This document discusses properties of pure substances, specifically steam. It defines steam as water in its gaseous state or as a liquid-gas mixture. Steam is commonly used in steam engines and turbines for power generation. The document outlines key steam properties including specific sensible heat, latent heat of vaporization, enthalpy of dry-saturated steam, enthalpy of superheated steam, wet steam properties, dryness fraction, and enthalpy of wet steam as a function of dryness fraction. These properties are important thermodynamic concepts used to characterize the state and behavior of steam.

1. Unit No:01
Thermodynamics
Prof. Yash B. Parikh
M.Tech (Computer Integrated Manufacturing)
B.E.(Mechanical Engineering)
Assistant Professor
Department of Mechanical Engineering

2. Properties of Pure Substance (Steam)
2
 A pure substance is one whose chemical composition
is uniform and it remains invariant during heat &
work transfer with the surroundings.
e.g. water, steam, nitrogen, etc.
 Water in gaseous state or a two phase mixture (liquid
+ gas) is called steam.
 Steam is mostly used in steam engines, steam
turbines for power generation.

3. Difference between Gas & Vapour
3
 Gas :
it is the state of substance in which the
evaporation from the liquid state is complete.
 Vapour :
it is the homogeneous mixture of minute
liquid particles in suspension with true gas of the
same substance.
 With changes in t & p, gas remains in its gaseous
state (except under extreme conditions)
 With changes in t & p, a vapour can undergo
condensation or evaporation.
 Gas obey the gas laws whereas vapour doesn’t.

4. Properties of Steam
4
Specific Sensible Heat or Specific Enthalpy of Water :
 It is assumed that enthalpy of water at 00C is zero.
 It is the amount of heat energy to be supplied to
convert 1 kg of water at 00C to its saturation
temperature at given steam pressure.
 It is denoted by hf and unit is kJ/kg.
 The value of enthalpy of water at different pressures
can be read from steam tables.

5. Properties of Steam
5
Latent Heat of Vaporization:
 It represents the amount of heat energy needed for
complete evaporation of saturated liquid into dry-
saturated steam at a given pressure.
 It is denoted by hfg and unit is kJ/kg.
 It’s value can be read from steam tables.

6. Properties of Steam
6
Enthalpy of Dry-saturated Steam:
 It represents the amount of heat energy needed to
convert 1 kg of water at 0oC into dry-saturated steam
at a given pressure.
 It is denoted by hg and unit is kJ/kg.
hg = hf + hfg

7. Properties of Steam
7
Enthalpy of Superheated Steam:
 Any further heating of dry-saturated steam will
increase the temperature of steam at a given
pressure.
 This steam is called superheated steam and its
temperature is denoted by tsup in 0C.

8. Wet Steam
8
 Steam inside a boiler invariably contains minute
water particles held in suspension with dry-saturated
steam called wet steam or vapour.
Mass of wet steam = Mass of dry-saturated steam +
Mass of saturated liquid (moisture)
Mass of wet steam = ms + m

9. Dryness Fraction
9
 The ratio of mass of dry-saturated steam in a given
mass of wet steam is defined as dryness fraction and
its denoted by x.
Dryness fraction, x = ms / ms+ m
 If mass of wet steam is 1 kg.
i.e. ms+ m = 1
Then, x = ms
 Dryness fraction represents the mass of dry-
saturated steam in 1 kg of wet steam.
 Value of x lies between 0 to 1.

10. Enthalpy of Wet Steam of Dryness Fraction
10
Consider 1 kg of wet steam of dryness fraction x.
Mass of dry-saturated steam in wet steam = x kg
Mass of moisture in wet steam = (1 - x) kg.
Specific Enthalpy of wet steam,
h = hf + x hfg