A turbine is a mechanical device that extract energy from a moving liquid and follower into useful work. The turbines are basically used to produce electricity power.
Turbines achieve this one of two through mechanical gearing or electromagnetic induction to produce electricity. Because of the turbine’s many function in a wide variety of technologies, research is still ongoing to perfect turbine and rotor performance.
Types of Turbine
- Water Turbine
- Steam Turbine
- Gas Turbine
- Wind Turbine
Water turbine (Pelton wheel)
A Pelton wheel is an impulse type water turbine. It was invented by Lester Allan Pelton in the 1870s The Pelton wheel extricate energy from the desire of moving water, as antithetical to water’s buried weight like the classical bound water wheel. Profuse modification of impulse turbines existed preceding to Pelton’s design, but they were less efficient than Pelton’s design. Pelton’s paddle calculus was designed so that when the rim ran at half the speed of the water jet. which allowed for a very efficient turbine.
A Pelton turbine or Pelton wheel is a type of turbine used generally in hydroelectric plants. These turbines are generally used for ground with heads exceeding 300 meters. This type of turbine was conceive during the gold rush in 1880 by Lester Pelton.
When used for generating electricity, there is commonly a water accumulation located at some height above the Pelton turbine.
As well, the efficiency of these wheels is highest when the speed of the movement of the cups is half of the speed of the water jet.
Water Turbine (Hydraulic turbine)
A turbine that extracts energy from moving water and converts it into electrical energy, is called water turbine or hydraulic turbine.
According to The Type of Energy Available at Inlet
If the energy available at the inlet of the turbine is only kinetic energy, the turbine is known as impulse turbine
If the energy available at the inlet of the turbine is kinetic energy as well as pressure energy, the turbine is known as reaction turbine.
According to the Direction of Flow
Tangential Flow Turbine
If the water flows along the tangent of the runner, the Turbine is known as tangential flow turbine. For Example: Pelton turbine.
Radial Flow Turbine
If the water flows in the radial direction through the runner, the turbine is called radial flow turbine.
Axial Flow Turbine
If the water flows through the runner along the direction parallel to the axis of rotation of the runner the turbine is called axial flow turbine.
Mixed Flow Turbine
If the water flows through the runner in the radial direction but leaves in the direction parallel to the axis of rotation of the runner, the turbine is called mixed flow turbine. For example: Kaplan Turbine
According to the Head at the Inlet of Turbine
High Head Turbine
- The net head varies in this turbine is from 150 m to 2000 m or even more. It requires small quantity of water. Eg: pelton turbine.
- Medium Head Turbine: in this turbine, the net head varies from 30 m to 150 m. It requires moderate quantity of water. Eg: Francis turbine.
- Low Head Turbine: In low head turbines, the net head is less than 30 m. it requires large quantity of water. Eg: Kaplan turbine.
According to the Specific Speed of the Turbine
- Low Specific Speed Turbine: it has specific speed less than 50. Eg: pelton turbine.
- Medium Specific Speed Turbine: The specific speed varies from 50 to 250. Eg Francis turbine.
- High Specific Speed Turbine: The specific speed is more than 250. Eg: Kaplan turbine.
A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft.
Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator about 85% of all electricity generation in the United States in the year 2014 was by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process.
Types of Steam Turbine
According to the Mode of Steam Action
If the steam available at the inlet has only kinetic energy, the turbine is called impulse turbine.An impulse turbine is a turbine that is driven by high velocity jets of water or steam from a nozzle directed onto vanes or buckets attached to a wheel. The resulting impulse (as described by Newton’s second law of motion) spins the turbine and removes kinetic energy from the fluid flow. Before reaching the turbine the fluid’s pressure head is changed to velocity head by accelerating the fluid through a nozzle. This preparation of the fluid jet means that no pressure casement is needed around an impulse turbine.
Most types of turbine exploit the principles of both impulse turbines and reaction turbines. However, a few, such as the Pelton turbine, use the impulse concept exclusively.
Main types of impulse turbine
A Pelton turbine has one or more free jets discharging water into an aerated space and impinging on the buckets of a runner. Draft tubes are not required for impulse turbine since the runner must be located above the maximum tail water to permit operation at atmospheric pressure.
