What is Steam Turbine | How does a Steam Turbine work?

Turbines are using all over the world for the production of electricity. The use of turbines is increasing day by day. There are multiple types of turbines that are used according to the application requirements. A steam turbine is a most common type of turbine which is also used worldwide to generate cheap electricity. According to the working fluid, turbines have four major types:

  1. Water turbine
  2. Gas turbine
  3. Wind turbine
  4. Steam Turbine

In the previous articles, we discussed gas turbine, water turbine, and wind turbine. Therefore, in this article, I will only explain different aspects of the steam turbine with the help of diagrams.

What is a Steam Turbine?

A steam turbine is a mechanical device that transforms the thermal power of steam into mechanical work in form of rotational energy. This turbine is known as a steam turbine because it uses steam as a working fluid. In 1884, the first steam turbine was discovered by Sir Charles A. Parsons.

In this turbine, the mechanical work generates with the help of the turbine shaft. This shaft is coupled with the steam steam generator which converts the shaft mechanical power into electrical power. 

steam turbine

Steam has a benefit over water: the amount of steam expands very quickly. The speed of the steam turbine is directly proportional to the output power. Therefore, the steam turbines must work at the highest speed if you want to achieve the highest output. The wheel turbines can’t rotate at high speed like a steam turbine.

It can attain maximum efficiency by using the kinetic energy of the steam. These turbines have many advantages over other types of turbines such as: it produces inexpensive electrical, and steam energy doesn’t pollute to the environment.

Due to these reasons, these turbines have displaced reciprocating engines as prime movers in large power plants. The steam turbines work on the basic principle of thermodynamic. That is, when the steam expands, its temperature drops.

Steam Turbine Working Principle

The steam turbine working principle is very simple. A steam turbine works on the basic principle of thermodynamic.  

steam turbine working
Fig: Steam Turbine Working

Image source: https://schoolworkhelper.net/

During the working of steam turbine, first of all,  water from an external source (such as a river, sea or canal)  is transfered into the boiler section with the help of a pump. Then, the water in the bowler is heated to a very high temperature so that water can convert it into supersaturated steam.

In the boiler, the steam generation varies according to the combustion heat and the flow rate, and the heat transfer surface area used. As the boiler is produced steam, the steam is directed into the turbine area. Actually, in these turbines, the pressure energy of the steam is transformed into K.E  after passing the steam via a nozzle.

As the steam strikes with the rotor blades, it creates dynamic pressure on the shaft and rotor blades. Due to this reason, both the shaft and the blade start rotating in an identical direction. Due to this  process, the steam’s thermal energy  transforms into the rotational energy of the rotor blade, and the rotor starts rotating. A shaft is coupled with the turbine rotor. The shaft receives rotational energy from the rotor, and starts rotating. 

A generator called a steam generator connects to the shaft via a coil. The shaft rotates the generator  coil in a magnetic field. As the coil rotates in a magnetic field,  the electricity generates and flows inside the wires. 

Due to the simple structure of these turbines, the vibration is much lower than with other engines with the same speed. 

Steam Turbine Types

There are multiple steam turbine types according to their different operations and their industrial importance. The most major types of steam turbines are given below:

1) Based on the point of steam entry

  1. Center admission turbine
  2. End admission turbine

2) Based on the application

  1. Marine turbine
  2. Industrial turbine
  3. Utility turbine

3) Based on the pressure of the turbine

  1. Low-pressure turbine
  2. Medium pressure turbine
  3. High-pressure turbine

4) Based on the exhaust condition of the turbine

  1. Extraction cum condensing turbine
  2. Extraction turbine
  3. Backpressure turbine
  4. Straight condensing

5) Based on steam flow

  1. Radial flow turbine
  2. Axial flow turbine

6) Based on blade design

  1. Reaction Turbines
  2. Impulse Turbines

1) Based on the Exhaust Condition of the Turbine

In this category, the steam turbine has the following three types.

i) Condensing Steam Turbines

In these types of steam turbines, steam introduces into the turbine by a control valve. As the name of condensing turbine represents that the steam inside the turbine can’t expand because this turbine is for condensing purposes. Besides, the blade will get wet during the final phase. The exhaust steam condenses in the condenser and condenser transforms this steam into water. This condensed water is again used in the boiler to generate steam. These turbines are most common in hydroelectric power plants.

Condensing Turbine

ii) Back Pressure Steam Turbine

In this turbine, the steam in the turbine doesn’t expand completely. After partial use of the steam thermal energy inside the turbine, all of the steam is released at a specific temperature and pressure. The steam parameters at the discharge are determined according to the process requirements.

