In an impulse turbine steam is admitted through a nozzle and directed against one or more rows of blades. Prior to passing through this nozzle, the steam is at high pressure but low velocity. The nozzle normally consists of a convergent and divergent section. In the former, the steam suffers a drop in pressure, but its velocity is increased. The function of the divergent section is to reduce to a minimum the tendency of the fluid to turbulence, and thus to ensure that the fluid flow is as smooth possible.
On emerging from the nozzle at its maximum velocity, the steam impinges on the row of moving blades which project radially from the turbine shaft. In this axial-flow type of turbine, the steam flow is along the axis of rotation of the shaft, and therefore the blades radiate outwords from the shaft. On entering the blades, which are set at a definite angle to the steam flow, the steam is deflected from its original path. In being deflected, it exerts an impulsive force on the blades, which causes them to rotate. While passing over the blades, the steam suffers a slight reduction in velocity through friction. In a simple turbine, it is then passed out into the atmosphere, or to a condenser, where it is condensed and led back to the boiler.
However, after leaving the blades of the turbine, the steam still possesses a considerable velocity, and this may be utilised in another type of turbine by passing it through a series of two or more turbine wheels. This is known as velocity-compounding. On passing through the first row of moving blades, the steam encounters a row of stationary blades which deflect the steam on to a second row of moving blades, and so on. Each time part of the kinetic energy of the steam is lost through friction, and therefore the velocity of the steam is progressively reduced. In order to compensate for this, the blades in each successive row are made progressively larger in cross-section, and their pitch is increased. In this way, a larger proportion of the kinetic energy of the steam can be utilised than in the simple turbine.
Another type of turbine in common use is known as the pressure- compounded turbine. It incorporates several rows of blades, but each one is enclosed between diaphragms to form a separate pressure stage. After passing through the first set of blades, the steam is directed through nozzles set in the succeeding diaphragm, and inpinges on the following row of blades.
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