A Turbo turbine is a variation on the Pelton. The Turbo runner is a cast wheel whose shape generally resembles a fan blade that is closed on the outer edges. The water stream is applied on one side, goes across the blades and exits on the other side.
A cross-flow turbine is drum-shaped and uses an elongated, rectangular-section nozzle directed against curved vanes on a cylindrical shaped runner. It resembles a “squirrel cage” blower. The cross-flow turbine allows the water to flow through the blades twice. The first pass is when the water flows from the outside of the blades to the inside; the second pass is from the inside back out. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross-flow was developed to accommodate larger water flows and lower heads than the Pelton.
According to the Direction of Steam Flow
Axial Flow Turbine
The steam flows through the runner in the direction parallel to the axis of rotation of the runner, the turbine is called axial flow turbine.
Radial Flow Turbine
The steam flows through the runner in the radial direction, the turbine is called radial flow turbine.
According to the Exhaust Condition of Steam
A turbine in which the steam is condensed, when it comes out of the turbine is called condensing turbine. It produces large output with single unit.
Condensing Steam Turbine
Condensing steam turbines are most commonly found in thermal power plants.
In a condensing steam turbine, the maximum amount of energy is extracted from the steam, because there is very high enthrall difference between the initial (e.g. 6 MP a; 275°C; x = 1) and final (e.g. 0.008 MP a; 41.5°C; x = 0.9) conditions of steam. This is achieved by passing the exhaust steam into a condenser (called a surface condenser), which condenses the exhaust steam from the low-pressure stages of the main turbine (decreases the temperature and pressure of exhausted steam). The exhausted steam is condensed by passing over tubes containing water from the cooling system.
Non-Condensing Turbine: In non-condensing turbine, the exhaust steam leaves the turbine at atmospheric pressure or low pressure. There is no condensation of steam takes place in this turbine.
According to the Pressure of Steam
- High Pressure Turbine
- Medium Pressure Turbine
- Low Pressure Turbine
According to the Number of Stages
Single Stage Steam Turbine
In single stage steam turbine, the steam after leaving the nozzle, impinges on one end of the blade, glides over the inner surface, leaves the blades and exhaust into the condenser.
Multi Stage Steam Turbine
In multi stages turbine, sometimes the steam after leaving the moving blade is again made to flow through a fixed blade ring and again impinges on second moving blade. Here the fixed blade ring is used to make the steam to flow at a desired angle. In multistage steam turbine more than one set of rotor blades are used.
Efficiency of Steam Turbine
The efficiency of any turbine or engine can be defined as its ability to convert the input energy into useful output energy which is expressed in the form of the following equation.
Efficiency (ɳ) = Output / Input
An ideal turbine with 100% efficiency is the one which converts all its input energy into output work without dissipating energy in the form of heat or any other form. But in the real world, it is not possible to build a turbine with 100% efficiency because of friction in the parts of turbines, heat loss, and other such losses. In the case of steam turbines following factors decides the overall efficiency f the turbine.
Velocity of input steam (which in turn depends on the temperature and pressure of steam)
- Angle of guiding vanes
- Blade angle on the rotor
- Radius of rotor
The electrical generating efficiency of standard steam turbine power plants varies from a high of 37% HHV4 for large, electric utility plants designed for the highest practical annual capacity factor, to under 10% HHV for small, simple plants which make electricity as a byproduct of delivering steam to processes or district heating systems.
Efficiency (ɳ) = Work Done / Input Kinetic Energy
Blade efficiency of impulse and reaction steam turbine.
The maximum efficiency of impulse steam turbine is achieved at zero degrees angle of inlet blades because this angle keeps the friction at the minimum by reducing the surface area of the blade. It is also possible to link several turbines in series to utilize maximum energy from steam before sending it back to the condenser. In this type of arrangement stage efficiency calculation method works best. An important point to note here is that all this discussion did not include the energy loss in heating water and condensing steam. Commercial industries also calculate efficiencies of these operations to find out the overall efficiency of the entire setup.
Different Efficiencies of Steam Turbines
Isotropic Efficiency: This is the efficiency which compares the actual output with the ideal isotropic output to measure the effectiveness of extracted work.
