Thermodynamics

568 | ThermodynamicsandOpen Feedwater HeatersAn open (or direct-contact) feedwater heater is basically a mixing chamber,where the steam extracted from the turbine mixes with the feedwaterexiting the pump. Ideally, the mixture leaves the heater as a saturated liquidat the heater pressure. The schematic of a steam power plant with one openfeedwater heater (also called single-stage regenerative cycle) and the T-sdiagram of the cycle are shown in Fig. 10–15.In an ideal regenerative Rankine cycle, steam enters the turbine at theboiler pressure (state 5) and expands isentropically to an intermediate preshth 1 q outq in2143.3 kJ>kg 1 0.450 or 45.0%3896.1 kJ>kgDiscussion This problem was solved in Example 10–3c for the same pressureand temperature limits but without the reheat process. A comparison ofthe two results reveals that reheating reduces the moisture content from19.6 to 10.4 percent while increasing the thermal efficiency from 43.0 to45.0 percent.TLow-temperatureheat addition212'INTERACTIVETUTORIALSEE TUTORIAL CH. 10, SEC. 4 ON THE DVD.Steam enteringboilerSteam exitingboilerFIGURE 10–14The first part of the heat-additionprocess in the boiler takes place atrelatively low temperatures.43s10–6 ■ THE IDEAL REGENERATIVE RANKINE CYCLEA careful examination of the T-s diagram of the Rankine cycle redrawn inFig. 10–14 reveals that heat is transferred to the working fluid duringprocess 2-2 at a relatively low temperature. This lowers the average heatadditiontemperature and thus the cycle efficiency.To remedy this shortcoming, we look for ways to raise the temperature ofthe liquid leaving the pump (called the feedwater) before it enters the boiler.One such possibility is to transfer heat to the feedwater from the expandingsteam in a counterflow heat exchanger built into the turbine, that is, to useregeneration. This solution is also impractical because it is difficult todesign such a heat exchanger and because it would increase the moisturecontent of the steam at the final stages of the turbine.A practical regeneration process in steam power plants is accomplished byextracting, or “bleeding,” steam from the turbine at various points. This steam,which could have produced more work by expanding further in the turbine, isused to heat the feedwater instead. The device where the feedwater is heatedby regeneration is called a regenerator, or a feedwater heater (FWH).Regeneration not only improves cycle efficiency, but also provides a convenientmeans of deaerating the feedwater (removing the air that leaks in atthe condenser) to prevent corrosion in the boiler. It also helps control thelarge volume flow rate of the steam at the final stages of the turbine (due tothe large specific volumes at low pressures). Therefore, regeneration hasbeen used in all modern steam power plants since its introduction in theearly 1920s.A feedwater heater is basically a heat exchanger where heat is transferredfrom the steam to the feedwater either by mixing the two fluid streams(open feedwater heaters) or without mixing them (closed feedwater heaters).Regeneration with both types of feedwater heaters is discussed below.