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Here is an essay on the ‘Types of Turbines’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on the ‘Types of Turbines’ especially written for school and college students.
1. Essay on Gas Turbines:
A gas turbine is a heat engine that uses high temperature, high pressure gas as the working fluid to spin the turbine and generate power. Combustion of fuel in air is usually used to produce the needed temperatures and pressures in the turbine, which is why gas turbines are often referred to as ‘combustion’ turbines. To capture the energy, the working fluid is directed by vanes at the base of combustor nozzles to impinge upon specially designed airfoils (turbine blades).
The turbine blades, through their curved shapes, redirect the gas stream, which absorbs the momentum of the gas and produces power. A series of turbine blade rows, or stages, is attached to a rotor/shaft assembly. The shaft rotation drives an electric generator and a compressor for the air used in the gas turbine combustor.
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This process of imparting potential energy to a gas working fluid by adding heat and pressure, and translation of the potential energy to work through interaction of gas and blades, is called a Brayton cycle. In the simple Brayton cycle, the turbine exhaust is typically vented to the atmosphere.
The gas turbine cycle and inside of a gas turbine are shown in the figures below:
Improvement of efficiency in gas turbines:
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Efficiency of the gas turbines can be improved significantly using following technologies improvements as given below:
(i) Improved turbine blades material (Spar-shell technology).
(ii) Avoiding fluid leakages.
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(I) Improved turbine blades material (spar-shell technology):
The simplest turbines have one rotor assembly which is a shaft with blades attached. The moving fluid acts on the blades so that they rotate and impart energy to the rotor. In the case of natural gas turbines, the hotter the working fluid (the gas), the more energy can be extracted from the turbine, and the more efficient the turbine is. Currently, the efficiency of natural gas turbines is limited by the temperature that the turbine materials can withstand.
Significant investment has already been made to achieve higher and higher turbine temperatures – through high temperature nickel super alloys, cooling air, thermal barrier coatings, film cooling, and other technologies developed in the last century. However, any further improvements in these technologies will serve only to incrementally increase the temperature above where standard operating temperatures are today.
The Spar-Shell technology allows the turbine operator to achieve significantly greater temperature increase because the turbine blade material is no longer the nickel alloy that is currently used, but a higher temperature metal of the refractory type. Refractory materials are a class of metals that are extraordinarily resistant to heat and wear.
Examples of refractory metals include Niobium, Molybdenum, Tungsten, and Tantalum. Spar-Shell technology leverages the excellent heat resistance of the refractory metal and combines it with an advanced design approach – the Spar-Shell Blade – in which the external surface of the blade exposed to the turbine’s hot gas, “the Shell”, is composed of a refractory metal while an internal structure “the Spar” (composed of more conventional materials) provides support.
The Shell protects the Spar’s conventional materials from the hot working fluid of the operating engine. The Spar-Shell Blade allows the metal temperature of the turbine blade to increase by 100 degrees F, saving 50 – 75% of the air required to keep the blades “cool”. These changes allow for the overall turbine to operate 3.5% more efficiently.
(ii) Avoiding fluid leakages:
Another step of efficiency improvement in gas turbines is minimizing or avoiding the leakage of working fluid in the turbines. While a wind turbine is open to the environment, gas, steam, and water turbines are constructed with cases around the blades to contain and control the working fluid. Every molecule of working fluid that the blade does not extract work from as it passes by, is called “leakage” which also reduces turbine efficiency. The amount of leakages in turbines can be minimized through advanced clearance control schemes and sealing technologies.
Apart from the above mentioned steps to improve the efficiency of gas turbines, there are other ways also to increase turbine efficiency. These steps include Improvements in the surface finish of blades and cases which help to minimize losses and which in turns improves efficiency.
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These efficiency improvement steps when followed can contribute to about 5% saving in energy or we can say that additional 5% of energy is generated by using the same amount of fuel. This in turn will also benefit in reduction in CO2 emissions in the environment. For large scale use of combined cycle based gas turbines worldwide, these efficiency improvements steps for gas turbine can significantly help in energy conservation efforts.
