Steam and its properties

steam and its properties

1. Properties of Pure
substances

Prepared by:

Pradeep Kumar Gupta

Assistant Professor

Department of Mechanical Engineering

2. Pure substance and phase

All substance is composed of numerous numbers of particles known
as molecules. Pure substance is defined as substance that is made of
only one type of atom or only one type of molecule. Its chemical
composition is uniform and remains invariant during heating or
work transfer with the surroundings. Such as water, diamond, ice,
nitrogen, oxygen, gold etc.

Phases

1. Solid,

2. Liquid and

3. Gas

3. Properties and important definitions

Boiling point: It is defined as that point (temperature) in the system at which
vapour pressure equals to the atmospheric pressure and at point
(temperature) phase transfer from liquid to gas (vapours) takes place.

Melting point: It is defined as that point (temperature) in the system at which
phase transfer from solid to liquid takes place.

Saturation point: It is defined as that point at which phase transfer takes place
without any change in pressure and temperature.

Saturation pressure: It is defined as that pressure at which phase transfer

takes place at constant temperature. Such as at any given temperature water

will be converted into steam at a definite temperature only. Such as boilin:
oint of water is 100°C at 1 atm pressure and 122°C at 2.1 atm pressure. So tha
atm and 2.1 atm will be the saturation pressure.

Saturation temperature: It is defined as that temperature at which phase
transfer takes place at constant pressure. Such as saturation temperature of
water at 1atm pressure is 100°C.

Triple point: It is defined as that point in the system at which all the phases
(solid, liquid and gas) exists in equilibrium.

4. Steam

Steam is a vapour of water, and is invisible when pure and dry. It is used as a working substance in the
operation of steam engines and steam turbines. It obeys the Iaws of gases, when it is perfectly dry.

Terminology for steam
Wet steam: When the steam contains moisture or particles of water, it is termed as wet steam.

Dry saturated steam: When the steam does not contain any moisture or suspended particles of water, it
is termed as dry saturated steam. It is obtained when the wet steam is further heated.

Superheated steam: When the dry saturated steam is further heated at constant pressure thus raising its
temperature, it is termed as superheated steam, Because heating of steam at constant pressure therefore
volume of superheated steam increases. It may be noted that the volume of 1 kg of superheated steam is
greater than the volume of 1 kg of dry saturated steam.

Dryness fraction: It is the ratio of the actual mass of dry steam to the mass of same quantity of wet
steam. It is represented by x. Mathematically it is written as,

mg
Mg tm My

Where,

ly = actual mass of dry steam
m; = mass of water
My = mass of wet steam = (mg+my)

5. Sensible heat of water: It is defined as the amount of heat absorbed by
one kilogram of water, when heated at constant pressure, from 0°C
(freezing point) to saturation temperature (formation of steam). It is also
known as liquid heat.

Latent heat of vaporization: It is defined as the amount of heat required
to evaporated one kilogram of water at its saturation temperature
(boiling point) without change of temperature. It is represented by h,.. Its
unit is kJ/kg. The latent heat of vaporization of water or latent heat of
steam is 2257 kJ/kg. If the steam is wet with a dryness fraction x, then
the heat absorbed by wet steam is Xhy.

hisg = (hg—hy)
Enthalpy: It is defined as the amount of heat absorbed by water from 0°C

(freezing point) to saturation point (sensible heat) plus heat absorbed
during evaporation (latent heat). It is represented by h,.

So that,
Enthalpy = sensible heat + latent heat

6. Some expressions for enthalpy,
Wet steam: Enthalpy of wet steam is given by;

h= hy +xhpg
Dry steam: Enthalpy of dry steam is given by;
h=h; hr +hgg eo... Since x =1 for dry air.

Superheated steam: From the definition superheated steam it may be
written as,

hsup = total heat for dry steam + heat for superheated steam

Asup = hr + pg + CplTsup — T)
Rsup = Ng + Cp(Tsup — T)
Where,
Cp = specific heat at constant pressure
Tsup = temperature of superheated steam
T = saturation temperature at constant pressure.

7. Specific volume: It is the volume occupied by the steam per unit mass at given
temperature and pressure. It is represented by v. It is expressed in m*/kg. The
reciprocal of specific volume represents the density.

Some expressions for volumes occupied by the steam,
Wet steam: Consider one kilogram of wet steam of dryness fraction x. It means
that this steam will have x kg ol dry steam and (1-x) kg of water. Let v, be the
yolume of one kg of water then, volume of one kilogram of wet steam is given
y,
=x09 +(1-x)07

Since v; is very small in comparison to v,, so the term (1 — x) vp may be
neglected.
So that,

Volume of one kilogram of wet steam

= xv, mé
Or
Specific volume of wet steam
v = xv, m* /kg

8. Dry steam: As studied earlier in dry steam, the mass of water is zero and dryness
fraction is unity. So that

Specific volume of dry steam
v = vy m3/kg

Superheated steam: As studied earlier when the dry saturated steam is further
heated at constant pressure thus raising its temperature, it is termed as superheated
steam. Because heating of steam at constant pressure therefore volume of superheated
steam increases. According to Charles’s law

Vsup _ Vg

T. T

sup

Or
_ UT sup
Vsup = T

Where,
Vsup = Specific volume of superheated steam
Tsup = temperature of superheated steam
vg = specific volume of dry saturated steam
T = saturation temperature.