Back Pressure Steam Turbine

iii) Extraction Cum Condensing Turbine

Extraction cum condensing turbine has two inlet valves. The first stage of the turbine is known as the “High Pressure (HP) stage,” and the second stage is known as the “Low Pressure (LP) stage.” As the HP phase completes, some steam is released. The remaining steam enters into the LP stage, where it is condensed further at low pressure.

extraction cum turbine

2) Types According to the Heat Drop Process

In this category, the steam turbines have the following types:

i) Condensing turbine with generator

In this type of turbine, steam is sent to the condenser chamber at a pressure below than atmospheric pressure. In this turbine, the steam  discharges from the intermediate stage and used to heat the feed water. The exhaust steam’s latent heat during the condensation process is dropped completely .

ii) Condensed turbines with various intermediate extraction stages

In this steam turbine type, the steam is discharged from the intermediate phase and used for industrial heating applications.

iii) Back Pressure Turbines

 In the back pressure turbine, the exhaust steam utilizes for heating or industrial applications. It is also possible to use a reduced vacuum turbine where the exhaust stream can be utilized for heating and processing applications. These turbines are also known as non-expanding turbines.

The turbine mechanical energy uses to operate mechanical or electrical devices such as compressors, fans, and pumps, etc. These steam turbines  have an easy configurations. They need very low or no cooling water. These turbines have a low price as compared to extraction steam turbines. Back pressure steam turbine doesn’t reject heat during the condensation; therefore, it has high efficiency.

turbine
        Fig: Back Pressure Turbine
iv) Topping Turbine

In the topping turbine, the exhaust steam uses in low and medium pressure condensing turbines. The topping turbine turbine work under higher initial steam temperature and pressure conditions and are used primarily to expand the capacity of power plants.

topping turbine
        Fig: Topping Turbine Cycle

3) Types according to the steam conditions at the turbine inlet

The steam turbine has the following types in this category: 

i) Supercritical Pressure Turbines

These turbines use steam with a pressure greater than 225 atm.

Supercritical Pressure Turbine

ii) Ultra-high Pressure Turbine

It uses a temperature of 550° C or more and a steam pressure of 170 atm or more.

Turbine

iii) High-Pressure Turbine

It uses steam with pressures more than 40 atm.

iv) Medium Pressure Turbine

These turbines consume up to 40 atm of steam pressure. 

v) Low-Pressure Turbines

These types of steam turbines use steam with a pressure of 1.2 atm to 2 atm.

4) Steam turbine types according to industrial use

According to the industrial applications, the steam turbine has the following types.

i) Stationary turbines with constant speed

These turbines are mainly used to drive alternators.

ii) Stationary turbine with variable speed

These turbines are used to power pumps, air circulators, turbofans, and more.

iii) Variable speed transient turbine

These turbines are typically used on railroad locomotives, ships, and steamers.

5) Types According to Blade Design

According to the design of blades, steam turbines divide into two main types.

  1. Reaction Turbine
  2. Impulse Turbine

Impulse vs reaction turbine

i) Reaction Turbine

Main article: Reaction Turbine

In the case of the reaction turbine, the steam flows through the vanes. Then, it expands on both the moving vanes and fixed vanes of the turbine. Moving and fixed vanes have a continuous pressure drop. The Reaction turbines are a little bit dissimilar from the impulse turbines, which consist of fixed nozzles and moving vanes. As compare to impulse turbines, reaction turbines have a lower pressure drop per stage. A reaction turbine is generally more efficient.

reaction turbines

An example of a reaction turbine is a Parson’s turbine. The reaction turbine requires twice as many rows of vanes as the impulse turbine for the conversion of the same heat energy. So, this builds the reaction turbines heavier and longer.

ii) Impulse Turbine

Main article: Impulse Turbine

It includes the famous steam turbine types. In the case of an impulse turbine, the steam exits the fixed nozzles at a very high speed and hits the fixed vanes around the rotor. The blade deflects the steam flow deflection without changing the pressure. The shaft of the turbine rotates due to the changes in impulse.

impulse

In these turbines, steam, which is injected at very high speed from a fixed nozzle, hits the blades attached to the rotor. The blade changes the path of the steam flow without varying the steam pressure. The force generated by the change in torque causes the turbine shaft to rotate. Examples: Rateau, Curtis, and De-Laval, Turbine.

Components of a Steam Turbine

The steam turbine has the following major parts:

1) Housing :-

The housing bears all low weight and high weight operating loads. The housing has a rotor, blades, governor, and many other internal components. It designs in such a way that it minimizes the thermal load. It provides safety to all the internal parts of the steam turbine.