CHP Electrical Efficiency: Combined Heat and Power (CHP) electrical efficiency measures the amount of boiler fuel converted into electrical energy or electricity. It can be calculated by following equation
CHP electrical efficiency = Net electricity generated/Total fuel into boiler
Total CHP Efficiency: This efficiency measures total output including electricity and steam energy by the boiler fuel. It is calculated by following formula.
Total CHP efficiency = (Net electricity generated + Net steam to process)/Total fuel into boiler
Effective Electrical Efficiency: This efficiency is calculated by the formula
(Steam turbine electric power output) / (Total fuel into boiler – (steam to process/boiler efficiency))
It is equivalent to 3,412 Btu/kWh/Net Heat Rate and
Net Heat Rate = (total fuel input to the boiler – the fuel that would required to generate the steam to process assuming the same boiler efficiency/steam turbine electric output (kW)
Heat or power ratio is also an important factor in this discussion and it can be calculated by the formula
Power/Heat Ratio = CHP electrical power output (Btu)/ useful heat output (Btu)
Steam turbines may be classified into different categories depending on their construction, working pressures, size and many other parameters.
Difference between Impulse and Reaction Turbine
The main distinction is the manner in which the steam is expanded as it passes through the turbine.
Steam turbine types based on blade geometry and energy conversion process are impulse turbine and reaction turbine.
Steam Turbine The impulse turbine is composed of moving blades alternating with fixed nozzles. In the impulse turbine, the steam is expanded in fixed nozzles and remains at constant pressure when passing over the blades. Curtis turbine, Rateau turbine, or Brown-Curtis turbine are impulse type turbines. The original steam turbine, the De Laval, was an impulse turbine having a single-blade wheel.
The entire pressure drop of steam take place in stationary nozzles only. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be a small pressure drop across the moving blades also.
Impulse vs Reaction Turbine – comparison
In impulse turbines, the steam expands through the nozzle, where most of the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades, changes its direction, which in turn applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. The main feature of these turbines is that the pressure drop per single stage can be quite large, allowing for large blades and a smaller number of stages. Except for low-power applications, turbine blades are arranged in multiple stages in series, called compounding, which greatly improves efficiency at low speeds.
Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery. The rotor blades are usually designed like an impulse blade at the rot and like a reaction blade at the tip.
Turbine Blade – Impulse and Reaction
Since the Curtis stages reduce significantly the pressure and temperature of the fluid to a moderate level with a high proportion of work per stage. An usual arrangement is to provide on the high pressure side one or more Curtis stages, followed by Rateau or reaction staging. In general, when friction is taken into account reaction stages the reaction stage is found to be the most efficient, followed by Rateau and Curtis in that order. Frictional losses are significant for Curtis stages, since these are proportional to steam velocity squared. The reason that frictional losses are less significant in the reaction stage lies in the fact that the steam expands continuously and therefore flow velocities are lower.
Compounding of Steam Turbines
Compounding of steam turbines is the method in which energy from the steam is extracted in a number of stages rather than a single stage in a turbine. In all turbines the rotating blade velocity is proportional to the steam velocity passing over the blade. If the steam is expanded only in a single stage from the boiler pressure to the exhaust pressure, its velocity must be extremely high.
A compounded steam turbine has multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the turbine in number of stages. For example, large HP Turbine used in nuclear power plants can be double-flow reaction turbine with about 10 stages with shrouded blades. Large LP turbines used in nuclear power plants are usually double-flow reaction turbines with about 5-8 stages (with shrouded blades and with free-standing blades of last 3 stages).
In an impulse steam turbine compounding can be achieved in the following three ways:
- velocity compounding
- pressure compounding
- pressure-velocity compounding
A velocity-compounded impulse stage consist of a row of fixed nozzles followed by two or more rows of moving blades and fixed blades (without expansion). This divides the velocity drop across the stage into several smaller drops. In this type, the total pressure drop (expansion) of the steam take place only in the first nozzle ring. This produces very high velocity steam, which flows through multiple stages of fixed and moving blades. At each stage, only a portion of the high velocity is absorbed, the remainder is exhausted on to the next ring of fixed blades. The function of the fixed blades is to redirect the steam (without appreciably altering the velocity) leaving from the first ring of moving blades to the second ring of moving blades. The jet then passes on to the next ring of moving blades, the process repeating itself until practically all the velocity of the jet has been absorbed.