2. Essay on Steam Turbines:
Steam turbines are the most widely and commonly used turbines in energy conversion processes involving coal and nuclear energy. These are used in coal and nuclear based power plants in which boilers are used to generate steam which is then used in steam turbine to produce requisite mechanical power to rotate electrical generator to produce electricity.
Steam turbines work on the same basic principles as gas turbines, but use steam as the working fluid. This steam is typically generated in an external boiler and fired by an external heat source. The process of imparting heat to pressurized water to produce a high potential energy steam, and translation of the potential energy to work through interaction of steam and blades, is called a Rankine cycle.
In the Rankine cycle, the turbine exhaust (steam), now at low temperature and pressure, is condensed and recycled back to a boiler or heat source in a closed loop. There is huge potential of energy savings in the process involving steam turbines. Improvement in efficiency of steam turbines leads to huge energy savings.
Optimising process operating conditions can considerably improve turbine water rate, which in turn will significantly reduce energy requirement. Various operating parameters affect condensing and back pressure turbine steam consumption and efficiency. Energy conservation benefits depend on the adopting minor or major modifications and using the latest technology.
Energy conservation does not mean curtailing energy use at the cost of industrial and economic growth. In the large process industries, steam turbines are the main energy consumers. Savings achieved here will be significant, with a better return on investment than for most other equipment.
Factors affecting the performance of steam turbines:
Turbines are designed for a particular operating conditions like steam inlet pressure, steam inlet temperature and turbine exhaust pressure/ exhaust vacuum, which affects the performance of the turbines in a significant way. Variations in these parameters affect the steam consumption in the turbines and also the turbine efficiency.
Theoretical turbine efficiency is calculated as work done by the turbine to the heat supplied to generate the steam. The main types of steam turbines used are condensing type and back pressure type steam turbines.
The various factors which affect both these types of steam turbines and efficiency improvement steps related to these factors are as follows:
(i) Steam inlet pressure:
Steam inlet pressure of the turbine also affects the turbine performance. All the turbines are designed for a specified steam inlet pressure. For obtaining the design efficiency, steam inlet pressure shall be maintained at design level. Lowering the steam inlet pressure will hampers the turbine efficiency and steam consumption in the turbine will increase.
Similarly at higher steam inlet pressure energy available to run the turbine will be high, which in turn will reduce the steam consumption in the turbine. Results show that an increase in steam inlet pressure by 1 kg/cm in condensing type turbine reduce the steam consumption in the turbine by about 0.3 % and improves the turbine efficiency by about 0.1 % respectively.
In case of back pressure type turbine increase in steam inlet pressure by 1 kg/cm reduces the steam consumption in the turbine by about 0.7 % and improves the turbine efficiency by about 0.16 %. Improvement in back pressure type turbine is more than the condensing type turbine.
(ii) Steam inlet temperature:
Enthalpy of steam is a function of temperature and pressure. At lower temperature, enthalpy will be low, work done by the turbine will be low, turbine efficiency will be low, and hence steam consumption for the required output will be higher. In other words, at higher steam inlet temperature, heat extraction by the turbine will be higher and hence for the required output, steam consumption will reduce.
An increase in steam inlet temperature by 10 deg C in condensing type turbine reduces the steam consumption in the turbine by about 1.1 % and improves the turbine efficiency by about 0.12 % respectively. In case of back pressure type turbine increase in steam inlet temperature by 10 deg C reduces the steam consumption in the turbine by about 1.5 % and improves the turbine efficiency by about 0.12 %. Improvement in back pressure type turbine is more than the condensing type turbine.
(iii) Exhaust pressure/vacuum:
Higher exhaust pressure/ lower vacuum, increases the steam consumption in the turbine, keeping all other operating parameters constant. Exhaust pressure lower than the specified will reduce the steam consumption and improves the turbine efficiency. Similarly exhaust vacuum lower than the specified, will lower the turbine efficiency and reduces the steam consumption.
