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 (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
- 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.
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.
- 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, sg = specific entropy of moist vapour, liquid, vapour
sfg = sg – sf
|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.
- 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.