This method of velocity compounding is used to solve the problem of single stage impulse turbine for use of high pressure steam (i.e. required velocity of the turbine), but they are less efficient due to high friction losses.
Pressure Compounding – Rateau Turbine – Zoelly Turbine
A pressure-compounded impulse stage is a row of fixed nozzles followed by a row of moving blades, with multiple stages for compounding. In this type, the total pressure drop of the steam does not take place in the first nozzle ring, but is divided up between all the nozzle rings. The effect of absorbing the pressure drop in stages is to reduce the velocity of the steam entering the moving blades. The steam from the boiler is passed through the first nozzle ring in which it is only partially expanded. It then passes over the first moving blade ring where nearly all of its velocity (momentum) is absorbed. From this ring it exhausts into the next nozzle ring and is again partially expanded. This method of pressure compounding is used in Rateau and Zoelly turbines, but such turbines are bigger and bulkier in size.
Pressure-Velocity Compounding – Curtis Turbine
Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. The pressure-velocity compounding is a combination of the above two types of compounding. In fact, a series of velocity-compounded impulse stages is called a pressure-velocity compounded turbine. Each stage consists of rings of fixed and moving blades. Each set of rings of moving blades is separated by a single ring of fixed nozzles. In each stage there is one ring of fixed nozzles and 3-4 rings of moving blades (with fixed blades between them). Each stage acts as a velocity compounded impulse turbine.
The steam coming from the steam generator is passed to the first ring of fixed nozzles, where it gets partially expanded. The pressure partially decreases and the velocity rises correspondingly. It then passes over the 3-4 rings of moving blades (with fixed blades between them) where nearly all of its velocity is absorbed. From the last ring of the stage it exhausts into the next nozzle ring and is again partially expanded.
This has the advantage of allowing a bigger pressure drop in each stage and, consequently, less stages are necessary, resulting in a shorter turbine for a given pressure drop. It may be seen that the pressure is constant during each stage; the turbine is, therefore, an impulse turbine. The method of pressure-velocity compounding is used in the Curtis turbine.
Reaction Turbine – Parsons Turbine
Reaction Turbine – schema The reaction turbine is composed of moving blades (nozzles) alternating with fixed nozzles. In the reaction turbine, the steam is expanded in fixed nozzles and also in the moving nozzles. In other words, the steam is continually expanding as it flows over the blades. There is pressure and velocity loss in the moving blades. The moving blades have a converging steam nozzle. Hence when the steam passes over the fixed blades, it expands with decrease in steam pressure and increase in kinetic energy.
In reaction turbines, the steam expands through the fixed nozzle , where the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades (nozzles), changes its direction and undergo further expansion. The change in its direction and the steam acceleration applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. There is no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. In this type of turbine the pressure drops take place in a number of stages, because the pressure drop in a single stage is limited.
The main feature of this type of turbine is that in contrast to the impulse turbine, the pressure drop per stage is lower, so the blades become smaller and the number of stages increases. On the other hand, reaction turbines are usually more efficient, i.e. they have higher “isotropic turbine efficiency”. The reaction turbine was invented by Sir Charles Parsons and is known as the Parsons turbine.
In the case of steam turbines, such as would be used for electricity generation, a reaction turbine would require approximately double the number of blade rows as an impulse turbine, for the same degree of thermal energy conversion. Whilst this makes the reaction turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.
Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery. The rotor blades are usually designed like an impulse blade at the rot and like a reaction blade at the tip.
Turbine Blade – Impulse and Reaction
Classification of Turbines – steam supply and exhaust conditions
Steam turbines may be classified into different categories depending on their purpose and working pressures. The industrial usage of a turbine influences the initial and final conditions of steam. For any steam turbine to operate, a pressure difference must exist between the steam supply and the exhaust.
