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 boiling
point of water is 100oC at 1 atm pressure and 122oC at 2.1 atm pressure. So that
1 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 100oC.
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 laws 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,
𝑥 =
𝑚 𝑔
𝑚 𝑔 + 𝑚 𝑓
=
𝑚 𝑔
𝑚 𝑤
Where,
𝑚 𝑔 = 𝑎𝑐𝑡𝑢𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑡𝑒𝑎𝑚
𝑚 𝑓 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
𝑚 𝑤 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑒𝑡 𝑠𝑡𝑒𝑎𝑚 = (𝑚 𝑔+𝑚 𝑓)

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 0oC
(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 hfg. 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 xhfg.
ℎ 𝑓𝑔 = (ℎ 𝑔−ℎ 𝑓)
Enthalpy: It is defined as the amount of heat absorbed by water from 0oC
(freezing point) to saturation point (sensible heat) plus heat absorbed
during evaporation (latent heat). It is represented by hg.
So that,
𝐸𝑛𝑡ℎ𝑎𝑙𝑝𝑦 = 𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 + 𝑙𝑎𝑡𝑒𝑛𝑡 ℎ𝑒𝑎𝑡

6. Some expressions for enthalpy,
Wet steam: Enthalpy of wet steam is given by;
ℎ = ℎ 𝑓 + 𝑥ℎ 𝑓𝑔
Dry steam: Enthalpy of dry steam is given by;
ℎ = ℎ 𝑔 = ℎ 𝑓 + ℎ 𝑓𝑔 … … … … 𝑠𝑖𝑛𝑐𝑒 𝑥 = 1 𝑓𝑜𝑟 𝑑𝑟𝑦 𝑎𝑖𝑟.
Superheated steam: From the definition superheated steam it may be
written as,
ℎ 𝑠𝑢𝑝 = 𝑡𝑜𝑡𝑎𝑙 ℎ𝑒𝑎𝑡 𝑓𝑜𝑟 𝑑𝑟𝑦 𝑠𝑡𝑒𝑎𝑚 + ℎ𝑒𝑎𝑡 𝑓𝑜𝑟 𝑠𝑢𝑝𝑒𝑟ℎ𝑒𝑎𝑡𝑒𝑑 𝑠𝑡𝑒𝑎𝑚
ℎ 𝑠𝑢𝑝 = ℎ 𝑓 + ℎ 𝑓𝑔 + 𝑐 𝑝(𝑇𝑠𝑢𝑝 − 𝑇)
ℎ 𝑠𝑢𝑝 = ℎ 𝑔 + 𝑐 𝑝(𝑇𝑠𝑢𝑝 − 𝑇)
Where,
𝑐 𝑝 = 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 m3/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 of dry steam and (1-x) kg of water. Let vf be the
volume of one kg of water then, volume of one kilogram of wet steam is given
by,
= 𝑥𝑣𝑔 + (1 − 𝑥)𝑣𝑓
Since vf is very small in comparison to vg, so the term (1 − 𝑥)𝑣𝑓 may be
neglected.
So that,
Volume of one kilogram of wet steam
= 𝑥𝑣𝑔 m3
Or
Specific volume of wet steam
𝑣 = 𝑥𝑣𝑔 m3
/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 = 𝑣𝑔 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
𝑇𝑠𝑢𝑝
=
𝑣𝑔
𝑇
Or
𝑣𝑠𝑢𝑝 =
𝑣𝑔. 𝑇𝑠𝑢𝑝
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 0oC (freezing point) of water. It may be obtained by
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 ∗ 𝑃 ∗ 𝑥 ∗ 𝑣𝑔 = ℎ 𝑓 + 𝑥ℎ 𝑓𝑔 − 100 ∗ 𝑃 ∗ 𝑥 ∗ 𝑣𝑔 𝑘𝐽/𝑘𝑔
Dry steam: Internal energy of dry steam is given by;
𝑈 = ℎ 𝑔 − 100 ∗ 𝑃 ∗ 𝑣𝑔 𝑘𝐽/𝑘𝑔 … … … … 𝑠𝑖𝑛𝑐𝑒 𝑥 = 1 𝑓𝑜𝑟 𝑑𝑟𝑦 𝑎𝑖𝑟.
Superheated steam: From the definition superheated steam it may be
written as,
𝑈 = ℎ 𝑠𝑢𝑝 − 100 ∗ 𝑃 ∗ 𝑣𝑔
𝑈 = ℎ 𝑔 + 𝑐 𝑝 𝑇𝑠𝑢𝑝 − 𝑇 − 100 ∗ 𝑃 ∗ 𝑣𝑠𝑢𝑝 𝑘𝐽/𝑘𝑔

