- 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
The T-s diagram of the Rankine cycle shows that at state 2-2’, the working fluid enters the boiler at a low temperature. This low entry temperature lowers the cycle’s efficiency.
Regeneration
We can solve this problem by raising the temperature of the working fluid (water) before it enters the boiler. This process is known as regeneration in steam power plants.
Conventional way of doing regeneration in a power plant is by extracting the steam from the turbine after partial expansion or partial work done. This steam is used to heat the feed water and the device in which it happens is called a feed water heater or a regenerator.
Regeneration improves the cycle efficiency by increasing the initial feed water temperature before the water, water enters the boiler and also helps in controlling the large the large flow rate of steam at the turbine exhaust.
Regeneration is commonly used in all power plants where efficiency is of importance and fuel saving is the motto.
A feed water heater is a type of heat exchanger where partially expanded steam from the turbine transfers heat to the feed water. This heating can be done by:
- Directly heating (in a tank)- Direct heating of feed water is performed in tanks or vessel also called open feed water heaters; or
- By indirect heating (in shell and tube type heat exchanger)- Indirect heating of steam and water is performed on shell and tube type closed heaters
Regeneration with Open Feed Water Heaters
Open or direct feedwater heating comprises of the vessel were extracted steam and feedwater directly mix with each other. Heated mixture leaves the tank at a temperature which is in line with the pressure of the mixing chamber. An example of the power plant operating with single stage regenerative cycle on the T-s diagram given below.
In a regenerative Rankine cycle, steam enters the turbine at the boiler outlet pressure (point 5). The steam then expands isentropically until it reaches an intermediate pressure (point 6), where it is extracted.
At the state (6) some steam is taken out or extracted and directed towards the feedwater heater while the rest of the steam continue to expand in the remaining stages of the turbine till the end i.e. condenser at a pressure corresponding to condenser pressure at state (7). The condensate in the condenser is at the saturation temperature corresponding to the condenser pressure at (7).
From condenser Condensate leaves as a saturated liquid at condenser pressure (1). Condensate or feedwater from here enters into the open feedwater heater via pump (1), where it comes in direct contact with the steam extracted from the turbine at (6). Mixture leaves the open feedwater heater as saturated liquid corresponding to heater pressure at (3). Second pump raises the feed water pressure equal to boiler pressure (4), in boiler chnage of state from water to steam and then superheating of steam took place to match the turbine inlet parameters.
For every 1 kg of steam coming out of the boiler, some amount of steam (y) kg expands partially in the turbine up to (6) and extracted to heat the feed water. Remaining quantity of steam (1-y) kg worked completely over the rest of the turbine stages to condenser pressure.
If the boiler generates m kg of steam, then it is (1-y)m steam enters the condenser. Analysis of the heat and work interaction for single feed water heater per unit mass of steam flowing through the boiler is given by:
Where,
Regeneration boosts the thermal efficiency of the cycle and reduces the boiler heat input requirement by improving the feed water temperature entering the boiler. The number of feed water heaters directly impacts Rankine cycle efficiency, but each addition should be considered carefully to balance benefits and costs.
