0. ME 400 Energy Conversion Systems Topic 2 Presentation 6 Power Cycles
Emad Jassim & Ty Newell Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign
© 2012 University of Illinois Board of Trustees. All Rights Reserved.

1. Rankine Cycle - Feedwater Heating
Feedwater heating is the second method we will examine for improving a Rankine cycle
Feedwater heating taps steam from the turbine at intermediate temperature and pressures for heating the water pumped to the boiler
This heating reduces T between liquid going to the boiler and the heat source
Feedwater heating, unlike reheat, should always improve the cycle efficiency

2. Rankine Cycle - Feedwater Heating
From our example problem, boiler heat input:
We did not examine heating of the liquid (2 to 2’) in detail
The liquid changes from 23C to 295C before reaching saturation and starting to boil
Feedwater heating adds energy to the low temperature liquid by using some steam that has performed work (which reversibly drops the steam temperature) s T
23C
295C
600C 3 2’ 2 1

3. Rankine Cycle - Feedwater Heating
An energy balance on the portion of the boiler where the liquid water is heated to saturation results in:
At state 2’,
Therefore,
~ 34% of boiler energy is for heating liquid to saturation

4. Rankine Cycle - Feedwater Heating
2 types of feedwater heating:
“Open” feedwater heater
2 liquid pumps and a mixing tank required
“Closed” feedwater heater
1 liquid pump, 1 heat exchanger
Isolates steam pressure from boiler feedwater pressure
Abbott PP uses one of each
Open for low temperature heating (15psig steam)
Closed for high temperature (~130psig steam)

5. Rankine Cycle with Reheat and Open Feedwater1 2 2’ 2” 3 3’ 3” 4 Qc
Wp1
Wp2 Qb
Wt1 Qr
Wt2
mtot-mfeed
mfeed
Open feedwater heater

6. Open Feedwater Heater at Abbott PP
Gases in water evolve from the heating and are collected in the top
Open feedwater heater
(~15psig~200kPa abs & 120C)

8. Open Feedwater Heater Level Controller
“Torque tube”
Level control in the feedwater heater is similar to other industrial processes where a liquid level must be maintained
Often, some type of “float” mechanism
Other common systems are “dip tubes”, optical sensors, and capacitance sensors
Pneumatic controller
pneumatic pressure operates valve in the feedwater pump line
Small tank with float

10. Open Feedwater Heater 3 3” 2’” 2” 2’ 2 3’ 4 1 mfeed=?
Goal: find fraction of mass flow to be diverted to the feedwater heater
P2 = P2’ = P3’ = P3”
x2’ = 0
P2” = P2’” = P3
mfeed=?

11. Closed Feedwater Heater1 2 2’ 3 3’ 3” 4 Qc
Wp1 Qb
Wt1 Qr
Wt2
mtot-mfeed
mfeed
Closed feedwater heater
valve
P2 =P2’ =P3
T2’ ~T3’

12. Closed Feedwater Heater at Abbott PP
Boiler feedwater is at boiler pressure (~500psig for gas boilers and ~1200psig for coal boilers)
Steam pressure ~115psig (high pressure extraction from turbines)
Closed feedwater heater is often a “shell & tube” heat exchanger

13. Closed Feedwater Heater3 3”
Goal: find fraction of mass flow to be diverted to the feedwater heater
P2 = P2’ = P2” = P3
T2’ ~ T3’ 2” 2’
mfeed=? 2 3’ 4 1
Valve (const h)

14. Example - Rankine Cycle with Reheat and Open Feedwater
Goal: Find feedwater mass flowrate.
Then, determine cycle efficiency – 3’ 2 2’
Assume conditions found for previous Rankine cycle example with reheat.
Formulate 1st Law on the feedwater heater.

15. Example - Rankine Cycle with Reheat and Open Feedwater3’ 2 2’

16. Example - Rankine Cycle with Reheat and Open FeedwaterTo be
continued…..