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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Limitations of Throttling calorimeters:
• This type of calorimeter fails when the steam
is not superheated after expansion.
Limitations of Separating calorimeters:
• 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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COMBINED CALORIMETER
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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COMBINED CALORIMETER
The steam first passes through a separating calorimeter
and then
through a throttling calorimeter.
Let mf = mass of water collected in the separating
calorimeter
Let mg = mass of steam passed from the separating
calorimeter into the throttling calorimeter.
It is then finally condensed in a condenser.
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COMBINED CALORIMETER
Therefore
mf + mg = total mass of steam
entered into the separating
calorimeter,
So, x2 = mg /(mf + mg )
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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:
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This dryness fraction x1 can be
determined with the help of
equation: h f 1  x1h fg1  hg 2  c p  ts  t f 
Where,
x  dryness fraction of steam before throttling.
h f 1  liquid enthalpy of steam before throttling.
h fg1  enthalpy of evaporation of steam before throttling.
h fg 2  enthalpy of dry steam after throttling
c p  specific heat of the superheated steam.
(taken as 2.01 kJ/kgK if not given)
ts  temperature of steam after throttling.
t f  saturation temperature of steam after throttling.
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Now if x be the actual dryness fraction at
the steam main, then
Then x = x1 * x2
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Example 4.9 : A combined calorimeter is used to
determine the dryness' fraction of steam at a
pressure of 10 kgf/cm2 abs. (9.8 bar abs.). 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 from the
throttling calorimeter. Find the dryness fraction of
steam. Assume cp=0.48 kcal/kgK (2.01 kJ/kgK)
Dept. of Mech & Mfg. Engg.
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