Compound Engines
Expansion valves were also fitted to compound steam engines. Both techniques are an attempt to achieve greater efficiency, even at the cost of more complexity.
It was usual for expansion valves to be fitted only to the HP (high-pressure) cylinder. Steam supplied to the following LP (low-pressure) cylinder has already been supplied to the engine, so there is little benefit to conserving it. Any early cut-off of the steam inlet to a LP cylinder may also represent throttling the exhaust of the preceding HP cylinder, and a reduction in the efficiency of that cylinder.
Later compound mill engines with sophisticated valve gears often fitted the complex gear to the HP cylinder whilst retaining a simpler traditional slide valve for the LP cylinder. Examples existed with four different sets of valves: drop valve HP inlets, Corliss HP exhausts and a LP slide valve with a Meyer expansion valve.
Read more about this topic: Expansion Valve (steam Engine)
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