This classification includes
- Condensing Steam Turbine
- Back-pressure Steam Turbine
- Reheat Steam Turbine
- Turbine with Steam Extraction
Condensing Steam Turbine
Condensing steam turbines are most commonly found in thermal power plants. In a condensing steam turbine, the maximum amount of energy is extracted from the steam, because there is very high enthrall difference between the initial (e.g. 6 MP a; 275°C; x = 1) and final (e.g. 0.008 MP a; 41.5°C; x = 0.9) conditions of steam. This is achieved by passing the exhaust steam into a condenser (called a surface condenser), which condenses the exhaust steam from the low-pressure stages of the main turbine (decreases the temperature and pressure of exhausted steam). The exhausted steam is condensed by passing over tubes containing water from the cooling system.
Decreasing the turbine exhaust pressure increases the net work per cycle but also decreases the vapor quality of outlet steam.
The goal of maintaining the lowest practical turbine exhaust pressure is a primary reason for including the condenser in a thermal power plant. The condenser provides a vacuum that maximizes the energy extracted from the steam, resulting in a significant increase in net work and thermal efficiency. But also this parameter (condenser pressure) has its engineering limits
Decreasing the turbine exhaust pressure decreases the vapor quality (or dryness fraction). At some point the expansion must be ended to avoid damages that could be caused to blades of steam turbine by low quality steam.
Decreasing the turbine exhaust pressure significantly increases the specific volume of exhausted steam, which requires huge blades in last rows of low-pressure stage of the steam turbine.
In a typical condensing steam turbine, the exhausted steam condenses in the condenser and it is at a pressure well below atmospheric (absolute pressure of 0.008 MPa, which corresponds to 41.5°C). This steam is in a partially condensed state (point F), typically of a quality near 90%. Note that, the pressure inside the condenser is also dependent on the ambient atmospheric conditions:
air temperature, pressure and humidity in case of cooling into the atmosphere
water temperature and the flow rate in case of cooling into a river or sea
An increase in the ambient temperature causes a proportional increase in pressure of exhausted steam (ΔT = 14°C is usually a constant) hence the thermal efficiency of the power conversion system decreases. In other words, the electrical output of a power plant may vary with ambient conditions, while the thermal power remains constant.
The pressure inside condenser is given by the ambient air temperature (i.e. temperature of water in the cooling system) and by steam ejectors or vacuum pumps, which pull the gases (non-condensates) from the surface condenser and eject them to the atmosphere.
The lowest feasible condenser pressure is the saturation pressure corresponding to the ambient temperature (e.g. absolute pressure of 0.008 MPa, which corresponds to 41.5°C). Note that, there is always a temperature difference between (around ΔT = 14°C) the condenser temperature and the ambient temperature, which originates from finite size and efficiency of condensers.
Back-pressure Steam Turbine
Back-pressure steam turbines or non-condensing turbines are most widely used for process steam applications. Steam is a principle energy source for many industrial processes.
The popularity of process steam as an energy source stems from its many advantages, which
high heat capacity,
The process steam can be produced by back-pressure steam turbines, which also generates mechanical work (or electrical energy). Back-pressure turbines expand the live steam supplied by the boiler to the pressure at which the steam is required for the process. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. Back-pressure turbines are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are needed. The electric power generated by the back-pressure turbine is directly proportional to the amount of process steam required.
Reheat Steam Turbine
Rankine cycle with reheat and super heat the low-pressure stage
Reheat turbines are also used almost exclusively in thermal power plants. All turbines, that have high-pressure turbine and low-pressure turbines use a steam reheat between these stages. Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle (i.e. thermal efficiency increases). This requires the addition of another type of heat exchange called a reheated. The use of the reheated involves splitting the turbine, i.e. use of a multistage turbine with a reheated. It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage.
High pressure and low pressure stages of the turbine are usually on the same shaft to drive a common generator, but they have separate cases. With a reheated (moisture separator-reheated – MSR), the flow is extracted after a partial expansion (point D), run back through the heat exchange to heat it back up to the peak temperature (point E), and then passed to the low-pressure turbine. The expansion is then completed in the low-pressure turbine from point E to point F.
The steam must be reheated or super heated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades. To prevent this, condensate drains are installed in the steam piping leading to the turbine. The reheated heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and, as can be seen from the picture, the steam is in a partially condensed state (point F), typically of a quality near 90%, but it is much higher vapor quality, than that it would be without reheat. Accordingly, super heating also tends to alleviate the problem of low vapor quality at the turbine exhaust.
