Phase transformations

PHASE TRANSFORMATION

Phase transformation means change of ice to water and from water to steam. It takes place at constant temperature and at constant pressure respectively.

SUB-COOLED ICE

Ice at a temperature lower than its freezing temperature 00C is called sub-cooled ice. Hence ice at -60C, -130C, -290C will be sub-cooled ice. Consequently ice at -60C will have 6 degree of sub-cooling.

SUPER HEATED STEAM

Steam at a temperature higher than the boiling point at a certain pressure is called a super heated steam. Steam at 1120C, 1280C, 1430C at 1 atm pressure will be super heated steam.

Fig. below represents the various phase transformations from sub-cooled ice to super heated steam.

 CALCULATIONS ARE BASED ON SPECIFIC VALUES (PER KG)

Specific heat of ice = cp ice = 2.1 kJ/kg0C, Specific heat of water = cp water = 4.18 kJ/kg0C and Specific heat of steam = cp steam = 2.3 kJ/kg0C

Latent heat of ice at 0C = 335 kJ/kg and Latent heat of steam at 100 0C = 2257 kJ/kg

(a)Process AB

Sensible heating of sub-cooled ice from -200 C to 00C. At point A, ice is at -200C and at point B ice is at 0C but there is no phase change.

SUB-COOLED SOLID

Any substance at a temperature lower than its freezing temperature is called a sub-cooled solid.Sensible heat gained during process AB

QS = 1x cp ice(0-(-12)= cice x 20

= 2.1 x 20= 42 kJ

(b) Process BC

Melting of ice at 0C. It involves only latent heat.

DRY SOLID

A solid at its freezing temperature at a certain pressure is called a saturated solid. At point B, ice at 0C is a dry solid

WET SOLID

Any solid at its freezing temperature along with its liquid is called a Wet Solid. Hence from point B to point C is a phase change process. Remember a phase change process always takes place at constant temperature as well as at constant pressure. Thus it is an iso-thermal and an isobaric process. Thus heat gain during BC will be = latent heat of ice = 335 kJ

(c) Process CD

Sensible heating of water from 0C to 1000C

SUB COOLED LIQUID

Any liquid at a temperature lower than its boiling temperature at a certain pressure is called a sub-cooled liquid. For example, water at 130C or below 1000C at 1atm pressure will be sub-cooled water. Water at 1120C at 2atm will also be sub-cooled water because boiling point at 2 atm is

SATURATED LIQUID

Any liquid at its boiling temperature corresponding to certain pressure is called saturated liquid. For example, the boiling point of water at 2 atm pressure is 1200C. Therefore, water at 1200C at 2 atm pressure will also be saturated water.

Heat gained during CD =m x cp water dt = 1 x 4.18 x 100 =418 kJ/kg

(d) Process DE

 On further heating at point D, water starts changing into steam vapor at 1000C and vapor formation is complete at point E at 1000C. During phase change, temperature and pressure remains constant. Thus phase change is isothermal as well as isobaric. Thus heat gained during process DE is latent heat of water = 2257 kJ/kg

DRY SATURATED VAPOR

Any vapor at its boiling point at a certain pressure without any liquid content is called dry saturated vapor. Therefore water vapor at point E at 1000C will be a dry saturated vapor. No liquid should be there along with the vapor.

(e) Process EF

 On further heating at constant pressure, temperature of steam will rise and say becomes 1300C. Steam at 1300C at 1 atm will be called a super heated steam or super heated steam vapor.

SUPER HEATED VAPOR

Any vapor at a temperature higher than its boiling temperature at a CERTAIN PRESSURE is called a super heated vapor.Thus heat gained in super heating to 1300C will be = m (cp vapor)dt =1 x 2.3 x 30 = 69.0 kJ/kg

DEGREE OF SUPER HEAT

Temperature difference between the actual temperature of vapor and its boiling temperature at a certain pressure is called DEGREE OF SUPER HEAT. Thus at 1 atm pressure, when vapor is at 1300C, the degree of super heat will be 30 and at 2 atm pressure, when vapor is at 1390C, the degree of super heat will be 19 (139-120=19).