In the ideal regeneration cycle , the efficiency is increased by exchanging the heat internally between the expanding steam in the turbine and feed water entering the boiler . Ideal regenerative cycle is not practically possible because the heat exchanger cannot be installed in the turbine as there are more chances for the erosion of turbine blades . In the practical regeneration cycle high temperature steam is extracted from the intermediate stage of turbine which is utilised to increase the temperature of feed water before entering the boiler.
Types of regeneration
Open feed water heater (OFWH)
It is basically a mixing chamber where high temperature steam is mixed with feed water before entering into the boiler to achieve equilibrium conditions . Mixing is highly irreversible and there is always a loss of energy due to mixing . Therefore the two streams should be mixed at equal pressure . These are generally not preferred as more number of pumps are required along with the losses due to mixing . Now a days there are only one OFWH is known as deaerator . The main purpose of deaerator is to remove the non condensable gases especially air from the feed water before entering into the boiler.
Close feed water heater (CFWH)
These are the proper heat exchanger in which heat is transferred from the extracted steam to the feed water without mixing takes place . So these streams can be kept at different pressures.
Effects of regeneration
- Cycle efficiency increases .
- For the same mass flow rate turbine work decreases.
- It provides a convenient way of removing the air from the feed water before entering the boiler.
- Condenser size decreases as the mass flow rate decreases .
- For the same power output steam flow rate increases therefore it requires large size boiler.
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