Transcript ChemE 260 - Thermodynamics
ChemE 260 The Rankine Cycle
Dr. William Baratuci
Senior Lecturer Chemical Engineering Department University of Washington TCD 9: A & B CB 9: 1 & 2 May 17, 2005
Vapor and Gas Power Systems
• • • • Baratuci ChemE 260 May 16, 2005
Vapor Cycles
– Use evaporator and condenser for isothermal heat transfer steps
Gas Cycles
– No phase changes – Use isobaric heat exchangers for heat transfer steps
Closed Cycles
– Not piston-and-cylinder devices – Working fluid remains inside the system – Use two HEX’s to absorb and reject heat – Power generation turbines
Open Cycles
– Fresh working fluid drawn into the cycle – Spent working fluid discarded after 1 pass through the cycle – Usually air is the working fluid – Only 1 HEX because the fluid doesn’t actually complete the cycle – Automobile and jet engines
PV Diagram
P
• This is a PV Diagram of a Gas Power Cycle • A Vapor Power Cycle looks the same, but the 2-phase envelope is also shown on the plot.
1 4 Q H Q C 2 T H 3 T C V W Sh,cycle
Baratuci ChemE 260 May 16, 2005
m
2
1
2 3
3 4
4 1
Enclosed Area
• In Vapor or Gas Power Cycles, the area enclosed by the cycle path is equal to the specific shaft work produced by the cycle.
TS Diagram
• This is a TS Diagram of a Vapor Power Cycle • A Gas Power Cycle looks the same, but the 2-phase envelope is not shown on the plot.
Baratuci ChemE 260 May 16, 2005
W Sh,cycle m Q cycle
2 1
W Sh,cycle
Q
T
3 2
int rev
4 3
1 4
Enclosed Area
• In Vapor or Gas Power Cycles, the area enclosed by the cycle path is, once again, equal to the specific shaft work produced by the cycle.
Carnot Is Not Practical
• Why is the Carnot Vapor Power Cycle impractical ?
– Most pumps do not handle vapor-liquid mixtures well • Pumps that do are more expensive and less efficient.
– Turbines do not work well with low quality • Quality must be greater than 90% at the outlet • This is difficult to achieve without superheating in the reboiler – Using strictly isothermal heating prevents… • Subcooled liquid feed to the boiler which would make the pump more efficient • Superheating in the boiler effluent which would increase turbine effluent quality Baratuci ChemE 260 May 16, 2005
The Rankine Cycle
• • •
A practical vapor power cycle
– Minimizes cavitation problems in the pump – Allows for the possibility of superheating in the boiler
The Rankine Cycle
– Step 1-2: – Step 2-3 – Step 3-4: – Step 4-1: Boiler Turbine Condenser Pump Heat added at constant pressure Isentropic expansion Heat rejected at constant pressure Isentropic compression
Internally Reversible
– External irreversibility due to heat transfer through a finite temperature difference in the boiler Baratuci ChemE 260 May 16, 2005
Baratuci ChemE 260 May 16, 2005
TS Diagram
Baratuci ChemE 260 November 23, 2004
Cavitation
Efficiency & Boiler Pressure
• W Sh • Q H
th W Sh Q H
• th Baratuci ChemE 260 May 16, 2005
Efficiency & Condenser Pressure
• W Sh • Q H : same
th W Sh Q H
• th Baratuci ChemE 260 May 16, 2005
Next Class …
• CIDR Focus Group Interview • After that … Problem Session • And then … – Improvements to the Rankine Cycle • Superheat the turbine feed • Pump to supercritical pressure • Use a two-stage turbine with a reheater between stages • Use regeneration with a two-stage turbine and a two-stage pump • Use two vapor power cycles to cover a wider T range – Non-ideal behavior in vapor power cycles • Boilers and condensers cannot operate at constant P because of friction • Turbines and pumps are neither adiabatic nor internally reversible.
• Pump feed must be subcooled to avoid cavitation.
Baratuci ChemE 260 May 16, 2005