Comparison between Impulse and Reaction Steam Turbines

In Impulse Turbine, steam completely expands in the nozzle itself. Hence its pressure remains constant on both ends of the moving blades.

In Reaction Turbine, Fixed blades act as nozzles. Hence steam expands both in fixed and moving blades continuously as it passes over them. Thus the pressure drop occurs gradually and continuously over both the fixed and moving blades.

In Impulse Turbine, Blades Passage is of constant cross section area, as there is no expansion of steam.

In Reaction Turbine, Blade passage is of variable cross-sectional area (converging type) due to expansion of steam.

In Impulse Turbine, As pressure remains constant in moving blades, the relative velocity of steam passing over the moving blades remains constant

In Reaction Turbine, Continuous expansion of steam means relative velocity in the moving blades increases.

In Impulse Turbine, Blades are of symmetrical profile types : hence, manufacturing of blade is simple.

In Reaction Turbine, The blade shapes are of aerofoil and non-symmetrical type; hence manufacturing is difficult.

In Impulse Turbine, Because of large pressure drop, the steam speed and the running speed are high.

In Reaction Turbine, Due to small pressure drop, the steam speed and the running speed are low.

In Impulse Turbine, Because of large pressure drop in the nozzles, the number of stages are less. The size of an impulse turbine for power output is comparatively small.

In Reaction Turbine, Because of small pressure drop in each stage, the number of stages are more for the same pressure drop. Hence the size of the reaction turbine for the same power output is large. Reaction turbines are multi-stage turbines only.

Impulse Turbine Occupies less space per unit power

Reaction Turbine Occupies more space per unit power

Impulse Turbine is Suitable for small powers

Reaction Turbine is Suitable for medium and higher powers.


Reaction turbine

Description of reaction turbine
It consists of a wheel or rotor, casing, fixed and moving blades. In this type, equal number of fixed and moving blades are attached alternately to the casing and the wheel respectively. The fixed blades is similar to a nozzle where velocity increases with decrease of pressure.

Working Principle

In reaction turbine, the steam is not expanded in the nozzle, but expands as it flows over the blades.

The steam passes over the fixed blade F. The fixed Blade changes the direction of steam and at the same time allows it expand to a higher velocity, with decrease of pressure.
Then the steam passes over the moving blade M. The moving blade converts the kinetic energy into mechanical work with decrease of velocity; but at the same time steam expands as it flows over the moving blade and there is a fall of pressure. This produces a reaction on the blades, by the expanding steam.

Thus in the reaction turbine the steam expands both in fixed and moving blades continuously as the steam passes over them. Therefore, the pressure drop occurs gradually and continuously over both fixed and moving blades. Parson turbine is an example of reaction turbine.


Compounding of Impulse Turbines

The disadvantage of De Laval type of turbine is that it’s extremely high speed, of the order of 30,000 rpm, cannot be employed for practical purposes. To reduce the high speed, more than one set of blades are used. This is called “compounding of impulse turbine”.

In the compounding method, the steam jet velocity or the steam pressure is absorbed in stages as it flows over the rotor blades. When “steam velocity” is absorbed in stages, it is called “Velocity compound impulse turbine”. When “steam pressure” is absorbed in stages it is known as “pressure compound impulse turbine”.

Velocity Compound Impulse Turbine:

In velocity compound impulse turbine, moving and fixed blades are placed alternately. Moving blades are fitted with the wheel while the fixed blades are fitted with the casing.

The steam is expanded in the nozzle from the boiler pressure to condenser pressure, to a high velocity. It is then passed over the first ring of moving blades. Only a portion of the high velocity of steam jet is absorbed by this blade ring, the remainder being exhausted on to the next ring of fixed or guide blades. These fixed blades change the direction of steam jet.

The jet is then passed on to the next ring of moving blades. A further portion of the steam velocity is now absorbed by this second moving blade ring. The process is then repeated as the steam flows over the remaining pairs of blades until practically all the velocity of the jet has been absorbed and the kinetic energy is converted into mechanical work.

It should be noted that the entire pressure drop takes place in the nozzle itself, the pressure remaining constant, as the steam flows over the blades. Hence the turbine is an impulse turbine. The Curtis turbine is an example of velocity compound impulse turbine.

Pressure compound impulse turbine:

In this type the expansion of steam takes place in more than one set of nozzles and each set of nozzles is followed by a set of moving blades. The total pressure drop of the steam does not take place in the first set of nozzles, but is divided up equally between all the sets of nozzles.


Impulse Turbine

Description of impulse turbine:

Wheel or Rotor :
The wheel or rotor is fitted over a shaft from which the useful power is available. It is a rotating element of the turbine on which moving blades are fixed.

The nozzle is a passage for the flow of steam where pressure energy is converted into kinetic energy. Its main function is to produce a jet of steam with a high velocity.

Blades :
De Laval turbine shown in the image below is an example of simple impulse turbine.
In this, only one set of impulse type blades is rigidly fixed to the rim of the rotor or wheel. It converts the kinetic energy of steam into mechanical work.

Casing :
The casing is the outside cover of the steam turbine fixed over a frame. It is fitted with nozzle.

Working Principle of Impulse turbine :

If a jet of steam is discharged from a fixed nozzle at a high speed over a flat stationary plate, a steady force will be exerted over this plate. This force is nothing but an impulse. No work is done as the plate is fixed. But, if a number of such plates are fixed on the rim of a wheel, the wheel may be rotated due to the impulse of steam. Curved plates are used instead of flat plates to utilize greater amount of energy.

