8/11/2019 WK2a

1/48

CHAPTER 2:

PROPERTIES OF PURESUBSTANCES

CHE 433 THERMODYNAMICS

8/11/2019 WK2a

2/48

PURE SUBSTANCE2

Pure substance: Substance with fixed chemical composition.

Single element or compound: e.g. N2, H2, O2, H2O

Mixture of single element or compound : e.g. Air

Mixture of 2-phase system: e.g. ice and liquid water

Nitrogen and gaseous air are pure

substances.

8/11/2019 WK2a

3/48

PHASES OF A PURE SUBSTANCE

3

The molecules in a

solid are kept at their

positions by the

large springlike inter-

molecular forces.

In a solid, the attractive and

repulsive forces between the

molecules tend to maintain

them at relatively constant

distances from each other.

The arrangement of atoms in different phases: (a) molecules are at relatively fixed positions in a solid, (b)

groups of molecules move about each other in the liquid phase, and (c) molecules move about at randomin the gas phase.

8/11/2019 WK2a

4/48

8/11/2019 WK2a

5/48

Phase-change

processes

Saturated vapour Superheated vapour

8/11/2019 WK2a

6/48

8/11/2019 WK2a

7/48

Phase change processes of water7

At 1 atm and 20C,

water exists in the

liquid phase

(compressed

l iquid).

At 1 atm pressure and

100C, water exists as a

liquid that is ready to

vaporize (saturated

l iquid).

As more heat is

transferred, part of the

saturated liquid vaporizes

(saturated liquidvapor

mixture).

At 1 atm pressure, the

temperature remains constant at100C until the last drop of liquid

is vaporized (saturated vapor).

As more heat is transferred,

the temperature of the vaporstarts to rise (superheated

vapor).

T-v diagram for the heating process of

water at constant pressure.

Compressed liquid liquid that is not about to

vaporize

Saturated liquid liquid that is about to vaporize

Saturated liquid-vapor mixture liquid and vapor

phases coexist in equilibrium

Saturated vapor vapor that is about to condenseSuperheated vapor vapor that is not about to

condense

8/11/2019 WK2a

8/48

Saturation Temperature and

Saturation Pressure8

The temperature at which water starts

boiling depends on the pressure;

therefore, if the pressure is fixed, so is the

boiling temperature.

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 changesphase at a given temperature.

8/11/2019 WK2a

9/48

Phase-change

processes

Latent heat

Latent heat of vaporizationLatent heat of fusion

Amount of heat absorbed or

released during a phase-

change process

The amount of energy

absorbed during melting. It is

equivalent to the amount of

energy released during

freezing.

The amount of energy

absorbed during vaporization

and it is equivalent to the

energy released during

condensation.

8/11/2019 WK2a

10/48

10

A plot of Tsatvs Psatis called a liquid-vapor saturation curve.

Tsatincreases with Psat. A substance at higher pressures boils at

higher temperatures.

The atmospheric pressure, and thus the boiling temperature of

water, decreases with elevation.

The liquidvapor

saturation curve of a

pure substance

(numerical values are

for water).

8/11/2019 WK2a

11/48

PROPERTY DIAGRAMS FOR

PHASE-CHANGE PROCESSES11

T-v diagram

99.6

45.8

179.

9

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.

8/11/2019 WK2a

12/48

12

The saturated liquid states can beconnected by saturated liquidline.

The saturated vapor states can

be connected by saturated vaporline.

Compressed liquid region: leftregion of saturated liquid line.

Superheated vapor region: rightregion of saturated vapor line.

Saturated liquidvapor mixtureregion (wet region): regionunder the dome - involve bothphases in equilibrium. T-v diagram of a pure substance.

8/11/2019 WK2a

13/48

Property diagram T-vdiagram

8/11/2019 WK2a

14/48

P-v Diagram14

The general shape of the P-

v diagram of a pure

substance is very much like

the T-v diagram, but the T =

constant lines have adownward trend.

P-v diagram of a pure substance.

8/11/2019 WK2a

15/48

Property diagram T-vdiagram

8/11/2019 WK2a

16/48

Extending the Diagrams to Include

the Solid Phase16

P-v diagram of a substance that

contracts on freezing.

P-v diagram of a substance that

expands on freezing (such as water).

8/11/2019 WK2a

17/48

17

Sublimation: Passing

from the solid phase

directly into the vapor

phase.

At low pressures (below the

triple-point value), solids

evaporate without melting first

(sublimation).

At triple-point pressure and

temperature, a substance

exists in three phases in

equilibrium.

For water,

Ttp= 0.01C

Ptp= 0.6117 kPa

8/11/2019 WK2a

18/48

Property diagram Triple point

8/11/2019 WK2a

19/48

P-T Diagram19

P-T diagram of pure substances.

8/11/2019 WK2a

20/48

Property diagram P-T diagram

8/11/2019 WK2a

21/48

Property diagram P-v-T diagram (P-T)

8/11/2019 WK2a

22/48

Property diagram P-v-T diagram

8/11/2019 WK2a

23/48

Property diagram P-v-T diagram

8/11/2019 WK2a

24/48

PROPERTY TABLES24

EnthalpyA Combination Property

The combination u +Pv is

frequently encountered in

the analysis of control

volumes.

