562 ENGINEERING THERMODYNAMICS

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\M-therm\Th15-2.pm5

(v)Modified Rankine cycle efficiency, ηmR :

ηmR =

()( )hh p pv

hhf

12 2 32

(^13)
−+ −
−

647 4
3263 9 289 3
..−
= 0.217 or 21.7%. (Ans.)

12.4. Regenerative Cycle

In the Rankine cycle it is observed that the condensate which is fairly at low temperature
has an irreversible mixing with hot boiler water and this results in decrease of cycle efficiency.
Methods are, therefore, adopted to heat the feed water from the hot well of condenser irreversibly
by interchange of heat within the system and thus improving the cycle efficiency. This heating
method is called regenerative feed heat and the cycle is called regenerative cycle.
The principle of regeneration can be practically utilised by extracting steam from the tur-
bine at several locations and supplying it to the regenerative heaters. The resulting cycle is known
as regenerative or bleeding cycle. The heating arrangement comprises of : (i) For medium capacity
turbines—not more than 3 heaters ; (ii) For high pressure high capacity turbines—not more than
5 to 7 heaters ; and (iii) For turbines of super critical parameters 8 to 9 heaters. The most advan-
tageous condensate heating temperature is selected depending on the turbine throttle conditions
and this determines the number of heaters to be used. The final condensate heating temperature is
kept 50 to 60°C below the boiler saturated steam temperature so as to prevent evaporation of water
in the feed mains following a drop in the boiler drum pressure. The conditions of steam bled for
each heater are so selected that the temperature of saturated steam will be 4 to 10°C higher than
the final condensate temperature.
Fig. 12.15 (a) shows a diagrammatic layout of a condensing steam power plant in which a
surface condenser is used to condense all the steam that is not extracted for feed water heating.
The turbine is double extracting and the boiler is equipped with a superheater. The cycle diagram
(T-s) would appear as shown in Fig. 12.15 (b). This arrangement constitutes a regenerative cycle.

(a)

Boiler

T u r b i n e

Superheater

1 kg

O

(^12)
3
(1–m –m) 12
(1–m –m) 12
(1 – m ) 1
(1 – m ) 1
Condenser
m 1 m^4
2
L.P. heater
H.P. heater
5
6
1 – m 1
1 kg
m 1 m 2
O
6
5
4
1
2
3
s
T
(b)
m 1
m 2
(1––)m 12 m
Wout
Pump
1 kg
(1–m ) kg 1
(1––m 12 m ) kg
Fig. 12.15. Regenerative cycle.
Let, m 1 = kg of high pressure (H.P.) steam per kg of steam flow,
m 2 = kg of low pressure (L.P.) steam extracted per kg of steam flow, and
(1 – m 2 – m 2 ) = kg of steam entering condenser per kg of steam flow.