Regenerative Cycle and its Advantages & Disadvantages

Regenerative Cycle and its Advantages & Disadvantages

1. The practical regenerative cycle involves the bleeding the steam at suitable points during the expansion of steam in the turbine and using this bled steam for heating the feed water in feed water heaters.
2. A feed water heater is a heater exchanger consisting of tubes through which the feed water flows.
3. The bled steam is passed over these tubes where it condenses and transfers its latent heat of vaporization to the feed water in the tubes.
4. The schematic arrangement of actual regenerative feed heating cycle employing single stage feed water heating is shown in Figure A (1) and on (T-S) diagram in Figure A (1).
5. It should specifically be noted that the (T-S) diagram represents the flow path of steam and in no case it bears any relation to the net heat added in the cycle because of different mass flow rates at various points.
6. Consider 1 kg of steam from boiler.
7. One kg of steam at pressure P₁, enters the turbine at state 5 and it is expanded upto pressure P₂ represented by state 6.
8. Let m₁, kg of steam is blade for feed water heating at this intermediate state of the high pressure (H.P.) turbine of pressure P₂.
9. The remainder steam, (1-m₁,) kg, expands in the low pressure (L.P.) turbine upto back pressure P_b.
10. The condensate (1-m₁) kg from condenser is pumped into feed water heater working at constant pressure P₂ where this feed water is heated reversibly from the bled steam.
11. The amount of steam is so bled that it is just able to heat the feed water upto saturation temperature of bled steam i.e. it becomes saturated liquid at pressure P₂, represented by state 3 at the exit of feed water heater while the bled steam transfers its latent heat of vaporization.
12. The condensate of bled steam is pumped into feed water line with the help of drain pump as shown in Figure A (1).–
13. Finally the mixed condensate of bled steam and heated feed water at pressure P₂ is pumped into the boiler with the help of feed pump.
14. In case of Rankine cycle i.e. without feed water heater, the steam will expand from state 5 to state 7 and the condensate from state-1 will return directly to the boiler with the help of feed pump.
15. Therefore, mean temperature of heat addition process in case of regenerative feed heating cycle and Rankine cycle will be,

(TP₃)_{Regeneration =} h₅ – h4/s₅ – s_4; (Tm₂)_Rankine= h₅ – h₂/s₅ – s₂

It is obvious that the efficiency of regenerative cycle will be more than Rankine cycle because of the increased mean effective temperature (Tm₁>Tm₂) during heat addition process, while the mean temperature of heat rejection process remains the same.

Advantages of Regenerative Cycle :

1. It increases the thermal efficiency of the plant.
2. The temperature stresses in the boiler are reduced due to decreased range of working temperatures.

Disadvantages of Regenerative Cycle :

1. Steam rate increases due to reduced work done per kg of steam flow from boiler. Therefore, for a given capacity of the power plant output the boiler size increases.
2. Though the heat rejected in the condenser reduces for unit flow of steam, however, the size of condenser almost remains the same for a given capacity of power plant.
3. The cost of the plant increases.