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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. ==Compound systems== 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 a cut-off valve (archaically known as a regulator). The steam pressure forces the piston down the cylinder, until it reaches about 25%–33% of its stroke (although this can be varied by the operator in many engines), when the cut-off valve shut. After the steam supply is cut off (hence "cut-off valve"), 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" allow much more work to be extracted, since the expansion of the steam is doing much of the work, rather than just using pure boiler pressure, which would simply exhaust high-pressure steam to the atmosphere at the end of the stroke As the trapped steam expands in a high-pressure engine, its temperature drops; because (theoretically speaking) no heat is released from the system; this is known as adiabatic expansion and results in steam entering the cylinder at high temperature and leaving at low temperature, purely through the normal effect of expanding a fixed mass of heated fluid (ignoring the small amount of heat lost through normal heat radiation). This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency. The steam cut-off point when using a slide valve is less than 30% of the stroke. Early cut-off causes the turning moment on the shaft to be more uneven, requiring a larger flywheel to smooth this out.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Compound steam engine」の詳細全文を読む スポンサード リンク
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