Ans. When all the power is furnished by the turbine, it is designed for high pressure; when operated in combination with a reciprocating engine, low pressure is used for constant load, and mixed pressure for variable load.
Fig. 2,754.—Sectional view of a semi-double flow turbine. This is a modification in which the intermediate section of reaction blading is single flow, and the low pressure section only is double flow. This would be analogous to a four cylinder triple expansion engine, that is, one with one high pressure, one intermediate pressure and two low pressure cylinders—a design not at all uncommon in very large engines in which the required dimensions of a single low pressure cylinder would be prohibitive. Such turbines are useful for capacities greater than is desirable for a single flow turbine, and which are still below the maximum possibilities of a double flow turbine of the same speed. In such machines the best efficiency is secured by making the intermediate blading in a single section large enough to pass the entire quantity of steam. A "dummy" similar to those used on the single flow Parsons type, shown at the right of the impulse wheel, compels all of the steam to pass through the single intermediate section of the reaction blading, and balances the end thrust due to this section. When the steam issues from the intermediate section, the current is divided, one-half passing directly to the adjacent low pressure section, while the other half passes through the holes shown in the periphery of the hollow rotor and through the rotor itself, beyond the dummy ring, into the other low pressure section at the left hand end of the turbine.
NOTE.—There are logical engineering reasons for the existence of the several types of turbine, viz., single flow, double flow, and semi-double flow. The double flow turbine is not inherently superior to the single flow design, but is used under conditions for which the single flow machine is unsuitable. Similarly, the semi-double flow is recommended only for conditions which it can meet more satisfactorily than either of the other types.
NOTE.—Low pressure turbines use exhaust steam from non-condensing engines and are valuable as an adjunct to existing plants for the purpose of increasing economy and capacity with a minimum outlay for new equipment.
NOTE.—Bleeder turbines are for use in plants which are required to furnish, not only power, but also considerable and varying quantities of low pressure steam for heating purposes. In these turbines a part of the steam after it has done work in the high pressure stages may be diverted to the heating system, and the remainder expanded through the low pressure blading and exhausted into the condenser. In this way none of the energy of the heating steam, due to the difference of pressure between the boiler and the heating system is wasted. On the other hand if no steam is required for heating purposes, the turbine operates just as efficiently as though the bleeder feature were absent.
