1 Chapter 1: PROPERTIES OF STEAM Steam: Vapour form of water is called STEAM. Water in solid phase: ICE Water in liquid phase: WATER Water in gaseous phase: STEAM Dept. of Mechanical & Mfg. Engg.
Dec 15, 2015
1
Chapter 1:
PROPERTIES OF STEAM
Steam:
Vapour form of water is called STEAM.
Water in solid phase: ICE
Water in liquid phase: WATER
Water in gaseous phase: STEAM
Dept. of Mechanical & Mfg. Engg.
2
Application of steam
Food processing industry.
Cooking: hotels, restaurants etc.
Used as a working fluid in steam engines and steam
turbines.
Used in industries for process heating.
Petrochemical industry.
Washing / drying / sterilizing in hospitals.
Health clinic / gym.
Dept. of Mechanical & Mfg. Engg.
3
Properties
Definition:
The characteristics of a system that can be used to define
its condition/state is called as a property.
Example:
Suppose if you were to specify the condition of ice cubes
in a freezer. You may find out that the ice cube is at -4oC
and occupies 1m3 volume upon mesurement.
Here, the temperature and volume are the properties of ice.
Dept. of Mechanical & Mfg. Engg.
4
FORMATION OF STEAM EXPERIMENT (Constant Pressure)
Consider 1 kg of water at
0oC taken in a cylinder
fitted with a freely moving
frictionless piston as
shown in figure.
Cylinder
I kg of
water at
OoC
Pressure
“P”
Dept. of Mechanical & Mfg. Engg.
5
The initial condition of water at 0oC is represented by the
point “A” on the Temperature – Enthalpy graph
A
Temperature
Enthalpy
(h)
Temperature
(ToC)
Dept. of Mechanical & Mfg. Engg.
6
A
Temperature
Enthalpy
(h)
Temperature
(T)oC
TSat B
hf
Sensible heat
hfg
Latent heat
TSup
AOS
C
D
DOS
Fig. 1
W W
W W
W
Fig. 2 Fig. 3 Fig. 4 Fig. 5
A B C D
Dept. of Mechanical & Mfg. Engg.
7
Definitions
Sensible heat (hf): (Enthalpy of water)
It is the amount of heat required to raise the temperature
of 1 kg of water from 00C to the saturation temperature
Tsat0C at a given constant pressure “P”.
hf = m x Cp x ΔT kJ
Where, m = mass of water in kg.
Cp = specific heat of water
= 4.1868 kJ/kg0K
ΔT = Temperature Difference.
Dept. of Mechanical & Mfg. Engg.
8
Saturation temperature (Tsat):
It is the temperature at which the water begins to boil at
the stated pressure.
Latent heat (hfg): (Enthalpy of evaporation)
It is the amount of heat required to evaporate 1 kg of
water at saturation temperature to 1 kg of dry steam at the
same saturation temperature and at the given constant
pressure “P”.
Superheated temperature (Tsup):
It is the temperature of the steam above the saturation
temperature at a given constant pressure.
Dept. of Mechanical & Mfg. Engg.
9
Amount of superheat (AOS): (Enthalpy of superheat)
It is the amount of heat required to raise the temperature
of dry steam from its saturation temperature to any
desired higher temperature at the given constant
pressure “P”.
AOS = m x Csup (Tsup - Tsat) kJ
Degree of superheat (DOS):
It is the difference between the superheated temperature
and the saturation temperature.
DOS = (Tsup – Tsat)
Dept. of Mechanical & Mfg. Engg.
10
Different states of steam
The steam as it is being generated can exist in three
different states,
1. Wet steam
2. Dry saturated steam (dry steam)
3. Superheated steam
Dept. of Mechanical & Mfg. Engg.
11
Wet Steam:
A wet steam is defined as a two-phase mixture of
finely divided water particles and steam at the
saturation temperature corresponding to a given
stated pressure.
Dept. of Mechanical & Mfg. Engg.
12
Enthalpy A
B C
Sensible
Heat
Latent Heat
hfg
Tsup
Temperature
Amount of
Superheat
Degree of Superheat
Ts
hf
D
Temperature
hg
Dept. of Mechanical & Mfg. Engg.
