific enthalpy and entropy, heat capacity, thermal conductivity, viscosity, speed of sound or surface tension — you obtain them as functions of pressure and temperature for the liquid and the vapor phase, including states on the saturation line.
The International Association for the Properties of Water and Steam (IAPWS) coordinates international research on the properties of water and, with the IF97, has created a mathematical formulation of the thermodynamic state properties optimized for industrial use; the transport properties viscosity and thermal conductivity as well as the surface tension follow separate IAPWS correlations. These formulations are the globally authoritative standard for power plant and process calculations and supersede the older steam tables (IFC-67).
In practice, the module supplies the input data for almost every thermal calculation in equipment and plant engineering: heat exchangers with water or steam, steam power cycles, condensers, feedwater systems and safety assessments. Derived quantities such as Prandtl number, thermal diffusivity, isentropic exponent, compressibility factor and dielectric constant are available directly.
Scope
- Properties of water
Typical workflow
Define the state point
The user specifies pressure and temperature — optionally for the liquid phase, the vapor, or both. For saturation states, one quantity suffices; the corresponding saturation temperature or saturation pressure follows from the vapor pressure equation of the IF97.
Determine the IF97 region
The IAPWS-IF97 divides the state space into regions (liquid, superheated steam, supercritical, saturation), each with its own fundamental equations. The module automatically assigns the state point to the correct region and evaluates the corresponding Gibbs or Helmholtz function.
Calculate the thermodynamic properties
From the derivatives of the fundamental equation follow, consistently, the density or specific volume, specific enthalpy, internal energy and entropy, the isobaric and isochoric heat capacities, the isentropic exponent, the speed of sound, the compressibility factor and the coefficient of thermal expansion.
Determine the transport properties
Dynamic viscosity and thermal conductivity are calculated using the separate IAPWS correlations, the surface tension using the IAPWS equation for the saturation line; the dielectric constant is output in addition.
Form the dimensionless numbers for heat transfer calculations
From the basic quantities, the kinematic viscosity, Prandtl number and thermal diffusivity are formed — the direct input quantities for Nusselt correlations, pressure drop calculations and the design of heat exchangers.
Frequently asked questions
What distinguishes IAPWS-IF97 from the scientific formulation IAPWS-95?
IAPWS-95 is the scientific reference formulation with the highest accuracy, but with a computationally expensive Helmholtz fundamental equation. The IF97 (Industrial Formulation 1997) is a formulation derived from it and split into regions, optimized for fast, highly repetitive evaluations in power plant and process calculations, and reproduces the IAPWS-95 values within tight tolerances. For equipment design, the IF97 is the governing industrial standard.
Over what range is the IAPWS-IF97 valid?
The IF97 is valid from 0 °C (273.15 K) to 800 °C at pressures up to 100 MPa; for the high-temperature range up to 2000 °C, a separate region up to 50 MPa is defined. This covers practically all states encountered in power plant and equipment engineering, including supercritical steam states above the critical point (373.946 °C, 22.064 MPa). Outside this range, for example for subcooled metastable water, the equations do not apply.
Why does the Prandtl number of water change so strongly with temperature?
The Prandtl number Pr = η·c<sub>p</sub>/λ of liquid water falls from around 13 at 0 °C to about 7 at 20 °C and below 1 near saturation at high pressures — mainly because the viscosity decreases exponentially with temperature, while thermal conductivity and heat capacity change only slightly. For heat transfer calculations, the properties must therefore be evaluated at the correct reference temperature (usually the mean fluid temperature); properties taken at the wrong temperature are one of the most common sources of error in Nusselt correlations.
What are the compressibility factor and isentropic exponent of steam needed for?
Superheated steam behaves approximately like an ideal gas only at low pressures; near the saturation line and at high pressures, the compressibility factor deviates significantly from 1. The isentropic exponent enters the calculation of nozzle and valve flows, critical pressure ratios (e.g. for safety valves) and compression processes. Taking both quantities from the IF97 instead of calculating with ideal-gas values (κ ≈ 1.33) avoids systematic errors in blowdown and flow calculations.
Worked example
For a heat transfer calculation, the derived dimensionless quantities of liquid water at 20 °C and 1 bar are required. From the basic IAPWS properties (density, dynamic viscosity, heat capacity, thermal conductivity), the kinematic viscosity, thermal diffusivity and Prandtl number are to be calculated — a worked example of a typical water property calculation.
Given
| Temperature T | 20 °C |
|---|---|
| Pressure p | 1 bar |
| Density ρ | 998.2 kg/m³ |
| Dynamic viscosity η | 1.002 · 10⁻³ Pa·s |
| Specific heat capacity cp | 4,182 J/(kg·K) |
| Thermal conductivity λ | 0.598 W/(m·K) |
Solution
Kinematic viscosity
ν = η / ρ = 1.002 · 10⁻³ / 998.2 = 1.004 · 10⁻⁶ m²/s
Thermal diffusivity
a = λ / (ρ · cp) = 0.598 / (998.2 · 4,182) = 0.598 / 4,174,472 = 1.433 · 10⁻⁷ m²/s
Prandtl number
Pr = η · cp / λ = 1.002 · 10⁻³ · 4,182 / 0.598 = 7.01
As a check: Pr = ν / a = 1.004 · 10⁻⁶ / 1.433 · 10⁻⁷ = 7.01 — both routes give the same value.
Result
| Kinematic viscosity ν | 1.004 · 10⁻⁶ m²/s |
|---|---|
| Thermal diffusivity a | 1.433 · 10⁻⁷ m²/s |
| Prandtl number Pr | 7.01 |
Technical content source: LV-Solved-Problems-Content 628d313247e7