Water and Steam Properties

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Standard Species Model		Stream Properties								Special Cases
Standard Species Model	Stream Properties Calculations	Density	Heat of Formation	Heat of Dilution	Specific Heat (Cp)	Solubility	Boiling Point Elevation	pH	Charge	Steam and Water	Sulfuric Acid

Introduction

As H₂O (water / steam) is a common and important component of most models, the property equations for both water and steam are built into SysCAD. These properties are hardcoded and cannot be changed.

However, the user may select the properties of H₂O to be calculated using either High Fidelity (IF97), Fast IF97 Sat or Simple equations (discussed in more detail below). High Fidelity equations will return the most accurate results for T and P, but the calculations do require more computing power, slowing the solver speed.

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, it is possible to "trick" SysCAD using a species with Definition 'O1H2' and a Component 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! (H2O(l) and H2O(g) cannot be removed from the species database.)
SysCAD does a range check on streams and many units to determine if liquid water, H2O(l), is present above the critical temperature (Tc) 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)

IF97

Full details of the algorithms used to calculate the thermodynamic properties of steam/water, such as heat capacity and enthalpy, saturated temperature/pressure, etc., are not presented here, as they are available in the listed references.
Calculations are based on the IAPWS-IF97 standard, described in:
- W. Wagner et al. (2000) "The IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam", ASME J. Eng. Gas Turbines and Power, Vol. 122
- Some good internet references for further information are:
  The International Association for the Properties of Water and Steam (http://www.iapws.org/)
  
  Thermodynamic and Transport Properties of Water and Steam (http://www.cheresources.com/iapwsif97.shtml)
The implementation in SysCAD has been optimised to maximise performance and solver speed.
Due to differences in reference points, numerical methods and implementation techniques there can be very small (generally much less than 0.1%) differences between IAPWS results and SysCAD results.
In SysCAD it may be necessary to perform calculations outside of the IF97 temperature range. This is required for a numerical robust solver and for cases where (correctly or incorrectly) conditions are outside if IF97 valid ranges.
The IF97 temperature range is 273.15 to 2273.15 K (0 to 2000 °C). The critical temperature for water is 647.096 K and the critical pressure for water is 220.64 bar.
The IF97 ranges are described in the image below, from IAPWS:

For SysCAD implementation outside of IF97 ranges:

An absolute maximum pressure of 1000 bar is applied (including IF97 Region5 which has a maximum of 500 bar).
For steam no upper temperature limit is applied (IF97 Region5 has a maximum temperature of 2273.15 K).
For water for temperatures below 273.15 K we have implemented an extrapolated correlation. This is intended for use in aqueous solutions.

IF97 Equation Names

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

Water Properties below 0°C

A set of equations for water Cp, H, S and density between 235K and 273.15K are available from Build 139.

To enable these equations, please check the tick boxes for H2O(l).UseNewLowT.Cp and or H2O(l).UseNewLowT.Rho on the View - Plant Model - Species Tab.

If these options are selected, then water Cp, H, S and Density between 235K and 273.15K are calculated based on references below. For temperatures below 235K (-38.15°C), the properties are extrapolated.
If UseNewLowT options are not selected, then water Cp and Density are extrapolated below 0°C. (This is original method used in Build 138. and earlier, retained here for backward compatibility purposes.)
UseNewLowT options are auto selected for any new project or existing project with a minimum temperature >0°C.
For projects with below 0°C temperatures, user can select this option to improve the accuracy of the enthalpy and density values between 235K and 273.15K.
If these new UseNewLowT options are not selected in a project, a warning is given in Message Window - Project settings Tab: PlantModel - Using old Water Density extrapolation below 0C. Review PlantModel.H2O(l).UseNewLowT.xxx. (where xxx = Cp or Density)

References:

V. Holten, C. E. Bertrand, M. A. Anisimov, and J. V. Sengers (2012) "Thermodynamics of Supercooled Water", J. Chem. Phys. 136, 094507
Smith, Van Ness, Abbott (1996) “Introduction to Chemical Engineering Thermodynamics", 5th Ed

IF97 Equation Version Change Notes

From Build 139.27886, a single equation, "IF97" replaces the old equations ("IF97" and "IF97_2") that were available in previous versions of SysCAD. Projects created using older builds will automatically upgrade to always use the updated "IF97" form of the equation. However, it is important to note that this change may affect results if the project was previously using the old equation with steam at high temperatures.

