Comparisons of Steam and Water Formulations
SteamTab Quad gives us the ability to generate water and steam properties using any one of the following formulations:
- The IAPWS Formulation for General and Scientific Use (IAPWS-95)
- The IAPWS Formulation for Industrial Use (IAPWS-IF97)
- The NBS/NRC Formulation; approved by the IAPS for General and Scientific Use in 1984 (NBS/NRC-84)
- The IFC 1967 Formulation for Industrial Use; also known as the "ASME Steam Tables" (IFC-67)
A question that our clients have repeatedly asked us is: What is the difference between these formulations? With the help of SteamTab Quad, we can now answer this question. The following sections describe the differences in the computed values of enthalpy, entropy, density, etc as computed by these 4 formulations.
Many SteamTab users find it is necessary to use more than one set of steam property formulations to determine whether steam system performance issue is related to steam properties or equipment or relying the assessment of plant performance based on the most accurate set of formulation and conducting commercial transactions with another contractually agreed set of formulation.
Results for additional parameters will be published here as soon as they are completed. If you wish us to examine any specific parameters or scenarios, please email your request to clc.support@chemicalogic.com.
Note
In all of these comparisons, we use the IAPWS-95 formulation as the baseline, as this formulation provides the most accurate representation of the thermodynamic properties of water an steam over a wide range of temperature and pressure.
Also, all of our analysis is presented in Metric/SI units.
Comparing Fundamental Constants
One source of difference among these formulations is the use of different fundamental constants of water, such as the critical properties. The following table compares the various fundamental constants used by the different formulations:
Property | IAPWS-95 | IAPWS-IF97 | NBS/NRC-84 | IFC-67 |
Molecular weight, kg/kmol | 18.01526800 | 18.01525808 | 18.01528000 | 18.01534000 |
Triple point temperature, ºC | 0.01 | 0.01 | 0.01 | 0.01 |
Triple point pressure, bar | 0.00611657 | 0.00611657 | 0.006117313 | 0.006112444 |
Critical point temperature, ºC | 373.946 | 373.946 | 373.976 | 374.150 |
Critical point pressure, bar | 220.64 | 220.64 | 220.55 | 221.20 |
Critical point density, kg/m3 | 322.00 | 322.00 | 321.89 | 315.46 |
Comparing Vapor Pressure
This sections presents the results of the difference between the various formulations for the vapor pressure of steam, calculated from the triple point to the critical point.
Comparisons of the saturated vapor pressure, from the triple point to the critical point is summarized below:
| Relative Error | IAPWS-97 | NBS/NRC-84 | IFC-67 |
Maximum absolute error | 0.018% | 0.096% | 0.135% |
Average absolute error | 0.004% | 0.053% | 0.060% |
Root Mean Square error | 0.007% | 0.062% | 0.070% |
Note: The IAPWS-95 formulation is used as the baseline for comparisons.
A chart of the deviations is available in PDF format. Also, the Excel spreadsheet used to generate this summary and chart is also available:
Comparing Latent Heat of Vaporization
Some of the largest deviations occur in the computation of the latent heat of vaporization, especially near the critical region. The results are summarized below:
Relative Error | IAPWS-97 | NBS/NRC-84 | IFC-67 |
Maximum absolute error | 36.327% | 60.610% | 113.136% |
Average absolute error | 1.730% | 2.165% | 3.534% |
Root Mean Square error | 6.534% | 8.304% | 15.311% |
Note: The IAPWS-95 formulation is used as the baseline for comparisons.
A chart of the deviations is available in PDF format. Also, the Excel spreadsheet used to generate this summary and chart is also available:
Comparing Saturated Densities
While specific density is not typically used by engineers, it is an extremely important property as the scientific formulations are explicit in temperature and density from which other properties are derived.
Comparisons of the saturated liquid densitiy, from the triple point to the critical point is summarized below:
Relative Error
(Liquid Density) | IAPWS-IF97 | NBS/NRC-84 | IFC-67 |
Maximum absolute error | 2.293% | 7.931% | 10.907% |
Average absolute error | 0.123% | 0.196% | 0.204% |
Root Mean Square error | 0.407% | 0.828% | 1.092% |
Comparisons of the saturated vapor densitiy, from the triple point to the critical point is summarized below:
Relative Error
(Vapor Density) | IAPWS-IF97 | NBS/NRC-84 | IFC-67 |
Maximum absolute error | 2.666% | 8.000% | 11.862% |
Average absolute error | 0.141% | 0.315% | 0.341% |
Root Mean Square error | 0.471% | 0.955% | 1.276% |
Note: The IAPWS-95 formulation is used as the baseline for comparisons.
A chart of the deviations is available in PDF format. Also, the Excel spreadsheet used to generate this summary and chart is also available: