Liquids and Vapors

If a pure liquid is heated at a constant pressure there is a fixed temperature at which bubbles of vapor form in the liquid; this phenomenon is known as boiling. The states of substance at this condition represents a point on the properties diagram, known as boiling point; e.g. point 1. A slight addition of heat to the liquid at this state changes some of it into vapor.

Saturated liquid line, SLL, is formed by connecting a series of boiling points. Boiling temperature known also as saturation temperature, T, for a pure liquid is only a function of pressure, P, i.e.

	T2 = T1 = f(P)

Vaporization continues by further heat supply to the system until no liquid is left. This state is known as dry saturated vapor, e.g. point 2. If the system is slightly cooled at this state, then droplets of liquid will begin to form. Connecting a series of points at dry saturated vapor builds a line, known as saturated vapor line, SVL. The state of substance between saturated liquid and dry vapor is called wet vapor. Further heating of a dry saturated vapor at constant pressure causes a rise of vapor temperature and it becomes superheated. The state of substance is completely defined by its pressure and temperature if it is in liquid or superheated vapor phase i.e.

	h = f1(P,T) = Specific enthalpy  
	v = f2(P,T) = Specific volume 
	s = f3(p,T) = Specific entropy

These properties for different substances are either tabulated or can be calculated by certain equations, e.g. IFC formulation for water and steam properties. The state of wet vapor can not be defined by just pressure and temperature until one other property is given. The condition or quality of wet vapor is often defined by its dryness or wetness fraction.

dryness fraction, x =  the mass of dry vapor in 1 kg of the mixture,  
and, 
wetness fraction, 1 - x =  the mass of liquid in 1 kg of the mixture.

For wet vapor with the dryness fraction, x,

	h = (1-x) hf + x hg  
	v = (1-x) vf + x vg  
	s = (1-x) sf + x sg

where f and g indicate the property of the substance at saturated liquid and dry saturated vapor states respectively. The heat supplied to the liquid for the complete phase change is called the specific enthalpy of vaporization.

	hg - hf = hfg