In the impulse turbine, steam is expanded in the fixed nozzle only. In the nozzle the velocity of steam increases with decrease of pressure. As the steam passes over the blades, the pressure remains constant with a decrease of velocity.

As the high velocity steam impinges against the baldes, it changes the momentum of jet causing impulsive force on the blades. The wheel is thus made to rotate in a definite direction.
Here the kinetic energy is converted into mechanical work, only by one set of blades. It is simplest type of impulse turbine.


Steam Turbine | Prime Movers

The steam turbine is universally used as prime mover in steam power plants.

Flow over Blades:

The steam turbine obtains its motive power from the change of momentum of a jet of steam flowing over a curved blade. The steam jet, in moving over the curved surface of the blade, exerts a pressure on the blade owing to its centrifugal force. This centrifugal force is exerted normal to the blade surface as shown in figure and acts along the whole length of the blade.

The resultant of these centrifugal forces plus the effect of change of velocity is the motive force on the blade. It should be realized that the blade obtains no motive form any impact of the jet, because the blade is so designed that the steam jet will glide on and off the blade without any tendency to strike it. In principle, it is analogous to a train passing around a railway curve. The train exerts a radially outward force on the line due to the centrifugal force.

Moving and Fixed Blades

In a steam turbine, a number of small blades are fixed to the ring of a revolving wheel or rotor. Jets of steam of a high velocity are obtained by expansion through nozzles and are directed on to the blades. The effective force of these jets, acting on the blades, rotates the wheel.

In modern turbines several of the wheel of moving blades are keyed to the same shaft, having a ring of fixed blades between each wheel of moving blades. These fixed blades are fixed to the turbine casing. Their object is to receive the steam jet from the moving blade ring and to divert it on to the next ring of moving blades by changing its direction as shown. This diversion may continue over several rings of moving and fixed blades until the whole of the kinetic energy of the steam jet is expended.

In the next post, I will explain about the impulse turbine and reaction turbine.


Combined Cycles

Combined cycle are obtained by coupling a steam power plant (or sometimes a diesel engine plant) with a gas turbine installation. These systems are used to increase the overall efficiency of the gas turbine cycle, as the efficiency of the basic gas turbine plant is as low as 20 to 25%.

There are a number of combinations of combined cycles. However, we shall discuss the following two important combined cycle systems:

Combined cycle with reheat of exhaust:

The image above shows a gas turbine plant couple with a steam plant.

Gas Turbine :
Atmospheric air is drawn into the air compressor, when it is compressed to a high pressure. Fuel (Natural gas or Gasified Coal) is injected into the combustion chamber, CC and burns in the steam of compressed air. The products of combustion, comprising a mixture of gases at high temperature and pressure, are passed to the turbine. Products of combustion are expanded in the gas turbine and electric power is generated in the generator G1.

It should be remembered that the exhaust gases from the gas turbine have a temperature of 400 to 500’C and contain about 40 to 50% of the initial heat energy.

Steam Plant with Reheat Arrangement:

As seen in the image above, the exhaust gases from the gas turbine are heated in a reheater (RH). The hot flue gases then pass through the boiler to generate steam. This high pressure steam from the boiler is used to drive a steam turbine. Thus electric power is generated in the generator G2.

This combined cycle recovers much of the exhaust heat energy by reheating the high temperature exhaust gases from the gas turbine and passing to the heat recovery boiler for power generation.

Combined cycle with cogeneration arrangement:

The topping cycle system shown in the above image is another type of combined cycle with cogeneration arrangement. In this arrangement, the exhaust heat from the gas turbine is used for steam generation in a waste heat recovery boiler. Steam from the boiler is used for process plant.

Advantages of combined cycles

  1. The overall efficiency of the combined cycle plant is nearly 47 to 42% which is much higher than a simple gas turbine or stem plant.
  2. The capital cost of combined plant with supplementary firing is less than the combined cost of separate units of gas turbine and steam turbine plants for the same power capacity.
  3. The cooling water requirement of a combined cycle is much less than a pure stem plant having the same output. The reason is no cooling water is required for the gas turbine.
  4. The combined plant is more suitable for quick start and shut down than steam plant. Whether natural gas or gasified coal is the fuel, combined cycle greatly reduces CO2 emissions. Thus environmental pollution is minimized.
  5. By using combined cycle plants, energy resources could be used efficiently so that they last longer. Thus the concept of combined cycle systems will promote energy conservation.


Applications of Cogeneration

Industrial cogeneration systems have received an impetus in recent years. The potential savings of cogeneration are significant. A gas turbine cogeneration system has been installed at ONGC, Uran with a capacity of 40 MW.

Gas turbine with heat recovery, steam turbine with heat recovery and diesel engine with heat recovery is the different cogeneration schemes used in traditional power plants.
Cogeneration systems are eminently suitable for various process industries like rayon, pulp and paper, chemical process, textile and fertilizer where both power and process system are used.

A few typical examples of industries where cogeneration systems could be utilized are:

  1. In industries, such as rayon, pulp and paper, chemical processing, and textile, which require simultaneous steam and power, it is possible to meet either part or full heat and power requirements using steam turbine, gas turbine with heat recovery boiler.
  2. Cement kilns and brick kilns require a large amount of high temperature process heat. The gas turbine exhaust, with or without supplementary firing, can supply this heat and produce electric power for the factory.
  3. In glass melting furnaces, heat from the exhaust gases can be recovered in waste heat boiler to produce steam. The steam can be expanded in steam turbines to produce electrical power.


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