Table of Contents

What is Steam Nozzle | Types Of Steam Nozzle shapes

Introduction to Steam Nozzle ;

A steam turbine converts the energy of high-pressure, high-temperature steam produced by a steam generator into shaft work.

A steam nozzle may be defined as a passage of varying cross-section, through which heat energy of steam is converted to kinetic energy. Steam Nozzle major function is to produce steam jet with high velocity to drive steam turbines.

The energy conversion is brought about in the following ways:
1. The high-pressure, high-temperature steam first expands in the nozzles emanates as a high-velocity fluid stream.
2. The high-velocity steam coming out of the nozzles impinges on the blades mounted on a wheel. The fluid stream suffers a loss of momentum while flowing past the blades that is absorbed by the rotating wheel entailing the production of torque.
3. The moving blades move as a result of the impulse of steam (caused by the change of momentum) and also as a result of the expansion and acceleration of the steam relative to them. In other words they also act as the nozzles.

A steam nozzle is a duct or passage of smoothly varying cross-sectional area which converts heat energy of steam into kinetic energy. The shape of the nozzle is designed such that it will perform this conversion of energy with minimum loss.

Working of Steam Nozzle:

During the first part of the nozzle, the steam increases its velocity. And then the later part of the nozzle, in which the steam derives more in volume than in velocity.

Note:

1. Since the mass of steam which is passed through any section of the nozzle remains’ constant.
2. The variation of steam pressure in the nozzle depends upon the velocity, specific volume, and dryness fraction of steam.

Uses: The main use of a steam nozzle in steam turbines is to produce a jet of steam with a high velocity. The smallest section of the nozzle is called the throat.

Types of Nozzles:

1. Convergent Nozzle
2. Divergent Nozzle
3. Convergent-Divergent Nozzle

Convergent Nozzle:

A typical convergent nozzle is shown in fig. in a convergent nozzle, the cross-sectional area decreases continuously from its entrance to exit. It is used in a case where the backpressure is equal to or greater than the critical pressure ratio.

Divergent Nozzle:

The cross-sectional area of the divergent nozzle increases continuously from its entrance to exit. It is used in a case, where the back pressure is less than the critical pressure ratio.

Convergent-Divergent Nozzle:

In this case, the cross-sectional area first decreases from its entrance to the throat and then increases from throat to exit.it is widely used in many types of steam turbines.

Flow-Through Nozzles

A nozzle is a duct that increases the velocity of the flowing fluid at the expense of pressure drop.

A duct which decreases the velocity of a fluid and causes a corresponding increase in pressure is a diffuser.
The same duct may be either a nozzle or a diffuser depending upon the end conditions across it. If the cross-section of a duct decreases gradually from inlet to exit, the duct is said to be convergent.
Conversely if the cross-section increases gradually from the inlet to exit, the duct is said to be divergent.
If the cross-section initially decreases and then increases, the duct is called a convergent-divergent nozzle.
The minimum cross-section of such ducts is known as the throat.
A fluid is said to be compressible if its density changes with the change in pressure brought about by the flow
If the density does not change or changes very little, the fluid is said to be in-compressible. Usually the gases and vapors are compressible, whereas liquids are incompressible.

Shapes of nozzles

1. At subsonic speeds (Ma<1) a decrease in the area increases the speed of flow.
2. In supersonic flows (Ma>1), the effect of area changes are different.

Significance of Mach number –

If the Mach number is less than one, flow is sonic, and the nozzle is convergent.
If Mach number is equal to one, flow is sonic.
If Mach number is greater than one, flow is supersonic and the nozzle is divergent.

Nozzle efficiency:

It is the ratio of useful heat drop to the isentropic heat drop. It is denoted by K

K = Useful Heat Drop / Isentropic Heat Drop

How were we finding Useful heat drop & isentropic heat drop, so let’s discuss some given useful points for finding nozzle efficiency?

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 Figure:

Process of h-s Diagram:

1. First of all, locate the point A for the initial conditions of the steam. It is a point, where the saturation line meets the initial pressure (P1) line.
2. Now draw a vertical line through A to meet the final pressure (P2) line. This is done as the flow through the nozzle is isentropic, which is expressed by a vertical line AB. The heat drop (h1 – h2) is known as the isentropic heat drop.
3. Due to friction in the nozzle the actual heat drop in the steam will be less than (h1- h2). Let this heat drop be shown as AC instead of AB.
4. As the expansion of steam ends at the pressure P2, therefore a final condition of steam is obtained by drawing a horizontal line through C to meet the final pressure (P2) Line at B’.
5. Now the actual expansion of steam in the nozzle is expressed by the curve AB’ (adiabatic expansion) instead of AB (isentropic expansion). The actual heat drop (h1 – h3) is known as a useful heat drop.

The efficiency of a nozzle generally varies from 0.85 to 0.95.

Factors affecting nozzle efficiency

1. Material of the nozzle.
2. Size and shape of the nozzle.
3. Finish of the nozzle.
4. Angle of divergence.
5. Nature of the fluid and its state,
6. Friction.
7. Fluid velocity.
8. Turbulence in the flow passages

Selection Criteria of nozzle.

Situation first: It is used when the backpressure is equal or more than the critical pressure ratio. It is also used for non – compressible fluids.

Convergent nozzle: Cross-sectional area is decreased continuously from entrance to exit.

Situation second: When back pressure is less than the critical pressure divergent nozzle is used.

Divergent nozzle: Cross-sectional area is increases continuously from entrance to exit.

Situation third: When back pressure is less than the critical pressure convergent-divergent nozzle is used.

Convergent and Divergent nozzle: Cross-sectional area of nozzle first continuously decreases and then increases from entrance to exit.

Application Of Steam Nozzle :

Steam and Gas turbine –
Jet Engine
Rocket Motors-
It is used to measure the discharge of fluid.- e.g. Venturimeter
Injectors for pumping feed water to boilers.
The supersonic gas turbine engine: for the air intake when the air requirement of the engine is high.
Rockets: for providing sufficient thrust to move upwards.
For removing air from the condenser using the injector.
Spray painting
Steam jet refrigeration system