Water and Steam Properties

From SysCAD Documentation

Contents 1 Introduction 2 High Fidelity 3 Low Fidelity 3.1 Water Density 3.2 Water Heat Capacity 3.3 Steam Density 3.4 Steam Heat Capacity 3.5 Vapour Pressure

Introduction

As H₂O (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 H₂O 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.

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:

• 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

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:

Where T is in degrees centigrade. The valid temparature 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.

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.

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 H₂O 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.