Inlet vs Exhaust Control

[vederstein]

In industrial pneumatics, to control the speed of air cylinders, air motors, and the like, a flow control is added to the exhaust side of the device. This puts a restriction on the outgoing air thus acting as a cushion for the device and controlling speed.

On industrial hydraulics, the speed control is on the inlet to the device because the fluid (oil) will not expand or contract because it's incompressible.

Now steam engines (which are pneumatic devices) have a governer which controls the speed of the engine (which is really just a pneumatic cylinder) via a valve on the inlet of the cylinder which opens and closes proportional to the speed of the engine.

Now for the question:

If current industrial pneumatic design has the flow controls on the exit side of the cylinder, why do steam engines have the control on the inlet?

Perhaps I need to do some experiments, but it seems that control on the exit side would make the entire system simpler (no need for the governer, just a valve).

Controlling a steam engine speed on the exhaust side would have the engine speed somewhat proportional to steam pressure, but my experience designing industrial machines indicates that the proportionality is relatively small.

(Although governers are cool )

Anybody have some insights?

Thanks,

...Ved.

 

[Charles Lamont]

Controlling a steam engine speed on the exhaust side...

With pneumatic cylinders the (thermodynamic) efficiency does not matter particularly. In a (full size) steam engine it usually does, and back pressure is not good. In the most efficient steam engines the governor worked on the valve cut-off, not on a throttle. Constricting a flow is bad for efficiency.

 

[gbritnell]

Hi Ved,
The reason steam is controlled on the inlet side is for economy. Depending on the type of linkage used the steam admission can be controlled to give the greatest efficiency and economy while running.
gbritnell