Chapter 8 Properties of Pure Substance - Thermodynamics, Mechanical Engineering Mechanical Engineering Notes | EduRev

Chapter 8

PROPERTIES OF PURE SUBSTANCE

• A pure susbtance is a substance of homogeneous chemical composition throughout its mass. It is one component system. It may exist in one or more phases.
•  A saturation state is a state from where a change of phase may occur without a changes of pressure or temperature.
•  Critical Point:
  •  Critical Temperature : At critical temperature a liquid completely changes to vapour and viceversa. Also above critical temperature a vapour cannot be liquified by any amount of pressure.
•  Critical Pressure : At critical temperature the minimum pressure required to transform a vapour to liquid is called critical pressure.
•  Transformation of solid to vapour directly is called sublimation.
•  Transformation of vapour to solid directly is called ablimation.
•  Triple point is the fixed point (fixed temperature and pressure) at which solid, liquid and vapour phase co-exist in equilibrium

• Following is the decreasing arrangement of substances triple point

• For water
  •  Critical pressure (p_c) = 221.2 bar
  •  Critical temperature (t_c) = 374.15ºC
  •  Critical volume (v_c) = 0.00317 m3/kg
  •  Triple point (P) = 4.58 mm Hg
  •  Triple point (T) = 273.16 K

• The slope of vaporization and sublimation curve for all substance are positive. The slope of the fusion curve for most substances is positive but for water, it is negative.
•  Water expands upon freezing while volume of other substances decreases upon freezing.
•  Mollier diagram (or h-s diagram)

• The slope of constant pressure lines is given as

• Dryness fraction : It is indicates the mass fraction of vapour in a liquid.

m_v = mass of vapour
m_l = mass of liquid.

•  For saturated liquid, x = 0
•  For saturated vapour, x = 1
•  Dryness quality lines originate from critical line in various diagrams (P–V, h–s diagram etc.).
   
•  Various properties of pure substance based on dryness fraction.

• For given pressure and temperature

v, v_f, v_g = specific volume of moist vapour, liquid, vapour
v_fg = v_g – v_f
h, h_f, h_g = specific enthalpy of moist vapour,liquid, vapour
h_fg = h_g – h_f
s, s_f, s_g = specific entropy of moist vapour, liquid, vapour
s_fg = s_g – s_f

•  At triple point

• A gas or a pure susbtance require two known properties (P, V, T, H etc.) to describe it completely i.e. it have two degrees of freedom.
•  A liquid and vapour in equilibrium state (saturated state) has one degree of freedom.
•  A liquid, vapour and solid in equillibrium (triple point) has zero degree of freedom.
•  The difference between the temperature of the superheated vapour and that of saturated vapour at the same pressure is called degree of superheat.
•  Throtting caloriemter
  •  It is used to measure dryness fraction of pure substance.
  •  Throttling is an irreversible and adiabatic but not isentropic.

• It is very difficult to measure quality of two phase system (liquid+vapour) so it is throttled to bring it to single phase that is vapour phase and then the measurment of pressure, temperature and other properties of two phase system can be done.