- Rankine Cycle Definition: The Rankine cycle is defined as a process in steam power plants where water is heated, turned into steam, and then used to produce mechanical work.
- Regeneration Purpose: Regeneration raises the temperature of water before it enters the boiler, improving efficiency in steam power plants.
- Feed Water Heaters: Feed water heaters, also known as regenerators, heat the feed water using extracted steam, either directly or indirectly.
- Open Feed Water Heating: Open feed water heaters allow steam and feed water to mix directly, resulting in a heated mixture that improves cycle efficiency.
- Regenerative Rankine Cycle: The regenerative Rankine cycle enhances thermal efficiency by extracting steam at an intermediate pressure to preheat the feed water.
Rankine Cycle
In the basic Rankine cycle, compressed liquid enters the boiler at state 2 and is heated through a wide temperature range. Raising the feedwater temperature with heat taken from within the cycle increases the average temperature of external heat addition and can improve thermal efficiency.
Regeneration
Regeneration preheats the working fluid before it returns to the boiler. In steam power plants, this normally uses steam extracted from an intermediate turbine stage.
For conventional regeneration, part of the steam is bled from the turbine after partial expansion. It transfers energy to the feedwater in an open or closed feedwater heater. The remaining steam continues through the later turbine stages.
Regeneration reduces the external heat required in the boiler for each unit of main steam and reduces the mass flow through the low-pressure turbine and condenser. It usually improves cycle thermal efficiency, but extraction also reduces turbine work from the bled steam.
Plants choose the number and pressures of heaters by balancing efficiency, output, equipment cost, complexity and operating constraints.
A feedwater heater is a regenerative heat exchanger in which extracted steam transfers energy to the feed water. Two common arrangements are:
- Direct heating in a tank: Extracted steam mixes with feedwater in an open feedwater heater.
- Indirect heating in a surface heat exchanger: Steam and feedwater remain separate in a closed feedwater heater.
Regeneration with Open Feed Water Heaters
In an open feedwater heater, extracted steam and pumped condensate mix at the heater pressure. Under ideal analysis, the outlet is often taken as saturated liquid at that pressure. A real heater can leave the water slightly subcooled and must also remove non-condensable gases and control level. The diagrams below show a single-stage regenerative cycle.
Steam enters the turbine at boiler outlet state 5. In the ideal cycle it expands isentropically to the extraction pressure at state 6. A real turbine has entropy generation, so its actual outlet enthalpy differs from the ideal value.
At state 6, a fraction of the steam flows to the feedwater heater. The rest expands through the remaining turbine stages to the condenser pressure at state 7. Ideal analysis commonly treats the condenser outlet as saturated liquid, while real condensate may be subcooled.
Condensate at state 1 passes through the condensate pump, which raises it to the open-heater pressure at state 2. It mixes with extracted steam from state 6. In the ideal model, saturated liquid leaves the heater at state 3. A second pump raises the pressure to boiler pressure at state 4. The boiler then heats, evaporates and, where specified, superheats the water before it returns to the turbine.
Use 1 kg of steam leaving the boiler as the calculation basis. A fraction y is extracted at state 6, while the remaining fraction 1 – y continues to the condenser. The open-heater mass and energy balances determine y.
If the boiler flow is m kg, then (1 – y)m kg continues to the condenser. The following expressions show the ideal mass, energy, heat and work balances for one open feed water heater on a unit-main-flow basis:
Where:
Regeneration can increase thermal efficiency by raising the feedwater temperature entering the boiler. More heater stages can approach a better temperature profile, but the benefit is not unlimited. Each stage adds pressure losses, pumps or drains, controls, maintenance and capital cost, while the extraction changes turbine output. A complete Rankine cycle calculation must include those effects.
