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Transcript of PROPERTIES OF STEAM - EngineeringDuniya.com Year/Mech Engg... · PROPERTIES OF STEAM ... FORMATION...
MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 1
Chapter 1:
PROPERTIES OF STEAM
Steam:
Vapour form of water is called STEAM.
Water in solid phase: We call it as ICE
Water in liquid phase: We call it as WATER
Water in gaseous phase: We call it as STEAM
MADHWESH N., Faculty, Dept. of Mech & 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.
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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”
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The initial condition of water at 0oC is represented by the
point “A” on the Temperature – Enthalpy graph
A
Temperature
Enthalpy
(h)
Temperature
(ToC)
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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
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Definitions
Sensible heat (hf): (Enthalpy of water)
It is the amount of heat required to raise the temperature
of 1 kg of water from 0 0C to the saturation temperature
Tsat0C at a given constant pressure “p”.
hf = m x Cp x Tsat kJ
Where, m = mass of water in kg.
Cp = specific heat of water
= 4.1868 kJ/kg0K
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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.
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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)
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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
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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.
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Enthalpy A
B C
Sensible
Heat
Latent Heat
hfg
Tsup
Temperature
Amount of
Superheat
Degree of Superheat
Ts
hf
D
Temperature
hg
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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 mixture.
Mass Total
Steamin Wet present SteamDry of Mass x fraction, Dryness
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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
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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.
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Superheated Steam:
A superheated steam is defined as the steam at a
temperature higher than the saturation temperature at the
given stated pressure.
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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.
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Disadvantages of Superheated Steam:
The high superheated temperatures poses problems in
the lubrication.
Higher initial cost.
Energy content of steam:
Superheated
steam
Dry saturated
steam
Wet
steam > >
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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
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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 hg = hf + x hfg kJ/kg
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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
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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
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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
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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
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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
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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.
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Problem 3:
Given enthalpy of 1 kg of steam at 30 bar is 3681 kJ. Is the
steam is wet or superheated?
If it is wet; find its dryness fraction.
If it is superheated; find its degree of superheat.
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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
Superheated temperature
Degree of superheat
Enthalpy of superheat
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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.
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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
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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.
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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.
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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 600 kJ/kg
of heat. 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.
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Problem 10:
The steam initially at a pressure of 9 bar and dryness
fraction 0.98. Find the final quality and temperature of
steam at 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
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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?
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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
MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 36
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.
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Throttling:
Throttling is a type of expansion where steam passes
through a narrow passage and expands with a fall of
pressure without doing an external work.
In this case, there is no interchange of heat similar to an
adiabatic process.
The enthalpy remains constant during this operation.
So,
Enthalpy before throttling = Enthalpy after throttling
In this process, steam becomes drier and nearly
saturated steam becomes, superheated.
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Measurement of Dryness Fraction
Dryness fraction of a sample of steam may be
determined by means of steam calorimeters.
Types of calorimeters:
1. Tank calorimeter
2. Throttling calorimeter
3. Separating calorimeter
4. Combined calorimeter
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Throttling calorimeter:
It is known that at the time of
throttling the steam becomes
drier and nearly saturated
steam becomes superheated.
Limitations:
This type of calorimeter fails
when the steam is not
superheated after expansion.
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Separating calorimeter:
If the steam is very wet then
the dryness fraction will be
measured by this type of
calorimeter.
Limitations:
Only an approximate dryness
fraction is measured, because
the steam discharged from the
calorimeter is not absolutely
dry.
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Combined calorimeter
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Separating calorimeter does not give an accurate result
and the throttling calorimeter fails if the steam is not
superheated after throttling.
A combination of separating and throttling calorimeter is
therefore found most suitable for accurate measurement of
dryness of steam.
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The steam first passes through a separating calorimeter
and then through a throttling calorimeter.
Let m' = mass of water collected in the separating
calorimeter
Let m = mass of steam passed from the separating
calorimeter into the throttling calorimeter.
It is then finally condensed in a condenser.
Therefore, m + m' = total mass of steam entered into the
separating calorimeter,
So,
2
mx
m m
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Let “x2” be the apparent dryness fraction determined by
the separating calorimeter.
So,
Let “x1” be the dryness fraction of wet steam which enters
into the throttling calorimeter. This dryness fraction can be
determined with the help of equation:
2
mx
m m
1 1 1 2g pf fg s fh c tx th h
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Where,
x1 – dryness fraction of steam before throttling
hf1 – liquid enthalpy of steam before throttling
hfg1 – enthalpy of evaporation of steam before throttling
hg2 – enthalpy of dry steam after throttling
ts – temperature of steam after throttling
tf – saturation temperature of steam after throttling
cp – specific heat of superheated steam
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So the actual amount of dry steam passes from the steam
main is “x1m”.
Now if “x” be the actual dryness fraction at the steam
main, then
11 1 2
x m mx x
m m m mx x
MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 47
Problem 1:
A combined calorimeter is used to determine the dryness
fraction of steam at a pressure of 9.8 bar. The steam is
discharged from the calorimeter at atmospheric pressure
and at a temperature of 105°C. During the test 0.2 kg of
water is collected into the separating calorimeter and 2 kg
of steam is discharged to the throttling calorimeter. Find
the dryness fraction of steam. Assume Cp = 2.25 kJ/kg0K
MADHWESH N., Faculty, Dept. of Mech & Mfg. Engg. 48
Solution:
m = 2 kg, m’ = 0.2kg, P = 9.8 bar, ts = 105°C, tf = 100°C
With the help of separating calorimeter,
With the help of throttling calorimeter,
2
20.909
2 0.2
mx
m m
1 1 1 2g pf fg s fh c tx th h