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.

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

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

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

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)

Rankine Cycle with Reheat and Open Feedwater 1 2 2’ 2” 3 3’ 3” 4 Qc Wp1 Wp2 Qb Wt1 Qr Wt2 mtot-mfeed mfeed Open feedwater heater

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)

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

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=?

Closed Feedwater Heater 1 2 2’ 3 3’ 3” 4 Qc Wp1 Qb Wt1 Qr Wt2 mtot-mfeed mfeed Closed feedwater heater valve P2 =P2’ =P3 T2’ ~T3’

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

Closed Feedwater Heater 3 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)

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.

Example - Rankine Cycle with Reheat and Open Feedwater 3’ 2 2’

Example - Rankine Cycle with Reheat and Open Feedwater To be continued…..