Steam

1. BASICS OF MECHANICAL ENGINEERING
Dr. Sunanda Roy
Mechanical Engineering

2. STEAM

3. If water is heated beyond the boiling point, it vaporizes into steam or water in the
gaseous state. However, not all steam is the same. The properties of steam vary
greatly depending on the pressure and temperature to which it is subject.
STEAM

4. Steam constitutes an important power source for industrial society. Water is
heated to steam in power plants, and the pressurized steam drives turbines that
produce electrical current. The thermal energy of steam is thus converted
to mechanical energy, which in turn is converted into electricity.
STEAM: The energy fluid
The steam used to drive
turbogenerators
furnishes most of the
world’s electric power.
Water Steam
> Boiling
Condensatio
n

5. What is sensible heat?
The heat which is responsible for the change
in temperature of the substance is known
as Sensible heat.
The sensible heat can be sensed manually
because of the rise in the temperature of the
object.
During the exchange of the sensible heat, the
object doesn’t change its phase.
Example: The heat required for the heating or
cooling of the water.

6. Latent heat is responsible for the change in phase of the substance while the
sensible heat is responsible for the change in temperature of the substance.
Latent heat is the energy absorbed by or released from a substance during a
phase change from a gas to a liquid or a solid or vice versa.
ic
e
1. The amount of heat responsible for the change in phase of the substance from solid to liquid or from liquid to
solid is known as latent heat of fusion/melting.
2. Latent heat of vaporization: Liquid to vapor or Vapor to liquid
3. Latent heat of sublimation: Solid to direct vapor state
Sensible heat is the amount of heat required to increase the temperature of water at constant
pressure without changing its liquid state, while Latent heat is the amount of heat required to
change the state from Liquid to Vapor at constant temperature and pressure.

8. Formation of steam
Superheated steam is steam that is hotter than its boiling point for a given
pressure. For the example above superheated steam would be hotter than 212°F
(100°C), but still at atmospheric pressure.
P= 1 atm
A
A
’
B
C
D

9. Cont…
 Consider 1 kg of water at 0 degree Celsius contained in a piston-cylinder
arrangement as shown in the above figure (i) the piston and weights maintain
constant pressure in the cylinder. If we heat water in the cylinder, it will be
converted to steam.
 The volume of water will increase slightly as an increase in temperature hence it
will cause the piston to move upwards, and work is obtained.
 On further heating temperature reaches boiling point, the boiling point of water is
100 degree Celsius at 1.103 bar but increases with an increase in pressure.
 When boiling point is reached, the temperature remains constant and water
evaporates thus pushing the piston up at constant pressure. The specific volume
of steam increases.

10. Cont…
 At this stage steam will have some particles of water in suspension and is termed as
wet steam. Process will continue till the whole water is converted to steam.
 On further heating the suspended water particles will get converted into steam.
 The entire steam in such a state is called dry steam (iv). It acts as a perfect gas.
 On further heating the temperature of steam starts rising and is termed as
superheated steam(v).

11. 100 °C Evaporation
tsup
D
Wet
steam
Saturated
steam
hf
hfg
Cp (tsup –t)
Cp = Specific heat capacity for
superheated steam
hf= sensible heat of
water/enthalpy of water
hfg= enthalpy of evaporation
latent heat of evaporation
ts= saturation temp of water/
fluid
tsup= temp of superheated steam
hg
hsup
Saturated water Dry saturated steam
Vf= Vol. of saturated water
Vg= Vol. of dry saturated
steam
Critical point
Vsup= Vol. of superheated
steam
X=0 X=1

12. Total heat or enthalpy of wet steam (h).
It is defined as the quantity of heat required to convert 1 kg
of water at 0°C into wet steam at constant pressure.
It is the sum of total heat of water and the latent heat and
this sum is also called enthalpy.

Superheated steam.
 When steam is heated after it has become dry and saturated, it is called
superheated steam and the process of heating is called superheating.
 Superheating is always carried out at constant pressure.

Where, Cps is the specific heat of superheated steam at constant
pressure.

13. Cont…
Specific volume: It is the volume occupied by the unit mass of a
substance. It is expressed in m3/kg.
Entropy: It gives about the usefulness of an amount of heat
transferred in performing work
Internal energy : It is defined as the difference between the enthalpy
of the steam and the external work of evaporation.
usup = hsup – p vsup

14. Dryness fraction
Cont…
x varies from 0 to 1. For saturated
water, x=0 and saturated vapor x=1

15. Q2: In 1 kg of wet steam, dry steam is 0.9 kg and water particles is 0.1 kg.
What is the dryness fraction?
Mass of dry steam= 0.9 kg
Mass of water particles or liquid = 0.1 kg
Total mass of wet steam =1 kg
X = Vapor/ total
=0.9/1
= 0.9
Mass of steam = 50 kg
Mass of water = 1.5 kg
Q1: Calculate the dryness fraction of steam which has 1.5 kg of
water in suspension with 50 kg of steam.
Class A: In 5 kg of wet steam contains 1 Kg of water in suspension. Calculate
the dryness fraction of the steam?

17. Sensible heat of water (hf )
It is defined as the quantity of heat absorbed by 1 kg of water
when it is heated from 0°C (freezing point) to boiling point.
It is also called the total heat (or enthalpy) of water.
It is expressed by the symbol hf.
Latent heat or hidden heat (hfg)
It is the amount of heat required to convert water at a given
temperature and pressure into steam at the same temperature
and pressure.
It is expressed by the symbol hfg.
•Dry steam - All water molecules are in the gaseous state
•Wet steam - Wet steam is a mixture of steam and liquid water. It exists at a
saturation temperature containing more than 5% water. It is said to be a two-
phase mix: steam contains droplets of water that have not changed phase.

18. Steam Table
Steam table is a tabular presentation of properties of steam
and saturated water such as specific enthalpy, specific
entropy, specific volume, etc. at different saturation pressures
and temperatures.
Steam table may be on a pressure basis or on a temperature
basis.

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