9. Internal energy: Internal energy of steam is define as the energy stored
in the steam, above 0°C (freezing point) of water. It may be obtained b
subtracting the work done during evaporation to the enthalpy of steam. It
is represented by U. Mathematically it is written as,

Internal energy of steam

= Enthalpy of steam — Workdone during evaporation
Some expressions for enthalpy,
Wet steam: Internal energy of wet steam is given by;

U=h-100*Px*xx*vy= hy +xhpg —100*P*x*v, kJ/kg
Dry steam: Internal energy of dry steam is given by;

U=h, —100*P*v, k]J/kg .........since x = 1 for dry air.
Superheated steam: From the definition superheated steam it may be
written as,

U = hsup — 100 * P * Ug
U = hy + Cp(Toup — T) — 100 + P + Vsupy kJ/kg

10. Steam Table and its use

Absolute

| o. .

028 pressurein — Specfic volume in m*/kg

“Temperature based

Specific enthalpy in kJ/kg,

water

Specific entropy in kJ/kg K

water steam

water(v) steam(v) (h) Evaporation(h;) steam(h) (s) Evaporation(ss) (s,)
Es 000657 | 0001000 19261 42 24992 25034 0015 9116 9131
001704 0001001 77978 629 24661 25291 0224 8.559 8783
aus? one sm mo US ww um wes
Absolute
PAT remperaturein °c | Specfic volume in m?/kg Specific enthalpy in kJ/kg Specific entropy in kJ/kg K
bar
water steam water steam
(0) water (vj) steam(v) (6) — Evaporation(hy) (0) (5) Evaporation (sq) (s,)

26.69 0.001008 39479 1118 2438.6 25504 0391 8132 8.523
3618 0001006 23741 1515 24160 25675 0521 7810 8331
| oo | 43.79 0.001009 16204 1833 23978 25811 0.622 7.566 8188

11. Rankine cycle

Rankine cycle is thermodynamic cycle associated with Carnot cycle to eliminate
its limitations .Rankine cycle is a modified form of Carnot cycle. The schematic
diagram of steam engine plant is shown in fig.

Process 1-2 : Isothermal heat addition (in boiler)
Process 2-3 : Adiabatic expansion (in turbine)
Process 3-4: Isothermal heat rejection (in condenser)
Process 4-1 : Adiabatic Pumping (in pump) 2

12. Pressure ————»

Temperature ———>

Volume

Entropy

13. Process 1-2: The saturated water at point lis isothermally converted
into dry saturated steam in the boiler, and the heat is absorbed at a
constant temperatureT, and pressure P,.The state of dry saturated steam
is shown by point 2 on P-v and T-s diagram. The temperature T, and
pressure P, is equal to temperature T, and pressure P,. This isothermal
process is represented by curve 1-2 on P-v and T-s diagram. We know
that the heat absorbed during the isothermal process by water is stored
in the form of latent heat and is responsible for vaporization of water (i.e.
hrgı = hyg2), corresponding to pressure P, or P, (since P,=P,).

Process 2-3: The dry saturated steam at point 2, expands isentropically
in turbine. The temperature and pressure falls from T, to T; and P, to Pz
respectively with a dryness fraction x3. Since no heat is absorbed or
rejected during isentropic process at constant entropy. The isentropic
expansion is represented by curve 2-3 on P-v and T-s diagram.

14. Process 3-4: The wet steam at point 3 is condensed isothermally in
a condenser and rejects heat at constant temperature and pressure
T; and P respectively until the whole steam is converted into water.
The temperature T,and pressure P, is equal to temperature T; and
pressure P3. This isothermal process is represented by curve 3-4 on
P-v and T-s diagram. The rejected by the steam is its latent heat (=
X3hfg3).

Process 4-1: The water at point 4 is now heated in a boiler at
constant volume from temperature T, to T, and pressure P, to P..
This heating operation is represented by the curve 4-1 on P-v and T-
s diagram. The heat is absorbed by the water during the operation is
equal to the sensible heat to pressure P, (i.e. equal to sensible heat
at point 1- sensible heat at point 4).

15. Let
Sensible heat or enthalpy of water at point 1

sensible heat or enthalpy of water at point 4

So that, heat absorbed during heating operation 4-

Inga Mya
ha Mya

‘The heat absorbed during the whole cyck

at absorbed during isothermal oper
1.2 4 heat absorbed during heating operation 4-1
= ya (a = ha)

ya + Mor = ys
12 hi

(Since, hz oF ga = Mya + Xliyge sand for dry steam x = 1,50 that iz = Aa + ga)

‘The heat rejected during the cycle 3-4;
= hye

I= ys oli = ya = ola

(Since, ya = Mya)

‘The work done during the cycl

W =Heat absorbed - Heat rejected

(ince, + xao = hs)

Rankine efficiency;

(The quantity (ahs) is termed as isentropic heat drop)