Lecture 4: Regenerative Rankine cycle (Closed Feedwater Heaters)

1. Closed Feedwater Heater
With Drains Pumped Forward

2. Regenerative Cycle with a CFwH with Drains Pumped
Forward
5
1
6
7
(y)
QH
QC
WT
10 9
Steam
generator
WP
Turbine
2
3 4
(y/)
12 13
14
8
(1-y-y/)
11
(1)
TTD for h.p. & l.p. heater = ?
TTD or TD = Terminal temp. difference = Saturated temp. of bled steam – Exit water temp.

3. Analysis of Regenerative Cycle with a CFwH with Drains
Pumped Forward
1
2
3
4
5
6
7
8
s
T
9
11
10
12
(1)
(y)
(1-y-y`)
(y`)
13
14
Lets do the thermodynamic
analysis.
1. Energy balances on h.p. & l.p.
heaters?
2. Energy balances on h.p. & l.p.
mixing chambers?
3. Wp = ?
4. WT = ?
5. QH = ?
6. QC = ?
TTD or TD = Terminal temp. difference = Saturated temp. of bled steam – Exit water temp.

4. Closed Feedwater Heater
With Drains Cascaded Backwards

5. Regenerative Cycle with a CFwH with Drains Cascaded
Backwards
TTD or TD = Terminal temp. difference = Saturated temp. of bled steam – Exit water temp.
DCA or DC = Drain Cooler Approach = Sub-cooled temp. of bled steam – Inlet water temp.
5
1
6
7
8
y
QH
QC
WT
CFwH (cascaded
backwards) with DC
Trap
10
9
Steam
generation
unit
WP
Turbine
2
3 4
Trap
y/
11
12
(y + y/)
CFwH (cascaded
backwards) without
DC
(1)

6. T-s diagram for a Regenerative Cycle with a CFwH
with Drains Cascaded Backwards
DC = T11 – T7
For h.p. heater,
TTD = Tsat(@P2) – T8
i.e. < 0 because of
superheated steam.
For l.p. heater,
TTD = T9 – T7
i.e. > 0.
Note: TTD is often of the order of 3 °C. The HEI recommends that DCA not be less
than +10 °F.
1
2
3
4
5
6
7
8
s
T
9
11
10
12
(1)
(y)
(1-y-y`)
(y`)
(y + y`)

7. Analysis of Regenerative Cycle with CFwH with
Drains Cascaded Backwards
10
9
12
11 ; h
h
h
h 

• Apply mass and energy
balance on h.p. & l.p.
heaters.
• Also,
)
`)(
(
)
`)(
1
(
)
`)(
1
(
)
)(
1
(
?
5
10
5
4
8
1
4
3
3
2
2
1
h
h
y
y
h
h
y
y
Q
h
h
Q
h
h
y
y
h
h
y
h
h
W
W
C
H
T
P



















1
2
3
4
5
6
7
8
s
T
9
11
10
12
(1)
(y)
(1-y-y`)
(y`)
(y + y`)

8. A steam power
plant with one
open and three
closed feedwater
heaters.

9. (Source: See page 62 of your textbook.)

10. 1. Plants can have upto 8 feedwater heaters, one of which is usually
an OFwH (usually placed near the middle of the feedwater
system).
2. Designers use computer programs to help them decide on the
number of heaters to use and their types.
3. Usually, CFwH with drains cascaded backwards has a DSH and
DC section in the h.p. stages but no DSH section in the l.p. stages.
4. One CFwH with drains pumped forward is often used as the
lowest-pressure feedwater heater.
5. The advantages of OFwH are lower cost and high heat transfer
capacity as opposed to CFwH.
6. The disadvantage of OFwH is the necessity of a pump at each
heater to handle the large feedwater stream as opposed to CFwH.
Notes on Feedwater Heaters

11. Regenerative Cycle with CFwH with Drains
Cascaded Backwards
1
2
3
4
5
6
7
8
s
T
9
11
10
12
(1)
(y)
(1-y-y`)
(y`)
Example Problem
Find the amount of steam extracted at each FwH, the pump work, heat input and
thermal efficiency of the cycle shown below if DC = 6 °C, the TTD at the l.p. heater is
2 °C and at the h.p. heater is -1 °C.
3 MPa
400 C
800 kPa
100 kPa
5 kPa
------

12. Assignment # 3
1
2
3
4
5
6
7
8
s
T
9
11
10
12
(1)
(y)
(1-y-y`)
(y`)
13
14
Find the amount of steam extracted at each FwH, the pump work, heat input and
thermal efficiency of the cycle shown below if the TTD at the l.p. heater is 2 °C and at
the h.p. heater is 0 °C.
3 MPa
800 kPa
------
400 C
100 kPa
5 kPa

13. The natural question
Placement of Feedwater Heaters

14. The extraction pressures for multiple feedwater heaters are chosen to
maximize the cycle efficiency.
As a rule of thumb, the extraction pressures for the feedwater heaters
are chosen such that the saturation temperature difference between
each component is about the same i.e.
Placement of Feedwater Heaters
FwH
to
boiler
FwH
to
FwH
FwH
to
cond T
T
T 




It should be noted that the bleed temperature may be higher than the
saturation temp.
In general, for n feedwater heaters, the optimum temp rise per heater is
given by:
1




n
T
T
T condenser
boiler
optimum
No serious effect on efficiency of small variation from the optimum
positions.

15. Example
An ideal regenerative steam power cycle operates so that steam enters the turbine at
3 MPa, 500o
C, and exhausts at 10 kPa. Two closed feedwater heaters are to be used.
Select starting values for the feedwater heater extraction pressures.
0 2 4 6 8 10 12
12
0
100
200
300
400
s [kJ/kg-K]
T
[C]
3000 kPa
815 kPa
136.2 kPa
10 kPa
Steam
 T  62.68
 T  62.68
 T  62.68
C
C
C
233.9 C
45.85 C
Placement of Feedwater Heaters

16. Comparison