Extraction Turbine – Turbine with Steam Extraction
Extraction type turbines are common in all applications. In some applications, when required, steam can be extracted from turbine before steam flowing through the last stage, named extraction turbine. As in back-pressure turbines, extracted steam can be used for many industrial processes or it can be used to improve the efficiency of thermodynamic cycle. The second case is usually known as the heat regeneration.
Almost all large steam turbines use the heat regeneration (i.e. they are extraction turbines), since it reduces the amount of fuel that must be added in the boiler. The reduction in the heat added can be done by transferring heat (partially expanded steam) from certain sections of the steam turbine, which is normally well above the ambient temperature, to the feed water. Note that, most of energy contained in the steam is in the form of latent heat of vaporization. Extraction flows may be controlled with a valve, or left uncontrolled.
For example, most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine with 3 or 4 self-regulating extraction lines and three parallel multi-stage LP turbines with 3 or 4 self-regulating extraction lines.
The high pressure feed water heaters are usually heated by extraction steam from the high pressure turbine, HP, whereas the low-pressure feed water heaters are usually heated by extraction steam from the low pressure turbine, LP. Both are usually self-regulating. It means that the greater the flow of feed water the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
Steam turbine governing
Steam turbine governing is the procedure of controlling the flow rate of steam to a steam turbine so as to maintain its speed of rotation as constant. The variation in load during the operation of a steam turbine can have a significant impact on its performance. In a practical situation the load frequently varies from the designed or economic load and thus there always exists a considerable deviation from the desired performance of the turbine. The primary objective in the steam turbine operation is to maintain a constant speed of rotation irrespective of the varying load. This can be achieved by means of governing in a steam turbine.
There are many types of governors
Steam Turbine Governing is the procedure of monitoring and controlling the flow rate of steam into the turbine with the objective of maintaining its speed of rotation as constant. The flow rate of steam is monitored and controlled by interposing valves between the boiler and the turbine. Depending upon the particular method adopted for control of steam flow rate, different types of governing methods are being practiced. The principal methods used for governing are described below.
In throttle governing the pressure of steam is reduced at the turbine entry thereby decreasing the availability of energy. In this method steam is passed through a restricted passage thereby reducing its pressure across the governing valve. The flow rate is controlled using a partially opened steam control valve. The reduction in pressure leads to a throttling process in which the enthalpy of steam remains constant.
Throttle governing – small turbines
Low initial cost and simple mechanism makes throttle governing the most apt method for small steam turbines. The mechanism is illustrated in figure 1. The valve is actuated by using a centrifugal governor which consists of flying balls attached to the arm of the sleeve. A geared mechanism connects the turbine shaft to the rotating shaft on which the sleeve reciprocates axially. With a reduction in the load the turbine shaft speed increases and brings about the movement of the flying balls away from the sleeve axis. This results in an axial movement of the sleeve followed by the activation of a lever, which in turn actuates the main stop valve to a partially opened position to control the flow rate.
Throttle governing – big turbines
In larger steam turbines an oil operated servo mechanism is used in order to enhance the lever sensitivity. The use of a relay system magnifies the small deflections of the lever connected to the governor sleeve. The differential lever is connected at both the ends to the governor sleeve and the throttle valve spindle respectively. The pilot valves spindle is also connected to the same lever at some intermediate position. Both the pilot valves cover one port each in the oil chamber. The outlets of the oil chamber are connected to an oil drain tank through pipes. The decrease in load during operation of the turbine will bring about increase in the shaft speed thereby lifting the governor sleeve. Deflection occurs in the lever and due to this the pilot valve spindle raises up opening the upper port for oil entry and lower port for oil exit. Pressurized oil from the oil tank enters the cylinder and pushes the relay piston downwards. As the relay piston moves the throttle valve spindle attached to it also descends and partially closes the valve. Thus the steam flow rates can be controlled. When the load on the turbine increases the deflections in the lever are such that the lower port is opened for oil entry and upper port for oil exit. The relay piston moves upwards and the throttle valve spindle ascend upwards opening the valve. The variation of the steam consumption rate ṁ (kg/h) with the turbine load during throttle governing is linear.
