Thermodynamics process sand mollier chart

thermodynamic process to find the enthalpy.

1. BY INDRAKUMAR R PADWANI BE MECHANICAL MBA MARKETING PGDCA LECTURER IN MECHANICAL ENGG DEPT GOVERNMENT POLYTECHNIC GODHRA GUJARAT GUJARAT THERMODYNAMICS PROCESSES AND MOLLIER CHART

2. A thermodynamic process is the transformation of a system from an initial state to final state This transformation is accompanied by changes in the pressure , volume and temperature of the system THERMODYNAMIC PROCESSES ARE REPRESENTED BY PRESSURE- VOLUME LINES THERE ARE FOUR TYPES OF CYCLIC PROCESSES ISOCHORIC PROCESS ISOBARIC PROCESS ISOTHERMAL PROCESS ADIABATIC PROCESS Thermodynamic Process

3. P-V graph showing an different processes 1. ISOTHERMAL 2.ISOCHORIC 3. ISOBARIC 4. ADIABATIC p v 1 4 3 2

4. In an isochoric process the volume is kept constant.(i.e . ∆ V=0) THIS IMPLIES THAT THE WORK DONE BY THE GAS IS ZERO ( i.E , W=0 ) The change in internal energy during an isocharic process is equal to the heat supplied to the system.( I.E.∆U=q) The p-v graph of an isocharic process is a vertical line. P=pressure Q=heat supplied T=temperature ∆U=change in internal energy V=volume W= workdone p v 1 4 3 2

5. ISOBARIC PROCESS In a isobaric process the pressure is kept constant ( i.e ∆P=0).The work done by the gas is given by P∆V. The change in internal energy during an isobaric process is given by ∆U=Q- p∆V . The p-v graph of an isobaric process is a horizontal line p v 1 4 3 2

6. ISOTHERMAL PROCESS In an isothermal process, the temperature is kept constant( i.e. ∆T =0).However there may be heat exchanged between the system and its surrounding . The p-v graph of an isothermal process is a curve p v 1 4 3 2

7. ADIABATIC PROCESS In an adiabatic process there is no transfer of heat between the system and surrounding s( i.e.Q =0)However the temperature may not be constant The change in internal energy during an adiabatic process is equal to the work done on the system….( i.e. ∆U =W) p v 1 4 3 2

8. PROCESS CHARACTERISTICS ISOCHORIC ∆V=0,W=O,∆U=Q ISOBARIC ∆P=0 ; W=P ; ∆ ; ∆U=Q=−P∆V ISOTHERMAL ∆T=0 ADIABATIC Q=0 ; ∆U=W

9. PROPERTY SYMBOL UNITS PRESSURE P Pa TEMPERATURE T K VOLUME V m DENSITY Þ Kg/m GAS CONSTANT R J/K ENTHALPHY H J ENTROPY S J/K INTERNAL ENERGY U J HEAT CAPACITY U J/K THERMAL CONDUCTIVITY k W/( m.k ) Surface Tension N/m v 3

10. Before going to the mollier chart it is important to the study of various properties of steam : SATURATION TEMPERATURE ( T ) At given pressure the temperature at which water boils is known as saturation temperature . SUPERHEATED TEMPERATURE (T ) At a given pressure temperature of steam which is higher than its saturation temperature is known as superheated temperature. DEGREE OF SUPERHEAT (T - T ) For a given pressure the difference between superheated temperature of steam and its saturation temperature is known as the degree of superheat MOLLIER CHART sat sup sup sat

11. Dryness fraction of steam (x ) :- Dryness fraction of wet steam is the ratio of mass of dry steam contained to the total mass of the wet steam . Suppose M kg of wet steam contains Mg kg of actual dry steam and Mf kg of water particles within it so DRYNESS FRACTION x= mass of dry steam total mass of wet steam x = Mg Mg+Mf Latent heat of vaporization (h ): It is the amount of heat absorbed to evaporate 1 kg of water at its boiling point or saturation temperature without change of temperature. It is denoted by h and its value depends upon the pressure. fg f g

12. Enthalpy or total heat of steam(h ):- It is the amount of heat absorbed by water from freezing point to saturation temperature plus heat absorbed during evaporation Enthalpy or total heat of steam=Sensible heat + Latent heat It is denoted by h .The expression for the enthalpy of wet steam , dry Steam and superheated steam is given by 1.Wet steam : The enthalpy of Wet steam is given by H = h + x.h 2.Dry steam : The enthalpy of dry steam H= h + xh For dry steam x=1 H=h + h g g fg f f fg fg f

13. Superheated steam : The enthalpy of superheated steam is given by H = h +h +Cp(t -t ) = h + Cp (t - t ) sup f fg sup sat g sup sat Where Cp= specific heat of water at constant pressure is usually taken as 4.2 KJ/kg =K

14. IT IS GRAPHICAL REPRESENTATION OF STEAM TABLES IN WHICH THE ENTHALPHY (h) IS PLOTTED ALONG THE ORDINATE AND THE ENTROPY(S) ALONG ABSCISSIA. THE MOLLIER DIAGRAM HAS FOLLOWING LINES 1. DRYNESS FRACTION LINE 2.CONSTANT VOLUME LINE. 3.CONSTANT PRESSURE LINE 4.ISOTHERMAL LINES . The Mollier diagram is useful when analyzing the performance of adiabatic steady flow processes, such as flow in nozzles , diffusers , turbines and compressors. MOLLIER DIAGRAM

15. Calculate the enthalpy of 1 kg of steam at a pressure of 10 bar A . DRYNESS FRACTION = 0.75 B . STEAM DRY AND SATURATED C . STEAM IS SUPERHEATED TO 230 USING 1. MOLLIER CHART 2.STEAM TABLE NOW FROM STEAM TABLE P=10 bar At pressure 10 bar tsat =179.9 M = 1kg hf = 762.6 X=0.75 hfg =2013.6 tsup =230

16. H= M( hf +x.hfg) =1(762.6+0.75(2013.6) =1(762.6+ 1510.2) =2272.8 KJ/kg….. B. H = M( hf+hfg ) =2776.2 KJ/kg C. superheated steam H = M(Hg ) + CP( tsup – tsat ) =2776.2+2.0 (230-179.9) =2776.2+2-0(50.1) = 2776.2+100.2 =2876.4 KJ/kg .

17. a b c MOLLIER CHART

18. From the mollier diagram chart We have to calculate enthalpy See the mollier diagram Mollier diagram contains pressure line , dryness fraction line and superheated temperature. A. In this example pressure is 10 bar and dryness fraction 0.75 See the pressure line 10 bar line and dryness fraction line 0.75 in mollier diagram at which they intersects that gives us ENTHALPY A. BY MOLLIER DIAGRAM ENTHALPY H IS 2275 KJ/kg

19. B. When x = 1 and Pressure 10 bar line intersect in the Mollier diagram that gives enthalpy. H = 2776.2 KJ/kg C. Now, when pressure is 10 bar and T = 230 C in Mollier diagram that intersect that give enthalpy 2881.41 KJ/kg sup

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