Thermodynamic Properties of Steam

THERMODYNAMIC PROPERTIES OF STEAM

Various properties of water at various conditions of steam (i.e. wet, dry and superheated) are discussed in this topic.

(i) Enthalpy of steam (h):

It is the amount of heat added to the water from freezing point to till the water becomes wet or dry or superheated steam.

For wet steam, hwet = hf + x hfg

For dry steam, hdry = hg = hf + hfg

For superheated steam, hsup = hg + Cp(Tsup - Tsat)

where

h = Enthalpy of steam in kJ/kg

(Tsup - Tsat) is called degree of superheat.

(ii) Specific volume of steam (v):

It is defined as the volume occupied by the unit mass of steam at the given pressure and temperature.

For wet steam, Vwet = Vf + x Vfg

For dry steam, Vdry = vg= vf + vg

For superheated steam, vsup= vg Tsup / Tsat

where

v = Specific volume in m3/kg

(iii) Density of steam (ρ):

It is defined as the ratio of mass to the unit volume of steam at given pressure and temperature. Its value for wet, dry and superheated steam is the reciprocal of specific volume of the steam.

(iv) Work done during expansion (W):

During the evaporation process, there is a considerable increase in its volume when the pressure remains constant. Thus, some amount of external work is done during evaporation. In other words, it is the energy required for absorption of latent heat for increasing the volume of steam.

For wet steam, Wwet = pv = p (vf + xvg)

For dry steam, Wdry = pvg

For superheated steam, Wsup = pvsup

where

W = Work done in kJ

p = Pressure at which evaporation takes place in bar.

(v) Internal energy of steam (U):

Internal energy of steam is defined as the actual heat energy stored in steam above the freezing point of water at the given conditions. It is the difference between enthalpy of steam and external work done.

h = W + Δu

Δu = h - W

For wet steam, Uwet = (hf + xhfg) – pv = (hf + x hfg) - p(vf + xvfg)

For dry steam, udry = (hf + hfg) – p vg

For superheated steam, usup = hsup - pvsup

where

u = Enthalpy of steam in kJ/kg

(vi) Entropy of steam (s):

It is the property of the steam which increases with increase in temperature and decreases with decrease in temperature.

where

s = Entropy of steam in kJ/kg K