Adiabatic Compression : PV Diagram

Ch 8, Lesson C, Page 11 - Adiabatic Compression : PV Diagram

• On the last page, we saw that no adiabatic compressor can also be isothermal.
• Too bad, too!  Let’s see why.
• Consider the equation we derived in lesson 8B for SS, SISO processes with negligible changes in potential and kinetic energies.
• The compressors we are considering conform to all of these assumptions, so WS,dot over mdot is equal to the integral of Vhat dP.
• This means that the area to the LEFT of the process path for our compressor is proportional to the amount of shaft work we must supply to raise the pressure from 0.1 to 0.5 MPa.
• The good news is that the isentropic compressor requires the minimum work of any adiabatic compressor that doesn’t violate the 2nd Law.
• The bad news is that many compressors, including the isothermal compressor, require even LESS work. Too bad they violate the 2nd Law.
• What this PV Diagram is telling us is that if we could somehow COOL the working fluid as we compressed it, the process would require less shaft work ! Excellent!
• So, what is the minimum amount of shaft work we need to supply if we cool the fluid while we compress it?
• Flip the page and let’s see.