The Rankine Cycle

Ch 9, Lesson B, Page 2 - The Rankine Cycle

• The first change from the Carnot Cycle to the Rankine Cycle is that the boiler is not isothermal.
• The feed is always a subcooled liquid and the effluent is sometimes a saturated vapor, but more often it is a superheated vapor.
• The fact that the boiler is not isothermal makes it VERY difficult to transfer heat to the working fluid in a reversible manner.
• This new source of irreversibility is a small price to pay for saving the pump and turbine from costly damage.
• The isentropic turbine in a Rankine Cycle takes in saturated or superheated vapor and exhausts a high-quality vapor liquid mixture.
• The quality of the turbine effluent is usually greater than 90% and often greater than 95%.
• The condenser in the Rankine Cycle produces a saturated liquid.
• The isentropic pump raises the pressure of the saturated liquid feed, producing a subcooled liquid and completing the cycle.
• How do you think these changes effect the efficiency of the cycle ?
• Well, the 1st Carnot Principle tells us that the efficiency cannot be better than the efficiency of a Carnot Cycle, right?  But did the efficiency go down ? Yes, in fact it did.
• QH increased.  You can tell this is so by considering the area under the path from point 1 to point 2.
• QC also increased because the area under the path from point 3 to point 4. But which increased by a larger percentage ?
• Because the TH isobar dips downward after it leaves the 2-phase envelope, QH increases by a smaller percentage than QC.
• Remember that QC is wasted energy and the ratio of QC to QH is larger for the Rankine Cycle than it is for the Carnot Cycle.
• The result is that the efficiency of the Rankine Cycle is less than the efficiency of the Carnot Cycle.
• This isn’t a big surprise, but it is very important that you are able to see why this is true by looking at the cycles on a TS Diagram.
• Why did the efficiency go down ?  Which step became irreversible ?
• The boiler became irreversible because it no longer operates at a single temperature.
• No matter what temperature we have available in our high temperature reservoir, the heat transfer into the boiler will be irreversible.
• Consequently, the efficiency of even the ideal Rankine Cycle, with an isentropic turbine and an isentropic pump, drops below the efficiency of the Carnot Cycle.
• Now let’s take a quick look at each process in the Rankine Cycle.