13
The quality of wet steam is specified by the dryness
fraction which indicates the amount of dry steam present in
the given quantity of wet steam and is denoted as “x”.
The dryness fraction of a steam is defined as the ratio of
mass of the actual dry steam present in a known quantity of
wet steam to the total mass of the wet steam.
Steam Wet of Mass Total
Steamin Wet present SteamDry of Mass x fraction, Dryness
Dept. of Mechanical & Mfg. Engg.
14
Let,
mg = Mass of dry steam present in the sample quantity of
wet steam
mf = Mass of suspended water molecules in the sample
quantity of wet steam
The dryness fraction of wet steam is always less than 1.
The dryness fraction of dry steam is equal to 1.
g
f g
mx
m m
Dept. of Mechanical & Mfg. Engg.
15
Dry Saturated Steam:
(dry steam)
Dry saturated steam is the steam at saturation
temperature and at a given pressure having no water
molecules entrained in it.
Dept. of Mechanical & Mfg. Engg.
16
Superheated Steam:
A superheated steam is defined as the steam at a
temperature higher than the saturation temperature at the
given stated pressure.
Dept. of Mechanical & Mfg. Engg.
17
Advantages of Superheated Steam:
Superheated steam possess very high energy compared
to dry saturated steam or wet steam at the same pressure,
hence its capacity to do the work will be higher.
It doesn’t create any problems like rusting or corrosion of
blades of turbine / engine cylinder.
Dept. of Mechanical & Mfg. Engg.
18
Disadvantages of Superheated Steam:
The high temperature of superheated steam poses
problems in lubrication.
Higher generation & initial cost.
Energy content of steam:
Superheated
steam
Dry saturated
steam
Wet
steam > >
Dept. of Mechanical & Mfg. Engg.
19
Enthalpy equations for different states of
steam
a) Enthalpy of Dry saturated Steam (hg):
hg = hf + hfg kJ/kg
Enthalpy A
B C
Sensible
Heat
Latent Heat
hfg
Tsup
Temperature
Amount of
Superheat
Degree of Superheat
Ts
hf
D
Temperature
hg
Dept. of Mechanical & Mfg. Engg.
20
b) Enthalpy of Wet Steam (h):
A
B C
Sensible
Heat
Latent Heat
hfg
Tsup
Temperature
Enthalpy
Amount of
Superheat
Degree of Superheat
Tsat
hf
D
Temperature
h = hf + x .hfg kJ/kg
Dept. of Mechanical & Mfg. Engg.
21
c) Amount of superheat (AOS): (Enthalpy of superheat)
A
B C
Sensible
Heat
Latent Heat
hfg
Tsup
Temperature
Enthalpy
Amount of
Superheat
Degree of Superheat
Ts
hf
D
Temperature
AOS = Csup (Tsup - Tsat) kJ/kg
where, Csup = Specific heat of the superheated steam
= 2.25 kJ/kg0K
Dept. of Mechanical & Mfg. Engg.
22
d) Enthalpy of Superheated Steam (hsup):
A
B C
Sensible
Heat
Latent
Heat
hfg
Tsup
Temperature
Enthalpy
Amount of
Superheat
Degree of Superheat
Tsat
hf
D
Temperature
hg
hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg
Dept. of Mechanical & Mfg. Engg.
23
e) Degree of superheat (DOS):
A
B C
Sensible
Heat
Latent
Heat
hfg
Tsup
Temperatur
Enthalpy
Amount of
Superheat
Degree of Superheat
Tsat
hf
D
Temperature
DOS = (Tsup - Tsat) 0C
Dept. of Mechanical & Mfg. Engg.
24
a) Enthalpy of Dry saturated Steam:
hg = hf + hfg kJ/kg
b) Enthalpy of Wet Steam:
h = hf + x .hfg kJ/kg
c) Enthalpy of Superheated Steam:
hsup = hf + hfg + Csup (Tsup - Tsat) kJ/kg
d) Degree of superheat (DOS):
DOS = (Tsup - Tsat) 0C
e) Amount of superheat (AOS):
AOS = Csup (Tsup - Tsat) kJ/kg
Dept. of Mechanical & Mfg. Engg.