The equation :
Where a is the steam rate in kg/kWh, ‘L’ is the load on turbine in KW and C is no load steam consumption.
In nozzle governing the flow rate of steam is regulated by opening and shutting of sets of nozzles rather than regulating its pressure. In this method groups of two, three or more nozzles form a set and each set is controlled by a separate valve. The actuation of individual valve closes the corresponding set of nozzle thereby controlling the flow rate. In actual turbine, nozzle governing is applied only to the first stage whereas the subsequent stages remain unaffected. Since no regulation to the pressure is applied, the advantage of this method lies in the exploitation of full boiler pressure and temperature. Figure 2 shows the mechanism of nozzle governing applied to steam turbines. As shown in the figure the three sets of nozzles are controlled by means of three separate valves.
2-D schematic of nozzle governor
Occasionally the turbine is overloaded for short duration. During such operation, bypass valves are opened and fresh steam is introduced into the later stages of the turbine. This generates more energy to satisfy the increased load. The schematic of bypass governing is as shown in figure.
2-D Schematic of bypass governor
Main Parts of a Turbine.
The main parts of a turbine are
It guides the steam to flow in designed direction and velocity.
It is the rotating part of the turbine and blades are attached to the runner.
It is that part of the turbine on which the fast moving fluid strikes and rotates the runner.
It is the outer air tight covering of the turbine which contains the runner and blades. It protects the internal parts of the turbine.
A gas turbine, also called a combustion turbine, is a type of continuous combustion, internal combustion engine. There are three main components:
An upstream rotating gas compressor;
A downstream turbine on the same shaft;
A combustion chamber or area, called a com buster, in between 1. and 2. above.
A fourth component is often used to increase efficiency (turboprop, turbofan), to convert power into mechanical or electrical form (turbo shaft, electrical generator), or to achieve greater power to mass/volume ratio (afterburner).
The basic operation of the gas turbine is a Bray-ton cycle with air as the working fluid. Fresh atmospheric air flows through the compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process. The turbine shaft work is used to drive the compressor; the energy that is not used for shaft work comes out in the exhaust gases that produce thrust. The purpose of the gas turbine determines the design so that the most desirable split of energy between the thrust and the shaft work is achieved. The fourth step of the Brayton cycle (cooling of the working fluid) is omitted, as gas turbines are open systems that do not use the same air again.
Gas turbines are used to power aircraft, trains, ships, electrical generators, pumps, gas compressors, and tanks.
Examples of gas turbine configuration
- Turbo shaft (electric generator)
- High-bypass turbofan
- Low-bypass after burning turbofan
The turbojet is an air breathing jet engine, typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine (that drives the compressor). The compressed air from the compressor is heated by the fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust.
A turboprop engine is a turbine engine that drives an aircraft propeller.
In its simplest form a turboprop consists of an intake, compressor, com buster, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then com busts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.
In contrast to a turbojet, the engine’s exhaust gases do not generally contain enough energy to create significant thrust, since almost all of the engine’s power is used to drive the propeller.
Turbo shaft (electric generator)
A turbo shaft engine is a form of gas turbine that is optimized to produce shaft power rather than jet thrust.
In concept, turbo shaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. They are even more similar to turboprops, with only minor differences, and a single engine is often sold in both forms.
Turbo shaft engines are commonly used in applications that require a sustained high power output, high reliability, small size, and lightweight. These include helicopters, auxiliary power units, boats and ships, tanks, hovercraft, and stationary equipment.
The turbofan or fan jet is a type of air breathing jet engine that is widely used in aircraft propulsion. The word “turbofan” is a portmanteau of “turbine” and “fan”: the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion,and the fan, a ducked fan that uses the mechanical energy from the gas turbine to accelerate air rearwards. Thus, whereas all the air taken in by a turbojet passes through the turbine (through the combustion chamber), in a turbofan some of that air bypasses the turbine. A turbofan thus can be thought of as a turbojet being used to drive a ducked fan, with both of these contributing to the thrust.
