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

Mechanical Engineering SSC JE (Technical)

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

The document Chapter 8 Properties of Pure Substance - Thermodynamics, Mechanical Engineering Mechanical Engineering Notes | EduRev is a part of the Mechanical Engineering Course Mechanical Engineering SSC JE (Technical).
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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

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

  • Following is the decreasing arrangement of substances triple point

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

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

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

  • 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)

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

  • The slope of constant pressure lines is given as

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

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

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

mv = mass of vapour
ml = 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.

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

  • For given pressure and temperature

v, vf, vg = specific volume of moist vapour, liquid, vapour
vfg = vg – vf
h, hf, hg = specific enthalpy of moist vapour,liquid, vapour
hfg = hg – hf
s, sf, s= specific entropy of moist vapour, liquid, vapour
sfg = sg – sf

  •  At triple point
internal energy = 0
entropy = 0
enthalpy > 0
  • 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.

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

  • 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.
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