Ques. What is the application of the two systems?
Ans. Induced draft is installed mostly in new plants, while forced draft is better adapted to old plants.
Steam Turbines.—It is not the author's intention to discuss at length the steam end of the electric plant, because too much space would be required, and also because the subject belongs properly to the field of mechanical engineering rather than electrical engineering. However, because of the recent introduction of the steam turbine for the direct driving of large generators, and the fact that it is now almost universally used in large central stations, a detailed explanation of its principles and construction may not be out of place.
Fig. 2,748.—Longitudinal section of elementary Parsons type steam turbine. The turbine consists essentially of a fixed casing, or cylinder, and a revolving spindle or drum. The ends of the spindle are extended in the form of a shaft, carried in two bearings A and B, and, excepting the small parts of the governing mechanism and the oil pump, these bearings are the only rubbing parts in the entire turbine. Steam enters from the steam pipe at C and passes through the main throttle or regulating valve D, which, as actually constructed, is a balanced valve. This valve is operated by the governor through suitable controlling mechanism. The steam enters the cylinder through the passage E and, turning to the left passes through alternate stationary and revolving rows of blades, finally emerging from them at F and flowing through the connection G to the condenser or to the atmosphere, depending upon whether the turbine is condensing or non-condensing. Each row of blades, both stationary and revolving, extends completely around the turbine and the steam flows through the full annulus between the spindle and the cylinder. In an ideal turbine the lengths of the blades and the diameter of the spindle which carries them would continuously and gradually increase from the steam inlet to the exhaust. Practically, however, the desired effect is produced by making the spindle in steps, there being generally three such steps or stages, H, J and K. The blades in each step are arranged in groups of increasing length. At the beginning of each of the larger steps, the blades are usually shorter than at the end of the preceding smaller step, the change being made in such a way that the correct relation of blade length to spindle diameter is secured. The steam, acting as previously described, produces a thrust tending to force the spindle toward the left, as seen in the cut. This thrust, however, is counteracted by the "balance pistons," L, M and N, which are of the necessary diameter to neutralize the thrust on the spindle steps, H, J and K, respectively. These elements are called "pistons" for convenience, although they do not come in contact with the cylinder, but both the pistons and the cylinder are provided with alternate rings which form a labyrinth packing to retard the leakage of steam. In order that each balance piston may have the proper pressure on both sides, equalizing passages O, P and Q are provided connecting the balance pistons with the corresponding stages of the blading. The end thrust being thus practically neutralized by means of the balance pistons, the spindle "floats" so that it can be easily moved in one direction or the other. In order to definitely fix the position of the spindle, a small adjustable collar bearing is provided at R, inside the housing of the main bearing B. This collar bearing is adjustable so as to locate and hold the spindle in such position so that there will be such a clearance between the rings of the balance piston and those of the cylinder, that the leakage of steam will be reduced to a minimum and, at the same time, prevent actual contact under varying conditions of temperature. Where the shaft passes out of the cylinder, at S and T, it is necessary to provide against in-leakage of air or out-leakage of steam by means of glands. These glands are made tight by water packing without metallic contact. The shaft of the turbine is extended at U and coupled to the shaft of the alternator by means of a flexible coupling. The high pressure turbines are so proportioned that, when using steam as previously described, they have enough capacity to take care of the ordinary fluctuations of load when controlled by the governor through the valve D, thus insuring maximum economy of steam consumption at approximately the rated load. To provide for overloads, the valve V is supplied to admit steam to an intermediate stage of the turbine. This valve shown diagrammatically in the illustration, is arranged to be operated by the governor and is, according to circumstances, located either as shown by the illustration, or at another stage of the turbine.
Fig. 2,749.—Arrangement of blading in Parsons type turbine, consisting of alternate moving and stationary blades. The path taken by the steam is indicated by the arrows.