# Water and Steam Properties

**Related Links:** Species Table 9.3, Standard Species Model, Plant Model - Species Property Overrides, Fast IF97 Sat Properties

## Contents

## Introduction

As H_{2}O (steam / water) is a common and important part of most models, the property equations for both water and steam are built into SysCAD. The user cannot change these properties.

However, the user may select the properties of H_{2}O to be calculated using either High Fidelity (IF97), Fast IF97 Sat or Simple equations. (These are discussed in more detail below.) Obviously, the High Fidelity equations will return more accurate results for T and P, but the calculations do require more computing power. Therefore the solver speed will be less.

**Notes:**

- Properties for solid H2O (ice, or water trapped in a solid crystalline structure) are not included. To include these, add a species to the Species Database in the normal way with Phase = 'Solid' but the compound must NOT be 'H2O' and the Definition must NOT be H2O1, rather use
**O1H2**. - If alternate user defined properties for water or steam are required, then it is possible to "trick" SysCAD using a species with a Definition 'O1H2' and a Compound that is not "H2O".
- IMPORTANT: Caution should be used when including alternate properties for steam/water as the hardwired steam/water will also exist in the project!

- SysCAD does a range check on streams and many units to determine if liquid water, H2O(l), is present above the critical temperature of water. If it detects water above Tc it will generate a warning message. Please also see Plant Model - RC for more information.

## High Fidelity (IF97)

The algorithms used to calculate the thermodynamic properties of steam/water, such as heat capacity and enthalpy, saturated temperature/pressure, etc, are not given in this document, as they are available in the reference. The implementation in SysCAD has been optimised to maximise performance (speed). The calculations are based on the IAPWS-IF97 standard. This is described in a paper by W. Wagner et al. called "The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam", published in ASME J. Eng. Gas Turbines and Power, Vol. 122 (2000).

Some good internet references for further information are:

The valid temperature range is:

- Steam: 273.16 to 3000K
- Water: 273.16 to 647K

**NOTES**:

- The IF97 equations are applied for temperatures up to 1073.15 K. Above that temperature SysCAD calculates any steam/water properties using Region 5 properties and linear extrapolation. Thus the higher the temperature, the more inaccurate the steam properties will becomes. Please use data with caution.
- Due to differences in reference points, numerical methods and implementation techniques there can be very small (generally much less than a tenth of a percent) differences between IAPWS results and SysCAD results.

Reference: Properties of Water and Steam in SI-Units, 2^{nd} Revised and Updated Printing, Springer 1979.

### Improved IF97 for Region 5 (IF97_2)

SysCAD 9.3 has improved equations for steam in Region 5. Projects upgraded from SysCAD 9.2 will use old IF97, but these should be changed to use IF97_2.

### IF97 Equation names

In the SysCAD species database, the equation names for full IF97 correlations are: **IF97WaterRho(), IF97WaterCp(), IF97SteamRho(), IF97SteamRho2(), IF97SteamCp() and IF97SteamCp2**.

## Fast IF97 Sat

SysCAD 9.3 has a new set of lower fidelity equations called "FastIF97Sat" for water and steam Density, Cp, Enthalpy and Entropy. These equations are based on accurate curve fits to the IF97 data along the Saturation pressure line. The equations are therefore a function of temperature only, pressure is ignored and it is assumed that the pressure is the saturation pressure. These equations are significantly faster and for many operating conditions, especially for water, are suitable for most simulation requirements where the full accuracy of IF97 is not required. Please refer to Fast IF97 Sat Properties for more detail.

### Water Density

The equation for water density is an equation fitted to IF97 water density along the saturation line. The valid temperature range is 260K to 647.096K. If the temperature is outside of the valid range then the minimum or maximum temperatures are used to calculate the density.

In the SysCAD species database, any species can be assigned this density by use of a special density function **LiqH2ODensity()**.

### Water Heat Capacity

The equation for water Cp is an equation fitted to IF97 water heat capacity along the saturation line. The valid temperature range is 260K to 573.15K.

In the SysCAD species database, any species can be assigned this Cp by use of a special Cp function **LiqH2OCp()**.

### Steam Density

The density is calculated using the IdealGasDensity equation with the temperature ranged between 260K and 4000K.

In the SysCAD species database, any species can be assigned this Density by use of a special Density function **VapH2ODensity()**.

### Steam Heat Capacity

The equation for steam Cp is an equation fitted to IF97 steam heat capacity along the saturation line.

In the SysCAD species database, any species can be assigned this Cp by use of a special Cp function **VapH2OCp()**.

## Simple

The "Simple" equations were called "Low Fidelity" in SysCAD 9.2. These are retained for backward compatibility. For Water Density, Water Cp and Steam Cp the new "FastIF97Sat" equations should be used instead as faster more accurate equations valid for larger temperature ranges.

### Water Density

The **SIMPLE** (less accurate) equation for water density is:

[math] \frac{a_0+a_1T+a_2T^2+a_3T^3+a_4T^4+a_5T^5}{1+b_1T} [/math]

- T is in degrees centigrade.
- The valid temperature range is -30°C to 300°C.
- If the temperature is outside of the valid range given above, then the minimum or maximum temperatures are used to calculate the density.
- For example, if the stream temperature is 400°C, then the water density will be calculated using the maximum temperature of 300°C.

In the SysCAD species database, any species can be assigned this density by use of a special density function **LiqH2ORho()**.

### Water Heat Capacity

The **SIMPLE** (less accurate) equation used for liquid heat capacity is:

HSC_Cp(a,b,c,d)

- [math] C_p = a + b.10^{-3} T + \frac{c.10^5}{T^2} + d.10^{-6} T^2\, [/math]

where

a = 16.749

b = 62.120

c = 32.798

d = 90.391

T = temperature in Kelvin

This equation is used in the range from 273.15 to 500K.

In the SysCAD species database, a species can be assigned this heat capacity as **HSC_Cp(16.749, 62.12, 32.798, 90.391):Range(K, 273.15, 500)**.

### Steam Density

The density is calculated using a rearrangement of the Ideal gas law. The **SIMPLE** (less accurate) equation used is:

[math]\rho = \frac{PM}{RT}[/math]

where

P = Pressure

R = universal gas constant

T = Temperature in Kelvin

M = molecular weight of water

This equation is used for all temperatures and pressures.

### Steam Heat Capacity

The **SIMPLE** (less accurate) equation used for heat capacity of the vapour is:

- CRC_Cp(a,b,c,d)
- [math] C_p = a + 10^{-3}b T + \frac{10^5c}{T^2} + 10^{-6}dT^2\, [/math]

where

a = -38.14955

b = 463.4602

c = 6.24e-5

d = -762.3604

T = temperature in Kelvin

This equation is used in the range from 298.15 to 500K.

### Vapour Pressure

The **SIMPLE** (less accurate) equation that is used to calculate the vapour pressure of H_{2}O is

[math]\rho = 0.1333224\times10^{\frac A T +B\log_{10}(T)+CT+D}[/math]

where

T = Temperature in Kelvin

A = -3433.74

B = -12.0063

C = 0.004782

D = 41.1767

(0.1333224 converts from the pressure from mm Hg to kPa.)

This equation is used from 0 to 1000K.

## Water and Steam Properties in PGM

The user may obtain some water and steam values in PGM code using the Species Database Class.

Some of the values that are available to users are:

- Saturated Temperature;
- Saturated Pressure;
- Density;
- Heats of Formation; and
- Enthalpy.