Triple-expansion steam engine

A compound steam engine unit is a type of steam engine where steam is expanded in two or more stages. A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger-volume low-pressure (LP) cylinders. Multiple-expansion engines employ additional cylinders, of progressively lower pressure, to extract further energy from the steam.

Invented in 1781, this technique was first employed on a Cornish beam engine in 1804. Around 1850, compound engines were first introduced into Lancashire textile mills.

There are many compound systems and configurations, but there are two basic types, according to how HP and LP piston strokes are phased and hence whether the HP exhaust is able to pass directly from HP to LP (Woolf compounds) or whether pressure fluctuation necessitates an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver (receiver compounds).

In a single-expansion (or 'simple') steam engine, the high-pressure steam enters the cylinder at boiler pressure through an inlet valve. The steam pressure forces the piston down the cylinder, until the valve shuts (e.g. after 25% of the piston's stroke). After the steam supply is cut off the trapped steam continues to expand, pushing the piston to the end of its stroke, where the exhaust valve opens and expels the partially depleted steam to the atmosphere, or to a condenser. This "cut-off" allows much more work to be extracted, since the expansion of the steam is doing additional work beyond that done by the steam at boiler pressure.

An earlier cut-off increases the expansion ratio, which in principle allows more energy to be extracted and increases efficiency, but as the trapped steam expands its temperature drops. This temperature drop would occur even if the cylinder were perfectly insulating so that no heat is released from the system (see adiabatic process and § adiabatic heating and cooling). As a result, steam enters the cylinder at high temperature and leaves at a lower temperature. The changing steam temperature alternately heats and cools the cylinder with every stroke and is a source of inefficiency which increases at higher expansion ratios. Beyond a certain point, further increasing the expansion ratio will actually decrease efficiency due to the increased heating and cooling.

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