CHOICE OF TYPE
In large power station using high pressures and temperatures the compound impulse and the axial flow reaction are most common although radial flow machines up to 40 MW, 1,500 rpm, have been adopted. The singleshaft turbine is sound, simplifies operation and is general for small and medium sizes.
With radial flow turbines two alternators and two shafts are usual. Another case requiring two shafts is where it is economically justifiable by reason of high steam pressure to have a high pressure section running at a higher speed than a low pressure section. In deciding upon the number of cylinders the efficiency is nearly always of primary importance, and if this is to be a maximum with a large high-pressure turbine at least two cylinders will be necessary. A single-cylinder machine is cheaper in first cost than a multi-cylinder machine of the same output. It is possible to build single-cylinder turbines up to 80 MW at 1,500 rpm and up to 30 MW at 3,000 rpm, but general practice favours multi-cylinder sets for these larger sizes and also to separate high-pressure and low-pressure cylinders if the initial steam conditions are high. In the latter case the multi-cylinder turbine has the advantage that the separate high-pressure cylinder and its components which are subjected to the initial high pressure and temperature may be kept reasonably small. In this way the stresses in. these rotating and stationary parts may be kept within the safe limits of the materials available for use.
Further advantages of the use of multi-cylinder sets are that the diameters of the shafts may be kept within reasonable dimensions and designed to ensure that the critical speed is well above the running speed. The multi-cylinder turbine has resulted in a reduction of clearances rendered possible owing to the extremes of temperature in any one casing being reduced, thus enabling a turbine to be run up to speed much quicker than with a large single cylinder. The reduction in diameter of the wheels and shortening of the shafts reduces the stresses and tendency to whip. In some designs of multi-cylinder turbines the H. P. cylinder is of the "pure-reaction" type or even combined impulse and reaction. Some manufacturers do not employ reaction blading in H. P. cylinders on account of the small clearances which are necessary to obtain reasonably good efficiencies. The higher the initial steam pressure, the smaller will be the blade heights at the H. P. end, and it therefore follows that' the blade tip clearance with unshrouded blades must be very small to keep down the leakage over the blade tips. Alternatively, if the blades in high-pressure reaction turbines are shrouded to permit of safe blade tip clearances, the axial clearances must be kept very fine.
The disadvantages are that the overall length of the turbine is increased thereby necessitating larger building space and introducing additional losses by the use of interconnecting piping. The number of exhausts to be used will depend chiefly on the size of the turbine. The output of a single exhaust turbine is governed by the area of the exhaust annulus, the latter being limited by the blade tip speed. Losses at the exhaust are composed of the leaving losses and exhaust losses. The former are due to the carry-over velocity of the steam leaving the last row of blades. This loss may be reduced by using a double or triple flow exhaust arrangement, which in turn increases the output of the set. On the other hand the gain is offset by the additional floor space and cost of accommodation. A small drop in pressure must exist if steam is to flow from the last wheel to the condenser, and the heat energy required to produce this flow and make up for the losses due to eddies, etc., is termed the exhaust loss. With a given maximum exhaust area and given back pressure the output is limited if the efficiency is to be maintained and not impaired by high leaving and exhaust losses. To overcome this difficulty at the exhaust end turbines are usually of the multi-cylinder type arranged with single or double flow in a low pressure cylinder. With large output and low speed, a two cylinder turbine with a single-flow low-pressure cylinder can be used, as the low speed enables the requisite exhaust area to be obtained in a single exhaust.
The simplest type is the single-cylinder turbine, for it is compact and has few parts. Single-cylinder turbines with duplex exhausts are also adopted. The duplex exhaust turbine consists of two sets of low-pressure blading of the rotor, through which the steam flows in parallel, the two streams being brought together in the exhaust branch. With the double-flow turbine the axial thrust is balanced, since the flows are in opposite directions. In turbines having an intermediate cylinder the steam flow may also be arranged in the opposite directions, thus balancing the thrust. The volume of steam leaving the last wheel of a large turbine is enormous and it is more efficient and cheaper to discharge it to two or more condensers.
The performances of the various types for a given output are very similar and the choice of make is usually decided by the capital cost, steam conditions, output, speed, efficiency, and the opinions of the engineers concerned.
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