Engineering Thermodynamics one

1. ENGINEERING
THERMODYNAMICS
For Second Year Mechanical and
Electrical Engineering Students
By : Mihretu W/M (MSc)

2. Chapter Two
Pure substances
• Pure substances- A substance that has a fixed chemical composition
throughout. Such as water, air, and nitrogen.
• A pure substance does not have to be of a single element or
compound. A mixture of two or more phases of a pure substance is
still a pure substance as long as the chemical composition of all
phases is the same.
• Air is a mixture of several gases, but it is considered to be a pure
substance.
Figure 2.1: Nitrogen and Gaseous air are pure substances 2

3. Cont…
Figure 2.2: A mixture of liquid and gaseous water are pure
substances, but a mixture of liquid and gaseous air is not.
3

4. Phase of pure substances
 A pure substance may exist in different phases. There are three principal phases
solid, liquid, and gas.
 A phase: is defined as having a distinct molecular arrangement that is homogenous
throughout and separated from others (if any) by easily identifiable boundary
surfaces.
 A substance may have several phases within a principal phase, each with a different
molecular structure.
 Intermolecular bonds are the strongest in solids and the weakest in gases, b/c the
molecules in solids are closely packed together where as in gases they are separated
by relatively large distances.
 Solid phase- the molecules are arranged in a three‐dimensional pattern (lattice)
throughout the solid. The molecules cannot move relative to each other; however,
they continually oscillate about their equilibrium position.
 Liquid phase- the molecular spacing in liquid phase is not much different from
that of the solid phase (generally slightly higher), except the molecules are no longer
at fixed positions relative to each other.
 Gaseous phase- the molecules are far apart from each other, and a molecular order does not
exist. Gas molecules move randomly, and continually collide with each other and the walls of
the container they
 Molecules in the gas phase are at a considerably higher energy level than they are in
liquids or solid phases.
4

5. Phase change processes of pure substances
 Consider a process where a pure substance starts as a solid
and is heated up at constant pressure until it all becomes
gas. Depending on the prevailing pressure, the matter will
pass through various phase transformations. At P=1atm:
Compressed liquid (sub cooled liquid): A substance that it
is not about to vaporize.
Saturated liquid: A liquid that is about to vaporize.
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6. Phase change processes of PS Cont…
Saturated vapor: A vapor that is about to condense.
Wet vapor or Saturated liquid–vapor mixture: The state
at which the liquid and vapor phases coexist in equilibrium.
Superheated vapor: A vapor that is not about to condense
(i.e., not a saturated vapor).
6

7. Cont…
• If the entire process between state 1 and 5 described in the figure is
reversed by cooling the water while maintaining the pressure at the
same value, the water will go back to state 1, retracing the same path,
and in so doing, the amount of heat released will exactly match the
amount of heat added during the heating process.
Fig X. T-V diagram for the heating process of water at constant pressure
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8. Saturation temperature and Saturation pressure
 Water boils at 100℃ at pressure (101.325kpa or 1atm)
 If the pressure is changed by adding weight on the top of the
piston, the temp at w/c water boils will change.
 That is the temp at w/c water start boiling depends on
pressure. If the pressure is fixed, so is the boiling temp.
 Saturation temperature Tsat: The temperature at which a
pure substance changes phase at a given pressure.
 Saturation pressure Psat: The pressure at which a pure
substance changes phase at a given temperature.
 During a phase‐change process, pressure and temperature
are dependent properties, Tsat = f (Psat).
8

9. Cont…
Fig. The liquid–vapor saturation curve
of a pure substance(numerical
values are for water).
9

10. Cont…
• Latent heat: The amount of energy absorbed or released during a phase-
change process.
• Latent heat of fusion: The amount of energy absorbed during melting. It is
equivalent to the amount of energy released during freezing.
• Latent heat of vaporization: The amount of energy absorbed during
vaporization and it is equivalent to the energy released during
condensation.
• The magnitudes of the latent heats depend on the temperature or pressure
at which the phase change occurs.
• At 1 atm pressure, the latent heat of fusion of water is 333.7 kJ/kg and the
latent heat of vaporization is 2256.5 kJ/kg.
• The atmospheric pressure, and thus the boiling temperature of water,
decreases with elevation.
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11. PROPERTY DIAGRAMS FOR PHASE CHANGE
PROCESSES
 The variations of properties during phase-change processes are best studied and
understood with the help of property diagrams such as the T-v, P-v, and P-T
diagrams for pure substances.
Fig. T-v diagram of constant-pressure phase-change processes of a pure substance at
various pressures (numerical values are for water)
11

