Editing Compound steam engine (section)

{{Short description|Steam engine where steam is expanded in stages}}
{{For|compound steam locomotives|Compound locomotive}}
{{Use dmy dates|date=March 2020}}
[[File:Triple expansion engine animation.gif|thumb|Double-acting triple-expansion marine engine<br/><small>High-pressure steam (red) passes through three stages, exhausting as low-pressure steam (blue) to the condenser</small>]]
[[File:TMW 677 - Triple expansion compound steam engine.jpg|thumb|Cutaway of triple expansion compound steam engine, 1888]]
[[File:Cross-compound Robey steam engine, close-up of cylinders, Bolton museum.jpg|thumb|[[Robey & Co|Robey]] horizontal cross-compound steam engine: small high-pressure cylinder (left) and large low-pressure cylinder (right)]]
A '''compound steam engine''' unit is a type of [[steam engine]] where steam is expanded in two or more stages.<ref name="Van Riemsdijk, Newcomen, Compound Locomotive" >{{Citation
  |last=van Riemsdijk |first=John
  |title=The Compound locomotive, Parts 1, 2, 3
  |journal=Transactions of the Newcomen Society
  |year=1970
  |issue=2
}}</ref>{{sfn|Van Riemsdijk|1994|pages=4–9 }}
A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) [[Cylinder (engine)|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.{{sfnp|Hills|1989|p=147}}

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<!-- , and the pressure of the steam increased, and the principle was applied to horizontal rotative engines, which became the standard engine in later mills -->.