Key equations for Process Engineers Thermodynamics

Basic properties of steam

Besides steam pressure and steam temperature, other important steam key properties include as a minimum the following:

Enthalpy

Put is simply, enthalpy is a general measure of the internal stored energy per mass unit of a flowing stream. It is expressed in kJ/Kg (SI units) or in Btu/lb. It is usually represented with letter H.

Entropy

On the other hand, entropy, is a measure of the thermodynamic potential of a flowing stream in the units of energy per mass unit and absolute temperature. It is expressed in kJ/kg×K (SI units) or in Btu/lb×F. It is usually represented with letter S.

Specific Volume

As its name implies, specific volume is a measure of the volume of a flowing stream per mass unit. It is expressed in m3/kg (SI units) or in ft3/lb. It is usually represented with letter v. - steam quality

Steam Thermodynamics

Under superheated or subcooled conditions, properties such as enthalpy, entropy and specific volume are dependent on temperature and pressure. However, at saturated conditions where a mixture of steam and water coexist, an additional parameter, steam quality, needs to be defined. Steam quality, usually represented with letter x, is defined as follows:

x = ms / (ms + mw), where:

ms is the mass of steam and
mw is the mass of water

Steam quality is frequently recorded as a percentage of steam by weight after being multiplied by 100%

The mixture enthalpy, entropy and specific volume of a steam-water mixture can then be easily defined as follows:

H = Hf + x *(Hg-Hf)

s = sf + x*(sg-sf)

v = vf + x*(vg-vf),

where subscripts f (from fluid) and g (from gas) refer to properties at saturated liquid and vapour conditions respectively.

Widely accepted formulas for above mentioned steam properties have been developed by the International Association for the Properties of Water and Steam (IAPWS). These formulas are widely available from various sources over the Internet as well as add-on functions to spreadsheets or relevant software programs.

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