The ratio of the mass-flow of air bypassing the engine core divided by the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working together; engines that use more jet thrust relative to fan thrust are known as low-bypass turbofans, conversely those that have considerably more fan thrust than jet thrust are known as high-bypass. Most commercial aviation jet engines in use today are of the high-bypass type,and most modern military fighter engines are low-bypass.Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.
Modern turbofans have either a large single-stage fan or a smaller fan with several stages. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit.
Low-bypass after burning turbofan
The turbofan or fan jet is a type of air breathing jet engine that is widely used in aircraft propulsion.Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.
Types of Gas Turbine
- According to the Path of Working Substance
Closed Cycle Gas Turbine: In closed cycle gas turbine the, the air (gas) is continuously circulates within the turbine.
Open Cycle Gas Turbine: In this types of turbine, the air is not circulates continuously within the turbine. The air after flowing over the blades of the turbine exhausted in the atmosphere.
Semi-Closed Gas Turbine: As the name indicates, the semi-closed gas turbine is the combination of both the turbines, one working on the open cycle and the other on the closed cycle.
- According to the Process of Heat Absorption
Constant Pressure Gas Turbine: A turbine in which the air is heated at constant pressure in the combustion chamber is called constant pressure gas turbine.
Constant Volume Gas Turbine: A turbine in which the air is heated at constant volume in the combustion chamber is called constant volume gas turbine.
Working Principle of a Turbine
- A fast moving fluid (it may be water, gas, steam or wind) is made to strike on the blades of the turbine.
- As the fluid strikes the blades, it rotates the runner. Here the energy of the moving fluid is converted into rotational energy.
- A generator is coupled with the shaft of the turbine. With the rotation of the runner of the turbine, the shaft of the generator also rotates. The generator converts the mechanical energy of the runner into electrical energy.
A turbine which extracts energy from the fast moving wind and converts it into electricity, is called wind turbine
Wind power is currently the fastest-growing source of electricity production in the world with about a quarter of a million wind turbines operating around the globe in over 90 countries. We
now have an output of 336 GW (June 2014), with wind energy production at 4% of total worldwide electricity usage, and growing rapidly; this has prevented 518 million metric tons of carbon dioxide from entering the ecosystem annually. While windmills have been in use for about 1,500 years, from their first development in Persia around 500 AD, it has never been more prevalent than in modern times.
Current wind turbines have experienced a 15-fold increase in output since 1990, and now stand at an astonishing 59% efficiency. Make no mistake: we can continue to increase that figure and generate more power with the same number of installations by further increasing that efficiency. A single wind turbine can now power 500 homes in the energy hungry United States, but thousands in a less demanding countries that have much lower demands per household. As of May 2014, the United States has 46,000 units installed. It is going to be a while before they hit 55% like parts of southern Australia have already done.
Lots of Watts
That’s a lot of power we’re making, and although wind doesn’t blow constantly, it does blow day and night, so it’s more useful for more hours of the day than solar power alone. It certainly does not eliminate it, but does decrease the need for power storage. With its small land-use footprint, virtually water less operation, and its pollution free design with cost less fuel, we have excellent reason to celebrate Global Wind Day on June 15th annually.
Currently, 95% of wind turbines are installed on private land, but there are projects all over the world that are taking advantage of offshore installations. Despite the fact that it is somewhat more costly, the greater continuity of wind and the decrease in aesthetic impact are worth the additional cost. It also helps to minimize the radar interference that has occasionally identified wind turbines as aircraft or unusual weather patterns.
Types of Wind Turbine
The wind turbines are of two types
Horizontal Axis Wind Turbine (HAWT)
A wind turbine in which the shaft of the turbine is horizontal to the ground is called horizontal axis wind turbine. In other words if the axis of rotation of the turbine blades are horizontal to the ground than it is known as horizontal axis wind turbine.
Vertical Axis Wind Turbine (VAWT)
A wind turbine in which the shaft of the turbine is vertical to the ground is called vertical axis wind turbine. In other words, if the axis of rotation of the turbine blades are vertical to the ground than the turbine is known as vertical axis wind turbine.
This is all about the different types of turbine. If you find anything missing or incorrect than don’t forget to comment us. And if you enjoy the article than like and share us.