Results indicate that improvement in exhaust vacuum by 10 mm Hg reduces the steam consumption in the turbine by about 1.1 %. Improvement in turbine efficiency varies significantly from 0.24 % to 0.4 %. In case of back pressure type turbine reduction in exhaust pressure by 1-0 kg/cm2, reduces the steam consumption in the turbine by about 0.8 % and improves the turbine efficiency by about 0.14 %.
Vacuum ejector system creates and maintains the vacuum in the surface condenser by removing the air ingress. Removal of air ingress is important, as accumulation of this hampers the performance of surface condenser, which reduces the surface condenser vacuum. Motive steam condition shall be maintained as specified.
Inter-after condenser shall be cleaned in the available opportunity, as they get choked due to foreign material coming with cooling water. Flange joints shall be tightened properly to avoid any ingress of air. Exhaust side of the turbine shall be properly steam sealed to avoid any ingress of air.
In many condensing turbines it is observed that the exhaust vacuum of these turbines is much less than the vacuum at the condenser. Mainly, it is due to the higher pressure drop in the exhaust pipeline from turbine exhaust to the condenser. In order to improve the vacuum at turbine exhaust so as to reduce steam consumption in the turbine, exhaust pipeline of these turbines can be replaced with higher size.
Apart from above mentioned factors affecting the performance of steam turbines, old steam turbines having low performance can be replaced with new steam turbines which have improved water rate and better efficiency as compared to old turbines.
3. Essay on Hydraulic Turbines:
The use of hydraulic turbines has a long history. Depending upon the head and flow, the hydro turbines are classified as Pelton turbine, Francis turbine and Kaplan turbines. Hydraulic turbines are not only used to convert hydraulic energy into electricity but also in pumped storage schemes, which is the most efficient large-scale technology available for the storage of electrical energy.
Separate pumps and turbines or reversible machines, so called pump turbines, are used in such schemes. During their long history there has been continuous development of the design of hydraulic turbines, particularly with regard to improvements in efficiency, size, output and head of water being exploited.
Recently, the use of modern techniques like computational fluid dynamics (CFD) for predicting the flow in these machines has brought further substantial improvements in their hydraulic design, in the detailed understanding of the flow and its influence on turbine performance and in the prediction and prevention of cavitation phenomenon.
The perfect design of the hydro turbines can be achieved with the help of computational flow dynamics. This helps in improving the efficiency of these hydro turbines. The perfect design of buckets in Pelton wheel helps in extracting the maximum energy from striking water jet and converts the useful energy into mechanical energy with minimum losses.
In case of Francis turbines the spiral case of a Francis turbine is designed such that the velocity distribution in the circumferential direction at the inlet to the stay vanes is uniform and the incidence angle over the height of the stay vanes varies only little. The main function of the stay vanes is to carry the pressure loads in the spiral case and turbine head cover. Their second purpose is to direct the flow towards the adjustable guide vanes with an optimal incidence angle.
The adjustable guide vanes are the only device available to control the flow and thus the power output of a Francis turbine. Leakage flow through the gaps between the guide vane tips and facing plates causes efficiency losses and can cause local erosion. To avoid the leakages in Francis turbine, suitable seals must be provided. This will help in stopping the leakage and in turns will improve the overall efficiency of Francis turbine.
A major problem affecting the performance of hydro turbines is the phenomenon of cavitation. Cavitation occurs in the flow of water when, owing to regions of high-flow velocity, the local static pressure decreases below the vapour pressure and vapour bubbles appear. Cavitation may occur on the blade suction surface in regions of low pressure or at the runner leading edge at off-design operation.
The effects of cavitation are harmful, both on performance and on erosion of material. Cavitation erosion is caused by the extremely high pressure peaks that occur during the implosion of cavitation bubbles in the vicinity of a solid surface. Cavitation imposes restrictions on blade loading and blade design. This problem of cavitation can be well addressed using the modern computational flow dynamics techniques for designing the hydro turbines.