The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. Velocity drop occurs in the moving blades and not in fixed blades. It can be explained mathematically from the following formula i. In second set of nozzles, the remaining pressure fall takes place but the velocity here raises and the drop in velocity takes place in the moving blades of the second wheel or rotor. The next ring of moving blades absorbs the velocity obtained from this second ring nozzle. It is done by the fixed blades which act as nozzles.
Because they are usually used in the entrance high-pressure stages of a steam turbine, when the specific volume of steam is low and requires much smaller flow than at lower pressures, the impulse blades are short and have constant cross sections. The effect of absorbing the pressure drop in stages is to reduce the velocity of the steam entering the moving blades. Hence, velocity is very less as compared to the previous case. A detailed numerical techniques to solve complicated thermal engineering problems will be covered throughout this course. The minimum cross-section of such ducts is known as throat. Steam turbines are employed as the prime movers together with the electric generators in thermal and nuclear power plants to produce electricity. Consider a typical example to understand it better.
This process is shown diagrammatically in figure 5. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. Proceedings of the South African Sugar Technologists' Association. It also follows that the sonic velocity can be achieved only at the throat of a nozzle or a diffuser. Based on types of compounding techniques, it could be velocity or pressure.
The steam then enters the next ring of moving blades; this process is repeated until practically all the energy of the steam has been absorbed. The steam pressure remains unaltered. A steam turbine is basically an assembly of nozzles fixed to a stationary casing and rotating blades mounted on the wheels attached on a shaft in a row-wise manner. This is used to solve the problem of high blade velocity in the single-stage impulse turbine. The pressure partially decreases and the velocity rises correspondingly. Discharge Mode, Example, Tutorial Sheet 8.
Conversely if the cross section increases gradually from the inlet to exit, the duct is said to be divergent. At subsonic speeds Ma1 , the effect of area changes are different. In this type of compounding, steam is expanded more than once as in velocity compounding. The velocity diagram in figure 2, shows the various components of steam velocity and the blade velocity of the moving blades. This is explained in figure 6. The fixed blades act as nozzles i.
Curtis patented the velocity compounded de Lavel turbine in 1896 and transferred his rights to General Electric in 1901. The fixed blades redirect the steam into the next set of moving blades. However, the work produced in each stage is not the same. The steam passes over a series of alternate fixed and moving blades. Both methods have their Pros and Cons, hence choice is made as required.
The enthalpy drop per stage of one row fixed and one row moving blades is divided among them, often equally. This means that maximum power can be produced at much lower blade velocities. This process is shown diagrammatically in figure 5. Velocity compounded stage is also called Curtis stage. Velocity drop occurs in the moving blades and not in fixed blades. The moving blades move as a result of the impulse of steam received caused by change in momentum and also as a result of expansion and acceleration of the steam relative to them.
Therefore the pressure decreases and velocity increases partially in the nozzle. They are usually symmetrical and have entrance and exit angles respectively, around 20 °. A schematic diagram of the Curtis stage impulse turbine, with two rings of moving blades one ring of fixed blades is shown in figure 1. This high velocity steam is directed on to the first set ring of moving blades. In this type of turbine the pressure drops take place in a number of stages. As the steam flows over the blades, due the shape of the blades, it imparts some of its momentum to the blades and loses some velocity. It is done by the fixed blades which act as nozzles.
This does not imply any heat transfer to the stages from outside. Hence, the pressure thereafter remains constant. This leads to an increase in the velocity of the steam. Each stage acts as a velocity compounded impulse turbine. The point 3 corresponds to a metastable equilibrium state of the vapour. The symbols in the figure have same meaning as above. The steam expands equally in all rows of fixed blade.