Rankine Cycle Deep-Dive for Turbines
Beyond the Basic Loop
The simple Rankine cycle — pump, boiler, turbine, condenser — is a useful starting mental model, but no real utility power plant actually runs it in that bare form. Real cycles add two major refinements that meaningfully improve efficiency: reheat, which reheats steam partway through expansion, and regeneration, which uses extracted turbine steam to preheat feedwater before it reaches the boiler. Both refinements exist for the same underlying reason — to extract more useful work from the same fuel input.
Reheat — Recovering Temperature Mid-Expansion
After steam expands through the HP turbine, it has given up considerable energy and cooled significantly, but it's still well above condenser conditions. Rather than continuing to expand that cooler steam straight through IP and LP, a reheat cycle routes HP exhaust steam back into the boiler through a dedicated reheater section, raising its temperature back up close to the original throttle temperature. That reheated steam then re-enters the turbine at the IP section and continues expanding.
Reheat delivers two distinct benefits. First, it raises the average temperature at which heat is added across the whole cycle, which directly improves thermal efficiency (a core Rankine cycle principle: higher average heat-addition temperature means more of that heat converts to work). Second, it reduces the moisture content of steam in the final LP stages — expanding cooler HP exhaust steam all the way to condenser pressure without reheat would leave the LP stages handling significantly wetter steam, accelerating blade erosion.
Reheat doesn't add steam mass to the cycle — it's the same steam, reheated mid-expansion. The benefit is thermodynamic (higher average heat-addition temperature) and mechanical (drier LP steam, less erosion).
Regeneration — Preheating Feedwater with Extraction Steam
The second refinement works on the feedwater side. Instead of sending cold condensate straight back to the boiler, a regenerative cycle bleeds off ("extracts") small portions of steam at several points along the turbine's expansion and routes that extraction steam to a series of feedwater heaters. Each heater uses its extraction steam to raise the temperature of feedwater flowing back toward the boiler.
This might look counterintuitive — extraction steam skips the LP stages, giving up potential shaft work it otherwise would have produced. But the trade pays off: raising feedwater temperature before it reaches the boiler means the boiler has to add less heat (burn less fuel) to bring that water up to steam conditions. The net effect, worked out across the whole cycle, is a real efficiency gain — which is why virtually every utility-scale steam plant uses multiple stages of feedwater heating rather than none.
The Deaerator — Where Heating Meets Water Chemistry
One feedwater heater in the train does double duty: the deaerator is a direct-contact heater (extraction steam mixes directly with feedwater rather than through tube walls) that also mechanically strips dissolved oxygen and other non-condensable gases from the water. The deaerator sits at the dividing point between the low-pressure feedwater heaters (before it) and high-pressure feedwater heaters (after it), and its outlet typically serves as the suction source for the boiler feed pump.
Feedwater heater strings are graded by pressure to match each heater to an appropriate extraction point — pulling steam from too high or too low a pressure stage for a given heater wastes potential efficiency gain or risks poor heat transfer. If a feedwater heater is taken out of service, feedwater arrives at the boiler colder than design, and the boiler compensates by burning more fuel — a real, measurable heat rate penalty covered further in Module 1.4.
Putting It Together — The Full Cycle
A modern reheat-regenerative Rankine cycle, in sequence: feedwater is pumped to boiler pressure, heated and boiled to superheated steam, expanded through the HP turbine, reheated in the boiler, expanded through the IP and LP turbines (with extraction bleeds along the way feeding the feedwater heater train), condensed in the condenser, and returned through the feedwater heaters and deaerator back to the feed pump to repeat the cycle. Every one of these refinements — reheat, extraction, regeneration — exists purely to squeeze more usable work out of the same fuel input, and together they're the difference between a textbook-simple Rankine cycle and the real machine you operate on shift.