12. Cont…
• saturated liquid line
• saturated vapor line
• compressed liquid region
• superheated vapor region
• saturated liquid–vapor mixture region (wet region)
 At supercritical pressures (P > Pcr), there is no distinct phase-change (boiling)
process.
 Critical point- The point at which the saturated liquid and saturated vapor states are
identical.
12

13. Cont…
Fig. The pressure in piston-cylinder device can be reduced by
reducing the weight of the piston.
13

14. P-T or Phase Change Diagrams
 This is called phase diagram since all three phases are separated from
each other by three lines. Most pure substances exhibit the same
behavior.
 One exception is water. Water expands upon freezing.
Fig. P-T Diagram of pure substances 14

15. Cont…
 There are two ways that a substance can pass from solid phase to
vapor phase
i) it melts first into a liquid and subsequently evaporates,
ii) it evaporates directly without melting (sublimation).
 The sublimation line separates the solid and the vapor.
 The vaporization line separates the liquid and vapor regions
 The melting or fusion line separates the solid and liquid.
 N.B. These three lines meet at the triple point.
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16. Property Tables
 For most substances, the relationships among thermodynamic
properties are too complex to be expressed by simple equations.
 Therefore, properties are frequently presented in the form of tables.
 Some thermodynamic properties can be measured easily, but others
cannot and are calculated by using the relations between them and
measurable properties.
 The results of these measurements and calculations are presented in
tables in a convenient format.
 The subscript “f” is used to denote properties of a saturated liquid and
“g” for saturated vapor. Another subscript, “fg”, denotes the difference
between the saturated vapor and saturated liquid values of the same
property.
16

17. Cont…
17

18. Saturated liquid and Saturated vapour states
 Table A–4: Saturation properties of water under temperature.
 Table A–5: Saturation properties of water under pressure.
A partial list of Table A–4.
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19. Cont…
19

20. Quality and Saturated liquid-vapour mixture
Quality, x : The ratio of the mass of vapor to the total mass
of the mixture.
Quality is between 0 and 1
0- for sat. liquid, 1-for sat. vapor.
Consider a mixture of saturated liquid and saturated vapor.
The liquid has a mass mf and occupies a volume Vf. The
vapor has a mass mg and occupies a volume Vg.
20

21. Cont…
21

22. Cont…
Note, quantity 1- x is often given the name moisture.
The specific volume of the saturated mixture becomes
22

23. Cont…
 It is noted that the value of any extensive property per unit mass in the
saturation region is calculated from an equation having a form similar
to that of the above equation. Let Y be any extensive property and let y
be the corresponding intensive property, Y/m, then
The term yfg is the difference between the saturated vapor
and the saturated liquid values of the property y; y may be
replaced by any of the variables v, u, h, or s.
23

24. Cont…
 The Lever Rule is illustrated in the following figures.
 Quality is related to the horizontal distances on P-v and T-v diagrams.
 The v value of saturated liquid-vapour mix lies b/n vf and vg values at
the specified T or P.
24

25. Cont…
Examples of Saturated liquid-vapor mixture states on T-v
and P-v diagrams.
25

26. Super heated vapour table
A substance is said to be superheated if the given temp is
greater than the saturation temp for the given pressure.
State 5 in Figure X (page 7) is a superheated state.
Superheated vapor is characterized by:
26

27. Cont…
• NB: In this region, temperature and pressure are
independent properties.
• Partial listing of Table A–6.
27
At a specified P, superheated
vapor exists at a higher h than
the saturated vapor.