Rankine Cycle and Regenerative Feed Heating
Rankine CycleFrom the T-s diagram of Rankine cycle it is evident that at the state 2-2’ working fluid enters the boiler and this temperature is very low temperature at which water is entering the boiler. As a result of its efficiency of the cycle is lower.
RegenerationThere is a way to overcome this problem by raising the temperature-of the working fluid (water) before it enters into the boiler and this process is called 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 basically a heat exchanger where heat is transferred to the feed water by extracting the partially expanded steam from the turbine to heat the feed water. Heating of feed water 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 HeatersOpen 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 Rankine regenerative cycle steam enters the turbine at the boiler outlet pressure at (5). After entering the turbine the steam expands isentropically in the turbine till the point (6) or intermediate pressure (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 helps in boosting the thermal-efficiency of the cycle and also the boiler heat input requirement by enhancing/improving the feed water temperature entering the boiler. No. of feed water heaters is directly having a positive impact on the Rankine cycle thermal efficiency, but addition of every heater should do judiciously after studying the real benefit and the cost incurred (capital expenditure required).