IF97 Equation Change - Detailed notes on how it may affect projects upgraded from Build 138 and earlier.
Prior to Build 139, there were two equations for Steam Density and Steam Cp: "IF97" and "IF97_2". The original "IF97" (from SysCAD 9.2) was found to have errors in steam Enthalpy and Density equations at very high temperatures (Region 5). A corrected version, "IF97_2", was implemented and the original "IF97" was retained for backwards compatibility. In the SysCAD species database, equation names for the "IF97" and "IF97_2" correlations were: IF97WaterRho(), IF97WaterCp(), IF97SteamRho(), IF97SteamRho2(), IF97SteamCp() and IF97SteamCp2() From Build 139.27886, the "IF97" and "IF97_2" equations have been merged into a single equation "IF97" (equivalent to the old "IF97_2"). "IF97" is now the only choice. This uses the correct steam Enthalpy and Density equations for Region 5. Projects and CFG files are auto-upgraded to always use "IF97". "IF97_2" is removed as an option from various dropdown lists (e.g. Plant Model - Species Tab). Notes for project upgrade: For project upgrade from Build 138 to Build 139, the CFG files are auto-upgraded to use the new "IF97" form. The updated CFG files are not backward compatible, so if they are reused in an earlier build there will be load errors/warnings. The change of equation may affect results if the project was previously using the old equation with steam at high temperatures.

Fast IF97 Sat

A set of lower fidelity equations called "FastIF97Sat" for water and steam Density, Cp, Enthalpy and Entropy is available from Build 139. 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.

Function	Equation	Notes
Water Density	Special density function LiqH2ODensity() See Fast IF97 Sat Properties.	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 Cp	Special Cp function LiqH2OCp(). See Fast IF97 Sat Properties.	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	Special Density function VapH2ODensity(). See Fast IF97 Sat Properties.	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 Cp	Special Cp function VapH2OCp(). See Fast IF97 Sat Properties.	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" set of equations is less accurate and is retained for backward compatibility. However, for Water Density, Water Cp, and Steam Cp, it is recommended to use the new Fast IF97 Sat equations - these equations are faster, more accurate, and valid for larger temperature ranges.

Function	Equation	Notes
Water Density (less accurate)	[math]\displaystyle{ \cfrac{a_0+a_1T+a_2T^2+a_3T^3+a_4T^4+a_5T^5}{1+b_1T} }[/math]	T = Temperature in Celcius 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 Cp (less accurate)	HSC_Cp(a,b,c,d) [math]\displaystyle{ C_p = a + b.10^{-3} T + \cfrac{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 (less accurate)	[math]\displaystyle{ \rho = \cfrac{PM}{RT} }[/math]	where P = Pressure R = Universal gas constant T = Temperature in Kelvin M = Molecular weight The density is calculated using a rearrangement of the Ideal gas law. This equation is used for all temperatures and pressures.
Steam Cp (less accurate)	CRC_Cp(a,b,c,d) [math]\displaystyle{ C_p = a + 10^{-3}b T + \cfrac{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 of H₂O (less accurate)	[math]\displaystyle{ \rho = 0.1333224\times10^{\tfrac A T +B\log_{10} \;(T)+CT+D} }[/math] (0.1333224 converts pressure from mm Hg to kPa.)	where A = -3433.74 B = -12.0063 C = 0.004782 D = 41.1767 T = Temperature in Kelvin, this equation is used from 0 to 1000K.