UNIT 1.1: STEAM AND ITS PROPERTIESIntroductionSteam formation at a constant pressureTypes or conditions of steamDryness fractionSimple numerical to understand use of steam tablesINTRODUCTION A perfect gas does not change its phase during a thermodynamic process Pure substance is homogeneous, retains its chemical composition even in phase change during a thermodynamic process Water is one of the pure substance existing in three phases ICE in solid phase WATER in liquid phase STEAM in gaseous phase Ice transforms to form water Water heated beyond boiling point forms steam (vapor of water) During vaporization two phase mixture (water + steam) exists After vaporization pure gaseous phase (steam) exists Different states/types/conditions of existence and associated properties of steam viz., pressure, temperature, specific volume, enthalpy, internal energy and entropy are to be discussedSTEAM GENERATION EQUIPMENT In steam generation, water at atmospheric pressure and temperature is converted into steam by the application of heat Water is heated from 00C at a given constant pressure Hence amount of heat energy supplied to convert water into steam will be equal to its enthalpy Consider 1kg of water at 00C in a cylinder with freely moving frictionless piston

Total weight (piston & weight) exert required pressure P on water (point A) As heating at constant pressure rises the temperature of water to its boiling point, a slight increase in water volume results Temperature at which water boils is saturation temperature Ts (point B) Line AB indicates heating of water from 00C to Ts0C at constant pressure Amount of heat required to raise temperature of 1kg of water from 00C to Ts0C is sensible heat/heat of liquid/enthalpy of liquid hf Further heat initiates water evaporation at Ts as water will be saturated with heat Phase change from liquid to gaseous (point C) Line BC indicates constant pressure & constant temperature addition Amount of heat to evaporate 1kg of water at Ts to 1kg of dry steam at Ts and constant pressure is called latent heat of evaporation/enthalpy of evaporation hfg Further addition of heat at constant pressure increases Ts to Tsup This process is called superheating giving out superheated steam Inclined line CD indicates this Amount of superheat/enthalpy of superheat is the heat required to increase dry steam temperature from Ts to any desired higher temperature at constant pressure Tsup - Ts is degree of superheatADVANTAGES OF SUPERHEATED STEAM Possess more heat energy hence capacity to do work will be higher Superheating by exhaust gases saves energy and improves thermal efficiency of boiler Reduces and prevents condensation thus giving better economyDISADVANTAGES OF SUPERHEATED STEAM Lubrication problems due to high superheated temperature Higher depreciation and initial costTYPES OR CONDITIONS OF STEAM Steam can exist in wet, dry saturated and superheated states Water hated beyond saturation state at constant pressure gives a two phase mixture (water + steam) called wet steam in thermal equilibrium Wet steam has different proportions of water molecules and dry steam To state quality of wet steam dryness fraction x is specified x = mg/(mf + mg) = mass of dry steam in wet steam/total mass of wet steam