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Power Engineering and Refrigeration 45 Gas Turbines & Jet Propulsions, , , , Working of Constant Volume Gas Turbine :, , Air from atmosphere is sucked into compressor and compressor increases its pressure around 3 kN/m. This, compressed air is admitted to constant volume combustion chamber through inlet valve where fuel is injected by, fuel pump. This mixture of air and fuel is ignited by spark plug to raise pressure upto 12 kN/m* where volume, remains constant. Thus, combustion takes place at constant volume., , This high pressure and high temperature gas mixture is expanded in turbine to get work of expansion and, exhaust is released into atmosphere., , The part of power developed by turbine is used to drive compressor and remaining power is used to drive, generator. The starter motor is used to start the compressor and is disconnected once the turbine starts to develop, the power,, , The main disadvantage of constant volume gas turbine is that the pressure difference and velocity of hot gas do, not remain constant and therefore speed of turbine fluctuates., , , , , , 4.5 OPEN CYCLE GAS TURBINE, , , , , , A simple open cycle gas turbine consists of a ‘4s Fuel combustion, inFig. 43., , Fresh air is drawn into the compressor from the, atmosphere and heat is added by combustion of fuel in, combustion chamber., , The exhaust from the turbine is released in the, atmosphere. Therefore in open cycle gas turbine, a, arrangement of continuous replacement of the working 1, medium is required Ar Exhaust ges, , Fig. 4.3 : Open cycle gas turbine, , , , , , Starter Compressor Turbine Wy, , , , , , , , , , , , 4.6 CLOSED CYCLE GAS TURBINE, , , , , , Instead of rejecting exhaust from turbine into the atmosphere like open cycle, closed cycle turbine circulates the, same working air continuously. Thus, the working air remain confined within the plant., , Since the closed cycle gas turbine continuously %, Circulates the same working gas, the gases of higher Neatexchanger, density than air like hydrogen or carbon dioxide are s woe Th a, , The heat addition is supplied through a heat, exchanger which is generally called as ‘air heater’ as “™w'*" —*5} Compressor Tare “, shown in Fig. 4.4 and heat rejection takes place in oo, another heat exchanger with cooling medium which is, generally called as ‘precooler, , Thus, in closed gas turbine, the gas is compressed, from pressure P, to P, in the compressor. oe, , Fig, 4.4 : Closed cycle gas turbine, , The compressed gas is heated in air preheater, where fuel and air are supplied from outside, After this, the, compressed and heated gas expands in turbine, , The exhaust gas from the turbine is cooled in a precooler and is recirculated to compressor., Advantages of Closed Cycle Gas Turbine (s-18), , 1. A high pressure can be maintained throughout the cycle with consequent reduction in volume of air, , circulated causing reduction in the size of components., 2, The cycle operates with the limited amount of clean air, which is used again and again. Therefore, the, compressor is not troubled by the deposits of atmospheric dust and dirt., 3. Since the combustion is external, any type of fuel can be used, , , , , , , , , , , , , , , , , , , , , , Cooler