2) Rotor :- 

The rotor has multiple buckets that rotate with the rotor movement. It has a shaft. One side of the shaft uses to connect the driven pump, and the other end of the shaft uses for the speed controller and the quick speed trip system. It is a key part of a steam turbine that converts the thermal energy of the steam into mechanical power.

3) Blades :-

These blades use to extract the energy of the high-velocity steam and transfer it to the rotor. The design of these blades plays an important role in turbine efficiency.

4) Governor :-

The governor system is a speed-dependent control system that installs in a steam turbine. It is also known as a Controller. It uses to control the speed of the turbine. A governor valve installs to control the turbine speed by changing the flow of steam by the turbine. It has a servo motor system, a counterweight with a spring return, and a steam valve.

This turbine component records the speed of the turbine shaft via a direct assembly or a magnetic pulse from the gear. The variation in the outlet and inlet conditions of the steam turbine and changes in the power required from the pump cause changes in the speed of the turbine. This variation in speed causes the governor weights to be rearranged, followed by the governor valves.

Steam Turbine Efficiency

Many aspects affect the steam turbine’s efficiency including the size and type of the turbine as well as temperature, and pressure of the inlet steam and exhaust. It also depends on the flow rate of steam.

These turbines are best appropriate for large thermal power plants. There are different sizes of turbines up to 1.5 GW for generating electrical power.

How to Calculate Steam Turbine Efficiency

A steam turbine has two different types:

  1. Impulse reaction turbine
  2. Reaction steam turbine

These both types of steam turbines work on different principles, as discussed above. Therefore, these have different efficiency, but the below-given formula can calculate the efficiencies of these turbines:

efficiency formula

In the above-given equation, the input kinetic energy varies according to the absolute velocity of the steam at the turbine inlet. While the work done is dependent on many factors such as the steam’s relative velocity, reduction in the amount of steam heat in the turbine, the angle of the blade, and the guide vane’s angle at the turbine inlet. In some cases, due to these factors, it is very hard to compute the work done, and sometimes it is impossible to precisely calculate some specific features such as steam pressure, temperature, and velocity.

Following are two various methods to calculate steam efficiency:

  1. Blade efficiency (ɳb)
  2. Stage efficiency (ɳs)

The steam velocity is used to calculate the blade efficiency (ɳb). In contrast, the variation in the steam enthalpy uses to calculate the stage efficiency (ɳs). The enthalpy describes the heat capacity of steam. In both cases, the guide vane angle on the inlet side is indicated by α1, and it performs a vital function in turbine efficiency. The cosine of this angle performs a central function in determining the impulse and reaction steam turbines efficiencies.

The below-given diagram represents the blade efficiency for the impulse and reaction turbines.

blade efficiency of impulse and reaction turbines
Figure: Blade efficiency of the reaction and impulse steam turbines

The above diagram is indicating clearly that an impulse turbine is less efficient than a reaction steam turbine.   

The highest efficiency of the impulse turbine can be attained by setting the angle of the inlet blade at zero. This is because this angle minimizes friction by decreasing the blade’s surface area. You can also connect multiple turbines in series to maximize steam energy before the steam is returned to the condenser. The stage efficiency computation technique works best in this type of turbine assembly.

Steam turbine efficiency formula

The steam turbine efficiencies can be calculated by the following formulas:

Isentropic Efficiency :- It is a ratio between the actual work and the Isentropic work of the turbine.

Isometric efficiency

CHP Electrical Efficiency :-  It is a ratio between the net electricity generated and total fuel in the boiler. The following equation uses to calculate it:

CHP electrical efficiency = Net electricity generated/Total fuel into boiler

Total CHP Efficiency :- It uses to measure the electricity and steam produced by the total fuel inside the boiler. Total CHP efficiency can be calculated by the following formula.

Total CHP efficiency = (Net steam to process + Net electricity generated)/Total fuel into boiler

How to Improve Steam Turbine Efficiency

There are different methods to improve the efficiency of the steam turbine, and some of them are given below:

  • Spill bands may also use to lessen throttling loss.
  • The friction losses can be decreased by using highly efficient blades/nozzles.
  • An exhaust air baffle can be used to reduce the pressure in the exhaust housing.
  • Specific properties used in a particular application are generally based on a trade-off between capital investment and the cost of generating steam over the turbine lifetime. So, this is an optimization method.
  • In a water preheater, under specific conditions, the expanded steam extracts by the turbine. After extraction, this steam utilizes for water heating purposes just before transferring it to the turbine. In a water pre-heater system, the energy from the extracted steam is fed back to the system, and the loss from the cold source is much less than in a simple Rankine cycle. In addition, this extraction process completes in various phases. Theoretically, the power plant efficiency is directly proportional to the number of steam extraction stages. If the steam extraction stages are more, higher will be the efficiency of the power plant.

