In high-pressure steam boilers, the total pressure drops from boiler pressure to condenser pressure in a nozzle due to the expansion of steam. And it causes high-velocity steam at its exit.
The required speed of the rotor would be approximate 30000 RPM if the high-velocity steam expanded over the single ring of the moving blade. As this speed is too high for practical purposes, various methods are adopted to absorb either the steam pressure or the jet velocity in stages to reduce the rotor speed.
The method of reducing speed is known as the compounding of steam turbine.
Each compounding method consists of multiple rotor systems keyed to the shaft with several stages arranged in series.
There are four methods for the compounding of steam turbine.
- Velocity Compound Impulse Turbine (Curtis Turbine)
- Pressure Compound Impulse Turbine (Rateau Turbine)
- Pressure-Velocity Compound Impulse Turbine
- Reaction Turbine
Velocity Compound Impulse Turbine (Curtis Turbine)
In a velocity compound impulse turbine, the steam expands in a set of nozzles, from the boiler pressure up to the condenser pressure that converts the pressure energy into kinetic energy.
The high-velocity steam passes over the ring of moving blades. Each ring of moving blades is separated by a ring of fixed blades.
A part of high-velocity steam is absorbed in the first ring of moving blades and the remaining is passed to the next ring of fixed blades.
The fixed blades are used to change the direction of the flow of steam. Hence, it can glide over the second ring of moving blades.
The velocity of steam that passes over the fixed blades is constant except for the energy lost for overcoming the friction losses.
Similarly, again a part of steam velocity is absorbed in the second ring of moving blades, and the process of absorbing the steam velocity continues till it is finally wasted to exhaust.
The below figure represents the variation of pressure and velocity of steam passing over the sets of fixed and moving blades along the axis of the turbine.
Advantages of Curtis Turbine
The advantages of a Curtis turbine are listed below.
- Due to relatively large heat drop, a velocity compounded impulse turbine required a comparatively small number of stages and less space.
- The cost of a turbine is low.
- The optimum blade speed ratio decreases with an increase in number of stages.
- The pressure drops in nozzles. Therefore, the turbine casing is not required to be designed to withstand high pressure.
Disadvantages of CurtisTurbine
The disadvantages of a Curtis turbine are listed below.
- The velocity of steam is high. Hence, the friction loss is high.
- Efficiency is low and continuously decreases with a number of stages.
- Power developed in later stages continuously decreases while these stages require the same material space and cost of fabrication.
Pressure Compound Impulse Turbine (Rateau Turbine)
In a pressure compound impulse turbine, the total pressure will not drop in a single ring of nozzles. But it is divided up between the set of nozzle rings.
In the first ring of a nozzle, partially steam is expanded. Then it is passed over the ring of moving blades till its velocity is absorbed.
For further expansion, the exhaust from the moving blade ring is passed over the second ring of the nozzle. And its increased velocity is absorbed in the second ring of moving tables.
A similar process repeats till the steam is expanded up to the condenser pressure.
The velocity of steam is not very high as the steam is partially expanded in each ring of nozzles. Therefore, the turbine velocity is low.
Pressure-Velocity Compound Impulse Turbine
Both the previous methods of velocity and pressure compounding are utilized in this method.
The total pressure drop of steam is divided into stages. And due to expansion in each stage, the velocity of steam is obtained.
In this method, the number of stages is less. Hence, the size of a turbine is small compared to a pressure-compounded turbine.
The illustration of a pressure-velocity compounded impulse turbine is shown in the figure below.
In an impulse turbine, nozzles are provided. But, in the case of a reaction turbine, the nozzles are not provided.
In a reaction turbine, pressure continuously drops in the rings of fixed and moving blades.
The fixed blades act as nozzles and it is used to change the direction of steam. Hence, it can enter the next ring of the moving blade without shock.
The steam expands over the ring of moving blades giving a reaction to the moving blades. Therefore, this turbine is known as a reaction turbine.
The steam is continuously expanding. So, the steam velocity and turbine velocity are not high.
The diameter of the turbine must increase after each set of blades to feed the increased specific volume of steam at lower pressure in each of the successive stages.
So, we have discussed all four methods for the compounding of steam turbine.
Difference Between Impulse and Reaction Turbine
The difference between the impulse and reaction turbine of a thermal power plant is shown in the table below.
|Pressure drops only in nozzles, not in moving blades.||Pressure drops in nozzles and moving blades.|
|Steam does not enter all around the cross-section.||Steam enters from all around the cross-section.|
|It cannot develop much power.||It can develop much power.|
|For the same power, it requires less space.||For the same power, it requires more space.|
|The impulse turbines are suitable for small power generation. Hence, these turbines are suitable for industries.||The reaction turbines are suitable for medium and high-power generation. Hence, these are suitable for large power plants.|
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