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• A turbine is a rotary engine that exerts energy from a
fluid flow and converts it into useful work.
There are two basic types of turbines according to
the mode of steam.
1. Impulse turbine
2. Reaction turbine
It runs by impulse of steam.
Nozzle directs the steam on the curved blades, which
causes them to rotate.
The blades are in the shape of buckets.
The energy to rotate an impulse turbine is derived
from the kinetic energy of the steam flowing through
The potential energy is converted into kinetic energy
when it passes through the nozzle.
The velocity of steam is reduced when it passes over
In an Impulse turbine, the whole of the available energy of the
fluid is converted to Kinetic Energy before the water acts on the
moving parts of the turbine.
Pelton Wheel is an example of Impulse turbine.
Pelton Wheel (Impulse Turbine)
Pelton Wheels in a hydroplant Components in a Pelton Wheel
Water is blasted at these cups by one or more jets mounted in the
surrounding casing. Momentum is transferred from water to cups,
and a torque is created, causing the wheel to rotate.
This type of turbine is highly efficient.
It has no nozzle.
Two rows of moveable blades are separated by one row of
Fixed blades are attached to the casing & act as nozzles.
Blades are like the wings of a plane.
Velocity of steam is increased when it passes through
the fixed blades.
The enthalpy drop in moving blades is called degree of
A common arrangement can have 50% of enthalpy
drop in moving blades, it is said to have 50% reaction.
If all enthalpy drops in moving blades then it is said to
be 100% reaction.
There are two main types of Reaction Turbine – Francis and
Sectional and Top View of a Francis Reaction Turbine
Various types of water turbine runners.
From left to right: Pelton Wheel, two types
of Francis Turbine and Kaplan Turbine
Pressure velocity graph of impulse and reaction
Difference between impulse and
S.NO Impulse turbine Reaction turbine
The steam flows through the
nozzle and impinges on the
The steam first flows through the
guide mechanism and then through
the moving blades.
The steam impinges on the
buckets with kinetic energy.
The steam glides over the moving
vanes with pressure and kinetic
The steam may or may not be
admitted over the whole
The steam pressure is reduced
during its flow through the moving
4. The blades are symmetrical The blades are not symmetrical
Euler Head and Efficiencies of Hydraulic Turbines
Efficiency of turbines is a function of the available head.
Euler's Head: It is defined as energy transfer per unit weight.
Hydraulic Efficiency - It is the ratio of power developed by the
runner to the head of water (or energy) actually supplied to the
Mechanical Efficiency - It is the ratio of actual work available
at the turbine shaft to energy imparted to the wheel.
Overall Efficiency – The overall efficiency is based on the
useful work output divided by the water power input.
To maximize the efficiency of steam turbine the steam
is expanded, generating work in a number of stages.
Multiple stages turbines are highly efficient.
Most steam turbines use a mixture of both impulse
and reaction design.
Higher pressure sections are impulse type and lower
pressure sections are reaction type.
Advantages of steam turbine
It can develop higher speeds.
The steam consumption is less.
All the parts are enclosed in a casing so it is safe.
It requires less space and lighter foundations.
There is very less friction in the turbine due to few
Turbines are used in force draft blowers, pumps and
main propulsion turbines.
Used in the jet engines and air crafts.
They are also used in power plants , ships and
The working principle of a gas turbine also known as
‘ Internal Combustion Turbine ’ is similar to a steam
turbine, but its working fluid is the products of
combustion of fuel with air instead of steam.
A simple gas turbine consists of an air compressor
combustion chamber and a turbine. The function of
the air compressor is to compress the air required for
The fuel is burnt in the combustion chamber and the
combustion products flow through the turbine. The
function of the turbine is to convert the heat energy
into mechanical work.
GAS TURBINES AND ITS TYPES:
In this type of turbine, the compressed air from an air
compressor C admitted into the combustion chamber A
through the valve U.
When the valve U is closed, the fuel admitted into the
chamber is ignited by means of a spark plugs.
The combustion takes place at CONSTANT VOLUME with
increase of pressure.
The valve V opens and the hot burning gases flow to the
turbine T and finally they are discharged into the
The energy of hot gases is thereby converted into
mechanical work. These operations are repeated for
running the Turbine.
CONSTANT VOLUME GAS TURBINE :
The compressor C is coupled with a turbine shaft and
it is driven by the turbine T. Air taken from the
atmosphere is compressed by the compressor to a high
The compressed air then flows into the combustion
chamber M where the fuel is injected y means of a
The fuel burns at CONSTANT PRESSURE and the hot
gases flow through the turbine and finally discharged
into the atmosphere.
CONSTANT PRESSURE GAS TURBINE:
It is suitable where the supply of water is a difficult
Water required for a steam turbine imposes a serious
problems,when it is used as a portable engine.Even in
ships, when it is floating over a vast amount of water it
produces much difficulty as sea water is not suitable
for the engine.
A steam turbine requires a boiler for supply of steam
and it therefore requires additional space than what is
required by a gas turbine.
THE ADVANTAGES OF GAS TURBINES OVER