A compound turbine is a steam turbine

Steam turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

 in which there are two casings, a high-pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 casing and a low-pressure casing, operating in concert to extract work from a single source of steam. The steam is partially expanded in the high-pressure casing, then exhausted to the low-pressure casing.

Tandem compound or cross compound

The rotor arrangement can be either tandem-compound in which the two axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

s are joined end to end, or cross-compound in which the two turbines have separate axles. In the cross-compound case two separate generators are usually supplied, although a gearbox can reduce this to one.

Advantages

The principal advantages of compound turbines are the reduction in size of any one casing, the confinement of the highest pressure to the smaller casing (which may be made of stronger and more expensive materials) and the possibility of divided flow in the low-pressure casing for the purpose of equalizing end thrusts.