The productpressure volume

has energy units.

8/11/2019 WK2a

25/48

8/11/2019 WK2a

26/48

Property Table Saturated Liquid and Saturated vapour

vg= specific volume of saturated vapour

vf= specific volume of saturated liquid

vfg= difference between vg and vf

(vfg= vg-vf)

8/11/2019 WK2a

27/48

8/11/2019 WK2a

28/48

8/11/2019 WK2a

29/48

Property Table Saturated Mixture of Liquid and vapour

Quality, x

x = mg/mT

8/11/2019 WK2a

30/48

Property Table Saturated Liquid and Saturated vapour

8/11/2019 WK2a

31/48

Property Table Quality,x

x= (vavvf) / vfg

8/11/2019 WK2a

32/48

8/11/2019 WK2a

33/48

Example 3-133

A rigid tank contains 50 kg of saturated liquid water at 90C.

Determine the pressure in the tank and the volumeof the tank.

Pressure

P = Psat@90oC

= 70.183 kPa

Specific volume

v = vf@90oC= 0.001036 m3/kg

Total volume

V = mv = (50 kg)(0.001036 m3/kg)

= 0.0518 m3

Refer to Table A-4

8/11/2019 WK2a

34/48

Example 3-234

A piston cylinder device contains 0.06 m3of saturated water vapor

at 350 kPa pressure. Determine the temperatureand the mass of

the vapor inside the cylinder.

P =350 kPa

V = 0.06 m3

Temperature

T = Tsat@350 kPa

= 138.86oC

Specific volume

v = vg@350 kPa= 0.52422 m3/kg

Mass

m = V/v

Refer to Table A-5

8/11/2019 WK2a

35/48

Example 3-335

A mass of 200 g of saturated liquid water is completely vaporized

at a constant pressure of 100 kPa. Determine the volume change

and the amount of energy transferred to the water.

8/11/2019 WK2a

36/48

36

The v value of a saturated

liquidvapor mixture lies

between the vfand vg

values at the specified T orP.

y v, u, or h

For internal energy and enthalpy,

If x is known or has been

determined, use above relations to

find other properties. If either , u, h

are known, use it to find quality, x.

8/11/2019 WK2a

37/48

Example 3-437

A rigid tank contains 10 kg of waterat 90oC. If 8 kg of the water is in

the liquidform and the rest is in the vapor form, determine the

pressurein the tank and the volume of the tank.

8/11/2019 WK2a

38/48

Example 3-538

An 80-Lvessel contains 4 kg ofrefrigerant-134aat pressure of160 kPa. Determine:

(a) The temperatureof the

refrigerant(b) The quality

(c) The enthalpy of the refrigerant

(d) The volumeoccupied by the

vapor phase.

Answer:

T = -15.6oC, x = 0.157, h = 64.2 kJ/kg,

Vg= 0.0775 m3

8/11/2019 WK2a

39/48

Superheated Vapor39

In the region to the right of the

saturated vapor line and at

temperatures above the critical

point temperature, a substance

exists as superheated vapor.

A partial

listing of

Table A6.

At a specified P,

superheated

vapor exists at a

higher h than the

saturated vapor.

8/11/2019 WK2a

40/48

Example 3-6 & 3-740

Determine the internal energy of water at 200 kPa and

300oC.

Answer: 2808.8 kJ/kg

Determine the temperature of water at a state of P = 0.5

MPa and h = 2890 kJ/kg.

Answer: 216.3oC

8/11/2019 WK2a

41/48

Linear Interpolation41

T, C

h, kJ/kg

T2

h1

T1

h2

T = ??

21mslopemslope

1 2 1

1 2 1

T T T T

h h h h

2 1 1 1

2 1

T TT h h T

h h

h

8/11/2019 WK2a

42/48

Compressed Liquid42

A compressed liquid may

be approximated as a

saturated liquid at the

given temperature.

At a given P and T, a

pure substance will

exist as a compressed

liquid if

8/11/2019 WK2a

43/48

Example 3-843

Determine the internal energy of compressed

liquid water at 80oC and 5MPa using

(a) Compressed liquid table

(b) Saturated liquid tableAnswer: 333.82 kJ/kg, 334.97 kJ/kg

8/11/2019 WK2a

44/48

Property Table Superheated vapor

Lower Pressures (P < Psatat given T)

Higher temperatures (T > Tsatat given P)Higher specific volumes (v > vgat a given P or T)

Higher internal energies (u > ugat a given P or T)

Higher enthalphies (h > hgat given P or T)

8/11/2019 WK2a

45/48

Property Table Subcooled/compressed Liquid

Higher Pressures (P > Psatat given T)

Lower temperatures (T < Tsatat given P)

Lower specific volumes (v < vfat a given P or T)

Lower internal energies (u < ufat a given P or T)

Lower enthalphies (h < hfat given P or T)

8/11/2019 WK2a

46/48

Property Table Subcooled/compressed Liquid

Given: P and T

v vf @ Tu uf @ T

h hf @ T

8/11/2019 WK2a

47/48

8/11/2019 WK2a

48/48

Problem 3-2548

Complete this table for H2O:

T, oC P, kPa h, kJ/kg x Phase

description

200 0.7

140 1800

950 0.0

80 500

800 3162.2