The working pressure of steam

[Tantra]

I am trying to clarify a question I have about the pressures that drive a steam engine or turbine. Does vapor pressure correspond to absolute pressure? Do these properly describe the pressures involved in operating a steam engine or turbine?
Or more specifically, is the difference in vapor pressure of 0.1014 (MPa) at 100°C and 22.064 (MPa) at 374°C properly describing what happens in a steam engine by saying that the high pressure is approximately 220 times greater than the low-pressure and in the most simple terms, this is what drives the engine? And, is a steam turbine more effective, in part because the low end (through condensation) is even lower, perhaps 0.0386(MPa) at 75°C? And is this properly described as being a pressure of approximately 571 times less than a high pressure of 22.064 (MPa)?
And,
what is the range of pressures involved in an efficient steam turbine, the inlet pressure versus the outlet pressure?
I am working on a design and would really appreciate some clarification about this, thanks.

 

[byrdj]

Instead of delta presure, you might want to look at delta enthalpy. because it is a "heat engine",

 

[danberry]

Yes, vapor pressure is absolute. Pressures relative to atmospheric are termed "gauge".

It is not false to say the inlet pressure is 220 times the outlet pressure but it doesn't mean much. The maximum theoretical work that can be obtained from a machine is the difference in enthalpy between the initial steam condition (pressure and temperature), and the enthalpy of the steam at the final condition (final pressure and initial enthropy).

With a real machine (eff < 100%) the final enthropy is higher than the initial enthropy. The enthalpy difference will be lower for the same pressure drop.

The higher the inlet pressure and temperature, the more powerfull the machine (for a fixed exit pressure). Inlet pressures can be as high as materials will allow. The exhaust pressure can be as low as your heat sink will allow. Exhaust pressures in the 5 kPa range are common in power plants; this is under vacuum.

Does this help?