To make things somewhat simplistic when superheated water turns to steam and
is allowed to escape it will expand something like 1600 times(this is why a dry
crown sheet is deadly) its volume as water. When you compress air say too 150
psi that is only 10 times the general atmophieric pressure. Compressed air
quickly looses its power when allowed to expand in the cylinder where as steam
keeps pushing as it expands. We are currently building a 12" guage steam
locomotive and have it running on compressed air at the present time. It will
run nicely down to about 25psi, but of course this is wheels up and no load.
Our hope is to have at least a running boiler pressure of 150psi if not alittle
higher.

tim