P-V Diagram of Steam Turbine

A steam turbine works on the base of the Rankine cycle. A Rankine cycle is an ideal thermodynamic cycle of a heat engine that transforms the heat energy into mechanical work while undergoing a phase change. 

A steam generator works in the following way :-

steam turbine PV diagram

  • Isentropic compression :- In the above diagram, the line 1-2 represents to isentropic compression. In this cycle, the liquid pumps from low pressure to high pressure.  During this process, the pump needs very low power for pumping liquid.
  • Isobaric Heat Supply :- Line 2-3 represents to isobaric heat supply process. The high-pressure water goes into the boiler, where it is heated via an external heat source at constant pressure to convert it into dry saturated steam.
  • Isentropic expansion :- Line 3-4 represents the isentropic expansion process. During this process, the dry saturated steam is expanded by the turbine to produce electricity. In this process, condensation occurs due to the reduction of pressure and temperature of the steam vapors.
  • Isobaric heat rejection :- Line 4-1 represents to Isobaric heat rejection process. During this process, the wet water vapors enter into the condenser, where they condense to a saturated liquid at constant pressure.

Advantages and Disadvantages of Steam Turbine

The steam turbine has the following advantages and disadvantages:

Advantages of Steam Turbines

  1. It is a type of rotating heat engine that is particularly suitable for driving generators.
  2. The reciprocating engines have low thermal efficiency as compare to a steam power generator.
  3. The power to weight ratio is very high competed to the reciprocating piston engine.
  4. These turbines have low revolving components as compared to the reciprocating engine.
  5. These turbines are best appropriate for large thermal power plants. There are different sizes of turbines up to 1.5 GW for generating electrical power.
  6. Generally, steam contains a large amount of enthalpy (especially in the form of heat of vaporization). It means, steam has less mass flow as compared to gas turbines.
  7. Steam turbines are more reliable, especially in applications that require continuous high performance.
  8. The electricity produces by a steam turbine has relatively low cost.

Disadvantages of Steam turbines

  1. It has high initial cost.
  2. In partial load operation, the efficiency of this turbine is lower than that of the reciprocating engine.
  3. It has a longer start-up time as compared to a gas turbine and is longer than a reciprocating engine.
  4. Compared to the reciprocating engine and gas turbine, it reacts less to changes in energy demand.

Applications of Steam Turbines

  1. These turbines use in power generation.
  2. It is used in renewable energy applications
  3. These are also used in producing electricity through steam.
  4. These uses in Waste Plants.
  5. Steam turbines use in oil and gas companies.
  6. These generators also use in manufacturing industries.
  7. These turbines use in missiles, airplanes, and ships for producing propulsion.

Advantages of Steam Turbines over Steam Engines

  1. Higher thermal efficiency.
  2. No reciprocating components for perfect balance and no need for a heavy foundation.
  3. A high-speed range is possible with the steam turbine.
  4. Lubrication is very easy as there are no friction components.
  5. No flywheel is required because the rate of power generation is uniform.
  6. In the case of a steam turbine, the steam consumption is low.
  7. During working, steam turbines require less attention and more compact.
  8. Best appropriate for large power plants.
  9. The elimination of parts such as connecting rods, cross arms, piston rods, and pistons dramatically simplifies construction and operation and reduce maintenance costs.
  10. Significant overload can occur, but overall efficiency is slightly reduced.

We take a deep look at the steam turbine working principle and some other aspects. So, I hope that you have clear all concepts related to this topic. If you need any more help on this topic then let me know in the comment box. I will try my best to give you a good answer.

See More :-

  1. Turbine and its Types
  2. Gas Turbine

FAQ Section

what change of energy occurs between the steam generator and the turbine?

Between the turbine and steam generator, thermal energy transforms into mechanical energy (rotational energy). Due to this rotational energy, the turbine rotor starts rotating, which further turns the coil of the generator, and the generator converts the mechanical energy into electrical energy.

The steam turbine works on which cycle

A steam turbine works on the base of the Rankine cycle.

Who Invented the Steam Turbine?

In 1884, the first steam turbine was discovered by Sir Charles A. Parsons.

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