K J SOMAIYA COLLEGE OF ENGINEERING,
MUMBAI-77
(CONSTITUENT COLLEGE OF SOMAIYA VIDYAVIHAR
UNIVERSITY)
Module 4 : Steam Nozzle
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Introduction to Steam Nozzles
1.A steam nozzle is a passage of varying cross-section, which converts heat
energy of steam into kinetic energy.
2. A well designed nozzle converts the heat energy of steam into kinetic energy
with a minimum loss.
3.The main use of steam nozzle in steam turbines is to produce a jet of steam
with a high velocity.
TYPES OF STEAM NOZZLES
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How pressure decreases and velocity increases in
divergent section
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Introduction to Steam Nozzles
1.
During the first part of the nozzle, the steam increases its velocity.
2.
But in its later part, the steam gains more in volume than in velocity.
3.
Since the mass or steam, passing through any section of the nozzle
remains constant, the variation of steam pressure in the nozzle depends
upon the velocity, specific volume and dryness fraction of steam.
4.
A well designed nozzle converts the heat energy of steam into kinetic
energy with a minimum loss.
5.
The main use of steam nozzle in steam turbines is to produce a jet of steam
with a high velocity.
6.
The smallest section of the nozzle is called throat.
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Flow of Steam through Convergent-Divergent Nozzle
1.The steam enters the nozzle with a high pressure, but with a negligible velocity.
2.In the converging portion (i.e. from the inlet to the throat), there is a drop in the steam
pressure with a rise in its velocity.
3.There is also a drop in the enthalpy or total heat of the steam.
4.This drop of heat is not utilized in doing some external work, but is converted into
kinetic energy.
5.In the divergent portion (i. e. from the throat to outlet), there is further drop of steam
pressure with a further rise in its velocity.
6.Again, there is a drop in the enthalpy or total heat of steam, which is converted into
kinetic energy.
7.It will be interesting to know that the steam enters the nozzle with a high pressure and
negligible velocity.
8.But leaves the nozzle with a high velocity and small pressure.
9.The pressure, at which the steam leaves the nozzle, is known as back pressure.
10.Moreover, no heat is supplied or rejected by the steam during flow through a nozzle.
11.Therefore, it is considered as isentropic flow, and the corresponding expansion is
considered as an isentropic expansion.
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Velocity of Steam Flowing through a Nozzle
Assumptions
1. Steady state
2. Adiabatic boundaries (fluid velocity is very high so there is no
time available for heat exchange with surroundings)
3. Equilibrium states at inlet and outlet.
4. Change in potential energy is negligible.
5. No shaft work
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We know that for a steady flow process in a nozzle,
Neglecting losses in a nozzle
V2 
2000 h d  44 .72 h d
In actual practice, there is always a certain amount of friction present between the
steam and nozzle surfaces. This reduces the heat drop by 10 to 15 percent and
thus the exit velocity of steam is also reduced correspondingly. Thus the above
relation may be written as :
V2  44 .72 K h d
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where K is the nozzle coefficient or
nozzle efficiency.
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Friction in a Nozzle or Nozzle Efficiency
As a matter fact, when the steam flows through a nozzle, some loss in its enthalpy or
total heat takes place due to friction between the nozzle surface and the flowing
steam.
This can be best understood with the help of h-s diagram or Mollier chart, as shown
in Fig.
Now the coefficient of nozzle or nozzle
efficiency (usually denoted by K) is defined as
the ratio of useful heat drop to the isentropic
heat drop.
K
Useful heat drop
AC h1  h 3


Isentropic heat drop AB h1  h 2
 Nozzle effeciency , K 
PR
PQ
In general, if 15% of the heat drop is lost in
friction, then efficiency of the nozzle is equal
to 100-15 =85% =0.85.
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Effects of friction
1. Reduction in actual enthalpy drop.
2. Reduction of exit velocity.
3. The final dryness fraction of steam is increased as
the K.E. converted into heat due to friction and is
absorbed by steam.
4. The specific volume of the steam is increased as
the steam becomes more dry due to frictional
reheating.
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Efficiency for convergent nozzle
How to find h1 and h2
1) Using steam table
h = hf + x hfg
s = sf + x sfg
2) Using Mollier diagram
V2th = 2 ℎ1 − ℎ2 ∗ 1000
V2act =
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2 ( ℎ1 – ℎ2 ’) ∗ 1000
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Problem 1
In a steam nozzle , the steam expands from 4 bar to 1 bar . The
initial velocity of steam is 60 m/s and the initial temp 200 0C.
Determine the exit velocity if the nozzle efficiency is 92 %
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Mollier chart
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Problem 2
Dry saturated steam enters a steam nozzle at a pressure of 15 bar and is
discharged at a pressure of 2 bar . If the dryness fraction of discharge steam is
0.96 . What will be final velocity of steam ? Neglect initial velocity of steam .
If 10 % of heat drop is lost in friction , find the percentage reduction in the
final velocity.
Solution : -
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Problem 3
Steam initially dry and saturated is expanded in a nozzle
from 15 bar at 300 0C to 1 bar . If the frictional loss in the
nozzle is 12 % of the total heat drop . Calculate the mass of
steam discharged when exit diameter of the nozzle is 15 mm.
Solution : -
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• Problem 4. A convergent divergent adiabatic steam
nozzle is supplied with steam at 10 bar and 250°c.the
discharge pressure is 1.2 bar. assuming that the nozzle
efficiency is 100% and initial velocity of steam is 50
m/s. find the discharge velocity.
• Initial pressure(p )=10bar Initial
• Temperature(T )=250°c
• Exit pressure(p )=1.2 bar
• Nozzle efficiency(η )=100%
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Solution
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Problem 5
Dry saturated steam at 6.5 bar with negligible velocity
expands isentropically in a convergent divergent
nozzle to 1.4 bar anddryness fraction 0.956. De
termine the final velocity of steam from th e nozzle if
13% heat is loss in friction. Find the %reduction in the
final velocity.
Given data:
Exit pressure (P2) = 1.4 bar
Dryness fract ion (X2) = 0.956
Heat loss = 13%
Solution: Enthalpy of saturated steam
h1 at 6.5 bar h1=2760.3 kJ/kg (from steam table)
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• Similarly, at 1.4 bar,
• hfg= 2231.9 KJ/Kg
• hf = 458.4KJ/Kg
• h2 = hf2 + X2 hfg2
• = 458.4 + (0.956) 2231.6
• h2= 2592.1 KJ/Kg
• Final velocit (V2) = 2000(ℎ1 − ℎ2)
• V2=577.39 m/s
for case 2; Heat drop is 13%= 0.13
• Nozzle efficiency (η) = 1- 0.13 = 0.87
• Final velocity 𝑣2′ = η𝑛𝑜𝑧𝑧𝑙𝑒 2000(ℎ1 − ℎ2)
• =0.87 2000(2760.3 − 2592.1)
• 𝑣2′ = 538.55 m/s
• %reduction in the final velocity.= (𝑣2
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- 𝑣2′ )/ 𝑣2 = 6.72%
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Practice problem
A convergent divergent nozzle receives steam at 7bar and
200οC and it expands isentropically into a space of 3bar
neglecting the inlet velocity calculate the exit area
required for a mass flow of 0.1Kg/sec . when the flow is
in equilibrium through all and supersaturated with
𝑃𝑉 1.3 =C
• Given Data:
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