A steam turbine is one powerful turbomachine where thermal energy is utilized to produce mechanical energy. All steam turbines can be classified into different types based on the number of cylinders, general flow, means of heat supply, Operating principle and direction of flow.
Further, they can also be classified based on exhaust conditions, casting or shaft design and type of drive. But the two basic types of steam turbines are impulse and reaction type.
Based on the number of cylinders steam turbines are of two types; single cylinder and multi-cylinder turbines. While based on the flow of steam they can be classified as a single flow, double flow and reverse flow turbines.
Similarly, based on exhaust conditions, there are five main types of steam turbines; condensing, non-condensing, automatic extraction, mixed pressure, and regenerative extraction.
We will discuss in details about all these turbine types and their classifications down below. But before we proceed you must have a clear idea about; what is turbine, how does it work, Its basic parts and applications.
Types of Steam Turbine & Their Classifications
A ) Its Types Based On Principle of Operation:
( i ) Impulse Turbine: Impulse turbines have this assembly of the fixed nozzle and moving blades; where the steam flow through the system with little to no pressure change between the inlet and outlet of moving blades.
Thus rotor blades are subjected only to kinetic energy. Since the drop in steam pressure took place across nozzles and not the rotor blades.
In Impulse turbine, since there is no effect of rotors on the acceleration of steam flow. These are generally multistage turbines with higher operating speed and better efficiency than a reaction turbine and often require less maintenance.
( ii ) Reaction Turbine: Also known as the impulse reaction turbine there is a pressure drop across both nozzle and rotor blades. Here the pressure drops as it passes through the moving blades adding more kinetic energy to the subsequent blades in front.
Here the blades are subjected to both pressure and kinetic energy. This is achieved by having a varying cross-section area for the blade passage.
Used mainly for electrical production these are a little less efficient but silent. In Reaction turbine, there is a gradual drop in pressure across the turbine blades. Thus resulting in a large or longer turbine size for the same power.
B ) Its Types Based On Number of Cylinders
( i ) Single Cylinder Turbine: Single cylinder steam turbine has all its stages housed within the same casing. Here the turbine uses one shaft for all its different stages. this allows for better flexibility, simple design and lower cost.
In a typical single-cylinder turbine after the expansion of steam to its lowest pressure it is rejected only to be used for heating purposes.
The best thing about single cylinder turbines is that they can be run with much lower steam flow than normal. Overall they satisfy a higher demand for large capacity at a much lower cost than multi-cylinder turbines.
( ii ) Multi-Cylinder Turbines: These are large output turbines with too many multiple stages to support all of them on one shaft. Under these circumstances, their stages are housed under different casing with separate shafts and bearings.
Here the low, medium and high pressure cylinders leans against the transverse pin mounted on the bearing housing.
Further, they need to maintain minimum axial clearances between adjacent rotor and stater blades. The number of cylinders in a multi-cylinder turbine depends on its terminal conditions and design considerations.
For example a multi stage turbine with 900 MW output capacity would need two LP turbines, one intermediate or MP turbine and another one HP turbines.
C ) Its Types Based On Means of Heat Supply
( i ) Single Pressure Turbine: These are the typical turbines assembly used for generic use and have a single source of steam supply. Other than that they are also used in electrical applications and small industries.
( ii ) Reheat Turbines: For large multi-stage turbines there needs to be steam reheating before the steam exit the turbine. This is done to avoid corrosion of blades and improve efficiency. Such turbine types are commonly called as reheated turbines.
This is achieved by a re-heater incorporated within the boiler; that increase the steam temperature to maintain them at superheated condition throughout the entire stage.
( iii ) Dual Pressure Turbine: A mixed or dual pressure turbine use two separate source of steam at different pressure. Typically used in nuclear reactors they are also used for heavy industrial operation with varying demand for load.
These turbines usually operate at low-pressure steam but have inbuilt provisions for admitting high-pressure steam to the high-pressure stages.
Here one source of steam is always more economical than others while the other provides quick speed and more power. For example, they may be required to start on one pressure and operate under different steam pressure.
D ) Its Types Based On Direction of Steam Flow
( i ) Axial Flow Turbine: The most suitable turbine design for large turbo-generators; here the direction of flow is parallel to the axis of the shaft. An axial turbine has one or more stages where the flow is accelerated in a nozzle and passed on to rotor blades.
Axial flow turbine is similar to axial compressors in construction but operates in reverse order. Further because of their simpler design they are easier to use and maintain.
The most simplest example of an axial turbine is “Kaplan turbine”. Now since these turbines work with high positive gradient. We can have a much larger pressure drop across stages.
( ii ) Radial Flow Turbine: As the name suggests here the steam flows in a radial direction to the rotor shaft axis. Here we have a turbine incorporating two shafts with each driving one shaft. Further, they have a disc with radial flow blades fitted to the shaft.
Here we achieve a relative speed twice that of the running speed. this is achieved by having two sets of blades fitted counter to each other.
These turbines can be quickly warmed and started; making is quite suitable at times of peak load. But due to formidable design challenges at the larger size, they are largely in smaller size up to 300-350 MW demand.
( iii ) Tangential Flow Turbine: These are types of turbines where the steam flows tangentially to the rotor shaft axis. These are generally quite robust and efficient machinery used mostly as the auxiliary turbines in power generation.
Tangential flow turbines are generally used in hydropower generation. Thus generally not operated by steam. The most common example of a tangent flow turbine is the Pelton turbine.
E ) Its Types Based On Exhaust Condition
( i ) Condensing Turbine: These are turbines with multiple stages and outlets where extract energy from high-pressure steam while the other stage extracts the remaining energy from the low-pressure steam extract from the first stage or outlet.
From the second stage, the steam goes to the condensation chamber then hot well and back to the boiler. Now because here the steam recirculates within the same system this turbine are also termed as a regenerative steam turbine.
Further, these turbines have a steam regulating valve so as to regulate the output as per the change in demand.
Because of their complex configuration and costly installations, these are limited to large industries and power plants generating electric power.
( ii ) Non Condensing Turbine: Its a simple turbine configuration that uses a high pressure steam for rotating the blades. The steam pressure at the outlet depends on the load, thus can be anywhere in between atmospheric pressure and low pressure steam.
These turbines are quite cheap and simple in construction. Have a better efficiency at a fixed load but are highly flexible.
Non-condensing turbines are used mostly in CHP plants to produce steam for industrial or heating uses. Here the output steam is often wet steam with a pressure always above 1 atm. These are used where the installation and running cost needed to be less and substantial amounts of heating is required.
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