Phase transformations

Phase transformations

Keywords: Sub-cooled solid- – phase change- sub-cooled liquid- saturated liquid, phase change, super heating

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.

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. at -60C, degree of sub-cooling will be 60C.

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.

 Note: no gas laws are applicable to vapors. thus resulting equations for analysis of vapors is quite complex and laborious, and hence tables and charts have been prepared for vapors and the same are used in the analysis of vapors. steam and refrigerants are used in the vapor states.

 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

Specific heat of steam = cp steam = 2.3 kJ/kg0C

Latent heat of ice at 0C = 335 kJ/kg

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. 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

  • Process BC: Melting of ice at 0C

SATURATED SOLID

A solid at its freezing temperature at a certain pressure is called a saturated solid.

At point B, ice at 0C and at point C, water is at 0C. Between point B and point C ity is partially solid and partially liquid. Such a solid is called a wet 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.

Heat gain during BC will be = latent heat of ice = 335 kJ

  • 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 1200C.

At point C it is water at 0C and at point D, it is water at 1000C.  Water at 1000C is at its boiling temperature at 1 atm pressure. It is called saturated water.

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

  • 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 remains constant. Thus phase change is an isothermal as well as isobaric process.

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.

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.

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

At 1 atm pressure, when vapor is at 1300C, the degree of super heat will be 30.

At 2 atm pressure, when vapor is at 1390C, the degree of super heat will be 19 (139-120=19).