In single-flow turbines the steam enters at one end, flows once through the blading in a direction approximately parallel to the axis, emerges at the other end, and enters the condenser. This is the most common arrangement in small and moderately large turbines.

In double-flow turbines, which are generally lowpressure machines, the steam enters at the center and divides, the two portions passing axially away from each other through separate sets of blading on the same rotor. This type of unit is completely balanced against end thrust and gives large area for flow through two sets of low-pressure blading. In these turbines the two low-pressure exhausts are brought together within the casing and led to the condenser; in some cases, separate exhaust connections are used for the two ends.

Compound turbine is the term applied to a machine in which the steam passes in sequence through two or more separate units, expanding in each. Most often the high-pressure and the intermediate-pressure portions are in one casing and the low-pressure portion in another. Two units may be arranged tandem compound1, in line on the same shaft and driving the same electrical unit, or cross compound, on separate shafts and driving separate electrical units. In the latter case, the low-pressure unit may rotate at a lower speed, which is advantageous for handling large steam flows, but the speeds must have a certain definite relation dictated by generator frequency requirements. The two units of the cross-compound turbine are operated as one, being held rigidly in step2 by the electrical phase relation. Tandem-cross-compound arrangements are also possible, where the high-pressure unit and its generator are on one shaft and the intermediate- and lowpressure units are arranged tandem and drive two generators.

The two units of a cross-compound turbine may be gear-connected to a single generator or to a single propeller shaft. In some marine installations, separate high-, intermediate-, and low-pressure units have been gear-connected to a single propeller shaft. Various divided-flow arrangements have been used in the past, with splitting of the flow of partially expanded steam from the high or intermediate sections between two other units. These are now little used in stationary practice, but in marine installations, where two propeller shafts must be driven independently but at approximately equal speeds, except when manoeuvering, they find application.

 

(Church E. F.)