The efficiency of a basic Rankin cycle is very less due to the enthalpy loss in the condenser. So the reheat and the regenerative cycle are used to increase the efficiency of the Rankin cycle.Here we will discuss how regenerative cycle is used in thermal power plant.
(Rankin cycle with regenerative feed heating)
If the bled steam is made to give up its latent heat and any superheat it possess to a heater then it is called Regenerative. For feed water heating, we extract some part of high enthalpy steam from the various stages of the turbine after being partially expanded in the nozzles of the turbine and sent to the feed water heaters. Feed water heaters are the heaters that transfer the heat from the high enthalpy steam with high pressure and temperature to the feed water in the cycle.
Advantages of Regenerative Feed Heating:-
1.Due to the steam extractions ,the steam flow at the initial stages or blades of the turbine is increased and the steam flow at the end of the turbine shell is reduced,which results in longer blades at the initial stages of the turbine and smaller blades at the exiting end of the turbine for increasing the efficiency of the turbine.
2.The amount of steam dumped in the condenser is also reduced,which results in smaller size of condenser and saving the capital investment.
Here no work is done by the extracted steam so efficiency can decrease, but here the enthalpy of the steam is utilized to heat the feed water, so we supply the sensible heat to the water before it enters the boiler, so we ultimately reduce the coal consumption in the boiler and the overall efficiency is increased.
Types of Heaters
Depending upon the method of heat transfer:-
1) Direct Contact type heater: In case of direct contact type heater, the steam extracted from the turbine is allowed to mix with feed water. So latent heat present in the steam is utilized to heat the feed water and the condensed steam mixes with feed water.
2) Indirect type heater: it consists of many tubes and a shell. Here the shell contains extracted steam and tubes contain the feed water. High temperature steam is utilized for increasing the temperature of feed water and after condensation of extracted steam, it supplies back to system/condenser through the UFT (Unit Flash Tank).UFT is nothing but a flash tank where the flashing of the drip occurs and after the flashing the steam is vented out and the condensed fluid is dumped into the condenser.
(Heater Internal)
Depending upon the working pressure:-
Low Pressure Heater(LP): A heater located (with respect to the feed water flow) between the condensate extraction pump and the boiler feed pump. Here the steam is extracted from the low pressure turbine.
High Pressure Heater(HP): These type of heaters are located between the boiler feed pump and the FRS(Feed Regulating Station). Here the tube side pressure is very high, and the steam extraction is taken from the high pressure turbine.
(HP Heater)
Design and Construction
In heaters, there are mainly three basic parts-
1.A shell with a steam connection
2.A nest of tubes
3.A metal box, with water connections on the inlet and outlet headers
Heaters consist of a cylindrical body made of mild steel, which houses a nest of solid brass U-tubes. The ends of the tube are trapped between a metal flanges on the body. Inside the tubes many baffles are provided for even distribution of the steam and between the inlet and outlet tubes of the heater insulation is also provided in order to maintain a temperature difference.
Different Zones in a feed water heater:-
Condensing Zone: In this zone, the overall coefficient of heat transfer is high. All the extracted steam is condensed and any remaining non-condensable gases are removed. A larger percentage of the heat energy added by the heater occurs here.
Drain cooling Zone: The condensed steam enters into this zone at the high temperature and is cooled by convection mode of heat transfer from the incoming feed water.
De-superheating Zone: In this region the incoming steam from the extraction enters the zone by giving up its most of the superheat to the feed water while exiting from the heater.
