Lecture 3: Regenerative Rankine cycle

1. An Introduction to Rankine cycle with
Regeneration
A technique to raise the average
temperature of the heat addition process.

2. T
1
a
3
s
Internal heat transfer to feed water heater.
4
2
Principle of the Regenerative Cycle
Extract a portion of the steam from the turbine and send to (regenerative)
feedwater heater(s) to preheat the condensate before entering the boiler.

3. Open Feedwater Heater
Introduction

4. Closed Feedwater heater (Figures)

5. The ideal Regenerative
Rankine cycle with an
open feedwater heater.
Using Open Feedwater Heaters
Note: Heat loss from the feedwater heater is considered negligible.
(1)
(1)
(y)
(1-y)
(1)
(y) (1-y)
What is Pump II called?

6. OFwH ( mass fractions [y] )
y m m
  / 
6 5
)
1
(
1
/
/
5
2
5
2
5
6
5
3
2
6
y
m
m
m
m
m
m
m
m
m
m
m
m out
in








 













Let be the fraction of mass extracted from the turbine for the feedwater
heater. Conservation of mass gives:
 
  
 ( )  
E E
m h m h m h
ym h y m h m h
y
h h
h h
in out

 
  



6 6 2 2 3 3
5 6 5 2 5 3
3 2
6 2
1
Conservation of energy for the open feedwater heater:
y
From the outlet
of the condenser
and first feedwater
pump. (1-y kg.)
From the
turbine.
(y kg.)
To the second
feedwater pump.
(1 kg.)
1-y
6
2
3 Open
FWH

7.  
  
4
5
1
2
1
,
3
4
2
,
7
6
2
,
6
5
1
,
1
)
1
(
h
h
Q
h
h
y
W
and
h
h
W
h
h
y
W
and
h
h
W
H
IN
IN
OUT
OUT












h
s
1
3
4
2
WOUT,1
QH
QC
WIN,1
5
6
7
WOUT,2
WIN,2
(1 kg)
(y kg)
(1-y kg)
The h-s diagram of Regenerative Cycle with OFwH
       
4
5
1
2
3
4
7
6
6
5 )
1
(
)
1
(
h
h
h
h
y
h
h
h
h
y
h
h













8. OFwH (Deaerating heater or DA)
§Purpose is also to vent dissolved gases from the cycle.
Procedure is called deaeration. Note that solubility of
these gases decreases with increasing temperature.
§ Helps to minimize corrosion.
§ In general, only one is used.

9. Closed Feedwater Heater
Introduction

10. Closed Feedwater heater (Figure)

11. )
)(
1
(
;
)
)(
1
(
)
(
)
)(
1
(
1
8
5
6
8
7
7
6
3
4
1
2
h
h
y
Q
h
h
Q
h
h
y
h
h
W
h
h
y
h
h
y
W
C
H
T
P















(1)
(1)
(y)
(1-y)
(y)
(1-y)
TTD or TD = Terminal temp. difference = Bled steam exit temp. – Feedwater exit temp.
The ideal regenerative Rankine
cycle with a closed feedwater
heater pumped forwards

12. The ideal regenerative Rankine
cycle with a closed feedwater
heater cascaded backwards
)
(
)
)(
1
(
;
)
)(
1
(
)
(
1
5
1
8
3
6
8
7
7
6
1
2
h
h
y
h
h
y
Q
h
h
Q
h
h
y
h
h
W
h
h
W
C
H
T
P














(1)
(1)
(y)
(1-y)
(y)
(1-y)
5
4
: h
h
Note 

13. OFwH
§The extracted steam and condensate are physically mixed (direct-contact).
§ Always requires additional pump per heater.
CFwH
§ The extracted steam and the condensate are not mixed (shell-and-tube HX).
§ May or may not require pump (depends on type).
Some differences between OFwH and CFwH