ChemE 260 Improvements and Non-Ideal Behavior in the Rankine Cycle

ChemE 260 Improvements and Non-Ideal Behaviorin the Rankine Cycle Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 9: C & DCB 9: 3 - 6 May 20, 2005

Improvements on the Rankine Cycle • Superheat Rankine Cycle • Almost always used, improves h and turbine effluent quality • Supercritical Rankine Cycle • Increases h. Not common b/c, for steam, T and P are very high & materials become very expensive. • Reheat Rankine Cycle • Very common way to improve turbine effluent quality • h drops slightly unless regeneration is used as well. • Regeneration Rankine Cycle • Preheating boiler feed reduces irreversibility of heat transfer in the boiler • Increases h. • Binary Rankine Cycle • Not very common. Main advantage is TH >> TC • Big increase in h, but also increases cost to build. • Rankine Cycle with Cogeneration • Use some of the HP turbine effluent in another process. Baratuci ChemE 260 May 20, 2005

Superheat Rankine Cycle Baratuci ChemE 260 May 20, 2005

Supercritical Rankine Cycle Baratuci ChemE 260 May 20, 2005

Reheat Rankine Cycle Baratuci ChemE 260 May 20, 2005

Regeneration Rankine Cycle Baratuci ChemE 260 May 20, 2005

Binary Rankine Cycle Baratuci ChemE 260 May 20, 2005

Irreversibilities • 4 main sources of irreversibilty in a real power cycle: • Heat losses to the surroundings • Effects every process as well as the pipes that connect them. • Fluid friction • Effects every process as well as the pipes that connect them. • Result is pressure drop. This causes the temperature to drop as well in the boiler and condenser. • Mechanical losses (friction & rapid expansion & compression) • Effects the turbine and the pump the most. • Causes entropy to increase. • Subcooling in the condenser • Necessary to avoid cavitation in the pump. Baratuci ChemE 260 May 20, 2005

Irreversibilities on a TS Diagram Baratuci ChemE 260 May 20, 2005

Irreversibilities & Lost Work • From Lesson 8D: • Now, include the effect of direct heat lost to the surroundings: Baratuci ChemE 260 May 20, 2005

Next Class … • Prepare for Test #2 • After that … Test #2 • TCD Ch 5 – 8 • CB 4.3 – 4.5 and Ch 5 & 6 • And then … • Gas Power Cycles • Air-Standard Power Cycles • The Brayton Cycle • Variations on the Brayton Cycle • Regeneration • Reheat • Intercooling • Regeneration with Reheat and Intercooling Baratuci ChemE 260 May 20, 2005

Example Problem • Net Power, Heat Transfer and hth in an Ideal Rankine Cycle with Reheat • Water is the working fluid in an ideal Rankine cycle. The pressure and temperature at the turbine inlet are 1200 lbf/in2 and 1000oF, respectively, and the condenser pressure is 1 lbf/in2. The mass flow rate of steam entering the turbine is 1.4 X 106 lb/h. The cooling water experiences a temperature increase from 60 to 80oF, with negligible pressure drop, as it passes through the condenser. The ideal Rankine cycle is modified to include reheat. In the modified cycle, steam expands through the first-stage turbine to saturated vapor and then is reheated to 900oF. If the mass flow rate of steam in the modified cycle is the same as in ideal Rankine cycle, determine for the modified cycle: • the net power developed, in Btu/h. • the rate of heat transfer to the working fluid in the reheat process, in Btu/h. • the thermal efficiency. Baratuci ChemE 260 May 20, 2005

Example Answers Baratuci ChemE 260 May 20, 2005

ChemE 260 Improvements and Non-Ideal Behavior in the Rankine Cycle