Nav wrote:

But I'm puzzled by your statement that recips don't need as great a
vaccum for peak efficiency. Isn't thermodynamic efficiency directly
related to the inlet and outlet pressures -or do you mean that most
reciprocating engines run at higher outlet pressures?

Reciprocating steam engines alternately see cool steam and hot steam on
the same end of the cylinder. As the steam expands during the stroke it
cools. As it cools, it cools the cylinder and some of it may condense on
the cylinder walls.

When steam is admitted to the previously exhuast side some of it
condenses immediately then reevaporates as the cylinder temperature
rises again. This phenomenon is exaggerated at the low pressures and
temperatures associated with greater vacuum. The thermodynamic
efficiency of a recip steam engine is limited due to this effect.

That is why steam turbines like superheated steam and exhaust into the
greatest vacuum obtainable. The only place we want steam to condense is
in a condenser and the only place we want to evaporate feedwater is in
the boiler.

Rick