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Gas Turbines - Classification, Applications and Limitations, INTRODUCTION:, Gas turbine is a prime mover like an internal combustion engine. It is a rotary type of I.C engine. It utilizes the kinetic energy of a jet of, burnt gases and air. Its constructional features resemble a steam turbine. The gas turbine requires an air compressor for supply of, compressed air for combustion. The compressor being coupled to gas turbine itself a portion of the power developed by the turbine is, spent to run the compressor. This results in the fall of overall efficiency of gas turbine. It generally employs oil, producer gas etc., as its, fuel., CLASSIFICATION OF GAS TURBINES :, Gas turbines are generally classified as follows:, 1. According to the path of the working fluid., Ex : (a) Open cycle Gas Turbine, (b) Closed cycle Gas Turbine, (c) Semi-closed cycle Gas Turbine, 2. According to the process of heat absorption, Ex : (a) Constant pressure or continuous combustion type gas turbine, (b) Constant volume or explosion type gas turbine, OPEN CYCLE GAS TURBINE :, The schematic diagram of an open cycle gas turbine is shown in figure., , Air is drawn in from atmosphere by the rotary compressor (1) and is compressed adiabatically. It is then delivered to combustion, chamber (2). In combustion chamber fuel is burnt and the hot gaSes formed mixup with compressed air. The air gets heated up and its, mass increases. A jet of these hot gases flow over the blades of a turbine (3) and expand over them adiabatically producing work, (power) at the output shaft. The gases are exhausted out into atmosphere. The output shaft may be coupled to an electrical generator, (4) for electric power generation, it may be coupled to a propeller shaft of an air craft., An open cycle gas turbine is also called a continuous combustion gas turbine as the combustion of fuel is continuous. It works on, Brayton cycle (Fig (a) and (b))., , Note:, 1. Here the process 4 — 1 has no practical importance as the gases are exhausted into atmosphere at point 4, and fresh air is sucked by, compressor at point 1., 2. Turbine shaft is also coupled to compressor shaft so that no other source is required to drive the compressor. But a portion of power, developed by turbine is utilised in driving the compressor., A heat exchanger may also be employed in an open cycle as shown in figure., Here the exhaust gases from turbine are not let out to atmosphere. They are passed through heat exchanger. In this heat exchanger,, heat from hot gases is transferred to compressed air before entering the combustion chamber. Thus, by pre-heating the air, the heat of, exhaust gases is utilised to some extent and overall efficiency of the plant is increased. Fuel consumption per unit mass of air is reduced., CLOSED CYCLE GAS TURBINE :, The schematic diagram of a closed cycle Gas Turbine is shown in figure.

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In this cycle the working fluid (generally air) is confined within the plant and receives heat from the external source. Air is first, compressed adiabatically in compressor and is delivered to heating chamber. Here air is heated by an external source, say, by a burning, fuel, but without coming in direct contact with fuel. The hot air is now expanded adiabatically over the turbine blades producing power., The expanded air is then passed through a cooling chamber where it is cooled by circulating water to original temperature. Afterwards,, air passes to compressor and the cycle is repeated. It is to be noted that throughout the cycle of operation, air is circulated inside the, plant itself and air alone circulates through the compressor and turbine. The closed cycle gas turbine works on Brayton cycle shown in, fig (a) and (b)., SEMI-CLOSED CYCLE GAS TURBINE, This is a combination of two turbines namely, open cycle and closed cycle turbines. The open cycle turbine drives the main generator, and works within pressure limits of atmospheric and 16 bar. The closed cycle unit drives the compressor and works within the pressure, limits of 2 bar and 16 bar approximately. This type of turbine is not used on commercial basis., CONSTANT PRESSURE GAS TURBINE :, This is a turbine in which fuel is burnt and air is heated at constant pressure in a combustion chamber. Almost all of the modern gas, turbines belong to this class only. Air is drawn from atmosphere and is compressed in a multi-stage compressor (Fig). The compression, ratio varies from 4 : 1 to 18 : 1. The compressed air passes into a combustion chamber where fuel is injected into it. A portion of this air, is utilised in burning the fuel and rest of the air gets mixed up with the products of combustion thus reducing the temperature of, products., , The mixture of air and combustion products enters the turbine forming a jet to impinge on turbine blades. As the gases expand over the, blades the pressure energy is converted into kinetic eneru. The K.E is absorbed by the turbine rotor and thus power is developed. The, expanded gases are exhausted into atmosphere. The speed of turbine can be controlled by regulating the fuel supply. The turbine shaft, may be coupled to compressor to drive it. Or else a separate electric motor is used to drive the compressor., Note :, (i) The p-V diagram and T-Φ diagram for the cycle are shown in figs (a) and (b)., (ii) The student, however, may draw only a line diagram of the cycle as shown in fig., CONSTANT VOLUME GAS TURBINE :, This is a turbine in which fuel is burnt and air is heated at constant volume in a combustion chamber. Basic elements of the turbine are, shown in fig., , The different parts of Constant Volume Gas Turbine are, 1. Air Compressor, 2. Combustion Chamber, 3. Air Valve

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4. Fuel Injector, 5. Spark Plug, 6. Nozzle valve, 7. Gas. Turbine, 8. Heater, 9. Steam Turbine, 10. Water Jacket, Air under pressure (2 to 3 bar) is supplied to combustion chamber (2) by an air compressor (1). Its quantity is regulated by air valve (3)., Fuel is injected through an injector (4) into the combustion chamber and is ignited by means of a spark plug (5). The pressure of mixture, of air fuel rises from 10 to 14 bar at constant volume. At this instant nozzle valve (6) opens and the combustion products pass through it, into the turbine (7). When they pass through nozzle valve pressure energy is converted into kinetic energy which is absorbed by turbine, rotor as the gases expand over turbine rotor as the gases expand over turbine blades. Thus mechanical energy is produced and the, turbine develops power rotating its output shaft. The hot expanded gases are exhausted into a heater (8) where water is evaporated by, the heat in exhaust gases. Steam produced in the heater is discharged into a steam turbine (9) to which the air compressor is coupled., Thus whole of the power developed by gas turbine may be made use of to run the generator etc. Combustion chamber may be cooled, by water jacket (10) in which water is circulated., In this turbine combustion is not continuous, but it is intermittent. Hence it is also called explosion type turbine. Its cycle of operations, follows Atkinson cycle., FUELS USED IN GAS TURBINE:, Most commonly used fuels in gas turbine are :, Oil, Producer gas, Blast furnace gas, Coal gas, Pulverized coal etc., APPLICATIONS OF GAS TURBINES :, Gas Turbines are generally preferred where high speed and high power are of primary consideration. They are employed, • in jet propulsion units for air crafts, • in ships as propulsion units, • for electric power generation, • to run locomotives and automobiles, • in the process of supercharging I.C engines, LIMITATIONS OF GAS TURBINE :, • Compared to thermal efficiency of I.C engines that of gas turbines is less. ,, • It is not possible to use closed gas turbine cycle in aeronautical engines because it requires cooling water for its operation., • The maximum combustion pressure in gas turbine as compared to I.C engines is less., • Constant volume explosion type gas turbine is more bulky on account of heater etc., and makes unfit for air craft applications., • The maximum gas temperature is limited to about 750°C only because of possibility of blade material to fail. The blade material is, nickel cobalt alloy., , Contact us Privacy & cookies, , Terms of use ©Swapdial