All
The reason the cylinders are larger as the steam pressure drops is that
the lower pressure requires a larger square inch surface (piston dia) to
output an equal amount of work as the high pressure cylinder & so on.
If the work rate of each clinder fell with the drop in pressure then the
engine would be extremely unbalanced with the consequent travails.
Some ships were fitted with 4 cylinder triple expansion engines because
the sq" required at the low pressure end would have resulted in a low
pressure cylinde diameter that was just not practical. The low pressure
was therefore split between two smaller cylinders.
Sam E
"Shyvers, Steve" wrote:
>
> Regarding SaltyChief's statement: "Take a look at the old three cylinder
> marine engines. They had three different sized cylinders with the exhaust
> from one cylinder feeding the next
> cylinder. To use the exhaust from one set of cylinders to power another set
>
> they have to be different sizes."
>
> It took a while for the lightbulb to turn on in my brain, and then some more
> time to figure out how to put it into words. The reason that each
> successive cylinder must be larger is because the steam enters the cylinder
> at a lower pressure than when it entered the previous cylinder. Because
> volume is inversely proportional to pressure the steam's volume is larger.
> If the lower-pressure cylinder is not big enough then it cannot hold all the
> steam coming from the previous, higher-pressure cylinder. I believe this is
> a fair explanantion of why a compound must have different sized cylinders.
>
> A small complication is that the steam loses temperature also as it passes
> through the high pressure cylinder and into the lower-pressure cylinder.
> This will reduce the steam's volume compared to what it would be if the
> steam temperature remained constant.
>
> My conclusion from all this is that to get efficiency from a compound
> requires a lot of calculation regarding steam temperature changes and
> pressure drops through the cylinders and piping, and expected work output
> from each set of cylinders. An inquiring mind could look up the published
> specifications for a particular compound locomotive, do a few
> straightforward calculations, and determine whether the loco manufacturer
> was fibbing about the loco's hauling capability.
>
> Steve
>
>
