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 
Related Links: Species Table, Standard Species Model, Plant Model  Species Property Overrides, Fast IF97 Sat Properties
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)
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 IAPWSIF97 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:
 http://www.iapws.org/
 http://www.cheresources.com/iapwsif97.shtml
 The valid temperature range is:
 Steam: 273.16 to 3000K
 Water: 273.16 to 647K
 Steam: 273.16 to 3000K
IF97 Equation Names
In the SysCAD species database, equation names for full IF97 correlations are:
 For Builds 139: IF97WaterRho(), IF97WaterCp(), IF97SteamRho() and IF97SteamCp()
 For Builds 136 to 138: IF97WaterRho(), IF97WaterCp(), IF97SteamRho(), IF97SteamRho2(), IF97SteamCp() and IF97SteamCp2().
General Notes
 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 SIUnits, 2nd Revised and Updated Printing, Springer 1979.
Water properties below 0°C
In Build 139, a new set of equations have been added to calculate water Cp, H, S and density between 235K and 273.15K.
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 Reference given 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 Build138 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)
Reference 1: V. Holten, C. E. Bertrand, M. A. Anisimov, and J. V. Sengers, "Thermodynamics of supercooled water", J. Chem. Phys. 136, 094507, 2012.
Reference 2: “Smith, Van Ness, Abbott – “Introduction to Chemical Engineering Thermodynamics", 5th Ed, 1996
IF97 Equation Version Change Notes
 For Build 139.27886 and later, the “IF97” and “IF97_2” equations have been merged into a single equation "IF97" (SysCAD 9.3 Build139 Version).
 user now only has one simple choice – namely “IF97”. This uses the correct steam Enthalpy and Density equations for region5 (ie the IF97_2 code).
 Projects and cfgfiles auto upgrade to always use “IF97”.
 “IF97_2” is removed as an option for various dropdown lists (eg: Plant Model  Species Tab).
 For Builds 136 to 138, there are two equations for Steam Density and Steam Cp: “IF97” and “IF97_2”.
 During the release of SysCAD 9.3 Build136, we have found the original "IF97" (SysCAD 9.2 version) have errors in steam Enthalpy and Density equations for region5 (Caution: 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 as the temperature gets higher, the steam properties are less accurate).
 A corrected version (IF97_2) was implemented and for backward compatibility we have retained "IF97" (SysCAD 9.2 version).
 NOTE for Build 136 to Build138: SysCAD has improved equations for steam in Region 5. Projects upgraded from SysCAD 9.2 will use old IF97 (SysCAD 9.2 version), but these should be changed to use IF97_2.
 Implications:
 May affect results if the project was using the old equation with steam at high temperatures.
 Cfg files are upgraded in Build139, the files are not backward compatible, so if they are reused in earlier build there will be load errors/warnings.
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.
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" (less accurate) 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.
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] 
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
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
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
T = temperature in Kelvin This equation is used in the range from 298.15 to 500K. 
vapour pressure of H_{2}O (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

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.