25
Properties of steam: Numerical
Problem 1: Find the enthalpy of 1 kg of steam at 12 bar
pressure when,
(a) steam is dry saturated,
(b) steam is 22% wet and
(c) Steam is superheated to 250°C.
Assume the specific heat of superheated steam as 2.25
kJ/kg°K
Dept. of Mechanical & Mfg. Engg.
26
Problem 2: Determine the conditions of steam from the
following data:
a) Pressure is 10 bar and temperature 200°C,
b) Pressure is 12 bar and enthalpy of 2600 kJ/kg.
Problem 3:
Given enthalpy of 1 kg of steam at 30 bar is 3681 KJ.
Is the steam wet or superheated?
If it is wet; find its dryness fraction.
If it is superheated; find its degree of superheat.
Dept. of Mechanical & Mfg. Engg.
27
Problem 4:
By actual measurement, the enthalpy of steam at 6 bar is
found to be 2500 KJ/Kg.
a) What is the quality of steam?
b) If 500KJ of heat is added to this steam, what is the
1. Superheated temperature
2. Degree of superheat
3. Enthalpy of superheat
Problem 5: 2 Kg of water at 300C is heated continuously
at constant pressure of 5 bar. The total amount of heat
added is 500 KJ. Determine the dryness fraction or
degree of superheat of the resulting steam as the case
may be.
Dept. of Mechanical & Mfg. Engg.
28
Problem 6:
5 kg of water is heated from 400C to superheated steam
at 1500C with constant pressure of 3 bar. Find,
a) The total amount of heat added in the heating process
b) Amount of superheat
Dept. of Mechanical & Mfg. Engg.
29
Problem 7:
2 boilers, one with super heater and another without super
heater are delivering equal quantities of steam into a
common main.
The pressure in the boiler and main is 20 bar. The
temperature of steam from a boiler with a super heater is
3500 C and the temperature of steam in the main is 2500 C.
Determine the quality of steam supplied by the other boiler.
Dept. of Mechanical & Mfg. Engg.
30
Problem 8:
1000 Kg of steam at a pressure of 16 bar and 0.9 dry is
generated by a boiler and it enters the super heater, where
its temperature is raised such that the degree of superheat
is 1800C. If the temperature of feed water is 300 C,
determine
a) Total heat added to feed water in the boiler.
b) Total heat absorbed in the super heater.
Dept. of Mechanical & Mfg. Engg.
31
Problem 9:
A dry saturated steam at a pressure of 16 bar is
generated in a boiler. Dry saturated steam leaves the
boiler to enter a super heater, where it looses heat equal
to 600 kJ/kg. And in the super heater, steam is super
heated to temperature of 380oC. If temperature of feed
water is 30oC, determine:
Total heat supplied to feed water in the boiler
Dryness fraction of steam at the entry of super heater
Total heat supplied in the super heater.
Dept. of Mechanical & Mfg. Engg.
32
Problem 10:
Steam is generated in a boiler at a pressure of 9 bar and
with a dryness fraction of 0.98. Find the quality and
temperature of steam under each of the following
operations.
a) When steam loses 50 kJ/kg at constant pressure
b) When steam receives 150 kJ/kg at constant pressure
Dept. of Mechanical & Mfg. Engg.
33
Problem 11:
The steam initially at a pressure of 0.9 MPa and 2% wet
expands in a turbine such that it losses 80 kJ/kg at
constant pressure.
a) What is the quality of steam after expansion?
b) If it receives 160 kJ/kg of heat before expansion, what
would be the final state and temperature of the steam as
it comes out of the turbine?
Dept. of Mechanical & Mfg. Engg.
34
Critical Temperature &
Pressure.
Pc = 221.2 bar
Tc = 374.150C
At a particular pressure
water is directly converted
into dry steam without
going through the phase of
evaporation. i.e., hfg = 0 .
This point is called critical
point.
Temperature
Enthalpy
Tc
Pc
Pi
P3
P2
P1
hf hfg hf
Dept. of Mechanical & Mfg. Engg.
35
Critical pressure:
It is the pressure at which the water is directly converted
into dry steam without undergoing the state of
evaporation.
Critical temperature:
It is the corresponding temperature at the critical point.
Dept. of Mechanical & Mfg. Engg.