10. Steam Table and its use
Temperature based
Temperature in oC
Absolute
pressure in
bar
Specfic volume in m3/kg Specific enthalpy in kJ/kg Specific entropy in kJ/kg K
(T) (P) water (vf) steam (vg)
water
(hf) Evaporation (hfg) steam (hg)
water
(sf) Evaporation (sfg)
steam
(sg)
1 0.00657 0.001000 192.61 4.2 2499.2 2503.4 0.015 9.116 9.131
15 0.01704 0.001001 77.978 62.9 2466.1 2529.1 0.224 8.559 8.783
20 0.02337 0.001002 57.838 83.9 2454.3 2538.2 0.296 8.372 8.668
Pressure based
Absolute
pressure in
bar
Temperature in oC Specfic volume in m3/kg Specific enthalpy in kJ/kg Specific entropy in kJ/kg K
(P) (T) water (vf) steam (vg)
water
(hf) Evaporation (hfg)
steam
(hg)
water
(sf) Evaporation (sfg)
steam
(sg)
0.035 26.69 0.001003 39.479 111.8 2438.6 2550.4 0.391 8.132 8.523
0.060 36.18 0.001006 23.741 151.5 2416.0 2567.5 0.521 7.810 8.331
0.090 43.79 0.001009 16.204 183.3 2397.8 2581.1 0.622 7.566 8.188

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)

13. Process 1-2: The saturated water at point 1is isothermally converted
into dry saturated steam in the boiler, and the heat is absorbed at a
constant temperatureT1 and pressure P1.The state of dry saturated steam
is shown by point 2 on P-v and T-s diagram. The temperature T2 and
pressure P2 is equal to temperature T1 and pressure P1. 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.
ℎ 𝑓𝑔1 = ℎ 𝑓𝑔2), corresponding to pressure P1 or P2 (since P1=P2).
Process 2-3: The dry saturated steam at point 2, expands isentropically
in turbine. The temperature and pressure falls from T2 to T3 and P2 to P3
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
T3 and P3 respectively until the whole steam is converted into water.
The temperature T4 and pressure P4 is equal to temperature T3 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 (=
𝑥3ℎ 𝑓𝑔3).
Process 4-1: The water at point 4 is now heated in a boiler at
constant volume from temperature T4 to T1 and pressure P4 to P1.
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 P1 (i.e. equal to sensible heat
at point 1- sensible heat at point 4).

15. Let
ℎ 𝑓1 = ℎ 𝑓2 = Sensible heat or enthalpy of water at point 1
ℎ 𝑓4= ℎ 𝑓3 = Sensible heat or enthalpy of water at point 4
So that, heat absorbed during heating operation 4-1;
= ℎ 𝑓1 − ℎ 𝑓4
= ℎ 𝑓2 − ℎ 𝑓3
The heat absorbed during the whole cycle = Heat absorbed during isothermal operation
1-2 + heat absorbed during heating operation 4-1
= ℎ 𝑓𝑔2 + ℎ 𝑓2 − ℎ 𝑓3
= ℎ 𝑓2 + ℎ 𝑓𝑔2 − ℎ 𝑓3
= ℎ2 − ℎ 𝑓3
(Since, ℎ2 𝑜𝑟 ℎ 𝑔2 = ℎ 𝑓2 + 𝑥2ℎ 𝑓𝑔2 , and for dry steam 𝑥2 = 1, so that ℎ2 = ℎ 𝑓2 + ℎ 𝑓𝑔2)
The heat rejected during the cycle 3-4;
= ℎ3 − ℎ 𝑓4
ℎ3 = ℎ 𝑓3 + 𝑥3ℎ 𝑓𝑔3 − ℎ 𝑓4 = 𝑥3ℎ 𝑓𝑔3
(Since, ℎ 𝑓3 = ℎ 𝑓4)
The work done during the cycle;
W =Heat absorbed – Heat rejected
= (ℎ2−ℎ 𝑓3) − 𝑥3ℎ 𝑓𝑔3
= ℎ2 − ℎ 𝑓3 + 𝑥3ℎ 𝑓𝑔3 = ℎ2 − ℎ3
(Since, ℎ 𝑓3 + 𝑥3ℎ 𝑓𝑔3 = ℎ3)
Rankine efficiency;
𝜼 𝑹 =
𝑾𝒐𝒓𝒌 𝒅𝒐𝒏𝒆
𝑯𝒆𝒂𝒕 𝒂𝒃𝒔𝒐𝒓𝒃𝒆𝒅
=
𝒉 𝟐−𝒉 𝟑
𝒉 𝟐−𝒉 𝒇𝟑
(The quantity (ℎ2−ℎ3) is termed as isentropic heat drop)