Water and Steam Properties
Related Links: Species Table, Standard Species Model
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 calculated using either High Fidelity equations or the Low Fidelity equations. (These are discussed in more detail below.) Obviously, the High Fidelity equations will return more accurate results, 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 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 - Range Checks for more information.
High Fidelity
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 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.
Low Fidelity
Water Density
The equation for water density is:
- 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, as shown below:
LiqH2ORho() |
Water Heat Capacity
The equation used for liquid heat capacity is:
HSC_Cp(a,b,c,d)
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 shown below:
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 equation used is:
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 equation used for heat capacity of the vapour is:
- CRC_Cp(a,b,c,d)
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 equation that is used to calculate the vapour pressure of H_{2}O is
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.