1Rankine Cycle - Feedwater Heating Feedwater heating is the second method we will examine for improving a Rankine cycleFeedwater heating taps steam from the turbine at intermediate temperature and pressures for heating the water pumped to the boilerThis heating reduces T between liquid going to the boiler and the heat sourceFeedwater heating, unlike reheat, should always improve the cycle efficiency
2Rankine Cycle - Feedwater Heating From our example problem, boiler heat input:We did not examine heating of the liquid (2 to 2’) in detailThe liquid changes from 23C to 295C before reaching saturation and starting to boilFeedwater heating adds energy to the low temperature liquid by using some steam that has performed work (which reversibly drops the steam temperature)sT23C295C600C32’21
3Rankine 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
4Rankine Cycle - Feedwater Heating 2 types of feedwater heating:“Open” feedwater heater2 liquid pumps and a mixing tank required“Closed” feedwater heater1 liquid pump, 1 heat exchangerIsolates steam pressure from boiler feedwater pressureAbbott PP uses one of eachOpen for low temperature heating (15psig steam)Closed for high temperature (~130psig steam)
5Rankine Cycle with Reheat and Open Feedwater 122’2”33’3”4QcWp1Wp2QbWt1QrWt2mtot-mfeedmfeedOpen feedwater heater
6Open Feedwater Heater at Abbott PP Gases in water evolve from the heating and are collected in the topOpen feedwater heater(~15psig~200kPa abs & 120C)
8Open Feedwater Heater Level Controller “Torque tube”Level control in the feedwater heater is similar to other industrial processes where a liquid level must be maintainedOften, some type of “float” mechanismOther common systems are “dip tubes”, optical sensors, and capacitance sensorsPneumatic controllerpneumatic pressure operates valve in the feedwater pump lineSmall tank with float
12Closed 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
13Closed Feedwater Heater 33”Goal: find fraction of mass flow to be diverted to the feedwater heaterP2 = P2’ = P2” = P3T2’ ~ T3’2”2’mfeed=?23’41Valve (const h)
14Example - Rankine Cycle with Reheat and Open Feedwater Goal: Find feedwater mass flowrate.Then, determine cycle efficiency –3’22’Assume conditions found for previous Rankine cycle example with reheat.Formulate 1st Law on the feedwater heater.
15Example - Rankine Cycle with Reheat and Open Feedwater 3’22’
16Example - Rankine Cycle with Reheat and Open Feedwater Tobecontinued…..